Backlash ? In my CNC ?( NOTE : This article references FANUC controls but is basically applicable to all CNC controls. )A machine is a machine is a machine. Just because the words CNC are attached to your machine tool doesn't mean it doesn't get old or lose it's accuracy. And one of the main reasons your CNC machine losses it's accuracy is due to the ever infamousbacklash.What is backlash ?The axis motion that makes up your machine tool is done through the use of ballscrews attached to your machining center's table and spindle housing or your lathes tool turret. The nut for the screw is usually attached to the table or turret and is connected to the ballscrew which is connected to your drive motor. As the motor turns the ballscrew, the nut moves the table or turret and your machine has motion. All ballscrew assemblies have some "slop" or backlash at assembly - the match between the screw and the nut. Basically backlash is the amount of motion the screw has to make when reversing direction before the nut and therefore the table or turret start to move.How is backlash compensated?Using the machine tools CNC controller, the builder can tell the controller how much motion is lost when the axis reverses direction due to the backlash. This value is stored in the machines parameters and when the particular axis goes to change direction, it looks in this parameter to know how much motion it needs to have (how many revolutions of the screw it needs to make) before the axis will physically start to move. The value of the parameter is usually in MM, although they may be in INCH settings in some instancesWhy should I care ?As the machine tool wears or as contaminants get onto the ballscrew and therefore in the nut, the original backlash settings lose their accuracy and therefore effect the accuracy of the machine tool. Positioning problems arise, straightness problems arise, as do a host of other related problems. Basically, the machine does not meet the specs like it did when it was new.As mentioned above, sometimes contaminants can get onto the screw and then get carried into the nut. Although most nuts are protected against chips and debris, poor conditions can sometimes force the debris into the nut causing premature wearing of the screw and a pronounced backlash problem. Those contaminants can range from coolant to cutting chips. That is why it is essential to keep the machine areas clean and free from an excessive amount of chips. If chips are allowed to accumulate, they can become packed and when the machine tool moves, it forces the chips under guards and into areas where they shouldn't be. Eventually they get forced into the screws and nut areas causing un-repairable problems. Ballscrew replacement is not a cheap repair. Keep the expression: "An ounce of prevention is worth a pound or cure" in mind when planning your maintenance efforts.What can I do about backlash ?The normal method for adjusting the machine's backlash involves adjusting the backlash parameter values. This can be done by a qualified technician or you can give it a try. Outlined below is a brief but complete explanation of how to check for backlash and how to adjust it in FANUC controlled machine tools.How often should you check it ? Recommended time frame would be about every 3-6 months. If you create the following sample programs in your memory and leave them there or upload and download them from a shop floor PC, you shouldn't spend much more than one hour or so keeping your machine accurate and at the same time you'll be checking for any other damaging problems. For example, if you see the backlash changing drastically, you might find a way lube problems or chip build up problem before they cause bigger problems.How much backlash compensation is acceptable ? As mentioned above, all machines have some backlash adjustment, even when brand new and at ship time. As the machine wears, that value needs to be increased. Normal wear might have .005" - .010" adjustment in a ballscrew. If the value needs to be more than .010", it might be time to take a deeper look. Also, you need to check the backlash at various areas of the screw as it might be wearing more in one area than another. One example might be on a machining center where the set-up people always mount the vise or fixture in the middle of the table. Looks good but also causes a massive amount of wear in one confined area. the best scenario is to mount the vise or fixture all over the table, changing the location for every job - speading the wear around evenly.The best way to check the backlash is to first clear out the current parameter value in the control. The various parameter numbers for the variety of FANUC controls are listed further down in this page. First, write down the current values, then clear them by setting them to zero. Then make the machine move through the memory mode. We have found discrepancies in the past between the machine's handle or MPG mode and the memory mode, so we recommend you run the machine through MDI or through the machines memory mode. Below are a couple of sample programs for FANUC controls that you can use to gather your backlash data. Remember, the backlash is the amount of wasted motion when the particular axis changes direction.If possible, check the backlash at different areas of the screw. On a machining center, mount the block in different areas of the table and check. On a lathe, check the backlash as various distances away from the chuck. If the values are different in the different areas, this could mean that the screw is worn in one place different than others. On a lathe, this tends to happen close to the chuck where the majority of the cutting is performed. You can's do much about to prevent it on a lathe but on a machining center, you can help yourself by mounting the chuck or fixture in various places on the table to allow for even wear. If you find big differences in the backlash in different areas, it may be too late and you may have to replace the screw.Machining Center Backlash Adjusting Program.If you have a Vertical or Horizontal machining center, the following program will give you an idea of how to create a program to test the backlash for each axis.The following is a sample program for the X axis. Start the program with an indicator mounted to the spindle, touching a block mounted on the table, touching the right side of the block.

You can let the program run a couple of times to make certain that you get the same readings at the M00's in the program. The difference between Reading #1 and Reading #2 is the amount of backlash in your X axis.You can use the same style program making changes as required to perform the same function for the other axis as well. Basically, you just want the machine to move one way then back, stop so you can and collect the indicator reading, then move the other way and back and collect that reading.CNC Lathe Backlash Adjusting Program.If you have a CNC lathe, the following program will give you an idea of how to create a program to test the backlash for each axis.The following is a sample program for the Z axis. Start the program with an indicator mounted to the spindle or chuck, touching a block mounted on the turret or the tool turret itself, touching the spindle side of the block or turret.

As above, you can let the program run a couple of times to make certain that you get the same readings at the M00's in the program. The difference between Reading #1 and Reading #2 is the amount of backlash in your Z axis.You can use the same style program making changes as required to perform the same function for the other axis as well. Basically, you just want the machine to move one way then back, stop so you can and collect the indicator reading, then move the other way and back and collect that reading.What to do with the Numbers.Once you collect the value and know the backlash for your machine, you'll need to adjust the parameter values. Parameter values for FANUC controls are usually given in MM values, without the use of decimal point. So, for example, a parameter value of 30, actually means .030 mm - the decimal point is imaginary and placed three places from the right. You can use the following conversion formula to change your backlash data to mm, then enter that value into appropriate parameter - don't forget to drop the decimal point and add any zeros as required.MM = inch x 25.4For reference, 1mm = .0394 in.On a CNC lathe, the value can either be a radius or diameter value. Since there is no easy way to tell, input a radius value then re-run the test program. Adjust as necessary and make a note so next time you will know.When your done, you should re-run the particular axis program again to double check that you did the backlash adjustment correctly. When you re-run the program, you should see less than .0001" backlash.FANUC Backlash Parameter Numbers.Listed below are the parameter numbers for the various FANUC control models. One note, lathe controls are T models whereas machining centers are M models.FANUC Version 6T :X Axis = Par # 115Z Axis = Par # 116FANUC Version 6M :X Axis = Par # 115Y Axis = Par # 116Z Axis = Par # 1174th Axis = Par # 118FANUC Version 10/11/12T :Par # 1851Seperate line for each axis.FANUC Version 10/11/12M :Par # 1851Seperate line for each axis.FANUC Version 0T :X Axis = Par # 535Z Axis = Par # 536FANUC Version 0M :X Axis = Par # 535Y Axis = Par # 536Z Axis = Par # 5374th Axis = Par # 538FANUC Version 16/18/20T :Par # 1851Seperate line for each axis.FANUC Version 16/18/20M :Par # 1851Seperate line for each axis.NOTE :This 16/18/20 series of control can have a seperate backlash amount when moving at a feedrate and for moving at the rapid rate. This is an option - check with your machine tool builder. If this is the case, Parameter number 1851 is for feedrate and # 1852 is for rapid. You can use the programs above, just change from G00 to G01 and add a feedrate to test for the feedrate backlash amount.Disclaimer: CNC TIPS are provided without charge or obligation by Kentech Inc.. It is the responsibility of the reader to perform any action outlined here in a safe and responsible manner. The reader assumes all responsibility for service or actions taken as a result of the information contained here. KENTECH INC. and its employees assume no responsibility for personal or property damage, any type of monetary losses or losses caused directly or indirectly from the material provided in this Web page or any pages contained in the WebSite.THIS DISCLAIMER OF WARRANTY AND LIMITATION OF LIABILITY IS GOVERNED BY THE LAWS OF THE COMMONWEALTH OF MASSACHUSETTS

Eliminating Backlash, Part 1: BasicsIf you're looking over this page, I'll assume you wonder why you should eliminate backlash (or how much can your stand), you want to build a CNC machine from scratch, or you are converting a manual machine that has measurable backlash. If you have a machine already converted that has the "good" parts (like ballscrews), and still have too much backlash, tryPart 2for ideas to solve your problem.What is Backlash?According to theCNC Dictionary, backlash is any kind of unexpected play in an axis due to clearance or looseness of mechanical parts.When the axis is commanded to move, the drive motor may turn briefly before movement begins.That delay is the backlash.Backlash has a variety of causes.The most common is play between the leadscrew threads and those of the nut.ACME screws can have considerable backlash of this kind, while ballscrews may have almost none.Another source is any tendency for the screw to move axial in the bearings that hold it, or any other such play in the system.Precision angular contact bearings with preload are often used to combat this tendency.Gears, belts, and chains can all introduce backlash into a mechanical system.Even loose fasteners or flex in the mounting plates or chassis can be a source of backlash.Why Eliminate Backlash?Backlash is a subject that comes up frequently in CNC machine discussions, and it seems machine makers are willing to go to no end of trouble and expense to eliminate backlash. Why is it considered so bothersome?There are several reasons. First, CNC machines are largely blind and even most closed loop systems lack much ability to sense that the axis hasn't moved even though the motor has. A system that has both encoders on the shafts and some form of linear encoder may be able to sense the axis hasn't moved, but even a machine that "knows" the backlash is there suffers from the other problems.A second issue for backlash can occur while climb milling (for a definition of climb and conventional milling, try theCNC Dictionary). Climb milling is often preferred to conventional milling because it produces a better surface finish and places less stress on the machine and cutter. However, if an excess of backlash is present, the action of the cutter can operate to pull the workpiece suddenly into the cutter by a distance equal to the backlash. At the very least this is counterproductive to surface finish and at the worst, it can result in a broken cutter, scrapped workpiece, or even injury to the operator from flying debris. Very bad! Clearly this one doesn't go away even if the machine knows how to compensate for the backlash. The benefits of climb milling are so great that even some of the later manual milling machines at the high end came with ballscrews or "backlash eliminators" just so climb milling could be practiced on these machines. You can still buy retrofit kits today to add ballscrews to say a Bridgeport mill for this purpose. If you don't plan to CNC the machine, make sure your ballscrew installation is properly spec'd to avoid "back driving". This is where the friction on the ballscrew is so low that it won't hold the table in place against the cutting forces. It's largely a matter of specifying the right lead for the ballscrew or in some cases have a friction mechanism on the axis that can be engaged and disengaged as needed.The last issue is one of smooth continuous machining when direction is reversed. It's possible for a CNC machine to do things a manual operator spinning the handwheels could never do, like cutting a smooth circle. The operator needs a rotary table to do this, but a CNC machine can smoothly operate two axes precisely enough to generate a circle, unless there is backlash. It turns out a circle forces several changes of direction throughout the course of the cut. See theStepper/Servo/Backlash Simulatorfor more details of this, but here is an example of what such a cut looks like when there is backlash:

Backlash "Ears": 1" diameter circle, 0.020" backlash on both X and Y axes...The little kicked out "ears" happen at each point an axis changes direction on the circle. You can imagine also that when a part is being profiled, the spindle is moving back and forth and up and down to follow the contours of the part. Every time the spindle goes down and then shifts back up, that is a reverse of direction. Backlash is going to cause inaccuracy or dwell marks at those points where the direction is reversed.As a matter of fact, cutting circles is an acid test for a mill. Try one and see how it turns out to get a rough idea how your backlash is doing as well as a number of other factors.It's worth noting that lathes have it a lot easier than mills where backlash is concerned. There is no equivalent of climb milling on a lathe, and a reversal of direction need not occur smoothly while cutting as often.Lastly, the CAM software you'd like to use to generate your g-codes assumes your machine has no backlash. It doesn't produce code that operates the machine the way a manual machinist sensitive to avoiding backlash problems would.Okay, hopefully we've made a believer out of you that you want to get rid of backlash. BTW, for some applications, such as plasma cutting, the resolution needed may be so low that quite a lot of backlash is acceptible. Try to keep that in perspective as well.What About Backlash Compensation?When a manual machinist is faced with backlash, he naturally follows a routine that for the most part allows him to ignore it. That routine involves taking up the slack (or backlash) on the handwheels before beginning to cut. Machine control software such as Mach 3 can automatically follow the same routines for backlash. It isn't quite as good as the operator, because it lacks his "feel" of exactly when the lash is gone and instead relies on being told how much backlash is present. It's very hard to get that exactly right and it may change as your leadscrew wears or your machine needs adjustment (for example the gibs need adjusting). Remember also the problems with climb milling and reversing direction. Our manual machinist probably avoids both because he knows he'll get into trouble. We're not getting full value out of our CNC machine if we force it to avoid climb milling, cutting circles, or the host of other operations that involve changes of direction during a cut.So, backlash compensation is a weak bandaid for the problem. It's better than nothing, and may tide you over until you break out the checkbook and fix your backlash, but it isn't something you'd probably like to live with long term. If your machine is a lathe, you may be able to live with just backlash compensation.How Do I Measure the Backlash in My Machine?Okay, next question. How do I know how much backlash I have? A DRO is handy if you have one, because it will directly measure how far the axis really moved. Failing that, put a dial indicator in the spindle (attach one with an Indicol or similar and don't turn the spindle on for Heaven's Sake!). Set it up to read against a 1-2-3 block or other flat reference, put a little tension on it with the handwheel (or manually jogging a CNC machine) to get a reading, and zero the indicator. Now move the axis in the direction that takes the tension off the indicator a distance that is greater than any possible backlash and read that off the handwheel. Turn the handwheel in the opposite direction exactly the distance you read off from the first movement. Check your dial indicator. The amount it did not get back to zero is your backlash.A DRO will directly measure the distance travelled, and when I first got myIndustrial Hobbies mill, I measured 0.010" of backlash on the X-axis using my DRO.Sources of Backlash in Conventional MachinesOkay, so where is that backlash coming from and what's it going to take to eliminate it?In all likelihood, the majority of your backlash is coming from your leadscrew and associated nut. Most manual machines will use an ACME leadscrew. Because of the nature of these screws, if the nut is made tight enough to eliminate the backlash, it introduces too much friction and the screw becomes impossibly hard to turn. In addition, the nut is often made of a material that wears more rapidly than the screw, so that it may be replaced more cheaply than replacing the leadscrew itself. Such nuts often have an adjustment to take out some of the excess backlash as they wear. When they don't or when they're not adjusted properly, or when all the adjustment is used up on an old heavily used machine, you get lots of backlash. It is not unusual to see backlash of 0.005" on a good quality manual machine with ACME screws, and that figure can easily deteriorate to 0.025" or worse on a badly worn machine.There are other sources too, such as the gibs or the mounting of the nut or screw. There can be play in how a handwheel or motor is attached to the shaft as well. Since the leadscrew is the biggest source of trouble, we'll consider leadscrew alternatives first.The "Good Parts": Leadscrew AlternativesThe most commonly discussed alternative to a conventional ACME leadscrew for CNC use is a ballscrew. Ballscrews are typically intended for CNC work, and so are made to minimize backlash. Because they turn with a lot less friction than an ACME screw, they can be built to much tighter tolerances. Ballscrew come in rolled and ground flavors, with the latter being more precise and more backlash free. A decent rolled ballscrew will deliver 0.003" backlash while a poorly made one has perhaps 0.010". A ground screw ought to be no more than 0.001" and probably should be less. There are ways to reduce this further having to do with the nuts and preloading of oversized balls, which I'll talk about below.Besides ground versus rolled, ballscrews come in different accuracy grades that you should be aware of:C0 - 3um or 0.0001" per 300 mm / 12"C3 - 7um or 0.00027" per 300 mm / 12"C5 - 14um or 0.0005" per 300 mm / 12"This refers to how close the position will be after the screw has turned through 12" of motion. Note that in this area, there are ACME screws available that are every bit as accurate, so the ballscrew has no special advantage here. Another thing to be aware of is that a lot of machine control software, including Mach 3, has a feature known as "ballscrew mapping". This feature lets you measure the true position reached at various points along the ballscrew and use it to compensate for errors in the ballscrew. This function works very well, and should be taken advantage of if you have the means to accurately measure the deviations. This alone is a good reason to install aninexpensive DROat least temporarily on your machine until you can get the compensation tables calibrated.It is also interesting to note that this error can vary as the temperature changes based on the room the machine is in and how hard its working. Companies such asHeidenhainsell special controls based on linear scales (i.e. scales like a DRO uses) that dynamically compensate for such errors on very high accuracy machines. It's quite interesting to read theirtechnical articlesabout this and gain an understanding of how much error can accrue from such factors as temperature variations. The effects of a fully warmed up machine over the full length of the ballscrew was about 0.004", which is significant to many applications.There are some alternatives to ballscrews that I won't discuss here. They're not seen very often, and you can research them on the web if you like.Nuts, Nuts, and more NutsLots to know about nuts and backlash. First, you can get nuts made to reduce the backlash of an ACME screw? Wait a minute, you say, we just spent all that time hearing you can't do that without getting too much friction, what gives? Well, it turns out that if you make the nut from Delrin or similar low friction material, you can pretty well get rid of a lot of the normal ACME backlash and things work well for a time and with limited cutting forces. I feel this is a good solution of something like a plasma or router table, but I'm skeptical about how well it would work if you need high cutting forces, for example, to cut metal. In addition, such nuts will wear out rapidly and need to be protected from chips and other contamination.Secondly, one can arrange a scheme whereby there are two nuts with a spring (such as a Belleville Washer) between them to preload the backlash out of the system. This works for ballscrews, and it also works for ACME screws, albeit with a lot more friction. This is normal approach to eliminate backlash with ground ballscrews. It adds a fair amount to the cost, but is worth it.The last trick is to load the ballscrew with some oversize balls to take out the slop. This is the normal approach to fine tune a ground ballscrew, and it will work to an extent with rolled screws, but there is a problem with the latter. Since rolled screws are not made to the same degree of precision as ground, too much preloading with oversized balls can lead to binding. The grooves are simply not laid out precisely enough to use this method to eliminate all the backlash.Using these techniques carefully, one should be able to get the backlash in the ballscrew itself down to the tenths level (0.0001" to 0.0005") or perhaps less with a very high quality screw.Mounting the Leadscrew and Nut Properly: Angular Contact What? They Cost What?!??Now you've get a ballscrew with either double nuts or preloaded properly with oversized balls. The next step is to ensure the ballscrew is mounted properly. Unfortunately, this is neither an easy, nor particularly inexpensive thing to do. By design, ballscrews are intended to be secured at their driven end by a pair of angular contact bearings. The other end of the screw is either left to float free in some designs, or secured by a bearing in such a way that if the screw expands or contracts due to heat, it has the freedom to do so at this end. The two angular contact bearings are typically installed in a preloaded configuration which holds the ballscrew firmly and prevents any motion along the screws axis, but still lets the screw turn freely when driven. There is a lot to know to propely design and build a ballscrew mounting of this type!First thing is to get familiar with the bearing maker's literature on angular contact bearings:BardenTimkenNSKFAGIt's really not all that painful to read through these documents, and there is a lot of good interesting information there. If you learn nothing else, be sure to study the standard nomenclature used to identify these bearings. If you're going out fishing on eBay or the Internet to find them, you've got to know how to identify what you're looking for or looking at.Now given those resources, we can start to explore some of the issues, or at least some rules of thumb that can be used in this area. First, how does the double bearing mounting look and work? Here's a basic schematic representation:

Mounting a Ballscrew with 2 Angular Contact Bearings...Note the crossed lines. These are thecontact anglesfor the angular contact bearings. Because they are opposed, this provides the resistance to back and forth motion along the axis of the ballscrew. In addition, the bearings are preloaded to take out any internal slop. The preload is provided by the spacer between the bearings, the shoulder of the ballscrew, and the lock nut that is threaded on the end of the ballscrew and bears against the other end of the bearing pack. The ballscrew shoulder and locknut bear on the inner races of the bearing, while the mounting block and cover bear on the outer races. The torque on that nut establishes the preload, and all of these surfaces need to be finished with some fair precision if the end result is to work precisely. The cover is simply holding the bearing assembly in place with respect to the machine, but it does need to be tight with respect to the outer races, so some shimming or spacers may be required.In this case I have shown the two bearings mounted in the DF, or duplex face to face configuration. The pressures against axial movement are outward, and you can see the bearing balls are supported in that way. You can reverse both bearings to create a DB, or duplex back to back configuration. This still works, and in fact, the bearings will be even stronger at fighting axial movement. The advantage of the DF configuration is that the assembly can tolerate misalignment a lot better, so this is the recommended configuration in this application.Okay, let's assume you've carefully perused the bearing literature, you understand the way they're to be employed, how do you select the best bearings for your application? There are a number of selection parameters to consider:Bore SizeYou're going to want a bore that works with your ballscrew. In all likelihood, you may have to turn and thread the end of the ballscrew. You want as precise a fit as possible for best accuracy. The larger the bearing, the stronger it is likely to be as well, though you can check the specs to see for sure. You can't run the ballscrew inside a sleeve to make it fit a bearing that's too large and hope for any accuracy, however.

Contact AngleRemember the crossed lines above. The greater the angle, the stiffer and better the bearings will be. 15 degrees is the low end of the scale, 60 degrees is the high end of the scale.

PreloadMore is better. You shouldn't exceed the recommended preload of the bearing, but bearings are made to different preload specs. Get the ones with more preload for this application. It is amazing how much preload can be beneficial in eliminating as much backlash as possible. Think 500 lbs and up if you really want to control backlash to minute levels. A typical 15 degree contact angle bearing with high preload might only tolerate up to 125 lbs of preload.Alternatives:You can concievably stack 4 bearings instead of 2 in order to do better, but I confess I do not know much about this higher level of vodoo!In addition, while I said not to exceed the preload, if you have a cheap set of bearings (not a $1000 set!), you might experiment with creeping up on more preload. The downside is that it is going to introduce friction and wear that at some stage may be self defeating.

Quality, Accuracy, and TolerancesThe bearing is going to have some slop in it. The higher the accuracy rating, the less there will be. Here is the secret decoder ring:DINJISABECBore DiameterWidthRadial RunoutApplication

P0Class 01(0.00012)0.00012

P6Class 63(0.00008)0.000120.00008

P5Class 55(0.00008)0.000080.00006Machine Tools

P4Class 47(0.00006)0.000040.00003Machine Tools, Spindles

P2Class 29(0.00004)0.000020.00002High Speed Spindles

There is at least one highly regarded low end CNC mill being sold today that uses ABEC3 bearings to mount the ballscrews. OTOH, if you want the best, ABEC 7 is the way to go. Note how small the actual differences are between these two grades: 0.00008" runout vs 0.00003" runout. Half a ten thousandth! I can see why for many applications the ABEC 3 works well enough. You need to decide what will be good enough for you, as this particular specification drives up bearing prices faster than anything else. It's easy to pay $1000 or more for a set of ABEC 7 bearings.

Axial Load RatingIf your bearing manufacturer offers this number, you've got the diving rod right there. This is the measurement of what it takes to cause axial motion with the bearing. You want the biggest honking axial load you can get!

Understanding Angular Contact Bearings Specifications for Ballscrew ApplicationsWhen it comes to mounting these little jewels, keep in mind that you probably want ground spacers between the two bearings and the preload nut probably wants to be ground as well. All that can be done for you by a qualified shop, or in some cases you can purchase the parts already ground. Try to buy your bearings as a matched pair (they'll say "DUL" or "DUH" in the bearing name) and they won't need the spacer between them either. As mentioned before, you may need to shim or spacer the bearings in the mounting block to make sure they're properly locked in there on the outer races. You will also want some form of dust seal to keep the junk out of your bearings. Here is an exploded view of a professional ballscrew bearing block:

Note that the spacers shown here are not for preload, they simply allow more bearings to be mounted in the block for even greater stiffness, or different spacers would allow some bearing substitutions. Also note that this block has the bearings mounting in DB configuration, rather than DF.I suspect when you first start shopping for these bearings, particularly if you set out looking for ABEC-7's, your initial reaction is going to be one of discouragement. Great bearings are really expensive. It's easy to spend $1000 without hardly getting started. Your application may justify it, but most don't. Here's a couple of less expensive compromise bearings I came across that are worth looking at:7204CTDULP4: $200 for a duplex pair by Nachi (good name) that can be run in DF config, and are ABEC-7's. The downside? There's only a 15 degree contact angle and the preload is light. Still, not a bad starting point.7204: $200 too much? How about $24 each for ABEC3's? The downsides--they're not ABEC-7's and they're not a matched duplex pair. OTOH, they have a better contact angle at 40 degrees. Can't tell from this what the preload spec might be, which is bad. Note that what they're calling it, just "7204", is an incomplete desription by bearing nomenclature standards. It would be like calling a person "Frank". If you know there person well, you know which Frank, but in this case "7204" only tells us the size of the bearing. There's a lot we don't know about these bearings, and most of it is probably to the detriment of performance. You don't want to just go ask for a "7204". Read the information above and learn how to fully specify your bearing if you want the best performance.What if you do want the all the performance possible? Try to find some bearings such as "7204A5TYDUHP4". They'll have a higher contact angle and preload than the "7204CTDULP4" bearings I mentioned, which as we've discussed makes it a better choice. The real expensive bearings, the ones designed for ballscrew applications on machine tools and used by pros, would be something like "20TAC47BDF". That nomenclature is a little different, but you can find them out there in the bearing catalogs. They won't be cheap, but they have a huge contact angle, 500 lbs of preload, and they are ABEC-7's. Don't expect to find the 20TAC's sitting in an eBay auction or expect to buy them from a skate bearing supplier. You might get lucky, but it's unlikely in the extreme. Get ready for some sticker shock on the price, but if they're what you need for ultimate performance, that's what it takes. Many bearing manufacturers keep these types of bearings in a separate "precision" or "machine tool" bearing catalog. Be sure to sniff around their web sites for those catalogs to see what these bearings are and to learn more about how to employ them in this sort of application.There are endless combinations and trade offs. Shop carefully. Your mileage may vary, and by all accounts, you do get what you pay for!The other end of the ballscrew can be left unsupported, and indeed has been in at least one CNC mill out there, but this limits the speed you can drive the ballscrew without whipping. In this case, support it with a normal deep groove ball bearing such as you would use on an electric motor shaft. It's not providing precision, it's just providing support and the leeway for the ballscrew to expand and contract axially with temperature changes.I Want to Use Cheaper Bearings Instead of Matched Duplex PairsThis can be done. The matched pairs have simply had either the inner or outer race ground so there is a differential size between the inners and outers and preload can be had by compressing the shorter of the two until the races from the two bearings are in contact. One can achieve the same effect by using spacers on one or the other race (but not both!) to create that differential instead of grinding. Doing this is going to require a certain amount of skill and perhaps a lot of trial and error.The first step is to measure the bearing race deflection with a given amount of preload. Use the amount recommended by the bearing manufacturer for these bearings. This given amount can be placed on the bearings using weights. Measure how much the races deflect with that amount of weight, and then make yourself a spacer with the same thickness.For a DB configuration (stiffest configuration, but most alignment sensitive), you will be shimming the outer bearing rings. For a DF configuration (less stiff, but more tolerant of shaft misalignment), you will shim the inner bearing rings.A variety of things can be used to make the bearing spacers, but they need to be very flat, and they will be very thin. In a pinch, aluminum foil could be used, but you'll need to carefully cut it to the correct size to serve. A more elegant solution is to draw up the appropriate washer shape in your 2D CAD program, send it to a laser cutting house, and have them cut washers out of shim stock. You'll want to get some in 0.010", some 0.020" and some 0.002 and 0.005's. Be sure to consult your bearing handbook to make sure the washers are sized properly before you have them made up.

A note of warning: don't try to set preload by a torque wrench on the nut you're tightening or some sort of calculation on the force that nut delivers! It is almost impossible to get it right. A lot of the force is eaten up as friction, and you will not have enough data to do proper preloading this way. The required information is held closely by the bearing manufacturers. When properly installed, tightening that nut will just tighten down the races on the shoulders and spacers. The nut will come to a stop when things are tight and you should not over tighten further.A second warning: don't think you can get the preload right by feel. It is very easy to damage your bearings this way!You really do have to rig up a test rig with weights and measure deflection!You should also make sure the bearings are not fit too tightly on the shaft or in the bearing block. A hand slip fit is all you need! Too tight a fit can damage the bearings or make it impossible to accurately set preload.For more thoughts and details on angular contact bearings, check out mymill belt drive spindle project page.Align the High Spots When Installing the Bearings!I want to stop here and mention an important point, particularly if you are about to disassemble a ballscrew mounting assembly. Be sure to record the orientation of the bearings relative to the housing so you can put them back exactly as you found them. When assembling a new bearing system for the first time, most of these bearings have the high spot marked on the bearing. Align the high spots for the two bearings. This ensures that the ballscrews moves eccentrically in the same direction as the two bearings rotate, rather than wobbling, and will result in better performance.Okay, that's the basics. I will assume at this stage you have a good ballscrew, proper nut to minimize backlash (or preloaded oversized balls), and a good mounting scheme using high quality angular contact bearings. I would hope your backlash is measuring considerably less than it had been before all those new goodies got installed. Hopefully its under 0.005". The next section discussed refinements to further reduce backlash.

Eliminating Backlash, Part 2: RefinementsIf you're looking over this page, I'll assume you have a machine already converted that has the "good" parts (like ballscrews), you've gotten rid of most of your backlash (let's say you're down to 0.005" or less), but you still have too much backlash. If instead you either, wonder why you should eliminate backlash (or how much can your stand), you want to build a CNC machine from scratch, or you are converting a manual machine that has measurable backlash, tryPart 1.I Installed , And Still Have Backlash. What Should I Do?First, make sure you do in fact have the Good Parts on your machine and that they are installed in the most advantageous way. If you have any doubt, you might just quickly perusePart 1.Okay, so you have the good parts and still have backlash. How to go about diagnosing? Simply put, there are just a few areas that can be sources of the backlash on a given axis: Leadscrew & Nut Leadscrew and/or Nut Mounting Motor Drive Mechanism Slideways and or Gibs Machine Flexure and RigidityLet's look at each one, describe how backlash can develop there, and discuss what you might do to eliminate that cause if you diagnose it as a problem.GibsLet's start here because it is the easiest to work with and likely the source of a fair amount of your problem. I'm assuming you're not running commercial ball bearing linear slides, but have conventional ways of one kind or another with adjustable gibs. Start by making sure the slideways are properly lubricated. I hate to leave out that essential point, but while we're on it, be sure your ballscrew, ballnut, and angular contact bearings have proper lube as well. Remember, any unnecessary friction can translate into forces that want to bend or deflect something and can lead to backlash.Unfortunately, adjusting your gibs is a matter of art coupled with much trial and error. I will try to provide some insights. The recommendation is to run the gibs as tightly as possible but no tighter. Easier said than done. If they are too tight, that yields the unnecesary friction that then leads to flexure or stick/slip which leads to backlash. If too loose, there can be slop in the system which translates to backlash and in the worst case, binding and more backlash. In the end, you need to creep up on it by gradually tightening the gibs and taking backlash measurements at each tightening. The backlash should gradually reduce until you've gone too far, at which point it'll jump back up. Back off in small increments and try again to find the sweet spot, being better informed about when to stop the next time around. I have often wondered if measuring the torque applied to the adjusting screws wouldn't be a way to make this a little more systematic and scientific. It can take you literally hours to get this right the first time, but it's worth it if you're chasing out the last increment of backlash.Of critical importance is balancing the forces if you don't have tapered gibs. Tapered gibs have a single adjustment screw that varies tension along the whole length of the gib. Non-tapered gibs use multiple adjusting screws along the length of the gib, and the challenge is to get them all about even. Again, I wonder whether a digital torque wrench wouldn't be a God send to balance that all out properly.Trial and Error: Tighten, Measure Backlash, Rinse and RepeatIn terms of more analytical approaches, I have two suggestions. First, you can simply adjust your gibs while systematically measuring backlash and quit when you have as little backlash as possible and it is still possible to turn the leadscrew. Note that minimum backlash may not turn out to be at maximum gib tightness, so that's why we're measuring backlash each time we tighten the gibs.Set Gibs Based on Slop Perpendicular to the AxisA second analytical approach is one I was told is the Bridgeport factory procedure (and is also recommended by Fadal) for setting up the gibs. Use a 0.0001 reading indicator and measure the slop in the slide. Example: For the X axis, place the mag base on the end of the saddle and put the stylus on the table. At that end of the table push and release. Then pull and release. The differance is the amount of clearance in the slide. Repeat at the other end of the saddle. Adjust gib in a like new machine with little wear to give a reading of 0.0005 (note, Fadal recommends 0.0003", whilePyramid, a rebuilder, recommends 0.0004"). A machine with more wear will have to be checked with the table closer to the end of travel. The same procedure is used to set the saddle to knee gib. There must be some clearance for the oil film and that film also helps dampen vibration. On a machine with hardened and ground box ways and turcite on the moving member the procedure is to set the clearance to almost nil. 0.0001 is a good number. This is again a practice of checking backlash while setting the gibs, the difference is the Bridgeport factory knew what backlash to expect on a newly manufactured machine.Prototrak wants you to adjust their lathes so the cross slide has not more than 0.001" of slop perpendicular to the axis.Use Your Load MetersNow suppose you have a CNC machine. Are there load meters on each axis? If so, you have a shortcut to consistent gib adjustment because the load meter will tell you how tight you have them. I plan to install load meters on my upcoming IH mill CNC conversion for this and other reasons. The load meter is just an ammeter on the axis DC supply before it gets to the driver board, so it wouldn't be hard to add these. One account I read suggests setting the gibs so that your axis load is about 30%.Use a Torque WrenchSouthwestern Industries (ProtoTrak) sets the gib tightness on their CNC lathes using a torque wrench. They recommend 15 in/lbs of torque be all it takes to turn the ballscrews.Linear Slide AdjustmentsIf you are running ball bearing linear slides, and you have two of them on an axis, are they truly parallel, or are they binding up because they're not? The latter will result in flexure if you overpower it with a strong motor and leadscrew combination. You've got to get them parallel to an acceptible standard,Leadscrew & NutThe gibs are adjusted to best effect, and you're still on the hunt for backlash. What's next?Is everything bolted up tight with no play or flexure back to the machine? Just check it out carefully, perhaps even disassembling and reassembling to make sure everything is torqued well. Make sure the leadscrew runs parallel to the direction of axis travel, or you're going to get binding at some point that may force flexure and therefore backlash into the system. If you have access to do so, try to place your indicator against the mounting points of the ballscrew and ballnut in order to check for small amounts of flexure where there should be none. If you find some, you either need a beefier bracket, beefier mounting method, or less friction in the system (gibs too tight? everything lubed properly? ways in good shape?).You can also try setting the indicator's magnetic base on the table, and the indicator tip on the ball groove of the screw. Try to move the table by hand. You should not be able to move it far at all (