FOR THE SCHOOLS

Top: Lower bracket-bearing pin andhand control wheel. Centre: Openingin enlarger head for negative carriertwin columns with their lower-bearing60 deg. points. Bottom: Upper bracketand bearing pin. The platform tiltingscrew is visible on the enlarger body

MODEL ENGlNEER

D. H. DOWNIE describes the further

stages in the making of an enlarger

LENS MOUNT HASTO BE

As the optical system of anyenlarger is perhaps its mostimportant part, it pays to be

as accurate as possible in any workclosely associated with the lens andcondenser mounts.

To make the lens mount castinghold the small diameter set true, face,and leave a spigot 3/32 in. deep x4-1/2 in. dia. With a pointed tool,scribe a line on the lugs for the pitchcircle of the bayonet slots.

Using the index collar on the crossslide, move the tool 65 thou each sideof the line and scribe two more lines.These are an accurate guide to cuttingand filing out the bayonet slots. Boreout the boss and screw to 10 t.p.i.Using lubricant, rough out the thread.

Hone the sides of the tool andfinish to full depth with light cuts togive a clean, smooth thread. With thetool set at the last cut, recess awaythe first two threads. This will giveyou the correct diameter to turn thelens focusing tube for screwcutting.Reverse the casting in the chuck, facethe small boss and clean up theexterior surface with light cuts.

Use any dissimilar metal, brass orsteel, for the lens focusing tube. Boreout, leaving a 1/8 in. thickness wall inthe bottom. This is to drill and taplater when you mount the lens.

Make the smaller bore in this a nicelocating fit on the lens mount spigot.Cut the thread to fit the 10 t.p.i. inthe lens mount casting. A goodsmooth hand fit is required here.Finishing with a hand chaser will givethe best results.

The two packing strips which makethe negative carrier opening can bemachined or filed up parallel and toequal thickness.

The wall brackets and support pinswill require flat steel 2 in. x 1/4 in.thick. Braze or weld the bosses on,drilling or boring them for the 5/8 in.dia. support pins. Use 5/8 in. dia.b.m.s. for the pins, bevelling the endsfor good appearance. On the pins,set out the bearing holes accurately to3-1/2 in. with dividers, and centre dot.

514

ACCURATEConcluded from April 12, page 457

Drill each pair of holes in a pin atone setting in the vice, or hold thepin in a pair of V-blocks. To pick upyour marking-off accurately, start eachhole first with a centre drill. One pinhas two 3/8 in. holes and the other twofull-centre drill holes, to make bearingpoints for the two columns and allowthem to float and revolve freely.

The lamp house is made fromlight-gauge steel sheet and two discs ofsteel plate, the lower one recessed todrop on to the diameter of thecondenser tube. The top plate is boredthrough to take the boss which carriesthe tube and lamp holder. The platesare brazed to the top and bottomhalves of the lamp house. You canuse your own ideas here, if you arenot handy at sheet metal working.

Completed lamp house

26 APRIL 1962

,

i/

Ifi

//,I!

Sliding head casting-being machined Top: Screwcutting the bayonet lens mount casting, within the top picture, finished off by a focusing tube to hand. Centre: Cutting 1-1/2 in. pitch helixtechnician student in the centre, and with travelling steady; a simple spring tool-holder makes forbored to jit the columns at centres of clean, easy work. Above: Tilting head pIatform; machining3-1/2 in. in the picture immediateIy above the recess for the spigot of interchangeable lens casting

26 APRIL. 1962 515 MODEL ENGINEER

Make a plain cylindrical shape, orlook around the stores for a suitabletin container; it will serve just as well.Drill four holes in the top and bottomfor ventilation, and cover them withsmall baffle plates. Use a brass lampholder on the tube, as plastic ones maycrack under the continual heat.

To assemble, hinge the tiltingplatform to the sliding head: usegood solid brass butts. Sink thebutts in the castings to make a goodiob. Locate and drill the holes in theplatform to let the 3/4 in. rods passthrough. Elongate these holes toallow the platform to rise and fall.Drill and tap for the tilting adjustmentscrew.

Mount the cover plate, packingstrips beneath on the platform. Lineup the sides of the two rectangularopenings with a square and clamptogether. Set out the holes for fixingand drill No 0 BA tapping size.Remove and open out the holes in thetop plate and strips to clearance size,

then countersink the four holes intop plate. Tap the platform holes,and you can then assemble. Thisarrangement makes the opening forthe negative carriers.

Drill and file out the bayonet slotsin the lens mount casting to the linesscribed in the lathe. These will be at120 deg. intervals. Locate the castingin its recess under the platform andmark out and drill the tapping holesfor the BA studs. Use a taper tap;do not take it right through the holds;thus making sure that the studs willlock tightly into position whenassembled.

Make round knurled nuts for thestuds. This arrangement enables youto remove the lens mount with afraction of a turn, and change to along focus lens for larger negatives ifyou wish to add one later. Drill andtap the hole for the filter mount clearof the lens mount casting. This isjust an added refinement but is quiteworth while. Smear a little petroleum

jelly on the screwed focusing tube forsmooth action.

Make the negative carriers fromplywood or plastic sheet, two orthree for different size negatives.Two cover glasses are required foreach.; your local glazier will cut themto stze. The clamp pressure springsare made from short pieces of clockspring. Turn a hand control wheel2-1/2 in. dia. with knurled edge to fix tothe lower end of the column whichhas the helix cut on it. Wood, plasticor alloy is suitable for this.

Finish all internal surfaces withBerlin Black to avoid internal reflec-tions. The outside of the lamp houselooks well in black crackle finish. Theexterior of all the castings can bemedium blue in eggshell finish.

Mount the brackets on a 7 in.width of mahogany faced 3/8 in. or1/2 in. plywood. Arrange the distancebetween the brackets so that thecolumns revolve freely with a positiveminimum of end play. The pin with

Top: Parts ready to be assembled onthe twin columns. Above: Here the 10

Top: Finished parts. Condenser tube with condensers;

t.p.i. is being cut on focusing tubebayonet lanes casting; screwed focusing tube and lens.Above: Lamp houseparts. “ You can use your own ideas”

MODEL ENGINEER 516 26 APRIL 1962

the centre bearing holes will be at thelower end.

Stain and wax finish the mountingboard. Fix the enlarger to the wallabove the table on which you willwork. With a spirit level adjust thetilting negative platform to the levelof the table and you are all set forwork.

For 35 mm. negatives you will needa lens of 2 in. focus; for 2-1/4 in. squareor 3-1/4 in. x 2-1/4 in. a longer focus lensof 4-1/4 in. is needed. You can use thetilting table for correcting convergingverticals in pictures which includehigh buildings.

After pushing the head of theenlarger up to its approximate positionfor the size of enlargement. engage theplunger and rotate your controlwheel, moving the head up or downfor the precise size and composition ofyour picture. q

* The Looking Ahead issue pub-lished on April 12 included the first partof this article and also a six-pagepull-out giving al l the drawings forbuiIding the enlarger.

FOR YOUR BOOKSHELF

D A. BOREHAM’s Narrow GaugeRailway Modelling (Percival

Marshall, 21s.) is an authoritative andspecialised work by the secretary ofthe Model Railway Club. It isintended for those who are alreadydevoted to the construction of Britishnarrow-gauge railway systems, but isso absorbing that beginners who readit will be keener on narrow-gauge thanon standard, and the standard-gaugeenthusiast (one of Mr Boreham’s“ miscreants. ” on page 24 ?) willwant a narrow-gauge feeder somewhere along his main line.

The author makes a strong appealfor putting on record, in the form ofaccurate models, the little railwayswhich have long since disappearedand those which have happily beenpreserved. He rules out completelythe “ high-speed main-line,” such asthe 3 ft 6 in. gauge railways of SouthAfrica, and the metre gauge in Malaya,as they are not “true narrow gaugerailways.” But here. perhaps. overseasreaders may not entirely agree.

As mixed-gauge is a peculiarity ofsome narrow gauge railways-especially on the Continent-MrBoreham has dealt with its construc-tion in detail. Examples of mixed-gauge junctions are clearly shown inthe drawings and photographs. Special

26 APRIL 1962

attention is given to wheel andcompensated bogie design.

One chapter is mainly concernedwith the practical construction ofrolling stock, and another with metalwork for coaches, underframes andcouplers. On locomotive constructionMr Boreham provides a number ofuseful suggestions from his ownexperience in dealing with frames.crankpins, axles, bodywork, and theextra weights essential for adhesion.

The owner of a model narrow-gaugelayout intending to give a demostra-tion at an exhibition or garden feteshould note what the author says:“The first thing to remember aboutexhibition layouts is that the publicpays to see them, so they must work.Furthermore, the public at large isnot interested in the sort of activitieswhich really make a model railwayrailwav-like: station-work. shunting.timetable operation add similaractivities. All the public wants to seeare trains in motion, and the more themerrier.” And so the owner andwould-be exhibitor is warned.

Suggestions are made for theconstruction of the more unusual typesof railways: the Irish Lartigue mono-rail, a rack railway, and a pier railway.We may like to add the name of the33 in. gauge Ravenglass and EskdaleRailway, now running as a 1.5 in. line.

In one chapter, and in the bibli-

517

ography, Mr Boreham offers help tothe modeller in getting information onnumerous narrow-gauge railways. Forthe lone hand this approach can bevery irksome; he is strongly advisedto join a club.

Useful advice is also given on thepreparation of scale drawings oflocomotives and rolling stock. Twentyplans of station layouts are appended,with seventeen full-page scale drawingsof locomotives, coaches and wagons.They are fine examples of draughts-manship.

The main emphasis of NarrowGauge Railway Modelling is on thewell-established 4 and 7 mm. scales,but there is, of course, plenty of scopefor the modeller interested in 10 mm.scale layouts (the standard gauge is45 mm ). Thus, for a 2 ft narrow-gaugeline the gauge would be 20 mm. andfor a metre-gauge system the gaugewould be precisely 32 mm.-gauge 0.

Mr Boreham recommends themodeller “ to use the largest scale hehas room for.” He also advocatesscaling to the exact gauge. There isa variety of gauges below the 4 ft 8-1/2 in.,and so if narrow-gauge is to be mademore popular some measure ofstandardisation may be advisable.

The book is entertaining throughout,is a good example of careful compila-tion, and is recommended for anyrailway modeller’s bookshelf. E.A.S

MODEL ENGINEER