OVERHAULING THE LATHEby B. J. Whitehead

M ANY READERS must be in possession of lathesand other machinery that have slide wear con-centrated in sections of most use. 1 expect thatsome have heard of, if not come in contact with,the old craftsman held in awe because his mach-ine is in such a bad condition that he is the onlyone able to produce good work from it. Howmuch better he could perform on a good machinewhere full concentration could be given to theworkpiece rather than the eccentr ic i t ies andshortcomings of his machine tool.

An accurate machine is a joy to use. but wearcreeps in insidiously, one day it is found imposs-ible to adjust the gibs to give full travel and yetbe tight enough for accurate work in the sectionof most use. When this situation arises it doesnot necessarily mean an expensive regrind foryour lathe bed for example, my aim is to showhow small machine tool slides may be restoredby the amateur with a minimum of equipment. Inthis age of enormous labour costs few reasonablypriced machines have hand-finished slides, soowners of new machines may profit from a littleof the work I shall attempt to describe.

Rather than describe the overhaul of a par-ticular make and type of machine 1 shall givea procedure for a 3-1/2 in. lathe, I am sure readerswill be able to apply the principles and methodsinvolved to their own make and size of lathe,also millers, shapers etc.

Some of the methods I use may not be theo-retically correct and there may be better waysgiven sophisticated equipment but if they areemployed intelligently and dextrously they work;I don’t want to be jumped by the “Talk in halfthou’s, work in half yards” brigade.

Before stripping, overhaul the headstock man-drel and bearings. I do not intend to enlarge onthis, volumes have been written on the subject,adjustment is usually built in anyway. Good bear-ings are essential for letting up the slides laterand the lathe is still intact if new bearings haveto be machined.

Strip the lathe right down to the bed, nowcheck the mounting onto bench or stand thatthe bed is not twisted by the holding downbolts, this can be checked with a sensitive spiritlevel across headstock seating and unworn sec-tions of bed, readings should be exactly thesame, if not shim or adjust the jacking boltsto correct. Much more care must be taken witha bed mounted on feet at each end than thecantilever type.

Clean the bed thoroughly with carbon tet. orsimilar solvent and take a good look at it inconjunct ion with the saddle , headstock, tail-stock and sketches Fig. 1, identify the main guidefaces, these will always be opposite the gib faces.

Now to check the wear present, I doubt ifmany readers have a fishback surface plate as

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Some examples of machine sl ide and gib design

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sh ims some-

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x deno tes ma in gu ide facey - g i b f a c e and z - c l e a r a n c e

Fig. 1.

MODEL ENGINEER 15 August 1975

Check the bed all over including guide edgesin this manner, you will probably be comfortedto find that very little wear is causing all that

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JJ_j_j_____ Al Jig in use to check parallelism

‘1, / of main and gib faces of a lathe bed

Jig for measurementaccross vee slides Fig 2

long as the bed, but a 24 in. straight-edge can,with care, be used equally well. The thick typeof straight-edge as used in combination sets iscer tainly the best and easiest to use. Thesestraight-edges can be obtained plain withoutgraduations at a considerable saving In cost.

Try along the top flats of the bed with thestraight-edge, if the headstock seating is on thesame level as the bed, bridge the gap from un-worn seating to bed as far back as possible andtry to insert a .0015 in. feeler gauge in wornsection; unless wear is excessive you will prob-ably find that It won’t go. Wear will have to bedetected by holding the straight-edge firmly downon the bed carefully keeping it vertical and flex-ing it sideways, a little practice and you will beable to feel where the bed is hollow or worn.

present can be taken by using micrometers,vernier or ordinary calipers over and between allbed surfaces. To measure over vee slides a jig orjigs must be made as shown in Fig. 2. Screw a

trouble. A check on the actual amount of wear

1-1/2in. length of ground stock to a piece of 1-1/2 in.x 1/4 in. flat with one face trued flat.

Before scraping the bed a good surface plate atleast 6 in. x 6 in. will be necessary and also an offstandard one about 2 in. x 1 in. x 8 in. with oneedge machined and scraped flat at about 45degrees. a piece of old cast iron strip from alarge machine is excellent, see Fig. 3.

Cast iron tends to form a thin but very hardskin, so a few words on scrapers and the methodof attacking the surface are in order. I have notyet found a commercial scraper that is hardenough for use on machine slides, 1 have foundthe best is a good quality parallel flat file thinnedby grinding see Fig. 3 and the end hardened rightout by quenching in water; for the work in hand3/4 in. wide is about right.

Two different methods of sharpening are used,one for roughing and one finishing. The rough-ing scraper is flat and straight ended, not curved,as the book says and the finishing scraper is

for vee s l ides oilstone

Fig. 3 FINISHING SCRAPER SHARPENING

ROUGHING SCRAPER SHARPENING

4-_--oilstone oilstone

MODEL ENGINEER 15 August 1975 801

Fig 4. W H E R E T O M A I N T A I N P A R A L L E L I S M I NS A D D L E A N D C R O S S - S L I D E

sharpened as Fig. 3. This scraper is used toremove the high spots and is worked with asort of forward-sideways-rocking motion, likedigging the high spot out. I am sorry, but thisis the best description I am able to give, thosethat are able to mottle will understand what Imean and also the difficulty of describing it, how-ever do not despair if you are not able to mottle,a very attractive frosted effect may be obtainedquite easily.

Start scraping down the top of the bed withthe roughing scraper. Work at about 45 degreesacross the bed, use long even strokes and putsome pressure on, try to remove metal evenly.If difficulty is encountered breaking the skin usea fine 6 in. flat file pressed firmly, again at about45 degrees across the bed and worked slowly andevenly. This will score the surface to give thescraper a bite. With the skin removed use level,straight-edge and feelers to take the high areasdown evenly to the lowest point of wear. Thescraper should be sharpened frequently on amedium India stone, do not use anything coarseras it will cause the scraper to score the bed. Eachpass along the bed should be at 90 degrees tothe previous one.

When you are satisfied the bed is dead flatto the s t raight-edge smear the large surfaceplate with engineers’ marking blue and slide itup and down the bed a few times, the high spotsrevealed will probably be few and far between,but if the preceding work has been correctly

carried out no perceptible rock should be felt inthe plate. Still using the roughing scraper, take thehigh spots off and repeat a few times until areasonable contact is shown all over the bed. Bynow smal! bright spots surrounded by blue shouldbegin to appear, using the finishing scraper takethese off. A word of caution. large areas of heavyblue do not mean a good bearing, it means toomuch blue on the plate, 1 repeat, the very highspots will show bright surrounded by blue. If youpick the high spots off with a short forwardand sideways motion working at 45 degreesacross the bed and the next time the plate isused, at 45 degrees the other way the high spotswill appear in more dense clusters and a verynice frosted effect will take form on the bed. Itis now up to the individual how much time,patience and dexterity he puts in to obtain abearing surface, the more and closer the highspots, the greater bearing and flatter surface, butwhenever the work is stopped the bearing shouldbe even all over the bed. All the low areas sur-rounded the high spots are “oil wells” whichwill help to retain lubrication on the surface.

Locate the main guide face and carry out thesame procedure on it, if this face serves as theheadstock location work on this as well to obtaina straight flat face right through. The tailstockis probably guided by the same face as thesaddle but if not, this is the next for attention,measurements must be taken from the completedface to ensure parallelism. The secondary or gib

802 MODEL ENGINEER 15 August 1975

faces for the saddle and tailstock are worked onnext, not only maintaining straightness and flat-ness but parallelism is of paramount importance.Working clearances are so small that half a thoumakes the difference between binding and slackin the assembled machine, so if you can measurea difference, scrape it out. The top and verticalfaces complete, the bed can be removed from itsstand and turned over for work on the undersideof the shears, make sure these faces are kept deadparallel with the top of the bed.

It may be advantageous to remove some veetype beds from the stand at an earlier stage thanthis but every effort should be made to get themain guide faces completed while firmly held toprevent twist.

The completed bed can be refitted to its stand,take great care to avoid distortion.

Before commencing work on the saddle youshould look at the consequences of your actions.Removing metal from the bed means that thesaddle will lie in a different plane from theoriginal, lower and to one side, this will bemagnified by the time the saddle is trued up,there will be misalignment between leadscrewbearings and the split nut. Since the intention isto deal with a few thou of wear and not areally “clapped out” machine, there will also bewear in the nut and bearings and it is doubtfulthat the slight misalignment will have any adverseeffects.

When wear is excessive and the bed has to bemachined or ground the usual procedure is tomachine out the saddle and tailstock and screwon plates to come back to s tandard, othermethods can be used depending on the designof the machine, bearings can be repositioned,apron modified etc.

Every individual must use a fair amount ofinitiative to apply these principles to his ownmachine, so a warning to use forethought andenvisage the consequences of metal removal andthe possible action necessary to correct, look evenfurther, the consequence of this action. Be s u r ethat curing one problem does not create a biggerone. Clean the saddle off with solvent and checkfor wear on the bearing surfaces by means ofstraight edge and feelers, the surface plates canalso be of use here, in addition take measure-ments as Fig. 4. It is essential to scrape thebearing faces so that when complete the cross-slide or boring table top face will be dead parallelwith the bed. Lathes used for normal workalmost always suffer most wear on the saddlemain guide face, a straight edge will probablyshow this face to be bowed, when trued it mustbe square with the cross-slide.

Many small lathes have a boring table withvee slides longer than the mating ones on thesaddle, it is better to work on this first leavingthe saddle until afterwards. The boring tableslides will probably resemble those in Fig. 1d,except that the gib is more likely to be as Fig.lc. Scrape the under bearing faces true and flatto the large surface plate, taking care to keepmeasurements to the top face exactly the sameall round. Scrape the main guide face true tothe vee surface plate and dead parallel to thefront edge of the table, use a piece of groundstock to take measurements, see Fig. 4. Blue theboring table slides and use as a reference toscrape the flats and main guide face of thesaddle cross slide surfaces, take the points ofthe vees off with a file to give clearance in thecorners if necessary. Using the vee surface plateand pieces of ground stock for measurementscrape the gib face parallel with the main guide.If the gib is of the type shown in Fig. lc, it isonly necessary to scrape the bearing face flat tothe surface plate and the cross-slide can becleaned, oiled and assembled. Test by pushingback and fore by hand, it will be heavy to startbut should then move with an even drag with-out tight or slack spots.

It is likely that the gib strip is steel, if so, usea spot of soluble cutting oil on the scraper, thiswill prevent pick up and give a bright clean cut,the scraper will work much more cleanly andsmoothly.

Some machines may have a different type ofgib strip. there are many different designs in use,a few are shown at Fig. I. The gib at Fig. Id isretained by screws up through it or down throughthe slide, it is a dead fit and to take up slack itmust be carefully filed on the top face to let it in.Sornetimes peeler shims are fitted to help, a goodbearing must be obtained on both taper and topface and retaining screws kept dead tight. Thistype needs a lot of time, patience and skill to fitwell. The gib at Fig. la may be a dead fit andhave to be treated as Id but it is more likely thatthere are adjusting screws behind it and theretaining bolts pass through clearance holes in thegib to allow for adjustment. It is unlikely that ataper key will be met with in small machines butif so it is fitted to a good bearing throughoutits length on both faces.

To true the saddle main guide face square withthe cross-slide put the saddle with boring tablefitted upright on the surface plate supported onthe front edge of the table, this is parallel withits slides, use a square between plate and guideface, file out bow true with the square.

To be continued

MODEL ENGINEER 15 August 1975 803

T

A 3-1/2 in.gaugeL.M.S“Duchess”built byT. A. Bottandloanedby J. E.Langrishe.t’hotographs:KeithLauderdale.

Right: Model steam yacht “Alva” by I a n Sharp ofthe Burrow Ship Model Society.

Z-gauge one from the Manchester M.R.S. occu-pying only 6 ft. x 3 ft.

The Barrow Ship Model Society again put ona show of ship models, both sail and power.Perhaps the most outstanding ship model was thesteam yacht by Ian Sharp, powered by a Stuart1OV engine and a spirit fired boiler.

Among the locomotives was the very fine 3-1/2 in.gauge Vale of Rheidol engine by Alan Green ofUrmston. A. Walshaw of Kendal showed hismodel beam engine, a Silver Medal winner at theM. E. Exhibition, and there were several horse-drawn vehicles, such as a model tip cart by BillWhidborne, and an Oxfordshire hoop-ravedwagon and a gypsy straight-sided wagon.

In the School yard, the F.M.R.C.‘s 5 in. gaugeportable track was kept busy, the Club’s Simplexgiving a good account of itself.

Overhauling the latheBy B. J. Whitehead

Part II From page 803

FILE AND SCRAPE out any measurable differencein under-saddle to cross-slide thickness. blue thebed and bed in the saddle flats and main guideface to it; it is an advantage to lighten thebearing in the centre of the guide face keepinga really good bearing at both ends. True up thegib, clean and oil saddle and bed, fit and adjustthe gib and push the saddle from end to endof the bed, any tight spots will be felt, an even

drag will reward accurate work. Remove the giband fit the keep plates. this is best done bymeasurement , stepping will probably be neces-sary. Fit them one at a time using blue u n d e rthe bed shears to obtain a good bearing. theyshould be just making contact with the retainingbolts hardened up ; again feel the drag.

Assemble saddle. apron. leadscrew. cross-slideand screw and check that everything i s workingsmoothly and correctly. Refit the headstock andchuck a piece of I in. 1-1/2 in. dia. ground stockabout 7 in. 8 in. long, set it to run perfectlytrue through-out its length. fix a dial test indicatoron the cross-slide. a magnetic stand is excellent

MODEL ENGINEER 5 September 1975 865

for this if you have one, so that it may be trav-ersed over the ground bar by means of thecross-slide ; take the highest readings at the chuckand unsupported ends of the bar, any discrep-ancy between these readings indicates the amountthe headstock mandrel is out of parallel with thebed. Scrape the underside of the headstock. main-taining a good bearing to correct. With the head-stock refitted and the test bar still chucked andnow dead parallel vertically with the bed set theD.T.I. against the side of the bar at centre height ;traverse it by means of the saddle along thelength of the bar. The difference in reading endto end must be corrected by scraping the head-stock locat ing face or using any adjustmentincorporated in the machine.

When you are satisfied that the headstock isdead true with the bed, make the acid test oncross-slide squareness. Attach the “clock” to thefaceplate so that by swinging it, it makes con-tact with each end of the true front face of theboring table or alternatively each end of a lengthof straight ground stock laid in the front vee ofthe cross slide with the table removed. It is usualto set the cross-slide not absolutely dead squarebut so that it will face .001 in. - .002 in. concavein the diameter of the faceplate, this ensuresthat faced components will mate together withoutrock. You will probably find that it will benecessary to again strip the saddle etc. and scrapethe main guide face to obtain this setting for thecross-slide.

The next problem is the tailstock. Blue the bedand scrape the tailstock to a good bearing withit. Extend the barrel fully and lock, set the D.T.I.

on the faceplate so that it can be swung aroundthe barrel, lock the tailstock on the bed withthe D.T.I. at the front end of the extended barrel,take readings around the barrel and move theset-over device to centralise the barrel with theheadstock; note any discrepancy in height, Nowmove the tailstock bodily up the bed and againlock so that readings can be taken as near tothe casting as possible. If the barrel at this pointis out of centre and a different height the tail-stock flats and main guide face will have to bescraped to correct. The barrel should now beparallel to the bed but it is probably low in rela-tion to the headstock, the cures for this are eitherfile and scrape the underside of the headstockto bring it down in line or machine out and fitplates where tailstock is split for set over to bringit up. A quick way out is to use shims instead ofmachining out and fitting plates. If the barrel isbadly worn, some may consider fitting the tail-stock in front of the saddle, just take up thelock to bear and bore out using the saddle topush it along, it is then bound to be in line,either bush or fit an over size barrel.

The top-slide is dealt with in the same wayas the cross-slide, anyone that has gone this farwill not find it any problem.

I hope these notes will be of some use toreaders who are prepared to use time, patienceand elbow grease to improve their machines,but are not quite sure how to go about it, I canassure them that their efforts will be amplyrewarded, I will bet they will think twice aboutusing emery without precautions on the latheafterwards.

AROUND THE TRADENew “Braze-Welder”Kellers Welding Centre of 32133 Cattle MarketStreet, Norwich: are manufacturing a new style ofwelding equipment designed for brazing as well aswelding. Known as the Kel-Arc Braze-Welder, thisworks from the domestic 230-250 volt electricitysupply and comprises a fan-cooled transformer, a“Chem-Arc“ torch with pencils, a set of weldingleads and electrode holder and earth clip. and anindustrial head mask. A supply of 16 gauge elec-trodes and flux-coated brazing rods are alsoincluded.

The Braze-Welder is designed to braze or weldmetals from thin steel sheet of 24 s.w.g. up to1/8 in. plate.Drawings of farm cartsWe have received samples of the “Mode1 Wheel-wright” plans produced by John Thompson of 15Darset Avenue, Fleet, Aldershot, Hants.

The drawings include a sketch of the item toshow the general appearance, information on thepresent location of the item, its origin and historyetc., and the correct names of the various parts.

The drawings are to scale, key dimensions onlybeing given. They are all of items accessible atthe public museums, etc.

Two instruction charts are available, coveringtools, materials and methods of construction.

The supplier’s list draws attention to the ModelHorse Drawn Vehicles Club-Mr. J . B. Pearce, 4Heron Drive, Westgate, Morecambe, Lanes.

16,000 Official Drawings now available tomodel engineersMany readers of the Model Engineer have alreadyavailed themselves of the opportunity to securecopies of the authentic works drawines. which havebekn collected, filmed, and marketed by British Rail/Oxford Publishing Co. over the past 18 months.However, as the total number of plans located hasnow reached the staeeerine figure of 16.000. andthere are perhaps seevra1 model engineers w h o haveyet to realise just what a chance is here I havebegged space from the Editor to try and indicatethe progress of this collection, and to give some ideaof its coverage.

Briefly-its origin came about through the co-operation of an enthusiastic Public Relations Office

MODEL ENGINEER September 5 1975

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