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COLCHESTER STUDENT HEADSTOCK ADJUSTMENT

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Hugh Gilhespie01/06/2014 09:39:58
130 forum posts
45 photos

There have been a couple of interesting recent threads on Colchester lathes and there are clearly some very knowledgeable owners on this forum. Unfortunately although I am a Colchester Student owner I am definitely not very knowledgeable and I would like some help about how to adjust the headstock to make the lathe cut true.

Some background. The lathe is a Student 1800 and has six feet. The lathe was relocated into a new workshop at the beginning of the year and one of the first projects in the new shop was to make a set of screw jacking feet that make it straightforward to level the lathe. This was pleasantly successful and so the lathe was duly levelled. I used a Hilger & Watts clinometer that I bagged from fleabay for not very much at all - I can thouroughly recommend these. Not only do you get a very sensitive level but with a bit of basic trig you can actually calculate how many turns are needed for the raising and lowering.

My feet use M20 x 1.5 threaded bar which is readlily available and since this thread is used in automotive applications, sets of taps are also easy to find and fairly cheap.

Anyway, back to the main story. I got the lathe nicely level but when I did some test cuts with a length of 1.5 inch steel bar it was still cutting a taper. About 4 thou over six inches which was too much. So I looked at the manual for the lathe with a view to using the headstock adjustment to set the lathe true. To be honest, I chickened out. It wasn't at all clear to me how to access the adjuster without what looked like some serious dismantling.

So, Plan B was to set the cut true by adjusting the feet. Harold Hall's website has a very clear set of instructions for this and by following these I did indeed manage to reduce the degree of taper to less than one thou over six inches which I could live with. BUT - a big but too - to get the degree of adjustment needed the lathe is a long way from level to the extent that the rear foot at the tailstock end is now completely off the ground. This surely can't be right and so I really do need to learn how to use the built in adjuster. Then hopefully I can have both a level lathe and one that cuts true.


Could someone who has used the headstock adjuster please give me some details about how to do it? I think that you need to remove the banjo to access the adjuster - if so is this difficult and are there any issues when you put it all back together ?


Thanks, Hugh

clogs01/06/2014 19:29:08
578 forum posts
12 photos
Hi Hugh, welcome...I'm a newbie 2 but ur problem is an easy fix.....
 
HI Hugh, welcome...I'm a newbie 2 but the problem was an easy fix....
 
I had the same problem with my sqare top student......I guess they'll b simillar, finger's crossed.......
 
I had 2 remove the rear cover and the external gear's for access and getting on ur knees look at the lathe bed at the back under the head....in the filth....u'll find 2 Allen bolts, give them a turn or so 2 loosen them and the same likewise under the chuck at the front......it was said that if the lathe is level, just by undoing the bolt's and then nipping them up again it was probably going 2 do the job but not 4 me I'm afraid......
I loosened them again and with a 2" bar about a foot long in the chuck (tight).....pulling on the bar in the direction I wanted 2 go, I GENTLY TAPPED the chuck with a copper mallet and then tightened the bolt's up again...the problem fixed....I did not feel any movement when I tapped the chuck but it did the job.....apparently at the back of the lathe around those bolt's there is some sort of a screw jack arrangement that the factory used when a new lathe was being set up.....everybody said that it probably won't be nec. to use it.....for me they were correct.....
I guess that moving them around by truck must shake them up a bit .....
hope this works 4 u.....
good luck.....Frank in France
Hugh Gilhespie03/06/2014 13:06:56
130 forum posts
45 photos

Frank in France,

Many thanks for your reply. I was hoping for info about using the 'screw jack' to set the headstock true as I am a bit worried about clouting the chuck - I know you said GENTLY TAPPED but even so it can't be good for the bearings and they cost a fortune to replace. If all else fails I will go this way but surely somebody must have experience of using the 'screw jack' and can tell me if I need to remove the gear banjo to do so?

Regards, Hugh

Stuart Bridger03/06/2014 13:49:17
476 forum posts
26 photos

I have a chipmaster which was exhibiting similar problems. I can't help with the specifics of the screw jack adjusting arrangements. But the chippy also has four allen bolts holding the headstock down. Getting these loose on a 50 year old lathe was no mean feat, in the end i think I only got three undone. Instead of tapping on the chuck, I just used a rubber mallet gently on the headstock casting, which would eliminate any possible issue with the spindle bearings. It worked fine..

Regards

Stuart

Clive Foster03/06/2014 14:27:16
2389 forum posts
77 photos

Hugh

Probably worth hunting around for a download of the manual which includes all the adjustment and parts diagrams along with care and feeding instructions.

As I recall things the adjustment jack is the same as on the Triumph 2000 being a simple block with a pin each end made flat on the outside and tapered on the inside. The two adjusting screws have tapers matching those on the pins so they push the pins out when screwed in. Theoretically you can get at things with the gear banjo (swing plate in Colchester terminology) in place but its best to bite the bullet and take it off so you can easily see what youa re doing. Its all pretty straightforward.

Procedure is to decide which way the head needs to move, slacken off one screw and tighten the other one to drive it across. Remember that it swings about a dowel around half way along on the far side of the headstock so there is, relatively speaking, a considerable sideways translation movement on top of the angular shift. Not a bad idea to do some quick sketches and trigonometry to get a rough idea of how much shift is involved. If doing the initial setting by measurement off a decently stout bar a foor or so long you may be surprised just how much angular shift there is. Always adjust against a loose pin. Tughening the second screw up merely locks the setting. You cannot work booth screws against each other in any satisfactory manner.

As you have discovered adjustment by bed twist is not possible on a machine of ths size.

Clive

Dave plus / minus 40 thou03/06/2014 15:11:55
avatar
19 forum posts
4 photos

Hi Hugh,

just adjusted the headstock on my 1800 for the same reason,

the method used as follows:

1. 6" x 1" bar in 4 jaw chuck + set up with DTI

2. remove back splash guard + both guards in front of headstock (to expose the allen bolts holding the headstock below the level of the chuck)

3. remove back gear cover, remove leadscrew gear drives (to expose the allen bolt)

4. fit DTI onto cross slide with tip on end of bar in chuck - this is to measure the movement / swing when adjusting.

XXXXX the head rotates around a dowel pin on the front of the headstock on the non operator side XXXXX

5.loosen all 4 allen bolts (had to shorten an allen key to fit)

6. adjust either the left or right hand small allen bolts in the block between the two hold down bolts at the rear of the machine, this will "twist" the head.

7. look for the movement on the dti.

8. when adjusted lock up all bolts and take small cuts and measure dia with a micrometer, i managed to get mine to within 2thou - i have a worn bed! but before the the adjustment i had a difference of 0.3mm over 150mm

dont be scared of it - you wont break the lathe!

good luck

Dave

Hugh Gilhespie04/06/2014 09:27:50
130 forum posts
45 photos

Hi,

That was a great response, many thanks to everyonefor the information. I will have a go at setting the lathe true this weekend and - hopefully - report a successful outcome.

Thanks again, Hugh

Nigel McBurney 104/06/2014 14:35:30
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744 forum posts
3 photos

Hi I have adjusted my master 2500 as described above using the push pull screws,now you will find the allen screws holding the headstock down are very tight ,use a allen key that is in very good condition,you will need pipe on the key or hit the key with a copper mallet, never never hit a lathe spindle to adjust the head.If I remember correctly some of the sheet metal of the back tray covers one screw, now spend some time in doing the job properly,it took nearly 2 days to get mine right. First of all set the lathe level then clean up the lathe lubricate the ways and tighten the lock screw on the top slide and take any looseness out of the cross slide ,this is so that when test machining you are testing the accuracy of the headstock and not any errors in the rest of the lathe. If you have a gap bed make sure that it is fitting perfectly . The test bar ideally free cutting mild steel needs to be about 40mm dia and protruding about 7 inches out of the chuck to avoid deflection, now you only need two areas on the bar for test machining about 1 inch long, one at the end and one near the chuck,relieve the length in between so that the tool only has to cut for a total of two inches,this to reduce time and tool wear affecting the results,dont forget tool wear can cause taper turning a potential problem when you are looking for half a thou. Use a HSS tool really sharp with small rad and honed with a oilstone,do not use carbide for this job. For a test turn use about 150 rpm and a slow feed to get a good finish,speed is lower than normally used ,this is to reduce tool wear. Use power feed for turning ,do not do one test with power then the next one with hand feed,believe it or not you can get different results,I do not know why as its a V bed and should not affect the saddle travel, I asked a friend who was a seriously good toolmaker (ex Vickers) about my results , when he had run his business he had found similar results with Colchesters though he was looking for tenths. Unless the lathe is badly worn I would be looking for about half a thou taper or better over 6 inch , the larger end of any taper should be at the headstock end. Once set up to your satisfaction ,the tailstock will need adjustment as the headstock will have swung out of its old position and give an error when turning between centres.

If you have a new workshop with concrete floor ,check the level in say a years time as the concrete can settle.

The Headstock adjustment is covered in the Colchester handbook,get the correct book for your machine as screw positions and dowel pin can vary.

Best of luck you will need plenty of patience, p/s make sure your test piece is firm in the chuck and does not move, a slack chuck will give funny results, the four jaw may grip tighter.

Pete07/06/2014 19:30:10
78 forum posts

Hi Hugh,

In my opinion I would be extremely careful and more than a bit paranoid about doing anything as radical as resetting a good quality lathes headstock without exhausting every other possibility I could think of about exactly why your lathe is turning a taper like that. This is again another method that's very much recommended by people who just don't fully understand or appreciate how intricate proper machine tool alignments really are. I barely know enough about it to know good advise from bad. But this is not and never will be a cheap hobby at any level. The equipment, tooling, accessories, and even the room it requires adds up very fast. Even the information, skills, and knowledge required alone to get any good at this is staggering. So your first stop needs to be finding either a new copy from Industrial Press of the 1950's written book called "Machine Tool Reconditioning". And no there hasn't been anything newer than that 1950's book written in the English language. And a brand new version is still going to cost you right around $100.00. And it's also not knowledge that forums like this can provide for free.

Or it's either that new version, or maybe if your lucky you might find one on Ebay. It's also an extremely dry and boring book to read. It's not at all imho optional for what your trying to do. Most of the book goes into great detail about exactly how to rebuild various machine tools to brand new or better alignments. The books contents are even too complex to properly explain here. But over and above the hand scraping information, it will give you the basic knowledge and understanding of good machine tool alignment, and I can say it doesn't start out by beating on spindles as an adjustment method. There's possibly dozens of reasons why your machine could be turning a taper that steep, and unless your lathe was hit really hard, or dropped, then adjusting the headstock is the very last place I'd adjust until I'd checked everything else. Good machine tool rebuilder's start out by doing what's called a " machine tool survey", this is basically a series of tests using mostly a micrometer and DTI to do most of it. Having a very expensive precision straight edge would be another required tool for some of it, but a GOOD micrometer and that DTI can prove a lot. They pretty much follow the German Shlessinger (sp?) developed machine tool tests. And while that information is online if you dig for it, you still very much need the book I mentioned to understand exactly what your results and test numbers mean.

And as a extra bonus, even if you never ever rebuild or adjust any machine tool, the book will completely change your opinion about just how clean or how well lubed you keep your machine tools ways. That alone has more than saved me the cost of the book. I'm certainly not saying it's impossible your headstock needs a realignment, but I am saying it's probably around 99% it doesn't need it.

And for anyone who wants to say I'm wrong? I'd expect you to be a machine tool rebuilder, or have at least read that Machine Tool Rebuilding book so we can use some logic instead of our best guess about this. Hugh asked for some good answers, that's the least he should get.

Pete

Edited By Pete on 07/06/2014 19:37:02

Pete07/06/2014 20:34:01
78 forum posts

And after some more thought Hugh.

There is a fairly quick method of giving you a quite good idea about your headstock's actual alignment using some simple test equipment. You can do all the precision test's and alignments you'd like. But the bottom line is just how well the machine performs under real world cutting conditions. Anything else can't and won't prove all that much. The finished parts and just how true and accurate they are show the machines actual alignments.

I know zero about your lathe, but they all work to the same basic design principles. All good lathes should be setup at the factory so the cross slide will cut a very shallow concave angle while doing a facing cut. Standard commercial quality lathes have a larger allowance for this than tool room quality lathes. If? I remember correctly. Tool room quality lathes should be set up and produce a concave cut of around .0015" over 12". My memory could be a bit faulty about that specification though. The exact numbers aren't that important for what you want to really test though. But that concave is ground in to the machine so parts don't rock as the faces are brought together, and the machine wears into alignment not out of alignment as you use it.

Since I haven't checked the Lathes UK site I don't even know what your lathe looks like. Do you have a faceplate for your lathe? Does your lathe have power cross feed? Or do you have a piece of scrap plate as large as you can possibly swing? Any of that will work for this test.

Bolt or thread your faceplate to your spindle, or figure out a way to attach and face off a plate to the maximum diameter your lathe is capable of. Ideally you have power cross feed. But it could be done using a hand feed. You want the slowest feed yet very best surface finish you can manage across the whole face until it cleans up. That will replicate the exact alignment your headstock has to at least the cross slide's way alignment. Now, you need to set up a good accurate and 100% completely dependable DTI on your lathes cross slide with the DTI's finger against the part you just faced. You do not want to check that faced part by running the DTI finger from the parts O.D. that's closest to you into the parts center. All that does is follow the cutting tools path and it can't and will not prove anything. You want to set the DTI's finger at the parts approximate C/L and then use your cross slide to run the DTI's finger across to the furthest point away from you. That would be towards the lathes rear bed way. The variation you record on the DTI dial will be twice as much as the lathe actually cut's. It's an extremely accurate way to at least test the cross slides alignment to the headstock's alignment. Longitudinal turning or finding a taper while measuring that longitudinal cut could show a taper due to many other reasons. But the test above should give you a very good idea if your headstock is misaligned or not. It would be without question my first test for your lathes basic headstock alignment.

Pete

Edited By Pete on 07/06/2014 20:42:41

Nigel McBurney 107/06/2014 21:56:41
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744 forum posts
3 photos

Pete, You have jumped to some conclusions despite not knowing what a Colchester looks like, Nowadays the Colchester is very popular for home use,as there were thousands of them made and spares are still obtainable. Many years ago they were not well regarded among skilled turners ,but they were popular for training schools and technical colleges and were available at a reasonable price ,by the 1970s they had improved ,beds and slideways were heavier and larger for example but they retained some features which were not ideal, one being the adjustable headstock, The majority of lathes had their headstocks fitted to the v or flat beds and finally aligned by fitting and scraping so despite a knock or two they could not move out of alignment any subsequent alignment problems were due to a lot of bed wear or misalignment on long bed lathes and would required very skilled fitting or a bed grind to correct matters. Colchesters went the other way.obviously to reduce costs why spend days scraping and testing a headstock when it can be made adjustable and very easy to align in the factory before all the bits ad pieces are assembled,and if one did get a bang it could be corrected.If you wanted a precision lathe then you went and bought a Dean Smith and Grace or a Holbrook and paid an awful lot of money for them. And a lot of Colchesters did get bangs at the training establishments ,the front edges of many topslides show rough encounters with chuck jaws. The big advantage of many of these ex training lathes is that despite a few knocks and dings on the machine surfaces and the odd broken handle they did not get much wear ,and were a bargain during the last 20 years though supplies appear to be now drying up. Also the advent of the hiab lorry crane has not helped matters, the correct lifting eyes in the bed get lost and the lifting slings are very often not correctly placed, for example around the spindle behind the chuck ,or a round sling through the spindle,the driver/ operators are just not careful enough.,they get certificates for safety not the skilled slinging of precision machine tools.So Your Colchester may need adjusting,and providing you take care it can be adjusted back to spec.

What really surprised me was about 25 years ago a large cnc vertical mill with only about 6 months round the clock operation needed the vertical slidebars replaced, the recirculatory ball bearing slide units had worn slight grooves in the hardened bars, thy were replaced (£10k) The machine was bedded onto 4 feet of concrete and the mounting bolts ran in sleeve nuts so that the machine could be jacked up or pulled down the vertical travel was tested and found to be out relative to the machine table so the fitter from the manufacturer using the sleeve nuts adjusted the column until it was vertical, the bed frame was a heavy steel fabrication so it was literally distorted ,to gauge the size of the machine the vertical head was just over a ton, There was no really skilled machine fitting about this repair, and we found out why the slides wore so quickly ,we were producing very precise aluminium castings but with very little machining allowance so the swarf was like dust,which had penetrated the slide covers and acted like a lapping paste,the manufactures had a similar problem with another customer who fine machining brass. I never thought I would see the day when an expensive machine was aligned by distorting the main bed.

Pete07/06/2014 23:45:41
78 forum posts

Hi Nigel,

Some interesting life experiences

I took the time and jumped over to the lathes UK site. Hugh's lathe is exactly what I expected, a very desireable and good quality British built machine tool. But it changes nothing about what I said. Obviously we both know all lathes are set up, aligned, and work to a standard set of very well understood basic alignments. It's impossible for them to perform to the level of what we expect if they were somehow "different". But I do respect and appreciate that you do have what seems to be a great deal of hands on experience with the Colchester machines. That alone makes my points a bit suspect.

And I lust after anything that's got a DS&G lettering cast onto it. Even more I'd like due to my recent finds one of those Holbrooks. Just possibly an even better machine than the DS&G's.

But, I didn't say it was impossible for Hugh's lathe to be out of alignment. It may well be. What I do disagree with is the automatic assumption by far too many on these forums that it's always a headstock misaligment. Going by your experience, then you'd have to agree that far too many just jump to that conclusion without fully checking the rest of the machine. But now with your additional experience and information, then yes it makes it slight bit more likely that Hugh's headstock might need a proper realignment. But .................................I did give him a test method that will quickly prove what direction he might need to go. If that facing test is even close to the factory allowable specifications, then there's no need to adjust the headstock.

Maybe in a way I worded and wrote my post due to reading far too many other posts where the adjustment of the headstock is the first thing mentioned. I still maintain it should be the last thing you touch until you've proved it is in fact the headstock that's out. That facing test I mentioned isn't obviously 100% reliable because it's a combination of the head stocks alignment and the built in alignment and wear on the cross slide. But it will help to pinpoint where the problems are or aren't. It's exactly where I'd start, but it's something I've yet to see even mentioned anywhere else.

FWIW, and it doesn't mean that much to this discussion. I started out with a little Emco Compact 5 lathe when they were still being built in Austria. I got deeply interested in why I was getting tapers on long parts. Not very much of a taper, but enough I didn't and wouldn't tolerate what I found. So I bought Emco's add on accessory, they call them "leveling plates". Those still didn't give me what I wanted on a wooden bench top, so I then mounted my little lathe to a 1" x 12.5" x 28" long mild steel plate.It was only then I could see just what this little and fairly cheap lathe was actually capable of. Unfortunately my shop floor isn't concrete, and due to it's standard stick built 2" x 6" floor construction, my methods aren't even close to being permanent. I can and have proven using a brand new Mitutoyo 2" digital mike that it's possible to adjust this lathe by leveling the lathe bed, and then making very slight adjustments after that where I can turn a 2" x just about 12" long shaft to under .0002" taper from end to end. Now if I shift my feet on the floor or move at all it shows up immediately in the size. The settings can't and don't last more than an hour or two due to temperature or humidity changes. But those accuracy's are possible with a bit of skill and knowledge. Maybe more than a bit of luck helps too.

Pete

Billy Mills08/06/2014 01:02:48
377 forum posts

Pete,

I think you need to SEE a Student 1800 and try to pick it up, even better use it for a while. The facing test is not that unknown but the bar test is far simpler because it relates the bed axis to the spindle axis. It is also far closer to most user's turning needs and is easy to do and well understood. The Colchester's have far stiffer beds than you are used to, leveling has a different order of magnitude effect ( as described in an earlier post) so headstock alignment also has a different significance.

Your Emco might like sitting on a concrete slab.

Billy.

PS I followed Tony Griffith's Buying advice and got a Student 1800 so can speak from experience.

Edited By Billy Mills on 08/06/2014 01:07:20

Pete08/06/2014 02:12:14
78 forum posts

Bill,

I email with multiple people on almost a daily basis. One who is a working industrial designer and certified machinist, he even owns a Colchester Chipmaster. And one who's a barely retired professional machinist after 50 plus years in the trade, and is in fact a more than well known Model Engineer. One of the best today in fact. I'm not saying this to brag, I'm saying this because I can hold my own with these people.

I don't need to SEE one of these lathes at all. I've been around lathes that had at least 30' between centers. I already know what a heavy well built lathe is quite capable of. Just because I don't own one doesn't mean my points are worthless. I said and I 100% meant that ALL lathes work on the exact same alignment principals, and it's very much pointless about who built it if it's a half decent machine. Hugh's is by being a Colchester a well made machine. That part is unquestionable.Just how good it was built and how well it's aligned right now is the question. In a way I can see your point about that bar test. I've done that more than a few times. Even Emco shows it in my manual. But material deflection could easily fool you just from the sharpness or lack of on the tool point. A poor set of chuck jaws could even sway the results. You didn't read or at least fully understand what I tried to explain. That facing test is only the very first step, It will prove 100% one way or another that something is out or it's not. If there's something wrong in the numbers, you can then start looking into further areas. It is possible that an incorrect cross slide alignment or even wear could change the results.But after that test you then might start going to your two point bar test over say 6" But with what I've learned the extremely hard way, I don't and won't ever depend on one single test or method. You could easily mistake a bed twist for a headstock misalignment without some prior experience with your bar test. Your bar test does work, but just like my facing test, there can be other factors that could change your test results.

And You and I may know about that facing test, but I've yet to see it ever mentioned on any forum like this.

My current home won't work with a concrete slab under my little Emco. But that doesn't prevent me from desperately wanting one. I did at one time have my setup on a proper concrete floor, and it spoiled the hell out of me. I didn't fully appreciate what I had then.

Pete

Edited By Pete on 08/06/2014 02:16:56

Edited By Pete on 08/06/2014 02:28:59

Edited By Pete on 08/06/2014 02:30:27

Hugh Gilhespie08/06/2014 09:56:20
130 forum posts
45 photos

Hi Everyone,

More interesting responses that I thank you for. It gives me more to ponder on - I haven't actually attempted the headstock alignment yet so all information is welcome.

Some points that might help clarify:

1. The lathe is mounted on a fairly hefty concrete slab

2. The jacking feet allow it to be set level with a reasonably similar torque on all six feet.

3. When it is level. it cuts a pronounced taper - 4 thou in six inches

4. By 'unlevelling' the front tailstock foot the taper is reduced to less than 1 thou in six inches.

5. At this point, the rear tailstock foot is no longer touching the ground.

6. This was a training lathe - for London Transport - so has had the usual knocks and bangs but is not badly worn - as far as my inexpert eye can tell.

7. It seems entirely possible to me that the apprentices being trained were shown how to adjust the headstock as part of their training so it's misalignment isn't a great surprise.

Regards, Hugh

Colin Heseltine10/06/2014 20:54:04
462 forum posts
130 photos

Watching with interest as I also have a Colchester Student (squarehead) lathe. Need to check its level and turning accuracy.

Best of luck with this Hugh.

Regards,

Colin

ronan walsh11/06/2014 00:14:10
542 forum posts
32 photos

If a length of thick silver steel is put in the four jaw chuck, leaving a substancial length sticking out (10 or 12 inches or so) and clocked up at the chuck end and the opposite end. Couldn't the alignment be checked with a dti mounted in the tool post, running up and down the length of the bar ? The clock wouldn't put any pressure on the bar giving misleading readings.

Edited By ronan walsh on 11/06/2014 00:14:51

Clive Foster11/06/2014 10:12:12
2389 forum posts
77 photos

Ronan

Theoretically a straight bar and DTI could be used in this manner but the practical application is fraught with difficulty because angular offset in any plane will cause a deviation so it is impossible to be certain that any error lies in the horizontal plane where it can be adjusted out. There are ways of dealing with this sort of thing but it isn't easy and you have to keep your wits about you.

In science its often said, and all to often forgotten, that "What You Measure Is What You Get" pretty much the equivalent of the engineers KISS "Keep It Simple Stupid". Putting these precepts into practice boils down to making the most direct measurement possible to determine the error. As a taper turning error means that the axis of the workpiece is at an angle to the lathe bed so that traversing the saddle with a fixed tool position produces a different diameter close to the headstock than it does close to the tailstock clearly turning two collars at opposite ends of a suitably stiff bar gives a direct measure of the error. More complex and indirect methods therefore being redundant. It helps that measurements can be made with micrometer or vernier which are probably the most accurate and consistent instruments in the average model engineers workshop.

I advocate using a bar pretty much as large as can be fitted in a headstock collet with nylon or delrin collars. Nylon or delrin turn easily with low tool pressure so bar deflection can be neglected. Just checked my Smart & Brown 1024 VSL using 40 mm(nominal) diameter by 20 mm long nylon collars on a 1" diameter, 1 ft long bar. The variation is essentially unmeasurable appearing merely as a change in feel when the micrometer is set on one collar then slid over the other. Set using the ratchet at the headstock end its slightly free at the other end whilst if set at the tailstock end its a little stiff at the headstock. Sub tenths I guess. She'll do!.

Whatever you do don't follow the advice from Pete about the facing test. As is so often the case a short description can be somewhat misleading and the whole process is fraught with potential errors. Especially on a machine of unknown provenance and unknown starting condition. Good for verifying that a factory fresh machine has been returned to its original alignment after delivery and pretty much an essential step when setting long be machines with extra support in the middle but so much harder than the simple bar and collars.

Clive

ronan walsh11/06/2014 13:03:07
542 forum posts
32 photos

Thanks clive. What we were shown were shown while in the training centre while doing apprenticeship block release was the dial gauge method, for headstock to bed alignment, tailstock to headstock alignment etc. it was only the basica of machine tool alignment and maintenance really, but gave a good idea of the principles involved. I was talking to a chap who's trade was machine tool fitter, he told me the best way of setting up a machine of any description was to have the base mounting holes over some threaded rod that had been concreted into the ground, and the whole machine aligned with nuts on the threaded rod. When the machine is level and aligned , the base id grouted to the floor.

Clive Foster11/06/2014 14:38:35
2389 forum posts
77 photos

Cheers Ronan

As you say you were being taught the basics of machine alignment to help understand what can go wrong and how things are measured so that the problem can be fixed. The bar and dial gauge method is a great, and quite fast, method of checking that things are still OK. Providing you have a known good bar which is known to fit repeatably in the headstock taper and a way of mounting the dial gauge accurately on centre. There is also an implicit assumption that the machine was OK recently prior to whatever occurred to give rise to the suspicion of machine alignment error. In an industrial or tool room shop environment you will almost always be ruling out machine alignment errors confirming that the real cause was a workpiece set-up problem.

In the home shop where you most likely don't have any of the comforting knowns the collar method is more appropriate as everything derives from first principles. Naturally once you have the collars turned a dial gauge makes adjustment quick and easy. Just offset to halve the difference in collar sizes before skimming to check. Only a couple of minutes more to do the tailstock too whilst you are set up.

Its probably sensible once things are aligned to make a long custom test bar by press fitting a decent diameter rod or tube onto whatever size of blank end taper fitting arbor fits your headstock, turning roughly true and fitting collars as before which can then be turned true and accurate. Typical ME size machines will have trouble dealing really accurately all along a suitably long bar. Thick wall tube makes for slighter, easier to handle bar. If you are confident about your assembly skills a section of motorcycle telescopic fork inner tube would work well but it has to go on close enough to true 'cos you won't be able to turn it. If the first couple or three inches after the taper are made true to a good finish, whether by direct turning or fitting a turnable collar if you use a piece of fork tube or similar, that part can be held in the three jaw chuck to verify its accuracy. The taper fitting doesn't get involved as it won't reach far enough back to engage in the headstock. In practice chucks are never totally true so one would record the run out error at the tailstock collar end. Prudent to mark for no 1 jaw to get best repeatability although theoretically it shouldn't be necessary.

Although taper fitting test bars can be bought affordable ones tend to be on the small side, typically 5 to 6 inches long and 1/2 to 3/4 inches diameter or metric equivalents. Two or three times the size would be much easier as the absolute measurement for any given error is larger and the consequences of minor dial gauge alignment errors much smaller.

Clive

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