|71 forum posts|
I've discovered that my 3 jaw chuck doesn't hold work 'on-axis', I've put a DTI on various diameters with the following total runout measured:
1. With a ground 1/2" steel bar held in the internal 3 Jaw Chuck: 0.25mm
2. Same bar held in 1/2" collet: 0.04mm
3. Mandrel, just next to headstock bearing: 0.0mm (no discernable movement)
4. Mandrel, inside the collet taper: 0.0mm (no discerable movement)
The run-out using the chuck is too much to allow me to turn something and then reverse it to complete machining.
What do I do to fix it? Is it possible to fix it?
This is on a Smart & Brown, Model M, Mk 1.
Grateful for your advice.
|old mart||20/03/2021 18:10:30|
|3416 forum posts|
That runout is common on three jaw chucks. You would be best getting a four jaw independent, they can be dialed in to whatever you want. With the chuck you have, the easy way to loose most of the error is to reduce the fit between the backplate and the chuck register by 0.3mm so you can get it running better before the bolts holding the chuck on are tightened.
|71 forum posts|
Hi Old Mart,
Just to avoid misunderstanding your statement 'reduce the fit between the backplate and the chuck register by 0.3mm' means to allow more of a gap between the locating circle of the chuck and the raised portion of the back plate: I would then nip up the bolts once I've checked the chuck runout.
Apologies for what is probably a dosy question!
|Derek Lane||20/03/2021 19:05:14|
569 forum posts
Check out Joe Pieczynski video
|old mart||20/03/2021 19:07:41|
|3416 forum posts|
With a little clearance in the register fit,you can gently tap the chuck body into line while the bolts are loosened slightly. It is a hit and miss method, and needs to be done after the workpiece is clamped tightly in the jaws. The runout should be rechecked after tightening the bolts. A different diameter of workpiece may run differently, depending on how the scroll is worn and wear generally. At the museum, we have a Smart & Brown model A which has the same size spindle fitting as your machine, and over the years I have ammassed about 6 three jaw scrolls and 2 four jaw independents. Only the four jaws and one of the three jaws have a tight register, the others can be adjusted if necessary. The ones that can be adjusted are all very good and have only been adjusted once or twice, their runout at different diameters is well within my idea of a good chuck.
Watch the video before doing anything to the chuck.
I have watched that video some time ago, and it gives a lot of very valuble tips on how tighten up work properly.
Edited By old mart on 20/03/2021 19:11:26
Edited By old mart on 20/03/2021 19:15:46
|Dave Halford||20/03/2021 19:21:07|
|1820 forum posts|
Before you do any of the above try a larger piece of turned metal and see if it's the same.
I've got a 5" chuck that throws out 15thou at 1/2" dia, but only 3 thou over 1 1/4" bar. You can feel the scroll go tighter as you wind the jaws out.
7695 forum posts
Another possibility is the jaws are worn. New jaws, or try regrinding them by clamping the jaws against an outer ring and truing up the ends. I expect someone else will be able to explain better! If the chuck body is worn or damaged, replace the chuck
Before doing anything too wild though, strip it down and for a good clean. Might be swarf in the works, especially at the register surfaces
|old mart||20/03/2021 20:13:37|
|3416 forum posts|
As S O D mentions, a good strip down and clean can work wonders.
|not done it yet||20/03/2021 20:15:35|
|6440 forum posts|
First, do your jaws have the same number stamped on them? Next, does it make a difference if you change the jaw positions by one slot each way (not all chucks have the slots numbered).
Then try the ideas above. There are very few self centring chucks where the workpiece can be removed and remounted without losing concentricity. Collets go a long way towards, but turning between centres is the time-proven method for items which need dismounting and remounting.
|old mart||20/03/2021 20:39:45|
|3416 forum posts|
The other thing we all forgot to mention is which of the three key positions gives the best results. Only testing with different size work, preferably precision ground on the od, using each of the key positions will find the sweet spot, and a small mark made to remember which one. Any of them can be used for undoing the chuck, but always use the best for tightening up.
For the best critical work, soft jaws bored correctly or four jaw independents are unbeatable.
Edited By old mart on 20/03/2021 20:42:04
|martin haysom||20/03/2021 20:49:22|
58 forum posts
i too have a smart and brown model m mk 1 i get similar numbers to you when that is not good enough i use the 4 jaw
|bernard towers||20/03/2021 22:24:38|
|338 forum posts|
Why not try to resurrect it by grinding the jaws. In a previous issue of MEW it shows how to make a petal plate and from personal experience it’s nothing short of brilliant. I have done 3 chucks and have also lent it to 3 others who were really pleased with the results, what have you got to lose?
|Neil Lickfold||20/03/2021 22:52:33|
|720 forum posts|
You want to check a lot of different sizes. The scroll on some early lathes is 8tpi so every 1/4 inch size will likely be the same runout. So 1/4 bar is often the same runout as 1/2 inch. But 5/16 and 3/8 are on a different part of the scroll. Is it the same when held by the full jaw length and at 1/2 the jaw length. Older chucks can have damage to the chuck body by gripping too tight on short pieces, and racks the jaws track in the chuck body.
Another option is to loosen the holding screws, and the spigot, then put Belleville washers under those screws or nuts. It will allow you to make small adjustments to the part being held , a bit like a grip true does, but without the adjuster cams.
|Mike Poole||20/03/2021 23:13:50|
3095 forum posts
This old chestnut goes round and round, if three jaw scroll chucks were capable of extreme accuracy then why make collets and 4 jaw independent chucks. A scroll chuck can be made to a highly accurate tolerance and when new and unabused may perform quite well. The day will come when the chuck is over tightened or a jam up strains the chuck and it will now have lost whatever class of accuracy it was manufactured with. I was taught to not regard the scroll chuck as accurate and to plan operations to avoid removing the work from the chuck, if removal cannot be avoided and concentricity is important then a collet or 4 jaw independent should be used when remounting the job. If after a thorough clean and inspection the runout persists then check at different diameters and if you have a similar runout on all diameters then grinding the jaws may improve things but this improvement may not last forever and certainly a collision could undo it all in a moment. The mechanism of the 3 jaw is not well protected from dirt and swarf so it can eat a chip at any time and adversely effect the accuracy. Unless you clock job then it could be unexpectedly out at any time. I couldn’t tell you what the runout of my scroll chucks is because if accuracy is important then I won’t be using it.
|226 forum posts|
Mike Poole is absolutely spot on. I struggle to believe that so much has been written here about backplates, grinding jaws etc on a three jaw chuck.
When yo turn something in your three jaw chuck does it produce correct size concentricity, straightness etc? If it does then you are doing fine.
Having part turned your piece, turning it in a three jaw chuck is a totally pointless exercise. You have no chance of the second end running true to match the first turned end. So either use a collet on both ends, mount the piece between centres, turn the piece complete and merely part it off leaving a simple facing operation to finish, or if real accuracy is not required bore out a set of soft jaws to accept the first turned end. Yes you could turn the part and mount it in a four jaw chuck but the methods I have describes are more consistent. HTH. Happy turning.
5505 forum posts
Issue 267 of MEW has the full article on regrinding chuck jaws. There is more to it than a simple ring. The "petal' or clover leaf plate is very simple to make and a Dremel grinder held in the toolpost with a simple bracket will do the rest.
Three jaw chucks should hold true within about .003" or 0.07mm so the OP's 0.25mm is clearly unacceptable. First step is to give it a good clean out and check over. Check mountings on backplate etc. And check to see if the jaws are bellmouthed by looking for a gap near the front of the jaws when lightly gripping a ground bar etc.
The several Myford and Drummond chucks I have reground now have a runout of less than .003" at any given diameter of job. Very happy with the result.
|David George 1||21/03/2021 08:02:46|
1675 forum posts
I bought a 4 jaw self centering chuck with jaws that were mixed up and didnt match the body but we're numbered 1 to 4 with diferent set numbers. It was cheap at the Doncaster show. I mounted it up on a new back plate and with a test piece there was about 0.65 mm run out and needed grinding. I made a set of holders for the jaws, a four hole cloverleaf plate for the inside grinding and a set of rings for the outside faces. After grinding the run out was less than 0.02 mm. You have to load the jaws in the direction of his they are used and direction of clamped.
I borrowed a toolpost grinder from a friend and ground both inside and outside faces on the jaws.
Edited By David George 1 on 21/03/2021 08:05:10
|larry phelan 1||21/03/2021 09:36:01|
|1119 forum posts|
I used to think 3 jaw chucks were dead on, not anymore !
But then, I used to believe in fairies at one time too.
The 4 jaw is your only man.
|Paul M||21/03/2021 10:07:57|
|70 forum posts|
Never used a 3 jaw chuck that is spot on accurate. To ensure concentricity I always use over size bar and turn down work to diameter if in a 3 jaw. Anything else is first clocked in a 4 jaw. Others may disagree but I reckon it's a waste of time fiddling about trying to improve the run out of a 3 jaw chuck apart from occasional grinding of the jaws when worn, if you have the kit.
7695 forum posts
Certainly don't disagree with Mike and MadMike comments, but Avon's 0.25mm error suggested his problem is worse than usual.
My Chinese 3-jaw measures about 0.05mm 50mm away from the chuck jaws. If Avon's chuck has a 0.25mm error at the jaws, I think something is wrong.
But provided work isn't moved during the job, the absolute accuracy of a 3-jaw often doesn't matter because cuts are taken relative to the lathe's stable spinning axis, not the imperfect chuck. When work has to be moved, collets and 4-jaws are the answer when accuracy matters. Expensive 3-jaws provide an enhanced level of safe interchangeability, which can save a lot of time, but no 3-jaw is entirely trustworthy for spot on work. I suspect for many amateur purposes, an ordinary 4-jaw is better value than a costly super 3-jaw. The 3-jaw pays off in a busy workshop - hours of turning, day after day, not grandad firing up his Connoisseur for a Royal visit!
Edited By SillyOldDuffer on 21/03/2021 10:28:01
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