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Chuck out of true

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Grotto26/07/2019 22:00:20
124 forum posts
71 photos

After several items I turned on my ML7 turned out bad I’ve discovered my 3 jaw chuck is not running true.

when I removed it I noticed that it was very tight on the head stock thread. I’ve pulled the jaws out and inspected them, they look fine.

I've put a dial gauge against the flange on the head stock and that's perfect. I've been using ER collets since I realised the 3 jaw was dodgy and they seem fine.

I suspect that some swarf has got into the tread of the 3 jaw and is causing it to run off centre. I've cleaned the internal thread on the 3 jaw backing plate as best I can but it's still tight when I try to fit it (4 jaw goes on fine).

Any ideas on how best to cure the problem?

The positive from this is that I've got really quick setting up the 4 jaw. I'd be fine continuing with the 4 jaw but it’s no good for turning hex stock.


David George 126/07/2019 22:49:25
1303 forum posts
447 photos

Hi Grotto! It sounds like there is some damage of swarf in the system somewhere. You have to strip the chuck give it a clean and check for damage. If you remove the chuck from the back plate you will find some screws three will hold the key bevel gears in and three will hold the back plate on to hold the scroll gear in place remove all but label the plates and gears with a dot from a centre punch to make sure that they are replaced in the same orientation. Clean the back plate and screw on to the spindle and clock it to make sure it runs true. If it dosnt unscrew and check for damage. You can shim the face to true if nessesary. Rebuild chuck and assemble on to back plate. Put a dowel about 20mm dia in to chuck and check for run out. If it dosnt run true you have to clock the chuck and maybe adjust the location diamiter.


Bob Stevenson26/07/2019 23:39:42
426 forum posts
7 photos

...........Chuck been dropped lately?

Grotto27/07/2019 02:25:49
124 forum posts
71 photos


I was a bit nervous about pulling the back plate off in case I couldn’t get it back on true, but guess as it’s unusable as is there’s nothing to lose.

I haven’t dropped the chuck, suspect it got fitted with swarf in the threads by either myself or my son.

when screwing the chuck on, it gets tight before it’s fully on, so think the tread is a bit damaged. I don’t have a tap big enough to fit the thread, maybe put the back plate on the face plate and have a go at threading on the lathe?

Michael Gilligan27/07/2019 06:28:37
16193 forum posts
706 photos
Posted by Grotto on 27/07/2019 02:25:49:


I was a bit nervous about pulling the back plate off in case I couldn’t get it back on true, but guess as it’s unusable as is there’s nothing to lose.

I haven’t dropped the chuck, suspect it got fitted with swarf in the threads by either myself or my son.

when screwing the chuck on, it gets tight before it’s fully on, so think the tread is a bit damaged. I don’t have a tap big enough to fit the thread, maybe put the back plate on the face plate and have a go at threading on the lathe?


It is obviously difficult/impossible to diagnose at a distance, but: It seems likely that there is a small burr raised on the backplate thread, or perhaps a bit of impacted swarf

I suggest that you remove the backplate from the chuck and try gently screwing it reversed onto the spindle nose.

This should be sufficient for you to identify the location of the blemish; [and with luck on your side might even level it down] ... If the backplate fitted nicely before, you should find it only needs localised attention.


Mick B127/07/2019 09:13:21
1657 forum posts
88 photos

David and Michael are probably on the right track, but I'd add that I think you can expect the 3-jaw fit on the screwed spindle nose to be more exacting than it is for the 4-jaw even if everything's in good nick. That's the way it was on my Speed 10 - I had to align the 3-jaw more accurately when starting the thread to screw it on, or it would bind on the parallel bore in the backplate before the threads even met. The 4-jaw was a good deal easier.

The accuracy of that location is obviously less significant on the 4-jaw where the user and not the chuck is setting the workpiece centre of rotation.

I guess I'd start by looking at the bore in the backplate for bruising.

JohnF27/07/2019 09:14:25
1011 forum posts
143 photos

Grotto, There should be no problem removing and re-fitting the back plate providing it is correctly made - there should be a register to locate the chuck body on but do mark the back plate and the chuck body with a dot as has already been suggested. In reality it should not matter but as good practice its best to do so.

From what you said in the first post your chuck ran true before it became tight on the spindle ? This would indicate that there is probably some swarf or damage to the internal thread. Rather than screwcut as a first try I would hand clean the thread "scraper style" with a screw cut tool, if it is just a small piece of swarf you will probably feel this and be able scrape it off, if not then revert to your plan and screwcut it taking great care to align the tool with the existing thread.

Probably prudent to turn by hand and not under power for a start until you are sure all is well then use power.

For interest how much error was there on the parts you made -- what was the TIR error on the chuck/parts ?


not done it yet27/07/2019 09:35:39
4877 forum posts
18 photos

The above have it sussed, one way or another.

The back plate threads are likely not anywhere near a precision fit - they are there simply to hold the chuuck on the spindle threads and so do not need to be a really high precision job.

The chuck will be aligned by the registers between the spindle nose and back plate. One axial contact surface and at least one radial contact area.

If there are two radial contact areas, the chuck will be thrown off-line by the affected surface. It does not need any more than a slight dink or tiny piece of trapped swarf to seriously affect the alignment.

old mart27/07/2019 20:00:39
1911 forum posts
151 photos

Use a dental mirror to look for something stuck in the threads, there must be something there. I have chuck backplates with registers varying from 0.0005" to 0.015" clearance. They all repeat perfectly, the tight one can be difficult to get on and off though.

Once you manage to get your chuck screwing on and off more easily, then remove the chuck from the backplate after fitting on to the spindle and check the face and register for the chuck with a DTI. A light skim may be required in one or both planes.

Neil Wyatt28/07/2019 10:09:28
18133 forum posts
713 photos
77 articles

Once you are sure the mounting thread is OK, have you ever dismantled the chuck and cleaned it?


Grotto28/07/2019 19:53:15
124 forum posts
71 photos

I've pulled the back plate off the chuck. It doesn’t want to thread on to the head stock either front or rear first.

I was sure it was dirty threads in back plate as the 4 jaw goes on fine, but have tried screwing my face plate on and that is also tight.

Weekend was busy so didn’t get much time in the workshop, but have set up the change wheels to 12 tpi and will run an internal thread tool through (turning by hand) this evening. I've tried cleaning thread using a thread gauge, but that didn’t seem to make much difference. I can’t see any obvious swarf in the tread, but there is one point where thread may be a little damaged.

the thread on the head stock looks perfect, but as the face plate & 3 jaw are both tight maybe I being deceived.

Pete Rimmer28/07/2019 21:35:53
773 forum posts
50 photos

Put a dial gauge on the OD of the chuck, and then against the face. You'll have to pull the jaws out and turn it carefully so you don't catch the slots. If neither run out then you have a worn scroll or bell-mouthed chuck slots. If the OD is true but the face runs out it could be bell-mouthing of the face. If the OD is running out but the face is true then it could be it's not registered to the back plate properly, in which case slacken the bolts and tap it into truth with a soft faced hammer.

If the OD AND the face run out then mark them to ascertain where the high points are. If they are opposite each other on diameter remove the backplate and clock the register face. If that runs out check the rear face for dings and if it has none run a skim over the register face and bolt the chuck back up, true it radially as above.

To check for bell-mouthed slots remove the chuck jaws and turn the chuck so a slot is horizontal. Now mount a DTI on the cross slide and set the probe against the slot and wind the cross slide in. The DTI should not move.

Grotto29/07/2019 10:10:02
124 forum posts
71 photos

I found some time today so had a go at sorting the issues.

I was thinking it was the thread on the face plate since the 4 jaw goes on fine, but based on what Mick B1 has said that may not be the case.

i gave the head stock & backing plate threads a good clean, and the 3 jaw & face plate both fit better, although tighter than I recall.

I used an indicator on the edge & face of the face plate....

photos show maximum variation. Edge is true, face is a bit out



Indicator on head stock is OK


Indicator on backing plate is not so good



Indicator on 3 jaw with a 10mm dowel is way out



Vasantha Abey29/07/2019 10:40:45
16 forum posts


It seems that your chuck back registering area has a tiny protrusion may be by an object while keeping aside. Its like a single knurl which makes the chuck slightly off set on face. The register or the nicely polished neck of the spindle has to be spot on mating with the female area of the chuck. If all that is not the trouble, swarf inside the chuck scroll, or the jaws assembled in different numbering to the embossed numerals on the chuck body.

If all fail, what you can do is hold a circular ring on the chuck but locking by opening outward. Then use pencil like die grinder mounted on the tool post and slowly do an internal grinding of the chuck jaws.

usually after long use, the jaws develop end plays with reference to the face as there will be wear on the T slot on the chuck body. If its so, and you cannot do anything else then pls go for a new chuck. Any way any brand new chuck is out by a few thou, may be .01, /.02 mm

There is a lathe called Cazenuve made in France which has a separate chuck holding plate that has a protruding ring, and this protrusion has 4 threaded holes and 4 threaded bolts with square holes to adjust , that which shift the chuck by a few thous to correct out of roundness. The three jaw chuck is held to this back plate by alan bolts sunk in behind the plate, and holding the 3 jaw chuck from behind at a 90% torque so it is free to shift to allow correction.

JohnF29/07/2019 11:07:07
1011 forum posts
143 photos

Grotto, two points in the photo Indicator on the headstock - pic 1 you are checking the OD of the spindle nose and not the register -- register is the plain part immediately behind the thread. It probably will give the same reading but the part you have the indicator on will make no difference to the chuck mounting.

Indicator on the back plate - again where you have the clock bearing will not affect the chuck mount, you need to check the outer edge where the mount holes are, again if turned together as one would expect both areas should give the same reading- but ?? The same goes for checking the OD of the back plate the actual register is the narrower part that goes into the chuck body that really matters.

As a matter of interest was the back plate machined on your lathe ? Normal practice would be to machine the backplate on the machine it will be used on thus ensuring true centralisation.

You have not said if prior to this whether your chuck did run true? Or has it always had significant runout? Makes a difference to the solution.

In any event you need to get the tightness of the fit resolved first then if the backplate and or chuck still has significant run out it may be worth considering re-machining the backplate register providing there is enough material to allow this.


Mick B129/07/2019 11:40:37
1657 forum posts
88 photos

I'd guess there's a bruise that's stopping the backplate locating properly against its datum face, and tilting it. My Myford was a Speed 10, and in any case I can't remember whether the bottom of the backplate register bore or the back face of the backplate contacted first, but it could well be that engagement that's being compromised.

The runout of a straight parallel bar should vary with distance from chuck face if that's what's going on.

Pete Rimmer29/07/2019 15:59:27
773 forum posts
50 photos

I've had plenty of chucks that didn't use the OD register but ran perfectly true just seated on the face register and centred by the threads. If you think about it the threads will do a pretty good job of centring even though they are a helix. The face register is by far the most important. That front face of the spindle step and the back face of the chuck backplate must be flat with no dings in the edges. Screw on chucks pick up dings so easily you should check them every time before mounting the chuck.

old mart29/07/2019 20:32:14
1911 forum posts
151 photos

As already mentioned, your faceplate requires truing up, a difficult job on a lathe without power cross feed. The difficulty is in keeping a constant smooth feed while facing.

I have already mentioned that the register has little or no affect on the repeatability of a screw on chuck.

If the faceplate is trued up, then the backplate could be attached to it, adjusted for concentricity of the female register, and the threads very lightly skimmed. Then before taking anything down, the rear face of the backplate skimmed true. After that is done, the backplate needs to be screwed up tight and backed off several times to settle it down, before checking the runout of the chuck mounting face and the concentricity of the chuck register. Both may require truing up.

Howard Lewis29/07/2019 22:10:30
3536 forum posts
2 photos

Coming in lare, as usual, if the 3 jaw, 4 jaw and faceplate are hard to screw on to the Mandrel, it sounds as if that is where the problem lies.

It may well be that the register has been damaged. A burr here wouldmake the fittings hard to screw on, and hold them out of position, and possibly out of square.

From what you say, it sounds as if your ER chuck goes on OK, and runs trues, which is puzzling when the others do not.

REALLY daft question, the male and female threads are all the same pitch and size? Ie you haven't been sold a Myford, with accessories for another machine? You should be looking at 1.125 x 12 tpi threads. From memory, i think that the register should be 1.250

Myford cognoscenti please verify.


Grotto30/07/2019 04:59:09
124 forum posts
71 photos

Thanks for all the help

John F - Good point about where the indicator stalk is on the head stock. I did it again against the register and there’s no movement in the gauge.

also checked the back plate on the outer edge. Photo shows maximum...


The back plate came with the lathe & chuck. It came from a retired engineer who’d had it in his home workshop. He was real precious about it, wouldn’t let his middle aged sons touch it (they’d both done fitter/turner apprenticeship) in case they damaged it. It looks very well made (better than I could do). It previously ran true, I’m not sure when it went out as I had several months away from the workshop. I was initially think all the round stock I had was oval (I’m a real beginner).

Howard - the 4 jaw screws on fine, it’s only the 3 jaw & face plate which are tight.

I put the backing plate on back wards, and tried the indicator on that...




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