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How much can a chuck effect finish?

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Hollowpoint17/04/2019 20:44:28
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191 forum posts
27 photos

Guys i'm getting a bit frustrated with my little C1 lathe its a temperamental so and so. The way it cuts seems to depend on what day of the week it is.

I think I've narrowed the problem down to one of two things. I think its either the chuck or the head stock bearings. However both give me doubts.

1. The chuck looks to have been messed with. It doesn't centre up very well without anything in it but with a length of stock in it does appear to hold reasonably concentric and tight. However when turning the finish can be anywhere in the range from acceptable to god awful. I can only take very light cuts even in aluminium and I usually get judder and it sometimes screams like a banshee.

2. The head stock bearings are strange, I can spin them by hand and they feel OK, if possibly sounding a little dry. There does seem to be a slight tight spot but this is only noticable with the chuck removed and the belt disconnected. There appears to be no sideways play or movement whatsoever.

If I chuck up a length of stock directly into the spindle using a mt2 collet and draw bar the finish improves dramatically and I can take bigger cuts judder free even in steel.

So can the chuck really make that much difference? or is it simply that when i'm turning using collets i'm much closer to the head stock?

SillyOldDuffer17/04/2019 21:56:06
4425 forum posts
957 photos

I'm suspicious of the chuck. Possibly the jaws or the scroll have been damaged allowing the work to rock slightly as it spins and cutting pressure is applied. You might be able to feel a 200mm rod gripped in the chuck wobble or see a DTI jumping as you waggle the rod.

Another possibility is swarf inside the scroll, between chuck and adaptor plate, and/or between adaptor plate and the spindle flange. Try giving it all a good clean, and if necessary strip the chuck down and inspect the scroll and other innards for damage.

Bad bearings, excessive tool-overhang, difficult material, loose tool-post, and loose gibs seem unlikely causes because you get better results from a collet. But trust nothing, sometimes two or three small problems combine to produce scary symptoms.

Used to have a mini-lathe and it had no trouble with Aluminium. Had to work harder on mild-steel but that was OK too.

Good luck,

Dave

Karl Challis17/04/2019 21:57:49
8 forum posts

Hi Hollowpoint,

There are a couple of factors that can have an effect on surface finish which are directly related to the chucks performance, like you mention there is the headstock bearings, In my experience when the bearings start to wear you can usually get a better feel by putting the chuck under a load using your tailstock and then manually rotating and feeling / listening for issues.

If your hard jaws dont centre very well then it seems the scroll thread inside the chuck is worn matched with worn out jaw threads can create a lot of play and this can also cause temperamental finishes.

I don't mean to insult your knowledge on the subject but it seems worth mentioning, is your crossslide is tight, the slide screws are tightenened appropriately to remove play from the slides, tool fastened securely and on center height with a sharp cutting edge?

Hope some of this may be of use.

Karl

Hollowpoint17/04/2019 22:16:01
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191 forum posts
27 photos

I am not insulted at all Karl, all help is appreciated. I should have mentioned though that i'm not completely new to engineering and I own bigger machines. I have tried the more common problems which is why I don't believe it to be anything else.

I have adjusted all the slides and gibs and they are pretty smooth and solid.

I have tried brand new cutters both HSS and Carbide tipped.

Tool and work piece overhang is minimal.

I have tried holding tools in both the compound slide and a tool post fixed directly to the saddle.

I have tried to minimise chuck and back plate run out as best as I can.

not done it yet18/04/2019 08:25:44
3031 forum posts
11 photos

Choose your machining days in the workshop more carefully?!smiley

If your bearings feel rough, they are rough.

Any three jaw chuck could be off-centre by a few tous - but the cut will still be concentric.

Are these cuts carried out using a centre support? I expect so if you are an experienced machinist, but that would reduce or remove any end-float, provided the thrust bearings are not shot.

Is it both facing and sliding cuts? Dry bearings are not a good sign.

Somehow, I doubt it is the chuck. Holding power would have to be suspect, if it was - and tailstock support should help to keep everything held in position.

Clive Brown 118/04/2019 08:40:32
234 forum posts
7 photos

Is it a 3-jaw or a 4-jaw sc chuck? the OP hasn't said which. I ask because an inaccurate 4-jaw won't hold round stock firmly, whereas a 3-jaw will.

A separate point is that work overhang is greater with a chuck than a collet, giving more of a test for the spindle bearings

Clive

XD 35118/04/2019 10:34:44
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1276 forum posts
107 photos

Doesn’t sound like a bearing problem to me because when you change over to a collet chuck the problem goes away , if it were the bearings it would remain the same .

You could have a crook chuck - i would strip it down and have a look at it .

one thing you can check before pulling it apart is the contact pattern of the jaws , take the shank of drill bit (one with no damage on the shank  or a piece of silver steel ( drill rod ) and colour the shank in with a texta or sharpie for a length a little longer than the gripping surface of the chuck jaws then hold this shank very lightly in the chuck so you can grab the shank with pliers and and turn it so it slips in the jaws this will show if the jaw faces are parallel or bell mouthed . If they are you can re grind them but on a cheap chuck it is not worth the hassle .

Other things to check are how well the jaws fit in the chuck body slots - they should be smooth with no play or wobble - if there is some serious play try switching jaws around and see if you can improve it . The scroll should be a nice fit in the body and have no radial play and minimal end float ) if you put the scroll in the body and it floats around all over the place especially on the spigot it rotates on look at getting another chuck .

Check the register surfaces on the spindle and the rear surface of the chuck to ensure there are no burrs or high spots that could be stopping the chuck from seating properly - a very light rub with an oil stone will show up any high spots , don’t go mad with the stone you only want to show up high spots not remove any metal .

Dave Halford18/04/2019 10:35:36
402 forum posts
3 photos

Are the jaws going bell mouthed when tightened? Basically what XD is saying above

Bruised chuck mount?

Edited By Dave Halford on 18/04/2019 10:36:26

Pete Rimmer18/04/2019 17:02:53
337 forum posts
11 photos

Could be bell-mouthed jaws or it could be a raised burr on the rear register. If the back of the chuck doesn't seat firmly on the spindle all the way round then it'll rock on the threads when you put a cutting load on it.

John Reese18/04/2019 17:36:49
751 forum posts

+1 more on bellmouthed jaws.

Alan Hopwood18/04/2019 20:04:02
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29 forum posts

Risking being thought too simplistic, I have got into the habit of tightening all three stations of a three jaw before any cut. I also almost always wipe the contact edges of the jaws with my index finger before inserting the stock to be machined.

Regards,

Alan.

Dennis Pataki18/04/2019 21:00:47
6 forum posts

I assume yours is a type of lathe where the self act carriage feed is provided using the half nuts and lead screw.

If so, you might check for a bent lead screw. In such an instance, a bent lead screw will impart an oscillating motion to the carriage, causing cutting tool movements that would not occur if the lead screw were straight.

Ady Wilson18/04/2019 21:36:52
39 forum posts
14 photos

Mount some stock in the chuck and put dial gauge on it. Put some pressure on the stock and see how much deflection is there. Then move the dial gauge and put it against the head stock. Push on the bar again and see if there is any deflection. I you are getting deflection on the head stock then your problem will be there, might need tightening down. It happened to me once and baffled me for a while because I was not expecting the movement there.

Adrian.

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