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View Full Version : Steady-rest pulls workpiece out of 3-jaw



aostling
04-13-2010, 01:50 PM
I'm making a part consisting of one tube telescoping inside another with a sliding fit. PVC 3/4" white tubing is perfect for my application. The ID is not perfectly round, though, and I want to true it up with a boring bar, going in no more than about 1.5".

Trouble is, the steady-rest acts like a "screw," pulling the tube out of the chuck. It happens whether I chuck on the OD or the ID.

You may note that I have the steady rest installed backwards, with the adjusting screws for the three brass pads facing the chuck, where they are harder to access than if they are facing the tailstock. I first installed it the correct way, and only switched it to this backward way to see if that would cause the part to screw in rather than out of the chuck. But this had no effect.

The clamping force at the chuck is necessarily limited by the thin wall, but of course I clamp it up as tight as I practically can.

Any suggestions?


http://i168.photobucket.com/albums/u183/aostling/steadyrest.jpg

mochinist
04-13-2010, 01:53 PM
a small round piece with the same dia as the ID of your pvc right where you are chucking, then you can put a little more force on the jaws without collapsing the part.

2ManyHobbies
04-13-2010, 02:16 PM
3-jaw is horrible for tubing. The tube will torque then deform taking pressure off of a jaw. The other 2 don't really have a grip on anything at that point. 6-jaw, collet, or soft jaws cut to match the work would be better. A mandrel wouldn't help you here...

aostling
04-13-2010, 02:21 PM
a small round piece with the same dia as the ID of your pvc right where you are chucking, then you can put a little more force on the jaws without collapsing the part.

Thanks Donald, I'll try this next. Should have been obvious, perhaps, but I was thinking more in terms of why it was happening. Helps to ask a pro.

small.planes
04-13-2010, 02:22 PM
if the steady isnt exactly holding the work central then the workpiece will 'orbit' and as a result walk out of the chuck. not helped by the limited clamping force you can get on a thin wall tube.
Try resetting the steady carefully, and turn up a plug to fit the tube to allow you to clamp a bit tighter, or a close fitting collar and use the jaws internally on the tube.

hth

Dave

Oldbrock
04-13-2010, 02:31 PM
further to Small Planes point, get the tube running true by checking with dti then set the steady jaws with the steady right adjacent to the chuck. then move the steady away to the end of the tube then there is no side force pulling the work out of the chuck. The steady must not exert any side force on the work or it will always walk out of the chuck no matter how tight the chuck is. Peter

Mike Burdick
04-13-2010, 02:42 PM
...Trouble is, the steady-rest acts like a "screw," pulling the tube out of the chuck. It happens whether I chuck on the OD or the ID. ...

This may not be a solution to your problem but for me when the material is "unscrewing" from the chuck the problem is usually from the material not being exactly on center-line with the spindle. Since the steady is a little off centerline each rotations causes a little wiggle at the chuck and the result is the material gets pushed out. As one can see, the shorter the work piece the greater the problem. To solve this, I just adjust the steady more to to the spindle center-line.

Carld
04-13-2010, 02:47 PM
Yes, you don't have the tube centered at the steady rest in line with the center line of the chuck and that will cause the steady rest to pull the work out of the chuck.

Ummm, if you took that picture from the front of the lathe you have the steady rest on backwards.

djc
04-13-2010, 03:38 PM
Another option instead of or in addition to the chucking piece is to make up an expanding plug that sits in the tube, with a drawbar through the mandrel, to make darn sure it can't come out.

The principle of the plug is like a drain stopper:-

http://www.hygienesuppliesdirect.com/images/productpics/1311721.jpg

Three mud washers, three nuts, some rubber and a piece of allthread...

John Stevenson
04-13-2010, 03:41 PM
Ummm, if you took that picture from the front of the lathe you have the steady rest on backwards.

Read the first post.
.

clutch
04-13-2010, 05:21 PM
The part is turning into somewhat of a cone where your jaws are clamping. Cone meaning how the forces are distributed in clamping if you graphed them longitudinally.

The suggestion of using a plug is the solution and someone else already beat me to it.

Clutch

aostling
04-13-2010, 05:21 PM
get the tube running true by checking with dti then set the steady jaws with the steady right adjacent to the chuck.

I'm sure that would be a good idea, but I haven't started that fiddly process yet. I centered the part with the bullnose in the tailstock, then backed that out of the way. No wobble is detectable by eye. If the plug solution alone is not sufficient I'll get out my indicator.

oldtiffie
04-13-2010, 05:44 PM
Allan,

mount the job in the 3-jaw chuck, loosen the steady arms until they are just clear of the job and move the steady up to the chuck.

Adjust the steady arms until the bottom ones are just touching the job - the top arm should just (barely) clear the job. The steady is now aligned to the centre of the spindle axis and the job.

Move the steady back to the tail-stock end of the job.

Set your tools up-side down and at centre-height in your tool post.

Run your lathe in "forward" (ie as normal) and cut on the "back" side of the lathe/job centre line. The tool cutting - facing and boring - will cause the job to be forced down onto the two lower steady arms. Use a good cutting/tapping oil as a lubricant on both the cutting tools and the steady arm bearing/supporting arm pads/ends.

Its easier to see the bored hole this way as well.

I'd seriously consider putting a temporary wooden plug/"bung" into the bore where the chuck grips the tube so as to both support it and to stop the chuck tending to push it out.

Take it easy, do it in slow time until you get it set in your mind until you are confident in the process and away you go.

aostling
04-13-2010, 07:13 PM
Run your lathe in "forward" (ie as normal) and cut on the "back" side of the lathe/job centre line. The tool cutting - facing and boring - will cause the job to be forced down onto the two lower steady arms. Use a good cutting/tapping oil as a lubricant on both the cutting tools and the steady arm bearing/supporting arm pads/ends.


Tiffie,

I'm really glad I asked this question. Until today I had never taken the steady rest out of its package. Your response in particular describes setting up in a way I would not have thought of, having the forces press down on the two lower steady pads. Centering the pads near the chuck, I would not have thought of that either. Lubricant too, splattered walls be damned.

Spin Doctor
04-13-2010, 08:22 PM
Identical post in the Centering Large Pieces in the Lathe thread

We used to do a lot of work using TG&P stock. Some of it up to 3" or so. One thing I always liked to have on hand was short pieces of the sizes we used most of. I would indicate one of those in and set the steady rest to that. I could then move the steady rest out to where I needed it and put the stock I was using in the lathe. And we did everything in a 4 jaw chuck.

This is useful for those odds and ends you will acumulate

darryl
04-13-2010, 09:40 PM
I don't know how many times I've had a piece of pvc tubing come out of the chuck. I've developed a bit of a lean to the right which helps prevent me getting hit in the face by the projectile. What hurts the worst is that I never learn from it, and continue to just clamp down on the pipe without having made a plug for it first.

The plug is probably the best answer, as then you can clamp down much more without distorting the tube. The opposite to that is also good, which is when expanding the jaws inside a tube, you have a large washer type thing on the outside of the tube to give the jaw forces something to tighten up against.

oldtiffie
04-13-2010, 10:01 PM
Originally Posted by oldtiffie
Run your lathe in "forward" (ie as normal) and cut on the "back" side of the lathe/job centre line. The tool cutting - facing and boring - will cause the job to be forced down onto the two lower steady arms. Use a good cutting/tapping oil as a lubricant on both the cutting tools and the steady arm bearing/supporting arm pads/ends.

Tiffie,

I'm really glad I asked this question. Until today I had never taken the steady rest out of its package. Your response in particular describes setting up in a way I would not have thought of, having the forces press down on the two lower steady pads. Centering the pads near the chuck, I would not have thought of that either. Lubricant too, splattered walls be damned.

Thanks Allan.

Glad to pass on something that helps that was passed onto me with the expectation that I pass it onto others.

No need to be heavy-handed with the tapping/cutting oil either - just use a brush sparingly as required as its a lubricant and not a coolant. The higher viscosity of the cutting oil will keep it pretty well where you put it.

wierdscience
04-13-2010, 11:32 PM
Try sliding the steady down to just infront of the chuck jaws,set the steady rest fingers to the pipe OD there,then slide the rest back to where you need it.Pipe should be centered close enough for it to work.

Wax,candle or bees is excellent lube for PVC and it doesn't make a mess.Hope this helps.

aostling
04-14-2010, 12:47 AM
Wax,candle or bees is excellent lube for PVC and it doesn't make a mess.Hope this helps.

I like the idea of using bees wax. I'm climbing Camelback as usual tomorrow morning. I'll ask any ladies at the summit to recommend a candle shop.

gearedloco
04-14-2010, 01:46 AM
oldtiffie, what you said seems right on, but it makes me think steady rests are built upside down, now that I think about it!

-bill

small.planes
04-14-2010, 05:25 AM
something I just thought of:
Three jaw chucks rarely run true, so even with the steady set true there will still be an orbital motion due to chuck run out.
If you have a 4 jaw then you can dial it in for zero runout.
Dave