Help getting parallel faces in a lathe

[softtail]

I need to get a 3" dia, 4" long steel round to have parallel faces. I thought I nailed it pretty good, but on further inspection, somethings not right. I had it spinning via a nub in a 5c collet supported by a dead center on the other end, did both faces and the diameter (just for good measure) in one set up. The light finish cuts to the faces were done after taking the skin off the od. Both faces were finished with a very light power feed cut, carriage locked. The diameter doesn't matter, just that the faces are parallel to eachother.

Ideas what could cause this and how to get it right?

Thanks,


st

[Dr Stan]

Try doing it between centers.

[Tony Ennis]

What's not right? Are the faces not flat? Are they not parallel? How far off are they? What's your acceptable error?

[J Tiers]

If you have not got the two centers accurately lined up, you may find that you have made two cones..... which may be parallel, but not at all what you had set off to do.

Check to see if the turned OD has given identical diameters at both ends.

If you don't need the entire face, maybe you can undercut the ends so that just a ring is faced, which will be easier to get "acceptably perfect".

And you must do something to deal with the portion next to the centers, which you can't reach with the tool without hitting the centers. So undercutting the end for that area will allow you a little leeway there.

[tmc_31]

Softtail,

What kind of steel are you using? I know that cold rolled steel has internal stresses that are released when the "skin" or outer surface of the steel is removed causing the steel to squirm. Is it possible that this is causing your faces to be off? You might try some hot rolled.

Tim

[Jim Shaper]

Edit: scratch what I originally wrote (if anyone saw it).

Put a 3 jaw in there and face one side, flip it, then put it against parallels against the face of the chuck, remove parallels after clamping and face it again.

[Carld]

Use a four jaw chuck and since you have the OD turned you can chuck up an inch in the four jaw and then indicate both ends to "0" and face one end. Then flip it over and indicate both ends in again and do the other end.

You have to use a four jaw because you will never get it indicated to "0" in a 3 jaw chuck. It's important to get the end at the chuck and the free end running as close to "0" as you can possibly get it to have the faced ends parallel.

This is a time consuming job and nothing short of near perfection will do but remember, you will not get it perfect by machining it.

[Paul Alciatore]

You didn't say what's wrong. Are the faces not parallel? Or are they conical? Or what?

From what you describe, I would expect two faces that are parallel, but each slightly conical. The face on the tail stock end should be slightly concave and the one at the headstock end should be slightly convex. This is due to the fact that no lathe can have the cross slide exactly perpendiular to the main ways and all manufacturers, well most anyway, will make sure that the error is towards a concave face where facing is normally performed, on the tail stock end. By "slightly" I mean perhaps 0.0005" or perhaps 0.001" for a part your size. If that is your problem, you will have to turn it around to face the second side.

I am doing a similar job at the present. My spacers are only one inch in diameter and vary from 1/4" to 1" long. They have a center hole which also must be concentric so a stud is out of the question. I am making about ten or so. Chasing tenths is a b***h. I'm using a shop made "collet" that grips the 1" OD. It was turned on the lathe's spindle with a flat face for the parts to seat against that is dead true (because it was turned in place). The gripping area is only 1/8" deep but it holds well enough for a light facing cut. I have to be sure that it is dead clean when mounting each part.

Probably the best way to do a piece like this would be to use a custom face plate. Give it a final truing cut when it is mounted. You can use a three or four jaw for the OD and the first face with no problems. You could rough out the second face the same way, leaving 0.010" or so for the final cut. The part is held with the first face against the face plate with a threaded rod through the spindle or with several bolts in a circle through the face plate. In either case, you will need one or more tapped holes in the part. Or a center bore and a nut in a recess. Use whatever is best for your part. Take light cuts.

This should insure that the two faces are dead parallel and also that BOTH are slightly concave.

If you need a better finish on the ends, you can use abrasives to lap them. But be careful to keep the faces parallel. If you do this under power on the lathe, it should be a very even process which would keep the faces parallel, but they will tend to become convex in the process. If you do it by hand it will be easier to keep them flat, but they may stray from parallel and frequent checks will be needed.

Of course, a surface grinder would make fast and accurate work of it. And no concave/convex. But I don't have one either.

[Black_Moons]

You can allways face with your compound set to 90 degrees.
of course, Parallel implys 0.000" error.
Everything has error, so its more a matter of how long you wish to tweak and adjust and measure.

[softtail]

A thousand ways to skin a cat it seems.

I may have gotten it as good as I can. Sounds like a surface grinder is the only way to get it dead nuts.

It's pretty damn parallel. I'm using it as a 'whipping post' mounted on a surface plate, welded parts are then mounted and measured off that to check alignment, some points are 3' away, so error is magnified.

As far as I can tell, the faces are flat (one face on the table, other has part mounted to it, bolts through each end), just not parallel. I can take a parallel and see a slight gap at the top of the post on the side, slight gap at the bottom of the side when swapped 180, so the sides arent' perp to the table, which in itself doesn't matter, but in this case tells me the faces aren't right either, and I 'swept' the table with a long tube mounted on the post, indexing the post 1/4 turns, taking readings at 3 areas of the table,then back to it's original position, it absolutely points to the two faces being out of parallel. The first and last position of the post were the same, and the difference of the combined variance was 0.00078" , which shows good repeatability.. but the three readings themselves, taken at 3 different areas of the table for a given post position varied as much as 0.179, so I was able to get the same crap readings for a given 'index' of the post. Better than getting totally different crap readings, but I need this thing to read the same across the table.

Can a surface grinder handle a part 4" high? May just take it down to the local shop and have the guy throw it on the mag chuck. I assumed I would be able to get it 100%. I have a digital height gauge, and can't pick up any variance with the post mounted on the table, and taking measurements around the upper face.


st

ps, cold rolled, don't know what type. Machined really nice. I've heard of the stresses before that's why I took of a couple od passes, and then finished the faces, which were already rough faced. I did drill and tap it after facing. Could that screw things up?