Threading barrel tenon and pulling my hair out!

Hello,

I'm trying to thread a barrel tenon using a barrel holding fixture similar to this one:

https://shop.pacifictoolandgauge.com/index.php?main_page=product_info&cPath=5_28&products_id=1457&zenid=tnruh5dqr8p79p6kdhrn4nr2q3

Best I can tell, it's not holding the barrel tight, which I don't understand. I put the barrel in and indicated on a range rod to within 0.0001" and then cut the tenon and thread relief with no problem. I re-indicated it, and started threading. About 3/4 of the way to my final thread depth, the bit caught and jammed up the thread. It wasn't too bad and since I wasn't at my final thread depth yet, I re-indicated it, set up the bit to chase the threads and began to slowly pickup the threads to clean it up and cut it to final depth. It was during this that I realized that every time I made a pass, and I was taking 0.001" on the compound at 29.5*, that the barrel was getting knocked out of alignment with each pass. So, I started doing a cut, re-aligning, doing a cut, re-aligning, etc, etc. I found that the barrel was getting knocked out of alignment about between 0.003 to 0.010" with each 0.001" pass!!! Finally, one pass must have knocked it way out and it crashed the threads again and I don't believe they are repairable anymore.

I tightened the set screws for each alignment and believe me, that bugger was in there tight! I'm using a HSS threading bit from Arthur Warner and it's sharp. So, I'm a barrel down and I certainly don't want to put another one in there until I know why it's getting messed up. I just can't imagine how it's getting knocked around that much with such light passes.

Anyone have any experience with a similar fixture or suggestions?

Thanks,
Dave

Are the jaws in your chuck tight? Are they parallel under tension? What about the conection between the spindle and chuck? Is your spindle tight in it's bearings with no axial play? How about your lead screw is it tight?
Any of these could cause the tool not to follow correctly and knock the barrel around.
If the barrel sticks through the outboard side, support there could keep imbalance from moving things as the spindle is turning.

As far as I can tell my machine is fine. Prior to doing this cut, I took an old piece of aluminum (granted its a softer material) and cut these same threads because I wanted to check my major diameter measurement because it's not standard. I did that cut in my 4-jaw chuck and took heavier cuts and that thread turned out beautifully. When I say that the barrel is getting knocked out, it seems to be just the barrel. When I look close, I can actually see the wobble in the barrel as it spins. I would think that if the whole fixture were loose in the chuck jaws I'd see the whole fixture wobble, maybe not though.

It makes very little sense to me.
You say your moving 0.001 on the compound at 29.5 deg.
If the set up is perfectly rigid, you should only be taking 0.0017 off of the diameter, yet the part is moving 2 to 5 times that amount with each pass.

The part could be flexing and you may not be getting a full depth of cut each pass. This can gradually build, increasing the cutting load pass to pass.
Have you made a pass without advancing the compound?
I find that many parts flex quite a bit when threaded and every so often its useful to take a pass without advancing the compound.
I am always surprised to see a chip peal off without a change of settings, but even the old EE is not perfectly rigid.
This helps keep the chip load even, rather than slowly building it up with increased part flex.

I have also observed tools grab and pull in or have the part try to crawl over them.
If the tool grabs, the part can climb up and over the tool, potentially moving that much.
The place I experience this is a parting tool, especially in a small lathe like a heavy 10 SB where the saddle and compound flex a bit.

The set up your using lends itself to this.
Your part is cantilevered a long way out of the chuck and you have no outboard support shown.
Any chance you can use your tail stock to provide a bit more support.
Sure that may mess with the 0.0001 indication, but your part is walking around so your not maintaining that level of precision.

Another thing you can do to improve rigidity of the set up, work with the compound pulled back and the tool close to the tool post. The closer the cutting forces are to the horizontal pivot point of the compound the less it will flex. This can be a big deal on small lathes.

A tightly held part may not be a rigid part. Standing on ball bearings does not hold them in place.
The 4 jaw chuck grip may be an issue. Pop that holder out of the chuck and check your contact pattern. Do you have good even contact or is the part rocking?
If you cant tell, put some ink or layout die on the parts and try again. It is very possible you have ring of contact near the chuck face and almost no pressure near the jaw tips.
This is more common on worn 3 jaw chucks. Debris, trash, even burrs from clamping can all cause problems.

The part may feel rigid to you, but the compound pushes a lot harder than you can.

VERIFY your indicator is ok. Its not likely to be the problem but you could be on a wild goose chase.

All very good ideas and I will definitley check on the chuck jaws and whatnot. I did do some "spring" passes with zero feed. I agree, I can't really explain what is happening either. I find the tool grabbing theory interesting. I'm using really slow feeds and lots of oil, but its possibe. Thanks for the ideas!

It may be the angle of the shot but looking at your picture I would say that the holding fixture is not parallel with the ways of the lathe. On that basis I would hazard a guess that the fixture is loose in the chuck (ala FredC).

FWIW

-Ron

Ditch the 4-jaw and bolt that directly to a backing plate. Better yet, saw it off to about 2" long and make another for the tail end of the spindle. But that assumes you can actually reach the tail end with a barrel, which I know is impossible with a lot of older lathes. Been there and suffered with it.

If you are going to support at the back end of the spindle, why not just use the 4 jaw as is?

The more I think about it, 3 screws per side is not the best way to go, it invites bending of the barrel in the fixture.

I would call Bop Pastor- 269-521-3671 and talk to him about your issues. He makes and uses that fixture and a nice guy.

as far away from the chuck as your tendon is, i could bet it would move. I would put a live center it there for added support.
i can see problems with that setup.
Ken

ahall said:

If you are going to support at the back end of the spindle, why not just use the 4 jaw as is?

The more I think about it, 3 screws per side is not the best way to go, it invites bending of the barrel in the fixture.

Click to expand...

I think the idea of that device is to support an action. Three holes for short and long actions, you only use two of them. Then again, no reason it couldn't work on a barrel. I'd still only use two screws on each side, front and back for a barrel. I'd still like to have a spider on the opposite end to keep the barrel from whipping around, spider would just be there as a steady, not to indicate.

Art

Not familiar with that fixture at all, but it appears to only have the three screws in a row. The link you gave does not show it any better. Anyway, two, three, or eight screws tightened to the barrel will not have near the holding strength that the 4 jaw chuck would have unless there is a lot of taper on the barrel. The fixture could use a shoulder to bear against the face of the chuck jaws to secure it against tool pressure.

I have a feeling that when you did your aluminum test piece it was not sticking out 6 inches from the chuck jaws. Thinking some more on this since, I would bet at least 2 cents the fixture is moving in your 4 jaw. Bolting the fixture to a plate has been mentioned as well as a center for support. Either or both may fix things if it is the fixture moving. Have you indicated the fixture before and after a failure?

Why not just clamp the barrel in the 4 jaw? That fixture is only adding another contact point and you are a long way away from the chuck. Closer is better in my opinion.

accumulated error!!!!! the more you add to the setup the more to go wrong.
Ken

Ditch the fixture.

It looks to me that your set screws are too short, maybe only retaining a few threads in the fixture, the rest protruding through to the barrel. This may allow too much play as a whole. I would install longer screws that offer more lateral support.

I have a fixture about identical to that, only I machined a 1" thick steel plate couterbored for the fixture, welded it in, and installed on a backplate. Much more secure and brings the cutting location back a few inches closer to the headstock.

This isn't my pic. but it is something to consider.

Grumpy365 said:

View attachment 107233

This isn't my pic. but it is something to consider.

Click to expand...

This.
Its simple and direct.
There's no need for that jig.
That copper wire and a 4 jaw with an outboard cathead will get the job done.

Grumpy365 said:

View attachment 107233

This isn't my pic. but it is something to consider.

Click to expand...

Wow, first class work!


LOLz...