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armedandsafe
08-25-2011, 07:28 PM
I need a 7/8-14 x 3/8 plastic (probably Delrin) plug and thought to buy the rod and thread it. My idea was to thread 6" pieces, using 1/2" at the head stock and 1/4" at the tail stock dead center, then cut the 3/4" plugs from the long threaded portion.

Well, I've destroyed 18" of the stuff so far and I'm still not getting things right. If somebody would advise me on rotational speed and cut feed, I'd appreciate it.

Thank you.

Pops

SGW
08-25-2011, 08:28 PM
Can you be more specific as to what "not getting things right" means?

You are single-point cutting the thread on the lathe, I take it? Delrin cuts like a dream, so if it's really Delrin, it ought to be pretty forgiving as to feed and speed.

armedandsafe
08-25-2011, 08:54 PM
Two major problems:

1. Swarf clogging the point to the extent of stalling the lathe. I am using a double-ended threading HSS tool I got from littlemachineshop with my set-up gauge. I'm setting up to run as slowly as the machine will run.

2. Finish on the outside is very rough, although very clean and smooth in the grooves themselves. That actually is not a "major" problem, as this plug will never be removed.

Another annoyance is that I haven't learned how to get the thread indicator to rotate under power in both directions. It will rotate going one direction under power, but doesn't in the other direction. Tiny changes in the engagement will change which direction that it will indicate. It will rotate under hand crank in either direction.

Keep in mind that I am just getting into this skill after 50 years of talking to electrons. (Use small words. :D )

Pops

Black_Moons
08-25-2011, 09:02 PM
sure the gear is tight on the shaft? on my threading indicator it came from the factory very loose. nearly falling apart.

Sounds like you might be taking too deep of a cut, also plastic is VERY flexable, threading 6" of it might be a bit much, try down to 4"

also, threading is a VERY VERY slow operation, No I don't mean RPM wise, I mean cutting depth wise. A typicaly threading op might be like .007 .005 .004 .003 .002 .002 .002 .001 (repeat 0.001) untill done.

Because as your threading tool goes deeper, every pass has more of the cutting surface in contact with the tool (All form tools do this), So you need to take a lighter and lighter cut to compensate.

Also try the 29 degree infeed. (29 degrees from cross slide, Not 29 degrees from carriage! Ie, compound is closer to being aligned with cross slide then it is the carriage)

darryl
08-25-2011, 09:14 PM
For one I think you're having too much unsupported length, even though you're using a tailstock center. It will probably be faster to do them one at a time, then part off. If you can find a nut that matches that thread, you can chuck the nut once all the plugs are threaded and parted off. Then run each plug into the nut to face the rough end. The nut would need a stopper in it at the right depth- you could cross drill it and press in a pin to be the stopper.

Plastic has a lot of give, and that doesn't work well with threading. You may have other issues- dull threading tool, improper relief and helix angles- not to mention play throughout the entire tool positioning exercise, which means crosslide, compound, carriage ways, plus the threading leadscrew slop- all in addition to possible errors in using the threading dial. I've never been fond of the dial. My preference is to stop the lathe, back the tool out, run it in reverse, then bring it in again to start the next cutting pass. Leave the threading lever engaged all the while. This at least eliminates errors with the half nuts catching the leadscrew on the proper thread.

armedandsafe
08-25-2011, 09:18 PM
sure the gear is tight on the shaft? on my threading indicator it came from the factory very loose. nearly falling apart.

Sounds like you might be taking too deep of a cut, also plastic is VERY flexable, threading 6" of it might be a bit much, try down to 4"

also, threading is a VERY VERY slow operation, No I don't mean RPM wise, I mean cutting depth wise. A typicaly threading op might be like .007 .005 .004 .003 .002 .002 .002 .001 (repeat 0.001) untill done.

Because as your threading tool goes deeper, every pass has more of the cutting surface in contact with the tool (All form tools do this), So you need to take a lighter and lighter cut to compensate.

Also try the 29 degree infeed. (29 degrees from cross slide, Not 29 degrees from carriage! Ie, compound is closer to being aligned with cross slide then it is the carriage)

I was thinking that I might be threading too long, so I tried some 1" stubs. Not a wise choice. :eek: I spent an hour looking for the last one after the lathe flung it across the county. :D

I am definitely cutting too deeply, as I thought I could get deeper, cutting plastic. I've been taking .010 cuts.

The 29* infeed confuses me. I remember seeing this mentioned on one of the sites I visit, but didn't understand what was happening. It just doesn't make sense to me, yet. A 29* feed, but tool set straight to the work?:confused: I'll do some more research and see if I can find some pictures showing me what is happening there.

I was going to do some scrap for practice, but my only scrap is more expensive than the plastic, unless I go to wood.

Thank you.

Pops

Don Young
08-25-2011, 10:27 PM
The 29 degree infeed just moves the tool to the left at the same time it moves inward. That makes the left side of the tool do nearly all of the cutting. The right side of the tool is practically just moving along the right side of the Vee, doing very little cutting.

Note that when you have it set up properly the compound feed is pointing almost squarely toward the left side of the Vee. The normal threading tool is still set at 90 degrees to the work but it is being fed almost in line with the right side of its Vee shaped tip. It is mainly fed squarely into the left side of the thread.

Try cutting a very coarse thread into wood and it will be easier to see and understand. You should see the chips peeling off of the left side only.

armedandsafe
08-25-2011, 10:55 PM
For one I think you're having too much unsupported length, even though you're using a tailstock center. It will probably be faster to do them one at a time, then part off. If you can find a nut that matches that thread, you can chuck the nut once all the plugs are threaded and parted off. Then run each plug into the nut to face the rough end. The nut would need a stopper in it at the right depth- you could cross drill it and press in a pin to be the stopper.

Plastic has a lot of give, and that doesn't work well with threading. You may have other issues- dull threading tool, improper relief and helix angles- not to mention play throughout the entire tool positioning exercise, which means crosslide, compound, carriage ways, plus the threading leadscrew slop- all in addition to possible errors in using the threading dial. I've never been fond of the dial. My preference is to stop the lathe, back the tool out, run it in reverse, then bring it in again to start the next cutting pass. Leave the threading lever engaged all the while. This at least eliminates errors with the half nuts catching the leadscrew on the proper thread.

I definitely have the nuts for this, as that is what the plug is for: to fill the nut. The "plug" will actually be used to create an alignment guide for the floating die cutter, to align it with the cutting plate as the die is closed. After I get it installed in the nut, I will use the cutting plate as a guide to drill the hole through the plastic plug, and then counter sink the open end of that hole to guide the cutter into the hole of the plate.

The threading tool is brand new and has never touched metal, so I think it is sharp. It feels sharp to my fingers. The angles I assume to be right, although they seem strange to me, with one side being much speeper angle than the other. This is the tool sent me by littlemachineshop.

I like the idea of leaving the half nuts engaged. If I turn the motor direction to reverse, the forward/reverse gear lever promptly goes to neutral. The forward/reverse lever at the back of the machine is REALLY stiff, but I guess that is what I need to use.

I notice that threading is done from right to left, driving the cutting tool into the head stock. That really makes me nervous. Is it acceptable to cut from left to right, if I have the tool properly ground? (I know, I shouldn't introduce a new question into an existing thread, but ... ;) )

Pops

J. Randall
08-25-2011, 10:56 PM
Two major problems:



Another annoyance is that I haven't learned how to get the thread indicator to rotate under power in both directions. It will rotate going one direction under power, but doesn't in the other direction. Tiny changes in the engagement will change which direction that it will indicate. It will rotate under hand crank in either direction.

Keep in mind that I am just getting into this skill after 50 years of talking to electrons. (Use small words. :D )

Pops

Just so we are clear on that last statement, when your carriage is stationary and your leadscrew is rotating your thread dial should be turning, when you lock in the halfnuts and your carriage starts to move your thread dial should freeze on the line you engaged on and not move until the halfnuts are disengaged. That may be what you are doing, but it sounds like you are expecting the dial to turn while the carriage is under power.
James

armedandsafe
08-25-2011, 11:14 PM
Just so we are clear on that last statement, when your carriage is stationary and your leadscrew is rotating your thread dial should be turning, when you lock in the halfnuts and your carriage starts to move your thread dial should freeze on the line you engaged on and not move until the halfnuts are disengaged. That may be what you are doing, but it sounds like you are expecting the dial to turn while the carriage is under power.
James

That explains that one. DUH! I am learning. Just slowly, it seems. ;)

Pops

Scottike
08-25-2011, 11:27 PM
threading from right to left gives you right hand threads (normal)
righty tighty - lefty loosey
threading from left to right will give you a left hand thread
lefty tighty - righty loosey
edit: you can get around that by cutting your threads on the backside of your work, but I don't think you want to go there .. yet.

armedandsafe
08-25-2011, 11:37 PM
threading from right to left gives you right hand threads (normal)
righty tighty - lefty loosey
threading from left to right will give you a left hand thread
lefty tighty - righty loosey
edit: you can get around that by cutting your threads on the backside of your work, but I don't think you want to go there .. yet.

Got it. You are right, I DON'T want to go there. I'm backward enough as it is. :D

Pops

rohart
08-26-2011, 05:10 AM
Once you're OK making a thread, the alternative to the 29 degree method for large threads is to set the compound parallel to the bed as normal, and hit the thread on the right and left of the groove separately, and only go centrel when you've taken some meat out of the groove.

I know it sounds like flying by the seat of your pants, but it gets lots of metal/plastic out of the thread before you have to get accurate. The thread doesn't care how you started - just how you finish. If the last few passes feeding in straight cut on both sides of the tool, job done.