Threading problems

**Mcgyver** · 07-14-2006, 07:35 AM

. deleted first attempt, misread thought it was an internal thread you were attempting

I don't know the machine, but here's a couple of ideas. The problem seems strange, so I'm stretching a bit, but maybe it triggers something.

Is there anyway the stock is moving, ie getting pushed into the chuck? for your problem, there has to be some disconnect between the spindle and screw, or, the relative position longitudinally has changed.

how deep a cut are you taking? many lathes have a safety clutch on the screw - spring loaded or such that the lathe won't tear itself apart it the carriage is up against something, maybe your cut is hefty enough that its skipping a beat? (yeah, I know, grasping at straws)

maybe the tread dial is not working or engaging properly. Try an extremely light first cut, at the end of the cut, don't disengage, power off. retract the tool and run the lathe in reverse to get the carriage back to the start of the cut. If its now tracking properly, there must be something wrong with the tread dial. This is a way to thread in a lathe without an indicator btw

that's all i could come up with

**J Tiers** · 07-14-2006, 07:50 AM

Some of the asian machines have been supplied with wrong gears. That wouldn't directly do what you say, unless the dial gear is wrong.

Count the leadscrew turns per unit (turns per inch for you?) and then count the teeth on the dial gear. The gear should have a multiple of the turns per inch, such as 16 or 32 teeth for an 8 tpi leadscrew. Otherwise it won't work.

Dial gear loose?

Are you disengaging the leadscrew drive from teh spindle at any time? Don't.

Aside from that, not taking up the "lost motion" before the cut, not having the cutter cut on the side that the carriage is moving toward.

If the compound dial is tilted to your right, the motion should be to your left.

I would use 29 deg for a 60 deg thread, but that isn't your problem.

If the cutting force can push the carriage, you will get a drunken thread, and maybe what you say.

**fraker** · 07-14-2006, 08:14 AM

RE: threading problems

Mcgyver & J Tiers
Thanks for those suggestions.

I will check those things and try those ideas.
My lathe doesn't have a safety cluth on the leadscrew, so I know it isn't that.

The cuts I am taking are very light,0.02" So I am pretty certain the stock isn't moving in the chuck.

I have the compund slide set to the right ( \ ) and am cutting right to left (towards the chuck.)

As for the rest. Well, I shall try reversing the lathe and see what happens after I check the gears and dial indicator.

I'll get back to you and let you know how I get on.
Thanks again guys.
Bill.

**SGW** · 07-14-2006, 08:32 AM

As a way of eliminating some things...does the lathe reverse?

If so, try making a pass but not disengaging the halfnuts at the end; withdraw the tool, run it in reverse back to the start, then feed in the tool for the next pass.

If this works correctly you'll know that the problem is in the way you're disengaging/engaging the halfnuts.

**ljchipmaker** · 07-14-2006, 08:43 AM

On my chinese machine it is critical that I engage the half-nut at exactly the same position everytime if I have dis-engaged the half-nut after a pass. Exactly the same number or line, and at exactly the same position; if not the threading tool will not track the same on every pass. I realized that even the slightest change in engagement position would throw it all off. I had assumed the the half-nut was engaging at the same point, but after closer inspection realized that the engagement positions could vary slightly, which would cause the tool to not track properly, even though it appeared as though it had engaged on the same number or line.

So I went to the reversing method that has been recommended by the others to assure the same spindle/carriage timing.

Also, for threading .020" infeed on the compound is too much. You can start out taking .005-.010 on the first couple of passes only, but you should be finishing with infeeds of .001-.0005", with some intermediate passes and the last pass with no in-feed. With an in-feed of .020" your drive belts might also be slipping.

LJ

**jimmstruk** · 07-14-2006, 09:00 AM

Just to make sure all is right with the lathe and the change gears, make your initial light cut, then check the first cut with thread guage to insure allis OK .JIM

**fraker** · 07-14-2006, 09:04 AM

RE: threading problems.

Okay. Back again.
Just been out to the shed and checked.
The leadscrew has 16 T.P.I and the indicator has 16 teeth.
I guess the problem isn't there.
The Indicator dial wasn't loose either.

I WAS disengaging the leadscrew at the end of the cut, then winding the carriage back to the start, then reengaging the halfnuts as the same number came round on the dial.
This COULD have been where I was going wrong. 'Cause I just tried threading by stopping the machine, withdrawing the tool and reversing the lathe back to the start of the thread. Result?

A near perfect thread.

So was I doing it wrong, or has my lathe got a problem?

Thanks again guys.

Bill

**ulav8r** · 07-14-2006, 09:09 AM

Apparently your half nuts are not enganging properly each time. Disassembly and deburr might be in order.

**Benta** · 07-14-2006, 09:46 AM

Could it be that they delivered a machine with a metric leadscrew?

Benta.

**J Tiers** · 07-14-2006, 12:36 PM

If your leadscrew is actually a 1.6mm pitch, it could be close enough to look like 16 tpi (it would be 15.875 tpi), yet it would always be wrong if you ever disengage half nuts.

A very careful measurement may be in order.

It is by no means unknown for our chinese "friends" to use something "close" instead of the right item.

The next question is whether it actually cuts a correct thread at the desired TPI. If the leadscrew is not correct, odds are that it will cut a thread "close to" but not exactly "on" the TPI you expect.

I'd try cutting a couple different TPI, with at least one prime number thread pitch.... like your 16 tpi and also 13 tpi.

That should smoke out a gear train that compensates for an off-TPI leadscrew, since I think it is essentially impossible for an even metric pitch to be compensated correctly at all common imperial TPI.

**Mcgyver** · 07-14-2006, 01:29 PM

you're not doing it wrong, there is a problem of some sort. The point of the indicator is that you can disengage the feed, move the carriage back (without reversing the lathe) and then through the indicator, re-engage the leadscrew in exactly the position (relative to the spindle)

the 'reverse without disengaging' idea was to isolate the problem and convince yourself you weren't going insane,

but it's not how you want to thread if you've the luxury of a thread dial.

I'm trying to think this through (its a challenge) but if the leadscrew was close but not dead on say 16tpi, then the pitch of the worm on the thread dial indicator is going to be close, but not dead on. therefore, the indicator says the screws made 16/16th's of revolution, time to engage....but in fact its only made 15.75's (or whatever) parts of rev - this make any sense? hmmmm

**capperbar** · 07-14-2006, 04:15 PM

Check the gear in the threading dial. My Enco 13x40 used came with the wrong gear. Drove me nuts, I could cut threads with w/o disengaging the halfnut but never with the threading dial. Check your manual for the correct tooth count. Sav

**Millman** · 07-14-2006, 04:26 PM

Carbide?

A very sharp and stoned HSS tool is all that is required for that size thread. Why do you need the carbide? Why is the compound set at 27*? Should always be set at 29.5* There is also the top of the tool, try a 2* angle on the leading edge.

**Peter N** · 07-14-2006, 04:31 PM

[QUOTE=Millman] Why is the compound set at 27*? Should always be set at 29.5* [QUOTE]

He's cutting Whitworth threads, which are 55 deg included angle.

Peter

Threading problems

Threading problems

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment