View Full Version : Threading problems

07-14-2006, 07:12 AM
Hi again guys.
I have a problem I hope you can help me out with.
Here goes:
I have a Clarke CL300M 7" x 12" mini lathe.

(Its the same as the Grizzly 7" x 12" If that helps those on the other side of the pond.) :)

The problem I have, is that when trying to cut an external thread, I end up with what can only be described as an unholy mess.

What seems to be happening is, that the cutter cuts the first spiral, then on the second pass, cuts another, and so on, thus wrecking the piece.
Sometimes the tool seems to track correctly, other times not.

I am trying to cut a 3/4 x 16 thread for my woodworking lathe.
Using a single point 55 degree brazed carbide tip tool.

I have the compound slide set at 27.5 degrees ( this way. \ ) I have the tool square to the workpiece and centred. The cross slide set to 0 as prescribed in the archives, using the compund slide to advance the tool, and am setting the carriage moving at the same point on the threading dial every time.
I have even tried stopping the lathe and making sure I catch the dial indicator just right, all to no avail.
I have checked that there is little / no slop in the gears, leadscrew, halfnuts and all gib strips and now have run out of ideas.
I hope all this makes sense.

Can anyone point me in the right direction of what I may be doing wrong / not doing? :confused:

Thanks in advance.

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.

07-14-2006, 08:14 AM
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.

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.

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.


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

07-14-2006, 09:04 AM
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.


07-14-2006, 09:09 AM
Apparently your half nuts are not enganging properly each time. Disassembly and deburr might be in order.

07-14-2006, 09:46 AM
Could it be that they delivered a machine with a metric leadscrew?


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.

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, :D 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

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

07-14-2006, 04:26 PM
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.


07-14-2006, 04:32 PM
Must have missed the Whitworth part?

07-14-2006, 04:34 PM
I've experienced similar things, and the culprit in my case turned out to be the half-nut engagement. I've had a couple mini lathes, and, as delivered, their feed levers both felt a bit 'springy' to me. I found that if you are just a tiny bit late engaging the nuts the lever will lock down but won't really fully engage the leadscrew at first. As the leadscrew turns the inertia of the saddle retards its movement until the threads of the halfnut do align with those of the leadscrew, at which point the thread dial is actually halfway between one number and the next, and you're on your way to a cross-threaded workpiece.

Adjusting the engagement of the half-nuts fixed it for me.


07-14-2006, 04:40 PM
My Chinese lathe came without a threading dial and when I checked with the supplier I found that it was not supplied because the lathe can make both inch and metric threads and the threading dial will work on one but not the other--ie; if you have an inch lead screw and try to cut metric threads the dial will not work and vice versa if you have a metric lead screw and try to cut inch threads the dial will not work. I see by one tid bit you dropped that your lead screw is inch so is there any chance you are trying to cut a metric thread? If so you will get the results you describe.

Later Edit
On rereading you post I see you were cutting an inch thread so I would look very closely at the lead screw. One test would be to try cutting a metric thread and see if the dial magically starts to work


Peter N
07-14-2006, 04:54 PM
I don't think you missed it Millman, it was just a deduction. He's over this side of the Pond and talking about an imperial spec lathe rather than metric, which probably means he's cutting Whitworth.


07-14-2006, 05:18 PM
No Peter, I missed that fact. Hey, have I ever told you that I don't especially care for Metrics?? OR CNC?? Thanks for correcting me.

07-14-2006, 10:05 PM

Even if it is a metric leadscrew the thread dial may not work correctly unless it has more than one gear to engage the leadscrew. Metric thread dials usually have several gears, often four, and all these gears are needed to cut the full metric range. Each gear will only be accurate for one batch of thread pitches, and the correct gear for the pitch being cut must be set to engage the leadscrew.


J Tiers
07-14-2006, 10:57 PM
Franco.... you must be joking........

Metric is ALWAYS better and SIMPLER, and here you are mucking it up..... making Metric harder....

It would help to know if that particular machine right there is actually made to be imperial or metric.... for certain...... And that it isn't some odd hybrid, like the 4mm leadscrews of the 80s or 90s....

The 16 tpi screw and 16 tooth gear struck me as odd... but then that is how the "109" or "AA" lathes were made also....

07-14-2006, 11:48 PM
J T,

QUOTE: "Metric is ALWAYS better and SIMPLER, and here you are mucking it up..... making Metric harder...."

Yeah - it would have made life simpler if they'd used threads per cm. as the basis of the thread system, instead of the reciprocal of threads per mm (pitch), which involves lots of nasty fractions.


J Tiers
07-14-2006, 11:54 PM
Ironic that those nasty fractions "required" (not) in imperial are why metric is better..... isn't it?

I still can't quite believe the lead screw is actually a whole number tpi..... it has to be off, or the 16 tooth gear would work.....

The whole deal is that the dial works in terms of the "per inch", which repeats every inch....

Worse yet is the fact that he is cutting a 16 tpi screw, which means the LS simply needs to turn at the same rate as the spindle......

He could close halfnuts ANYWHERE THEY WILL CLOSE and it should follow the thread fine...... its an exact ratio with the LS tpi.... "one" in this case.

The fact that he has ANY trouble with it given what he has and what he is cutting inducates that there is a fundamental problem with the machine, probably the LS is NOT imperial, ain which case he most likely is NOT actually cutting 16 tpi either.

kap pullen
07-15-2006, 12:45 AM
Those brazed tools don't work well for threading.

They don't have the side clearance required.
You need to clear the lead of the thread plus a couple of degrees.

There isn't any top rake either. On 1020 or similar low grade material,
you get a chewed mess like the METALMITES been chewing on it.

There could be any number of problems here, some mentioned, some not.
Beginners get into lots of trouble, like newbees on a computer.

Old hands have some problems too.


You need to go back in your cave. Professionals only use cnc today!

I read that on some other discussion board.

If you ain't a button pusher, you ain't notting these days.

And you do what? Work with your hands! Ughhhhh!


metal mite
07-15-2006, 12:51 AM
There isn't any top rake either. On 1020 or similar low grade material,
you get a chewed mess like the METALMITES been chewing on it.


Kap Pullen,



07-15-2006, 01:09 AM
Bill, try setting your compound at 55 degrees. (double the degrees you want) I had to do this on my griz machine and it worked. Seems the Gooks don't know which way, why, or where things start from, or where angles or degrees are on their machines. I don't turn many threads but when I do, I use an adapter that inserts into the spindle and I turn by hand. Might not work for the pro's, but it works for me. just a thought

John Stevenson
07-15-2006, 05:05 AM
Forget the setover angle until you have sorted your problem out.
Just use the cross slide to plunge straight in.
That's one bit of error out the way.

Has anyone mentioned checking the drive train for the right setup ?
May have an odd gear in there.

First thing is to determine if it's imperial or metric setup, can't do ANYTHING until that's done.

Consult the book or chart with the lathe , measure the leadscrew over a long length, not just one inch as a small error is hard to see.
You say it's 16 tpi so is it still on the mark over say 6".

If this checks out post the screwcutting chart.

Where are you in the UK ?


07-15-2006, 12:22 PM
Hi guys.
Thanks for all the ideas and suggestions.
Seems like I shall have to go over the lathe with a fine tooth comb. No problem.

Leave it with me for a day or two and I shall report back on what I find.

John, for your information. I am in Dagenham Essex.

Thanks guys.

07-17-2006, 07:50 AM
Okay. Back into the fray once again.
Hope you all had a good weekend without TOO many serious incidents.

(Gotta feel for Alistair though)

Okay, so. On further inspection, and having tried cutting a longer thread than before.
(Earlier attempts were only about 1/2'' long)

It would seem that I have a Mickey Mouse leadscrew.
My thread gauge fits, seemingly okay, on my earlier, shorter attempts.
However, on cutting a thread a couple of inches long, the error makes itself perfectly clear.
I measured the leadscrew threads over six inches, and while it SEEMS to be right, I guess it must be out just enough to be a real PITA.
So, I suppose now, I shall have to swap out that leadscrew and try my metric leadscrew. See if that's as bad.

I feel it only fair to say. I feel I have wasted a lot of you guys valuable time. If I had cut a proper thread to start with, I suspect all this would have become clear an awful lot earlier.

May I take this opportunity to apologise to those who invested so much effort and thought into trying to help out a newbie.

Once again, I thank you for all your suggestions and help. Much appreciated.


P.S It seems my leadscrew is somewhere around 16T.P.I But not exactly.

John Stevenson
07-17-2006, 08:02 AM
Measuring screw threads to a newbie is hard as they often count one too may.

You may know this and if so I don't want to insult you but coming from Essex you must have a thick skin :D

Place the end of your ruler on the start of a thread and at the say 4" mark the next thread should be just starting.
It's not the start of one to the end of one, it's start to start.
Count how many threads you have and divide by 4 that's the tpi.

If over 4" you don't have one starting and one ending on the marks then it maybe you have the metric screw.

In which case turn the rule over and measure over 100mm.
You have to have one thread starting and one finishing on the marks. Metric is measured in pitch ie distance to go one thread like 2mm or 3mm and not so many in so long.

Best to check both screws and post your findings.


07-17-2006, 08:20 AM
As an addition, and as John Stevenson asked.
Here is a copy of the gearing list if its any help.





07-17-2006, 08:53 AM
Hi John.
I NEVER take offence with anyone who is trying to help me out of a fix.:D
Right. Having checked both leadscrews. The metric one seems fine at 1.5mm pitch.
The imperial one. Well, i'm not sure if I am doing it wrong. I followed your instructions, and came up with.
16.25 T.P.I. Although, my 16 T.P.I thread gauge fits just fine.

What do you think?


Peter N
07-17-2006, 09:35 AM
I wouldn't worry about wasting anyones time on here, it gives us a chance to try and prove how clever we are :D (or not...)

There was a similar problem that came up on a UK newsgroup some time ago, exact same symptoms as yours, but this was on a brand new £7K Myford. Turned out that Myford had fitted a spindle gear/tumbler gear (basically one of the gears that you don't ever change...) with the wrong number of teeth on it, so none of the 'correct' listed ratios would ever work for screwcutting.

It was only discovered when one of the NG members went down there with an assortment of change gears to try and sort the problem. Myford were too busy as I recall reading.

So - might seem simple but have you actually tried counting the number of teeth on the A & D gears you're using for the drive? It could be that this may have been incorrectly marked in manufacture. Ignore the B gear as this is just an idler and the number of teeth won't matter.

Other than that it's worth listening to Sir John's advice IMO, he has a way of finding simple solutions to many problems.

And welcome to the board btw.


07-17-2006, 09:37 AM
The thread gauge isn't long enough to show a small error. Measure over the longest distance you can with an inch rule. The thread crests should fall on the 1/16 marks all the way along.

John Stevenson
07-17-2006, 09:56 AM
OK so from Page three it's obvious that the leadscrew SHOULD be 16 tpi as this one is the only equal ratio, 40 driving 40.

Does what Evan says, start of tooth every 1/16" work.
If so then it's down to the gear train if not you have a duff leadscrew

Hang on 1.5 mm pitch is 16.9 tpi.
Have you got two metric screws ?

07-17-2006, 10:24 AM
Hi again John.
Thanks for your advice and help.
And of course to all the others who have offered help.:)

I have both metric and imperial leadscrews and they are definitely different.
I have compared them alongside each other.
I will have to check my imperial one again.

07-17-2006, 11:35 AM
Hi again guys.

Now, here's something you don't often hear...

I was doing it wrong all along...
In my haste to make chips. I (AHEM!) Kinda mis-read the manual.
I had a COMPOUND (I.E 2 gears) in the idler position, instead of just one.

BOY! is MY face red now.:o
I never for one second thought about the difference that would make. D'oh!!!

Guys, thank you, EVER so much for all your help. I just tried it and it cut a BEAUTIFUL 16 T.P.I thread.

Just goes to show, manuals CAN be useful after all.

Retiring quietly into the background with a VERY red face and a hung head.

07-17-2006, 11:40 AM
'When all else fails, read the instructions!' I've been there and feel your pain!

07-17-2006, 11:43 AM
HEY Guys,
change of plan. I WILLINGLY stand in the centre of the room and await your well deserved ridicule. Heh,heh,heh:D

Plus, if we ever meet, drinks are on me okay?

07-17-2006, 11:44 AM
Yesterday I was setting up a pair of wireless routers in repeater mode. I spent 1/2 hour trying to configure them with no luck. :confused: I finally gave up and read the instructions. They work fine.

John Stevenson
07-17-2006, 11:48 AM
What can you say to an Essex lad ?

Wota Plonker.............