View Full Version : thread chasing dial

Alistair Hosie
11-09-2004, 05:23 PM
I am currently reading about the thread chasing dial many books say it is permissable to keep the halfnut engaged and reverse the lathe.
Question ????
When doing this is it important to stop the lathe in between forward and reverse .I ask this as many operations can only be accomplished when the lathe is stopped putting the lathe into backgear etc what can be done safely with the lathe running or must it be switched off for all operations like this.
Needless to say I don't want to damage anything, as I did damage a lathe before when I changed it into backgear when it was running,and this has made me very wary! Alistair

11-09-2004, 05:59 PM
I've wondered about that too. On my lathe it recommends doing that with certain 1/2 pitch threads. I think I'll stay away from half pitches. Most threading i've done is up against a shoulder so the reversing thing sounds hairy... I'll stick to pulling me nut http://bbs.homeshopmachinist.net//eek.gif

11-09-2004, 06:03 PM
You'll need to reverse the motor to accomplish what you're trying to do, not change the gearing. Otherwise, you'll lose the relationship in the gearing that you're trying to maintain.

So...yes, you do need to stop the lathe. Hoffman brings up a good point though; unless you have an instant-reverse motor, there is likely to be a certain amount of coasting to a stop, so you'll need to switch off the motor "enough" in advance to allow for that, if you're going to a shoulder, and maybe do the last half-turn or turn by hand, if need be.

Even if you could get the result you want by changing the gearing, you'd still want to stop the lathe while you changed it.

[This message has been edited by SGW (edited 11-09-2004).]

Alistair Hosie
11-09-2004, 07:04 PM
They as I understand it don't actually recommend changing the gearing but just leave as is and reverse, then start without disengaging the half nut then it should pick up as it has never been altered the thread as it did on the first run.
So when the thread has been cut to the point you want to take it you do nothing but reverse it and then run again with a deeper cut.
That is my understanding perhaps I am wrong but can yo do this without switching the lathe off? or have I got it all wrong?Alistair

11-09-2004, 07:55 PM
As SGW says; when you are at the end of the thread, switch the motor to off. When the lathe stops, back out the cross slide a bit, engage reverse until you are passed the start of the threads and switch the motor off again. When all is stopped, return the cross slide to its original position, advance the compound a few thousandths and switch the machine back on in the forward direction. Don't touch any levers or disengage the halfnut at any time. As a general rule Alistair, never switch any gears or pull any levers while the machine is running. The obvious exception is engaging the halfnut when threading with the threading dial.

It is much easier and faster to just use the threading dial (when possible), then the lathe never has to be switched on or off during the entire threading operation. Only the halfnut get engaged at the beginning of the cut. At the end of the thread you disengage the halfnut, pull out the cross slide, then manually wind the carriage back to the start of the thread, return the cross slide to it’s original position, advance the compound a few thousandths, and start all over again until the thread is to full depth, keeping the motor running and in forward the entire time. Easy as pie!

Alistair Hosie
11-09-2004, 09:01 PM
Joel Pal don't you have to watch the thread dial come to a certain point before you engage the carriage and is this not dependant on which thread for which starting point as far as I knew you had to be careful only to employ this at certain times during it's circular travel.?
Also I agree it is better to not do anything with the motor running but my lathe has two things on the front of the apron a lever pulls in and out to engage the "crosslide" or "carriage" movement and a knob decides whether it travels sideways or accross the bed these can be used safely while the motor is running so far I feel fine with that should I be more anxious about these two things ?Alistair

[This message has been edited by Alistair Hosie (edited 11-09-2004).]

11-09-2004, 10:06 PM
If I remember correctly you can engage any mark on even threads and only odd numbers on odd threads. I still used to stick with only evens while threading even threads. Nothing like seeing that tool rip right down the middle of some almost done threads should you engage on the wrong mark. Man, Iwanna cut some threads...

Hoffman in Warner Robins Ga

11-09-2004, 11:16 PM
Alistair,on the lathe you have you should be able to drop the halfnut out and throw it back it on every thread pitch listed on the machine that isn't special or requires a special setup.The only thing that should require reversing the spindle would be special threads that require changing a transposing or stud gear(s)such as module or withworth pipe like 11-1/2.
Lathe mfgs could not design a lathe that would both have a quickchange box and still cut every thread imaginable.So as a happy medium they came up with a set ratio to provide for the most commonly used pitches.

If your thread dial has just numbers,you can use any number on the dial,if it has hash marks you can use any hash mark,if it has BOTH numbers and hash marks then you can use either numbers or hash,but not both on the same thread unless you are cutting a double start thread.
Reason for this is that on a change gear lathe the rules while mostly the same for a quickchange are slightly different.The change gear machine is based to operate on a ratio between the spindle and the leadscrew that you set when selecting gears to cut a particular thread.The quickchange operates on a set ratio between the spindle and the leadscrew and that ratio is multiplied for each thread to be cut,the term for this is geometric progression.
If your lathe is setup to cut 2,4,5,6,8,9,10,11,12,13 etc then you can use any number os has mark on the dial.If it will cut an additional number of pitches like 11-1/2,4-1/2,27 and so on with you having to change a stud gear or similar then you may find that on some of those threads you will have to leave the halfnut closed and reverse the spindle.

11-10-2004, 01:44 AM
If you are using the method you described ie stopping at the end of the cut and reversing etc,you can engage the halfnut at any time you like because you will not be disengaging until you have finished the thread.On some lathes I have used you can change speeds without upsetting the relationship between the headstock and the leadscrew,to find out just put the lathe gearbox in neutral with the leadscrew engaged and give the chuck a spin, if the leadscrew turns you can freely change gears while screwcutting.

Paul Alciatore
11-10-2004, 02:11 AM

From your posts, I wonder how your lathe reverses. Do you have a reversable motor with some kind of Forward/Reverse switch or does it have a mechanical reverse mechanism.

If it is an electric reverse switch, then you likely have no choice but to stop the motor between forward and reverse. My drum switch has the Off position in the middle.

If you have some kind of mechanical reversing arrangement, like the tumbler reverse for the change gears, then I would definitely recommend that you stop the motor and let it come to a complete stop before reversing it. Most lathe gears are just simple gears and not designed for meshing while turning like an automotive transmission. Especially not under load! Any engagement of such gears should be made with the motor stopped.

Another thought; you aren't confusing the tumbler reverse for the lead screw with reversing the lathe? The tumbler reverse should never be disingaged after you start cutting a thread. If it is, it will be very difficult to resynchronize the cut. The relationship of the lead screw to the spindle must be preserved while cutting threads. The tumbler reverse is not a way of reversing the lathe; it only reverses the leadscrew.

When cutting a standard thread (on the quick change chart) on a lathe with a quick change gearbox, the sequence would be:

1. For the first cut, engage half nut on any number and remember that number.

2. Disengage the half nut at the end of the first pass.

3. Crank the carriage back to the right, to the starting position.

4. Advance the in feed by whatever method you are using.

5. Engage the half nut at the same number or according to the rules below

6. Repeat steps 2 through 5 until the thread is cut to full depth.

Rules for engaging half nut on most (but not necessairly all) lathes with an 8 TPI lead screw when cutting English (inch) threads:

For even numbered threads (4, 8, 10, 32, 48 TPI etc.), engage at any mark.

For odd numbered threads (5, 9, 15, 27 TPI etc.), engage at any numbered mark (1, 2, 3, or 4).

For 1/2 threads (like 7.5 TPI) engage on even numbers only (or odd numbers only if first pass was on an odd number).

For 1/4 threads (like 3.75 TPI) engage on the original mark/number (from step 1) only.

I hope this helps.

Paul A.

11-10-2004, 02:43 AM

Paul left out one step between 2 and 3. You have to back the cutting tool out with the carrage before you move back to the start of the thread. When you get back to the start position you feed the cross slide in to the original setting which is usually 0 if you set it up at the start of the threading process. If you don't you will run the cutting tool through the threads you just cut.

Added: Even if you thread by leaving the halfnut engaged you want to back the tool out of the threads using the cross slide. This will prevent damage to the thread because of play in the half nut and lead screw.

Don't change any gears that have to do with speed and ratios with the motor running. The only thing that you can do with the motor running is to start and stop feeds like halfnut for threading, carrage travel and crossfeed. If you have a rapid traverse that can also be operated with the motor running. There are a few exceptions to this but until you know your lathe don't try anything else.

Hope this helps.



[This message has been edited by WJHartson (edited 11-10-2004).]

11-10-2004, 01:12 PM
Alistair, sounds like you're going to start practicing your threading, that's great! Just for practise, start with a thread that is the same as, or a multiple of your leadscrew pitch. i.e. 8 pitch leadscrew, cut 8, 16, 24, 32 etc. The halfnuts can be engaged at any point on the dial as long as the thread is the same as or a multiple of the lead screw. When you get more comfortable with the threading process, then use any mark on the dial for even pitches, any odd number for odd pitches. To cut metric threads you do as in your orginal question, make a threading pass, retract the tool(cross slide) and stop the motor, reverse the motor to take the carriage back to the start of the thread, 0 the cross slide, advance the tool with the compound, and restart the motor in the forward direction to take the next pass. This can be done with any thread, but learning to use the threading dial will be much more rewarding and valuable down the road.

Paul Alciatore
11-10-2004, 02:27 PM
Sorry about the missing step. I was answering late at night. Yes, you must back out of the thread before moving the carriage back to the starting position. That's step 2.5.

One more thing I found is that you always reach full depth about two or three passes faster than you figured you would. When you have cut the thread to about half depth, measure or gauge it after every pass.

Ain't threading fun. Personally, I love it.

Paul A.

11-10-2004, 02:39 PM
Alistair, among those Christmas presents your wife ran off with is a book you urgently need to read right now. It is Machine Shop Practice Volume 1. Beginning on page 341 (1981 copyright issue) is one of the best explanations/directions I have ever read for cutting threads on a lathe. Beginning on page 365 Molthrecht gives step-by-step instructions, including stopping the spindle - not the motor - in order to withdraw your cutting tool, and going on from there. Beg your wife to let you have at least this "present" before Christmas and then, with this book at your side, go to work cutting threads on your excellent lathe. You'll love it so much you'll find yourself cutting threads just for the fun of it. (One of my absolute favorite things to do on my Myford.)

Alistair Hosie
11-10-2004, 02:51 PM
I am going to try it before the weekend is up possibly tomorrow if I get a chance.
Thanks guys You are true brothers to me.
I speak from the heart when,I first started to build a workshop it was a real uphill struggle, and I was demoralized many times but you guys always gave me the strength and will to keep going.
Without your frienship heaven knows where I would be and of course my dear wife whom I love very much she has been there for me all the way god bless you all.Alistair

11-10-2004, 03:11 PM
I believe that Hardinge allows you to reverse the machine so that you only need to back out the cross slide then feed back in for the next pass.

Using a VFD on my lathe, I thread by running it forward and reverse (with a 1 second stop time) without disengaging the nuts nor using a thread dial. It works great but I won't be threading to a shoulder any time soon with this arrangement. Most of what I do can handle a groove to stop in.

added - every lathe will require backing out the cross feed when you reverse, otherwise backlash will damage the thread you're cutting and maybe the tool as well.

[This message has been edited by nheng (edited 11-10-2004).]

11-10-2004, 03:47 PM
Make one of these Alistair.
shown fitted to the same model as yours.
lets you take your time and study what's going on.


all the best...mark

Allan Waterfall
11-10-2004, 05:20 PM

Why don't you get "Screw Cutting In The Lathe" by Martin Cleave. It will more than likely answer any questions you might have.
It's one of the Nexus Workshop Practice Series.

You might even get it north of the wall. http://bbs.homeshopmachinist.net//biggrin.gif


Alistair Hosie
11-10-2004, 05:23 PM
Guero thanks to you I got vol 1 early and am reading it as we speak will be in touch thanks for the info pal.Alistair ps looks very good thanks again

11-10-2004, 06:41 PM
<font face="Verdana, Arial" size="2">Originally posted by Allan Waterfall:

Why don't you get "Screw Cutting In The Lathe" by Martin Cleave. It will more than likely answer any questions you might have.
It's one of the Nexus Workshop Practice Series.

You might even get it north of the wall. http://bbs.homeshopmachinist.net//biggrin.gif


I'll second that, it's a great text on the subject of threading in the lathe.

Paul Alciatore
11-11-2004, 01:04 PM
"Screw Cutting in the Lathe" is great and I am on the last few pages right now. I also highly recommend it.

However, I would suggest a simpler explanation for the rank beginner. Read this book only after cutting at least a few standard threads by the easiest technique possible. You will understand and appreciate it a lot more.

Paul A.