View Full Version : Lathe half nut

02-01-2010, 06:42 AM
Isent the lathe half nut/threading dial a rather inconvient system?

a 1 dog (or some kinda of keyed/indexable clutch) system connected directly to the spindle would work much better.

IE: just allow the spindle to rotate entire revolutions and only clutch it to the lead screw when it aligns up to whatever point it was unclutched from.

If im thinking right here, would'nt it allways produce correct engagement points even for metric threads on an imperial leadscrew?
I mean, if you rotate something 1 exact turn.. reguardless of its pitch (or indeed, number of threads), your right back where you started. Hence if you rotate any number of exact turns, your right back where you started..

The only downside I can see is you have to keep the leadscrew half nut engaged, and move the leadscrew to go back for a second pass...

But on the plus side, im pertty sure it should be possable to electronicaly (motor) rotate the leadscrew with the clutch disengaged, then reengage to the correct point anywhere.. since its 'indexed' to the rotation of the spindle, and not the leadscrew anymore. Or just reverse the lathe.. Plus side here is you don't have to *stop* the lathe to stop the thread feed on a dime, even with oddball threads, and you get an engagement point *every* revolution of the spindle, insted of having to wait for some thread dial to line up every 1/8~2"
(and having to swap gears in the thread dial if your doing metric!)

02-01-2010, 07:22 AM
You mean like a Hardinge HLV?

Or this conversion for a mini lathe?


02-01-2010, 07:30 AM
so why isent it seen more?

02-01-2010, 08:41 AM
Basically a dog clutch never knows how far along the shaft it is it may be in the right place to drive but numbers of revs out of sync.


02-01-2010, 09:32 AM
Isn't that what the half nuts do? Don't they engage the lead screw at the point you released them if you just look at the number on the thread dial? What is so hard about watching the number on the thread dial and engaging the halfnuts?

I don't thread over 120 rpm on my lathe and have no trouble catching the right number. If I were threading at 300 rpm or more for a coarse thread it may get interesting but you just start farther away from the thread so you can release the half nuts if needed. I usually thread at 75 rpm.

From the photo's in the posts here some of the thread dials have to many numbers on them. The older lathes many times only had four lines on the dial. Most have 8 and that is plenty because you can always catch the engagement between the marks if the lead screw will do that but a coarse lead screw won't.

If a dial has to many numbers then make a new dial with only 4 or 8 numbers and make it easy to use.

I don't know how a Hardinge HLV threading system works but it must be expensive and for that reason not used on other lathes and it may be patented by Hardinge.

I just don't see what is so hard about using the half nuts.

02-01-2010, 09:40 AM
Hardinge yes, and also Hendey.
No need for a thread dial with the single-dog clutch.
Sort of a nice kept secret from the newbees reserved
for the experienced machinist on real toolroom lathes.
Newbees gotta learn threading the hard way first.
That's just how it works! ;)
My 1917 Cataract toolroom lathe has it and
my 1951 Hendey tool and gaugemaker's lathe has it.
With these lathes, you can thread around 1000 rpm if you like.
Thread finish looks excellent.


02-01-2010, 09:45 AM
Newby's, hell, most the machinists I have known have never seen or used a Hardinge lathe and they are a LONG way from being a newby.

Are you refering to some elitist group or something related to Hardinge.

02-01-2010, 09:46 AM
I think it would be pretty cool to have the leadscrew on a stepper motor and not mechanically connected to the spindle. Computer would have to know the spindle angle and match carriage position to the thread.

I bet that's how a CNC lathe works.
If not, maybe we should build one. :cool:

02-01-2010, 09:50 AM
Hey, that's a good idea. Maybe you could use digital control motors and link them to a computer and design a program to do the machine work for you.

Now THAT is a novel idea and it may catch on. Should we call it computer controled machining, yes CCM works for me.

EDIT: Black_Moons, I'm just having a little fun with you, no harm intended but threading with a dial is not that hard.

02-01-2010, 09:54 AM
Just pushing your buttons, Carl.
It's easier than threading on a Hardinge. :)


Peter N
02-01-2010, 09:59 AM
Black_Moons, I'm just having a little fun with you, no harm intended but threading with a dial is not that hard.

I tried that once, and I couldn't get it to work at all.
So I gave up and put the toolbit back in, and that cut them just fine.

Peter :D

02-01-2010, 10:04 AM
I just don't see what is so hard about using the half nuts.

Ever cut a metric thread with an imperial lead screw, or vise versa?

Even if you've rigged up the right gear for the indicator, you might be waiting a long time for the marks to coincide.

02-01-2010, 10:13 AM
Yes, and you don't have to release the half nuts, in fact you can't release the half nuts but what has that to do with finding the right number on the dial.

Black_moons, that's a good one, I like that, it make me chuckle after I caught what you meant.

02-01-2010, 10:26 AM
Isn't that what the half nuts do? Don't they engage the lead screw at the point you released them if you just look at the number on the thread dial? What is so hard about watching the number on the thread dial and engaging the halfnuts?
I just don't see what is so hard about using the half nuts.

As the OP noted, there is usually an issue with the half-nuts when the leadscrew isn't the same language as the thread being cut, e.g. cutting a metric screw using an imperial leadscrew.

Plus, metric lathes generally need several little gears (interchangeable) for the threading dial to handle all the metric threads.

I added a dog clutch to my 7x12 and combined with a retracting tool holder it makes threading much faster and easier for both imperial and metric. I used to thread to a shoulder using a hand wheel, now I rely on the dog clutch and often thread at 300+ RPM. Higher speed seems to produce nicer threads on this little lathe.
See: http://www.gadgetbuilder.com/Dog_Clutch.html


02-01-2010, 10:30 AM
Black_Moons, I have to agree that it seems like in several hundred years they could come up with an easier way to engage the lead screw and all the manufacturers could use it.

When you look at the lathe we use now and the lathes they used in the 1800's there is little difference except in the head stock. That is belt drive versus gear drive while the rest of the lathe is much like the early ones.

The more complicated it becomes the higher the price and failures of the mechanism can increase because of complicated workings. Sometimes trade off's are necessary.

In trying make it easy the makers that put all the marks on the dial only make it worse and complicate things. It would be much easier if they just made a simple dial with four marks and put all the rest on a chart.

I guess no matter how you do it if your confused your confused and I must say I have been confused by the chart too. For that reason I mostly use the number 1 when there are several to choose from. On the other hand, if you using a lathe with a worn lead screw you best use the same number all the time because if you engage a number that is supposed to work for that thread the worn lead screw can put you in the wrong spot in the thread and spoil the thread. Been there, done that. I don't trust old worn lathes and that is what most shops I worked in had. I guess it's a habit I developed to save my butt from slopping up a thread.

02-01-2010, 10:42 AM
Electronic, or virtual lead screws exist and have replaced the half-nuts. CNC is not a requirement.

An example case study:

02-01-2010, 10:53 AM
Wouldn't it be easier to dog gear and have a reverse gear for the drive motor for the whole lathe? Assuming we are talking about single phase lathes here.

So leaving the half nuts engaged and all and just releasing the dog clutch for the whole lathe drive and than switching to reverse gear much like a tumbler and than engaging the clutch to reverse the lathe spindle and carrige back to the thread starting point again.

02-01-2010, 10:58 AM
GadgetBuilder, yes, reading the first post he is mostly talking about metric on an imperial lead screw or vice versa but to make a system to do that could be something subject to failure or to difficult to make for the standard lathe.

The following is my take on this and is not intended to be arrogant in any way so please don't take it that way.

It's not really that hard to leave the half nuts engaged and cut a thread. I have done it on lathes from 9" to 18" and all you have to do is pay attention to what your doing and know your lathe.

The problem is an comparing home shop to paying shop and the machinist doing the work. The average home machinist doesn't have the experience a working machinist does and many times he has to learn stuff on his own. Hand eye coordination and experience go hand in hand. A person who does machine work day in, day out has a better feel for threading. So, when the home machinist having little experience cutting threads, tries to cut a thread, imperial or metric, he has issues and until he cuts enough threads to become very comfortable he will continue to have issues. At some point he will master thread cutting if he persists.

If they put a mechanism on the lathe to make threading automatic it will increase the price of the lathe and expensive to repair. One of the essential parts of doing machine work is understanding the machines and mastering the methods to use the machine.

There is easy way to do the imperial/metric issue, at least easy in some ways. They could put two lead screws on a lathe and you engage the lead screw for metric or imperial as needed. Now, at face value that seems simple but to do it would require lots of gears and shift levers and complicated instructions. Guess what, we're back at something that will confuse an untrained machinist or any machinist for that matter.

Machine work is not easy and if it was everyone would be a machinist.

Why not just buy a CNC lathe?

02-01-2010, 11:08 AM
If your concerned with how long it takes to reverse the carriage when you can't release the half nuts then learn your lathe.

When I got my current lathe I tested it and found out I can change speeds without disengaging the spindle from the lead screw as long as I didn't use the high/low shift lever. This is something I did with all the lathes at work, that is I experimented to learn them.

Knowing that I could thread forward at 75 rpm, stop the chuck with the foot brake, back out the cutter, shift to 300 rpm and reverse the spindle back to the starting point and begin the next pass at 75 rpm.

With that knowledge I can also speed up or slow the spindle rpm if I want to while threading.

Isn't it nice to know exactly what your lathe can do that is NOT in the instruction manual?

02-01-2010, 11:11 AM
Yeah the half nut just took over other designs, but certainly many other systems have been around a long time.
I dig the threading system on my old Artisan which is probably 100 years old or so. It'll cut every thread my South Bend model A will and faster.
It has a dog clutch on the leadscrew and a cone clutch for the spindle drive.
Threading is done quite differently than on a typical half nut lathe by using the dog clutch on the .5" pitch leadscrew and the spindles cone clutch.
For threads divisible by 16, it's a no contest win in speed for the Artisan over the half nut, it's not even close. For all other threads, it's still quicker but you have to brake the spindle and count 1, 2 or 4 revs of the hand crank when returning (depends on the thread pitch).

These are the original threading instructions that were included with this lathe.

I removed the cover for this photo to show the hand crank and dog clutch setup. A lever beneath the hand crank controls it.

This shows the cone clutch on the jackshaft. A lever at the base controls it. It stops the spindle very fast.

And yes, I do actually use this machine.


02-01-2010, 11:16 AM
That's interesting S_J_H, but I can see why they don't use that setup on the standard engine lathes we use. It may work on the tool room lathes like the Hardinge, etc, not that it's the system Hardinge uses, I don't know what they use.

02-01-2010, 11:25 AM
Carl, I agree with you. It's interesting the way they built this old Machine though. Whoever designed it seems to have been looking for ways to improve or at least differentiate from the status Quo at the time.


02-01-2010, 11:32 AM
I think the reason a dog clutch is located in the gear train is because there isn't much torque needed to accelerate the gear train when the clutch engages - it isn't really a clutch, just one or more teeth that engage and must rapidly get the system up to speed. Accelerating the chuck, gear train, and work twice per pass would need a much more robust dog clutch.

The dog clutch concept I copied came from Martin Cleeve in "Screwcutting in a Lathe". Cleeve made much of his living threading modest runs of items for industry and added a dog clutch to his Myford to improve his speed and efficiency.

Adding a dog clutch to a lathe during manufacture would require planning how to incorporate it but shouldn't be particularly difficult.

The awkward part is connecting the trigger to the dog clutch and this could be done using a solenoid if a mechanical connection is particularly difficult. Also necessary is a method of winding the leadscrew back to retract the carriage, where an electric motor ala the HLV might be reasonable.

A point in favor of a dog clutch vs dual leadscrews is that thread pickup is automatic and takes at most one spindle revolution. Plus stopping at the end of each pass is automatic and accurate to a few thou even for inexperienced machinists.


02-01-2010, 11:43 AM
GadgetBuilder, I was just checking out your dog clutch setup on the little 7x. Fantastic work!!


02-01-2010, 11:44 AM
All that is true John but it would have issues for the standard engine lathe of any size. Perhaps the small engine lathes could use it.

All these methods are common knowledge with the lathe manufacturers and if it were something that could be used they would be doing it. The manufacturers aren't ignorant and they know what others have done and if they thought it would be good they would be using it if it's not under patent.

They are in the market to sell what the public wants and will work. It's a good idea for tool room lathes but I just don't see it as a good feature for the standard engine lathes most shops use.

I can see it on the home shop lathes to make it easier to cut threads but I think it should be an option. I don't think everyone would want to have one.

02-01-2010, 12:54 PM
Blackmoons : you can buy/make an Electronic leadscrew system for almost any lathe. There is a forum on Yahoo called the "Electronic Leadscrew" that has done just that. Cut any thread metric/imperial or design your own.. no matter what leadscrew/lathe you have. Software mods have been done to allow you to cut a programmed taper and many other things. Interesting to read. A couple of hundred $, some time, and you're running. It's on my "make one" list.