Heavy thread cutting on light lathe

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I have to cut a thread M35x3 on my Schaublin 102 VM. The heaviest I ever did, with some trouble , was a 1,25 mm pitch so a 3 mm pitch is a challenge. The traditional methods to cut a thread all end up with 3 mm facing the tool.

Traditional thread cutting.jpeg


So I decide to try cutting the groove in layers:

thread cutting in layers.jpeg


Basically when I advance in x - steps of 0,05 mm and in y - steps of 0,09 mm the resulting feed will be 0,10 mm under an angle of 29 degrees, which is fine. To actually do it I found it useful to draft a cutting schedule. My thread has a flat top of 0,5 mm wide and also a flat bottom of 0,5 mm wide, giving a depth of thread of about 1,65 mm. This I decided to do in two layers. I have ground my 60 degree thread cutting tool with a flat top of 0,5 mm. The resulting cutting schedule is:

Cutting schedule M35x3.jpeg


The thread I have to cut is an inner thread. I decide first to cut two outer threads to try out the difference. The first picture is the thread cut by the traditional method of advancing the tool by half the thread angle minus a little bit. I used a 60 degree sharp tipped tool. Beyond a depth of about 1,0 mm the forces on the tool pushed the tool back. After the last cut I did try to compensate for this spring-effect by cutting again with the same settings. A full five times the tool kept cutting while veering back to the correct position. The effect as you can see is that the resulting thread has become quite sharp. The thread flanges themselves are fairly smooth for the purpose of this thread.



M35x3 full cut.JPG


Doing the layered cut felt in the beginning cumbersome, but the cutting itself went quite smoothly. Every once in a while I did a second cut with the same settings and I found hardly any veering back of the tool. At the end the 0,5 mm wide tip of the tool caused some chatter. The thread profile came out as planned. See picture.

M35x3 layered cut.JPG


For my light lathe both methods will probably work wit some adaptions. I think the traditional method will work if I re-cut with the same settings e.g every second cut. The layered cut which felt much smoother in execution wil probably work fine if I grind my tool with a smoothly rounded tip instead of a square tip.
 
A high rake angle will a better finish.
I typically finish the thread with file.

Dave

I have to cut a thread M35x3 on my Schaublin 102 VM. The heaviest I ever did, with some trouble , was a 1,25 mm pitch so a 3 mm pitch is a challenge. The traditional methods to cut a thread all end up with 3 mm facing the tool.

View attachment 135169

So I decide to try cutting the groove in layers:

View attachment 135170

Basically when I advance in x - steps of 0,05 mm and in y - steps of 0,09 mm the resulting feed will be 0,10 mm under an angle of 29 degrees, which is fine. To actually do it I found it useful to draft a cutting schedule. My thread has a flat top of 0,5 mm wide and also a flat bottom of 0,5 mm wide, giving a depth of thread of about 1,65 mm. This I decided to do in two layers. I have ground my 60 degree thread cutting tool with a flat top of 0,5 mm. The resulting cutting schedule is:

View attachment 135171

The thread I have to cut is an inner thread. I decide first to cut two outer threads to try out the difference. The first picture is the thread cut by the traditional method of advancing the tool by half the thread angle minus a little bit. I used a 60 degree sharp tipped tool. Beyond a depth of about 1,0 mm the forces on the tool pushed the tool back. After the last cut I did try to compensate for this spring-effect by cutting again with the same settings. A full five times the tool kept cutting while veering back to the correct position. The effect as you can see is that the resulting thread has become quite sharp. The thread flanges themselves are fairly smooth for the purpose of this thread.



View attachment 135172

Doing the layered cut felt in the beginning cumbersome, but the cutting itself went quite smoothly. Every once in a while I did a second cut with the same settings and I found hardly any veering back of the tool. At the end the 0,5 mm wide tip of the tool caused some chatter. The thread profile came out as planned. See picture.

View attachment 135175

For my light lathe both methods will probably work wit some adaptions. I think the traditional method will work if I re-cut with the same settings e.g every second cut. The layered cut which felt much smoother in execution wil probably work fine if I grind my tool with a smoothly rounded tip instead of a square tip.
 
The Schaublin 102 VM is a 90 year old Swiss design, center height above bed 102 mm, distance between centers 500 mm. My lathe is about 60 - 70 year old and has 0,02 mm play in the headstock front bearing. I need to address that, but it requires a for me quite complex dismantling of the headstock so I keep putting it of.
 
Looks like great 👍 lathe
http://lathes.co.uk/schaublin/page2.html
I have South Bend 9A built about same time (1949) and it will cut 8tpi acme thread. Witch in metric is 3.175 mm.

The problem most have is trying to use a low rake angle. Use a rake angle of 7° to 12° and your lathe can do a great job.

HSS did give best finish.
I have use carbide threading insert and did give finish I wanted .

Dave

The Schaublin 102 VM is a 90 year old Swiss design, center height above bed 102 mm, distance between centers 500 mm. My lathe is about 60 - 70 year old and has 0,02 mm play in the headstock front bearing. I need to address that, but it requires a for me quite complex dismantling of the headstock so I keep putting it of.
 
I have to cut a thread M35x3 on my Schaublin 102 VM. The heaviest I ever did, with some trouble , was a 1,25 mm pitch so a 3 mm pitch is a challenge. The traditional methods to cut a thread all end up with 3 mm facing the tool.

View attachment 135169

So I decide to try cutting the groove in layers:

View attachment 135170

Basically when I advance in x - steps of 0,05 mm and in y - steps of 0,09 mm the resulting feed will be 0,10 mm under an angle of 29 degrees, which is fine. To actually do it I found it useful to draft a cutting schedule. My thread has a flat top of 0,5 mm wide and also a flat bottom of 0,5 mm wide, giving a depth of thread of about 1,65 mm. This I decided to do in two layers. I have ground my 60 degree thread cutting tool with a flat top of 0,5 mm. The resulting cutting schedule is:

View attachment 135171

The thread I have to cut is an inner thread. I decide first to cut two outer threads to try out the difference. The first picture is the thread cut by the traditional method of advancing the tool by half the thread angle minus a little bit. I used a 60 degree sharp tipped tool. Beyond a depth of about 1,0 mm the forces on the tool pushed the tool back. After the last cut I did try to compensate for this spring-effect by cutting again with the same settings. A full five times the tool kept cutting while veering back to the correct position. The effect as you can see is that the resulting thread has become quite sharp. The thread flanges themselves are fairly smooth for the purpose of this thread.



View attachment 135172

Doing the layered cut felt in the beginning cumbersome, but the cutting itself went quite smoothly. Every once in a while I did a second cut with the same settings and I found hardly any veering back of the tool. At the end the 0,5 mm wide tip of the tool caused some chatter. The thread profile came out as planned. See picture.

View attachment 135175

For my light lathe both methods will probably work wit some adaptions. I think the traditional method will work if I re-cut with the same settings e.g every second cut. The layered cut which felt much smoother in execution wil probably work fine if I grind my tool with a smoothly rounded tip instead of a square tip.
Left everything because I'm going to try and refer to the pics.

I would suggest that you rough out the V before finishing.

x
x x
x x
x x

is what you want

so maybe start with a rigidly held parting tool (held short for rigidity!!!!!)
making a rectangular shape stopping short of the sides of the V

or for seconds you grind a form tool and use it at two different angles

x x
x x x x
x x x x
x x x x
x x x x
a third operation (using possible a parting tool would then be used to hog that center pillar

(It appears that the formatting is being lost - - - argh!!!!!!)

I was taught to use hogging methods to cut larger acme threads (or square thread even more useful).
Gets useful when making worm gears.

I think that no matter what technique you are going to use that you'll find that things will get 'interesting'.
Likely because you're asking a little girl to do a big job (imo) and she is struggling.

What you show in your pics is that you've already got the idea covered.

I do remember my boss cutting the thread with maybe even 8 passes at the same 'finished' depth.
So just doing 1 'spring' pass - - - - maybe do a few more.
(That gets to be like burnishing but it did get good parts!)

This might be a time to use HSS tools.

You work planning is impressive!!!!
Thank you for sharing!!!!!!
 
The thread I have to cut is an inner thread.
Can you mill the thread? I use it sometimes for very small outer threads.

I saw the idea here



Required for milling is a tool, something like this.
1647610297918.png

And some sort of motorized spindle to mount onto the cross slide.

I usually turn the motor (thread milling tool, not the lathe) on.
Then I put the lathe in lowest possible gear or even turn it by hand using a spanner on the chuck jaws. (then the lathe is powered off entirely)

Just had a look, unfortunately 3 mm pitch mills are for M24. Big, expensive and difficult to get.


Greetings Timo
 
Last edited:
Thank you for the advice, Dave. I normally use a zero rake angle. But I wil try out your suggestion!
My father used 0 angle too.
But one day I was looking at drills and taps had high rake and the cut about same time was reading about automatic screw machines . So try it and work great and never look back.
The only material that needs a low angle or 0 angle is brass.

Dave
 
My father used 0 angle too.
But one day I was looking at drills and taps had high rake and the cut about same time was reading about automatic screw machines . So try it and work great and never look back.
The only material that needs a low angle or 0 angle is brass.

Dave
How do I grind the tool then?
Somewhere I saw a comment that the rake will mess with flank angle. I could not figure out if that is really true.
Making Module1 worms requires similar theading, so that would come handy to get a better tool geometry.

Greetings Timo
 
I use a gear test gauge for grinding the tool bit..
After grinding the profile then a grind the rake angle.

Dave

How do I grind the tool then?
Somewhere I saw a comment that the rake will mess with flank angle. I could not figure out if that is really true.
Making Module1 worms requires similar theading, so that would come handy to get a better tool geometry.

Greetings Timo
 
You mill your own profile too.
Not all worms have a easy to buy profile test gauge. It is easy to make from aluminum flat bar.

Dave

How do I grind the tool then?
Somewhere I saw a comment that the rake will mess with flank angle. I could not figure out if that is really true.
Making Module1 worms requires similar theading, so that would come handy to get a better tool geometry.

Greetings Timo
 
Thanks for all the ideas and advise! Hogging out seems especially useful as I have indeed som trapezium threads in the forecast!

You know - - - - one idea hasn't been mentioned yet!

A buddy with a bigger lathe.
Even a 500 mm dia spin (IIRC 250 mm radius is a more customary nomenclature) should make easy work of that.
Then its not even a big job!

You would have lots of things (functions) that you could trade!!!
(The work you've shown is impressive!)
 
If the machine needs service, it needs service. You're giving up a lot in that machine with worn or loose bearings.

Typically, each pass is taking the same volume of cut. Looking at the multipass schedule for the tipped off tool, you have a large number of low volume passes in the first few cuts. The idea is to maintain tool pressure. This is a typical CNC threading cycle. You can also finish the side of the thread independently by leaving the tip flat smaller than final size and adjusting axial position.

1647711244790.png


When adding rake the angle must be accounted for. For small rake angles and thread pitch it's likely within tolerance. If necessary, refer to the thread standards. Larger pitch has smaller tolerance on the angle.
 
Last edited:
Sound good but this used on old lathe for reading the dial. The old lathes dial was on radius not diameter. So angle the compound to about 30° and compound would move on the diameter.

When cut thread I feed on cross slide.
With high rake it feeds and the finish in great..

Dave

If the machine needs service, it needs service. You're giving up a lot in that machine with worn or loose bearings.

Typically, each pass is taking the same volume of cut. Looking at the multipass schedule for the tipped off tool, you have a large number of low volume passes in the first few cuts. The idea is to maintain tool pressure. This is a typical CNC threading cycle. You can also finish the side of the thread independently by leaving the tip flat smaller than final size and adjusting axial position.

View attachment 135257

When adding rake the angle must be accounted for. For small rake angles and thread pitch it's likely within tolerance. If necessary, refer to the thread standards. Larger pitch has smaller tolerance on the angle.
 
FYI
If you doing grinding on lathe you set the compound to 6°. Then compound will feed grinding wheel into the bar 0.000,1 per mark on compound.
The old lathes using radius feed use 3°

Dave

If the machine needs service, it needs service. You're giving up a lot in that machine with worn or loose bearings.

Typically, each pass is taking the same volume of cut. Looking at the multipass schedule for the tipped off tool, you have a large number of low volume passes in the first few cuts. The idea is to maintain tool pressure. This is a typical CNC threading cycle. You can also finish the side of the thread independently by leaving the tip flat smaller than final size and adjusting axial position.

View attachment 135257

When adding rake the angle must be accounted for. For small rake angles and thread pitch it's likely within tolerance. If necessary, refer to the thread standards. Larger pitch has smaller tolerance on the angle.
 
Hi Dieselpilot, your comment is justified. However the family tradition says that my grandmother had to drive the threadle of her Dad's lathe when he needed to concentrate on a fragile precision part. As apprentice in a Brussels factory I worked at a belt-driven lathe. I myself had a broad leather belt on, to which was attached a stick with which to handle the belt-shifter. You would do forward-neutral-backward with a wriggle of your hips. In the late sixties my project for my engineerings master was the design and realization of a numerical control for a medium sized mill. The control was OK, but I found that the conventional machinetools of the time were by far not stiff enough to handle NC, even with feedback-loops. Of course all that changed in the last 50-60 years. But on retirement I decided to go back to the classic manufacturing technology of up to the sixties, that is kind of inbred in my family.
Nevertheless you are right - all machinetools need their tender loving care. As I'm a slow operator its going to take me time to do the spindle revision of my Schaublin 102 VM. So the first step was the acquisition of another lathe to work in the meantime. A beautiful Myford Super Seven in mint condition from a friend of mine that was operated as a back-up lathe and never cut many chips. As soon as I have installed DRO's to read Metric on this Imperial machine I will start the revision of the Schaublin.
 
Hi Dieselpilot, your comment is justified. However the family tradition says that my grandmother had to drive the threadle of her Dad's lathe when he needed to concentrate on a fragile precision part. As apprentice in a Brussels factory I worked at a belt-driven lathe. I myself had a broad leather belt on, to which was attached a stick with which to handle the belt-shifter. You would do forward-neutral-backward with a wriggle of your hips. In the late sixties my project for my engineerings master was the design and realization of a numerical control for a medium sized mill. The control was OK, but I found that the conventional machinetools of the time were by far not stiff enough to handle NC, even with feedback-loops. Of course all that changed in the last 50-60 years. But on retirement I decided to go back to the classic manufacturing technology of up to the sixties, that is kind of inbred in my family.
Nevertheless you are right - all machinetools need their tender loving care. As I'm a slow operator its going to take me time to do the spindle revision of my Schaublin 102 VM. So the first step was the acquisition of another lathe to work in the meantime. A beautiful Myford Super Seven in mint condition from a friend of mine that was operated as a back-up lathe and never cut many chips. As soon as I have installed DRO's to read Metric on this Imperial machine I will start the revision of the Schaublin.
Last year at the farm show there was a lady all of 80 that did the wiggle wag and flipped a foot wide belt over on notch on the steam tractor driven saw mill. She also stacked the raw cut boards as they were being sawn . She also had what was probably a period correct colt holster on . It probably would have been a good idea to talk respectfully with her.
 
Please understand my comment about the CNC had nothign to do with modern machinery, but the method applied to achieve equal cutting forces on each pass. My 10" Clausing 4900 struggled to perform when the bearings became loose. Fortunately, it uses taper rollers and correcting preload was simple.
 

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