Turning Threads - FYI

[PE @ IHS]

Many people don't realize that you need to shim your threading inserts by a specific angle to achieve proper thread form. I learned this the hard way by making a nice big stainless paperweight. Your threading insert needs to be the correct form for the thread you are cutting, but it also needs to tilt for proper thread form. The bigger your thread, the worse the error. Good tooling catalogs will contain info on how much of a tilt you need, and specific shims are available from all cutting tool suppliers.

 

 

http://www.carbidedepot.com/resources.asp

[Jimmy Wakeford from Barefoot CNC]

Thanks for the info and link. Will prolly add a link from our sight to that.

 

Does anyone else have some cool informative links that would be good to link to from my website for customers?

[Andy]

Ditto,

Thanks

[Thad]

Peter,

 

Could you be more specific on the purpose of these "shims?" I don't understand what or how it needs to be shimmed. Or if shimmed is the correct word.

 

Thad

[Fig sign]

Peter !

 

I can generalize this and say that this also relies to milling threads and in general to every turnining and 2 axes moving of any tool .

I found it long time ago .

The less diameter of turning part and slot size ,the less deviation.

In milling the less diameter and width of cutter ,

the less deviation .

It can easily be seen in the folowing exemple .

Let`s say you milling threading toolpath on a cylynder on a vertical mill with a simple slot mill .

If the mill diameter and width would be zero the cutting points for the upper and lower edges of the slot mill would be on their theoretical places ,but in reality one is before and the other after their theoretical ones .

This turns the cutting process to the process of cutting with a different from theoretical section

and with slot example the width of path will be different ,and with the thread cutter also the

thread profile will be tilted .

That`s why always the less diameter for the mill threading cutter the better .

When I check my thread with thread micrometer I

always compensate this deviation ,but because my profile is standart one ,not tilted ,it will always have angle deviation .

That`s one of the causes ,why I always use thread cutters of the minimal possible diameter .

 

Iskander teh Thread^millman

[crazy^millman]

Well you are right in theory but when cutting a thread 3" deep into a part thread milling I want the bigger cutter. Just ribbing you my friend the wise old owl

 

 

Thad I think the idea behind the shims is that all tool are made for cutting thread are really only made for a spefic size range and that if you play close attention to the rake angle of the tip to the cutting surface it changes the bigger in diameter the threads your cutting are. I think the best way I can expalin it on a conventional machine. I sometimes when cutting threads will place my tool a little below cetner to keep constant pressure on the tool to keep chatter down. Work very when cutting threads on a long shaft and using a steady rest. The tool is now not vertical to the cut as it was when it was on center by being below center you put the rake of the tool back into the cut but you also use the mass of the material as a damper to limit vibration down. Well the same can be said about threading on a cnc lathe and best way around this is to cut the tool down so that it will be cutting below center in a CNC lathe if cutting bigger dia. If I was cutting anything under .5 I would always check my center and shot for .001 to .002 below center and anything a small as #10 or even a small as 1MM when doing screw threads I will shoot for .001 below center or dead if I could. I never shimmed my insert becuase it would always throw it above center and I never liked that you get more rub than cut. I would keep 2 sets of Standard brand 1/2 wide shim kits in my box and use them for the standard 1" tool that fit 99% of CNC lathes out there and they even work well on 1-1/4" and 1-1/2" wide tools also. Just me thinking out loud and it always worked for me right or wrong just what I think is all.

[Fig sign]

Well ,basically I wanted to say this :

In milling there is also deviation.

The less diameter tool less deviation.

Use the minimum APPROPRIATE tool and your deviation will be minimal.

The smaller tool also the bigger RPM and feed bigger too ,and less chatter

 

Teh

[PE @ IHS]

Thad,

 

Threading tools are flat and on center from the factory. That means that you can use the same threading bar for a variety of thread sizes by changing the insert. The thread lead is achieved by the rpm and feedrate synchronization. The thread shape is achieved by the form of the insert (i.e. acme, 60 deg, buttress, etc.) All threads also have an inclination angle

 

http://www.iscar.com/ProductLines/Turning/...asp?CountryId=1

 

The inclination angle varies with each thread size. The seat of the insert needs to be changed from a flat pad to one with an angle to achieve this tilt.

 

Most people keep changing their offsets and cut their threads oversize when the gage doesn't fit, instead if tilting the insert to the correct angle.

[Andy]

It sounds like you guys are talking about 2 independent tilting axes.

 

1. In a lathe, with the point on center, then tilting back (as is back rake)

 

2. Same situation except rotating (as in side rake)

 

In either case the thread form would change.

 

Seems to me that in case 2 you would rotate tool to the same as the lead angle to prevent getting say a 61 degree form. Same idea for case 1.

 

And, of course, as the Diameter increases, the lead angle changes requiring a different side rotation angle.

 

 

In the Laydown thread inserts (Vardex Style) I use in the Lathe it appears that they have somewhat compensated for this. They have ground in a side and back rake that is unique to its pitch range.

 

I used to use a flat top thd tool to cut a 11 tpi .750 dia thread. Did not work well. Changed to the Vardex style and now, like magic, I get perfect threads. It was a pretty dramatic improvement.

[John Summers]

Peter,

I think you have a good lesson for us all, here. Could you be more specific about what the thread was supposed to be and how it got out of tolerance? I'd be glad to contribute some computation programs via FPLOT or -.PST programs, once I understand what happened.

(I use -.PST programs like BASIC. MP is a powerful calculator).

[Philcott]

Here is a bit from iscars site pictorially showing the inclination angle.

 

iscar

 

Phil

[PE @ IHS]

Excellent link, Phil

 

John, I was turning an OD thread 1 3/4-5 UNC. I turned the OD to the correct major diameter, and used the correct threading insert for that lead. The thread gage didn't feel right, so I cut a little deeper. I measured over wires and was within tolerance, but the thread gage was still too tight. By the time the gage fit, my thread was cut too deep and the part was scrap. I didn't realize that I need to compensate for the inclination of the thread by shimming the insert.

[John Summers]

Peter,

The thread inclination is only about 2.5 degrees. When I twist the thread profile by that much, it doesn't change the thread profile enough to cause the error you got. There was only .00008 error per side. Are you sure the thread profile was 60 degrees on your tool?

[McRae]

Formulae for the calculation of the proper inclination angle

 

P=1/TPI

D2=Nominal Diameter of Thread

 

A=arctan(P/(Pi X D2)

 

Sandvik Lit Cat 03-T Page C85.

 

John, I think that you are not taking into account the requirement for flank clearance and this is why the inclination is required. Inserts are easier to manufacture in this manner and so the shims are used to account for the error.

 

Peter, these instances are why Lathes are much more difficult to machine and program...

[CNCme]

Good Day,

 

I am also at a loss about this shimming'

you shimmed the 60 deg...or the center line

to the part. I have made many threads up to

12 inch and never exp. this problem.

 

What kind of gage did you have...and was

it national thread form series or not

 

negative or positive insert...

single or cresting insert, there are many

Possibilities.

 

 

Tony G

Almost Employed Senior Programmer

N.E Massachusetts - Southern New Hampshire

_________________________________________

End mills and tooling are like The "AMMO"

And coolant and chips are like the enemy

Under your boots as you advance in the

Manufacturing Battle

--------------------------------------------------

[John Summers]

Andrew,

What I'm saying is that even if shimmed correctly, the tool should produce a thread form within limits. Unless I'm missing something. Don't throw away the toolbit, Peter.

[CNCme]

Good Day Peter,

 

I went to carbidepot.com, and didnt see

anything about threading...??

 

 

Tony G

Almost Employed Senior Programmer

N.E Massachusetts - Southern New Hampshire

_________________________________________

End mills and tooling are like The "AMMO"

And coolant and chips are like the enemy

Under your boots as you advance in the

Manufacturing Battle

--------------------------------------------------

[CNCme]

Good Day,

 

IMHO

 

After further review of this Information,

It seems very basic:

 

carbidedepot.com

 

Thank you

 

Tony G

Almost Employed Senior Programmer

N.E Massachusetts - Southern New Hampshire

_________________________________________

End mills and tooling are like The "AMMO"

And coolant and chips are like the enemy

Under your boots as you advance in the

Manufacturing Battle

--------------------------------------------------

[McRae]

John,

 

If the insert was not shimmed and layed in a horizontal plane it would cut an angle greater than 60 degrees and you would experience some flank rubbing on the sides of the "V". The inclined plane achieved by shimming - would put the thread form of 60 degrees perpendicular to the helix of the thread and produce a true shape. I think that you and I are speaking of the same effect here and that your error amount was measured across the inclunied "V" - Correct?

[John Summers]

Yes. The tipped 60 degree profile is off only about .00005 on each side from the true 60 degree profile. It's still not clear how the part got scrapped.

[McRae]

He didn't tip the insert...