I am trying to cut my first thread M8x1.25.

The pitch is OK but the thread depth is not deep enough.

I am using dia 7.9 as x start diameter and 6.446 as the minor diameter.

The x axis is calibrated by doing a skim cut and entering the resultant diameter into Mach3 as detailed in the documentation and Tutorial Video.

Attached is my G code and a photo of the end result.

I could cut deeper by specifying the minor diameter as less than 6.466, however I am reluctant to specify the minor diameter as less than the recomended 6.446mm.

Where have I gone wrong?

Regards

Chrisjh

What kind of tool are you using? If you command a move to 6.446 does it cut at that dia?
Hood

Hi Hood,

I am using a carbide tipped fully formed index threading tool.

You raise a good point, I should try a light commanded cut using the threading tool itself and measure that.

The method I used was to turn the 7.9 diameter in my manual lathe first, then use a 3mm drill shank to set the tip of the threading tool to diameter 13.9.  Maybe this where I am going wrong.

Regards

Chrisjh

Suppose it would depend how accurate the drill shank is. How much do you think you still have to cut to get full depth?
Hood

FWIW,
I always go to get the correct pitch diameter. If the tool has a radius it's accounted for.
So i back the tool away from the max diameter. So if the max allowable diameter was 8mm  and the tool had a radius of 0.1mm then i would set the tool back from the 8mm ( as if it was a true sharp V ground tool). The max cut depth then would be 0.866P. Doesn't matter what the actual piece od is since if all goes well ( ie; the lathe has the mechanical abillity and the cutting was actual ) the pitch diameter will be correct.  Any deviation from the pitch diameter is due to mechanical or the actual cutting differences of material.

RICH

I nailed it!!!

I checked the real diameter with a cut from the insert and found it to be very close to 7.9mm.  I ran a light cut in the cnc lathe to be sure, remeasured and recalibrated the x Axis.

I checked the specs on the index thread insert and found it was 0.4mm.  I entered this into the tool table and ran the program again.  Slightly better but still not cutting deep enough.

After running more unsuccessful tests with varying OD,  I bit the bullet and decided to decrease the Minor (Root) diameter in small steps.

The test nut fitted on at a root diameter of 6.4mm (down from the recommended 6.466mm)

Thanks for your help.  I am very new at programming cnc lathes.  It took me 3 weeks to stop thinking in cnc mill mode where the tool tip position is clearly defined in X, Y, and Z coordinates to thinking in terms of diameters in the X coordinates.  I am a slow learner

Below is a link to a poor quality video of the successful threading routine.

http://www.youtube.com/watch?v=PsP8mnMGV-4

Regards

Chrisjh