Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: chrisjh on September 08, 2009, 06:27:29 AM
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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
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What kind of tool are you using? If you command a move to 6.446 does it cut at that dia?
Hood
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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
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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
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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
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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) ;D
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