I really struggle at screwcutting

Hi. Could anyone make any suggestions as to what I'm doing wrong ....
I'm trying to machine an internal close fit 1.5" BSW thread in 303 stainless
with a partial profile tip and what I believe is "old style" G76.
G76 P020055 Q50 R0.025 G76 X38.905 W-25.0 R0 P17595 Q300 F2.1167
The nut is bored to 35.79mm but becomes too tight on the shaft.
What should the (general) relationship be for a partial profile insert, the depth of cut P value and the bored out (minor??) diameter?
The finish on the thread is not good either .... I'll reduce rpm on the next test.
Thanks Rob
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On Apr 30, 7:23 am, rjh_not snipped-for-privacy@vici.demon.co.uk (Rob) wrote:

Hi Rob,
I've found that Vardex has some good info on partial profile thread turning:
http://www.vargus.com/vardex/template/default.aspx?pCatId=8#1
You can download the tips in PDF files there.
HTH
-- PaulS
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On 30 Apr 2009 12:23:20 +0100, rjh_not snipped-for-privacy@vici.demon.co.uk (Rob) wrote:

Is there a reason you are using two spring passes?
You are not chamfering out, does this part have a thread relief at Z end?

Dimensions don't look right, do you have a thread spec.?
http://mdmetric.com/tech/thddat8.htm
What RPM are you running?
What insert are you using?

For 303SS slowing it down is probably the last thing you want to do for a better finish. If you are using water soluble for coolant you want to run a little rich?
If you are running a lot of parts and this is a course pitch 6 or 8 TPI, I would use an alternate flank Infeed method, but you can't do that with G76.
Tom
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Rob wrote: > Hi. Could anyone make any suggestions as to what I'm doing wrong .... > > I'm trying to machine an internal close fit 1.5" BSW thread in 303 stainless > with a partial profile tip and what I believe is "old style" G76. > > G76 P020055 Q50 R0.025 > G76 X38.905 W-25.0 R0 P17595 Q300 F2.1167 > > The nut is bored to 35.79mm but becomes too tight on the shaft.
You probably have already done this, but make sure the insert is the correct one for the thread and *pitch* that you are cutting. I have seen people go batty trying to cut a thread and the problem was simply that they were using an insert that was incorrect.
Also check and make sure your tool holder is not rubbing.
-- John
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My calculations show 36.194 for the minimum minor diameter.
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Thanks everyone
This is just for a locknut for my own use. I've had another look at things and made something acceptable:
% :8011 (Medium Fit 1.5" BSW Internal 12tpi) (Partial profile Major dia 39.26mm, Minor dia 35.869mm) (Depth of cut 1.696mm partial profile) M8 G21 G97S250M3 <* G00X24.0Z++5             (Start point) *> G76P020055Q50R0.025         G76X39.26W-43.0R0P1696Q300F2.1167 M30 %
The standards only quote a minimum major diameter. Which seems a little strange. I think it's this dimension that gave me problems earlier.
Here P=(39.26-35.869)/2 What is the effect on the thread if I choose a larger P ie one that causes the first cut to be in "fresh air"?
Thanks Rob
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too tired to think about this, but what if after your done raise up .001 and run it. then lower .001 and run it, climb milling of course. It cuts so fast the time is insignificant. Also, bust out a diamond or ruby stone and hone an edge.
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