Inventor General Discussion

*Expert Elite*
Posts: 4,212
Registered: ‎04-27-2005
Message 1 of 3 (1,245 Views)

NPT is wrong

1245 Views, 2 Replies
04-03-2012 12:14 PM

We noticed that the tap drill callout for a 3/8-18 NPT was wrong.  It was showing 35/64 for the tap drill.  The value that all my charts show is 37/64 (.5781).


When I opened up the Thread.xls file, there I found where it was getting this information. I changed the value in the Tap Drill column to be the correct .5781.  Now the call out is correct, but the model does not look correct.  The threads are not deep enough in the model.

How do I get both correct?




Here is what the model looks like when I changed the value to .5781. Notice how short the threads are.



Product Support
Posts: 193
Registered: ‎10-25-2006
Message 2 of 3 (1,191 Views)

Re: NPT is wrong

04-06-2012 02:19 PM in reply to: karthur1

The tap drill size isn't in the MH, so I'm not sure where is came from.


As far as the length of the thread, it is controlled by the blind hole distance, and then a formula is used to back calculate the thread length. To have more threads, you'd need a deeper hole.


Alternately, you can extrude a circle with a -1.7833 draft to create the 1 deg 47 min taper (if memory serves). Then use the thread command. Better in situations when you need more specific control than the dialog box provides.

Dave Anderson
Premium Support Analyst – DLS Mechanical Design
Autodesk, Inc.

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*Expert Elite*
Posts: 4,212
Registered: ‎04-27-2005
Message 3 of 3 (1,148 Views)

Re: NPT is wrong

04-10-2012 11:08 AM in reply to: dave.anderson


I used the hole command and changed the thread type to NPT. Seems logical that this should show the hole correctly. I tried changing the depth of the blind hole to get more threads shown. Did not make any difference.  Here is what I see in the idw.  Notice the two lines.  Should these not be at the same dimension from the top of the hole? When I measure o the ends of the threads in the model, this hole is .311 deep. So I'm not sure what the .41 depth is for.






Here is the tap chart that is my MH (25th ed).



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