Accurate threads using helical sweeps

[ptc-1932745]

Although 'accurate' threads are interesting to do and may look good in
documentation, for most purposes creating accurate fastener threads
doesn't seem really useful.



1) They do not help in global interference checks

2) Fastener threads must be simplified when doing mechanical FEA
analysis

3) They take a lot of 'horsepower' to shade and show on drawings,
mostly looking like a black 'blob' on plots.

4) Not useful for manufacturing features (NC machining)

5) Usually have fine features as compared to overall part (accuracy
issues)



Thread features do help for the following:



1) Acme drive thread (coarse) features

2) Inspection drawings of thread checks over pins





This is one area where PTC should really offer a solution. How about a
'thread' feature that:



1) Looks correct when cosmetically shaded, looks 'simplified'
otherwise

2) Is allowed for during global interference checking

3) Is allowed for in mass and c.o.g. calculations

4) Is allowed for during FEA (automatic bolted joint constraints)

5) Comes across as detailed in Arbortext.



Also, how about something similar for simple compression (die)
springs...





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[wattsteam]

Your statement "Although ‘accurate’ threads are interesting to do and may look good in documentation, for most purposes creating accurate fastener threads doesn’t seem really useful."



Except of course if you are planning to mould the thread in a plastic part.



Other than that, I try hard not to model threads for many of the reasons you mention.



JD

[A.DelNegro]

One other thought... it would be nice if they didn't show up as an
'interference' in interference checks when a bolt is inserted into a
threaded hole.



Maybe "lightning' will strike and it will happen.... But who knows when
that might be. (lol, sorry I couldn't resist)



T

[BrentDrysdale]

Hi Terry,
Given what the others have said I assume you have a serious need rather than
just a cosmetic one or that the need for cosmetic outweighs the overhead.

In my case as I was preparing a small part for diecasting I needed all the
detail. I made a helical sweep based on the OD of the thread and the
pitch. I left the sketched section (which I sketched from Machinery's
Handbook) parallel to the sketch plane rather than normal to pitch as that
is the way all thread systems are defined as far as I know. The sketch
could use a saved sketch if reuse is an issue. I used the centre of the
thread depth as the section centre but it could be offset.
I also gave the thread run out buy adding a tan arc at the head end (vary
the R value for the run out ramp). You can vary the shape of the run out
but it must get clear of the OD by more than the thread depth.
You can use a revolved surface and solidiy or just a solid cut. The surface
comes into its own when you have the case that your thread runs pretty much
up to a flange (e.g. head) of the part in which case some surface modelling
is required to trim the helical sweep so it does not intersect the flange.

Not too tricky. Hope this helps.


Regards, Brent Drysdale
Senior Mechanical Designer
Tait Radio Communications
New Zealand
DDI +64 3 358 1093
www.taitradio.com