Need help accurately modeling M77 x .75 mm thread 1

[sheafromme]

I am trying to get a custom optical part made and need to model the threads in SolidWorks for the machinist. The threads have to be M77 x .75 but I cannot find it in any of the typical resources I consult; either the charts stop around M50 or only do coarse threads in the M77 range. I need to know the profile, tolerances, minor/pitch diameter, etc. but nothing I have searched for has brought up anything. I do know that it's a typical mounting thread for optics in metric systems (similar to 3.035"-40 in imperial) does that mean I could use the standard metric specs just translated to M77? Looking for any input you all might have.
Thanks!

 

[3DDave]

Typically the thread profile is taken as a boundary condition - that is the perfect thread will have sharp crests and be exactly the nominal diameter. From there the thread form tolerances add clearance and that clearance goes to make up for imprecision of the form of the thread and the helical advance of the thread.

For a given thread form and nominal pitch the pitch and minor diameters should just be offsets from the nominal outside diameter; the minor diameter usually including a root radius to avoid damage to the cutter as well as reducing fatigue fracture initiation.

I don't see why the camera threads are any different. The main problem is deciding what tolerances you want to use - they are chosen by the user. See

https://www.accu.co.uk/en/p/134-iso-metric-thread-tolerances

It includes threads to 355 mm

I would likely not model threads in any CAD system as the machine tools will either be a form tool, such as a tap, or generated using in-machine advancing such as using a single-point cutter. There's no need to provide a CAD surface for thread-cutting though it might be useful to add a curve to show a place where the full thread cutting can stop and another where all thread cutting should stop.

 

[ctopher]

I also never model threads, unless for visual effect. The machinist will find the tools needed to make the threads.

ctopher, CSWP
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[drawoh]

sheafromme,

If you are modelling in SolidWorks, check out the description of Unified National threads in your Machinery's Handbook. I have modelled and 3D[ ]printed threads, although a pitch of 0.7mm could be a challenge.

Your mount almost certainly is being machined. The machinist will read the thread specifications off your drawing. The Unified Standard will call up all of your fabrication tolerances. Read the Machinery's Handbook, and be glad you don't need to call all that stuff up on your drawing.

In any sort of production, they will make an inspection gauge. In a one-off, they may want to see the mating part. If both parts are fabricated, consider fabricating both of them in one shop.

Don't have a Machinery's Handbook? It's the bible. Get one!

I have written instructions on Parametric Threads in SolidWorks. Note how I don't show individual threads, and how my thread depths are inaccurate, in a conservative direction.

--
JHG

 

[SSCon]

The standard you need is DIN 13-4 (ISO general purpose metric screw threads - Part 4: Nominal sizes for 0,75 mm fine pitch threads; nominal diameter from 5 mm to 110 mm). Although it doesn't have a nominal size of 77mm, goes from 76mm to 78mm.