Finding the Number of threads needed on an M3 screw, Autocad 2017

Hi I am really new at creating threads on Autocad and screws in general. I'm currently working on a project that requires M3 flat head screws. I was wondering how many turns I can do on AutoCad just knowing the thread depth I have.

I am clueless if I am to look at the thread pitch, or something else. Also, am I supposed to make the thread holes smaller than the actual diameter of the screw, so that it will catch?

Any help would be appreciated.

I don't know why it would matter? Are you modelling 3D threads, with the intent to use them with a CAM program?

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Originally Posted by ger21

I don't know why it would matter? Are you modelling 3D threads, with the intent to use them with a CAM program?

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Yes, that is my intention. I want the CAD schematic to Display the model as if it would be manufactured later.

Modeling threads in AutoCAD seems like an unnecessary and tedious task. Programs like Inventor and Fusion 360 have the 'Hole Tool' which simplifies this process and allows you to have visual threads or physical threads in your model, the latter of which takes up a lot of computing power if there are lots of threaded holes in your part. An M3-0.5 screw will have a thread pitch of 0.5 mm meaning the threads are 0.5 mm apart. Therefore a 10 mm deep threaded through hole would have 20 threads in it (10 mm / 0.5mm = 20). In AutoCAD I would just model the hole to be the tap drill diameter of the screw you will be using. For an M3-0.5 that would be a 2.5 mm hole, coarse metric fasteners are nice because the tap drill diameter is equal to the screw diameter minus the pitch (3 mm - 0.5 mm = 2.5 mm). You also have to remember that if you're creating a blind threaded hole (vs. through) that you need to drill deeper than you plan on threading so that the tap does not bottom out in the hole. All these hole specifics would be called out on the part drawing.

Just open up the Machinery's Handbook and all thread profiles are there to the zillionth of an inch.
Then just define one loop of the "theoretical" point helix (thread lead) and do a solid extrusion,
Question, My cad software from 20yrs ago has the all fasteners detailed

Except for exact threads

Easier yet, go to McMaster Carr, find the screw you want, download the SAT version of it and import in to your drawing. If you want a modeled thread going through or into another part copy the screw slice the head free and place the screw thread into the solid you want to model then boolean subtract the thread from the solid.

Edit: When you use this level of detail, solid model thread, the drawing size grows and the load time and sometimes working time increase unless you have a powerful processor and graphics card.

Hi, Thanks for the numerous details. Would you know anything about the tolerances of the screws? For example, how much tolerance would I need and what kind of interference or transition mating would I need to do.

Tolerances for thread differ between Unified (ANSI) and Metric. As bostosh suggested before look in a Machinery's Handbook if you need to figure out the tolerances accurately. I am more familiar with Unified threads which are defined 1A 1B, 2A 2B and 3A 3B, 'A' are external and 'B' are internal threads 1 being the loosest fit tolerances and 3 being the tightest fit. Most commercial and industrial use 2A and 2B. I do not have any actual experience machining the threads since I rely on either prefabricated screws and nuts or purchased taps and dies to create special needs.

All previous posts are right ... and wrong.

If you plan to make *accurate* 3d models with threads, the advice given is right.
BUT...
This makes huge models, and will not help you in any way.

The only reason to make a really accurate thread is if you are going to manufacture the part via cam.

SO, ...
if you use accurate stl files from parts manufacturers or stockists like misumi et al...
each and every threaded hole will be "huge" in terms of 3d parts, polygons, hexels etc.

A single 3d model of a ballscrew, fairly accurate, is about 10 MB in size.
I made one model each of accurate, manufacturable, and reasonable.

If you had similar screws, say 100 pieces, the model "might" be fairly reasonable.
This, if and only if you used "instances" and links in your model.
But the developed structure /assy would still be incredibly detailed and large.

It is common for my models and common industrial stuff to be about 1GB in 3d file size.
I use an nvidia industrial graphics card to deal with this, with 70+ processors and maybe 8GB of memory.
You probably don´t.

There is a good reason to make accurate 3d models of threads etc..
when You are going to use cam to manufacture the holes via thread milling.
Otherwise, not.
Make a cosmetic simple plug to rep the thread/hole.

?
Test this.
Make a rectangular block.
Import a single screw as stl from misumi etc of any size, say 3 mm x 5 mm.
Instance 100 pieces in an array into the model, doing a boolean subtract from the solid.

How fast does it render / rotate / scale ?
What is the size ?
Probably 500x200x30 mm block x 100 holes => 100-800 MB in size.

Your settings for accuracy/resolution/import files size/accuracy will have a major 10x impact.

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Originally Posted by TeddyLu

Yes, that is my intention. I want the CAD schematic to Display the model as if it would be manufactured later.

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Unless it's purely for aesthetics, you don't have to model the threads in for CAM. Call it out on the drawing along with the tolerance if needed.