Should a screw thread not have a small difference between positive and negative shapes?

Rhino Rhino for Windows
1

Hi,
V5


then

shows positive then negative being made, but I see no ‘tolerance’ put into this, in reality I would have thought the fit had a minute amount of ‘play’ as opposed to a zero gap ?
How would that gap be made ?

How can one dial in know pitch values for the thread ?

Steve

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In reality, when making a thread you use the standard code for that thread. All dimensions are specified there. You would model to the nominal value and production will need to be within the specified tolerances.

The Rhino command for helix has a pitch option.

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Thread specifications are usually called out with references to a specification where appropriate , not taken from the geometry of the CAD model. Besides clearances the thread profiles also depend on the particular type of thread and specification.

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Hi,
With the making of the negative from the positive, if specs called for e.g. a tolerance gap of 0.2mm here, seeing that threads were 5mm in this tutorial, (my invented value) would one use offsetSrf to create that in the female thread having used boolean difference as was done ?

Steve

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steve, since i 3d print alot of items that are functional and i need to model threads then i do just as you said. i take the bolt or screw, offsetsrf whatever i feel i need to then do a difference with that.

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…as opposed to take the female thread in enlarge it a tad with offsetSrf.

I find offsetSrf creates naked edges and all sorts of little issues, naked spots, theyre the worst, no edge, just a red dot when running showEdges. Is it better to offset outwards than inwards. I am offsetting things inwards, If ones skin expands its smooth, if you lose weight it wrinkles, wrinkles sort of equal those little errors !

By the way, I gather that 3D printing can print pistons within cylinders and the result can see pistons run up and down, what about the gel stuff that gets used to support the structures that is then washed off, how does that escape out of such confined spaces ?..or your screw thread, I guess you unscrew wash then reassemble.

Steve

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steve, 3d printing should be done no different than any other manufacturing process. you make things in pieces. you want a piston then print a piston. basically print all the parts individual and assemble. avoid support at all cost. its just a pia.

as for the threads, i usually use the boltgen plugin for rhino. it makes what you need then i offset outward a touch and use the offset surface to boolean with. cant say i have ever gotten a naked edge using this method or issues with the fasteners created by boltgen. with that said i really dont do a ton of this either. i usually make blank holes then tap them myself when i can. you can only print threads so small anyway. your limited to large threaded bolts.

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Hi, ok,
it was just the fact that I saw a internet video that showed a small toy car 3D printed and the engine pistons worked immediately, clever stuff, they also show a bike chain fully operational straight after printing. maybe they didnt use the gel support system as some printers use.
8 micron no doubt can go fairly fine on screws, but as you say tapping is an option.

I shall explore the boltgen plugin, thanks for that info.

Wish there was a metal bending and twisting plugin !

Steve

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In a case like that you could experiment with Scale2D.
For your purpose it might be good enough and a lot less work than offsetting.

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Steve, it works already.

2.jpg792×612 72.4 KB

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?
seems unrelated ?

or have I missed something ?

Steve

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Metal bending? Your thread quest can be answered here however., http://www.amesweb.info/Screws/AsmeUnifiedInchScrewThread.aspx