3D Modeling Best Practice 2

[bbook1]

If you are creating a 3D model that is going to be used to create a CNC program, how should the model be drawn in regards to the tolernacing. For example, if a diameter is 33mm +0.3/-0.0 how would you draw the diameter? Would you make it 33mm or dimension it at the center of the tolerance?

 

[ewh]

It is just a toggle on NX, but as I posted, I have never verified their accuracy.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[MechEng2005]

I know the McMaster Carr models/drawing have the thread modeled (as opposed to just a graphical representation). It's really irritating and can dramatically increase file sizes and slow performance. I have no idea what software they use to generate their models/drawings though.

-- MechEng2005

 

[SteveMartin]

Most of the McMaster hardware that I've seen/used does not have a thread modeled, just something that looks like one at quick glance. I agree that it's a pain, because it's both slow and wrong. I usually delete out all the faux-threads and put in a cosmetic thread if I'm using one of those models for something. It takes longer up front, but doesn't annoy me every time I use the model.

 

[KENAT]

This is a bit off topic but I've got a bunch of fastener models from McMaster that have threads and that I'm debating tidying up.

On topic, the more I think about it the more I think you'll have to talk to your machine shop(s) and ask what's best for them then come up with your own procedure.

For general machining I can see 'mean' perhaps being best but for some process I'd suspect otherwise. For instance traditional drill tolerances allow for the fact that holes tend to drill oversize. To me it still makes sense to model the nominal drill size, rather than the mean which would be typically a couple of thou' bigger and wouldnt' match a standard drill size.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[fcsuper]

You can turn off (suppress) the thread feature in the McMaster Carr screw files, which just leaves the nominally correct OD of the thread,

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[TheFeds]

I have no idea what software they use to generate their models/drawings though.

I'm pretty sure it's SolidWorks. It's the only native parametric file
type they offer.

If you are creating a 3D model that is going to be used to create a CNC program, how should the model be drawn in regards to the tolernacing.

It really comes down to discussing this with the person who's doing the CAM programming. That's the most reliable way to ensure that any offsets and adjustments will produce a part that is within tolerance. Sometimes that means creating a special version of the model, but most often, the CAM technician handles it based on the drawing's tolerances.

 

[dgallup]

Some CAD systems let you have your cake & eat it too. Pro/E will let you have a dimension like 5+0/-.2 which normally will measure 5.0. However, you can go into to the part setup/dimension bounds and change any or all dimensions to upper, middle, lower or nominal. Changing the above dimension to "middle" will regen to 4.9 while the drawing will still say 5+0/-.2.

Similarly, you can have a max dimension like R0.3 max regen to measure 0.15 by setting the upper tol to 0, the lower tol equal to the nominal and regening to the middle.

This has saved me tons of work by always having the model geometry at nominal but the drawing can show the fully associative parametric asymmetric model dimension.

 

[KENAT]

Completely off topic - I didn't download them, I just have to clean up the mess.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[aardvarkdw]

Autodesk Inventor allows you to apply tolerances to the model dimensions (a bit cumbersome the last time I used it but effective), these dimensions could then be toggled to Nominal, Mean, Upper, or Lower tolerance size and would allow you to check fits in the assembly.

Unfortunately this information is not carried over when parts are saved out as neutral format files and I don't know if there are many CAM programs that read native Inventor files.



David

 

[ajack1]

That is the problem, as I stated many systems (I do not have a full working knowledge of every CAD system on the market) will create some kind of feature recognition that works with a compatible CAM system, however again to the best of my knowledge this goes out of the window when used on a CAM system that is not directly compatible and you use .iges .stp or whatever neutral files.

 

[PeterStock]

dgallup;

"........However, you can go into to the part setup/dimension bounds and change any or all dimensions to upper, middle, lower or nominal.........."

Thanks, thats something I had not realized about Pro/E and just became part of my "best practices".

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[ewh]

Regarding McMaster-Carr threads, they are garbage, used only for appearance sake. If you look closely at them, they are only angled torii, nothing like a true helix.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[dmech]

Model to nominal and tolerance to standard fits (symmetric, bilateral, MAX, etc.). Why? There are two reasons.

1) You can perform an interference check with coincident faces considered as interference. This will allow you to quick identify, check,and update the fit of nominal interfaces.

2) NC programmers can provide an offset which will leave or take more material. This can be implemented at any point in the code. So if the overall size of your part is +/- .005" they'll run the cutter nominal. However, if you have a internal profile that needs +.002"/-.000 they'll run the rough nominal and run the finish pass nominal with a .001" offset. The point is tolerances are easily programmed by a capable NC Programmer.

Be consistent and work with your vendor and they will make the parts right.