OT - Let's Talk Tolerances

[DBC]

I am curious if someone has a thought pattern they go through when designing and considering tolerances.

I ask because I have never been confident when determining them, although no one has ever really complained to me about them in my drawings (although the aluminum extruder I/we used in my previous day job would constantly say that they could not keep the tolerances that the company wanted). Finally I just added a note to all extrusion drawings that tolerances were to keep within the Aluminum Association Standards (or however it was worded). I have tried to read documents about tolerances but my eyes just glaze over as I am just not a book learner. That is probably why I did so poorly in school, but that is another story with a whole lot of other factors at the time .

So I am just curious if you have a simple process you use while designing something in the mechanical arena. Lets say something not terribly large like the boring head I posted in the gallery. The attached was a typical table we used on drawings. Please use layman's terms if possible, as engineer-speak can be just as difficult to understand as reading an engineering text.

 

[Lew_Merrick]

I am curious if someone has a thought pattern they go through when designing and considering tolerances.

It really depends on the Industry you (and the shops that support you) work within. Tolerances and allowances for a device that needs to (say) relate things within .000010 inch are going to be built differently from thins that have an "allowable misalignent" in the ±.030 range. Things that operate in a +3g/0g environment have far different tolerances and allowances from those that have to operate in a (say) 500g/300g environment.

 

[NateLiquidGravity]

Like Lew says it depends on your needs.

My job is mainly structural steel buildings. We typically fabricate with dimensions to a 1/16" (and that depends on who's looking at the tape measure). Our holes are oversized for ease of fabrication and construction. For a 3/4" bolt we put a 13/16" hole in the beams and a 7/8" hole in the plates. If its a 3/4" concrete anchor bolt then the plate gets a 1-1/16" hole because concrete tolerances are even greater.

However if I'm designing sheet metal parts that we laser cut, form and assemble then I can design much tighter tolerances.

There are different types of things to be aware of to include tolerance for.
Design - like using oversized holes for assembly and rounding/ truncating of dimensions on prints.
Manufacturing Process - the roughness of a circular saw cut, cnc milling, laser cut... Stock mill finish, ground finish, polished finish...

 

[Jim C]

In general I have found the tighter the tolerance the greater the cost (or time if I'm doing the work). There are many times when a tolerance of +/- 0.01 or more is plenty good enough. But then something like 0.0005 is needed for things to work properly. Make as loose as possible without making a sloppy item.

Jim C

 

[jhiker]

A general tolerance box like the one you showed is acceptable for some individual parts. It gets a little more complicated when designing two (or more) parts that have to fit together. In that case I would suggest you need a basic understanding of 'hole & shaft' tolerances, positioning and concentricity, etc. It's really not that scary.
https://www.veryengineering.com/gdt-the-beginners-guide-to-geometric-dimensioning-and-tolerancing/

 

[Lew_Merrick]

A general tolerance box like the one you showed is acceptable for some individual parts. It gets a little more complicated when designing two (or more) parts that have to fit together. In that case I would suggest you need a basic understanding of 'hole & shaft' tolerances, positioning and concentricity, etc. It's really not that scary.

Hi Jeff -- A couple of years ago I had a design where everything was dimensioned and toleranced in accordance with (the then) ANSI Y14.5 and the "shop" howled until I pointed out that a ø.063 tolerance band gave them more "fit allowance" than ±.031 did. -- Lew

 

[DBC]

Thanks everyone for your input. I guess my desire to have a simple single process is not realistic, so I will adapt my expectations. I really appreciate all the replies as they are helpful to see a fuller picture.

 

[Happycamper]

Within GD&T especially the metric system, there is all the info for shafts and bore diameters. They are designated similar tap sizes as in GH4, GH5 etc.