deveation question 2

[beebo666]

is it acceptable practice to show two negatives in a tolerance. for example .756 -.001/-.002? thank you.

 

[ewh]

NX can handle interferences on the modeling side quite well, but sometimes gets confused on the drafting side. Instead of two edges for example, it sometimes won't show any.

 

[scripple]

Metric drawings tend to show a double - or a double + number. Their drawings use a standard mm dimension and then deviate with the tolerances.

 

[dingy2]

A tolerance with both on the minus side just doesn't make sense in the real world but I have seen them on some European drawings.

On the shop floor, one still has to calculate the centre value or nominal and convert to a bi-lateral tolerance and there is a bigger chance of math errors.

Don't do it. It does not help anyone and rather confuses.

Dave D.

http://www.qmsi.ca

 

[ajack1]

I must say I do find the view of Dingy 2 strange, is Europe not in the real world?

Using double plus or minus has long been a standard over here in the UK going back long before ISO or metric, the concept is simple you look what sort of fit you want, drive, slide, clearance etc look at the corresponding values and there you have it.

This of course makes all sizes standard, most bearings, shafts, bushes, couplings and the like are toleranced in this way, what could be simpler, unless of course you struggle with very basic maths?

I must say I am amazed this is not a standard in the USA. I am sure I have seen it on NAMMS standard flying cams for example, perhaps I am wrong.

 

[ewh]

It is used in the US, it just isn't standard. Our standards tend to reflect manufacturing and inspection. For design purposes, using a nominal value with double - or double + tolerance can be of value. In the "real" physical world, those values are never seen and thus we tend to not use them.

 

[drawoh]

ajack1,

My understanding is that when you place an accurate dimension on a diameter, limit dimensions are clearest to the fabricators and inspectors. As a designer, I like to see nominal dimension and tolerances. I do not care if they both go in one direction. It makes sense to me because I can see the mating part, although other people seem to get confused. When I finalize drawings, I try to remember to convert my tolerances.

You have to ask yourself who is going to be using your drawings. From my design point of view, a shaft goes into a hole. I mark the ANSI fit class on my assembly drawing or arrangement. The dimensions on the fabrication drawings are nothing more than the solution to an arithmetic problem.

JHG

 

[ajack1]

I must say I find this strange, to take the most basic example a dowel in a reamed hole we would call up for example a 12mm m6 dowel and a 12mm H7 hole. So do ½” dowels not exist as they are .5001 dowels and ½” reamed holes not exist, as they are .4998” ? The exact figures maybe wrong.

If you do have ½” dowels and reamed holes why is a shaft, bearing, bush or whatever any different?

 

[drawoh]

ajack1,

As far as I am concerned, 1/2" dowels do exist.

Try to distinguish between nominal diameter and mean or median diameters. For a 1/2" hardened, ground dowel pin, I claim that the nominal size is .5000", or 12.7mm. If the size range is between .5001 and .5003", the median size is .5002".

JHG

 

[ja500]

I am a UGS consultant and the as for mentioned tolerance is a valid callout. This is applied mostly in the Aerospace domain.