deveation question
deveation question
(OP)
is it acceptable practice to show two negatives in a tolerance. for example .756 -.001/-.002? thank you.
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RE: deveation question
While I am unaware of any, there may be CAD packages out there that will handle this type of dimensioning.
RE: deveation question
RE: deveation question
David
RE: deveation question
I was hoping that there was some software that could handle this (model at actual, dimension at two negative or positive limits - not actual).
RE: deveation question
RE: deveation question
David
RE: deveation question
RE: deveation question
David
RE: deveation question
RE: deveation question
http://w
RE: deveation question
I did not know there was a CAD program that did not support double (-) or double (+) tolerances. The advantage of this procedure is that your dimension shows the nominal dimension. This is useful design information, however weird it looks to the fabricator. What matters to you?
I suppose you need a convention in your CAD office about how you model dimensions. I model to exact nominal size, and I apply tolerances. If I have to design something that mates to your part, I must examine your drawings, or at least look at the tolerances you applied to your 3D_model. This is true even if I understand how you scale your features. The size of your model, however you did it, is not an adequate guide.
JHG
RE: deveation question
Please don't get me started on non-associative dimensioning. They cause nothing but problems for us.
RE: deveation question
What program do you use?
David
RE: deveation question
RE: deveation question
Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 01-18-07)
RE: deveation question
RE: deveation question
David
RE: deveation question
When you dimension nominally, is your part modeled at nominal?
RE: deveation question
David
RE: deveation question
Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 01-18-07)
RE: deveation question
RE: deveation question
The ANSI standard fits are based on ISO tolerance classes. Assume my nominal diameter is 3/4", that I want to use an FN2 fit, and that I want to check stresses.
Shaft: .750+.0019/+.0014, ISO s6
.7519
.7514
or we could go .7514+.0005/0
Hole: .750+.0008/0, ISO H7
.7508
.7500
The interference goes from .0006" to .0019". If you are working out stresses and torques, you can work with the nominal size and the maximum and minimum interferences. I cannot see that the difference between the nominal size and the median allowable size is significant.
Meanwhile, I want to change from an FN2 fit to an FN1 fit. If everything is modeled to nominal size, I review four tolerance values. If I used median values, I have to recalculate the two median values and then fix the four tolerance values.
JHG
RE: deveation question
I model to nominal mainly for CAM purposes.
If stress analysis is critical for the part, then I would create configurations of the tolerances.
Everywhere I have worked, this always has been an issue ... "what exactly do we model to?" We always end up with nominal.
Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 01-18-07)
RE: deveation question
I seem to recall double negative, in fact I think I've had to force Solid Edge to do it, it was a while back though and I can't recall how or why, I think it may have been rivet holes but I'm not sure that sounds right.
And while your at it don't have dimension in the model to an infinite number of decimal places while the corresponding dimension on the drawing is only displayed to 1, 2 or 3 DP.
RE: deveation question
That is the main reason we model as we do - CAM.
RE: deveation question
I agree wholeheartedly! That is one of my biggest pet peeves. I see it all the time where I work, people getting lazy with dimensions (not giving a value just eyeballing) and then the problem continues throughout the whole assembly because you can't force the software to accept a mate constraint that is .0001 off.
David
RE: deveation question
I understand your position on this, and agree from a drawing standpoint, but how do you allow for MBD and CAM if you only use nominal dimensions in your model? The CAM programmers I know seldom if ever look at a drawing when programming, but will go back and tweak their program after the fact. This is an opportunity for mistakes.
RE: deveation question
I don't know anything but the people that do.
RE: deveation question
The one exception I think I have to this is our standard hole settings which are based around drill sizes & tolerances which aren't unilateral.
Perhaps I'm misunderstanding your post as I thought you agreed on my understanding of nominal.
RE: deveation question
doesn't matter how the dimension appears on the drawing.
RE: deveation question
Kenat brings up a good point regarding drill sizes and their tolerances. I've always assumed +.005/-.000 for most.
RE: deveation question
RE: deveation question
Are you speaking for the tolerances on the drill bit or the tolerances on the drilled hole?
RE: deveation question
RE: deveation question
The tolerance of drilled holes is explained in the Machinery's Handbook. They have a table showing the dimension range you should expect from a given size drill in steel and cast iron given an average machine shop and a properly ground drill.
JHG
RE: deveation question
If you model to nominal the tolerances including the H7 and limits come from a drop down menu, as stated above as this is an ISO standard I have always assumed this is the correct way, could I be wrong?
RE: deveation question
Our hole settings are based on the tables in machinery’s handbook, adjusted to suit our needs. For instance one table we looked at the C'bore for the screws weren't big enough and didn't fully allow for float of the screw in the hole, so we enlarged them. We also worked out C'bore for button heads which may sound odd but means for our application we often can use buttons heads instead of CSK so preventing a 'fixed-fixed' tolerance situation.
ewh - perhaps for what I was using nominal isn't the best term, perhaps mean is better, but nominal is what I've always tended to hear used.
RE: deveation question
As I noted above, I would prefer that things are modeled to the nominal dimension. You process works fine for me.
As I also noted above, if I am going to fabricate your part or design a mating part, I have to learn your tolerances. I must carefully examine your drawing, and compare the tolerance text with the text on my drawing. It would be more convenient if each of us followed a standard, predictable procedure, but if you are outside my organization, I cannot make assumptions.
Here is another limitation of CAD. In the FN2 fit I described above, the MMC sizes, the median sizes and even the LMC sizes interfere. If I use any of these sizes in SolidWorks, I am not going to see the seam in my isometric drawing views. If both features are nominal, SolidWorks works fine.
What about nominal dimensions like 19.05mm?
JHG
RE: deveation question
RE: deveation question
RE: deveation question
RE: deveation question
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.
www.qmsi.ca
RE: deveation question
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.
RE: deveation question
RE: deveation question
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
RE: deveation question
If you do have ½” dowels and reamed holes why is a shaft, bearing, bush or whatever any different?
RE: deveation question
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
RE: deveation question