Fractional dimensions 5

[paulyg]

My company has a long history of designing all in fractional or architectural dimensions, except the rare case where you need a tolerance to the thousandth. We deal with mostly castings (sand castings) and fabricated (welded) items. Being a younger engineer I was only exposed to working in decimal until I started here. My personal rule of thumb up to this point has been to design anything machined all in decimal and anything fabricated, especially big stuff (some of the fabrication you could fit inside) in feet & inches (architectural). I am dealing with more and more castings now and am note sure if I should continue to use fractional measurements or decimal. Because of the loose nature of sand casting only two place decimal is really needed. The head of our department doesn't care which method I use. I am curious what the collective experience here has to say about this choice. What is your personal or your company's position? Pros & cons of either choice?

 

[KENAT]

I think I just found where the designer got the idea for the foam. Just saw an old template which as well as having different tols for different inch dp also had I think it was +-1/16 for fractionals. Our current template doesn't have this.

Manufacturabily above should be of course Manufacturability, curse my clumsy fingers/slow brain. I see now why my boss always says DFMA.

Thanks Ctopher. 99.999-100.001% sounds good to me.

btrue, was a time when If you'd brought me a half I'd have thought you were questioning my masculinity.;-)




KENAT, probably the least qualified checker you'll ever meet...

 

[powerhound]

MintJulep,

You said:
The purpose of tolerances is to define how much variation from the nominal dimension can be tolerated before the part is no good.

While I agree with this, I don't agree with:
Sorry, but tolerances have nothing to do with manufacturing.

You must have a different perception of manufacturing than the people that actually do the manufacturing have. Tolerances completely define the process used to manufacture a part. If I see a hole on a print that is a standard drill size and has a tolerance of +/-.010 then I'm going to drill the hole with a drill bit and be done with it. If the hole is a standard drill size but has a tolerance of +.001/-.0005 then I'm going to drill it smaller and then follow it up with a reamer and ream the hole to size. If I see a large hole like something around 1 or 2 inches with a tolerance of +/-.005, I'll just interpolate the hole on the CNC mill, if the same hole has a tolerance of +/-.0005 then I'll interpolate the hole small then finish it out with a boring bar. I could also interpolate the hole small then sneak up on the dimension with more passes until the size is right. These are just a few example of tolerances having something to do with manufacturing. Are you approaching the concept of manufacturing from a different angle?

I don't see the rationale in your statement.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MintJulep]

Powerhound,

I think we are actually on the same page. My language was sloppy.

If you'll go back through the thread you will see that my statement was in reply to "Block tolerances are usually found to be reasonable manufacturing tolerances".

What I was trying to get at is that the part designer needs calculate the required tolerance based on the functional requirements for the part, not because a "standard tolerance" is easy for the shop to hold.

Your job, as the manufacturing guy, is to select the appropriate process to make parts to the print, accepting that the engineer has done his job correctly, and has defined the tolerances that the part needs.

So perhaps what I should have stated was:

Calculating what tolerance is necessary for proper function has nothing to do with manufacturing.

 

[ctopher]

Interesting info:

http://www.access-board.gov/research/tolerances/design.htm

http://feh.eng.ohio-state.edu/Lectures/191au04/11-01 Fits and Tolerances.ppt

http://adcats.et.byu.edu/Publication/99-6/ToleranceAlloc.pdf

http://www.stat.washington.edu/fritz/Reports/isstech-95-030.pdf

Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 09-17-07)

 

[powerhound]

MintJulep,
Being that the intended statement was as your last sentence, I now understand what you meant. In that context, I agree with you.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[paulyg]

Wow! I didn't think this message got posted. When I hit submit I recived an error message. After a few tries I gave up. Good discussion.

In regards to MintJulep's sidetrack about tolerancing: Just because the standard tolerance is good enough for most of the dimensions on the drawing doesn't mean the designer didn't put thought into them. There is plenty of occasion (at least with the parts I design) where the tolerance required by the design is very loose. But using those very loose tolerances could make for a very ugly looking casting that still passed a QC check! And no self respecting manufacturer or design house wants to ship out ugly parts.

 

[CheckerRon]

Your right PAULYG, there is a point where asthetics trumps tolerance, but not function.
My buddy KENAT mused as to whether or not ASME Y14.8M(Casting and Forgings) covered fractions. The "M" in the designation is a big clue, moreover ¶ 1.1.5 of ASME Y14.8M-1996 says that "The methods of dimensioning and tolerancing shall be IAW ASME Y14.5M ...". I may be wrong, but I have a feeling that PAULYG's company, who still dimensions castings in fractions also measures them with a tape measure(yo-yo)and doesn't pay much attention to ASME Y14.8M.

 

[KENAT]

Doh, you're right Ron. The M should have given me a clue.

KENAT, probably the least qualified checker you'll ever meet...

 

[btrueblood]

I guess I'm wondering why the ASME decided that fractions are no longer a valid mathematical method of displaying a non-integer value. Better let my kid's math teacher know.

But, in the meantime, if my drawing tells your shop to cut the part 16-1/2 in. long, with a tolerance of +/-1/16 inch, and you cut it 16-3/4 inch long, you're gonna be in trouble with me.

 

[PeterStock]

But not as much trouble as if he cut it 16-1/4 inch long.

Peter Stockhausen
Pollak Switching Products

 

[jeandrews]

I have one for you. I have an engineer here who has come up with a new system. He sets his CAD to all 3 place decimals.

The unspecified tolerance blocks looks like this:

Fractional 1/32 1/64
Decimal:
.0 +/-.015
.00 +/- .010
.000 +/- .005
or:
.000 = +/- .060
*.000 = +/- .030
**.000 = +/- .015
***.000 = +/- .005
or:
.000 = +/- 2mm
*.000 = +/- 1mm
**.000 = +/- .5mm
***.000 = +/- .13mm
or:
.000 = +/- .060 2mm
*.000 = +/- .030 1mm
**.000 = +/- .015 .5mm
***.000 = +/- .005 .13

Then he will put a *, **, *** next to a dimension he wants closer than .06

any guess how many calls I get from confused shops? Especially when there are several sheets and each one has a different unspecified tolerance block, sheet A will have one variation, sheet B another, sheet C a third. Some prints have *, other don't.

Add the language barrier for overseas suppliers on top of it.

I say just poor lazy work.

 

[ctopher]

Then he will put a *, **, *** next to a dimension he wants closer than .06

Is that in inches or metric?
Yes, I can see how it can be very confusing to a machinist.
I say call out the tolerance next to the dimension as much as possible.

Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 10-07-07)
ctopher's blog

 

[jeandrews]

sometimes drawn in inches, sometimes in metric, sometimes dual dimensioning. The last one causes problems in assembly when overseas makes it per metric tolerancing and the US inspects per decimal. .060 = 1.52mm not 2mm etc.

they say they don't have time, let the assemblers fix it

never time to do it right but always time to make it over

 

[KENAT]

Glad it's not just me has to put up with people playing fast & loose with this type of thing!

KENAT, probably the least qualified checker you'll ever meet...

 

[ctopher]

I had tech change a tolerance on a dwg once. (He was not supposed to change any dwgs). He said the same about having the assemblers fix it. A whole batch of parts were a week late and had to be reworked. I think the cost was ~30k and customer was not very happy.
Make the dwgs right the first time, no questions or errors later.

Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 10-07-07)
ctopher's blog

 

[pierdesign]

An office furniture company I worked at long time ago dimensioned formed parts in fractions and sheet metal layouts in decimal.

Now on drawings I have no tolerancing instead reference: TOLERANCING ISO 8015 / GENERAL TOLERANCES
ISO 2768-mH (or other class), and gdt where required. Somewhat reduces revisions and assists global manufacturability and inspection.

 

[KENAT]

ISO2768 basically says the tolerances are just a suggestion and you don't have to meet them.

I'd caution anyone before using it to fully undertand the implications and thoroughly read it.

KENAT, probably the least qualified checker you'll ever meet...