Fractional dimensions
Fractional dimensions
(OP)
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?





RE: Fractional dimensions
Regards,
Cory
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RE: Fractional dimensions
You talk about only holding to 2 decimal points but this is meaningless withouth the title block tolerances. I've sean tol blocks where 2 dp was +-.005, more typically +-.010.
The number of decimal points only indicates tolerance if your title block is set up that way. In my last company in the UK the number of decimals had no significance. We assigned a singe general tolerance, typically +-.25mm and every dimension that required a different tolerance had it indicated on the dimension.
You can have a 1.375 dimension and apply +-.050 to it if it makes sense.
KENAT, probably the least qualified checker you'll ever meet...
RE: Fractional dimensions
Tolerance is the amount of part-to-part variation that is acceptable without preventing proper function of an individual part, or its parent assembly. It is a design function, and is separate from and unconnected to the nominal dimension.
If I have a part that must have a feature within the range of 20.200 and 20.206 to function the requirement does not change if I call the nominal dimension 20.203 or 20-13/64.
Or maybe the part must be between 19.997 and 20.003 to properly function. The nominal is 20.
The fixation on number of decimal places "defining" the required tolerance - as associated in a typical "standard title block" - is simply laziness, indicating that the engineer or designer could not be bothered to actually engineer the tolerances, and has simply made a guess, or more likely given no thought to tolerance at all and has simply let the CAD program variable the sets the number of significant digits decide the tolerance.
RE: Fractional dimensions
I agree with others comments above.
Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 04-21-07)
RE: Fractional dimensions
Give us a break...
Block tolerances are usually found to be reasonable manufacturing tolerances, and are no different than if every dimension had its own tolerance. The design should allow for this. It serves no purpose to put a tolerance behind every dimension if the part was designed with the noted tolerances in mind. Drawings are supposed to be CONCISE, not verbose.
When and where a block tolerance is not appropriate, then call out something different.
YOU did not write the Y14.5 standard. If you had, I'm sure that it would be a very different animal.
RE: Fractional dimensions
Lots of parts here either have potential interferences or unnecessarily tight tolerances because people don't think it through and just use the default 3dp our CAD system places which invokes +-.005. On an older format 3dp was +-.002, we had large invar pieces which could easily have been +-.01 or more on overalls held at .002 because the person doing the drawing didn't think about it!
Using block tolerance and checking to see if any dimensions can/need to be different is not laziness, just trying to keep a tidy drawing.
You think standard tolerance block is bad look at ISO 2768, especially the last paragraph in the notes.
KENAT, probably the least qualified checker you'll ever meet...
RE: Fractional dimensions
This was not fully applied then and most probably not applied today by CAD operators.
Anyone else recall this?
RE: Fractional dimensions
KENAT, probably the least qualified checker you'll ever meet...
RE: Fractional dimensions
RE: Fractional dimensions
Not quite. The Standard is directed towards metric dimensioning for the most part, I believe.
BRIEFLY STATED: THROW AWAY THE FRACTIONAL SCALES AND USE ONLY DECIMAL-INCH SCALES.
RE: Fractional dimensions
I agree that 14.5 doesn't apply to architectural etc. However I'd expect it does generally apply to castings which the OP is asking about. Don't know what ASME Y14.8M-1989 Castings and Forgings might say.
KENAT, probably the least qualified checker you'll ever meet...
RE: Fractional dimensions
KENAT is correct that the laziness is in the designer/drafter not paying attention to the accuracy for which he is asking. Having a tolerance on each dimension is no guarantee against this. A checker would (should) catch these mistakes. But who needs checkers....
RE: Fractional dimensions
I had some foam drawings the other day that didn't call for more than +-.03 (which we invoke with 1 dp), in fact this may have been a bit tight. The designer tried to get around it by using fractions for any dimensions that would otherwise be more dp. My preference in this case is to individiually tolerance the dims. Another option is to change the block tol but this can cause mistakes when people are used to the stanard block tol and don't look at differences.
Getting way off topic but...
In my opinion the checker should spot it the first few times a designer does it.
In future the designer should learn from this and catch most 'mistakes' themselves with the checker only catching any they miss, or the designer should be disciplined and eventually fired if they continue to produce sloppy work.
Checkers shouldn't just be catching mistakes/problems, they should be educating designers through their mark ups so that designers make less problems in future.
KENAT, probably the least qualified checker you'll ever meet...
RE: Fractional dimensions
Sorry, but tolerances have nothing to do with manufacturing.
Tolerances are a design requirement. They must be defined by the engineer.
Manufacturing processes result in variation from part to part.
The purpose of tolerances is to define how much variation from the nominal dimension can be tolerated before the part is no good.
RE: Fractional dimensions
In AutoCAD, you could design in decimal and then set your dimensions, rounding, etc. to fractional values and update the dimensions. There are probably similar capabilities in other packages. I.e., you could work in one system and convert.
Beware of mixing systems, e.g. using .31 or .32 when the real dimension is 5/16. It worked okay in pencil, but in CAD systems it causes awful stackup problems, where lines that should meet, don't, or overlap. You can waste a lot of time finding a .0125" error in a CAD model.
My advice: Get used to fractions, use them where appropriate (as has already been established by local custom)... and get used to the idea that you'll have to adopt a new set of local customs in your next job.
Speaking of your next job, you might as well memorize this now: 5/16" is pretty close to 8mm.
Mike Halloran
Pembroke Pines, FL, USA
RE: Fractional dimensions
Japanese carpenters use tapes that are divided into 33rds of a meter.
RE: Fractional dimensions
Remind me never to order you a half pint.
RE: Fractional dimensions
I basically agree with you Mint that tolerances should be driven first by function. However, manufacturabily should be kept in mind. If function appears to demand very tight tols you should start thinking if there's a way to modify the design so that looser tols are ok.
So I have no problem with block tols for different dp as long as they are used correctly.
Mike, I'm with you on the decimal/fractional modelling, I've seen it cause problems so many times. If you want you use .38 on drawing as 3/8 fine, but model .380 not .375.
KENAT, probably the least qualified checker you'll ever meet...
RE: Fractional dimensions
I agree 100 +/-.001%.
Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 09-17-07)
RE: Fractional dimensions
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...
RE: Fractional dimensions
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
RE: Fractional dimensions
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.
RE: Fractional dimensions
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Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 09-17-07)
RE: Fractional dimensions
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
RE: Fractional dimensions
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.
RE: Fractional dimensions
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 ΒΆ 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.
RE: Fractional dimensions
KENAT, probably the least qualified checker you'll ever meet...
RE: Fractional dimensions
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.
RE: Fractional dimensions
Peter Stockhausen
Pollak Switching Products
RE: Fractional dimensions
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.
RE: Fractional dimensions
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
RE: Fractional dimensions
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
RE: Fractional dimensions
KENAT, probably the least qualified checker you'll ever meet...
RE: Fractional dimensions
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
RE: Fractional dimensions
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.
RE: Fractional dimensions
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...