Dimensioning Question

[EdK]

Looking at dowel pins on the McMaster web site, they list a dimension for a 1/4" dowel as:

1/4" +.0001/+.0003

What does this mean?  

Ed

[dallen]

I'd take it to mean that the pins can be from .0001 to .0003 bigger then .250

[f350ca]

Think it means they don't know how to write tolerances
Or they're proof reader isn't Steve.

[rleete]

It's the tolerance. It can be larger (the +) by up to .0003, but might be only as large as .0001 more. So, the dowel will come to you at .2501 (smallest) to .2503 (largest) or anywhere in between.

This dowel is meant to be tightly fitting (I would classify this as a press fit) in an accurately drilled 1/4" hole.

[EdK]

(12-09-2014, 12:59 PM)rleete Wrote: It's the tolerance.  It can be larger (the +) by up to .0003, but might be only as large as .0001 more.  So, the dowel will come to you at .2501 (smallest) to .2503 (largest) or anywhere in between.  

This dowel is meant to be tightly fitting (I would classify this as a press fit) in an accurately drilled 1/4" hole.

OK, that kind of makes some sense.

Ed

[TomG]

Dowel pins are always 2-3 tenths oversize so they fit snugly in a nominally sized hole. In reality though, you would ream the hole undersize for a good press fit, or oversize for a slip fit.

Tom

[rleete]

(12-09-2014, 06:58 PM)TomG Wrote: Dowel pins are always 2-3 tenths oversize so they fit snugly in a nominally sized hole. In reality though, you would ream the hole undersize for a good press fit, or oversize for a slip fit.

Tom

Agreed, but I didn't want to cloud the issue.

I worked as a contractor for years.  Some jobs lasted weeks, others months. Some (most?) places wanted you to specify the diameter and tolerance of holes.  At the place I work currently, common practice is to specify the fit I want, and let the machinist determine the tolerances.  Thus, the hole note reads, "light press fit for .250 dowel pin"  They may drill, ream or swage the hole as they desire, as long as I get my fit.  It works.

[PixMan]

Here's one example of where the "inch" (ANSI) system has it all over the metric (ISO) system.

I much prefer this method of writing the tolerance right there with the dimension. With the ISO shafts and holes tolerance system, you get a letter and numerical character pair for each, and it's hard to remember upper or lower case, which (shaft or hole) gets which, and what value is assigned to each. If you want to know the size, you have to go to a chart. As the range of diameter of the hole or shaft changes, the value of that alphanumeric character set changes too. How ingenious.

Just put the dimension you need with the high and low limit right on the print please.

[DaveH]

Not only that I think the 'thou' is a nice unit to work with. Even though my machines are duel (mm & inch) I mainly work in thou's.
And for good reason, my compound slide dial is  1div= 1 thou, and metric 1div =0.03mm.
Who can count in 3's ................... goodness me! One has to admit that is silly.

   


DaveH

[Mayhem]

(12-10-2014, 06:50 AM)PixMan Wrote: ...With the ISO shafts and holes tolerance system, you get a letter and numerical character pair for each, and it's hard to remember upper or lower case, which (shaft or hole) gets which, and what value is assigned to each. If you want to know the size, you have to go to a chart. As the range of diameter of the hole or shaft changes, the value of that alphanumeric character set changes too. How ingenious...

That and you run the risk of misreading the tolerance (7h) as the actual hole size

(12-10-2014, 07:24 AM)DaveH Wrote: ...metric 1div =0.03mm.
Who can count in 3's ................... goodness me! One has to admit that is silly.

That is silly Dave. My friend's little lathe that (conveniently) resides in my shop has .001" and .02mm graduations.

Tolerance is still something that I am not fully comfortable with. I must admit that when I watched Tom's latest videos it seemed very strange to hear him say a part was -0.000 and +0.003 and so he was aiming for something in the middle. Even stranger to hear him say that if the dimension was only to two decimal places he wouldn't even bother to take a final measurement.

Of course, having him explain why made a lot of sense but it just seems strange when I have been trying really hard to nail a dimension exactly, when in hindsight, I probably didn't need to worry.

I'd still like to see an actual video on it, as I think reading about it doesn't really do it for me on such topics.