Drawing Dimensions

[Chuck in E. TN]

I know this has been posted before, but I can’t remember where.
I’m studying plans for my next engine project, one of Elmer’s’ Wobblers. The drawings have dimensions like Ø3H7, or Ø12P7. I forgot to save a link or print the page that explained these dimentions. Can anyone help?   
Chuck in E. TN

[Bernd]

Chuck

I have Elmer's book. Which engine is it. If you don't want the public to know PM me.

Bernd

[PeterE]

Hi,

The dimensions are shown with tolerance.

The first characters, the diameter sign and the number states the diameter, f ex dia 3 mm. The next part (H7) states the tolerance of the hole or axle.

If I remeber my mechanics correctly, the a captial letter "H" states the hole, and a minor letter "h" states the axle.  The numbers following tells how large difference from "0" to be used. 7 is an ordinary sliding fit without any shake.

A far better explanation and illustration is found on this wiki page:
http://en.wikipedia.org/wiki/Engineering_tolerance

Hope it helps.

BR

/Peter

[Lew_Merrick_PE]

Chuck,

http://www.roymech.co.uk/Useful_Tables/ISO_Tolerances/ISO_LIMITS.htm has posted links for both hole and shaft size values for each class of fit.  If you think of it as being the old ANSI/SAE fit class (Shrink Fit, Press Fit, Transitional Fit, Locational Fit, Sliding Fit, Running Fit, or Clearance Fit) tolerance, it will make a bit more sense.  The problem is (as always) that such a tabulated approach is fine for non-interference fits at moderate temperatures, but not for interference fits at moderate temperatures for non-basic steels or for any application out of the nominal 20°C ± 20°C range.

If I seem a bit anal retentive about this, I am.  I spent four months in the mid-1970's compiling a proposal for a modification of the ANSI/SAE fit standards as part of an ISO simplification effort.  Instead, the committee decided to continue using the data that was compiled for the Spanish-American War and WWI as the basis for the "standard."

[Chuck in E. TN]

Bernd, It's Elmer's #25.

Chuck in E. TN

[Xldevil]

The Letters H and P stand for tolerances,as said before.
A very good explanation on this side:
http://www.mitcalc.com/doc/tolerances/help/en/tolerancestxt.htm

To get a 3H7 boring,you have to use a 3H7 reamer.
Cheers,Ralph

[mklotz]

There's something amiss here.  I've never seen any Elmer's plans with dimensions in millimeters.  I have his book and the plans for #25 are mostly specified as (ugh!) fractions.  No tolerances (beyond the non-numerical close and smooth fits) are specified.

Are you certain you're looking at an Elmer's plan?

[lordedmond]

just had a look at Jon toms site and they are in imp fractions with one or two letters after certain parts but the letters are explained at the foot of the drawings   e.g. S = smooth , alas none of my favourite MM to be seen



Stuart

[Chuck in E. TN]

My bad! Confused the drawing of an Elmer's with one from the Netherlands.

It is all metric with  dimention callouts like I posted before. So, the dimention Ø3H7 is 3 mm dia. reamed to H7, correct?

Chuck in E. TN

[Bernd]

I guess Marv cleared that one up. I was ask the same questions.

Problem solved. 

Bernd

[Lew_Merrick_PE]

It is all metric with  dimention callouts like I posted before. So, the dimention Ø3H7 is 3 mm dia. reamed to H7, correct?

No, ø3H7 = ø3 +0.01/-0 mm.  In inches, the dimension would read: ø.1181 +.0004/-.0000.  A 7 class of fit for an ø3 part gives a 0.01 mm tolerance band (IT) and a minimum diameter of 3 (mm).  Yes, you can probably accomplish that with a reamer, but the designation does not specify how the hole is to be made.