Drafting Tolerances

[Rob 88"]

Evening folks,
I'm struggling to get my head around parts of one of my latest Uni assignments...I've to draft various components and designate different fits. For example a slight interference fit, hole H7 and the shaft K7 (nominal size 50mm). Following the calculations and establishing the max and min, can I simply model and draft the mean dimension of each component and note +/- of each dimension ? i.e Hole max 50.025 min 50.000 therefore cad model the hole as 50.015 and draft as +/- 0.015mm. Is this the typical engineering procedure of dimensioning an accurate component or am I completely off track ? Cheers

 

[Spark plug]

I would normally expec to see a tolerance like that expressed as 50.000 +0.025/-0.000.

Be carefull when useing "H7" type tolerances as H and h have different meanings depending if your toleranching a hole or a shaft.

 

[Spark plug]

Try this tolerance calculator might help you out:

http://www.amesweb.info/FitTolerance/FitTolerance.aspx

 

[Sidewinder]

I would tolerance it as for example 25mm h7 or 25mm H7 depending on what it was these days TBH.

 

[Sidewinder]

OP, I think there is a typo in your first post I suspect is should be H7 & k7, or h7 & K7?
Which is it?

This is one of the reasons I get a bee in my bonnet about case and correct terminology.
Terminology should be correct, this promotes correct understanding, and, does not dumb down the "trades".

 

[cr500dom]

Firstly Draft is the angular slope applied to surfaces when Casting or moulding to allow it to release from the mold.
In terms of CAD terminology, you model the part and then Detail the drawing (Not Draft)
What CAD package are you using ?
Many of them will allow you to Specify a Tolerance and a numerical value.

Google "limits and Fits" as a first step

 

[cr500dom]

This is a solidworks Dimension modifier showing Fit+Tolerance if needed

 

[nodeward]

to be honest what matters is what is on the drawring but as you say good practice is to 'model to mean'. i like to dimension how i would like the dimensions to be if i was making the part, so i would dimension it as 50mm-50.025mm. There is not reason why you cant dimension it +-0.15 or just h7 some companies will have a preference some wont having the correct tolerance is what matters

 

[Rocketman]

When 3D modelling or 2D drafting you model say the 50mm hole/shaft and add the tolerance when detailing out in the dimension.

So say you have a 20mm hole that needs a H7 reamed fit you 3D model/2D detail a 20mm hole and then call up a H7 fit on the dimension.

Depending on who you are working with a simple a dimension of - diameter 20.00 H7 - should be adequate unless you are working with idiots and then you need to put the limits on it.

 

[cr500dom]

The only time I would really deviate from that is if you are working with a supplier that will cut direct from cad data, and you know what tolerance they can hold on their CNC Machine, then I would model at Mid Tolerance, but you have to have a good working relationship with the machine shop for that to work without biting you on the Ar$e......

But in the right situation, working together, it can save you a lot of time

 

[Rocketman]

CR500dom TBH they should be doing that within the CAM software really because machines vary so much and it is so easy to do in the CAM software.

Drawings no days are becoming more of just a reference for thing like limits and fits.

 

[cr500dom]

I agree, as we used to put on all our drawings at one Motor Manufacturer "Printed copies are uncontrolled - CAD Model is master"