Tolerances. 2

[cadastrophe]

Sorry if this is enquiry is below this forum, but I am a keen-to-learn amateur, and new to all this.
I have searched for references and tables of interference and assembly tolerences. Elsewhere, someone pointed out the Machinery's Handbook. Unfortunately I do not have one, nor can justify the cost of one.
Can anyone here help please?

 

[testy]

Try this website

http://www.engineersedge.com

 

[apjdjd]

I would like to know the best way to dimension and tolerance a bore on a casting. The bore needs to be concentric to the casting as well as parallel to it.
The reason I am asking is that I was told, according to some casters, that there are may ways to specify this.
Thanks.

 

[gearguru]

Learn about GD&T; that's the way how to do it.
gearguru

 

[rangi]

Cadastrophe,
Its ok we were all there once,I myself find tolerances a bit hard to tolerate(pardon the pun)myself
but i bought a book by AW Boundy called Engineering Drawing 6 and it has all the tolerances in it.Explains what they are and offers detailed descriptions on what they are used for ,also what type of fit you can expect.
It is a BIBLE for any enthusiast or drafter.
Good luck

 

[funnelguy]

Cadastrophe,et al. Tolerances and finishes for certain components are available through the net, distributors or textbooks. These components generally include fasteners, bearings and seals.

Interference and running fits and such require a bit more consideration, and geometric tolerancing can push a lot of cost into a product by constraining the producer to tighter tolerances or more expensive processes than the product's utility requires.

Hobbyists will be content to putter about or polish to achieve the desired tolerance and/or finish. (No disrespect intended, most professionals have a bit of hobbist in them! )

Professionals do not have this luxury. My advice would be to get out from behind the desk/terminal and spend time out on the shop floor. Experienced machinists and foremen will be your greatest allies.

 

[knobhead]

I think what might have been referred to is tolerance frames. They are standard symbols referring to different charactiristics (runout, concentricity, roundness and so on) with a defined way to intepret them.

This page

http://www.cybermetrics.com/GDTFont/GDT_OVR.HTM

will give you a bit of a guide.

BTW, if your serious about your hobby, buy a copy of Machinery's. It is expensive, but it's the only book you will ever need. It will tell you EVERYTHING, and I still refer to it most days.





Excessive accuaracy is a sign of poor breeding. -Socrates.

 

[falcon4]

Proper Geometric Dimensioning and Tolerancing (GDT) does not drive a lot of cost into an object. It should drive the required amount of cost into it. It will allow you to define more accurately the tolerances you need to make your parts fit and perform at the least cost. What probably drives costs up is the inability of certain manufacturers to understand what the GDT feature control frames are telling them.

Falcon4

 

[Tombstone]

I agree GD&T are useful and if properly used shouldn't add more cost than would have been built into the design anyway. The thing is they can also be used with wider tolerances to specify the same thing since the tolerancing required is going to vary for different parts anyway. Close work is close work, no matter who does it, but one type of tolerancing doesn't cost more than others if used properly and properly understood by the machinist.

 

[funnelguy]

Falcon and Tombstone are correct, of course. I thought one thing, and typed another and did not make my point. My overall point was that thought and experience should be applied to every feature and tolerance. One of the best sources for experience are senior engineering peers and shop personnel. Actually, GD&T has caused some people to review their practices, always a good thing. (Projected tolerance zones, in particular, have had that effect.)

What is not good, and is all too easy with CAD, is to stumble along applying the same tolerance for any situation, no matter what. The largest tolerances that will assure functionality are the correct ones. Quite often, the less experienced designer/draftsman/engineer will over-constrain to cover their tracks when they are unsure.

I get prints constantly that are not properly dimensioned or toleranced. The people making these things cannot be bothered with understanding the part's functionality. Much of that understanding will come from experience and cross-pollinization. By the way, this situation is nothing new. I've been in manufacturing for 20+ years and not much has changed.

 

[funnelguy]

I went back and re-read the post. Cadastrophe and apjdjd are both newcomers looking for information on tolerances and functionality. Working with castings, in particular, requires experience. Cadastrophe, being an amatuer (hobbyist) probably has limited access to experienced people. Ask around. Try to befriend someone local to you who can provide insight. Experience is what you need.