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Engineering type question

Posted by n9jcv 
Engineering type question
March 12, 2017 09:02PM
I am not an engineer and have no metal working experience, thus the questions.

If I were to have metal parts made by a CNC shop, at the cheapest rate, what tolerance could I expect. For example, if my file calls for a 5mm hole, what would be acceptable 4.8 -5.2mm?? Again, I have no idea of what is typical.

If designing a hole, that is supposed to have a rod go thru it, but be held tightly, like a 8mm rod and a hole, If I call for my hole to be 8.1mm is that enough of a gap to fit or do I need more clearance. Mind you I want it to fit tightly, but not so tight that I can not get the rod in the hole without a sledge hammer lol

If I have a 5mm screw, that I want to thread the hole for, should I spec the hole at 4.5mm and then tap it or spec it smaller or larger?

Thanks for any advice!!
Re: Engineering type question
March 12, 2017 11:53PM
For smaller hole they gonna use a drill bit anyway so it will be the precision of the drill bit( unless you require a really high tolerance), I dont think it will be so off as what you think, not 4.8 or 5.2 it will be 5 something if you need tolerance at only part you name the tolerance requirement in your blueprint, a normal if its not so important say 0.003- 0.005 inch will cost WAY less than 0.001 but if you need a 0.001" somewhere specified the location.

for your 8mm hole its depend of your shaft diameter and how good the tolerance is on the shaft,you can use some bearing fitting tolerance from tight to loose. But the less tolerance requirement the less is gonna cost, like your 8mm holder, maybe you can redesign it to have a loose tolerance and put a slot and a screw so when you tight the screw the hole grip the shaft. You can even but those holder already made that grip the shaft and all you have to do is screw them on your plate and this will also allow adjustment of your shaft. vs a straight hole.

Also dont forget to put some tolerance on your tap hole, if two part screw together you can some tolerance on both hole that gonna match I dont mean the diameter here, I mean the distance from edge and stuff, if you screw two plate together the one without a tap can have a large hole and less tolerance and bonus allow you to adjust later.

As for tap in aluminum you want 75% thread so a 5mm hole not gonna be 4.5 but 4.2 ish. check some chart around the web you can even use some imperial bit also , some chart recommend what drill bit to use.

There a lot of design trick you can use to avoid tight tolerance
Re: Engineering type question
March 13, 2017 12:49PM
It's up to you to supply the tolerances. With no tolerances specified how can the fabricator know what you want? Bear in mind that the tighter the tolerance, the more it will cost. e.g a hole with a loose tolerance can likely be drilled, but a hole with a tight tolerance will likely be drilled undersize then reamed to the the correct size, meaning an extra operation, hence higher cost. Engineering drawings always have dimensions and tolerances. e.g 5mm +/- 0.01 mm meaning it can be 4.99 to 5.01 mm. Or 5mm +0.02/-0.00, meaning it can be 5.00 to 5.02 mm.
Re: Engineering type question
March 14, 2017 10:53AM
This site may help for calculating fit tolerances.

For threaded holes, I use one of these sites:
Re: Engineering type question
March 14, 2017 06:14PM
Why not talk to the CNC shop!
They will give you real answers and prices??

Hey they want to make the part right
and have happy customers!

confused smiley
Re: Engineering type question
March 27, 2017 03:21AM
Hi, I am possibly a bit late to the party on this but I might be able to help for future reference.

In general you are quite safe specifying the machining tolerances given in ISO 2768 MK for any non specific tolerances, save tighter tolerances for when you need a really specific fit or you will only add unneeded cost to the parts.

Drilling tolerances are given by the standard drilling capabilities on MIL AND10387-6. This gives the hole size tolerances a normal twist drill will achieve. Tighter will require more expense, so again stick to tighter tolerances than this spec when you need a specific fit.

This site brakes down the ISO standard fits well: [www.roymech.co.uk]
(seems to be over its bandwidth allowance at the moment, perhaps try the cached version).

For thread tolerances "6g" for external threads (bolts) and "6H" for internal threads (nuts) is fairly standard and all that is needed for most applications, tighter or looser fits are only needed when you have an unusual mating component.

Regards, Simon.
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