The Machinery Handbook lists the recommended clearance for an M6 bolt as 6.4 mm for a close fit, 6.6 for the preferred fit, and 7.0 for a loose fit.

Remember that the tighter your tolerance, the more expensive and difficult to assemble the whole thing will be, for a variety of reasons.

As a not-so-entry-level-engineer who was where you are 4 years ago, my advice to you if you're going to be doing any sort of machinery work is to get a copy of "The Machinery Handbook".  It'll set you back 75 bucks or so, but I guarantee you that it will pay for itself.  EVERYTHING is in there.

"Machinery's Handbook" is the name of it.  I'm a chimp.

It's in it's 27th edition right now, so you'll definitely be able to spot it if you see it.

I've got the large-print edition, so I can definitely spot it!

Software For Metalworking
http://mrainey.freeservers.com

It also depends on the type of head on the shoulder bolt.  A hexagonal head will require more room since a socket will likely have to fit around it, while an Allen type bolt will not require that extra clearance.

-Reidh

To do it properly isn't necessarily as quick as you may think.

While it seems a simple application you either need to do the math or use a table that was driven by the math.

Machineries is a good start, assuming you get to dimension both parts.  If one part is dimensioned/toleranced outside of your control then you probably have to do the math.

To find the counterbore diameter you need to take into account:

Is is fixed (mating part has thread in it) or floating (nut & bolt thru holes in both parts) application.

Diameter of the head of the bolt/screw.

Coaxiality of the head of the bolt/screw to the shaft.

Coaxiality of the counterbore to the through hole.

Reidh case has even more to consider.

If you're in the US take a look at Appendix B of ASME Y14.5M-1994 it gives some calculations.  There are a couple of websites that do it for you but it would be good practice to work it out yourself a few times.

http://www.mechsigma.com/newsletters/2003_05_Newsletter.pdf
http://books.google.com/books?id=2FodPhcpupcC&pg=PA252&;lpg=PA252&dq=floating+fastener+case&source=web&ots=gQdRPLb3ud&sig=1Z0LBy-uluyX9DNTfRnl1OTvegg#PPA253,M1

May have been posted before over on GD&T, for instance thread1103-110414: Fixed and Floating Fastener Fit Formulas thread1103-125608: Floating Fastener Formula

KENAT, probably the least qualified checker you'll ever meet...

And you are asking about clearance or fit, not tolerance.

Since you are just starting, learn the difference between "tolerance", "variation", "deviation", "clearance", and "fit", and use the terms properly.

In this website you can find any screw and needed bore data.

http://www.celofasteners.com/enwebcelofasteners/enproductoFasteners.html

Thank you very much for all the help everyone.  Looks like I could learn a lot from all of you.  I think I'm going to be spending a lot of time in this forum, heh.  

So I was looking in McMaster, and the M6 screws have a shoulder diameter of either 8 or 10mm.  I was asking about clearance for the shoulder.  Just to be clear, this is for mating two cylindrical parts (pipe and machined insert) coaxialy with a few shoulder bolts along outside.  The clearance I'm concerned with is for pipe and insert (not sure if it's the same for both).

In the UK a shoulder screws looks like this http://www.ondrives.com/products.asp?subcatid=91 and depending on the application the clearance will be different to a normal screw or bolt.

ajack1. those are the type of screws i'm talking about.  where would I go to find out the clearance for the shoulder (I want a sliding fit) to design the hole to the correct dimensions?  I assume there are some standards or a rule of thumb i need to apply here for different fits.

They are already slightly down on nominal, I believe the shafts are e9 so a “standard” reamed hole will give a sliding fit, or for more accuracy look at the shafts and holes data to give you what you want. You will get much better results if you also spot face around the screw, rather than just relying on the thread to position the shoulder screw.

Appologies Bruno, I misunderstood your original post.  I misunderstood the way you were using counterbore on you OP.

KENAT, probably the least qualified checker you'll ever meet...

Don't forget these mundane and boring items.

What clearance does the application require? Possibly a very close fit, or maybe not.

What are the capabilities of the machine shop. Talk to the shop and give them the reasons for any tolerances you require. Always be able to justify every drawing tolerance from a functional standpoint, avoid drawing block tolerances, and come to an agreement with the shop before hand.

Believe me it took me years to smarten up and get off of my high horse. The engineer needs to partner with the shop; it is not an adversarial role.

Paul Ostand
www.ostand.com

thanks Paul.  

I actually started off my engineering career in a manufacturing/tooling company that required it's engineers to start off in the shop.  I worked with the tool-makers in the shop for about 6 months, and I think it was very helpful in the reagrd you are talking about.

BrunoA

That was a very good policy. I would have avoided some errors years ago, if I had done the same.

Best wishes

Paul Ostand
www.ostand.com