Counterbore series spec... 1

[1x4x9]

I am looking for the spec sheet that calls out counterbores as a series, ie: 300 series cbore for #4 capscrew. I can remember from years ago something about it being .015 over the max of the head diameter. Is this right? Where can I find this actual specification?

 

[KENAT]

I would think it was based on drill size rather than just a nominal clearance.

We had a copy of such a chart but it was a bit unclear and had some mistakes. We made our own version of it for inch fasteners and released it as a company document. It also has suggested positional tolerance and hole tolerances etc.

I think there's something similar in Machinerys Handbook.

 

[eromlignod]

Counterbores are nuts. There are all sorts of versions and a given machine shop probably won't have all of them. There was a standard for them before 1960, then when the capscrew standard changed, they changed the counterbores too. But the tooling manufacturers went on making the old series as well. I remember seeing the pre-1960 counterbores being available in a machinists tool catalog as little as a few years ago.

If you look in the McMonkey-Carr catalog, you can also get "oversized" c'bores that are sized both 1/64" over and 1/32" over the "standard" size. Oddly, none of these sizes matches the standard 1960 size printed in my Machinery's Handbook.

Unless the c'bore is for some sort of critical application, I simply mark the drawing as "DRILL .136 DIA. & C'BORE FOR #4 SHCS" and maybe include a depth, if it is deeper than flush. That way I get the through hole clearance I want and the machinist can use whatever counterbore he has. It's just a counterbored hole after all.

Don
Kansas City

 

[KENAT]

I found a copy of the sheet we used to use, but I'm not sure it's what you were looking for. At the top it just says "FASTENER CHART CODE 26403 September 1980".

I just checked an old (21st ed) copy of Machinery’s and it has some information.

I'd be careful taking the approach Don suggests. You start to lose control over the item, this is especially an issue if you use positional tolerance GD&T on your hole patterns. For this to work your hole sizes need to be defined not left up to the machinist to make whatever he feels like/has handy. In fact we discovered that the size C’bore given on the above chart I mention didn’t actually cover the full tolerance stack up so we had to enlarge it slightly.

 

[1x4x9]

I would like to thank all that have answered so far. Let me clarify my question a bit further. I have an actual print from a customer and it denotes the hole as: 300 series c'bore for a #4 capscrew. This may not help gather any more information but I thought I would clear up the fact that I am not designing this feature. I am required to manufacture it. I found c'bore specs in the machinists hand book. It is alot larger than what they expect. Like I stated earlier, I thought it has something to do with .015 over the max head dia. As usual, the customer has no knowledge of this spec. or where it came from. But it is somehow on thier print. Thanks again,
Craig

 

[eromlignod]

Counterbores have nothing to do with positional tolerance. They are simply clearance for a screw head. The *through* holes are the more critical feature and are dimensioned and sized accordingly. They are produced in a separate operation from counterboring. And critical fits should be handled by dowels anyway, which also have nothing to do with the counterbore.

A counterbore is a tool with a pilot tip that loosely fits the through hole (which has already been produced by another method) and guides the tool. Farther up the shaft it has larger-diameter cutting blades that produce the counterbored portion of the hole. The pilot of virtually every c'bore tool I have seen is sized to the nominal screw diameter. In other words, a c'bore for a 1/4" screw has a 1/4" dia. pilot and the fit in the through hole is the same as the screw that will eventually go there.

A capscrew counterbore should have absolutely nothing to do with the location of the mating parts. It is clearance for the head of a screw and is arbitrary. In fact it can often be quite large: it is very common to include a large c'bore, for example, if the screw is a hex head, so that a socket wrench can be used. If the through hole has a clearance of only .010", then the c'bore can be 6" dia and the hole must still locate to the .010".

Don
Kansas City

 

[KENAT]

1x4x9 You say the C'bore in machinery's was bigger than the customer expected, does the customer have any old parts for you to inspect to try and establish the size they expect? Or, and I hesitate to say this, is there any chance of measuring off/scaling off the print to get some idea what 'standard' Cbore it may match? I take it when you enquired they didn't come back with an actual size from the fact you're asking the question.

Don, the point I was trying to make was that we had a supposedly 'standard' table that gave c'bore sizes for cap screws and we'd been using it for a good while. I can't remember why now but some kind of problem came up and one of the guys did the math and discovered that the size of c'bore suggested wasn't big enough to accomodate the possible tolerance stack up under certain conditions. You could get a situation where the edge of the screw head was bearing on the edge of the C'bore as I recall (it was a while back so I'm a bit fuzzy). If by putting your note a machinist used a similar table (which I believe corresponded to Cbores) then you could get the same problem.

 

[1x4x9]

Kenat, The piece I have from the customer has a c'bore of +.015 from the max of the screw head. I am planning on producing it to that number. I just like to have actual spec. call outs for my reference, and for final inspection reports. I will make note of it to them, and have a first article approved by them. Thanks everyone for the advice.
~Craig~

 

[dimjim]

I think there are two types of counterbore sizes. One for
socket head type capscrews which would only have a 1/16th
larger diameter than the head or washer diameter required for the capscrews. There are cases where oversize washers are required to accomodate the bearing stresses under the
washers. I think it would be a very poor design standard to not specifically call out the c'bore size as well as the
maximum bottom radius.

The second would be for hexhead screws which would require a diameter greater than the socket wrench outside diameter.

 

[Philrock]

If your customer can't tell you what the spec on their drawing means, then they have no basis for rejecting anything you deliver that accepts the screw head. But that doesn't mean they can't argue about it.

Find something reasonable in Machinery's Handbook, and get the customer to sign off on it before making the parts.

 

[1x4x9]

Philrock, thats what I ended up doing. they are going to modify the callout now. Thanks.

 

[joebk]

ASME B18.3-2003 has a table of standard c'bore and clearance hole sizes. This standard lists the c'bore diameter for a #4 SHCS as 7/32". It also lists the max head diameter of a #4 SHCS as .183". So radial clearance would be approximately .018". Of course this assumes the SHCS adhere to the ASME standard!

I do agree with Philrock, better safe than sorry!

JBK PE (and all around pain in the butt)