Counterbore series spec...
Counterbore series spec...
(OP)
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?





RE: Counterbore series spec...
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.
RE: Counterbore series spec...
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
RE: Counterbore series spec...
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.
RE: Counterbore series spec...
Craig
RE: Counterbore series spec...
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
RE: Counterbore series spec...
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.
RE: Counterbore series spec...
~Craig~
RE: Counterbore series spec...
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.
RE: Counterbore series spec...
Find something reasonable in Machinery's Handbook, and get the customer to sign off on it before making the parts.
RE: Counterbore series spec...
RE: Counterbore series spec...
I do agree with Philrock, better safe than sorry!
JBK PE (and all around pain in the butt)