Counterbore hole tolerances for iso 4762 screws

[ArturKwoka]

Hello All,

I want to ask you what tolerances you use for counterbore holes for iso 4762 bolts. I know that the pilot hole diameters have H12 for close fit, H13 for normal and H14 for loose fit. But how about the counterbore diameter and depth? How do you determine these tolerances? Do you specify them in a drawing?

Thanks,
Artur

 

[Scuka]

Does it matter? What happens if the counterbore is 0.1mm wider than some supposed "ideal" diameter? How about 1mm wider? How about 3mm wider? How about 10mm wider?

Same question for depth.

 

[ArturKwoka]

I would not have asked this question if it had not been important in some cases. Following your reasoning, what happens if the counterbore is 0.1mm narrower, how about 3mm narrower? Or how about 10mm narrower. It seems like it's disappearing...

 

[Jboggs]

I see so many comments/questions on here about "standards". Standards are well and good, and serve a real purpose, but sometimes the driver of tolerances should be in the application - not some standard. On the other hand, until one has enough experience to actually have some judgment on these questions, the only thing they have is "standards" or common practice. That's where you just ask the white-haired old farts in the office.

 

[Scuka]

I would not have asked this question if it had not been important in some cases

OK, and what's the importance in your case? You have told us exactly 0 information about your case. The questions I asked are meant for you to analyze your case and determine the range of acceptable values for your specific case.

Again, what happens if the hole diameter is too small? Under which size does it start creating trouble in your assembly?
What happens if the hole diameter is too large? Above which size does it start creating trouble in your assembly?
What happens if the hole depth is too large? Above which depth does it start creating trouble in your assembly?
What happens if the hole depth is too small? Under which size does it start creating trouble in your assembly?

There, those are your guidelines for determining the tolerance zone lower and upper limits.

 

[Scuka]

I mean, the dimensions of the counterbore have 0 effect on the performance of the bolt. It's all up to you and your specific situation.

If the hole is too large, the bolt won't perform any differently. Bolt doesn't even have to sit in a counterbored hole. It can just sit on a flat surface like your vulgaris hexagonal bolt. So, the upper diameter of your counterbore tolerance equals infinity as far as the bolt performance is concerned.
If the hole is too small, obviously, the bolt won't fit into it, so there's your absolute lowest limit. Also, if the hole is too small in relation to the positional tolerance of the mating holes, then the bolt and the holes might not align, so if you need a specially tight hole for whatever reason, you need to do some tolerance stack up analysis for your specific case.

If the hole is not deep enough, the bolt head will protrude over the surface of the part. If the hole is too deep, the bolt head will sink deeper below the surface of the part. Neither of those affect the performance of the bolt. It's all up to you and your specific use case. Do you want your bolt to be flush with the surface? Do you want it slightly below? Do you want it slightly above?

 

[MintJulep]

In addition to Scuka's guidance it should be apparent that the counterbore diameter should never be smaller than the maximum screw head diameter, and never shallower than the screw head maximum height.

The cutter will never get bigger than new, so cut max diameter is controlled by the tool's maker and the runout of the machine it's used in. Cut min diameter is controlled by tool wear.

Variation of depth will be determined by the condition of the machine.

 

[Scuka]

never shallower than the screw head maximum height

It may be if you don't care about the bolt head being fully submerged.

 

[Tmoose]

I'd be concerned about the radius/corner at the junction of the c-bore Ø its flat bottom VS the chamfer on head of the SHCS.
Also the actual position of all the tapped holes vs locations of the clearance holeS AND the counterbore diameterS.

A "larger" C-bore diameter, well positioned relative to its clearance hole might might gracefully sidestep some assembly issues for free.

A saying amongst hot rod engine builders is-
If you have a little too much clearance only you will know, but if you don't have quite enough clearance everybody's going to know.

I seem to recall C F Taylor, Professor Emeritus of Automotive Engineering at MIT, said something similar in " Internal Combustion Engine in Theory and Practice."

 

[drawoh]

ArturKwoka,

What are you trying to accomplish?

If you are fabricating two separate parts, your holes must line up accurately enough that the screws will pass through them. The keepout diameter for a bolt is the diameter of the bolt. The minimum size of your clearance hole must be the bolt diameter plus the positional tolerance. If your bolt is in a tapped hole, the keepout diameter is bolt diameter plus the positional tolerance of the tapped hole. Your minimum sized clearance hole must be that keepout plus its own positional tolerance. This is not a good job for ISO clearance holes.

You should be able to work out your positional tolerances and clearances from first principles. If there is a specific formal requirement for your holes, you need to read that requirement.

--
JHG