MMC mod on countersunk hole position 1

[MintJulep]

I recently reviewed a drawing that used the MMC modifier on the position of several countersunk holes.

It seems to me that the corresponding fastner will tend to self-center in the countersink, thus rendering the MMC modifier usless. It just creates more clearance around the fastener shank, but the assembly can't take advantage of the increased hole size because of the centering effect.

Am I missing something?

 

[btrueblood]

I think you are right in general. I've always assumed that the position tolerance applies to the through hole, and c'sinks or c'bores just go along for the ride (i.e. they are made using the thru hole as the pilot for a countersink or counterbore tool). Though, if I really cared, I'd dimension each part of the hole seperately, each with its own gdt block, or least no gdt block for the c'sink, just for the thru hole.

The point being that the designer probably does care about clearance and fit for the fastener and thru-bore. Depending on how flexible the fastener is, & how many other fasteners are being tightened "simultaneously", the head may or may not "center" on the csink, it may (and usually will) only contact along one side.

 

[ewh]

I have to agree with btrueblood about the fastener contacting on one side. While it may want to self center into the countersink, the mating hole is what is going to drive it's position. I would think that to minimize this type of contact, the feature would be RFS.

As an aside, unless separated out at the hole callout, both the hole and countersink (or counterbore) default to the same geometric control block.

 

[MintJulep]

In this application the mating hole is a clearance hole, with a nut on the far side. Should have noted that originally.

While I agree that contact will not be uniform, I still don't see that MMC gains anything. At some point there will be thru hole clearance that is not usable by the fastener.

Seems like the countersunk holes should be RFS, and the holes in the mating part MMC.

 

[btrueblood]

Umm. Sorry about long-winded, off-topic reply. Yes, MJ, I agree that the MMC is un-needed for the c'sink, in any practical fastener application.

 

[MintJulep]

btrue,

No need for an appology. It's been a long time since I actually designed parts for a living, and when I did the companies I worked for didn't make use of GD&T principles, so I'm not as up-to-speed with the concepts as I would like to be.

I hope this forum becomes an active one, seems like a lot to learn here.

 

[TheTick]

Generally, using countersink turns the fastener into a fixed fastener instead of a floating fastener. As a result, there is no MMC leeway on the component with the countersink. As I recall, the net effect is that it cuts your allowable tolerance on the other components in half (MMC still can apply).

 

[ewh]

Tick,

Good explanation. That deserves a star!

 

[Tunalover]

I disagree with TheTick. A flat head screw can float in its countersink in a fixed fastener case OR floating fastener case. The fit formulas both still apply to sizing the clearance hole. The MMC countersink diameter is then determined by adding the MMC across-sharps screwhead diameter to the positional tolerances of both holes!


Tunalover