Reamed hole finish and tolerance & suggested callout 1

[johnchrc]

I need a .375 +.003/-.000" 32RMS hole in 4130 22HRC part. I'm unsure about capabilities and callout responsibility.

1) Should I just callout hole size and finish and let manufacturing determine the reamer and ream tap size? OR
2) Should I specify tap drill size and call for reamer?

3) is 32 RMS a problem to achieve reaming?
4) Are there std reamer sizes & tolerance limits?

I have attached drawing of the reamed hole and call out.

 

[BrianE22]

Let the manufacturer figure out the tap drill size and drill hole size for the reamer. Just put your tolerances on the drawing.

A 32 micro finish is about the best you could do with a reamer. The machinist might need to hone the hole a bit to get to the 32 micro.

For that size range you can get standard reamers in .0005" steps (see

http://www.McMasterCarr.com).

The tolerance would of course be less than .0005". The hole tolerance is actually what you should be concerned with. A precise reamer can produce an imprecise hole if the machinist isn't careful. Again, just call out the tolerance you need and let the machinist figure out how to get there.

Super precise holes (+/-.0003 or so) would require jig grinding which is a step up in cost.

 

[ornerynorsk]

The hole dimension and surface finish callout is sufficient by itself. A capable shop does not need to have the manufacturing method spelled out. It is not uncommon to call out a tap drill size.

Reamers can be bought in nearly ANY size.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[unclesyd]

I agree with the above post concerning the callout and letting the shop set the process though I think you might want to look at drilling and broaching to meet your requirements. We have made thousands of holes in your size range using this method without any failure to meet specifications. The secret was good drill bits and sharp broaches.
One very critical application was to produce 33,000 holes in a 2" thick steel (4140, Rc 28) cylinder, we completed three drums without having to rework any holes.

http://www.dumont.com/broaches/round/