Which Reamers To Buy?

Paddy OFernichur said:
Just kidding on that one, really.

However, in all seriousness I have had some parts running on CNC screw machines where nailing down the hole size was troublesome, SOme L&I reamers cut oversize, some under, and I often found myself trying several sizes up & down to get one cutting the diameter where I needed it. This was always in applications requiring tolerances tighter than .0004" total variation.
Click to expand...

Here's a way to add or subtract a couple of tenths.
take the shank of a drill or a piece of drill rod tha just fits the goove of the reamer. hold it perpandicular to the edge and firmly draw it from back to front. Once for every land.
This raises the wire edge and buys a couple ot tenths. You can also decrease the edge by doing the same thing on the outside edge, just keep the bunisher parallel to the cutting edge.
This works for straight shank reamers only.
in an emergency you can reduce the diameter of a reamer a couple of thousanths or more by drilling a hole in a piece of hardwood the same diameter of the reamer. Split the hard wood to bisect the hole and coat the hole with lapping compound. Chuck the reamer in the lathe and run it in reverse for a RH cutting reamer. Run the loaded block up and down the cuttung edge a few times an mic it to check size.

Remember. The finish on the reamer is the finish in the hole.
 
Tony,
As a general rule - how much material do you leave for a reamer to remove? How far under-size do you shoot for in boring the hole?

Inquiring minds.... :D
 
No problem, Mueller, I do it all the time. Good idea about lapping it with a split hardwood, because a stone can bounce a bit on the flute edges.

HP, that's a loaded question. Depends on the material, and the hole size, and a few other things. As an example, in unhardened tool steel, a 0.5000 +0.0000/-0.0002 finished hole, I would leave 0.012 or so. Reason you need to leave a little is that the surface of a drilled hole in particular are work hardened and inconsistent. You don't want the cut to be in that zone. As much as .020 you might be OK, but more than that can create chip problems. Remember, the reamer cuts primarily on the end, where the beveled nose edges are. Obviously, that itself limits the amount that can be cut by the reamer, but you'd never want to leave that much anyway.

I've seen some guys radius the corners of the flutes in an effort to improve finish, but I'm no fan of that, myself.

Generally, softer materials leave a little more, harder materials a little less.
 
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