[gmenzies]

I have a few blind holes that I'm looking to drill out with flat bottoms.

Can anyone tell me what the rule of thumb is for determining the drill size prior to reaming.

I.e.

6.5mm hole reamed.

What drill bit should I use?

Additionally, I'm looking to ream 12.5 and 14.5 mm.

Thanks for any help.

Here is what I do.

If I am using a manual mill or radial arm drill. I never move the table or head until I finish spot drill/drill/ream each hole
On the cnc we have a hard coded all drill operations to feed to the hole position from the same direction. eliminates any backlash issue

spot drill first ( new tool) use dwell three revs
Drill hole peck with dwell ( 2 to 4 % smaller ) always use a new drill. -- no hand sharpened tools allowed
Ream Peck feed depth at least 2 to 4 X diameter ( coolant only)

stick to the feeds and speeds of the material.
carbide reamers follow same rules

if a hole requires a extremely tight tolerance we will usually hand lap or wire EDM

If your really concerned. test on a scrap piece.

when reaming 1/32 and 1/16 we order coated reamers and drills from vendor

one last thing -- use collets-- No drill chuck

Cheers Gord

[Superman]

hmmm... not bad... back to life after 11.5 years after 1st post.