Need some recommendations how to drill a flat bottom hole on a lathe.  I have a .375" x 1.75" hole.  I peck with a 118 degree twist drill to 1.75.  So far have tried using a .375 2 flute end mill pecked from 1.6" depth to get the flat bottom.  The end mill fills too quickly and the spindle over torques and shuts down.  The end mill is not creating chips but rather dust.  Last effort was  s700 f.25 peck of .05

What approach should I take instead?

TIA

RT

Hi RT,
I have used with my high speed spindle which is way too fast and not enuf torque for a drill so used a smallish diameter endmill with circular interpolation.

The hole you are drilling is quite deep so the biggest diameter endmill you can use is going to be required otherwise it will flex to hell. A long 1/4" endmill
might do the trick.

Craig

Seems like a sound plan if I was using a mill and not a lathe ;^)

Hi,
bugger, sorry about that. How about grinding spare drill with a flat instead of a point. I have done it before, it squares the outer diameter of the hole quite well
but I've never achieved a center cutting grind.

The most recent application was to square the bottom of a hole to accommodate the head of a capscrew with a smaller diameter through hole for the screw, ie
I didn't need center cutting.

Craig

that was my first thought but I had the end mill and it seems as they both have two flutes but of different geometry.  Is there really that much difference?

Can you remove most of the material with a little smaller and shallower drill then finish with the end mill?

Think I remember Hood being very fond of what he called UDrills. Might be an option, I'm not sure.

https://www.sandvik.coromant.com/en-gb/products/corodrill_880

I would drill out most of the junk, then bung an end-mill in the drill chuck and run that down to depth, works for me in the past.

Here is what I do and use in both the lathe and the mill.

- initially use a 135 degree drill and not a 118. Less material to remove on the bottom.
- I  prefer to  use a 4 flute center cutting end mill. Make sure the hole is filled with cutting fluid.
  Slow feed and speed to avoid chatter. Note that an end mill will not make the hole bottom truely flat as the end
  mill face has a clearance angle and which tapers towards the center of it.

- But usually just use one of my home  made drill bits to create flat bottoms in holes. You still need a taper along the cutting
  face, but it tapers outward from the center and the taper is approx only 1 degree. Much better than using an end mill.
  BTW, I made a complete set of drills in 1/64 th increments, ground them myself using a Darex drill holding attachment, and can
  make all kinds of weird drills that work well depending on need.

PIC's attached.

RICH