Does Small-Hole Drilling Demand a Fast Spindle?

A reader recently used the “Ask an Expert” feature of our Micromachining Zone to ask the following question.

Question

I'm trying to drill a 0.019-inch hole in 316 stainless steel that will need to be 0.092 inch deep. I have a drill press with a top speed of 10,000 rpm. I have tried solid carbide circuit board drills and out-of-the-book high-speed drills. I'm breaking drills and have not drilled one good hole. I can adjust the feed slower, but this doesn't help. We are holding the drill in a pin vise in an ER collet. Is there a better drill to use? What do you think?

Response from John Bradford, micromachining R&D team leader for Makino

I certainly recommend that you investigate drills from Union Tools.

However, for micro drilling, our main focus is not necessarily on the drill itself, but rather on the total accumulated runout that occurs in actual operation. Accumulating factors include: spindle runout at actual rpm, tool holder and collet runout, and flute runout relative to the drill bit shank.

Additionally, I would recommend spotting the hole with a spot drill that gets you a seated diameter that is 5% larger than your drill bit diameter.

After these items are taken care of, then you can start tuning in your actual machining process.

I have a feeling that, in this case, your culprit is likely the runout created by the accumulation of items starting at the spindle downward.

I recently completed in stainless steel, with a 0.002-inch drill going 0.020 inch deep. It is a very stable process, and much easier if you have negligible runout.

You will note from our specific part that we were using a speed above 30,000 rpm, but this is not a requirement. You can effectively drill holes as small as 0.005 inch with 10,000 rpm, as long as you minimize runout and vibration.