How Do You Machine a 30:1 Hole in Stainless Without the Drill Wandering?

A reader recently used the “Ask an Expert” feature of our Micromachining Zone to inquire about a difficult holemaking application:

Question

I am trying to drill a 0.0145-inch hole 0.400 inch deep in 303 stainless. I am having a hard time keeping the hole within 0.002-inch concentricity of a hole at the other end. Any suggestions?

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

There are several potential sources of error to evaluate. I recommend considering each of these points:

1. Drill tip geometry

Ensure that the flute tip geometry of the drill you are using is centered relative to the shank of the drill. During rotation, drill runout should be minimal. (Also important is to pilot with a small spotting drill prior to introducing this drill.)

2. Collet holding

Though it is difficult to verify tool tip runout during spindle rotation, it is very important to minimize drill tip runout at the operating rpm. This will require using on-size precision collet holders (such as Yukiwa Seiko or Big Kaiser). These precision collet systems offer dynamic tool runout of less than 2 microns. Traditional collets are not accurate enough.

3. Peck cycle retract amount

If you can reduce the retract amount of the peck cycle so that the tip of the drill does not exit the top of the hole, this will minimize the “rubbing” that occurs during entry and exit of the drill. This can help keep the sharpness of the drill intact, leading to a straighter hole. The duller the drill tip and flutes become, the more lead-off you will experience.

One final point is that, given the high aspect ratio of these holes—almost 30:1—it may not be practical for you to conventionally drill these holes and consistently hold straightness, even if all conditions are ideal.

The alternative process I would recommend would be fine-hole EDM. A hole with an aspect ratio of up to 50:1 could be machined through EDM with a straightness of 0.0002 inch. Machining times could be as fast as 60 to 90 seconds per hole.