[eliot15]

I drilled about a 2" through hole with a brand new replacable carbide tip drill. The tip is .476. I can slip a .472 gage pin (all I need) in either end of the hole and slide it through most of the way from either side. But I can't slide it all the way through from either side. Drill tip looks OK.

So...my theory is what's happening is the same sort of thing that happens when you wring gage blocks together.

1. Air pressure from the surrounding environment is locking the pin up in the center of the hole as the air between the pin and the through hole is squeezed out.

2. Surface tension from coolant or water vapor is "gluing" the pin and the surface of the hole together.

3. When two very flat surfaces are brought into such close contact with each other, an interchange of electrons between the atoms of the seperate surfaces creates an attractive molecular force.

Got all those from a Starrett web page about gage blocks months ago.

I had to rule out the theory that my drill is drilling larger on both ends and smaller in the center for obvious reasons.

Anyone ever experience this weird behavior before?

I'm drilling cast, and the drill has high pressure through coolant.

[juergenwt]

It's not a bored hole but a drilled hole and your drill is walking. You have a slightly curved hole.

[eliot15]

Hmm...my part is being held by the A axis (sub chuck) of a turn/mill, and I'm drilling it with a live tool in the X axis. I wonder if maybe the part is "walking" (moving slightly). The chuck is holding the front, and I'm drilling through a couple of bosses in the back which are just sort of hanging out in space.

I can't really seem to wrap my head around the curved hole concept.

[txcncman]

Drill walk, yes.

Drills also tend to drill triangular shaped holes. Something I teach in machining class. So, even though it measures correctly across the diameter, if you check it with a tri-mic, it will be on size at one point and 60 degrees from that it will be smaller.

The way to fix this phenomena is to follow up with a ream or bore it.

[eliot15]

Thanks for the replys.

What I don't get is why the problem is more pronounced on this set up then on the last, identical setup. Only thing I changed was the was the drill tip. Wonder if that could be a factor?

[txcncman]

Drill tip. Hardness, or lack of it, of the material. The way the sun was shining through the window that day.

[eliot15]

In other words...ream it or bore it.

[txcncman]

Yes. That or put some nice drapes over the window. LOL

[eliot15]

Thanks for the help. 'Course the upside of all this is at least it wasn't anything as evil as a case of attractive molecular force! Phew!!!!

[fizzissist]

Some people might wonder if there isn't some spring from clamping force (even with cast), anomalies in the material that cause the drill to want to wander, ..... some people.

Me? I'll check all that stuff,,, but after I make sure I'm wearing a clean shirt, the shop radio isn't playing rap or opera, and it's not a month that ends with an "r". And I didn't have tuna for lunch.

Flip the part around and measure concentricity of the exit side of the part in a lathe...that's where you'll see if you've got a Wander-Drill.

[eliot15]

I slowed my feed and took a smaller peck. No more wander drill.

[txcncman]

Cha ching! Now you are in the money.