Machinists - why are holes sometimes triangular?

Don Plumley · Don Plumley's Garage

I was drilling a bunch of holes in aluminum today. 7/16" or so. Sharp bit, drill press, low speed. Bit runs freely up and down when done.

Most of the holes look, well, not round. Kinda triangular. How can this be?

Just wondering....

Moneyguy1 · 03-29-2006, 09:13 PM

Harmonics. Try drilling a number of holes in the same piece of material next to each other at different rpms and see what happens.

jim72911t · 03-29-2006, 09:32 PM

Don,
Was the piece clamped down when you were drilling? Were you using a stub length or jobber length drill? Was the bit new, or resharpened?

If the piece was clamped down, either in a vise or with clamps, and you were using a sharp, decent quality bit, your holes should be round. My guess is that you were holding the workpiece with your left hand and feeding the drill with the right hand. It's very easy for the drill (especially if it isn't sharp, or has been sharpened incorrectly) to start tracking a bit squirrelly and create the triangular shaped hole you are talking about.

Also, make sure you use a center drill, or, at the very least, a punch, to get the drill started. One more thing: it's much harder to get a round hole in sheetmetal than it is to get one in a thicker piece of material.

A bit more info is needed here.

Good Luck,
Jim

URY914 · URY914's Garage

If you really need a round hole you need to ream the hole with whatelse...a reamer.

Jeff Higgins · 03-30-2006, 05:04 AM

Three fluted drills make triangular holes, two fluted drills make oblong holes.

Seriously, it is impossible to drill a round hole with any standard drill bit. There are all kinds of fancy drill bits available, like "core" drills, double margins, etc. that drill "rounder" holes, but reaming is the best way to get one as round as possible with normal drill presses or hand held drill motors.

You can actually do pretty good with a standard bit if you start with a pilot hole to keep the center of the bigger bit from doing any actual cutting. When the center is cutting is when it tends to walk around a bit.

5axis · 03-30-2006, 05:48 AM

Everyone else pretty much covered the drilling issue. I will add the precision part now. A good way to get round, precision sized holes that are also on location is.

1) center drill

2) drill (undersize)

3) bore (undersize -.001-010 depending on materials and size to leave material for reaming.
The single point tool cuts the hole concentric to the spindle and on location.)

4) ream ( hint, using oil and the hole will be tight/ under. Use a water based coolant and it will tend to cut larger/ over. hint #2 run the reamer 1/3drilling RPM
hint #3 when the reamer is through the hole, stop the spindle and retract. This helps prevent the chips that are trapped in the reamer flutes from scratching up the finish

Don Plumley · Don Plumley's Garage

Jim - piece was in a drill press vice (which is unusual for me). The bit is longish, probably the first time used, three fluted, but of marginal quality.

Because of the production (and hack) nature of what I was doing, I did not drill a pilot hole. What you said Jeff makes sense - it walks as it cuts. So Pilot Holes are the ticket.

I was fabricating pieces to help hold down channel drain covers (5" wide). For some reason, only one in 5 was installed in the drain that runs in front of my garage. I went to buy new ones from a construction supply store (Orco). Not only were they expensive (almost $5 each), but they ended up being too short. So I found some channel aluminum at the scrap yard, picked up some stainless machine bolts and nuts, cut the channel to length, drilled a hole in the middle. Then the nut barely fits in the channel, and is held in place with a little JB Weld. Tah Dah!

Thanks!

BGCarrera32 · 03-30-2006, 07:35 AM

You can fix most of your problem using a stub length drill bit. It will be about 1/2 as long as the average jobber bit. If you have a woodworking drill press or something from harbor freight, you probably also have some bearing runout at the spindle and a cheesy chuck which would also contribute to the bit wandering around upon starting the hole.

And clamp your work to the table when drilling, backing it up against the rear or the drill press properly...