Countersink bit speed

[cjensen]

i've heard that for countersinking, slower speeds are preferrable. anybody else know? i know that for drilling AL, higher speeds are best, but countersinking is moving a lot more material, so do slower speeds work best? just wondering if i should buy an additional battery for my cordless to do the spars.

[tshort]

I found the best results for me on the spar were with a single flute countersink in the air drill at high speeds. These produced nice, repeatable, smooth countersinks. I used the method where the nutplate was riveted on first and then used as a guide for the #30 c'sink - worked great; a little chatter initially but then as the bit dug in it centered well and made a good countersink. For me, this was way faster than using a piece of Al behind the spar as a guide.
The three fluted ones work a little better at low speeds, but I could never get as good a countersink with the three fluted cutters.
YMMV...

Thomas
-8 workshop (wings on pause for now...)

[cjensen]

so you countersunk for the nutplate rivets first, riveted those in place, then made the larger countersinks? if that works out, that's the way i was thinking as well, so i don't have to make a guide.

[tshort]

Yep - that's exactly what I did.
The bit will chatter a little before it centers in the nutplate, but once it does the chatter marks are removed as the bit goes deeper.
I highly recommend the single flute c'sink cutters.

T.

[prestwich]

I'm not a machinist, but I work in a company that also does machining, so I've had quite a bit of experience and exposure. I was always told, and experience bears it out, that countersinking should be slow. When I say slow, I mean around 60 rpm or less. Good countersinking can be done by hand, but concentricity and perpendicularity are critical, so if it's possible to clamp the workpiece and use the drill press (or mill,) do it.

The speeds in general that are widely advocated for this hobby, I think, are insane. Although I'm barely beginning, I'm using a variable speed electric drill and running it relatively slowly. Speeds and feeds work together. If you're feeding 500"/minute, you might want an air drill running wide open, but who does that with a handheld tool? Slow speeds, slow feeds, good control, nice chips.

If you get yourself a scrap piece of 1/2" aluminum, and practice drilling holes in it, you'll eventually (after maybe 1000 holes) be able to feel through your fingertips, the metal being cut. Then change the speed, and notice how your feed rate has to change as well. Your fingertips will tell you when to add lubricant, when to relieve the chips by pulling out, when the flutes are loading up with metal, when your feed rate is wrong, and when the tool is getting dull.

[captain_john]

Like P-wich, I am not a machinist either. Nor do I play one on TV. I *can* tell you thinks that I have heard and things that I have learned.

What Thomas says about the single flute countersink is supposed to be correct. They say the single flute ones chatter the least.

Also, I don't use one. I have a 3 flute carbide tipped set. It is very sharp and has held it's edge quite well! That being said, it works well with low speeds. I have a 12v DeWalt and when doing the spars it gave a good workout on the FOUR batteries I have for it! I could have gotten by with 2, and 3 would be just right. I was constantly rotating them. That being the case, it is really a perfect job for a CORDED drill! Lots of work in one physical location. I used a cordless because I like the drill.

I vote for slow.

CJ

[aparchment]

Hey Chad:

I was taught by Tom Emery that high speed for countersinking is best. I put my countersink bit/cage in my air drill.

Antony

[Thermos]

I vote for slow countersinking speeds as well, especially when doing all those countersinks on the wing spars where the csink pilot doesn't start to center until it engages the platenut hole. Seems to me that a faster drill speed would mean a higher risk of chattering in the hole and really screwing things up!

I used Van's recommended technique of a cordless drill set on high torque and low speed, and everything came out fine. At first I was reluctant to trust the nutplates to adequately center the csink pilot, and made a little jig to clamp on the wing spar. But I tried Van's nutplate method on the inspection plate screw holes, and it worked fine...

Dave