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A q&d project

Posted: Thu May 28, 2009 12:37 pm
by awake
If anybody needs a quick and dirty project to tackle, here is my latest. As I continue to cut the change gears for the minilathe, I have discovered that I need a few more blanks. Unfortunately, I don't have a hole saw that is well matched to the size needed -- what I have is either just too small or way too large. So I finally made something I've been meaning to make for years. Machinists would probably call this a trepanning tool, but woodworkers call it a "circle fly cutter":

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This is intended to cut wood or plexiglass. I don't know that I'd try it on aluminum, and definitely would not try it on steel. But for a quick, reasonably accurately sized, clean hole or circle in wood or plexiglass, it is the cat's meow. Here are a couple of samples cut with the tool, one a gear blank in plexiglass, and the other a circle cut out of some scrap pine. It worked well in both materials, though it would have been nice to slow down the rpms -- my little benchtop drill press only goes down to 500 rpm, and that was a little fast, especially on the plexiglass.

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Re: A q&d project

Posted: Thu May 28, 2009 2:17 pm
by Harold_V
awake wrote:This is intended to cut wood or plexiglass. I don't know that I'd try it on aluminum, and definitely would not try it on steel.
It will work perfectly well for both. I used a similar cutter in a ½" drill motor to make the openings in the CT can and 400 amp switch for 3" conduit fittings when I was wiring my shop (3 phase 400 amp service). Each were made of 16 gauge steel.

It's surprising what will work when it's all you have at your disposal.

Harold

Posted: Thu May 28, 2009 3:06 pm
by Black_Moons
Yea, it will work.. if you grind your bit correctly (some relief would be nice) and align it nicely, and use nice low rpms (at that radius anyway, think SFM)

Posted: Thu May 28, 2009 6:09 pm
by awake
It will work perfectly well for both. I used a similar cutter in a ½" drill motor to make the openings in the CT can and 400 amp switch for 3" conduit fittings when I was wiring my shop (3 phase 400 amp service). Each were made of 16 gauge steel.
Hmm -- I wasn't thinking of sheet metal, but yeah, I'd think it would work for that. To clarify, I wouldn't want to try this particular version on thicker steel -- not rigid enough. After all, this was a q&d project -- it could be improved upon greatly! But it did what I needed it to ...
Yea, it will work.. if you grind your bit correctly (some relief would be nice) and align it nicely
Do I sense some critique of the bit? :) Not surprising, since I just made a guess at grinding it. It does have relief, though, and it is much better aligned than it looks in the picture (not sure why it looks angled there).

I would very much welcome input on how a bit for this application should be ground -- I thought about several possibilities, and tried one other that worked so-so. This one worked very well indeed on the plexiglass and wood, so I stopped with it.

Posted: Thu May 28, 2009 7:24 pm
by Black_Moons
I would think more of a skinny cutoff looking tool, maybe even with a chip braker ground in for extra rake might help..

Posted: Fri May 29, 2009 12:10 am
by Harold_V
Black_Moons wrote:I would think more of a skinny cutoff looking tool, maybe even with a chip braker ground in for extra rake might help..
That's correct, but not too much rake, so the tool isn't prone to hogging. It is very desirable for chip control, plus it lowers cutting pressure. Tools ground like that are easy to break, but once you establish a full circle cut, by slightly leaning the drill such that the tool is the highest spot, you can keep reasonable pressure on the cut. The handle oscillates in a small circle, pivoting on the pilot. It's a good idea to use a solid pilot, not the drill flutes. You also have to use care when the cut starts breaking through. The cut is rarely uniform in depth, so it starts in an area and grows. You have to avoid any pressure to prevent the tool from hanging up when it meets the area where the cut has not broken through. Not nearly as hard as it sounds.

To make it more successful, if a slight taper is ground on the tool such that the outside edge cuts deeper, the cut can be stopped when the cut begins to break through, and the disc knocked out with a hammer. That minimizes tool breakage considerably.

Harold