School me on using annular cutters in my milling machine

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Brian Rupnow

Design Engineer
Project of the Month Winner
May 23, 2008
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Barrie, Ontario, Canada
I have just purchased three sizes of annular cutters for my manual milling machine. They have 3/4" Weldon shanks. I bought 1" diameter, 15/16" diameter, and 7/8" diameter. I also bought three R8 collets with 3/4" capacity to make dedicated holders for the three annular cutters. Here is what I know---they require much less torque to drive them than a drill of the same diameter. they should be ran at a fairly low rpm. They require lots of lubrication (squirt on cutting oil) while working. What I don't know is this---Do they require a center "pin" to extend out past the cutting edge to pick up a center punch mark in the piece being cut. Do I have to buy appropriate center pins from a manufacturer, or can I make my own. If I have to make my own, there is a great YouTube video on how to make your own with a compression spring to automatically eject the center plug that is left inside the annular cutter. I have never used annular cutters before, so any help is appreciated.---Brian
I often don't use the pin. Sometimes the slug drops out on its own, sometimes it resides inside the cutter ID. I just wiggle it out with pliers (mind the sharp edge).

Not quite sure what you have in mind for the arbor but I have an R8 & MT3 version of this pic from KBC. Weldon style maybe? Anyways I think you need the opposing set screws to engage the flats on AC shank.


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We call them mag drill bits. It's a mini drill press that is held in place with magnets in the base. The pin in the center is used to align with a center punched mark in the work piece. As the drill progresses through the work the center pin gets loaded with a spring in the spindle. When the cutter breaks through the work piece the slug is ejected by the spring loaded pin.

You don't need the pin but it can save time by not needing to remove the cutter from the mill or lathe to tap out the slug every time.
blair makes the ones i use for drilling spotwelds in autobody. they are the very best here in U.S think they also make precision cutters an arbors
Repeating whats said above, I have annular cutters with a sprung pointed centre pin used mainly alignment on a centre punch mark. I have also made small ones for creating valve guides in solid cylinder heads. Works with or without centre pin, best without if not sprung or depth to long or to be blind. Avoids pick up.


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The trade name here (UK, for a magnetic drill base with a built in drill press, used with these kind of annular cutters to cut big holes in (usually) structural steel sections - RSJs) is 'Rotabroach'. I agree with everything that's been said so far, so not much to add. The centres do tend to fall out on their own. I bought one to cut some circular depressions in a disk against which 6 springs had to locate, so I didn't go all the way through...unfortunately not one of the sizes we keep at work so I had to buy my own!
If your looking at buying a mag drill look no further then Hougen. I have 3 of these one just used for taping with a tapmatic head on it. I have personally drilled through 6 inch solid steel with annular cutters without issues. But they are not worth ten cents in sheet metal they tend to want to rip in sheet metal. You can buy the cutters in different lengths and you can even buy them in std. tap sizes. They are pretty nice cutters

In your set up the pin wouldn't be "Required", you're using the cutters in your mill so you already know where the hole's center will be. But it would give you a last minute sanity check that you're putting the hole about where you want it.

If you want more cutters, there are many relatively inexpensive annular cutter sets available for mag drills. Some are good, some are bad - you really do get what you pay for. With most of these cutting depth would be limited. If you routinely need to put big holes in 1" stock or more, those cutters are going to cost you. For a business the cutters' performance justifies their cost. For the hobbiest who needs to do something like that once in a blue moon, I haven't been able to justify the cost - yet.

In most of these mag drill cutter sets, the center pin serves 2 purposes. First, it gives you some idea of where you are going to be drilling the hole. Second, most mag drills come with a gravity flow coolant system for the cutter, the pin is used as a valve to shut off the coolant flow unless the cutter is in contact with the material.
If you are drill a lot of holes or deep holes you will be better off bypassing the coolant system that comes with the mag drill and hook up a garden sprayer that you can pressurize. To the port on the adapter. Or if you are drilling out of position. On your milling machine a nice flood coolant should work just fine. Depending on material and all the other factors involved you will find 400-1500 rpm will be most of your cutting. But like anything else play around with you cutter speed until you find what works best for you
Firstly, annular cutter are just brilliant! They simply chew up the metal no matter how thick. I use TCT ones and they pretty much last forever. The HSS not so much, especially as with annular cutters they get a bit more sideways movement. I always drill with the center pin, it not only allows precise aiming, but I also find it helps keep the cutter aligned a little better.

The TCT sets are a little more expensive, but looked after you'll never need to sharpen or replace them, so I consider it money well spent. In OZ I think a set of 6 long cutters was around $250 or so, but they've done many hundreds of holes with no sign or wear.

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