View Full Version : Cutting Large Diameter Holes in Aluminum Plate

04-05-2004, 12:15 PM
What is the best way to cut say a 3 or 4" hole in a 1/2" aluminum plate. I'd been using a jig saw but my lathe won't chuck the piece so I can't really clean it up. I have a Mini-mill but really don't know how to clean them up there either? Fly cutter? Never used on but thought that might be what I need? Thanks

04-05-2004, 12:22 PM
You should be able to cicular interpolate it using IJK or R values, depending on your controller. You could use a hole saw. You could use a fly cutter if it's adjustable.

There's several ways, and the best way will depend on your tooling, controller/machine etc.


04-05-2004, 12:27 PM
My mill isn't cnc

04-05-2004, 12:34 PM

How close is the tolerance? You could scribe your circle and trace along it manually if it's not too critical. Or if you have a fly-cutter, the type with the cutter held in with set screws, you can approximate the dia and bore finish bore it with that. Or you could get a hole saw.


04-05-2004, 12:40 PM
A 3 or 4" hole... hmmm that's a pretty big hole to cut with a flycutter on a small machine. The best that I've managed are 1 1/4" holes x 1 1/2" deep using a boring head on my old chinese mini mill.

This was at the lowest speed setting and it still cause a bit of squeeling and low frequency rumple (not good because I was living in an hi-rise apartment at the time)

The problem is that a 3" hole has a circumference of almost 9 1/2" and at only 100 rpm spindle speed you end up with the cutting tool travelling at over 78 feet per minute.

This means that you have to take very light cuts because the cutting tool is held so far away from its rigid support.

Of course a cnc'd hole is relatively easy if you don't have too much backlash in your setup.

Good luck.

04-05-2004, 12:47 PM
Looks like it may not be too much fun. I've used hole cutter for about 1.25" but never any larger. I'll figure something out. Thanks

04-05-2004, 12:55 PM
Good luck. Let us know how it turns out.


04-05-2004, 01:22 PM
Do you have a router? If co use a carbide straight bit with a bearing and cut out a circle using particle board, masonite... as a template for a good finish.

04-05-2004, 01:50 PM
rotary table?

04-05-2004, 02:10 PM
Thats an good suggestion pminmo. Thanks

04-05-2004, 03:23 PM
Believe it or not, use a good hole saw. With aluminium use a faster speed than you would for steel. Drill press (or mill) is best but a good angle 1/2 inch hand held drill will work. Clean chips frequently. Add some lube and have a good long handle to prevent winding up the hand held drill. I have cut large holes up to 4 inches in diameter in 1/2 steel and 1 inch aluminium as well as in stainless thinwall stainless steel with this method. Keeping chips cleared and tool cool is a must for tool life. An eye for perpendicular is also important when doing it hand-held to avoid catching on the cut through at the end of the cut. Of course a pilot hole for guidance is essential. Sometimes it is advisable to predrill a pilot hole and then to replace the hole saw's pilot drill bit with a piece of the same size drill rod notched to fit the locking set screw. The smooth drill rod won't enlarge the pilot hole as a drill bit may and keep the hole saw cutting trueer than would otherwise be the case, especially if you do not keep the process perpinduclar. Also, be sure to stop the saw from revolving before you remove it to clean chips. Then do not begin turning it until it is back in the cut. Do not rush it. Clean chipe often. But, do not be afraid to put some pressure on the down cut as a continuously cut chip is easier on the saw teeth than constantly wearing without really cutting, which builds up heat, but gets little progress. Remember, a lot of the heat is carried away with the chips, just like with a drill bit. The hole saw has more trouble clearing the chips, though. You hjave to help by stopping and removing them. Keep the saw cool. You will develope a feel for the right feed pressureis you have a mechanical affinity.

But, I have one more caution: Ease up on the downward pressure as you near the end of the cutting depth. It is also possible to turn the piece over to cut in from the opposite side, but I would advise against it. The two seldom align as well as a one-sided cut and cleanup is always needed. If extreem accuracy is not needed hand-held works well. Clean up can be accomplished with cartrige rolls, sanding sleeves or large cross pads (all are abrasive tools.) Of course if you have a mill or slow turning drill press you can use a fly cutter for clean-up. But, NEVER HAND HOLD A FLY CUTTRER! And they are for enlarging holes, not for making them. If you have a mill or drill press a hole cutter that uses a cutting tool similar to that in a fly cutter, but held at 90 degrees to the bar (parallel to the hole axis) will work if you are careful with the feed rate.

Good luck!

04-05-2004, 03:26 PM
So you use a standard bi-metal hole saw to cut your aluminum, say from home depot?

04-05-2004, 03:31 PM
Exactly! With care, you will be surprised at how many holes you can cut with one hole saw. Dozens in mild steel and more in aluminium.

04-05-2004, 03:35 PM
Hmmn, I've used them before but I'll try again and use lots of lube. Thanks

04-06-2004, 12:07 AM
I don't know about using lube with a hole saw, I'd try it dry first and don't let much Aluminum build up in the teeth. The lube will just let the aluminum shavings build up in the teeth and around the saw. Keep it clean and if you have some way of sending air to it through the shank of the saw do it ( you won't have to clean it out as much). At work I have to use hole saws to cut out stress proof around carbide buttons, not fun at all.


04-06-2004, 03:18 AM
I want to make a hole in aluminum with a counterbore so that top of the head cap screw (5mm screw) is even with the surface of the aluminum. What are the advantages/disadvantages with using an end mill for the hole and counterbore versus using a drill bit and counterbore bit (e.g. speed, accuracy, etc)? Suppose I also want to make a hole and hand tap that hole? Can the cnc machine make these holes accuratly with good dimension sizes?

One more thing. Can I use a stepper motor (150 oz-in) and use it to automatically make a tapped hole?


04-06-2004, 07:16 PM
Scribe 2 circles, one about .100" under on radius, second the finished hole size. Use a 1/2" diameter center cutting 3flute aluminum roughing end mill. Plunge completely through (need parallels or consumable base) and rough out a hole just touching the inside scribe. When you pass the outside scribe get a new piece of material and start over. Suggest making small steps using only one handwheel at a time at first until you get some experience. Then use a flycutter as a boring head to clean up the holes.

04-06-2004, 08:25 PM

Plunge drilling with a 2 flute center-cutting slot drill (which is just a special designation for a certain type of endmill) is one way. However, the cutter does tend to want to wobble as it cuts, because it has no centralizing conical point like a twist drill does. Even on a big mill, the cutter will wobble.

If you have good cadcam software, you can make nice holes and counterbores with an endmill. You might still select the 2 flute slot drill, however, select one that is some size smaller than the hole you will be cutting. Either predrill with a regular twist drill, or plunge drill with the slot drill. To do this, you should use a step drilling routine to break the chips and to assure that some coolant gets down into the hole.

Then, create a spiral helix in your cadcam software, with a radius based on the hole's (finish diameter less the cutter diameter) /2 The downward slope of the helix should be quite gradual, maybe 3 degrees of ramp.

04-07-2004, 06:44 AM
If the hole saw doesn't get you want you want pursue the router idea. I've used that several times and it works exceptionally well. You can even use a piloted round over bit to ease your edges. Be prepared to deal with the flying chips. A light mist of cutting oil will dramaticly improve the surface finish.