square holes

[JPMach]

I have to bid a job that has a rectangular hole through 2.5" of aluminum with .062" radiuses in the corners. What is the easiest way to do this with only a VMC. I know the best would be a waterjet or wire edm but I don't have one of those and am trying to figure out how to do it in the mill.

2.5" is a long way with a 1/8" endmill, even from both sides. I was looking at Horn's broaching tools that you can use almost like a shaper but I am not sure if they would work.

I would like some thoughts and suggestions please

JP

[NC Cams]

You didn't mention the size - only the depth of the hole.

I'd use a drill to poke a hole first, then trepan around with a larger end mill.

Finally, use a 1/8" dia mill to shape up the final form.

[JPMach]

It's basically an Ar-15 type magwell

JP

[Geof]

Your machine has a pretty high force capability on the Z axis. Do you have spindle orientation activated? You could mill the hole and then square up the corners by just driving a square HSS tool with the corner radiused to .0625" like a vertical shaper. I would just experiment a bit at a slow feed rate on the Z and taking only a small cut to see how the servo load goes. Even if you did drive the servo into overload and alarm out it is highly unlikely that anything will get hurt because it would not be an impact load.

[JPMach]

Thats kinda what I was thinking about with Horn's broaching tool. I am thinking maybe I can get one of the inserts made for chamfering the sides of a keyway and round the tip to the radius I need (possibly on my tool grinder) and then orient the spindle into the corners to make the cuts. Still though I think their shanks are fairly big and all. It might be worth trying the HSS if the job looks promising.

If only I could find a local shop with a wire edm for cheap!

JP

[Geof]

You could even bolt a carbide insert flat on the end of a piece of round bar. Even if you cannot get a .0625" nose radius plotting the X,Y coordinates to cut the corner radius would only take a few minutes.

[BobWarfield]

Drill the corners with a 1/8" bit. Mill out most of the core with a larger mill, such as a 1/2". Blend the corners with an 1/8" mill.

BW

[NC Cams]

A VMC machine tool spindle IS NOT A BROACH nor a SHAPER nor should it be used in place of one of in place of one for that matter. Nor are screw drivers chissels nor should you pound screws with hatchets.

The bearings in a machine tool spindle will brinnell quite easily if a static load load is applied to them when the spindle is NOT turning when axial load is applied to the spindle. Since you don't know if the overload protection of the spindle is in fact callibrated for the application of a static load as opposed to a dynamic one, you can't rely upon the machine tool maker's protection circuitry to keep you out of trouble.

If I caught an employee doing that (using the VMC as a shaper or broach), I'd fire them on the spot.

BTW, in my younger days I did that "no no" and learned the hard way how much it costs to rectify such stupidity when I did it to a a drill press.

Do what you want but I wouldn't do that....

[JPMach]

Originally Posted by NC Cams The bearings in a machine tool spindle will brinnell quite easily if a static load load is applied to them when the spindle is NOT turning when axial load is applied to the spindle. Since you don't know if the overload protection of the spindle is in fact callibrated for the application of a static load as opposed to a dynamic one, you can't rely upon the machine tool maker's protection circuitry to keep you out of trouble.

Thats kind of what I was thinking could possibly happen. I didn't really like the idea of use the spindle as a broach even in Aluminum.

I wish I could afford a good used wire edm! Does anyone know someplace on the internet where I can learn more about wire edm's? All they ever told us in school is that they exist, we only got to work with ram style. I want to try and learn about wire for several months so that maybe if the time does come to buy one I at least know what I am getting into.

JP

[Geof]

What NC writes is technically correct but before anybody starts flapping theirs wings and saying the sky is falling it might be worthwhile checking the static axial load rating on the bearings. It is probably a good deal greater than 2000 lbs which is what the servos alarm out at (approximately). On your machine it is not necessary to factor in the head weight because it is counterbalanced.

[JPMach]

Originally Posted by Geof It is probably a good deal greater than 2000 lbs which is what the servos alarm out at (approximately). On your machine it is not necessary to factor in the head weight because it is counterbalanced.

My machine does not have a counter balanced head. The newer machines have a large motor on the Z axis and they did away with the hydraulic counterbalances. I think the motor on the head put out more like 4200 lbs of thrust compared to 2500 lbs on the X and Y axis.

JP

[NC Cams]

The RATED axial load of bearing is based upon a number of assumptions that do NOT always end up being facts of reality in an operating machine.

In my bearing engineering days, I've seen any number of bearings that were axially brinnelled at loads that were well below the RATED static load of the bearing.

I"d suggest that you VERY CAREFULLY read the qualifiers in the bearing applications manuals when they talk about applying axial loads to bearings. You may be surprised at what you'll learn when you go thru a life calculation.

BTW: the spindle engineers don't always have total say on the design or use of a machine. Thus, it is concieveable that the guys who did the counterbalancing didn't have much interfacing with the spindle guys.

I simply wouldn't even use the VMC spindle as a ram - period. Do what you wish and live with the results, good bad or indifferent.