Who out there likes to hog? I mean big chips clinking on the guards in sporadic time. There is nothing like it and it is one of those things that just makes machining fun! Even if you're not a machinist and your in management you must love it because it means a high metal removal rate.
"They just don't build them like they used too". This statement really fits when you are talking about machine tool builders. Today's philosophy - get the metal off using the spindle. High surface footage with specially coated carbide inserts. Take a small cut, then try and make up the difference by turning up the spindle and boosting the feed rate. I wonder how much a new machine would cost if it was still built to last decades and could handle the kind of cuts I'm talking about. Most of the new machines are just like VCR's - Run it until it dies, throw it in the trash and go get another one.
I understand the reasoning behind the new machine philosophy. It will never get the same removal rate but, it's a good theory and it saves money. What new technology do we have though that will get that pounds per hour up as high as yesteryear. It's called chip thinning theory. It works, it works well, requires a little math and needs at least 40HP at the spindle. The spindle horse power will be your limiting factor.
Many factors are important when high feed milling. A constant depth of cut and a constant engagement percentage are foremost. Vertical turning mills - a VTL with live tooling and a "C" axis, lend themselves very well to this application. Roughing a face down can be done very quickly and it has many advantages over a single point operation.
It was necessary to rough a ring down that was 160" in diameter and had 130" ID from 4" burned plate to 3" thickness. One inch of material at these dimensions is a lot of hoppers full of chips. At 500 sfpm it took 2.5 hours to make a pass with a single point tool. At .15 depth of cut calculate this run time. Since this was burned plate and warped almost one half inch the amount to be removed is now 1.5 inches with a lot of hit and miss. Now recalculate.
Using a 5" high feed mill at 600 sfpm with 9 inserts the chip thinning theory calculation was faster than the table maximum feed in polar coordinates would turn so, 360 ipm was used with a .045 depth of cut. Your programmer will program 2 axis at the same time. A (X) axis distance to travel and a incremental (H) to control the engagement percentage. Be sure to climb cut.
For 70% engagement the above part's cutting line would read: G01 X127.5 H-1671. F360.; Position the mill at the correct Z with OD clearance and feed in to an ID (X) clearance. The H value at 70% engagement is the start X minus the finish X divided by two and again divided by (5 x 70%) times 360.0. and the feed, go as fast as you can without overloading your spindle. You should start around 100 ipm and gradually increase the feed, When chip thinning theory takes over and begins to work you will actually see the load drop as the feed goes up.
It is very important if you are using warped plate that you find the high spot on the plate and begin your downward decent from here. Otherwise, you will overload your machine and possibly damage the machine tool or cutting mill.
There is also another great advantage in using this technique on warped plates. If you understand macro programming, using an address for an angle and a WHILE loop you can cut out all the hit and miss. Find the high spot, enter the angle in degrees that you guesstimate the bow is and only mill that section of the plate by reciprocating back and forth between your addressed degrees. Move to the next spot and work it down in the same manor. On a part as large as this, you have saved an incredible amount of time. I hope this was useful. Good day.
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