50 IPM Passes on the Tapping Arm

I got some work done on the tapping arm project yesterday afternoon. I made the two swing blocks that are the uprights in the parallelogram linkage. Here is what the overall arm will be:

The swing blocks are the red parts…

While making the swing blocks, I wanted to play with some faster milling feeds that use radial chip thinning. Being able to do the speeds and feeds calculations to maximize performance is one of the reasons I started writing G-Wizard. I’ve got some limitations given that my mill only has a 1600 rpm top spindle speed–pretty darned slow for aluminum. That was going to seriously limit the top end of what I could accomplish.

To pick up the speed while profiling the swing blocks, I did a couple of things. First, I used a 1/2″ corncob rougher which can sail through the work a little faster. Second, I used a HSS 4 flute. I need more spindle speed to make a carbide endmill fly, but I’ve got decent juice for high speed steel. Running 4 flutes instead of 2 is like doubling your rpm, so in effect, I can go as if I had a 2 flute in a 3200 rpm spindle–still not very fast, but faster. You can only use 4 flutes in aluminum when there is plenty of chip clearance. In this case, I’m profiling around the outside, so its no problem. If I was slotting, I would want to stick to 2 or 3 flutes.

Lastly, a set up a radial chip finning cut that was 0.125″ deep but nearly full cutter diameter high. Any time your width of cut is less than 1/2 the cutter’s diameter, you can start cranking up the feedrate due to chip thinning.

The end result was a 50 IPM profiling run:

Flying along at 50IPM to rough the Swing Blocks.

As you can see, it ran pretty smooth, with no chatter or other problems. Most of the noise is the gearhead itself. I can’t wait to do a belt drive conversion, just a couple more projects before I start on that. A couple of things to note:

- I’m using Koolmist, and perhaps a bit more fluid than I really need. Hard to fine tune the Noga mister. You need to use some form of fluid at these speeds for lubrication, else the aluminum will stick to the cutter and things go downhill in a hurry after that. I’ve seen that happen at these higher rates of machining, though I often run dry when I’m not pushing it. The one thing I don’t like about the mist is it makes the chips clump together. There’s more recutting of chips going on here than I would like due to that. I need a second air blast nozzle to really get rid of them.

- You can see steam flying off the milling operation at various points. You’d think things are hotter than heck and the cutter is not long for this world. But, all that heat is in the chips, not the cutter. It was room temperature to the touch immediately after I finished the pass.

- This cutter was not even working hard, but you can tell the hardest part of the cut is in those inside corners if you listen to the sound. If I had more spindle rpm and could go faster, I might want to consider slowing down a bit in those corners. Of course that’s what high speed machining tool paths like trochoidal milling do automatically.

Fun stuff to see a low-end mill like this cutting a little more aggressively. Each Swing Block ran for just under 10 minutes to completion. I didn’t show the whole thing in the video, but there was a lot of air cutting at the beginning. If I was going to make a bunch of these, I would want to find a way to get my CAM program to quit doing that.

More details on the Tapping Arm Page for those who are interested, including the g-codes if you want to try one yourself.

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1. Next Project: Tapping Arm
2. Next Project: Tapping Arm