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4th axis turning

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Jonathans

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#1
Total CNC noob here. Perhaps a dumb question.
I've seen videos of folks using a high speed 4th axis on their mill and turn with it, with the index cutter being held in the spindle. If I had a typical 4th axis, that was slow, is there any reason I couldn't turn with it using an endmill? Would the outcoming finish be ok?
 

Tony Wells

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#2
The catch to that is that the end mill will cut best if it is offset half the diameter, but that will leave a scalloped cut. To get rid of that, you have to run the end mill on center, where it won't cut all that well due to the fact that in an odd sort of way, it is plunge cutting as the part rotates into it in the 4th axis.

It can be done, but it's not optimal. That's why they use an inserted cutter.
 

Jonathans

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#3
I see. Would it make much of a difference using a ball cutter?
 

Tony Wells

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#4
Ball nose end mill would leave a radius groove or scallop no matter whether it was on center or not. If this part needs to be machined that way, you can do it in two steps. Offset the tool to get the best cut, then bring it back to center for finishing. Leaving only 0.010 or so won't pose any problems with the finish cut, depending of course on the mill.
 

Jonathans

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#5
Thanks,
I'll have to expeiment with that once I get a 4th axis. Was checking out some high speed models like the InTurn but they are pretty expensive.
 

Wreck™Wreck

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#6
You will not know unless you try it on the machine. I suspect that the results will be slow and unsatisfactory compared to turning.
Do it and see how it works, if everyone stood by conventional wisdom and the belief that doing everything the way that works best at this time you would be riding your horse to work tomorrow.

Experiment, try a new approach and go out on a limb when needed.
 

JayMcClellan

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#7
I'm working on a 4th axis build right now, and I plan to post a video of the build soon. One of the final steps will be to turn a faceplate with a tenon to fit into the back of my 3-jaw chuck. I plan to do that on my mill with the faceplate mounted on the 4th axis and turning, slowly, while cutting the face with an end mill. I could do that step on the metal lathe but doing it on the 4th axis itself should minimize the runout. I did turn it close to final dimensions on the lathe though, so that I will be taking just a very shallow cut in the final step. I think this will work well, but it is a somewhat special case because I'm only cutting 2 surfaces parallel and perpendicular to the axis of the end mill, versus a more general turning application where Tony's comments about scalloped cuts would apply.

Another thing to consider in any such application is that some kinds of milling cuts will put considerable torque on the 4th axis. I'm building mine with an ultra-low-backlash harmonic drive and a 45:1 reduction ratio so it should be pretty beefy, but I expect that the more common belt-driven 4th axes are much less rigid and they have a much lower reduction ratio so the driving and holding torque will be much lower. And the worm gear designs have more torque but tend to have significant backlash unless they're adjusted tightly, which wears the gears faster. I don't think it would be a problem as long as you keep the cutting torque opposite the rotation of the axis, i.e. non-climb-cutting. Otherwise it would tend to jerk the work forward into the cutter if there's any appreciable backlash in the system.
 
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