Not expecting/demanding this, just musing/theorizing if it’s even possible–if one put a stock-holding collet in the spindle, and clamped a lathe toolbit to the table, and had the right software, would the Nomad work as a small-part CNC lathe? (tops and whatnot) NOTE: Am not planning on doing this anytime soon, just came up in discussion with a friend.
It would be tricky setting it up in the enclosure.
Another concern is that Grbl doesn’t directly support a 4th axis — that said, people have done this on Shapeokos: http://www.shapeoko.com/wiki/index.php/Upgrade_Overview#Additional_Axis
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The main problem you’re going to run into is the 3" working height envelope. What’s more likely to work well would be an indexing 4th axis rotary that you can mount to the bed, even if it’s a manual one, that you can do such objects with.
You’d cut one side, then spin it to cut the other, and you could do finer increments to do other details/relief on-axis.
It’s on my wishlist too!
Another big issue is that the spindle is nice high speed, but relatively low torque (people are having no problem stalling it with fairly small diameter cutters used a little too agressively). For a lathe use with any kind of workpiece diameter, the lever arm for the cutting force will be many times longer, so a correspondingly smaller cutting force will stall the spindle.
For a lot of “turned” things you could just do a 2-sided MeshCAM milling job. With a small calculation tolerance, MC does a very good job of circle/arc fitting. If it is a flowing shape a ball-end mill will do pretty good blended surface, as long as the waterline stepdown is smaller than 1/8 the cutter diameter or so. The “shallow” areas you’d still want to do with X-Y parallel finishing, again with stepover 1/8 or smaller than the cutter diameter.
Good points Randy, tag-teaming in to suggest Rob’s post on the MeshCam blog about the impact of radius of tool on step-over size, and on finish smoothness, where sometimes bigger is better!
Yes indeed, UnionNine. My general philosophy in 3D milling is to rough-cut as much material as possible, which means using a ball-end mill and unchecking Use parallel path so that the roughing follows the contour of the part, and then as large a diameter ball-end for finishing as will fit in the smallest concave area, with the 1/8 or less stepover/stepdown ratio when there isn’t bulk material removal to worry about. There are actually cusp-height calculators online, but I’ve never found a real correlation to real-world work beause on small cutters tool deflection etc. (and even the alternating climb and conventional cutting on alternate parallel finishing “lawnmowing” passes) swamp the theoretical values.
I was actually also wondering if I could make a jig that could hold stock at various degrees related to the cutter (eg a manual 4th axis rotary, as @UnionNine was suggesting). Would need to figure out how to turn the object and stock by the same degrees in the software as well though.
To rotate the design & stock in the software is a bit tricky—rotating the design by itself isn’t too bad, as you can enter custom angles when using the “rotate geometry” feature in the second row of buttons, between move and scale.
You’ll want to first move the piece and set the “center of part to zero” on all axis, so that your part is rotating around it’s middle, and then you can rotate on the X axis. I’ve rotated this part (which I would actually just cut 2D) by -30* on X to get this orientation:
For reference, in most CAD/CAM software X is red, Y is green, and Z is blue, if that helps keep you oriented!
-Jonathan
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