On Fri, 2008-06-13 at 09:40 +0100, Ian W. Wright wrote: ... snip > The big problem making a cutter is all down to the size and the > difficulty in measuring and working to exact tiny dimensions. The 5-leaf > pinion I need to make at the moment has a flat in the bottom of the > tooth spaces of just 0.2mm width and a tooth depth of 0.45mm or > thereabouts. Have you ever tried to get accurate measurements across > sloping faces at this scale?? ;-) ... snip > I have used my little cnc miller to successfully make cutters but it > usually takes a couple of goes at least and it is a problem to get > relief on the cutting edges. My idea for generating the pinions - (which > are one-offs and not multi productions, therefore the time involved in > making them is of no consequence) - is to use tiny grinding disks like > the thin cut-off disks they sell for Dremels but thinner ( dentists use > ones of 0.2mm thickness ) and grind the blank to shape. I have used this > technique to grind small shafts and it works fine provided the speed of > the disk is high and the feed is slow. I was actually very surprised how > well the disks worked and after half an hour's grinding, the disk was > still virtually the same size as when I started. So, if I can just work > out the way to compose a G-code file with the multiple loops, I think > the idea is well worth a try.
Do you have pictures of your setup? > Maybe for your purposes, you could consider using a similar idea by > finding a supply of larger grinding disks - angle grinder cut-off disks > maybe? > > -- > Best wishes, > > Ian My original concept was to use generic tooling and have the CNC magic do the custom shapes. It's turning out that there is no generic tooling that will work. So a compromise of being able to easily make generic tooling with basic shapes may be the way to go. I was thinking that the generic tooling would do the roughing and your Dremel style disks would do the finish work on the mesh areas. The roughing tools would be like slot saws, only with side relief, so the sides could take light cuts. You would still need a set consisting of square cutters for square bottom roots and half rounds for radiused roots. Different widths in round and square cutters would also be needed, but they should be easy enough to make, so you make them when you need them. I am thinking that regrinding end or face mill inserts might be the way to go. A small insert would be easier to get the proper geometry on. The first rough cuts would be a slot across the gear face for the right and left root corners. Then, in my setup: http://www.wallacecompany.com/machine_shop/gear_cutting-2b.png the A axis would be rotated a little, the Z would come up a little to match the first tangent of the curve. Y could stay the same if the cutter doesn't touch the opposite gear curve, otherwise it can be adjusted for clearance. Then the "slot" is cut. Since we a roughing, six or seven tangents may be all that are needed. The X sweep will always be the same, the coordinates of the tangent point can be derived by a drawing program, so you would have six or seven loops that increment Y, Z and A which can be mirrored for roughing the opposite gear face on the "back" side of the setup. These two loops would be run for each tooth by incrementing A by a constant. A gear with a few teeth should be managable to g-code by hand and could be a model for a software utility. The tips of an involute gear could use the same method with an standard end mill (shown in violet in the link above). The tips of a cycloidal gear should already be done. So, at this point, we have a rough gear. Since the cutting load would be light, I think a Dremel style diamond disk would work. The roots should not need to be touched, so only the side of the disk will be used, which is good because I don't think the circumference is meant to be used anyway. The same method of sweeping X on tangents could work for finishing but allot more tangents would be needed. I think sweeping A and incrementing X might make a smoother profile and I think my CAM has a five axis processor so the g-code should not be a problem. Either way, with sweeping X or A the key is to derive the tangent points of the mesh area (30 or 40?) and looping those points a gazillion times. If this system could work, the only semi-custom tooling would be the face mill inserts. A standard surface grinder would make square inserts. Adding an A axis might do the half-round inserts. I suppose you could cut a large number at one time. Darn, now I have to buy a surface grinder. Another thought comes to mind, could wire EDM be used to cut very small gears and pinions? -- Kirk Wallace (California, USA http://www.wallacecompany.com/machine_shop/ Hardinge HNC/EMC CNC lathe, Bridgeport mill conversion, doing XY now, Zubal lathe conversion pending Craftsman AA 109 restoration Shizuoka ST-N/EMC CNC) ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://sourceforge.net/services/buy/index.php _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
