On 03/05/2010 07:23 AM, Andy Pugh wrote:
> (This is one of potentially several reposts of questions that never
> made it to the list due to operator error)
>
> I am unsatisfied with the results of my attempts at gear-milling. I
> think this stems partly from me not knowing which of the three cutters
> I have is for what tooth count, or what the addendum and dedendum is
> meant to be for each cutter.
> I have decided that as a set of milling cutters comprises about 6
> cutters at £10-£20 each, a hobbing cutter which will make any size of
> gear seems like a good plan.
>
> Hobbing, as you almost certainly know, involves rotating a hob and the
> work on not-quite-right-angles axes in a fixed ratio. Some parts of
> that are very easy with an EMC-controlled CNC machine (mine is one of
> these one of these, convertulated)
> http://www.amadeal.co.uk/acatalog/CX23A-750_Multi-Purpose_Lathe_Milling_Machine.html
> I have an encoder on the lathe spindle and it is a simple
> matter of connecting that to the rotary table in HAL to keep them
> geared together permanently at any
> arbitrary ratio. (there is even an encoder_ratio module for this sort
> of thing, but that isn't exactly right for what I want)
>
> However, the stepper-driven rotary table tops out at 5rpm. I can swap
> the motor for a servo, and that gets me 17rpm. The lathe and milling
> spindles don't really like doing less than 200rpm. I want to cut
> 12-tooth pinions, and that really isn't enough overhead for the rotary
> axis to catch up and synch.
> It is also a major change to the control box wiring and the software
> setup to swap to servo motor. However here are advantages, and I have all the
> parts.
> Holding the rotary axis at the correct angle to a hob held in the
> milling head is difficult. (though obviously doable)
> clamping it to the table at an angle to the lathe spindle is easy, but
> then I can only make gears of one diameter.
> It would be nice to be able to rotate the milling head. There is
> presently no facilty to do that, but there is a joint between two
> castings where the facility could be added.
>
> I am thinking of making a faster rotary axis using an ER32 collet
> holder I have on a 3/4" ground shaft and some taper roller bearings. I
> would drive that with a spare stepper I have, at about 10:1 ratio. (or
> one of the little servos)
> I can't believe that there are very large rotating forces on a gear
> during hobbing, I think it is probably largely balanced.
>
> If I do make this rotary axis, then I can either mount it at an angle
> to the milling spindle, or at an angle to the lathe spindle. In the
> latter case I would need a vertical slide, but I do have a spare
> compound slide that the CNC machine no longer uses. In either case I
> would need a compound feed on the two translational axes, or the
> offset angle will mean that the gear moves down the axis of the hob,
> giving a very slight second-order helix angle (if I am visualising it
> right)
> Modifying the milling head to tilt avoids this problem.
>
> The lathe spindle already has an encoder, the milling spindle is still
> waiting.
>
> Ideally I would make a hobbing head to clamp to the lathe saddle to
> cut gears held in the lathe spindle, with a nice powerful servo motor
> and encoder just like a real gear hobbing machine. But if I had a
> suitable servo motor like that it would already be my lathe headstock
> motor.
>
> So, rambling over, does anyone have any ideas for ways of arranging
> the shafts and slides that I have not thought of yet? (one idea has
> just occurred to me, I could mount a flexible coupling in the milling
> or lathes spindle. mount a secondary pair of bearings elsewhere, and
> put the hob on that to get the required angle).
> I think my favourite so far is a combination of new, faster rotary
> spindle holding the gear mounted on a vertical slide on the lathe
> saddle, with the hob in the lathe spindle.
>
> --
> atp
>
>
Here is a stepper based 4th axis that may work the way you're describing:
http://www.cnczone.com/forums/showthread.php?t=100160
It is 4 to 1 ratio, and set up for microstepping. It should be able
to keep up with your spindle. I'm assuming that yow want to use a
spiral hob?. If so, put some shims under the front of the axis, to
match the helix angle of the hob.
The better way, would be to create a straight hob, and just index the
axis for each tooth. The idea is the same, it's just slower.
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