Re: [CAD_CAM_EDM_DRO] Re: unipolar or bipolar?

audiomaker2000 wrote:

> First, I've never retrofitted a machine before. I obtained a
> Bridgeport BOSS with smashed control to use as my first attempt.
>
> First, the axis motors are 8 wire NEMA 42 steppers.
>
> Second, an offlist email informed me that they are rated at 7-
> 12amps, but nobody seems to know the voltage ratings.
>
>
> I have no idea (nor would I) if these motors were unipolar, bipolar,
> full, or half wound, nor the type of original drivers or their
> ratings.

They are anything you want. 8 wire motors can be connected in a
variety of ways. Probably the best performing is to parallel the sets
of windings. You can measure the resistance with a DVM and figure
the voltage that will be dropped across that resistance at the rated
current. V = I * R ( where V is volts, I is current in amps and R is
resistance in Ohms.) You should add a 10 - 15% increase to the
R because the copper resistance increases as the windings heat
up. But, you really don't need to know this voltage.

> I simply have 3 motors with 8 wires coming out of them, a
> source of DC power, 3 210(42)'s, and a boatload of electronics
> removed from the original.
> I have no idea if the originals were microstepped or not, but I'm
> assuming they were since the lowest jog increment on the BOSS system
> is .0001.

No, I don't think the BOSS had microstepping.

>
> My goal is this....
> Get the machine running in the "correct" way under PC control. It
> can run the same, better, but no worse as far as speed, power, and
> efficiency are concerned (regarding the axis drives and motors) than
> the original system.
>
> My concerns are these...
> First, the 210's are rated at 7amps and according to rumor that is
> where the motors start.

Well, you may be forced to run these motors in series, which will cut the
maximum speed significantly. You could try it both ways and see which
gives the best performance at 7 A. Parallel will cut the holding torque
some, but the gain is the high speed torque will improve dramatically.

>
> Not knowing the original voltage of the motors (it doesn't say
> anywhere...even internally), I cannot do the 4-25X calculation, nor
> would I know which is best if I had a choice (would 25X be too close
> to the max and would I need to be constantly checking the motors for
> overheating...etc).
> Being that the original system had all these resistors in the line
> would I be better off with a higher amprege, lower voltage driver
> system like some of your competitors make (I only ask because they
> have nearly the same resistors pictured in on of their setups), or
> am I still doing the best with the 210's?

Since the original drive used 60 - 80 V or so, running the Gecko at
about 70 V would be quite reasonable.

> Although I know a fair amount about machines as a whole, I am quite
> ignorant regarding motor windings, microstepping, and all the
> formulas that are entailed in motion control of the axes, and I must
> admit that I am at the point where I am just tempted to hook it up
> and if it sorta works, say "well, I guess that's right".
> To make matters even worse, I've never done much more on the BOSS
> mills than jog the axes, so I don't have any basis to compare the
> original performance with the retrofit performance to allow me to
> tune the new system. I have more experience with machines with
> cutting speeds that more than likely far surpass even the rapid's of
> this BOSS. In other words, I don't know what I should expect out of
> it and when to be disappointed or delighted.

Unless it is a late stepper BOSS, the performance is not too hot. Good
drivers may help it perform much better than stock! Several people
have reported quite good results with the Gecko drives on the BOSS
machines.

Jon