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Steps per Unit
« on: February 10, 2009, 02:38:04 PM »
Hi,
I am a little confused on the steps per unit.  I have a Hurco SM1, .200" pitch on my ballscrews, directly coupled to the servo on the X axis, 1-1 timing pulleys on both the Y & Z axis.  1000 CPR encoders on all 3 axis, Larken Viper servo drives that have the option of 0, 4, 6, or 10 step multiplier.  I also have a smoothstepper.

So...
My screw revs per unit = 5
Motor steps per rev = 4000
Mach 3 steps per rev =  20,000  (5 x 4000)

OK, does the step multiplier have any bearing on the Mach 3 steps per rev?  Larken recommends 4x multiplier. 

Thanks

Tom
1986 Hurco SM1 Stock ElectroCraft Servos and encoders, Larkin Viper 200 drives, Smoothstepper, CNC4PC C23 BOB, Dura Pulse GS3 VFD, Mach 3

Offline Hood

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Re: Steps per Unit
« Reply #1 on: February 10, 2009, 02:57:25 PM »
Yes if you use a step multiplier you need to reduce your Steps per by that amount, ie if its 20,000 and 4x multiplier then your steps per would be 5,000.
However I would suggest that you probably do not want to use the multiplier (also called electronic gearing) as this can lead to cogging as slow revs. The usual reason that electronic gearing is used with Mach is the parallel port can only put out so many pulses per second and if you want the rapids up you either need low count encoders or use electronic gearing. As you have a SmoothStepper then you will not suffer from that problem.
 There may be another reason Larken recommend using the gearing, I doubt it, but may be best to ask.
Hood
Re: Steps per Unit
« Reply #2 on: February 10, 2009, 03:51:15 PM »
Hood,
Here is a post from Larken...

Step multiplier, Why use it ?

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There seems to be some confusion by some people on what the step multiplier is for. Some people want the system to have the highest possible resolution of the encoder ( or encoder X 4 as in the viper) , so they want to run no multiplier ( 1 ).

On a servo system the system works off feedback error. If there is no error, then the drive puts no power to the motor.
Rarely under cutting and moving a machine with any friction can the drive keep a zero error.

On the bench with no friction or load, the motor will respond to a 1X multiplier, but on the machine and cutting, its just an illusion and requires very high steprates to get any speed.

On our machines i never run it less than 4X.
1986 Hurco SM1 Stock ElectroCraft Servos and encoders, Larkin Viper 200 drives, Smoothstepper, CNC4PC C23 BOB, Dura Pulse GS3 VFD, Mach 3

Offline Hood

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Re: Steps per Unit
« Reply #3 on: February 10, 2009, 04:07:02 PM »
I am not really sure what he is trying to say by that, sort of sounds like he is saying to get a decent rapid you will need a high step rate and that is not always possible with the PP, as you have the SS then its not an issue.
 I have large AC servos on my lathe and dont use electronic gearing and it works great but then again my drives are Industrial type so maybe that would make a difference.
Best bet I suppose is try with 4x and with none and see what you think.
Hood