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servo selection.......
« on: October 01, 2008, 07:28:51 AM »
Hi all,
Can you guys help me out with servo selection  for milling machine.
I have an old milling machine and i want to fit it with mach 3 along with servos..............for better stability.
I have two cheap options for servo
                                                 A) Delta Electronics servo
                                                 B) Yashkawa  servo
I guess 4Nm will be good enough to start with but i am not sure what PPR to choose for encoder. 2500 PPR is the minimum available encoder resolution. And its a 3000 Rpm motor.
What is the max. encoder frequency that a mach3 can handle.
Would it be more accurate to use G100 with 600Mhz PIII  laptop  instead of high end desktop PC .

Thanks
Re: servo selection.......
« Reply #1 on: October 01, 2008, 10:37:35 AM »
I have just CNC'd my knee mill using servo motors from www.homeshopcnc.com.  They have 500 line/2000PPR encoders.  Mach3 never "sees" the encoders - they are only connected to the servo controllers (Gecko or whatever).  I could not be happier with the way my mill now works.  It's fast, powerful, and accurate.  The motors have enough torque to snap off a 1/2" endmill without losing position!  I used Geckos G320s, and a SmoothStepper (which was MORE than worth what it cost!), running Mach3 on a lousy 540MHz PC.  The servos are "geared down" 2.5:1 with GT2 timing belts.  (BTW - I can provide CAD drawings of the belt reducers I designed if you like - they are easily adaptable to other machines by changing the adaptor plate).  My power supply is 70V/20A.

Regards,
Ray L.
Santa Cruz, CA
Regards,
Ray L.

Offline Hood

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Re: servo selection.......
« Reply #2 on: October 01, 2008, 03:47:51 PM »
2500ppr will mean Mach needs to put out 10,000 pulses per rev. If you dont have electronic gearing and are using the parallel port you are looking at approx 1350mm/min (asuming 5mm ppitch). There  is a device called the Smooth Stepper (as Ray has mentioned) which pulses externally from the computer and it allows pulses up to 4MHz so it is perfect for high count encoders. I have 2000 line encoders and using 1:2 electronic gearing in my drives I was limited to 3500mm/min on my lathe, now with the SmoothStepper I am 13,200mm/min and could actually go more than twice that speed if the motors could but as the lathe is heavy 13m/min is quite fast enough.

Hood
Re: servo selection.......
« Reply #3 on: October 01, 2008, 06:24:46 PM »
Go with the minimum encoder count available.
Use a servo drive that has "electronic gearing" such as the
G340.  (Not the best, -but the best bang for the dollar)
Save your money and don't bother with a smooth stepper.
Get a 1.8 Ghz or better computer running XP or XP pro.
Use the parallel port to drive the servos (through a break out board)
and you will get all the rapid speed a knee mill can handle.
Forget the G100.....99.5% of all mach users use the parallel port. 
I just picked up a P4 at 1.8 ghz and 512mb ram with 40 Gb hard drive
for $199.
I use a similar machine on my BP series 1 with no issue. 

Scott



Offline Hood

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Re: servo selection.......
« Reply #4 on: October 01, 2008, 06:32:35 PM »
Scott, what do you reckon "all the rapid speed a knee mill can handle." is?
just curious

Hood
Re: servo selection.......
« Reply #5 on: October 01, 2008, 07:21:10 PM »
Original poster is obviously not going to use the machine in a commercial
environment.  If he was a good used VMC would be a much better
 than an "old knee mill"

He can get about 220 ipm rapids with the PP and a divide by 4 on the drive.
This would take the table 18" in 4.9 seconds. 
Are his ballscrews designed for that kind of rotational inertia?
Does he even have ballscrews?  Was his machine designed
to take high speed rapids?

Speed is nice to oooh and ahhh the bystanders but how much time does
it really save?  Very little unless you are making parts day-in and day-out.

I set my BOSS to 80 IPM rapids and I program so I spend my time cutting
parts and not rapiding.

A router, plasma, etc would be a different story as you are moving a
lower mass and usually moving it a much larger distance.

Scott

« Last Edit: October 01, 2008, 07:25:23 PM by polaraligned »

Offline Hood

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Re: servo selection.......
« Reply #6 on: October 01, 2008, 07:35:34 PM »
My Boss is 2500mm/min (100IPM ) and that is plenty fast for that but the Beaver is in a different league and 10,000mm/min  is what will be used on that. Dont know what machine the OP has but just when he mentioned he was thinking of 4Nm Yaskawa I presumed it was  probably more than just a Boss or similar, but maybe not.
Hood
Re: servo selection.......
« Reply #7 on: October 01, 2008, 08:26:06 PM »
What kind of travels do you have on the Beaver?

My next retro is a Series 2 CNC.  15" x 30" travel,
huge box ways, table never overhangs saddle, 5000#.
Pretty much as close as a knee mill can get to a VMC. 
The iron is fully rebuilt and I am heading into the controller
now.  I don't know if the SmoothStepper is worth the
extra money for this machine.   So many machines running solidly with
just the PP.  I ordered Granite drives for this one.  Geckos are to
"hobby' for me.  They run my Series 1 Boss-  but not without
occasional problems.

Scott

Re: servo selection.......
« Reply #8 on: October 02, 2008, 02:04:45 AM »
Dont know what machine the OP has but just when he mentioned he was thinking of 4Nm Yaskawa I presumed it was probably more than just a Boss or similar, but maybe not.
Hood


Thanks for all your valuable feedback.

Dear Hood I got two old cnc machine made by (Kirloskar Indian manufecturer) Its 10 yrs old actually the system was fitted with their proparitry hardware n now they don't support for it any more. Its been laying at my frriend factory n he was abt to retrofit it with Siemens 810 cnc  ( he is in cranes and hoist manufecturing business.).
So i came up with my idea of turning this one machine into mach3 Pro CNC. And let it stand far above Siemens 810 in terms of features and performance.

This mach3 replacement has to be industrial grade so, i go with yashkawa coz its available in india.

The system integrator told me that this servo have simulated encoder o/p. That means for 2500 PPR i can have any PPR for mach3 from servo itself.

Once again thanks alot guys.

Offline Hood

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Re: servo selection.......
« Reply #9 on: October 02, 2008, 02:34:45 AM »
What kind of travels do you have on the Beaver?
16 X 32 travels and around 2000 - 2500Kg if I remember correctly. Has Box ways and a motor on the knee which I am planing on fitting with a servo at some point. The quill is 4 inch dia 40 taper with 6 inch travel.


Thanks for all your valuable feedback.

Dear Hood I got two old cnc machine made by (Kirloskar Indian manufecturer) Its 10 yrs old actually the system was fitted with their proparitry hardware n now they don't support for it any more. Its been laying at my frriend factory n he was abt to retrofit it with Siemens 810 cnc  ( he is in cranes and hoist manufecturing business.).
So i came up with my idea of turning this one machine into mach3 Pro CNC. And let it stand far above Siemens 810 in terms of features and performance.

This mach3 replacement has to be industrial grade so, i go with yashkawa coz its available in india.

The system integrator told me that this servo have simulated encoder o/p. That means for 2500 PPR i can have any PPR for mach3 from servo itself.

Once again thanks alot guys.

 I dont have Yaskawa drives but use similar style Giddings and Lewis  drives with AC Servos and the encoders are 2000line. As long as the Yaskawa will accept Step/direcion signals you are fine but to get the full benefit you will need a SmoothStepper. I think Yaskawa call the Step/Direction signals something like Pulse/Signal.  The Yaskawa will have electronic gearing but personally I feel its not a good thing for a CNC machine as the higher you set the gearing the more cogging you will get at lower speeds. On the lathe I at first had the gearing set to 1:4 but had to turn it down to 1:2 and even that had cogging at crawling speed now with the SmoothStepper I dont need any electronic gearing and  still I am only using about 1/4 of the pulse rate its capable of.
 The lathe Has 10Nm motors amd the Beaver mill has 5Nm motors on Y and 3Nm on X and Z.
 You dont need encoder output from the drive for Mach, it is the other way round, the drive needs to accept Step/Dir in. Mach is open loop but the drive closes the loop.

Hood