Luckily, I got the tape measure out last night.
I can't fit anything bigger than the standard Bridgeport in! That only fits because the head is between the roof trusses, so that rules out practically all others I think as even an Interact is 90" tall which i think is 12" more than a standard machine.
So my options now seem limited to converting my own BP, buying another BP and converting that then selling my on on or not bothering.
Just had a lengthy chat with my supplier of bits and they seem to favour heavily towards 8NM stepper-servo's for this task. The specs read well, basically a closed loop stepper and DSP drive with variable current etc. Price was about £250 per axis for motor and drive with cables.
Being stepper also means i can use my existing knowledge of BOB's and USB motion controller etc.
More options to throw in the pot.
Stepper-servo motors, I know I need the drive plus a BOB plus a motion control - done that before and it works.
AC servo motors, I know I need the drive amp plus ? - what goes between the drive unit and the PC?
Just trying to price up, the price is identical between 8Nm stepper-servo motors and drives OR 750W servo motors and drives - I have no idea how to choose between one or the other, one I know the other I do not.
I am advised that 750W is ideal for the job so thats one decision, also the 8Nm step-servo is ideal it seems.
Anyone want to fill the gap?
I know the servo is rated to 3000rpm and the stepper to maybe 1000rpm?
The stepper torque is way down where it's needed but the servo is where, linear spread?
Looks like a ballscrew kit is going to be around £1300 so thats a fair chunk of cash in one hit, the rest I need to get right first time.
Ok well in theory there's no difference in requiremnets between Servo and stepper for connecting to PC. However in practise there is specification difference.
Servo's require much higher Pulse frequency and much cleaner signals. Now your average £5 Bob doesn't cut it and the Parallel port is just doesn't cut the mustard.
The speed and resolution you get from Servo is dependant on the encoder fitted. The encoder also determines the pulse frequency required.
For instance most Servo come fitted with 2500 count encoders which are quadrature encoders so this number is *4 so 10,000 pulses required for one revolution at full speed. If 3000Rpm that's 30,000,000 pulses in 60s. Break this down to seconds and that's 500,000 pulse per second or 500Khz required to achieve full motor RPM.
Now PP is only 25Khz or at realistic best 45khz 45,000 pulses. At best this means you'll only get 270rpm out of your 3000rpm motor.
Now most servo drives provide a pulse multiplier so one pulse becomes 2, 4 etc depending on amount set. This however comes at the cost of Encoder resolution which gets reduced by the same amount. IE: 1:2 Multiplier 10,000 = 5000ppr 1:4 2500ppr etc.
Not problem your thinking I don't need all that resolution anyway but it's not just resolution that suffers, so does smoothness of the motor and really the whole point of servos is for there superior Resolution and smoothness along with linear torque.
So bottom line is if using servos then really you need an external motion control card that gives you required pulses of good quality.
On more positive note there are other advantages to servos than just resolution and linear torque. 3000Rpm often is too fast so you'll use ratio which increases torque so smaller cheaper servo can be used.
Torque is linear and the rated servo torque is when at full motor speed unlike steppers which is when at stand still and drops quickly when rpm rise. Large steppers drop very quickly and not much use after 900-1000rpm.
Servo drives are powered directly from mains so NO PSU required.
If I was converting a mill then I'd use servos because they are much stronger than Steppers and smoother in operation. Torque is linear so much stronger when your pushing hard and deep.
Last edited by JAZZCNC; 14-05-2016 at 12:31 PM. Reason: Brain fade RPm wrong
The UC300 USB motion controller goes to 100kHz so that seems ok, I only suggest that one because I have used the smaller UC100 with total success and the price is pretty good too. However I do not know yet if that interfaces direct with the servo drive unit?
Not having a high power PSU is a big plus as well and as there is no price difference it looks like servos have won the toss :)
Would you go with 750W motors or smaller??
2:1 reduction for resolution & torque increase, more?
Servo drives seem to have lots of settings, what is used - speed & torque, positioning..... All new to me but i have the manual for the drives now.
Really Ethernet Smooth stepper is what you want or feeling really flush then go for Cslabs Controller.
Matt (kingcreaky) on the Forum used 750W servos with 2:1 ratio on BP Interact sized mill and was happy with them. (He's got a video some where on this site)
Difference between Step/Dir and Analog is mostly how they are controlled and how the Closed loop works. With S/D the loop is closed only between drive and Motor. Analog the Loop is extended between Controller drive and Motor.
The result is that Analog feeds the Encoder position back to the controller, which it must do to work, which then checks this position with the commanded position and if wrong sends corrective signals.
The upshot is that the DRO's always display the actual position of the table, no matter if mach or controller as commanded this or you whind it there by hand. In affect works just like Manual DRO's.
Step/Dir on the other don't do this. It sends commanded position to the drives and takes it on faith that it arrived there. The drive then takes control of getting to commanded position and any corrections needed.
So the DRO's only show the commanded position and if it's wrong or if for instance something is loose you'll never know your out of position. Essentially works just like your steppers now with the exception the drive is checking the motor actually moved the distance told.
Bottom line is S/D is easier to setup. Analog gives more feed back but takes more setting up and tuning of drives.
See this video I made showing 1.8Kw servos controlled via Analog.
Last edited by JAZZCNC; 14-05-2016 at 02:45 PM.
Nice video, that would be what i would like ideally - feedback.
So The option is now ESS at about £150 and no feedback or CSMIO/A at about £490 with all the bells and whistles.
The motors I was looking at are from the same guy as in your video, drives the same too.
The 750W suggestion comes from the CNCZone where a builder mentions doing 30+ conversions of this type and always uses 750W, 1kw was too large and 400W too small.
He uses 1:1 or 2:1 but I favour 2:1 as I want to get the most torque I can and the machine won't handle massive rapids.
Ballscrew kits seem to be 5tpi so 5.08mm pitch
Question is - is the benefit of real-time position DRO worth the extra cash?
Last edited by Davek0974; 14-05-2016 at 03:10 PM.
Then you have 24V I/O etc not to mention you don't need a BOB. This Alone is BIG Plus, esp when using servo's. Don't confuse the IP-S with ESS they are completely differant animals.
While they seem expensive they are not actually that bad when you consider that it's one complete unit nothing else is required. It's also Industrial spec and high quality.
By the Time you have Bought ESS and Decent quality BOB like PMDX126, which about the only one I'd use with servo's plus Spindle speed Control board then your not long way behind the IP-S or IP-A.
They really do not compare other than both use Ethernet and provide 4Mhz after that it's all down hill for the ESS.!!
Last edited by JAZZCNC; 14-05-2016 at 03:29 PM.
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Ok JAZZ, i'm sold on the CSMIO/A unit, does it have PWM output or is that an add-on? Controlling the VFD would be nice.
Will be reading up on that tonight.
Rigid tapping is a bugger because you need to get an encoder on the spindle - this is a nightmare the Bridgeport manual machines. Possible later add-on though.
Yes I think those motors will be Nema34 size.
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