Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: john15 on November 19, 2009, 05:20:48 AM
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Hi al.
Im trying to retro fit a bridgeport sized vertical miller
My leadscrews will be 5mm pitch ball screws
I am not looking for very high speeds, just home shop use and reliable / acurate
What sized stepper motors should I aim for (I think I would prefer to go steppers rather than servo)
What would be a decent driver size to use
Are any of the multi axis boards, max seems to be 3amp per axis, worth considdering.
Thanks for any advice
John
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John,
If you can give an indication of the size of your machine / model then someone will be able relate to theirs and make a better suggestion. You can also go to DanaherMotion site and download a sizing program, and after defining all the parameters, it will give you torque requirements etc for motors.
RICH
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I have Geckos and 916 Ozin steppers on my Bridgeport Interact series 1, works well enough. Gearing between motor and ballscrew is 2.5:1 and I get about 2500mm/min (100 ipm)
Hood
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I have used one brand of multi axis drivers. I would never consider going that route again. Mine did not perform well at all, blew chips like crazy, and was really more of a science project than a reliable driver.
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I have to second the notion to avoid multi axis setups.
For steppers, I have to also second using big steppers as noted by Hood on his machine. Definitely 900 oz-in or better.
For stepper drives, I can confidently recommend the Gecko203V (their top-of-the-line). My preference from those I have used is the Centent commercial drives. Noticeably better than the Gecko . . . and noticeably more expensive as well.
DC brush Servo setups are now comparable in price to steppers for mid to larger machines if you accurately compare the real world performance power wise. In my opinion, there is no reasonable comparison performance wise. The two types are worlds apart in their characteristics.
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Hi John,
I have a Gorton 1-22 mill, a little bigger than the bridgeport. It has really old NEMA 42 Pacific Scientific motors with Geckos on X and Y and a big DC servo on the knee, Gecko drives all-around. I would avoid going with a knee-drive, it is slow, heavy, and it was pretty difficult retro-fitting a ball screw in there.... I run 80IPM rapids all around and can take heavy cuts at 30IPM no problem. Gecko's are superb stepper drives, I have a small preference for Vipers for servo (the preference gets bigger as the motor gets bigger :-))
Hope that helps,
Dustin
P.S- Direct drive on the steppers, .2in pitch ballscrews.
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The mill is a cincinati toolmaster, the same size as a basic bridgeport. 4foot by 10" table approx, travel about 18" by 10", Quill about 4" travel.
Motor at present is 1.5HP 1000rpm with 8 speeds via pullys, driven by an inverter drive.
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Thanks for the replys so far.
I was planning to avoid the knee and have Z on the Quill.
I have already got on order 5mm pitch ballscrews which is 0.2 inch
If I were to go servo instead what size and type of motor would work
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Thanks for the replys so far.
I was planning to avoid the knee and have Z on the Quill.
I have already got on order 5mm pitch ballscrews which is 0.2 inch
If I were to go servo instead what size and type of motor would work
I did my BP clone with 850oz-in peak/170 oz-in continuous, 72V, 4200RPM NENA 34 servos from www.homeshopcnc.com, and Gecko G320s. The X/Y drives go through 2.5:2 GT2 belt reducers to 4-pitch 1" Nook XPR ballscrews. The knee goes through a 4.8:1 XL belt reducer to the stock bevel gears and leadscrew. I'll have a quill drive on it in a few days, at which point the knee will be used only for tool length compensation. X/Y will do at least 350IPM (I don't have the nerve to go faster than that, but the hardware does....), the knee does 75 IPM. All three axes are almost unstoppable - they'll snap off a 1/2" tool without losing position.
If you go with steppers, something in the 1000-1200 oz-in range (NEMA34) should be plenty, especially with 5-pitch screws, and direct drive will be fine, making the hardware a bit simpler.
Regards,
Ray L.
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I can second the recommendation for the HomeshopCNC servo motor. Note that is it similar to the Keling drive, but 72V instead of 90V. USdigital or Aveco encoders work well.
My enthusiasm for the Gecko stepper drives does not extend to their servo drives. The Geckos have very crude tuning and they 'sing' or bounce between encoder counts constantly, which is quite annoying. The functional problem is that their low bandwidth chokes the motor RPM with a high resolution encoder.
In any case, a 20A drive is not the best match for that motor (in my opinion), as you loose a lot of acceleration with the 20A limit. I ran that very motor thru its paces with the Gecko320/340 and several other drives and there is a very big difference in the preformance. You can read the review here: http://www.thecubestudio.com/ServoDriveReview.htm Pay particular attention to the braking resistor issue with larger motors.
My preference is for the CNCdrive Dugong servo drive, which I use with the NEMA34 motor referenced earlier. I have 5 of them now and am quite happy with the features, especially the tuning software. The only issue is availability. They are often sold out.
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Simpson,
I have a big machine with DC Servos, GE, the X ans Y axis are 34 Amps. Cont. and the Z axis is 51 Amps. cont. Do you thing that the Dugong drive will work with this DC ServoMotors??
Thanks,
Jorge
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You did not mention voltage, so assuming you are OK with that spec, then it is down to amps. From all of my research and also proved out in my own testing and experience, I can not reccommend anyone use the 'continuous' rating on a servo motor as the basis for sizing servo drives for a machine tool.
The 'continuous' rating is exactly that; the limit of heat dissipation if the motor is producing full power continuously. Unless you are powering a conveyor belt, fan, agitator, vibrator, or some other industrial machine that is truly continuous, you can pretty much ignore that figure. On a mill or similar machine, 'normal' operation is anything but continuous. Repeated momentary peaks (accell/decell) with plenty of 'rest' (running at feedrate) in between is the norm. My assessment in a nutshell is this: consider only the peak. if you have a 50A peak motor and you run it with Gecko, then what you end up with is a 20A motor. If you run the same 50A peak motor with a Dugong, then for practical purposes, you have a 35A motor. You will only have a 50A motor if you run it with a 50A drive.
To answer you question directly, yes a Dugong would 'work' (provided the voltage is within spec), but it would limit the available peak amps to 35 and your motors are thirstier than that. The motors will likely run fine, but you will not get the acceleration they are capable of, so you would need to realistically assess your needs in that regard. There is no rule stating that just because you have motors capable of pulling anchor bolts out of the floor, that you have to use all of that power. At least one poster here has 38A motors running off 20A Geckos and seems satisfied with the setup. HomeshopCNC was mentioned and they have a scope trace puiblished on their site clearly showing the Gecko shutting down the party at 20A, thereby loosing half of the motors performance. I am not saying this is neccessarly a bad thing, but it is a fact to be aware of and consider in your thinking. The performace at 20A might be fine for some applications. Similarly, 35A performance of the Dugong may be acceptable with your big motors, but you will not get the max out of the motors.
One would imagine the peak on your motors is far above the capabilities of any of the 'entry level' drives (including the Dugong) at this price point. Drives that would get the attention of your big motors would be far more expensive, so you have cost factor to weight. My advice is to be realistic in your expectations. If you make an informed decision to sacrifice some performance to save many hundred of dollars on the drives, and your application will perform acceptably without the max acceleration your motors are capable of, then you will have done a good job of selecting drives, IMO.
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Thanks for your response. They are 90V DC.
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I can sacrifice some performance on that machine, because that is better to a useless machine. Also the specs. from new of that machine are not so great regarding speed, It is a 1980 Brown & Sharpe 1500VC, and here in the manual say it is a 400in/min. rapid positioning speed ( I belived It was half of that ). I also send the information to cncdrive.com and will reply here the answer. Thanks,
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You might want to email these guys and see what their price is like, http://www.semis.hit.bg/servoe_d.html
Dont know what they are like but they have seemingly been in the industrial analogue drives market for a long time so should be decent.
Hood