Servo Help, Series 1 Retrofit

[Jet A]

Hello,

I have a series 1 bridgeport that i am in the process of revamping. (as you can probably tell by all my threads). I need to replace the older servo motors, as they are mostly broken or unusable. They are nema 42 baldor, no name plates, 7.3 amp 80v 1400 rpm rated. They have a .625 shaft.

Here is my question. What is the best replacment for this. The less modifcations the better. (outside machine work is expensive). I am ok with going with stepper but prefer servos based off other forum member opinions. I have been eyeing the Keling Technologys 3 axis package with the servo motors rated at 1200 ozin (or something close to that.) Problem is, they are rated at 4000 rpm. Do i need to run a reduction system with this?

Are nema 42 stepper motors a better choice , due to less modifications to my system. Would a reduction system be required of this?

I am really looking for some good ideas. I have no idea how much torque is required of these things.

Looking forward to hearing some good suggestions

[HimyKabibble]

Originally Posted by Jet A Hello, I have a series 1 bridgeport that i am in the process of revamping. (as you can probably tell by all my threads). I need to replace the older servo motors, as they are mostly broken or unusable. They are nema 42 baldor, no name plates, 7.3 amp 80v 1400 rpm rated. They have a .625 shaft. Here is my question. What is the best replacment for this. The less modifcations the better. (outside machine work is expensive). I am ok with going with stepper but prefer servos based off other forum member opinions. I have been eyeing the Keling Technologys 3 axis package with the servo motors rated at 1200 ozin (or something close to that.) Problem is, they are rated at 4000 rpm. Do i need to run a reduction system with this? Are nema 42 stepper motors a better choice , due to less modifications to my system. Would a reduction system be required of this? I am really looking for some good ideas. I have no idea how much torque is required of these things. Looking forward to hearing some good suggestions

Pretty much any modern DC servo motor will be rated at 4000RPM or so, and will *require* reduction to make them usable. 3 or 4:1 is pretty common. If your existing motors are direct drive, then your path of least resistance will be steppers. There is absolutely nothing wrong with steppers, if they are properly sized, and properly driven. In machining performance, they will be every bit as good as servos. Worst case, you'll give up a little bit in rapid speed, which is really not all that important in most jobs.

1200 oz-in servos are overkill for a BP. I have 850 oz-in servos with 2.5:1 reduction and 4-pitch screws, and I can break tools all day long without losing position. And, it'll do 400 IPM rapids. With 1200 oz-in servos, you'll gain nothing in performance, and increase the chances of breaking the machine if something goes wrong. Bigger isn't always better.

Regards,
Ray L.

[Jet A]

Will a stepper in this case require reduction? OK, do you have the 850's from Keling? where is the best source for the servo?

Speed is not the ultimate goal for me. i just want to get this thing running so i can start playing with cnc.

[Jet A]

Originally Posted by Jet A Will a stepper in this case require reduction? OK, do you have the 850's from Keling? where is the best source for the servo? Speed is not the ultimate goal for me. i just want to get this thing running so i can start playing with cnc.

By the way, i have 5:1 ballscrews

[HimyKabibble]

Originally Posted by Jet A Will a stepper in this case require reduction? OK, do you have the 850's from Keling? where is the best source for the servo? Speed is not the ultimate goal for me. i just want to get this thing running so i can start playing with cnc.

Steppers can be run with no reduction, as they have high torque at low RPM. The only trade-off there is resolution. But, realistically, a direct drive stepper will do just fine.

Regards,
Ray L.

[Sparky_NY]

Originally Posted by HimyKabibble Pretty much any modern DC servo motor will be rated at 4000RPM or so, and will *require* reduction to make them usable. 3 or 4:1 is pretty common. If your existing motors are direct drive, then your path of least resistance will be steppers. There is absolutely nothing wrong with steppers, if they are properly sized, and properly driven. In machining performance, they will be every bit as good as servos. Worst case, you'll give up a little bit in rapid speed, which is really not all that important in most jobs. 1200 oz-in servos are overkill for a BP. I have 850 oz-in servos with 2.5:1 reduction and 4-pitch screws, and I can break tools all day long without losing position. And, it'll do 400 IPM rapids. With 1200 oz-in servos, you'll gain nothing in performance, and increase the chances of breaking the machine if something goes wrong. Bigger isn't always better. Regards, Ray L.

I have done a stepper retrofit on a bridgeport, ballscrews and a 42in table. I used 920oz steppers, 2:1 belt drive and Gecko 201's with 60volts. The rapids were reliable at 130ipm. I had no complaints about its performance. Interestingly, 1200oz stepper in the same setup give LESS performance. That is because their torque drops off faster with speed than their smaller brother.

I recently retrofitted a bridgeport cnc with new drives that had the factory servos. The factory servos were 30lb continuous rated, peak being about 4x that. They used a 2:1 belt drive for each axis and the machine was rated 250ipm rapids. The servos were spec'd at 6000rpm max but only ran a max of 2500 in that machine. A bit of quick math shows those servos to be about 2000oz peak but limited to less than that with current limiting, and yes it is more than necessary but it is what bridgeport chose. I agree that too much power can be a bad thing in the event of a mishap.

[Jet A]

what would be the recommended stepper for a direct drive? Good source of that stepper?


Thank you

[Sparky_NY]

Originally Posted by Jet A what would be the recommended stepper for a direct drive? Good source of that stepper? Thank you

1200oz is the norm stepper for direct drive.

[polaraligned]

Don't bother with the steppers....

Go to MachMotion and for $650 per axis you can get AC servos and drives from Teco. They will take step and direction input. They are rated at 3000 rpm and come with a 2500 count encoder. The shafts are custom for MachMotion (5/8") and long enough to bolt on with a $10 adapter plate they sell. Not only that you don't have to deal with setting up a power supply, etc. The drives are just plug and play. Pre-tuned and parameter set up from Mach Motion.

Probably the best deal out there for a large mill retrofit.

[H.O]

1200 oz-in servos are overkill for a BP. I have 850 oz-in servos with 2.5:1 reduction and 4-pitch screws, and I can break tools all day long without losing position. And, it'll do 400 IPM rapids. With 1200 oz-in servos, you'll gain nothing in performance, and increase the chances of breaking the machine if something goes wrong. Bigger isn't always better.

Right, and another thing to remember is that you need to factor in the drive as well here... Even if the motor is capable of producing 850oz-in or 1200oz-in of torque (peak) you have to have a drive that can supply the current.

Let's take the Keling 850oz-in model as an example. This motor is rated at 850oz-in peak torque but to produce that the drive needs to push 33A thru it. With the typical Gecko G320 drive which can push 20A "all" you get is 495oz-in, peak. And as Ray's machine prooves it's STILL enough to break tools.

Going for the bigger 1125oz-in motor but still using the G320 won't do you much good. The motor has a slightly higher torque constant than the 850oz-in model giving you a whopping 20oz-in extra. To utilize the peak torque of that motor you need a drive capable of pushing 43A thru the motor.

/Henrik.

[HimyKabibble]

Originally Posted by H.O Right, and another thing to remember is that you need to factor in the drive as well here... Even if the motor is capable of producing 850oz-in or 1200oz-in of torque (peak) you have to have a drive that can supply the current. Let's take the Keling 850oz-in model as an example. This motor is rated at 850oz-in peak torque but to produce that the drive needs to push 33A thru it. With the typical Gecko G320 drive which can push 20A "all" you get is 495oz-in, peak. And as Ray's machine prooves it's STILL enough to break tools. Going for the bigger 1125oz-in motor but still using the G320 won't do you much good. The motor has a slightly higher torque constant than the 850oz-in model giving you a whopping 20oz-in extra. To utilize the peak torque of that motor you need a drive capable of pushing 43A thru the motor. /Henrik.

And, as one of the guys on the Benchtop Mills group learned, when he put those big servos on his RF45, a crash can wipe out the Geckos. I've crashed mine many times, and never hurt my Geckos. He crashed his once, and blew seveal of his Geckos. It's like putting a 500 HP engine in a Toyota econobox, running it through the standard gearbox and differential - somethings gonna break the first time you try to USE all that power.

Regards,
Ray L.

[H.O]

Yeah but that sucks and really shouldn't need to happend (I know it sometimes does though). The drive should be able to protect itself and not blow up even with a locked rotor etc. As long as the armature inductance is within the drives specs so the current limit circuit has time to respond I don't get why that has to happen.


As a side note, I had some G320X acting funny, faulting on me very easily for no apparent reason. Turned out the inductance was too low, the normal current limit circuit apparently didn't have time to react so the overcurrent/short circuit protection kicked in and disabled the drive. THAT is what it's supposed to do NOT blow up.... but again, I know it happens :-(