Thinking of going to servos on my B'port S2 CNC

[jacampb2]

I have a 1982 series 2 CNC that had boss 6 controls that I retrofitted to use Kelling drives and Mach 3 about 2 years ago. I have been running the factory superior steppers, and recent work has shown them to be lacking a bit. I am getting into more and more high speed acrylic/plastic engraving and machining and with the steppers I top out at about 100IPM rapids and 50IPM for average machining. The problem is, the acceleration is so low that I normally only get to about 25-30IPM while doing engraving which has a lot of small moves and direction changes.

Anyhow, I would like to get into higher feed rates for the plastic stuff, to minimize my machining time and optimize my feed rates. I was originally considering purchasing new 1000 oz/in steppers to replace the 30+ year old boss motors. I am told that I will see an improvement doing this, simply because the factory motors were over rated, and the permanent magnets loose strength as they age. I would hate to throw more money after steppers and still be unhappy with the performance, so I am thinking about taking the servo route.

I can't afford $500 servos, I have been researching the hell out of it, and it seems that the math tells me I need ~1.5HP servos. These are way out of my league, and everyone says that a servo retrofit does not need to be that much more expensive than steppers. I am not seeing it though!

I have been looking through some of the offerings out there and I found these motors from Kelling: First motor on the page, clicky, they are rated at 1125 Oz/in peak, 226 Oz/in continuous. Their peak torque compares well to the factory specifications for the "holding torque" of the boss steppers, but 226 Oz/in seams a bit anemic for a machine with a 42x18" table. The original axis drives are geared 2:1, and I could reuse the reduction drive instead of going 1:1 and I still think I will see an improvement, but are these servos going to be big enough?

I understand the stepper system well, and am starting to get a grasp on servo motion control, but the industry has not made it easy to compare. If a 1000 Oz/in stall torque stepper did the job, does that match up evenly to a 1000 Oz/in peak torque servo? Can anyone recommend any other suppliers for servos, either surplus industrial or new hobby oriented?

Thanks in advance, I appologize for the long winded post.

Sincerely,
Jason

[HimyKabibble]

Originally Posted by jacampb2 I have a 1982 series 2 CNC that had boss 6 controls that I retrofitted to use Kelling drives and Mach 3 about 2 years ago. I have been running the factory superior steppers, and recent work has shown them to be lacking a bit. I am getting into more and more high speed acrylic/plastic engraving and machining and with the steppers I top out at about 100IPM rapids and 50IPM for average machining. The problem is, the acceleration is so low that I normally only get to about 25-30IPM while doing engraving which has a lot of small moves and direction changes. Anyhow, I would like to get into higher feed rates for the plastic stuff, to minimize my machining time and optimize my feed rates. I was originally considering purchasing new 1000 oz/in steppers to replace the 30+ year old boss motors. I am told that I will see an improvement doing this, simply because the factory motors were over rated, and the permanent magnets loose strength as they age. I would hate to throw more money after steppers and still be unhappy with the performance, so I am thinking about taking the servo route. I can't afford $500 servos, I have been researching the hell out of it, and it seems that the math tells me I need ~1.5HP servos. These are way out of my league, and everyone says that a servo retrofit does not need to be that much more expensive than steppers. I am not seeing it though! I have been looking through some of the offerings out there and I found these motors from Kelling: First motor on the page, clicky, they are rated at 1125 Oz/in peak, 226 Oz/in continuous. Their peak torque compares well to the factory specifications for the "holding torque" of the boss steppers, but 226 Oz/in seams a bit anemic for a machine with a 42x18" table. The original axis drives are geared 2:1, and I could reuse the reduction drive instead of going 1:1 and I still think I will see an improvement, but are these servos going to be big enough? I understand the stepper system well, and am starting to get a grasp on servo motion control, but the industry has not made it easy to compare. If a 1000 Oz/in stall torque stepper did the job, does that match up evenly to a 1000 Oz/in peak torque servo? Can anyone recommend any other suppliers for servos, either surplus industrial or new hobby oriented? Thanks in advance, I appologize for the long winded post. Sincerely, Jason

Jason,

Steppers and servos are very different beasts, and have to be treated quite differently. Steppers have lots of low-speed torque, but torque falls off rapidly as speed goes up. The larger the stepper, the worse the high speed performance. Servos have torque across their entire speed range, which is MUCH wider than the range for a stepper. So, servos are typically reduced much more than steppers - 4:1 is not unusual.

As a point of reference, I have a S1 clone I converted to CNC using the little brother to those servos - the 850 oz-in model. With a 68V power supply, Gecko G320s, 2.5:1 belt reducers and 4-pitch screws, it will easily do 400 IPM, and has enough torque at 100 IPM to snap a 1/2" endmill without losing position.

For encoders, go to digikey.com, and for $30 each you can get the CUI capacitive encoders, which have switch-selectable resolution up to 2048 lines. They work beautifully with the Gecko G320X.

Regards,
Ray L.

[jacampb2]

Thanks, so long story short, those servos might be appropriate for my needs? Even geared 2:1 they should still be better than 2xs the speed of my steppers.

Thanks,
Jason

[HimyKabibble]

Originally Posted by jacampb2 Thanks, so long story short, those servos might be appropriate for my needs? Even geared 2:1 they should still be better than 2xs the speed of my steppers. Thanks, Jason

Geared 2:1, they will be FAR faster than your steppers - probably too fast....

[jacampb2]

The servos I linked top out at 3300rpms at 90Vdc, if I run them at 80Vdc to match most hobby drives they should top out around 3000RPMs. From what I understand you are supposed to set them at about 70% of top speed so there is room for the controller to maintain lockup. So figuring about 2000RPMs for max normal operation, that is 2x's faster than the steppers run to hit 100IPM. The steppers top out at 1000RPMs. Maybe I am missing something, but that was my logic.

I am seriously considering making my own servos by using some decent sized DC motors and adding encoders. It will take a more expensive drive than the Geckos, because I'll need ~160Vdc supply, but I should be able to get some serious motors for about the same price as the kelling ones.

Thanks,
Jason

[Sparky_NY]

Look at the dugong drives from cncdrives.com I used them in retrofitting my bridgeport that had large factory servos. The dugong drives are rated 140V and 40amps. Pricing is extremely attractive also. (If you buy 4 the shipping is free, making that 4th drive a real bargain).

As Himy mentioned, the AMD capacitive encoders work extremely well. I used them and have been very happy.

[Sparky_NY]

Originally Posted by HimyKabibble Geared 2:1, they will be FAR faster than your steppers - probably too fast....

His machine is a series 2 bridgeport, a lot larger than the series 1. I agree that 2:1 might not be the best ratio both because of speeds and the heavier load being moved.

[jacampb2]

Originally Posted by Sparky_NY Look at the dugong drives from cncdrives.com I used them in retrofitting my bridgeport that had large factory servos. The dugong drives are rated 140V and 40amps. Pricing is extremely attractive also. (If you buy 4 the shipping is free, making that 4th drive a real bargain).

I did take a look at those, and they look nice. Where is the company located though? The UK? Do you happen to know if their pricing on their website is in Euros or Sterling?

Thanks,
Jason

[jacampb2]

So, am I missing something in my speed calculation then? If I were to go 4:1 reduction, then it seems it would put these servos back in the neighborhood of my current speed...

What are the thoughts on buying some quality 1.5HP continuous duty DC motors and adding encoders? I found a web site with a guy that has done it that sparked the idea, but since I started researching it, I have seen it show up as a possibility on this board as well. Specifically, something like a higher end treadmill motor converted for servo use. I have no qualms about either extending the motor shaft, or turning down a dual shaft motors shaft to fit an encoder. Face mounting could also be solved by making a sleeve type heat sink like the boss stepper and incorporating a nema flange mount on the end... Good idea/bad idea?

Thanks for all the info so far...

Sincerely,
Jason

[Sparky_NY]

Originally Posted by jacampb2 I did take a look at those, and they look nice. Where is the company located though? The UK? Do you happen to know if their pricing on their website is in Euros or Sterling? Thanks, Jason

It is located in Hungary. The web pricing is in euros. Google euro dollar converter and you will find a converter at current rates, I used a yahoo one.
The cost was about $125 (US) as I recall per drive when I bought my 4. A unbeatable deal considering the drives features, their support and free shipping with 4pcs. Took about a week shipping time.

[Sparky_NY]

Originally Posted by jacampb2 So, am I missing something in my speed calculation then? If I were to go 4:1 reduction, then it seems it would put these servos back in the neighborhood of my current speed... What are the thoughts on buying some quality 1.5HP continuous duty DC motors and adding encoders? I found a web site with a guy that has done it that sparked the idea, but since I started researching it, I have seen it show up as a possibility on this board as well. Specifically, something like a higher end treadmill motor converted for servo use. I have no qualms about either extending the motor shaft, or turning down a dual shaft motors shaft to fit an encoder. Face mounting could also be solved by making a sleeve type heat sink like the boss stepper and incorporating a nema flange mount on the end... Good idea/bad idea? Thanks for all the info so far... Sincerely, Jason

Neither a good or bad idea. Many have adapted dc motors for servo use. But there is something to consider........ Servos, designed for the purpose, are designed for hard acceleration, excellent torque with varying speeds, quality bearings and maybe most importantly they hare designed with braking performance optimized. There is a fair amount of work to adapt standard dc motors, and the performance will not match a dedicated servo. The cost savings will be minimal. I assume you have looked at the cost of new motors for kelling? Another route is ebay motors.

So, retrofitting treadmill motors or similar is for cost savings only and there is a performance price to be paid. What about durability? I think its reasonable on something like a X2 mini mill or a 7x10 mini lathe but a series 2 bridgeport is a lot different beast.

[keebler303]

I would keep an eye on ebay. I know there are a lot of motors that would be good for your use. I am thinking of AC servos in the 2kW ballpark. You can use Granite Devices drives to run them and they will have plenty of power. I know of 3 matching servos on ebay right now with current price of $50 each. Add in the GD drives and you should be under $300 per axis, if you pick the right motors (already have correct encoder, etc.).

Good Luck
Matt