Newbie- How much power?

[MachMan]

Hi, I'm new to CNCzone and I love the stuff I'm learning here.
I have a Bridgeport style milling machine I want to CNC.
I have a Galil MC and AMC drives (15amps/80volts) but what
size of (servo) motor would I need? I can't find much on the 'net
concerning this sublject. I'll probably go 2:1 belt drive to the ball
screws(X&Y). I think 1/2 horsepower will be enough but you guys have
been there, so I'll listen before I buy.
Any info would be appreciated.
Thanks in advance.
MM

[Al_The_Man]

I think you will find that the BP and Excello's often used SEM DC servo motors, mine are 26lb/in 8.6 amps at 140vdc max. 2:1 reduction.
Al.

[HimyKabibble]

MM,

I just finished my conversion using servos from homeshopcnc.com, and Gecko G320s running at 70V with 2.5:1 reduction on 4-pitch ballscrews. I get 100 IPM Rapids, and the axes are pretty much unstoppable - I can snap off a 1/2" endmill without losing position!

Regards,
Ray L.
Santa Cruz, CA

[MachMan]

Thank for the info. About 400-500 oz-in servos should do it for me.
I got a lot of conflicting info on the 'net. I guess you need
more power if you are using steppers.
Thanks again.
MM

[HimyKabibble]

MM,
I wouldn't cheap out on the motors. Last thing you want is to do all that work, and end up with a machine that loses position if you don't program it very carefully. I'd go for larger motors than 400-500 oz-in. The ones I got from homeshopcnc.com are very nice, and VERY reasonably priced.
If you plan to CNC the knee, you'll need either a larger motor, or a greater reduction. I'm putting the same motor with 5:1 reduction on mine. At 2.5:1 reduction, it couldn't quite raise the knee. Moved it down just fine, though! :-)

Regards,
Ray L.

[MachMan]

RE: to Ray L.
I went to homeshopcnc.com and yes they do have good prices.
What is the exact model # of your servos? Don't you think 500 oz-in
(31.25 lb-in, 3.53 Nm) continuous torque (not peak) is enough?
Thanks,
MM

[HimyKabibble]

MM,
Nope, I'd go bigger, or gear them down more. Keep in mind, those are PEAK torque ratings - the continuous is MUCH lower. The incremental cost of a larger motor is a drop in the bucket, compared to what you'll have in the machine by the time you're done. This is not a place you want to be shaving pennies. If you decide to CNC the knee, you'll need to go bigger still, or gear down even further. With the 850-oz-in motors and 2.5:1 reduction, I could not raise the knee. I'm going to 4.8:1 reduction there (as soon as the new pulleys and belts show up....).

Regards,
Ray L.

[polaraligned]

The peak torque means nothing here.
Continuous torque is the number you are looking for. The peak torque can not be sustained for more than a fraction of a second.
Commercial retrofits use motors that have 28 in-lb (450 in-oz) CONTINUOUS torque. The motors from HomeShopCnc may work but they would never be used on a commercial retrofit. I don't see any motors there that are really adequate. When one speaks about a servo motors torque, generally it is understood to be the continuous rating.
A larger servo motor is generally more money but why not go with a motor size that is being proven day in and day out?

I just picked up TWO brand new 28 in-lb Baldor servos on the E-place for $210 for both. And they include 1000 line encoders. They are environmentally sealed and made for commercial machines. Keep looking. Lots of bargains out there with everyone worrying about their wallet these days.

40 in-lb motors are fairly common and a good choice for running the knee. 2:1 gearing is enough for those motors and they will run off of Gecko's. I know a guy who runs his knee in a commercial setting and spanks holes through 3/4" steel plate with this knee set up this way.

Scott

[asuratman]

Hello,
Which one better to put servo on knee or quil? I saw somebody here put servo on quil so it does not require big servo motor. If you put motor on knee, yes, you will need big servo. But its quite complicated to put motor on quil. looklike not too rigid. Thank. Asuratman.