Comments on Ametek 38v Servo (SurplusCenter)

[dgoddard]

I'm planning on converting my Grizzly G0602 10x22 lathe to CNC. Would like to hear from anyone who is using the Ametek 38v servo motor from Surplus Center (page 127). From calculations it looks like this servo may work.

Power supply - 38v
No load rpm - 1150
80% NL rpm - 920 which gives 184 ipm
Mach3 rpm limit 25k steps/in - 938 rpm
Mach3 rpm limit 35k steps/in - 1313 rpm
Calculated CPR at 25k - 408
Calculated CPR at 35k - 571

Thinking about using 1:1 drive, with either 400 or 500 CPR. Any comments/recommendations? Would like to hear pros/cons on these servos.

[dgoddard]

Anyone out there who has or is using this servo on either mill or lathe would like to hear your thoughts on it. As it is low rpm (1150) could hookup direct. Would rather do this on the smaller lathe than go with another servo which would require reduction. However if this servo not "good" then need to look at another option.

[Mariss Freimanis]

My comments:

1150 RPM @ 38VDC means Kv = 33V / KRPM

Kt = 1.351 Kv so Kt = 44.6 in-oz / amp

I've run into this motor before and I believe it is rated at 4A continuous. This means the continuous rated torque is 4A * Kt or 178 in-oz.

Assume rated RPM = 0.8 * No-Load RPM so rated is 920 RPM. Power is W = RPM * in-oz / 1351 so this motor is good for about 120 Watts mechanical. Not real good, not real bad.

Mariss

[springlakecnc]

I do a few retrofits, and Mariss is right! They’re not to good and not to bad. Personally I think it’s a fantastic deal, for 50 bucks each, if you use them in the correct application.

There is a posting, a guy says their junk, and don’t buy them, but he was running a knee up and down, without a ball screw, and no counter balance weight of any kind. No surprise, that it got hot and burned up! A motor 3 times that size would have a hard time doing that!

I hooked them up to the Gecko drives, and I thought: this is the perfect setup for someone with a smaller size machine, like a tabletop size mill, or a router that runs on linear rails, or maybe your Grizz, (i'm not familular with your lathe, but i think its a smaller one, right?) for a darn cheap price.

This is what I found with a bench test:

Using my old Pentium 200mgz computer
Gecko 320 drives
250 line differential US Digital encoders (E5D-250-187-T)
38.1 volts DC power

I programmed the computer, so the motors would go 10 turns per inch. Due to the slow computer, and no pulse multiplier like the Gecko 340 drives have, I was able to rapid traverse at 45 inches per minuet, (450 RPM) which in my opinion is very good for a small machine. You could go faster with a better computer, or the Gecko 340 drives using the pulse multiplier. My personal Excello knee mill with 23”x 15”y & 6”z travels, I set at 80 inches per minute rapid traverse, which is just fine.

I chucked the motor drive shaft up in the lathe, and stuck a live center into the little encoder shaft, coming out the back, and kissed the housing just enough to clean it up for a flat surface, for the “stick on” encoder.

I put on the differential encoders because I wanted to put on 12’ cables. The cheaper encoder would work, if you have short cables.

Good Luck with your project,

Buck
Spring Lake cnc

[dgoddard]

Thanks for your input. Agree that these servo's are not for the "big" machines. Don't think would use them on my RF-31 mill conversion. Should be good for my project. Sure can't beat the price even with adding the encoder.

Just received two of the servos. Did all the calculations and these servo's should work very well for my small (10x22) lathe project. Could run Mach3 at 35,000 hz but prefer 25,000 which limits rapids to about 90 ipm. Plenty fast for me. Numbers indicate should use 400 cpr for best match. Also plan to use differential as cables need to be greater than 6-8 ft.

[Mike Nash]

Originally Posted by dgoddard Also plan to use differential as cables need to be greater than 6-8 ft.

Okay, I've got to ask! How is a differential encoder going to help with a G320? You've got no place to put the /A and /B encoder leads.

[springlakecnc]

Call US Digital, and ask. They claim it will allow you to use way longer cables, even thou you don't use the A- & B-

[Mike Nash]

They seem to beat around the Bush somewhat, on their site, but it looks that they use line drivsers on the ED5 series capabable of sourcing or sinking 20mA, Even if using only tha A and B signals, they would be less prone to noisy interference since they represent a low effective impedance load.It looks like ED1s use only 8Ma Sink or Source, And I get the impression the HEDs units only have an open collector that may have only a pullup resistor,
So while you won't be using them as differential encoders, the chosen line driver will indeed give an advantage over an open collector transistor driver.