Servo Motor Recommendations (Mariss was right!)

[Bubba]

OK, for better or worse; I am going to throw in my .02 worth on this design.

I think Adam has done a decent job of the conversion,but missed one tiny detail. On the X and Y, he used Ball-screws (efficiency ~95%) so running the numbers, shows this to be a real conservative design. I personally would have gone for a lower ratio, but that is neither here or there.

Now lets look as the Z axis. The screw is and ACME so if we use the mythical 500# load that I usually use for calculation basis, we must adjust this for efficiency! So instead of a theoretical 500# load, we are looking at (using 25% efficiency) ~2000#! In order to hold the current to the area of the recommended 2.9 amps, we would need a gear ratio on the order of 6.5:1 for this motor setup. Oh and by the way, I am assuming a 38 V power supply for this motor and I have not considered acceleration or de-acceleration loads either which look to be considerable looking at the pictures. I also feel there will be a lot a "stiction" with that big an overhang load on the dovetails and there appears to be no counterbalance measures. Oh, and if a ball screw is used in this application, every time the power is turned off, the knee will probably drop to the bottom as the weight of the table/knee/work will back drive the ball screw unless you put in a brake!

On a side note, THANK YOU Mariss for the explanation of the "marketing hype" concerning max torque values on servo motors. Usually when I try to explain that to people, they act like I am crazy!

[ckm]

Someone asked for pictures of the AMT-102v encoders I mentioned upthread. Here are some pics... The mounting bracket and shaft bushing are plastic, but the encoder electronics are housed in a cast aluminum case.

[springlakecnc]

As long as people are throwing in their “2 cents”, here’s mine:

First of all, I have retrofit somewhere around 400 cnc mills, so I know a little something, but not much. The smallest was a series 1 Bridgeport, so I don’t know any of the answers about those little tabletop mills.

Second: most all Bridgeport’s that have the knee for the Z axis, have a air counterbalance system.

Back in the late 80’s or early 90’s, I retrofit a huge Heild? (I think it was made by Cincinnati) Bed mill, which had hydraulic servomotors that didn’t care how much the head weighed. The head probably weighed more than your machine, as the Heild had 30” of Y travel, so the head stuck out a long way, and had no weight counter balance of any kind. It had a 50 taper spindle.

If you want to get your knee to work, for cheap, consider this: what I did to the Heild machine was, I mounted 2, 16” stroke air cylinders to the sides of the head (it had 16” of Z travel) for the counterbalance. Set your air pressure regulator so you have just slight down pressure. (So backlash in the acme screw won’t be a factor) Make sure you have a “self relieving” pressure regulator! Better yet, they do sell a special dump valve that guarantees back pressure won’t be any higher than the pressure set on the regulator. In the 80’s, I bought some series 2 Bridgeport’s for cheap, because someone replaced the air pressure regulator with a non self relieving type, so when you had it set at 60 psi, for going up, it was great, but it would build to maybe over 150 psi on the way down, thus giving it control problems, thus, they sold it to me cheap, because it wouldn’t run right!

In the mid 90’s, I retrofit a Bridgeport series 2, that did have a ball screw in the knee, and the knee was the z-axis, but it didn’t have the air counter balance. It was originally some kind of tracer mill with 2 heads. I put a SEM 40 In. Lb. Servomotor on it with no air counterbalance, 20 motor turns per inch, and the machine is still running today.

About motor power rating: Very confusing to say the least. I go by continuous and peak amperage draw. Your 12 amp peak motor isn’t very powerful. A nema42, 29 in lb SEM servo motor, used in most new commercially sold knee mills, with quill kits for the Z axis, are (going by memory) about 12 amp continuous motors, 35 amps peak, which is about 3 times as powerful as your green motor, and has a way better housing to sop up heat. We would make an attempt to gear the machine so you were using no more than 1/3 of the motors continuous amp draw, while on the machine, and not cutting anything.

Now, your considering replacing your green motor with one of those Keling nema34 motors? My guess is you will still have to add counterbalance weight to the knee. The Keling motor has high specks for a physically small motor, which makes me wonder, but not for long, because I bought 3 of them and am going to try them on my Excello mill. A nema42 motor has way more surface area to dissipate heat, but those little scrawny brushless motors sure rocked my world, as to what a shorty can do!

Good Luck,
Buck

[Adamj12b]

So I just just got back from vacation and am posting an update. I had planed from the start to add a counterbalance system to the knee, but it mill had only been completed for a week and a half before I left and had to wait till now. I plan on trying 2-90 pound gas charged spring to give 180 lbs of counter balance. The table weighs about 90 lbs, the saddle about 40 lbs and the knee is about 120 lbs, Plus the motors and screws ect. So total weight without a vise or any part is about 270+ lbs. Will 2 of the springs be good or should I go with 3 to give me 270 lbs of counterbalance?

My other question is with the 4:1 reduction on the X and Y being overkill, How low can I go and still be good? The timing pulleys on the motor are 10 tooth aluminum ones. I was hoping to change them to 16 tooth steel ones. I had to bore them out to 5/8" to fit the motor shafts and am not a huge fan of how them came out. Would 2.5:1 be too low or would this still work good?

Here is a link to the gas charged spring I am looking at: http://www.surpluscenter.com/item.as...795-A&catname=

Also, I am planing on replacing My encoders with the ones from Digikey real soon to increase the speed and performance of the machine.

Let me know what you guys think.

-Adam

[Bubba]

With those servos and using the lead screws you have, I probably would go no less than 2:1 and probably about 2.22 (18:40) and still enjoy ~100IPM rapids:}) obviously, this is for the X and Y axis. I don't have enough experience to be able to advise on the mechanics of the knee for counterbalance.