BIG servos! (is there such a thing as too much power? =)

[damae]

I just stumbled into a matched set of brushless AC servo motors and drives!

Until now, I've been struggling to find an affordable set (3) of servo motors powerful enough for my 4'x4' gantry mill project. I want to hit 500 IPM and calculated that servos with a 1/2 HP continuous rating should be plenty powerfull. So when this set of monster 2HP servos fell into my lap, I could hardly believe my good fortune!!

But now it begs the question (and I still can't believe I'm asking it): Have I perhaps got motors that are overkill to the extreme? They are rated at 2HP continuous and can peak at 200% of power and torque ratings. They're much bigger than Nema 34, weighing 19.5 pounds! The mounting flange is about 6" square.

The servo amps are matched to the motors and have a lot of sophisticated programming options. For instance, I can program the amp to understand ratios between the motor rotation and linear travel (i.e. 0.2000" per rotation). And I can select how many steps pulses per rotation (up to 4096 per rotation, which is the resolution of the resolver). So I have a lot of flexibility to change the speed and resolution of the motor. And even though the motors are 3 phase, 240v, the servo amp runs off of single phaase. Not to mention that they are matched sets and I have 3 of them.

So, of course, I really want to use them. But when I start adding up the z-axis weight, I'll be pushing around the a 19 pound motor plus 2hp spindle motor and all the usual linear slides and ballscrew. Is this wasteful? It seems that this should not be a real problem since the x and y axis motors are also 2hp servos and should be able to overcome this greater inertia.

Another concern is that the frame may be underdesigned. Until now, I had planned to use 3"x3" sections of 80/20 extrusions for the frame. Does anyone have practical experience building machines with that size frame members? Do you think it would be rigid enough?

Or looking at it another way, (and this is the real question I am asking myself): What size machine should I be building to do justice to these motors?

I have a picture and would love to post it. Can anyone tell me how I go about doing that? (I'm new to this forum).

Opinions, warnings, advice and any other comment is welcomed!

-D

[damae]

I'm trying to attach a picture of the motors/amps now. You can see some 5 gallon buckets and a double-size propane tank in the image for size comparison.

By the way, I calculated that with my current setup, here's how the 1/2 HP motors perform vs. the 2HP motors:

Reliance Electric 1/2HP servos with 100v Rutex:
1300 RPM, 1.8 ft/lbs (continuous, not holding torque)
325 IPM with 172 lbf of axial force along ballscrew (the force pushing the end mill through the material)

Emerson 2HP servos:
3000rpm, 4.5 ft/lbs (continuous)
750 IPM with 430 pounds of axial force!

Comments, ideas?

[Al_The_Man]

You can get an idea of what size you need and also compare you units to see the inertia ration you can achieve with them, this is the only site now I believe where you can download the Emerson sizer.
http://cnczone.com/modules.php?name=...inkinfo&id=343
Al.

[ViperTX]

damae...they are much too large for your application.....send them to me and I'll give you $50 a pierce and free up your storage of these units....*smile*

[damae]

Haha! ViperTX, you may be right about it being overkill, but I think I'll hold on to them for now =)

Really, I want to figure out how to make the most of the motors given my 4' x 4' cutting area. If I undersize the frame or other components, I won't be able to use the full power of the motors, for fear of breaking something.

Al, thanks for the handy Emerson link. I haven't completely figured out all the inputs that "Emersize" asks for, but it should help me verify my calculations for acceleration, once I figure out the final mass of my gantry assembly.

Does anyone have any feedback about using 80/20 framing (3" square sections) with motors this powerful?

Thanks!
-D

[damae]

I have a screenshot here showing the proposed framing to show its size in proportion to the servo motors. The model is, of course incomplete and I know I need to add bracing on the lower legs, even if I go with smaller servos.

Does this look reasonable to everyone? 750 IPM with 430 lbs of force on each axis?

[svenakela]

With those servo's I would go for a direct mount.

--S

[damae]

Hi Svenakela,

I can direct mount the gantry axis, and I already have the couplers I need. However, if I direct mount it, I'll get 1500 IPM and 200 pounds of side-cutting force on that axis. That might be a little too fast (did I just say that? can it ever be too fast?).

Anyway, I think I'll prefer to have 400+ pounds of side cutting force, so I would have to use pulleys to reduce the speed on the gantry. The Z axis can be direct mount, but because of the large pitch Z ballscrew I have, I might need to mount that by pulleys.

The axis that moves the gantry along the two rails --is this the Y axis? The machine is square in dimensions, and I'm unfamiliar with the proper term. Anyway, I am running two ballscrews, one on each rail. So I'll have to couple the motor to the two ballscrews on that axis with timing pulleys and belts.

I still don't have a feel for how much side-cutting force I'll need to cut aluminum at those speeds, so I guess my decision on how to mount may hinge on that. If 200lbf is too weak to cut at 1500 IPM (which I suspect it is) then I will certainly have to gear down further than the ballscrews.

Are there any tips or suggestions as to why I might still want to do direct mount? Something I overlooked? This is my first CNC machine (well, my first homebrew, anyway) and I'm still trying to soak up the wealth of info.

Thanks!
-D

[Al_The_Man]

I think if you anticipate 200lbs of side force to machine aluminum, ridgidity is going to be your biggest problem, with the anticipated weight of the Gantry taken in to this figure you should not even come close to this figure, what do you intend to mill?
To get an idea of the HP needed for different tooling and material get something like the free ME consultant if you do not already have it. http://www.tucows.com/preview/356150
Al.

[miljnor]

(That link is not working Al.) NEVERMIND!

[damae]

Wow, that ME consultant software is pretty handy! Thanks for the link! I'm going to keep this one on my machine I use to generate gcode!

As far as power requirements, ME Consultant is a little vague when describing what the HP parameter means:

"It shows the machine power, expressed as horsepower, required to satisfy the current machining parameters.

The calculated HP value is based strictly on the material removal rate for the active material. No adjustment is made for the type of tool being used."

So I'm not sure if that is spindle HP or the HP required to drive the tool through the material. If it indeed is talking about the HP of the servo motors, I think I can work backwards and calculate what side cutting force is implied by the suggested feed rate.

My original target for this machine is to cut aluminum, brass, wood, foam and only occasionally steel. I set those targets when I bought a set of smaller, 1/2 HP servos(the Reliance Electrics model 0690 being sold on Ebay). The aluminum I wanted to cut as fast as possible for the motor power. The steel I was prepared to be patient and cut slowly.

You have a good point about frame rigidity. I may have to step it up to the 3" x 6" sections of 80/20. Man is that going to get expensive!!

[Cold Fusion]

Would you ever consider selling the Emerson system if you found out that smaller servos would work? I'm retrofitting a Bridgeport and need beefy motors like these.