# Thread: What size servo's?

1. ## What size servo's?

Hopefully this isn't a beaten to death topic, but I've searched around and haven't found a clear cut answer as of yet. I did see the FAQ on servo's, but it's a lot calculations etc and in all honesty I'm not really quite sure what the load requirements will be. The one true way to know is to ask someone who's been there, so I'm asking now

I plan on doing an X3 or RF-45 size machine in the near future with DC servo's and Gecko's, and it will have ballscrews on all three axies. I'm hoping to get reliable, accurate 3D cutting around 25-30ipm in metals.

So my questions are; what size servo motors would be adequate for this task? Best I can tell is something around 400oz/in for the XY and 600oz/in for the Z would be ideal. Also, I've seen a lot of DC servo's around the surplus market. Some are brushed, others are brushless. Is the Gecko compatable with both and which is better for this application? I suppose since I'm on a roll I'll ask what ballscew pitch would be a good match. 5tpi seems pretty popular but would it provide the torque multiplication I need?

Thanks everyone, I know it's a lot of questions but in the end it will be a lot of money for me too, I'd like to do it right the first time.

2. Sadly, it is the math associated with driving the loads you are dealing with at the speeds you are talking about that determines the amount power required - IE; power is work done in a particular amount of time (as in ft-lb/min or in-oz/sec). MOre work in less time needs MORE POWER (enter Tim Tool Man Taylor grunt here).

Thus, unless you know the mass of the X, Y & Z axis to be moved, you don't have quite enought info do figure out how much power you need to move at the target inches/min.

Oh, and just because you can MOVE at whatever in/min doesn't mean that you can cut metals at that speed - you're now into a whole new topic of cutter selections and spindle speeds and spindle HP requirements. IE: Burying a cutter into foam 1" deep and 1/2" dia at 30 in/min is a far cry from trying to cut 4140PHT with the same cutter size and cut depth.

Perhaps the industry standard for mills is the Bridgeport - the pitch of the screws is legenday and well documented. Ditto the drive train for them - it is probably easier to document the ones that are NOT at 2:1 than those that are.

With the appropiate sized motors, it is easy to break off huge cutters and/or rotate the head of the mill (did that with a 1.375" dia hogging cutter in my BPT) and neither the mill nor table stalled - only E-stop did the deed.

If you're trying to match the capability of a commercially sized and constructed CNC mill, you're pretty much going to have to spend the money for comparably sized servos or steppers - and those cost \$\$\$\$'s. The issue ultimately is that of NOT letting your dreams overpower your pocketbook.

Buy/build what you can afford.

If you're going to ultimately go servos, go that way and make the wiring suitable so you can plug and play as you upgrade.

Since the motors are probably the hardest things to find cheap, I'd be inclined to select a drive and wiring that you can use with your ultimate "dream" motor combo. This way, you buy it and it is done - you can then buy a surplus whatever and get the machine running and then upgrade later - your "power supplies and drive system" are already capable of whatever you want to run (save for a P/S voltage adjustment perhaps).

Sort of like buying a muscle car. There were may 6 cyl Camaros that GM built that first got small blocks dumped into them that ultimately ended up with the big blocks they truly deserve many years later (don't believe what people say about big blocks - even though they may have a hot small block now, the owner REALLY wants a 540" big block).

i'll let others more learned in the science of brush vs brushless, et al, chime in here. My point is, simply buy good ball screws of typical pitch, take a proven drive (ala 2:1 belts), build robust electronics and shop for what you can afford drive wise.

Your first machine doesn't have to be your ultimate machine - you have to start someplace.....

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