servos and gears for ZAY7045FG like Asian mills

[bogus105]

Hi,
i'm planning to convert my zay7045fg benchtop mill machine to cnc. Because of constrained budget i need to it without buying expensive new servos, new ballscrews etc...

i want to start from Z axis because it will eat the biggest part of my funds.
in order to make Z axis drive:

i bought used aerotech 1000dc series, NEMA42 sized servo motor. it delivers 1.48Nm stall torque and 7.1Nm in peak and nominal speed of 2000rpm. Rated power 220W, continous current 6A, terminal voltage 80V (max about 130V).

motor manofacturer's specs (model 1210):
http://www.aerotech.com/products/motors/1000specs.html

i have also bought occasionally bosch rexroth 25mm dia, 10mm lead ball screw.

i have also calcutaled timing pulley gear ratio 6:1 assuming 50mm/sec jog speed. My calculations is based on CNCmechanics v1.2.pdf from this thread:

http://www.cnczone.com/forums/showthread.php?t=8129

With 6:1 geardown i'll achieve 8,88Nm continous torque on ballscrew and over 42Nm in peak.

My question now is how to make gears? the problem is that gear ratio is pretty big i think. Because of 16mm dia motor drive shaft timing pulley attached to it should be an approx 25mm dia. For T5 type pulleys this mean 16 teeth pulley. To get 6:1 ratio i need 6x16=86 teeth on ball screw. I can't find such a pulley so it seems i need to make a two stage gear down: 3:1 and 2:1. Is it good idea? how about accuracy on ballscrew with such a two stage gear?

And of course the most important question: is this motor strong enough to manage with zay7045 mill Z axis assembly (it weight over 100-120kg)?

I know that 10mm screw lead is not the best solution but i can't find 5mm lead screw for the price i can accept.

Please join this thread and write me what you think about it guys.
thx!!

8061

[H.O]

Hi,
I don't have any experience with the ZAY7045 series mills but how much linear force do you really need? I mean, 8.8Nm on a 10mm pitch ballscrew, figuring 85% overall effeciency gives you ~4700N of linear thrust - continous, that's probably more than it would take to lift the whole machine. How much can the ballscrew handle?

T5-60 seems to be readily available, with 16 teeth on the motor pulley and 60 teeth on the screw you'll get 3.75:1 and a linear force of ~3000N continous and ~14000N peak - that is a lot of force. (provided your amplifier can source the current).

[bogus105]

Hi,
the whole Z axis assembly can weight let's say 150kg. That gives let's say 1500N of downward force. There are some friction forces in addition. The rest force will gain movement acceleration. But maybe it is good idea to consider 4:1 gear ratio instead of 6:1?

[H.O]

Oh so it's gained 30kg in weight since you first post? Just kidding ;-)

With 4:1, if you can get that in one stage, you'll get ~3200N continous, figuring 85% efficiency of the transmission and ballscrew together. Let's say it takes 1500N to lift the head and the friction "consumes" another 650N (which feels like a lot to me but I don't know), then you still have ~1/3 left to accelerate with without even going into the peak-region of the motor.

Obviously this is all theory and doesn't account for intertia of the motor, transmission and screw but I think you should be able to do it without going to a two-stage belt-transmission. If you look at the Kelings largerst servo motor it has a slightly larger continous torque than your motor but I think this motor (or even the 850oz-in model) is what many people are using on these sizes of mills.

You can always resort to counterbalancing the head with gas-struts or a pulley/counterweight system.

[bogus105]

yes my Z assembly grew up (just better assume too big weight than too small) besides maybe i will mount additional high speed spindle (Kress FME1050) on it.
if you are talking about 850 oz-in motor you mean peak or continous?
my motor has 210 oz-in 'only' of continousv torque.
i've checked ball screw load ratings: Dynamic load 15700N and static 27000N so i don't need to worry about that as i'll be operate in lower range

[H.O]

Hi,
OK, no need to worry about the screw then, plenty of load carrying capabillity there.

I was referring to the Keling "850oz-in model" motor (KL34-170-90) which have a continous torque rating of 1.2Nm and a peak torque of 6Nm providied your drive can supply 33A which most can't anyway...

/Henrik.

[bogus105]

thank you Henrik for respond.
now when i get back home i have to change irf540n mos in my UHU drive to irfp260n to upgrade to 120V 35A as my current UHU can provide 60V20A at the moment. There is one more problem - transformer. I can't find one delivering 80V and enough current. Unfortunatelly there is no peak current in motor tech specification! continous current is 6A.
but if the motor power formula states: Power=torque times rotational speed=voltage times current, so if motor can deliver 7Nm of torque while 2000rpm then P=1466W so it gives less than 20A peak current.

Now the question is: what transforet to choose? 80/1,41=56V secondary VAC but what current? i think there is no need to buy 20A transformer because 20A is needed in peak only. On the other hand while cutting complicated things when Z head needs to move up and down all the time
then motor has to accelerate/decelerate all the time and thus consumes bigger than rated current. So what transformer current to choose?

[H.O]

Hello,
The datasheet for the motor lists the torque constant, this is how much torque you'll get per amp thru the motor. Since you know the peak torque (7.1Nm) you can calculate the peak current. 7.1Nm/0.25Nm per A = 28.4A. This meshes nicely with peak torque/continous torqe*continous current which ends up at 7.1/1.48*6A = 28.8A.

Your formula doesn't seem to take the efficiency of the motor into consideration which is probably why we don't get the same numbers.

Shouldn't you factor in the X- and Y-axis too when sizing the transformer? Or are you going to keep that completely separate? In the end one powersupply for all three axis will end up cheaper separate supplies provided you don't really have something handy you can use of course.

A typical toroid transformer can be quite heavily overloaded during short periods of time. I'd say a 10-12A transformer would be more than enough for this single axis and perhaps a 20A for all three. After all 20A@56V is 1120W and your motor(s) are 220W continous so you'd already have quite a bit of overload capacity for tapping into the peak region of the motors.

Regarding the UHU, you do know that many users have had trouble with higher voltage/higher currents, don't you? That's why the HP-UHU came to life. I'm not saying it won't work in your case just be prepared that you might run into trouble.

/Henrik.

[bogus105]

hello,

yes, i plan to separate z axis power supply from xy because of much different voltages: 80V for Z and 48V for XY.

i wondering if torque constant is constant even in peak region? thank you for pointing out torque constant.

what do you think: does the 15mm width timing belt is enough to withstand considered maximum 42Nm load?

about UHU-HP: it's a nice design indeed but i can't find pcb layout to maka pcb myself. I don't want to buy i from usa because od taxes, customs, shipping fees and time... has anybody in europe selling these pcbs? Making pcb by myself would be the best.

Here in Poland fellas on cnc forum developed nice pcb for regular UHU and it was it two versions. as i wrote earlier with irf540n mos transistors drive delivers 60V 20A and with irfp260n -120V 35A. so i just change transistors in my UHU as i have weak version right now. I didn't read anything wrong about that stronger version.

about toroid i even thinking about 8A because of oveloading capacity. I send inquiry to one company in Poland that manofacture toroids on request. I wonder what the price would be.

/8061

[H.O]

Hi,
As far as I know the torque constant is just that - constant. You should never exceed the maximum rated current though as that may damage the magnets.

As for the belts you have to look at the manufacturers data for the belts, some are rubber, some are polyurethane, some are reinforced etc etc. You also need to check that you'll get enough number of teeth in engagement. Personally I'm running 16mm AT5 steel-reinforced belts on my kneemill, 2:1 reduction, DC-servos at 130V, 19A continous current rating (=9Nm).

If you have a UHU design that you know works at that voltage and current then that's great! No need to use anything else. s designs floating around and I'm not sure which ones are having problems at the higher currents/voltages. One fo the problems is the fixed deadtime of the MOSFET drivers used in the "reference design". But again if the design you have works don't change it.

As far as I know only Paul (tenmetalman) has HP-UHU PCB's and kits for sale.

/Henrik.