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Lee Roberts
17-05-2009, 07:46 AM
Hi,

I appreciate for the cost difference itís probably not really worth questioning but is it really necessary to have a high rated motor for the Z-axis?

This would be for a cnc router machine, flying gantry style, machining wood, shallow cuts, reasonable federates.

I'm currently designing a new machine, and the Z-axis is what Iím currently working on, I would like to use the smallest physical size motor I can, as the machine is specifically being built for the propose of sheet work and the material being wood (ply) for the most part.

I'm just wondering if Iím going down the right route thinking that I wonít really need a big powerful motor for the Z-axis, itís quite relevant for me as it would mean Iím happy to mount a smaller compact nema23 motor directly above the Z-axis lead screw, vs. going with a bigger physical size motor, mounted off the back of the z axis connected via a belt drive.

Thanks guys, Lee

John S
17-05-2009, 12:16 PM
Lee, When we were doing the X3 conversion we used the same motors all round. These were triple stack Nema 23's rated at 310 oz / in direct drive onto a 5mm pitch ball screw in X and Y and belt driven 2:1 reduction in Z

Bear in mind this was running an X3 head up and down without a gas strut which we left off on purpose to get the extra travels needed.

http://www.stevenson-engineers.co.uk/files/zmount2.jpg

This made a nice self contained unit with two angular contact bearings for preload and backlash.

In practise we were getting 4 metres an minute for X and y and 2 m / min from Z but we dropped these back to 1.5 and 1 m / min because it's scary on a machine this size when dealing with beginners.

You need to give then a chance to grab the E stop :evil:

In answer to your other question this morning about belts I like the HTD 5mm pitch series. They are actually more efficient than the trapeziodal belts but to be honest at this size and rating it's not an issue.
Cost wise they are cheaper, believe it or not RS Components are very good on both pricing and stock, order via the web and delivery is free.

If cost is a problem they are the easist ones to make, drill a circle of 1/8" holes on the PCD and turn down so 1/2 the holes are showing, literally that simple.

Where speed comes into it no one has addressed the actual issue, it's not about motor torque and size, it's all about voltage.

A stepper motor NEEDS 20 times the rated voltage to get up to speed so a motor rated at 2 amps 4 volts actually needs about 80 volts to work to spec, feeding this 24 to 30 volts from a cheap driver will only get you 2 amp and about 6 times the voltage with the result that it's working right on it's limit.

Now reckon up running costs, reliability and payback time, i.e. being able to cut faster and reliably and the cheap drivers loose out.

If you move up to say a 50 volt driver rated at 4 to5 amps you wil have a driver running at 1/2 currect and 8 times the voltage which will increase speeds but more important get the driver off the edge of the reliability curve.

Now go the whole hog and go up to 70 volt and 7 amp.
Power [ amps] are now about a 1/3 so read more reliable, less heating etc and voltage is approaching the magic number of 20.

Ok this drive is now 2 - 2 1/2 times as expensive but it's a drive that will last any upgardes you need, it's also a drive that is reliable and will pay back in a short time if you use your machine often. The combined drives like the Xylotex, Routout to name a couple only have to blow one axis to have cost more than one seperate drive.

That is the background behind the X3 conversion we used to do, it's also the same background behind the Chinese KX3 mills. After about fourty X3 conversions we have had to change 3 drives cause when a guy living next to a power station had a surge one day which took most of his electrial gear out including computers, televisions etc.

John S.

Lee Roberts
17-05-2009, 08:24 PM
Hi John, great reply.
I'm still trying to take it all in; I was planning to have about 100mm worth of travel on the z axis. I donít think Iím going to need allot of speed on the z axis as most of what I want to do is in really thin wood like 3mm ply and stuff so when I design parts to be cut I will use the minimum amount of Z rais I have to as it moves to each job and so on.

Iím just trying to put this into context with what youíve said above, I will be using the MSD range of drivers on all 3 axis's from zap/mcp. I really canít see that I need a 400+ oz-in motor and a matching driver for such a small axis thatís not or shouldnít really see too much in terms of cutting forces, I know Iím properly missing the point but thatís not intentional ? :confused:

Gary
17-05-2009, 09:24 PM
Generally the Z axis does not need a motor the same size as X and Y.
The X 3 is different though, on mine i have SY60 on X and Y and a SY86 of Z.
The X3 Z axis is a heavy axis.


Hi John, great reply.
I'm still trying to take it all in; I was planning to have about 100mm worth of travel on the z axis. I donít think Iím going to need allot of speed on the z axis as most of what I want to do is in really thin wood like 3mm ply and stuff so when I design parts to be cut I will use the minimum amount of Z rais I have to as it moves to each job and so on.

Iím just trying to put this into context with what youíve said above, I will be using the MSD range of drivers on all 3 axis's from zap/mcp. I really canít see that I need a 400+ oz-in motor and a matching driver for such a small axis thatís not or shouldnít really see too much in terms of cutting forces, I know Iím properly missing the point but thatís not intentional ? :confused:

Lee Roberts
17-05-2009, 10:56 PM
Yea that’s what I was thinking Gary; I’m trying to design so that it’s minimal but strong at the same time and not to heavy! Still waiting for your build log to show up.

John S
17-05-2009, 11:08 PM
Generally the Z axis does not need a motor the same size as X and Y.
The X 3 is different though, on mine i have SY60 on X and Y and a SY86 of Z.
The X3 Z axis is a heavy axis.

Very true Gary about the X3 head but the 312 oz in motor on 2:1 works just fine and this is without the gas strut.
My small Teckno Isel router that I bought minus all electronics and motors is now on 210 oz / in all round and running at 40 volts and it screams along for it's size but then again the head isn't that heavy on this one.
Again all direct drives onto 5mm pitch screws.
It remarkably accurate, here's a badge I did as an example.

http://www.stevenson-engineers.co.uk/files/llanfairpg.jpg

Note the g's on the tails, they are all exact and no undercutting and wandering, this was engraved in brass, raised letters at 1200mm / min.

John S.