I want to DIY.. some questions please!

[Jonathan]

On the datasheet for the motor is said 6.2A (phase) which I take to mean each winding will draw 6.2A. So if I wire in parallel it will draw 12.4A. In series will will need about 3A but twice the voltage. Won't it?

No... in series it's 6.2A and in parallel 12.4A. Until I finish my own drivers you're not going to find drivers that will deliver 12.4A, so have to use 6.2A. As you say, that means a higher voltage is required ... so the only driver worth buying for that completely excessive and uncalled for Nema 34 motor is mains voltage:

http://www.zappautomation.co.uk/2m22...?cPath=9_3_132

Each side column of the gantry rides directly on the centre point of the linear bearings, and I was going to use solid 12mm alu sheet as a cross-brace underneath. In that case I guess maybe a single central ball-screw with a powerful motor would be okay.

Not at all - how strong you make the gantry with a single screw is largely irrelevant with regards to racking (although in general you clearly want to make it as strong as you can afford) since you're stopping the gantry rotating about the central ballnut, not stopping it bending. To rotate (rack) the rails do not have to deflect much at all, so the only way to do it would be to have a very large spacing on the X-axis linear bearings, however doing that you're just compensating for bad design in the first place. Use two ballscrews - I guarantee you'll regret one.

This happens with almost every build log - starts off thinking one ballscrew will be fine, then by about page 3 we manage to persuade that it's not.

However you point that a nema 34 spins slower is worrying as I want all axis to be capable of the same speed. What do you think, is it okay to use a nema 34 only on the Y axis?

You won't need a Nema 34 motor.

The only way to get a Nema 34 to have decent torque at higher speeds is to use the mains voltage drivers, so lets add up the costs:

Driver, £176
8.6nm motor, £108
Two RM2510 ballscrews from eBay, roughly £190
4 * 30T HTD pulleys, £22
Long (to link screws) and short timing belts, £27
Ballscrew end bearings & mounts, £50 (bit of a guess but shouldn't be far out, maybe more due to the long belt?).

Total £563

The other option is, as Jazz suggested, to use rotating ballnuts. See here for my experience with this setup:

http://www.mycncuk.com/forums/showth...-design-ideas/

(On the last page you'll find pictures of what I'm suggesting below, but the rest is worth reading for a more in depth explanation of why they work so well.)

To get the same (feederate and acceleration) as the above, probably better and certainly more than you'll ever need from a rotating ballnut setup you'll need:

2* m752, £80 (eBay)
PSU for 2 motors is shared with Y/Z motors so about £15 extra.
2* 3nm motor, £50 (eBay)
Two RM1610 ballscrews from eBay, £150
4 * 30T HTD pulleys, £22
2 * short timing belts, £10
4 * 7206 angular contact bearings, £40
Ballscrew end bearings & mounts, DIY for £10.

Total £376

So if I wanted to I could charge £187 to machine two rotating ballnut mounts and you'd end up with a better system for the same price. Realistically I'd charge less so it would be cheaper overall.* When I've worked it out before the price difference has been greater since I worked it out with two nema34 motors, not one, but either way it's a significant saving.

Or for about the same price (£376) you could just use the RM1610 and spin the screws as normal. I'm confident that will be adequate and a better option than RM2005 since the critical speed (speed at which the screw starts whipping, so the limiting speed) for a RM2005 screw is well under twice the critical speed of a 1500mm RM1610 screw so it wont actually get a better feedrate. Plus it clearly requires much more torque to accelerate a 20mm ballscrew compared to 16mm, so you'd need a bigger motor anyway.

In your situation I'd go for RM1610 and either rotate the screw or the nut. I went for rotating nuts since my machine has 2000mm ballscrews so rotating a 16mm screw wasn't an option...and equally because I wanted to try something new(ish). I've left my machine on 10m/min with 1m/s^2 acceleration on X and 1:1 ratio. It will do about 15m/min, even more if I change the pulleys, but 10m/min is already plenty so no point stressing the components. That's with the same 3nm motors and drivers, so you can expect better.

To give you an idea of how much more stable a system rotating ballnuts are; I recently didn't notice that one of my ballscrews had worked loose. So one end wasn't fixed at all and the other wasn't tight either. I only noticed the problem when after a few hours cutting aluminium I leant over/on the ballscrew to remove the tool and it moved... i.e. it made no difference! Try that with a rotating screw and you'd probably have ended up with a bent ballscrew.

Why is running two motors from one controller not a good idea? Provided it can output the required current and voltage it should be okay, won't it?

Think what happens if one motor has a slightly different load.. then stalls.


*Other vendors are available.

[Tenson]

No... in series it's 6.2A and in parallel 12.4A.

Hmm, surely the parallel connection will draw twice the current of a single winding, not twice that of the series connections. Similarly the series connection will draw half the current of a single winding, not half that of a parallel set-up.

Not at all - how strong you make the gantry with a single screw is largely irrelevant with regards to racking....
To rotate (rack), the rails do not have to deflect much at all...
This happens with almost every build log - starts off thinking one ballscrew will be fine, then by about page 3 we manage to persuade that it's not.

Lol, okay I get it :tup: I'll go for two ball-screws.

I still wonder though; why isn't it better to use the same screw pitch for all axis, so that providing the motors don't run out of torque, each axis will have the same speed and resolution?

I really don't want to go the rotating ball-nut route, nor linking the ball-screws with a belt. Going with dual ball-screw I'd like to use two motors. I'll bear the tedious tuning procedure when I first get it up and running. At least I know to expect that now thanks to you guys!

Think what happens if one motor has a slightly different load.. then stalls.

My question was really about why it is better to power each motor and ball-screw from a separate drive circuit, rather than just using both motors from one circuit. The motors can surely still individually stall, even with two driving circuits.

[Jonathan]

Hmm, surely the parallel connection will draw twice the current of a single winding, not twice that of the series connections. Similarly the series connection will draw half the current of a single winding, not half that of a parallel set-up.

In parallel the current is twice the current for one winding. In series it is the same as the current for one winding as you have the same current flowing through both.
GCSE Physics...

I still wonder though; why isn't it better to use the same screw pitch for all axis, so that providing the motors don't run out of torque, each axis will have the same speed and resolution?

Each axis has a different mass and length of ballscrew, both of which affect the feedrate. So the same screw pitch on both will result in different speeds since other variables are still different. The resolution will be the same.

Your Y-axis is 600mm (?), so I'd still go for 10mm pitch on Y.

I really don't want to go the rotating ball-nut route

Why?

My question was really about why it is better to power each motor and ball-screw from a separate drive circuit, rather than just using both motors from one circuit. The motors can surely still individually stall, even with two driving circuits.

Same answer, but to put it bluntly if one steppers stalls when you have two on one driver then you can say goodbye to the driver. If one stalls with two drivers it's a bit annoying as the gantry is bent, but not the end of the world.

[JAZZCNC]

Tenson Jonathan's right on all counts here. I gave you the options he's giving you specifics about the rotating nut. If your wise you'll go with the rotating nuts at this length for best speeds verse's cost's.
The rotating nut will give you best of both worlds and save you some money.

The other reason for not using 1 drive with 2 motors is that if the pulse timing doesn't get to each motor at exactly the same time then you will slowly drift either infront or behind on 1 axis so all in all it's a lose lose situation and at best you'll be inaccurate and worst be letting out the magic smoke. . . Your choice.!!