Dual X Axis

[skaos]

Hello,

I'm a newbie to all this CNC stuff, im a software engineer working in the operational research field.

Im still in the designing process and i have a some questions

I didnt saw many DUAL Motor on the longer axis (either using 2 acme rod or ballscrew) WHY?

I saw many design using R&P, but not many dual driven.

I aim for 60" on my longer axis using rolled 2505 ballscrew from china.

Except for the cost issue, my understanding is that 2 ballscrew will never be the same and some adjustment will be needed ? Its not as easy as slaving a motor in Mach3 ?

Thanks a lot,


Lou

[CarveOne]

Originally Posted by skaos Hello, I'm a newbie to all this CNC stuff, im a software engineer working in the operational research field. Im still in the designing process and i have a some questions I didnt saw many DUAL Motor on the longer axis (either using 2 acme rod or ballscrew) WHY? I saw many design using R&P, but not many dual driven. I aim for 60" on my longer axis using rolled 2505 ballscrew from china. Except for the cost issue, my understanding is that 2 ballscrew will never be the same and some adjustment will be needed ? Its not as easy as slaving a motor in Mach3 ? Thanks a lot, Lou

Rack and pinion only became widely available to the CNC hobby market back in late 2008-early 2009 and was introduced by cncrouterparts.com. These have become very popular for 48" and longer X axis machines since then. I use one on my gantry Y axis, and two on my X axis (X and A) in slaved mode. My machine is 12 feet long.

There is no reason that dual motors can't be used with ACME or ball lead screws to help prevent racking of the gantry. It doesn't provide much of an advantage versus cost for small machines. By the time the machine size becomes a problem with racking people now tend to go to rack and pinion drives anyway. A few years ago that was not really much of an option unless you designed and built your on drives.

60" with ball screws is at the point where you need to consider rack and pinion drives. You will be limited in jog speed by lead screw whipping. Rack and pinion doesn't have that potential and 1,000+ inches per minute jogs are possible, although it's not very useful.

Racks and ACME rods are not always exactly the same either, but adjustments can compensate for most differences.

CarveOne

[wcarrothers1]

I'm not exactly sure when rack and pinion was invented.. How ever I've had rack and pinion drive on my machine since 2006.. I'm not sure why CNC router parts.com would have to be elected as the company who gave this technology to the DIY market. Seeing Mcmaster car has been selling rack and pinions for years before they came along..hehe

That said. I like my rack and planetary axis (long axis) and how it operates far better then I've ever liked my ball screw axis. On the Z axis ball is just fine I suppose. But for my current build all the axis are getting racked..

b.

[ger21]

I didnt saw many DUAL Motor on the longer axis (either using 2 acme rod or ballscrew) WHY?

You didn't look very hard.

The majority of larger machines being built use 2 motors to move the gantry.

Except for the cost issue, my understanding is that 2 ballscrew will never be the same and some adjustment will be needed ? Its not as easy as slaving a motor in Mach3 ?

I've seen one person in my 7 years here saying the the two screws weren't close enough for him.

For most people, the very small error between the two screws should be insignificant, maybe a few thousandths over the 60".

If you want to use slaving (which almost everyone with 2 motors does), you'll have to assume the two are identical and leave it at that. Mach's slaving function treats them as identical, and there's nothing you can do about it.

A bigger concern should be your 5mm pitch screws. They'll severely limit your speeds if using steppers. 10mm lead or even more is preferable.

[CarveOne]

Originally Posted by wcarrothers1 I'm not exactly sure when rack and pinion was invented.. How ever I've had rack and pinion drive on my machine since 2006.. I'm not sure why CNC router parts.com would have to be elected as the company who gave this technology to the DIY market. Seeing Mcmaster car has been selling rack and pinions for years before they came along..hehe That said. I like my rack and planetary axis (long axis) and how it operates far better then I've ever liked my ball screw axis. On the Z axis ball is just fine I suppose. But for my current build all the axis are getting racked.. b.

I didn't say or imply that Ahren invented rack and pinion drives, or that none existed before. He did introduce them at a price point that has caused a lot of us to change our 60" lead screws to R&P without hesitation.

I didn't know that McMaster has been selling them. I assumed that commercial machines used custom made drives designed by their own mechanical engineers.

There can be minor differences in racks also, especially between two manufacturers who make the same part, and in different productions runs at the same manufacturer.

CarveOne

[hemsworthlad]

Hi Lou,

I use twin 60"(total length) 20/05mm pitch chinese ballscrews with only 1 motor connected via beltdrive and it works great and very accurate. Nothing wrong with slaving motors just my personal preferance not too.
My machine cutts Ali 99% of the time and the 5mm pitch rapids maxs out around 4.5mt/min when driven 1/1 with 8Nm nema 34 motors. This is fine for 95% of the work I do but Like gerry says if you need faster rapids or cutting speeds then 5mm pitch is limiting.
The way my machine is designed it allows me to swap the motor pulley and gear it 1/2 or with a belt change as well 1/3 thou I never have had need for 1/3. See pic
I just have Mach3 profiles that I swap between with motor tuning suited to each gearing, I use taper lock pulleys and it literially takes me 5 mins and I,ve doubled the speed of the machine all be it with a slight loss of resolution but usually if I need to do this it's for cutting woods or plastic and the resolution is fine and still better than most machines using R&P.

With my 20mm screws whip hisn't a problem and when testing gearing I pushed it upto 15mtr/min, that said I wouldn't use it at these speeds and would think it would certainly shorten the screw life.
If you plan on working wood then defiantly go with 10mm pitch minimum but also be aware that the 25mm dia screw is conciderably heavier than 20mm(I have some 25mm as well, big differance) so inertial forces will be something to factor into your build esp if an heavy Gantry is being used and will require larger motors to handle this force.

Differance's between screws is just not an issue that can be detected on my machine and over 1200mm I see less than 0.02mm and some of that could be my alignment not the screw. I'm sure I cant measure it accuratly enough to be certain my figures are exactly accurate but I can tell you 100% it's not a problem that shows in work produced and i'm quite fussy. .

Hope this helps.