Hi all,

Would just like to know, wot u pros think of using rack and pinion on x axis with 2 motors, 1 either side, before i start to get parts made to fit this i would really like to know pros and cons,

My rack is module 2 20mm and my pinion is 12 tooth 20 degree angle, Now i know after having bought this rack and pinion and fixing it to my system that 12dp isnt ideal and 24 would be better, but will this work, before i spend more money on this headache, i was thinking of direct driving these pinions with 300 oz inch steppers, and each rotation of pinion moves 3".

Thanks in advance for any replies. Appreciated

DeeDee

Your idea will work but I worry about the fact that you have two separate motors not physically linked. If one motor would loose a step or two then that would put the gantry in a bind. A lot of commercial machines use one motor turning a drive shaft linked to the pinions on either side. With this method the only way it could get in a bind is if a pinion would jump the rack. If there is this much play in your system then you have done something wrong in your design. If the commercial guys do it then there must be a good reason for it.

K.I.S.S.- keep it simple stupid

Many of the large commercial tables use motion control systems that are capable of 2 motors running independantly, rack & pinion on either side, with one encoder slaved of the other (master).
Al.

I run a commercial router with the set-up you are refering to - two motors, two racks. It is a very common arrangement - especially on larger gantry machines.

Scott

I dont like the idea, even if professional companies do use it.
I have a new customer who has this arrangement on his machine, and he is finding he has problems with racking. Sometimes the steppers get out of sync and then continues to drive the gantry skewed for the rest of the cut. I cant imagine what this does to the linear bearings.
I have taked him into a retrofit ( I have jumped into the deep end here) I have suggested we use just one big servo and a drive shaft across the gantry to connect both sides to the one servo. To me there is something more honest about this approach. Its a mechanical link, its simpler, it only requires one servo and servo drive card, less chance for future problems. ETC

But that is just my opinion. Just remember, the more technical something is, does not necesarily mean it is better. Sometimes mechanical is still superior to electrical.

Benny, I have a couple of customers that have a couple of these Plasma machines with this system for over 10yrs and still running 16hrs a day.
Al.

I'm planning on using the two motor system with racks and pinions....

I would bet that those that have problems with the "slaved" drive are using under-powered motors or are not tuning their systems within its capabilities.

Otherwise I have heard and read of many successful dual drive systems.

No argument guys. I was just telling my story and my customers story with his "professionally" built machine.
Yes Bloy, the steppers on my customers machine are surprsingly small. I was very surprised when I saw them. Actually I mught have a picture of them.

Here are the pics. Wouldn't you agree these steppers are surprizingly small?

I hear ya on the horror stories....I have had issues before but they were caused by an abnormal event.

If the motors are sized properly for the equipment I can't see there being a problem under normal conditions. Having a dust collection boot hang up on the edge of the table or fence, for example, is not normal......not that it's ever happened to me.... :o

Squaring the gantry during referencing is a consideration and a must with open-loop, independent drives.

Scott

Generally, when this question comes up, you're going to have people tell you it works fine, and people tell you all the things that can go wrong. As long as you don't have underpowered motors, you really shouldn't have any problems at all.

Hi everyone,

Thanks all so much for the replys,

I have 2 300 oz inch steppers that i am intending to use, the gantry would weigh around 120lb's or less, would you all think that these motors are enough for this machine, also what all is involved with setting up master and slave steppers, i am starting with a hobby cnc board, but i am thinking of upgrading to to geckos, or are there any other boards that would best suit my application.

Thanks again.
DeeDee

If you use Mach2/3 to control your machine, all you have to do is check a box in the software to slave one axis to the other. You should also have a seperate home switch on each side, and Mach2/3 will square the machine when you home it.

Great to hear that Ger,

I havnt got mach as yet, how ever i think i will get it, could you also tell me do you think 300 oz inch steppers are good enough for the job, i was wanting to direct drive them as it saves me allot of agro trying to use gearboxes or pullies to get them geardown,

Thanks for your Help, much appreciated :-)

I've never used rack and pinion, but from what I've read, they should work fine IF you gear them down at least 3:1. I'm not sure how well they will work direct drive.

You can download MAch2 or Mach3 and start using it right away, to see how it works. At the end of the install, if you uncheck the box that says load driver, it'll run in simulation mode. It's free for use with less than 1000 lines of g-code.

ger21 -

3:1 - is this a decent rule of thumb for R&P?

Scott

ger21 -

3:1 - is this a decent rule of thumb for R&P?

Scott

Not sure, but you want to gear it to get decent resolution.

One method I use enables me to get the lowest inertia ratio and fastest accel/decel rates and that is to find out the top rpm of your motors, the travel/rev of the pinion, and the desired top feed rate and go with the highest reduction ratio you can, in keeping with the method you intend to use for gearing etc.
BTW here is a sizing program I have used for quite a while, I believe this is the only site you can get it now, it was originally put out by Emerson (Emersize) in the 90's.
The advantage is you can enter you own 3rd party motors and amps.
http://www.macphersoncontrol.com/navbar/mcptools.html
Al.

A good commercial dual motor system will use servo/encoder Vs steppers to avoid the issue of missed steps. I work in a shop with all kinds of CNC equipment (mostly Amada) The press brake back guages use dual servos...we also have a shear using a cross-over shaft to keep the guages locked together. If you're useing rack and pinion (on both sides) it would be as simple as just adding an idler shaft anywhere convenient to lock the two sides together.