Rack and Pinion--The wrong way to go

[darylprice]

I built my CNC Router about 3 years ago, and it works quite well, but as funds came slowly I opted for a Rack and Pinion system instead of Ball Screw, big mistake. It is virtually impossible too remove back lash from this kind of system. I have seen a number of DIY versions whcih are incorporating Ball Screws and having 2 parallel ball screws on the Larger X- Axis. Finally my question:
How do you make two Servo motors sense each other, so they both are moving exactly at the same time? I am using Mach4 software; is there something in that that does it? My machine is only 24"x36", so I probably would not need two(opinions on this), but I may increase the size someday and need two.

Thanks

[ger21]

Mach has a slaving function to sync the two motors.

[Greolt]

Originally Posted by darylprice ...... big mistake. It is virtually impossible too remove back lash from this kind of system.

.

I'll go one further and say "It is impossible to remove all backlash from any kind of linear drive system"

What we need to do is design and use a system that will keep backlash within the tolerances needed for the application.

R&P has proven to be very suitable for many CNC applications. Including routers, plasma cutters, water jet cutters, etc.

R&P is a very common drive system in many types of commercial built CNC machinery.

Greg

[darylprice]

Thanks all for the information I will research it all

[Al_The_Man]

Usually unless you have a very large table and/or a very heavy gantry, the one motor should suffice, if you carry a drive shaft across the table to the opposite rack, this is one method to prevent skewing.
I have even seen this method used on larger (6'x12') tables.
Al.

[darylprice]

Thanks, I am not sure what you mean about carrying a drive shaft across the table however

[Glacern]

Originally Posted by darylprice Thanks, I am not sure what you mean about carrying a drive shaft across the table however

A power transmission shaft to drive two R&Ps (one on each side) with one motor.

[darylprice]

Yes, that is what I am doing now, sorry about the mis-understanding

[harryn]

Hi - I wondered if you would mind posting some pictures and description of your system and components?

I am certainly here to learn, and while learning from the "it works great" projects is always helpful, I learn a lot more from the "I am not happy with the results" projects.

When I make errors, it is usually more than one. Maybe someone will spot something simple you can do to improve the system sufficiently for use until you can make the next step of investment.

You could either put some pictures here or make a "build log" thread post and just put a link here.

Thanks for posting.

HarryN

[darylprice]

I will get some pics up here

Thanks

[gasmasher]

I am very new to CNC but am building my own machine. Mine is desktop size with two motors on the X axis. I am planning to use the slave functionality of the 4 axis controller board to run the X from two of the outputs, one for Y and the last for Z. This requires four controllers for just three axis. Basically the controller takes the input that would normally be used for one output controlling one motor on X and signals two outputs.

I am not experienced with Mach so ger21's answer may be easier/cheaper to implement but I guess you still end up with a controller per motor.

[Mike Everman]

Originally Posted by Greolt . I'll go one further and say "It is impossible to remove all backlash from any kind of linear drive system" What we need to do is design and use a system that will keep backlash within the tolerances needed for the application. Greg

You must mean that you can't eliminate hysteresis. Backlash is easily removed in a number of ways. It is common to consider any lost motion when changing direction as backlash. I like to think of it as "zero stiffness dead-band". While very different dynamically, a suppose it is the same when it comes to making backlash corrections in your controller.