Rack n pinion or ball screw

[dcp]

Hi from Ireland
About to start my plasma table
Just starting out. I have got blade runner 620 oz package from Candcnc
Q1. Can I use direct drive from motor to the rack . ?
Why should I have indirect drive?
What pitch of rack and pinion is most suitable for these motors . I'm assuming that people are familiar with these motors
Q2 if I was to use ball screws 8 mm pitch . Would that be a better choice.

Also I was looking at linear rails as an option for the sliding along x and y axis

Regards

DeclanBreslin

Similar Threads:

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• Rack & Pinion vs Ball screw

[awerby]

People usually use an indirect connection between the motor and the pinion gear to gain some mechanical advantage, and to facilitate spring-loading the gear into the rack. Here's an example (which you can purchase as an assembly, rather than have to construct yourself): Standard Rack and Pinion Drive, NEMA 34 | CNCRouterParts While ball screws can potentially give you more precision than rack-and-pinion, they are also more delicate (as well as being more expensive) and aren't generally used in plasma cutters, since the process generates so much grit and debris that it's easy to ruin them, unless they're well-protected. R&P drives are more forgiving, although it's still advisable to mount them in a sheltered position to avoid a buildup of dust on them. Linear rails have similar issues; plasma cutters often use alternative systems like V-rails: http://www.bwc.com/products/componen...s/dualvee.html

[wizard]

awerby covered most of the issue below but I will try to add a bit of perspective.

Hi from Ireland
About to start my plasma table
Just starting out. I have got blade runner 620 oz package from Candcnc
Q1. Can I use direct drive from motor to the rack . ?

If you are designing your own machine you can do anything you want. In a rack driven system though one issue you run into is resolution, especially if you are using a stepper. You can run the math here but if you have a two inch pinion one step can move you a significant distance I a rack based system. There are other issues also including inertia matching but I think it is fair to say you need a gear reduction of some sort.

Why should I have indirect drive?
What pitch of rack and pinion is most suitable for these motors . I'm assuming that people are familiar with these motors

That would depend upon your machine cover all size

Q2 if I was to use ball screws 8 mm pitch . Would that be a better choice.

I'd have ot say no. Mainly because of the same issues awerby pointed out, it is a nasty environment with slag and dust going al over the place. You will need protection for either system you implement but a rack based system would be easier to maintain in my opinion.

Now that being said have you considered a belt drive? A plasma machine and a belt drive has significant potential, you still will need a gear reduction (effectively the belt is a rack in this sort of application). Protection from the elements would still be needed but repair and maintenance would be easier.

In any of the cases above you can certainly use a ball screw for the Z axis (Torch hight control). The same issues of protection are in play but the short stroke should allow for more reliable protection of the lead screw.

Also I was looking at linear rails as an option for the sliding along x and y axis

Again the issue here is contamination which can happen fairly easy on a plasma machine. Assuming you are talking round or profile rails I'd most certainly consider alternatives. Many commercial machines have gone the V-Roller route for more resistance to the gunk that gets thrown around on these machines. It probably would hurt to have the roller spring loaded if the occasional bit of splatter gets in the way.

The nice thing about plasma, water jet and similar machines is that you basically have no reaction forces coming from the tool being used. In the case of plasma you are not talking high precision and great surface finishes anyways. So you don't need the rigidity of say a router to get good results.

Regards

DeclanBreslin

You should mention the size of your machine though it really isn't a huge factor here. On a small table it might be cheaper to use ball screws if you can protect them. For many largish plasma tables though a rack or belt drive beats the leadscrew solution due to size even if you don't consider the contamination issues. In any event The most importune thing to remember is that you need to know what your expectations are before you build so that you can bias the design to meet those needs.

[dcp]

People usually use an indirect connection between the motor and the pinion gear to gain some mechanical advantage, and to facilitate spring-loading the gear into the rack. Here's an example (which you can purchase as an assembly, rather than have to construct yourself): Standard Rack and Pinion Drive, NEMA 34 | CNCRouterParts While ball screws can potentially give you more precision than rack-and-pinion, they are also more delicate (as well as being more expensive) and aren't generally used in plasma cutters, since the process generates so much grit and debris that it's easy to ruin them, unless they're well-protected. R&P drives are more forgiving, although it's still advisable to mount them in a sheltered position to avoid a buildup of dust on them. Linear rails have similar issues; plasma cutters often use alternative systems like V-rails: DualVee

Thanks Andrew that was a great help

[dcp]

Thanks Wisard for the info. The table will be 5x5ft

[Torchhead]

Best belt reduction ratio for a stepper plasma table is one that gives you 25mm of travel for on rev of the screw. More than that and you can hit the speeds you need to cut thinner material. Less than that (like direct drive) you may not have enough torque to accelerate the load (gantry weight to the target 35 ips/sec

Ballserws DIVIDE the speed by the TPI . So 8mm pitch is about 3 threads per inch. Steppers spin up to aout 800 RPM but need to stay in the 400 RPM range to have enough residual torque to be able to accelerate and decelerate on rapid direction changes. so divide the 400 RPM by 3 and you can predict that you will have only about 133 IPM of CUTTING speeds. That takes you down to about 3/16 steel. Your best target is to try and build a machine taht will do about 350 IPM cutting pseeds and 600 to 700 IPM rapids. Ballscrews also have a max RPM so smaller ones cannot spin as fast as larger ones. The longer thay are they less RPM they will handle. The 8mm pitch ones would have to handle the Motor RPM


The size of the table is less important than the gantry weight. Once that is established for a given width you can make it as long as you want with no changes except to wire lenghts

[dcp]

That's very good . I'm using a hypertherm 45 so I'd be limited to 5 to 6 mm thickness steel anyway. Thanks for your thoughts. By making the gantry out of aluminium would lighten the payload , and unfortunately the aluminium extrusion 80/20 isn't that available here. Would making the gantry from box iron make the gantry away too heavy ? .
From a few builds that I have seen on the net , they talk about an adhesive strip that they had used to adhere the rack, what stuff is that?.

[rodw]

I used 40mm x 80mm x 3mm Aluminium Box section for my gantry and that worked out OK in the end. All of my linear rails and gear racks are bolted on. And yes, I did drill and tap a lot of holes! For some of them I cheated and used laser cut parts so I just had to tap them. The gantry I was able to drill and tap on mylittle mill to keep it all square. For the table, I bought a magnetic tapping jig and hand tapped them all.

I have seen adhesive tape being used. I don't have any experience with that but if I was going to do this, I would look for a 3M ATG700 tape gun with the right adhesive tape.
https://www.getpacked.com.au/p/3m-ad...t/2-ATG-700-3M
Others might have other ideas as this would not be the most economical solution as it costs a bit to set up with.

[xalky]

Hi from Ireland
About to start my plasma table
Just starting out. I have got blade runner 620 oz package from Candcnc
Q1. Can I use direct drive from motor to the rack . ?
Why should I have indirect drive?
What pitch of rack and pinion is most suitable for these motors . I'm assuming that people are familiar with these motors
Q2 if I was to use ball screws 8 mm pitch . Would that be a better choice.

Also I was looking at linear rails as an option for the sliding along x and y axis

Regards

DeclanBreslin

Not sure how far along you've come with your build but I used timing belts for the x and y on my cnc plasma cutter. I have the same blade runner package that you have, only I purchased mine 5 yrs ago. But they are relatively the same . I made a video of the machine of me reviving it after 2 yrs sitting dormant after a long distance move. Check out the video here:

If you got any questions, let me know in the video comments I'll be glad to answer them and I'll probably make a video on it.

Marcel

[dcp]

What's the teeth per inch on the timing belt.. I haven't started the build yet . My supplier needed the exact order for the rack and pinion . What module and teeth per inch. I'd really confuse him if I was looking for belt drive. But not if I had the exact size.
What sizes or the drives for the belt

[xalky]

What's the teeth per inch on the timing belt.. I haven't started the build yet . My supplier needed the exact order for the rack and pinion . What module and teeth per inch. I'd really confuse him if I was looking for belt drive. But not if I had the exact size.
What sizes or the drives for the belt

I use 3/8" tooth spacing x 1/2" wide belts. I'm in the US. I believe they are module L . They're larger than the mxl series bit in the same family. You can use something similar size in metric, something in the 12mm width range. Much depends on the right of your gantry. My gantry weighs about 60 to 70lbs with everything on it.I'm gonna be posting another video soon with some more design overview. Subscribe to be notified. I have a feeling I'll be posting up a few more vids based on the questions I get.

[jcoldon]

check my youtube site koollakes showing how to make a rack drive and cheap gantry