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Building a CNC Plasma Table

After I complete my CNC lathe, and CNC my IH Mill, I'll start thinking about what kind of CNC Machine to begin building next. A CNC plasma table would not be a bad idea. When designing a table like this, there are many decisions to be made. Perhaps the most fundamental surrounds the choice of ways and drive mechanisms for each axis.

Choices for ways include:

- Linear Rails: Most precise, bolt on installation, can be costly. To purchase 2 x 8 foot 25mm rails and 4 trucks new from THK would cost in the neighborhood of $800 for the rails and nearly $500 for the trucks. You can do a lot better on eBay, possibly as well as $200 for the whole kiboodle, but it will take a lot of time (plan on as much as 6 months) before you get the right deal and win the auction.

- Shaft w/ Commercial Linear Bearings: A step down from linear rails and ball bearing trucks, but still pretty nice. For 2 x 25mm shafts, 5' long, and 4 ball bearing bushings, the cost would be about $120 from VXB. The trouble on a system like this is supporting the shaft in the middle on a long shaft. They make bearing sets that have an opening for the supports, so I would look into these for the X and Y axes, which can be pretty long.

- Bishop Wisecarver V-Groove Rails. These are a nice, relatively inexpensive commercial system that bolts together.

- Fabricated skate bearing systems that run on shafts, or rectangular sections. These are the most common seen on home shop built machines because they are very cheap.

- HMWP and aluminum extrusion systems. You don't see these very often, especially for larger systems, but I understand they work great.

Drive sytems include:

- Ballscrews: Smoothest, most precise, and most expensive.

- ACME screws: My impression is that these work great for this application, especially if fitted with anti-backlash nuts.

- Rack and pinion: A real simple way to go. Be sure to tension to reduce backlash.

- Chain and belt drives: If tension is kept, these systems can work too. They are less common.

In preparation, I have been accumulating some parts and making some plans:


- 2 x THK HSR30 rails with 2 trucks on each rail and 52.5" rails. Acquired for $199 on eBay.

I plan to power the X-Axis with a large stepper motor or motors driving a rack and pinion. I need to decide whether each side of the gantry needs to be driven or whether it will be sufficient to drive one side only. I also need to figure out how to create a flat surface to mount the slides to. The framework for the table will be 4" steel square tubing with 1/4" walls. I am thinking I should round up a piece of thick aluminum that can be machined flat and then bolted with shims to the top of the tubing. New SHS trucks for comparison would be $120 each.


- THK KR33 linear slide assembly, purchased from eBay for $179.50.

This is a slick little unit complete from soup to nuts even including home/limit switches. It incorporates a ballscrew, linear slides, and a servo motor with timing belt reduction drive. It would be very difficult for me to design and build an assembly of similar quality, so I figured it would not only be a worthwhile investment, but that I could probably learn a lot by studying it. The ballscrew has a 6mm lead, and the overall length is 310mm ~ 10" of travel. Perfect!

I have been compiling an Ideas Notebook to try to get the juices flowing on design.

I like the rack and pinion approach--it is cheap, and reasonably accurate. Certainly it is good enough for plasma, if not routers as well. I wonder sometimes if I should use the linear slides for this project as a skate bearing and rail system would probably work just as well and much more cheaply. We will see. Assuming I stick with rack and pinion, I did come across the following advice on what/where to buy:

A good source of rack and pinion is Standard Steel Specialty Co. They are in Beaver Falls, PA, 724-846-7600. Unless you are running a VERY large machine, I would recommend 20DP, 20 degree pressure angle, 1/2" x 1/2" x 6 feet length. Part #200011 is 6 feet long, #200010 is 4 feet long. Pricing for quantities of 2 - 24 of #200011 is $23.20 each; shipping is extra, but I think UPS will take 6 foot lengths. They are strapped to a 6' board.
As to matching pinions, I think Boston Gear has them. You should try to have more than 18 teeth in the pinion on a 20 degree PA setup to avoid undercutting of the teeth. My notes show a 24 tooth Boston Gear #YA-24 lists for $15.46 each. With 24 teeth the distance per revolution will be
24/20 x PI = 3.7699 inches per revolution. An 18 tooth would yield 2.8274 inches per revolution. Ideally the pinion face width should be greater than or equal to the rack face width.


Plasma Table Idea Notebook

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