|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 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.
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