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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:
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.
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