Hello All,
I’ve had my CNC plasma table up and running for a couple of months now, and am finally getting around to doing a post on it. I’ve been reading through the “Zone” for a year or so and have found a great deal of useful information. This project would not have been possible without cnczone.com. This post is my attempt to give back a little to the “Zone”.
The machines working envelope is 61” by 61”. This was limited by the availability of linear rails on ebay and space constraints in my shop. I had a very hard time finding anything over 80” at a reasonable price. I looked into new rails, but the $3500 price tag was beyond my budget. I ended up with two 78” NSK20’s for the Y axis, one 65” THK35 for the X axis and one 28” IKO15 for the Z axis. The Z axis rail was cut in half to give me two 14 inch lengths.
The design is an open gantry (i.e.. the cutting/water table is separate from the drive/linear motion system). I like the fact that heavy sheets of steel placed on the table cannot deflect the linear motion components of the machine. I also thought it may be handy to roll the table out with a pallet jack, and allow me to cut stuff like 12 inch I-beam that would not fit between the gantry and table. The gantry is constructed from 3” square steel tube with a wall thickness of .125” and is anchored to the floor with height adjustable anchors. These anchors made leveling the machine a breeze…….. well maybe not a breeze. The legs of the gantry were braced with 45 degree braces due to this nasty little thing called “Inertia”.
The X axis and both sides of the Y axis are driven via rack and pinion. Each of the three use the same type of drive (all parts are identical). Basically I designed 1 drive and made 3 of them. Each drive pivots, at one end, on a ½” diameter precision shoulder bolt and is pulled toward the rack with a spring on the other end. These drives consist of a 270 oz stepper motor coupled to a 14 tooth XL timing belt pulley which drives a 60 tooth XL timing belt pulley which is coupled to a 14 tooth pinion gear. How’s that for a run-on sentence…. I used 16 pitch 14 ½ degree racks and pinions. The 4.29:1 drive ratio, coupled with the 14 tooth pinion gear, achieves .641” of travel for 1 revolution of the stepper motor. This ratio seemed like a nice compromise of speed and accuracy. With a 10 micro-step G201 Gecko drive and a 200 step/rev motor, one micro-step equates to .00032 inches of travel (200 X 10 X .0003205 = .641”). I’ve tested the machine’s rapid speed at up to 850 ipm. I suspect the machine could run faster with Mach3’s new increase in kernel speeds, but 850 ipm rapids scare me enough as it is. I’ve limited rapid speeds to 450 ipm. The acceleration values in Mach3 are set to 45 inches/sec/sec. I’ve run acceleration values as high as 150 inches/sec/sec, however the sudden jarring of acceleration and deceleration seemed a bit excessive. I was afraid of shearing the roll pins that I had used to attach the pinion gears to their drive shafts.
The Z axis motor is connected to a 14 tooth XL timing belt pulley which drives a 28 tooth XL timing belt pulley which is coupled to a 4 start, 8 pitch, ½” diameter lead screw. I used a multi-start thread in order to increase the distance traveled per revolution and thus increase travel speed. With this thread I get 1/2” of travel per revolution of the screw. Unfortunately this was a little too much, with my original 1:1 pulley to pulley ratio, and the weight of the Z axis would back drive the screw and the Z axis would slowly drift downward into the cutting table when power wasn’t applied to the Z axis stepper motor. The new 2:1 pulley to pulley ratio stopped this. With this setup I get .250” of travel per stepper motor revolution. I haven’t tested the Z axis for maximum speed. However, I've set it to 150 ipm, which seems to be more than adequate for what I’m doing.
If you’re thinking about adding a water table to your setup, I have 3 words of advice…. “Just do it!!!” You won’t regret it. Prior to adding the water table to my system, I would open every window and door, in my shop, and turn on the fans. I would still end up with a layer of black dust that settled on every surface perpendicular to gravity. This dust would not just settle close to the machine, but on every square inch of my 20’ X 40’ shop. Nasty stuff!!! With the addition of the water table the dust problem has been reduced by at least 95%. I don’t have to open doors and windows anymore when I’m cutting. I have three 8” ports in the walls that can be hooked up to a ventilation system, and have yet to use them. Plus, I get to enjoy an air conditioned shop, as opposed to the 100 degree outdoor temperatures we’re currently having in Kentucky. The water table’s construction is similar to that shown in a PDF file from Thermal Dynamics that I found on the net, which uses compressed air to raise and lower the water level. Water in an inner tank is displaced by compressed air and flows out and up into the area below the sheet that is to be cut. The top of the support slats are ½” below the top lip of the table. The slats are 2 ½” tall and are set into slots in 3 “hangers” which are welded to the top of the inner tank. The distance from the bottom of the slats to the top of the inner tank is 1”. Therefore if I’ve adjusted the water height to just touch the underside of a sheet, I have 3 ½” of water below the sheet. Personally I think a water level of ¼” below the sheet works the best. The green/amber color of the water is due to an additive called “Plasma Quench”, which is available from Koal Industries. Plasma Quench keeps steel from rusting and keeps the water from going rancid. Since the steel doesn’t rust I don’t get brown murky water. The cut particles of metal just sink to the bottom for later removal. The water stays a transparent green/amber color with no smell. Based on my limited time using it (about a month), Plasma quench seems like a very useful product.
I am using Gecko 201’s to control each of the stepper motors. The Gecko’s are powered by a 42 volt power supply. The power supply was built according to the data sheets and information available from geckodrive. The parts to build the power supply were purchased from digikey. Once again, if you’re thinking about using Geckodrives, “Just do it!!!” They are a little pricey, however it’s definitely a case of “You get what you pay for”. You will be absolutely amazed at the amount of power Geckodrives can impart into a motor that’s barely larger than the palm of your hand (using sufficient power supply voltage, of course).
I am using cnc4pc’s C1G parallel port interface card . I’m using the latest lockdown release of Mach3 (Go Art!!!! Keep up the good work!!!) As for generating code, I think the simplicity and speed of Sheetcam is extremely hard to beat.
I’m using the THC300 torch height controller, available from Campbell Designs. As my local Hypertherm rep put it, “A torch height controller in NOT an optional item when it comes to a CNC plasma table. It’s a necessity”. After using my table with and without a THC, I wholeheartedly agree. It’s just something that you’ve gotta have. Figure it into your budget. Your cut edge quality, and consumable life will show for it.
If you’ve gotten this far, thanks, and I hope you’ve found some useful information.
Pictures of my machine can be found in the link to my gallery below.
Jcar,
http://www.cnczone.com/gallery/showg.../38704/cat/500