My scratch-built plasma table

[grahamcowan]

Since I needed a place to post pictures and tell my tale somewhere other than my own website which is really only looked at by family, I figured I'd go ahead and start a thread here that explained what I did and why.

Considering the table is done, (first cut on it tonight!!) I will try and go back to the beginning of why I did what I did and how I went about doing it.

Hope some of you guys keep up and let me know what you think, etc.

I'll try to post as quickly as possible, but a 7-month old can keep you on your toes!

[grahamcowan]

I have always wanted a plasma cutter so around the beginning of February I went and got a Miller Spectrum 625 after much debating on brand, capacity, etc.

Go ahead, give me flak about the Miller -- I hear it all the time. I chose the Miller because I'm in good with the guys down at the local AirGas, and I'm partial to the power of blue.

I used the machine off and on to trim stainless sheet and mess around with, but it was aggravating to try and cut a straight line (even with the straight line roller guide I bought from Miller) without it looking like a drunk did it.

So ... I got a hairbrained idea to make a CNC plasma table.

My shop teacher from high school heard of my idea and showed me the PlasmaCam table he got the school to buy before he retired. The table, the machine, no computer, and two art CDs were $13,000. A little high for me, but I think those systems are designed for people with not much tinker time or down time to spare.

I had some basic designs sketched out that I thought would work until I started seeing what everyone online had been doing. Needless to say, my designs rapidly changed.

I typically will not even start a project until I have the entire design finished and all the kinks worked out (not in the typical government contractor fashion, of course), so it was the better part of a month of sketching, drawing, trashing, redesigning, researching, and thinking before I had a plan.

I eventually went from a full frame from 80/20 extruded aluminum to tube steel.

And so the project began.

[grahamcowan]

I scrapped the 80/20 idea due to the prohibitive cost and the fact that the local shop that stocked everything 80/20 kind of blew me off even when I told them I was ready to spend some money and would they please help me out. Oh well; their loss.

The frame ended up being 2" and 1-1/2" square steel tubing. I have the Y-axis rails and supporting legs underneath them out of the 2" tubing and all the cross braces out of 1-1/2" tubing.

I used 23' of 2" x 2" x 1/4" tubing and 30' of 1-1/2" x 1-1/2" x 3/16" tubing for the frame. The leveling plates on the bottom of the 4 legs are 4" x 4" x 1/2" plates with a 3/4" long 1/2"-13 bolt in the corner. I have no idea if these will even be used; the garage floor seems pretty level right now to begin with. These plates were actually added later so they won't appear in the pictures for a while.

I have a DeWalt portable bandsaw I used to cut the steel into the pieces I needed, ground off the crud with a flapper disc on my side grinder, and tacked everything together with, yep, you guessed, my Miller welder.

The 1-1/2" cross braces needed to be exactly the same length to keep things square, so I welded them in pairs. I took B-Line unistrut 4-hole splice places and 4-hole T's and welded them together, then welded them to the ends of the cross braces. After tacking them up and making sure both pairs of braces were the same length, I cut them apart.

So now I had 2 Y-axis frames tacked together and 4 cross braces tacked up.

Time to drill and tap some holes.

[grahamcowan]

Little did I know that I would manage to drill around 650 holes and tap 240 of them, ranging in size from 1/2"-13 all the way down to 4-40.

I lugged my drill press off the workbench, set the two Y-axis frames up on buckets, and went to drilling the starter holes for the 1/2"-13 bolts I planned on using to hold the entire frame together.

What a royal pain in the rear. Balancing the frame, drilling the holes, and holding the vacuum to the shavings all at once was tricky, but I managed to pull it off.

The wife was even gracious enough to let me use her spot in the garage/shop whilst I made a mess. (I have to clean up afterwards, by her decree...)

[grahamcowan]

The gantry design gave me the most difficulty.

I knew I would like to use a piece of 80/20 aluminum because it is light and strong and fairly cheap. But how would I attach X-axis rails to it? How would I attach it to the Y-axis rails? How would I make the bracing strong enough so it wouldn't wobble? I tried to consider every way I could screw it up by design.

I found a guy on eBay that sells used but good condition 80/20 everything, so I got a 96" piece of #2040. The #2040 is 2" wide and 4" tall. I figured it would be good enough to hold the weight of two linear rails and all the crap that goes with it.

Conveniently enough, the holes in the ends are exactly the size of a #7 drill bit, which taps to a 1/4"-20. This was very handy later in the build process.

[grahamcowan]

The frame came together pretty quickly once I had all the holes drilled and tapped. A 3/4" box end wrench and a rubber hammer to get everything relatively squared up finished the first bolt-up of the frame.

I had seen supports for the metal that is being cut on a plasma table plenty of times in person and online. Most of the time they were made from thin strips of sheet metal either in straight lines or wavy patterns.

Logistically, this was a stupid design for me to try and copy. How was I going to get the metal cut, bent, and fitted into place to begin with? After using the table for a while, how was I going to replace worn out supports quickly and cheaply?

My answer? I wasn't. Time to think of something else, preferably cheaper, faster, and easier.

I came up with long threaded screws or bolts with a jam nut on either side in case you needed to adjust the height. I designed 13 pieces of 1" angle, each with 14 holes. In each hole would be a 4" long hex-headed cap screw with a fixed nut on the top and an adjustable nut on the bottom. That's 182 holes to drill and 364 tacks to make. I guess I'm just a sucker for tedious work.

So I bought 65' of the 1" angle, and the metal shop enjoyed my small orders so much that they threw in the 24" piece of 2" angle for free that I planned on using for motor bracket supports.

Now I'm up to the end of March and have bought nothing but steel and piddly hardware, all of which is dirty and heavy and doing anything with it makes lots of noise and mess.

The wife was thrilled, but optimistic, for my sanity's sake.

[grahamcowan]

With the exception of the plasma cutter itself, the four linear rails were the most expensive piece of hardware I bought. In the beginning I had designed some really slick ball bearing rollers on pipe, then migrated to using linear slides from 80/20, but in the end decided that the time and effort and error-generating manual labor wouldn't be worth it.

I found 55" long linear rails on eBay, and bought four of them; 2 for the Y-axis and 2 for the X-axis.

They were heavy, greasy, and the worst of all, METRIC.

I have everything you can think of when it comes to drilling, tapping, bolting, etc. but not in the wonderful metric assortment. The distances between bolt holes on the rails were metric, the tapped holes in the blocks were metric, and to top it off I had just received that huge piece of 80/20 for the gantry, which was NOT metric.

Oh well, live and learn. Until then, modify!

(Side note -- I have now have a personal relationship with McMaster-Carr. All the hex-head screws, plate support screws and nuts, drive shafts, cable carriers, etc. came from them. If you can't find what you're looking for in their catalog, THEY DON'T MAKE IT.)

I drilled out the rail mounting holes to a little bigger than #10-32 screws to give me some wiggle room. Then I center punched the holes into the Y-axis frames and went about drilling and tapping them to #10-32. I put the rails on, tweaked here and there, and tightened them down.

Laser alignment notwithstanding, the rails are exactly 58" center-to-center according to my trusty Stanley tape measure. How exact? When I put the gantry together and slid it onto the Y-axis linear rails, it never bogged down or slowed while traveling from one end to the other.

I was pretty satisfied with the degree of precision I had so far obtained, considering I eyeballed just about everything I couldn't measure or level. Even my good level would make a millwright cry in shame.

Oh, before I bolted the rails down, I flipped the whole table upside down and drilled and tapped the holes that would hold the plate support rails to the underside of the Y-axis rails.

Now that the linear rails were in place and the gantry was performing as imagined, I started assembling the plate support railes and all the hardware that went along with them.

There's a picture with a closeup of the splice plate I made to hold the #2040 and the #1010 together on each end of the X-axis.

The last picture shows where I mounted the control enclosure on the side of the Y-axis. (Y1 axis, to be exact!) There are four buttons on the front of the control enclosure. Two are for turning all the power on and off. The other two are the e-stop button and the illuminated e-stop reset button.

[txcowdog]

Excellent build journal and lots of pics. It looks like you were making pretty good time there. Looking forward to seeing a finished product and maybe a set of plans so the rest of us can build one also.

[grahamcowan]

Before I even got started ordering parts for my plasma table, I made sure that the software and hardware were going to be compatible. After all, why slave over the mechanical part if the electrical part won't do a thing?

The motors that I ended up getting were known to work with my software/hardware combination (which will be explained in greater detail in a later post) and seemed to be pretty beefy. The X-axis drive was a NEMA 34 motor, as well as the dual Y-axis drives. The Z-axis drive was a NEMA 23, only because I really didn't need the power on th Z-axis like I probably would on the X- and Y-axis.

The three big motors are all rated right around 1,000oz-in of torque with a 1/2" shaft. The smaller motor I think is about 250oz-in with a 1/4" shaft.

The driveshafts I started with were the cheapest grade steel (ungraded, actually)I could find on McMaster-Carr, in 6' lengths, Acme threaded to 1/2"-10, Rockwell hardness of 60,000psi. They seemed great considering they were about $12 each, but once I actually put them in and started testing the motors the cheapness stuck out. Talk about floppy. I know that drive shafts have a tendency to bow out and flop at high speeds, but this was absurd. And to top it off, they were ever-so-slightly warped when I got them, which translated into massive problems at high speed.

I went back and searched some more and went ahead and spend the $40 each for Grade B7, 125,000psi Acme threaded rods, 6' long, except I went with 1/2"-8. Less turns for more travel = possibly lower top speed needed.

What a difference they made. They even looked better -- smooth silver finish, not looking like they were forged, and straight as an arrow.

The needle bearings are to support the driveshafts. You really can't have the bearings of the shaft of the stepper motors supporting the weight of the driveshafts -- they'll burn up pretty quick. So I took this into consideration when I designed my motor mount brackets. I found some needle bearings that would accept a 1/2" shaft and drilled a hole to keep them in place. Each driveshaft system and motor has a needle bearing support on each end; one at the motor and one at the far end. There is also a 1/2" shaft coupler at the motor to bolt onto the rod.

The trickiest part of building the motor mount brackets was making sure the needle bearings and motor shafts were aligned. I didn't have my driveshafts yet so I found a piece of stainless 1/2" tubing that actually had a pretty tight fit. I slid the tubing into the first needle bearing and then into the shaft coupler. I rotated the bearing plate against the aforementioned 2" angle and tacked it in place.

Now that I think about it, the motors are somewhat clunky looking mounted where they are sticking out to the side. On the other hand, if I had turned them around to face the other way and used a pulley system, they might have been streamlined better with the overall look of the table, but dealing with pulleys and belts was not something I felt like messing with.

Besides, direct drive has less parts to have to fix/tweak/modify later.

[grahamcowan]

I took some more pictures of the details I was describing in the previous post.

The first picture shows the difference in the old 1/2"-10 ungraded rods and the new 1/2"-8 Grade B7 rods. They are a world apart, that's for sure. Lesson learned? Don't skimp on parts if you can afford it. You'll end up paying for it in the long run.

The second picture shows what a typical motor mounting bracket, shaft couple, and needle bearing look like. This one is the X-axis drive system. Space is kind of tight, but I really didn't mind. My TIG torch will fit in just about anywhere I can squeeze it, so welding it up wasn't a problem. The bearing sleeves are also nice because you can take a can of white silicon lubricant and fill up the space between the bearing sleeve and the outer race and not have to worry about it dripping everywhere.

The third picture shows the Y1-axis drive system. That ugly metal tab on the top of the bearing sleeve plate shifts the Y-axis to the positive about 2". I found out later that if I had left it off, X0,Y0 would have put the torch right above my cross brace and eventually it would have to be replaced.

The fourth picture shows (in not very much detail I'm afraid) the drive system for the X-axis carriage with the Z-axis motor. I built the bracket you see for the drive nut but then later I realized I didn't have anywhere to anchor the cable carrier. A lot of custom work went into this table and I can safely say that I am no longer afraid of working with stainless.

[grahamcowan]

My favorite part, considering I'm an electrician.

Before I ordered any parts, I started researching how I would actually control the CNC part of the table. I knew that Windows-based software tends to be pretty expensive (for me, anything more than free is expensive) but it certainly does everything you want it to do, and easily.

I found an interesting piece of software called EMC that runs on Linux, and since I have tons of experience with Linux, this was the best way to go. I had a spare computer so I formatted and installed Ubuntu, then installed EMC. I also found a free 2D CAD program, QCad, and a free CAM program, GCam, and a bunch of other assorted program that I went ahead and installed just in case.

Did I mention they were all free?

Back to the hardware. I went to the EMC site and found a list of assorted hardware that was known to work with EMC, so that's what I picked. It was probably a bit more expensive that the cheap off-the-shelf combination of hardware, but I wanted something that I knew would give me the least problems.

I settled on a controller board and 80V power supply from PMDX, four G201 drives from Gecko Drive, and stepper motors from Keling Inc.

Since I love motor controls, I designed and built the control circuit that would take care of relays, power, limit switches, and the e-stop circuit. A little bit of on-the-fly rewiring and everything worked as planned.

Since I am posting lots of pictures now that may or may not follow along with the attached post, I am going to start giving a brief explanation of what each picture is.

The first picture shows the connector I mounted on the plasma cutter to remotely control the torch. All it consists of are two jumpers between the torch trigger contact and eventually back to a relay on the controller board.

The second picture shows my controller board mounted on the four Gecko drives, and all those are mounted on an aluminum heat sink rail.

The third picture is the 80V power supply from PMDX.

The fourth picture is the front of the controller. Kind of obvious what everything does here...

The fifth picture shows the internals of the control enclosure. the large gray cables across the top are motor cables, the obvious controller board and power supply, the power and e-stop relays, the terminal block and massive toroidal transformer. I added a connector for the parallel cable, and let me tell you, #28 wires suck! The three black cables in the bottom right are controller power (120V), torch remote control, and the switched receptacle for the duct fan.

The sixth picture is the motor disable switch and the manual torch override buttons. The motor disable just tells the controller board to shut the current off to the motors so they can be jogged by hand. The torch override is so I can turn the torch on and off by hand. One button is momentary, the other push-on-push-off.

The interesting thing about the torch override is the way Miller makes their plasma cutters. When you squeeze and hold the trigger, the torch and air turn on for cutting. When you release the trigger, the torch turns off but the air stays on, I assume for cool down or whatever. The air will not turn off until the trigger is jogged, so I added a button to manually jog the air. Eventuall I will get the software to do this.

[grahamcowan]

Of course you should have at least one limit switch per axis, unless you just set your X0,Y0 in the middle of your table, but then you'd really just not be getting the maximum out of your table, right? I've got two typical roller limit switches on the X- and Y-axis, and a tiny micro limit switch on the Z-axis. I set the limits on the X- and Y-axis to be both Minimum and Home, while the Z-axis is just Maximum. In EMC I can set the height of the Z-axis to wherever I want and then click the Z home button and it automatically sets it to zero.

Getting the limit switches mounted in the right place was a little tricky. I had to custom build the brackets and plates that would activate the rollers, but once they were set, they did fine. I also opted to put the X-axis limit actually on the gantry, that way all the limit cables come off the gantry in one bundle.

I found some really slick cable carriers online made by Nylatrac, but they are incredibly expensive and I was pushing the limits of my budget already. So after digging around a while online I found them on McMaster-Carr. Go figure. I bought two 3' lengths, with the Y-axis length being wider due to extra motor and limit cables. Nylatrac and the makers of my plasma torch cable (Thermadyne, incidentally, makes the torches for Miller machines) said the minimum bend radius was 8". Eight inches? I asked are you sure you don't mean diameter? Nope, radius. That meant my carrier would have to have a 16" diameter loop and that was way too big for anything I could have used. So after looking at pictures online of commercial rigs and many home-brew tables, I said to heck with it, I'll make it work, and ordered the biggest radius carrier I could get that would still fit on my table. And it ended up working perfectly.

The first picture is the X-axis limit switch. Note that bracket I had to weld in place to trip the roller.

The second picture is the Y-axis limit switch. Since that picture was taken I had to shift the Y-axis +2", so there is a little piece of steel to push it up.

The third picture shows an up close fo the Z-axis limit. I accidentally ran the sled up into it before I got it wired in and bent the little spring roller so I had to go back and rebend it and then watch more carefully to what I was doing. Live and learn.

The fourth picture is of the Y-axis cable carrier. It has two motor cables, three limit switch cables, and the plasma torch cable. I think they look pretty slick, and they certainly add to the "professional look" of the whole table.

The fifth picture is the X-axis cable carrier. I've seen a lot of people that get a cable carrier long enough to go all the way across the axis, but what for? If you can properly support the cable bundle while it heads for the other end, there's no need to spend the money for that length. So I didn't.

The sixth picture shows the completed Z-axis. I've got a removable plate that the torch clamp sits in (it's actually the clamp ring from the Miller circle and line cutting guide) and the whole thing can be unbolted and removed. If I wanted to make an adapter that held a Dremel or a wood router or even a pen, I could bolt it on right there.