Building a CNC plasma table

[MikeGyver]

Hey guys, first post, I'm hoping I can get some advice on this forum.

I've started building a plasma table from scratch; this is my very first CNC adventure and I'm looking forward to it since I'm into electronics design and welding/fabrication. The table can accomidate a 4ft wide steel plate, and the X-axis travel will be about 3.5 ft (feed the 4x8 ft plate/sheet through to keep cutting out more parts).



My plan so far is to use a direct drive rack and pinion gear setup and the 305oz steppers from hobbycnc.com ( Stepper Motors | HobbyCNC ) This might be massive overkill since they should provide somewhere around 30lbs of "push". I'm also planning on using their 4 channel "Pro" controller ( HobbyCNC PRO Chopper Driver Board Kit | HobbyCNC ).
There will be 2 X-axis motor's, one on each end of the bridge, one Y axis motor, and exentually a Z-axis.

I'm using these V-groove bearings from ebay ( 3/8" V Groove Guide Sealed Ball Bearing RM22RS inch - eBay (item 360274593197 end time Feb-16-11 10:29:14 PST) ) and my tracks will be lengths of 3/4" 316 stainless angle iron 1/8" thick. 316 stainless is quite hard (Rockwell B91), and springy (60.2kpsi yield), but here is how the bearings sit on the angle. Do you guys think this will cause problems?



The rack gear will be on the bottom of the beam that holds the angle iron track, and the motor will pivot and use a spring to maintain backlash and keep the V-groove bearings tightly against the track.
I did a search and it seems everyones going with 20degree 20tooth/inch 1/2"x1/2" racks, is this what I'm goin to want?

Also does anybody have any info for choosing a pinion size. I'm making the X-axis bridge mostly out of 2"x3" aluminum tubing to keep inertia minimal.

Another thing I'm unsure about is when cutting slowly (like say 10ipm), the individual steps maybe be too large for a smooth cut unless I use gear reduction on the steppers... any thoughts on this?

If anybody has any ideas or recomendations I'm all ears.

[EBC]

I see you also posted on mechmate.

Before you go too much further I suggest you read a few thousand posts there (mechmate) and see why you should grind your rails and learn what else you will do wrong before you waste a lot of hard earned money.

Congrats on your build

[millman52]

I agree on the reading. There are many builds here & other sites to learn from.

Since the rollers are 90 deg. why don't you run them on the 90 deg. corner of the angle?

I don't feel your motors are overkill at all. Especially direct drive. You may find it a bit hard to achieve good acceleration especially when your gantry total weight is going to bump 100# or more once it's loaded with the stepper motors, cables, hoses, Z axis lifter, floating head, plasma torch, etc. All this stuff adds up pretty quickly to a good bit of weight that you have to get up to speed, stop & change direction almost instantly.

I wouldn't worry near as much about the 10 IPM as the 100 IPM & better if you are planning on cutting thin gauge materials.

Pinion size is recomended to not go below 1" because of wear. Of course for direct drive the smaller the better. I used 3/4" dia on a 4:1 reduction (My gantry goes about 200# all total) & haven't had issues in 2 years of service. But then again I cut with ox/fuel & 30 IPM is getting about as rapid as I cut

Just my 2 cents....

[MikeGyver]

Thank you for your input, I appreciate it. Do you have a picture of your machine?
I was estimating ~50lbs of mobile weight, the main span will be about 6lbs of aluminum, side steel plates, motors/cables, Y-axis, Z-axis, plasma torch (probably handheld style for now).

I looked around on mechmate for a couple hours. I think I'm going to stick with the 1/8" 316 stainless angle for tracks. I can't justify the extra time and cost in grinding angle iron tracks to get a 90° corner (mechmate style). My bearings will have a smaller contact patch, but the rail material is like 3 times harder and the gantry is going to be WAY lighter, also the machine never contacts the work like a router does. If it doesn't work out for whatever reason, the design is modular enough that I could simply swap in a ground angle iron rail.

[MikeGyver]

I also have a question about electronic torch height control. I understand the arc voltage is directly proportional to it's length, so the by monitoring the voltage the plasma table can automagically maintain the desired arc length even if the work warps up while it's cutting.

Is this a feature that can be implemented into any plasma cutter? Is a signal simply being sent to one of the printer port's inputs that tells the program what the arc voltage is?
Any input or links on this would be great.

[tubesilly]

my question is the same as millma52 if you are not going to grind the rails why not run on the corner of the angle instead of the edge? its 90deg or as close as it gets compared to the rounded edge

[MikeGyver]

That was actually the original plan but it would be more difficult to mechanically anchor the angle down. If I aim the corner up it also won't integrate quite as nicely, but I'm still throwing the idea around. I need to figure out a way to mount the angle without any welding.
I've revised my idea since my first post; it will be similar to the mechmates where the angle overhangs the table's support beam (corner-up or leg-up)and the rack gear mounts directly underneath the angle.
Keep the suggestions coming.

[EBC]

Original ShopDroid


have you seen this?

[jimcolt]

Torch height control is in fact a method of using arc voltage as a feedback to control arc length. Sounds rather simple, however accurate control is dependant on machine speed...in other words when your machine decelerates into a corner or on fine features, the arc voltage will increase (kerf width gets wider as cut speed drops as the plasma is still burning at a fixed kW output), and the THC electronics will move the torch closer to the plate to maintain the voltage, either affecting cut quality in that area or colliding with the plate and causing damage. Torch height control systems also need a method of finding the surface of the plate before every cut starts...as pierce height requirements for the plasma torch are different than cut height. Further, at the end of the cut there is normally a kerf crossing...the height control needs to recognize the kerf crossing so it does not dive at this point also.

Arc voltage height control is a simple concept that requires a lot of features to make it reliable and foolproof. There are some industrial plasma height control systems that sell for $15k, and there are some "low cost" plasma height controls that sell in the $2500 range. Cheaper plasma height controls are available that interface doirectly with Mach3 software, and are integrated into the cnc machine. The bottom line...the THC has to interface closely with both the cnc and the plasma to do its job.

Jim Colt Hypertherm

Originally Posted by MikeGyver I also have a question about electronic torch height control. I understand the arc voltage is directly proportional to it's length, so the by monitoring the voltage the plasma table can automagically maintain the desired arc length even if the work warps up while it's cutting. Is this a feature that can be implemented into any plasma cutter? Is a signal simply being sent to one of the printer port's inputs that tells the program what the arc voltage is? Any input or links on this would be great.

[MikeGyver]

Thanks for the reply, Jim.
Is there an inexpensive THC that you would recommend?

Also I suppose you're the guy to ask... I've been looking at getting the hypertherm powermax 45. My requirements are to be able to pierce and cut 1/2" mild steel (stainless would be nice too but I could just edge start that and not pierce).

My questions are should I get the machine torch or the handheld torch? The handheld torch would be nice for when I'll need to cut something by hand, but it looks like the machine torch pierces better?

Also, where is the best place to get one? They're about $1550 on ebay with a hand torch, and $1950 with the machine torch.

[jimcolt]

Hypertherms lowest price THC is about $5k...it is designed for 3 shift a day industrial use, I think Dynatorch has a standalone THC and Torchmate also has one that are about 1/2 that price.

The 45 will pierce 1/2" material very well as long as there is a height control that sets pierce height before each cut. If no height control, it pierces 3/8" well, or can edge start on thicker material. I have done a fair amount of 5/8" and 3/4" with my 45 with excellent results. On stainless the same thickness ratings should work fine.

The machine torch is more expensive, however it is the way to go for machine cutting. Easier to square the torch to the plate, easier to change consumables. Mine is mounted on a magnetic breakaway mount. I know it is more costly, but I recomend having both hand and machine torches...that way the machine torch stays on the machine and you don't have to fiddle with it every time getting it square to the plate. The hand torch plugs in with no tools in seconds....It is great for cutting up the scrap skeleton, and for other hand held uses.

The online prices are good....as the hand torch 45 system list prices at over $2100.00. Just be sure the distributor you buy it from is willing to service the unit if you have issues. I find that often the local distributor will come real close to the same price, and there is usually good value in dealing locally....they will stock consumables, maybe throw in an accessory or two...

You can find local distributors on the Hypertherm | Plasma Cutters | Plasma Cutting Equipment | Metal Cutting Equipment site, top of the page, click on "where to buy".

Jim

Originally Posted by MikeGyver Thanks for the reply, Jim. Is there an inexpensive THC that you would recommend? Also I suppose you're the guy to ask... I've been looking at getting the hypertherm powermax 45. My requirements are to be able to pierce and cut 1/2" mild steel (stainless would be nice too but I could just edge start that and not pierce). My questions are should I get the machine torch or the handheld torch? The handheld torch would be nice for when I'll need to cut something by hand, but it looks like the machine torch pierces better? Also, where is the best place to get one? They're about $1550 on ebay with a hand torch, and $1950 with the machine torch.

[C.Michael]

I use the same v bearings and use guides made from 1/8th plate..Works just fine!..I use the Campbell THC 300a for 3 years making decorative steel..
Also I use 1/2 acme thread stainless with a delron nut from Mcmaster Car..The V bearings are very hard and the rails are soft,so it just machines it's self to fit the bearings..The whip concerns of using a screw is not much because for some reason,the speed I use for cutting 16 gauge steel is only 90 inches a minute and it cuts like a laser..Those v bearings must be mounted using a slot instead of a hole so that you can adjust the bearings to fit tight to the rail..then if and when you do get a bit of wear, you can adjust it out..Michael