CNC Plasma Cutting Table

The situation

It’s 2016 and my small home business has been growing steadily for some time. The most used machine I have is the small CNC plasma table I built from structural aluminium offcuts and other bits and bobs. It had a cut area of just 600mm x 600mm but was easily the most useful machine in the shop.

Plasma cutters cut metal by creating a stream of ignited plasma by passing compressed air over an electrical arc inside a special cutting torch – this stream of plasma is hot enough to easily cut steel, aluminium, stainless etc and also fine enough to cut it with great detail. The little machine had a 30 amp plasma generator on it and would easily cut 6mm steel plate, albeit at a fairly slow pace.

The time had come when I needed a much bigger machine – this would enable bigger sheets to be used which greatly reduces wastage and therefore costs of raw material which means higher profit margins. The larger size also meant bigger products could be made.

Much research was done and I had a big enough electricity supply and compressor to run a 45 amp plasma torch/system – this would allow me to cut 12mm thick steel. The software was to be “Mach 3” again as I was used to working with that from other CNC builds. Drive would be via 4Nm stepper motors and rack & pinion motion. I could have used ball-screws for the motion but the cost is far higher and they are not really needed here.

Motion would run on linear rails and blocks for smooth action, the design would be a “Gantry” build with the gantry running front to rear as the Y axis and X axis running left to right along the gantry.

The Build

The machine was to have a cut area of around 1300mm x 1300mm, this would allow me to use 1/3 sheet steel stock. The extra size needed to allow for the mechanics of motion meant the finished build would be nearer 1800mm x 1800mm. Thinking of selling it in the future, this size was way to big to go through the workshop doors so a self-assembly build was called for.

2mm Steel box-section was chosen, this meant I could weld it easy with my MIG setup. Much steel was ordered and the fun began…

As can be seen from the pictures above, I welded on threaded blocks to allow the thin tube to take bolts, and a recessed threaded plate where a butt-joint was called for.

Fabrication

The frame was to consist of two U-shaped end frames, spaced apart by bolted-in beams to create a square cube. I added retractable castors to the feet so it could be moved when assembled but dropped on to fixed feet for stability.

The frames were fabricated on the welding bench and a trestle as my bench was nowhere near large enough. Care was taken to get the frame level and square before tack-welding the corners. Once all the corners were tacked, beads were run where needed.

I was happily surprised to see the two frames come out flat and same-sized, no signs of warp or twist.

The top beams on the side frames are bolted on, not welded.

The left & right top beams carry the linear rails for the gantry motion. These rails need some accuracy when fitting so steel strip was screwed along the beam and then the rails butted against this strip as shown below.

As the steel box section was too thin to carry threads for screws, steel backing strips were drilled and tapped then placed inside the box sections for the screws to fix into.

The gantry

The gantry brings its own set of unique problems, it needs to be light, very rigid and firmly connected to the motion system.

Simply welding a beam to the side plates would result in serious wobble from side to side – the wobble is induced when the software changes the movement of the cutting torch and Z axis – this can happen extremely fast and in rapid succession on detailed cuts.

The answer came in FEA – Finite Element Analysis software. With this you can input a design, set motion points, tell it the expected acceleration and deceleration figures, mass etc and it will apply those inputs to the design then show where the weak points are, stress points etc. From that you can redesign or change features to prevent undesired effects like wobble or bend.

My design is shown below, it’s made from steel, welded and has holes cut where possible to reduce moving mass on the gantry axis.

The Z axis

The Z axis is a complex part, it runs on a linear rail mounted on the gantry beam. It holds the X axis drive motor (shown below) plus the Z axis, Z drive motor, torch holder on a floating mount and various switches.

The Z axis plate holds the torch mount – this runs on a short linear rail as the torch needs to be allowed to move up as the Z axis moves down – this action is used to detect the surface height of the metal when the cut starts, its called a floating Z axis. As the Z axis moves down, the torch touches the metal sheet and stops moving, further downward travel force the torch holder to trigger a switch and the movement stops – this tells the software that metal has been detected.

Limit switches

As with all my CNC builds, I like to fit limit switches to all the axes, on the plasma table the Y axis has two, one on each side. These are used when homing the machine to force the gantry into a known square position left to right.

The cutting bed

A plasma table needs a bed to cut on – these can be slats, spikes, waves etc. Whatever is used, it is sacrificial and gets cut over and over until its worn away.

I opted for curved slats. A frame was welded up from 50mm angle steel and a smaller strip welded under it as shown below. The slat frame is not fixed to the machine frame, it just sits in it.

Paint job

The parts were taken outside and treated to some primer and a nice blue topcoat.

Fume extraction

The plasma cutting process has one major drawback – fumes. The process creates a lot of filthy muck, this can be dealt with by using whats known as a water table – this is a tank of water that sits just below the cut and catches the muck as the flame blasts into it. Water tables however need maintenance, they also mean having a lot of water and humidity in the workshop, not a problem for a big place but not good in a small shop.

I went for outside extraction as the area the shop was in was away from other gardens etc. A large steel hood was fabricated for me by a ducting workshop, it had a bottom tank with inspection door and a connection for 300mm ducting.

A 300mm extractor fan was fitted to an aluminium panel that was riveted to a steel frame. This panel was used to replace one of the concrete slabs that the workshop walls were made from, it vented at the rear into a wooded area.

Wiring up

Wiring to the moving parts was held in caterpillar chain and fixed wiring held in plastic trunking tucked away under the frame.

Finishing up and testing

A mesh sheet was added just below the cutting bed to catch parts that fall away, saves having to dig for them in the extract pit.

Videos

A full cut, apologies for early video issue – I dropped the phone 🙂 This is 2mm steel sheet. The video was shot after about a year of use on the machine.

Summary

The plasma cutter was my favourite machine, with one of these you can make just about anything in no time at all. It was the most used machine in the shop, turning out hanging basket brackets, fire pits, flower planters, house numbers and company logos, name plaques and more.

This machine basically built the company, once business grew into it, the plasma cutter was in use pretty much every single day. I was getting steel delivered two or three times a month just to feed it. If I started the business again, this would be the first machine I bought or made, the only better ways to cut shapes in metal are a water-jet or laser cutter, both of these processes are still way out of price range for a small shop but the results are far better than plasma, my personal choice would have been a water-jet cutter as there are no fumes at all and it can be very detailed.

Sample work