Plasma Attempt #2

[JAZZCNC]

Specially for Jazz I'm using unsupported round rail for the Y axis.

Nah for a skinny plasma with 30mm rails I'll let you have this one.!! . . . . . But all this Math or Arithmatic shit is making my crust hurt so just stick a bloody 3Nm motor on plug into national grid and watch the thing go like a rocket.!! . . . Don't matter all corners will all be circles and the thing will rip it's self apart trying to speed up/slow down and change direction unless you have a nice S-curve motion planner..

[Robin Hewitt]

But all this Math or Arithmatic shit is making my crust hurt so just stick a bloody 3Nm motor on plug into national grid and watch the thing go like a rocket.!!

A common mistake for those who can't be arsed to do the sums. Like 200 lb plasma gantries for people who can't be arsed to test something lightweight. Like stepper motors to do torch height control because they can't be arsed to figure out how to set pierce and cut height without one. I am amazed how people weld up their plasma cutter frames with no thought of checking if they run true, my experience of welding says...
Weld oversize then cut it back to square because sure as eggs, it won't be

[Robin Hewitt]

Progress...

This is the assembly that connects one end of the 4" box section X axis gantry to the 30mm linear rail Y axis. There is a 2mm deep cut out on the front that is a good fit on the box section.

There is a plate welded half an inch up inside the box section with a big iron nut in the middle so I can pull the box section firmly into the groove on this plate and hold everything nice and square with one M8 bolt.

This assembly drives the 15mm tooth belt for the X. To be clever I put the top of the belt inside the box section and the bottom about 1.5mm below it.

Have I made a plasma dust conveyor belt that will pump crut in to the box section?

[Robin Hewitt]

Bolting the HiWin rail from behind on to box section meant running spacers inside. I used 3/4" aluminium bar

I suggest we make a tool to place them, my son thinks he can push them into position using the file with a 1.5m handle we used to deburr the hole internals. The 23 bolts only took him a couple of hours, I cut the spacers for a good fit, after about an hour of hammering and cussing I remembered I had some copper-slip

Thought I'd get a pic before he welds in the end plates and the spacers are never seen again.

The rail mounting holes are drilled oversize so I can put the rail level even if the box section sags a bit.
I have a cunning plan to mill the gantry ends dead square

[luke11cnc]

Rob

Do you have a sketch/drawing of your cnc plasma cutter?

glad all the maths is out the way

James

[Robin Hewitt]

Do you have a sketch/drawing of your cnc plasma cutter?

Well, yes, sort of, my drawings are rarely illuminating though

Here's the bit I made today.

I have to keep the cutter holder light because it's a dead weight that needs accelerating. I got it down to one piece of aluminium channel, the Hiwin bearing is the X axis, the 8mm linear bushings are the Z. The sticky out arm flips a roller microswitch so I have time to brake before the head slams into it's home stop. A sort of early warning device.

A little Maxon rare earth motor with an epicyclic box raises and lowers the head (God bless ebay) with optical stops to set the cut and pierce heights.

Bill, I think the key to long gantry plasma is to make everything adjustable so you can pull it straight and level when it's all together. I let you know if that is a good idea or a crap idea later

[BillTodd]

I'm watching this with great interest Robin. :)

I had similar thoughts about long axis gantry design for a machine for my nephew Lee.

Bill

[BillTodd]

A little Maxon rare earth motor with an epicyclic box raises and lowers the head (God bless ebay) with optical stops to set the cut and pierce heights.

Are you assuming the plate you're cutting will be flat enough ? - I thought the whole point of steppers on the Z was to track any warp in the material ???

[Robin Hewitt]

Are you assuming the plate you're cutting will be flat enough ? - I thought the whole point of steppers on the Z was to track any warp in the material ???

I'm not so sure about that, I plan on torch height control but I'm hoping I won't need it.

I suspect THC is for people who weld up their plasma table frame, realise it simply isn't square, then go searching for a solution. I've watched lots of movies of plasma cutters in operation and have yet to see the plate lift.

I think people use steppers for the Z so they can set pierce and initial cut height, THC merely requires up and down control which a DC motor does admirably. I set cut and pierce heights using a mechanical memory for plate thickness, it's one of my favorite bits, more on that later.

I also want to separate the torch positioning assembly from the plate support and water tray. I want to be able to lift the ways and gantry off the table without removing a single bolt. I plan a 4 point contact, one locator at each end of the two 30mm rails that form the Y axis. The location points are all adjustable in X, Y and Z so I can set everything square.

While I'm rambling about machine philosophy, I think a plasma table frame is there to support the slats which the metal rests on. If you lose sight of that you may be in trouble.

[Robin Hewitt]

It would seem there is a generous tolerance on 1/4" extruded aluminium

I was expecting 6.35mm but I got less. On the drawing the clearance between my 8mm shaft Z axis and 15mm rail X axis dropped to 35 microns.

I put it together somewhat gingerly, but it runs fine and dandy.

To see if it really was 35um I rolled it over a piece of paper (80um) and it came out curly.
Next I need a 3 metre darning needle to thread the T5 belt through the gantry