# Thread: Plasma Attempt #2

1. Originally Posted by Jonathan
use the formula torque=angular acceleration * moment of inertia, where acceleration is in rad/s^2.

In post #4 I calculated the moment of inertia of your 10g (well not quite, see below) tube using the formula, courtesy of Wikipedia:
Attachment 6018
I'm sorry to do this to you, good try and all that, know you only want to help, but you can't bamboozle me with your arithmetic and Wiki-whatsit

force = mass * acceleration

If you want torque you add a distance component

Torque = mass * acceleration * radius

Of course the acceleration here is peripheral acceleration, not radians/s/s

If you insist on radians/s/s and moments of inertia... One turn is 2 pi radians and moves you by 2 pi radius so they simply cancel out after you have gone all the way round the houses.

The 1/8" wall tube weighs 2.35 kg, the peripheral acceleration is the same 2.94 m/s/s as the gantry, the worst case radius is 0.019 m

2.35 * 2.94 * 0.019 = 0.131 Nm

But geared 3:1 down the motor only needs to find an extra 0.044 Nm

2. Originally Posted by JAZZCNC
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

3. Fair enough, that's a nice intuitive way to work it out for a thin-wall tube since you can assume the mass is concentrated at the radius. This clearly is an approximation, since the tube does have thickness, so to work it out accurately you have to consider the integral of mass*radius^2 and you end up with the formula I quoted after touching upon real maths! In this context the approximation is of course valid (error is <10%), however this method wont get you very far if the tube was substituted for a simple shaft.

Originally Posted by Robin Hewitt
The 1/8" wall tube weighs 2.35 kg

Now your numbers make sense, it must be 2700kg/m^3 and therefore aluminium tube...I thought it was steel! Hmm to be fair you did say it was aluminium originally. Whoops...

4. 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?

5. 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

6. Rob

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

glad all the maths is out the way

James

7. 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

8. Originally Posted by luke11cnc
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

9. 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 ???

10. Originally Posted by BillTodd
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.

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