Plasma Attempt #2

[Robin Hewitt]

Me again

First plasma attempt a dismal failure, next attempt is a pile of bits.

A new, slightly bizzarre design which seems to work on paper. I'm typing this because explaining the logic behind it will force me to check my maths...

What happens when you put 10kg bag of lead shot in the middle of a 3 meter aluminium box section with a DTI underneath?



Answer: It bends down approx 0.5mm. Not bad for something that only weighs 10kg itself !

"Hang on twit brains", I hear you cry, "the gantry is usually the short axis".

Possibly so, but going the wrong way round does have advantages when it comes to positioning the machine and loading metal on to it. Also Gary managed to send me a 2.7m length of HiWin rail unbent and it would seem ungrateful not to give it a go after that.

Here's the maths...

Let me aim for a tight plasma cutting tolerance, say 0.1 mm of gantry flex allowed. Using Hookes Law, if 10 kgf bends it 0.5mm, 2 kgf will bend it the allowable 0.1 mm. Simples.

The whole gantry weighs 20 kg but only about 6 kg of that contributes to bending because the stuff at the ends doesn't count.

Obviously 6 kgf will accelerates 6 kg at 1G, that's what happens if you drop it. So it follows that my 2 kgf will accelerate it at 1/3 G. Maths is easy if you choose the right units. Tom Caudle reckons you need 0.1 G accelerations for plasma so I am well inside the ball park.

To accelerate the whole 20 kg lump at 1/3 G will require about 20 * 1/3 = 7 kgf, call it 70 Newtons.

I like to cut in mm and a stepper motor does 200 steps per rev. To keep the units sweet 20 mm travel/motor rev suggests itself, but that is dismally slow given that steppers tend to clack out beyond 5 rps. I think I prefer 40 mm travel/motor rev. That gives me end to end on a 2.5m sheet in 12.5 seconds. Slow, but not dismally slow.

6.28 divided by my 0.04m/rev gives the motor a mechanical advantage of 157:1

So if I want 70 Newtons to accelerate it, I need a motor which can deliver 70/157 = 0.44 Nm. Double that for luck and a 0.8 Nm motor should do the trick. Seems pathetically small for such a huge table but you can't argue with Mr Newton.

First job is to drill the box section to mount the HiWin rail which is the threaded from below variety. The plan is to run M5x110mm bolts in from the back with spacers in the middle. It is most important that I can get at the bolt heads because I intend to bend the rail slightly up in the middle so the torch flies at a constant height.

Specially for Jazz I'm using unsupported round rail for the Y axis. The Z axis is truly weird, took me the best part of a week to figure it out

best

Robin