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Author Topic: Physical buttons for plasma  (Read 87802 times)

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Offline Davek0974

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Re: Physical buttons for plasma
« Reply #290 on: March 26, 2016, 04:00:20 PM »
If its more than about 300mm wide, drive both sides.

Slaved motors would be the way forwards, have Mach auto-square the gantry on ref.

It may feel firm but when Mach is doing its thing with a plasma job in thinner (<12mm) steel, the whole table will be shaking on tight turns etc, you will be surprised how violent things can get on thinner stuff and cutting intricate designs - I have some parts with grass cut on one edge, its basically a saw-tooth cut and even on 3mm sheet it will shake the hell out out my table - one motor will just anchor one end of the gantry and t'other will be pissing in the wind ;)
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline beefy

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Re: Physical buttons for plasma
« Reply #291 on: March 26, 2016, 05:03:15 PM »
Got to back up what Dave said.

Let's do some maths to illustrate the problem. Assuming a cut speed of 6000 mm/min. That is 100mm per second. If you could write some test code that made you gantry move 5mm then reverse and come 5mm back, and repeat, the gantry would be changing direction and moving 5mm 20 times per second.

If you can picture that you can get an idea of the rigidity needed if you plan to do any thin stuff. Actually, if possible you also want to keep the gantry light so the inertia of fast direction changes doesn't try and move the table. I look at some of the gantries for plasma and they look heavy. That's OK if you don't need fast cut speeds.

A look at Hypertherm cut charts will show the cut speeds can get higher than that on thin stuff. I'm actually gobsmacked that Hypertherm are not realising this issue and coming out with a cutter that allows us to keep the cut speed down on thin stuff. We need low amps and a tiny cute little nozzle with a very narrow kerf. The Finecut nozzles just don't cut it and the speeds can get right up there.

Offline Hood

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Re: Physical buttons for plasma
« Reply #292 on: March 26, 2016, 05:03:58 PM »
Hopefully the table will be pretty solid once finished but you are probably right about the gantry, especially as it is 3m wide.
I was just surprised how rigid it was with it bolted down.
I don't plan on having two motors but can if I find it needs it.
What I was planning was a rack and pinion at both sides and a tubular shaft running through the 4inch Alu gantry and belts from that to the pinions, the motor will drive one side of that shaft.
The rack/pinions are good quality helical ones so I hopefully will never need to square things.

Also bought a Trafimet machine torch for the plasma, bloody expensive, nearly dropped at the price :D

Hood

Offline Hood

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Re: Physical buttons for plasma
« Reply #293 on: March 26, 2016, 05:07:08 PM »
beefy, yes I think that will be the case.
My Gantry will be fairly light, it is 4inch box with 1/4 wall 6082, the profile rails will add a fair bit to it mind you as they  are 35mm wide rails and there will be two.
Time will tell I suppose.

Hood

Offline Davek0974

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Re: Physical buttons for plasma
« Reply #294 on: March 26, 2016, 05:44:20 PM »
3m of 35mm rail x 2 plus box section is going to weigh, a lot ;)

My table was designed to reach 9000mm/min as that is top speed for the PM45, so far I have got up to 5000 and that was scary !

With the tube connecting your side gears, watch out for torque causing a twist as the direction changes, with only one motor it will need to be big - servo's ??

If not, watch your top shaft speed as big steppers don't do well at speed or acceleration.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Hood

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Re: Physical buttons for plasma
« Reply #295 on: March 26, 2016, 05:58:43 PM »
The Alu will weigh about 18Kg, the rails probably a wee bit more each.
The frame however is fairly sturdy being made out of 3 and 4 inch steel box with 1/4 wall, so plenty of mass there and if need be I have a steel cabin housing it that it can be fixed to :)
It will be servo driven via a 10:1 gearbox so max cont torque available would be in the region of 20Nm and peak 70Nm peak.
I have some heavy wall tube that may do the job but if not a solid shaft will probably be employed although I prefer to keep the weight down if possible, will have to do some calcs and see but I do know the original machine that the racks etc came off had a 30mm dia solid shaft of about 4m.
 It will just be spinning at a few hundred RPM max .
Using two motors would add more weight to the gantry than a solid 30mm dia shaft would be anyway.

Hood

Offline beefy

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Re: Physical buttons for plasma
« Reply #296 on: March 26, 2016, 06:33:16 PM »
What I was planning was a rack and pinion at both sides and a tubular shaft running through the 4inch Alu gantry and belts from that to the pinions, the motor will drive one side of that shaft.

Hood

That's what I have right now but I want to change it for dual rack, dual motor when time / money permit.

One thing to consider is torsional twist, especially at higher cutting speeds with 180 degree direction changes. Seeing as you have a length of 3m you could do a torsion twist test. Get the torque specs of your motors (+ any gearing to factor in), and on the bench apply that torque to one end of the tube/bar while the other is held fixed. If that torque twists it, you will probably get that on your table during a cut, possibly leading to shaky oscillations and a scalloped edge cut.

Perhaps a thin wall high strength tube will be more rigid than a solid bar. I know aeroplanes and race car frames can be made from thin wall 4140.

I've messed up plenty with my first cheapy build so if I can help you avoid it, all the better.

Edit:
Oops, just realised Dave got there before me on the twisting matter. Great minds think alike Dave LOL.
« Last Edit: March 26, 2016, 06:38:38 PM by beefy »

Offline beefy

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Re: Physical buttons for plasma
« Reply #297 on: March 26, 2016, 06:42:37 PM »
Using two motors would add more weight to the gantry than a solid 30mm dia shaft would be anyway.

Hood

Yes, BUT with 2 motors you have twice the drive power / torque, which would much more than compensate for the extra weight. So with 2 motors your power to weight ratio would be much better.

Offline Hood

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Re: Physical buttons for plasma
« Reply #298 on: March 26, 2016, 06:51:54 PM »
What do you not like about the single motor dual rack way?

Regarding the motors, I have two similar torque motors with 10:1 planetary boxes so I can go that route if required but can't see the advantage at this time but as you have the setup I am planning on I may change my mind depending on your answer :D

BTW below is a pic of the original machines gantry, just wish I had a place big enough as I could have used the whole machine and saved a lot of building :D
To give an idea of scale, the energy chain in the pic  is about 200mm wide.

Hood
Re: Physical buttons for plasma
« Reply #299 on: March 26, 2016, 11:09:40 PM »
Both sides (don't forget you want high acceleration too)

Rob
Rob

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