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Author Topic: My first CNC (plasma) machine  (Read 13452 times)

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Offline BR549

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Re: My first CNC (plasma) machine
« Reply #10 on: March 08, 2015, 02:35:27 PM »
Several things come to mind IF you are building a plasma table.

1. You have severely overbuilt the gantry making it very heavy with lots of inertia weight. Plasma gantryies need to be as LIGHT/Stiff as possible as themachine needs to move at high velocity AND accelleration values .

2. I do not see any gear reduction on the table axis.  To get the high acceleration values you NEED torque to get the gantry moving AND to stop the gantry without loss of steps.

3. The PromaTHC is NOT the best of solutions for a THC. You would want a system that CONTROLS the Z axis independantly from MACH3. Let Mach drive the XY and the THC drive the Z. Letting Mach3 control the THC is always a compromise at best.

Just a thought, (;-) TP
Re: My first CNC (plasma) machine
« Reply #11 on: March 08, 2015, 05:40:00 PM »
Hey, thanks for the input. And, those were exactly my worries as well. I know that gantry is heavy, and I am planning on redesigning it. I was thinking of maybe taking the back square tubes off and putting in a brace or two somehow. I also am toying with designs to cut out of the gantry side plates for weight reduction.i was also thinking about just cutting the square tubing in half and using that instead of the C channel. The tubing wall is only about 1/16" thick. The channel is much heavier.  Actually I think thats what ill do, those couple things should make a huge difference. As for gear reduction, I have been worrying about that, but I hope with some redesigning I can shed enough weight to get away without it. That would be a pita. The proma thc is my mistake. I did research before I bought it. But, after buying it, thats when I started coming across all the people warning about it. There are success stories so Im hoping I can get it to work. If not then I have been looking at some options at a company called CANDCNC. Have you or anyone have any experience with them?

I guess it might be helpful if I let all of you know that i dont plan on cutting thin materials often. And when I do the thinnest I will ever need to cut will be 14 guage.
« Last Edit: March 08, 2015, 05:48:46 PM by xtjoliverx »
Re: My first CNC (plasma) machine
« Reply #12 on: March 11, 2015, 11:53:59 AM »
Ok, with the advice of BR549 and Chaoticone I fiund some areas that need addressed. First, is the noise. I have added some further protwction against that and will be implementing that when I get to that point in the build. I have to admit, I knew it was an issue, but chaoticone made it clear that its a bigger issue then I originally thought. He also stressed and restressed the importance of proper planning and research. I have been designing and researching for about two years before I started actually building. I am far from being a knowledgable source on the topic, but I think in that two years Ive learned enough to get my feet wet, and continue my learning experience hands on. BR549 opened my eyes to the weight of the gantry, which made me rethink the design on that. So far, I have taken( in design) the weight from about 200 lbs down to 120ish lbs. I am in the process of alternate gantry plates from thinner material. Those 3/8" plates weigh about 28lbs each. So that is definately another area I could shed some weight. As soon as I get time that is my next step in the build. Also pointed out was gear reduction, and have researched and calculated a 3:1 reduction would suit my needs. I have already started looking around for the 3" DP 20 Pa 20 pitch gears I will need. Does anyone know of a good source for these gears? All I can seem to find are very expensive prices for a simple little spur gear. And the THC, was also mentioned as a problem area. I am hoping it will suit my needs, I will be cutting thicker plate most of the time. According to Hypertherms tables the fastest I will ever need to cut at is 150ipm. Im hoping that proma can keep up. If not, then I will cross that bridge when I get there.
Re: My first CNC (plasma) machine
« Reply #13 on: March 11, 2015, 01:24:28 PM »
Sorry, I forgot to add at the end of the last post, asking if anyone has any other suggestions. I should have been asking this from the start. But, it will still be easier to address changes now, rather then wait until the end.

Offline stirling

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Re: My first CNC (plasma) machine
« Reply #14 on: March 11, 2015, 02:07:08 PM »
Gear reduction only increases torque if you're prepared/able to accept the proportional loss of speed. Even then, with steppers, I'd argue you get better acceleration with direct drive. All the (stepper) plasma tables I've built have been direct drive - never had an issue and run 15m/min at 0.5+G no probs.

What I WOULD do is consider nema23's rather than 34's - you'll likely get better performance.
Re: My first CNC (plasma) machine
« Reply #15 on: March 11, 2015, 02:32:59 PM »
Could you elaborate a little please? How would nema 23s give me better performance? And thanks alot for your input on the direct drive. What size gearing were you using? My gears are 20 pitch 20 pa and 1" pd.

Offline TPS

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Re: My first CNC (plasma) machine
« Reply #16 on: March 11, 2015, 04:48:20 PM »
Hi,

i put the "iknoweverythingbettermodusON"
 
How would nema 23s give me better performance? And thanks alot for your input on the direct drive. What size gearing were you using? My gears are

if you whant speed an torque there is now other way than to use servos.

ok "iknoweverythingbettermodusOFF"

Thomas
anything is possible, just try it.
if you find some mistakes, in my bad bavarian english,they are yours.
Re: My first CNC (plasma) machine
« Reply #17 on: March 11, 2015, 05:59:50 PM »
I did not use servos because of cost. I would rather have torque than speed. Like I mentioned in a previous post, the fastest I will ever have to cut is about 150 (ish)ipm. This machine is not going to be a full out production machine so I dont need the fastest rapids etc. But, when I am cutting and change direction at 150ipm I dont want to stall or lose steps. That is why i am totally starting from square one on the gantry. The motors are not in yet. But, I have them hooked up at home to get everything working. When I am jogging it with the pendant and change direction very quickly I can hear the motor losing steps. I dont think i will know until I get them mounted on the machine and try it with weight on them. I might even have the speeds and acceleration set wrong. My plans are to (re)build the gantry as light as possible. And hook everything up and see what happens. Does anyone have any suggestions other than what I just mentioned? Is there a way to measure or test tis before hand?

Offline stirling

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Re: My first CNC (plasma) machine
« Reply #18 on: March 12, 2015, 06:43:04 AM »
Torque is a means to an end. What you NEED is the torque that gives you the speed to match your cut charts and the torque that gives you the acceleration to maintain that speed as much as is possible.

23's vs 34's: Have a search around there's a lot been said on this but briefly - bigger is not always better.

Gearing: I used direct 15 tooth mod1.

Ian
Re: My first CNC (plasma) machine
« Reply #19 on: March 13, 2015, 09:21:22 PM »
So, I started the gantry weight reduction phase. Unfortunately I forgot to take some pics before I left. I'm going to be working on it some more on Sunday. Ill take some pics then. I should have more to show by then. I eliminated the two steel channels that the rails were mounted to. I drilled and bolted them to the square tubes that were that the rear part of the original design. I found some 3/16 plate I might be able to use for the gantry plates, instead of the current 3/8" plates as well. I think that might be enough. Any input on this from anyone? Think that will be light enough for two 1232 oz. In. Steppers to move around?  Stirling, could you nudge me in the right direction with the 23 vs 34 topic? Ive searched but cant seem to find anything on this forum comparing the two.