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plasma table idea


robertspark

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Thanks Charles. We'll see when it arrives!

I went on the box section I have in the rack which looked pretty good. It varied from 50.05 to 50.20 and there is no noticable twist (by eye) over the 2m or so I have.

I've found the thin walled sections seem to be better - and it has much sharper corners with about 1.5mm external rad.

Si

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si,

looking at your materials list, missing is a power supply + 4 core cable suggest something like 0.75mm² (steppers are 8 core 22AWG [0.33mm²], but you'll parallel up the 8 cores to form four as an 8lead -> bipolar parrallel http://www.probotix.com/stepper_motors/unipolar_bipolar/)

I went for a 600W 42V 14.2A, don't be tempted to go any closer to 50v (absolute maximum voltage) as those TB6600's go pop (I thought about a 48v power supply which is the next voltage up) spec sheet here, 42v listed as the maximum voltage

datasheet here: http://www.toshiba.com/taec/components2/Datasheet_Sync/201311/DST_TB6600HG-TDE_EN_31288.pdf

We've actually got the same stepper motors (you'll notice the weight when they arrive, they use to be silver & black they are now all black.

have you got enough steppers? (thought you were driving each parallel rail from a stepper?) You could do it from one, but you'll need a 8/8 coupler to take the drive off the rear of the stepper,

you may also need some bearings for the silver steel rod, unless the the pulleys are just going to float on the rod

you may need two more pulleys (looking at your last picture, you've got 4 on the x and 2 on the y axis)

my steel i got from the local metal stockholder out of the offcuts pile I could then inspect it and it was beer tokens

Rob

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Rob - you are absolutely right. The BOM says 6 pulleys, don't know why I ordered 4! Wishful thinking!

I'd not included a power supply - as I have a selection. Obviously to build one you would need one though. I'm going to use a 24v supply as the drives don't need any more than that and PSU's are cheaper.

I'm only using one stepper per axis. Synchronising steppers to run in parallel is problematic. There is a length of silver steel 8mm bar which connects the two pulleys at one end of the table.

For bearings, I'm using these: IGUS MCM-08-03 which have proven very good in similar applications. They are pretty cheap in quantities of 10+

For cable - I figured most people have cable. On my mill, I used 7 core trailer cable as it's cheap and robust. It uses servos which require 6 wires (2 for motor + 4 for encoder) plus earth. On a machine like this, you could use 4 for the motor plus another two for an end stop switch / home switch?

Si

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Almost as an aside, I see earlier in the thread that a 'PC with parallel port' is required. What basic operating system is required on this PC? (MS-DoS, Windows (variant?), Linux (variant?) etc).

Thanks (and sorry if the answer is already in the thread, just hidden '3' pages back!!).

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Si -> those bearing are a v good find, thanks I'll be using some of those in the future!

David -> software, I'll say you can use Windows or Linux, never got my head around Linux despite trying several times, but there are free software offerings on Linux.

On windows, any PC running windows 98 onwards will be fine with a printer port on it (i.e. a parallel port). Generally, a wifi enabled PC is not recommended on the older operating systems (win 98) because they use to say that the speed of the PC was affected by polling of the wifi card and it could give your drive issues. In short you need a simple as can be, cheap as possible PC nothing fancy at all with a monitor, keyboard, mouse, complete with a printer port, enough ram to run the operating system, small hard drive, and a means to upload the g-code from your "working PC" (i.e. the one that you design on and outputs the g-code (yes you can do both on the same PC, but I prefer not to design in the garage).

On the cnc PC, you need windows and a cnc controlling software, by far the most common for the home DIY'r seems to be Mach3 http://www.machsupport.com/, demo versions (free) are limited to 500 lines of code, without THC (torch height control) for plasma applications. (Si has not included THC on his build list above, but it's not essential, and many run without a THC).

Mach 3 basically controls all of the drives by reading the g-code (or you manually typing in the codes, or by jogging (moving) the axis as you require manually)

On the "working PC" you'll need something to draw your object to be cut and then covert this drawing into g-code (a post processor), a common application seems to be sheetcam http://www.sheetcam.com/ (demo version limited to 180 lines of code).

There are however many many software packages you can choose from.

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Thankyou all for the PC information.

The thinking behind my question was that any PC I have with a parallel port struggles to run XP.

It wouldn't be sensible to trust an application like this to such a PC.

Your answers make it obvious that my experience in this regard is more limited than the other posters in this thread.

Thanks again.

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You can buy a parallel port pci card that you should be able to plug into a modern PC for a few quid, however just ask around a few friends and you'll probably find someone willing to give you an old PC to run Mach 3 on ...... it really isn't processor hungry at all.

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From http://www.machsupport.com/software/mach3/

Mach3 Minimum Recommended Requirements:

Requirements if using the parallel port for machine control:

  • Desktop PC (Laptops are not supported) with at least one parallel port
  • 32-bit version of Windows 2000, Windows XP, Windows Vista, or Windows 7 (64-bit versions will not work)
  • 1Ghz CPU
  • 512MB RAM
  • Non-integrated Video Card with 32MB RAM(Large G-code files, especially 3D files will require a video card with 512MB RAM or higher)

Requirements if using an external motion device for machine control:

  • An appropriate external motion controller (options can be found on the Plugins page)
  • Desktop or Laptop with Windows 2000, Windows XP, Windows Vista, Windows 7, or Windows 8
  • 1Ghz CPU
  • 512MB RAM
  • Video Card with 32MB RAM(Large G-code files, especially 3D files will require a video card with 512MB RAM or higher)
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I've been using Dell Optiplex GX280 PC's. A few months ago, one fizzed itself and I bought a bundle of 3 on eBay for £30 - so now I have 2 spares. One of which will go with this.

They have parallel ports and have been very stable. The PSU died is the one that fizzed itself - but 3 new PC's cost less than a replacement PSU.

Mach 3 is a fantastic piece of software.

Si

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I went around to my brother and collected his old PC's which he said I could have in exchange for taking the remaining bits I didn't want to the tip for him.

So I ended up with one PC for Mach 3 and I have used the case from another one to mount the drivers,psu's and break out boards in .... seems to be working fine so I better get my finger out and build the cnc table now! :)

I bet the Dell Optiplex GX280's that Simon bought even came with Windows XP Pro licences :)

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For Robert!

post-74-0-54270900-1413305329_thumb.jpg

I also added a micro-switch on Z. Some people touch the tip of the torch on to the plate being cut, then back off a suitable distance at the start of each cut instead of using torch height control. Since a microswitch only costs 50p - this is a more cost effective, if less cool strategy.

post-74-0-58844300-1413305475_thumb.jpg

post-74-0-74942800-1413305487_thumb.jpg

Si

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Si, how are you going to toggle the microswitch + set if for various metal thicknesses on the bed? (is it not the wrong way around, if you're going to put a magnetic (therefore adjustable) bar stop on the z-axis rail?)

I wanted THC because I want to have a go at treadplate on it.

Springs -> I'm waiting for him to come back to me I'll give him a bell tomorrow.

At the risk of either sounding a smart a****, I was wondering if you've run through the stepper calculations and what speed you can drive them at? I am no expert at this but you're 24v power supply (and my 42v) got me thinking which one is "more" correct and why.... (I basically went with the larger as I was said that the higher the voltage the more power + availability to drive the steppers faster, however having done some calcs (which I may have got really wrong!) that theory may not be right.

ps-> I've got mach3 running on windows 7 64bit home (well the software works, and it connects to the smooth stepper via USB), hoping to finish either the driver box tonight or weld up my rails (square!)

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You lower the head till the microswitch activates (makes or breaks depending on how you wire it) which will be caused by the torch head hitting the metal, at that point you know you have to retract the torch by a fixed amount (which you only have to measure once and will then work for any thickness of metal).

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If tread plate means ally chequer plate or steel durbar it's best to cut it flat side to the torch , even the

best THC systems can't cope with the rise and fall pattern side up

whilst on the subject of operating tips , when you want as round a hole as poss program for two full cuts continuously round the hole dia - it's basically what Truehole

does (Hypertherm Pro-cut)

cheers

Steveb

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Crazywabbit -> treadplate yes, you can do that too, hoping for a better quality of cut and also if the thin stuff warps (can always sell it later at no loss).

edit -> thanks Steveb

https://www.youtube.com/watch?v=rSABKHKKRb0

https://www.youtube.com/watch?v=1BdQKTQpxJ0

truehole? :https://www.youtube.com/watch?v=2GLnh5EdIJ8

https://www.youtube.com/watch?v=RNZysuoVTBQ

microswitch -> is it not on the top face of the z axis? it's not going to contact with anything is it?

http://forums.lr4x4.com/uploads/post-74-0-09975200-1413038759_thumb.jpg

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Robert, stepper motors are rated by current and usually specify a voltage at which that current will flow through them. If you have say a 4A motor, you limit the current via the driver to 4A. So long as the PSU that powers the driver is higher voltage than that required to push 4A through the motor, and it can supply the current, it will be capable of running the motor at its max power and hence speed too.

The motors will not run faster or with more torque with a 42v supply versus a 24v.

The z plate is mounted on slotted holes so it can move up (normally pulled down by gravity). The micro switch is attached to the z plate but the button on the switch touches the assembly the z plate is bolted to when the torch is in it's lowest position. When you lift the torch (or move down the z axis), the switch body is lifted away from the assembly, making or breaking the circuit.

Si

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Having had a read on this (including the info that I think Robert posted and then removed, maybe I imagined it) the higher the supply voltage the faster you will be able to get the stepper to spin.

There is a calculator and explanation on the following link http://www.daycounter.com/Calculators/Stepper-Motor-Calculator.phtml

Whilst the link doesn't make it clear the number of steps they are referring to are the number of native full steps of the stepper and not the microsteps as far as I can tell. The logic behind this is that the equations are based on time taken to go from 0 current to full current and back to 0, microsteps are are actually just current somewhere in between the 0 and full, or to put it another way just a point on the full swing of the waveform for a full step.

Anyway, a higher voltage supply will allow you to get to the max current quicker (remembering that you are fighting the inductance of the motor).

BTW I'm quite prepared to be shot down in flames here if I have it wrong. (which I may well do)

EDIT plugging the specs for my motor in give me the following speeds :-

24v = 2.94 RPS = 176 RPM

36v = 4.41 RPS = 264 RPM

42v = 5.15 RPS = 309 RPM

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CrazyWabbit -> I made a slight error in my post the other night, found some other information on the net, and decided to edit the post before either misleading someone with wrong information (or proved I knew little :) )

Anyway, long + short is, after a bit of discussion, I think that my post was wrong as I was focusing on inductance and then equating this to resonance (where the drive of the stepper would become unstable), unfortunately I forgot (school boy error) that resonance requires inductance and capacitance, and we have very little capacitance in a stepper drive.

In hindsight, some of it was right about the frequency, and I'll repost that bit, but the torque was defiantly wrong.

(I also wasted a night of practical work assembling the plasma, with a load of calcs that were a little wrong), anyway... still trying to get my driver case assembled, get some pics when I'm done with the CAD/PAD design (computer aided design / paper aided drilling) (dots on a bit of paper, and a bit of bluetack, quicker for me to use cad and offset measurements than mark up and measure the plate [perfectionist]).

Si - spring samples in post to me, need to do the bends and inspect + comment, then they'll be done next week, v-impressed with approach.

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CraztWabbit:


Well, having read the calculator that you posted & input the values for my stepper, I'm now more confused than ever before. (think I'll just plug the thing in an see what happens on 1/4 step mode, 4.2A, and set the Mach3 -> config -> motor tuning ->8steps /mm (1/4 step microstepping / 5mm belt pitch / 20T pulley / 200 steps per rev) , and the limiting speed at 2540mm/min (100"/min) for the x and y axis and see what happens


[100"/min seems to be about the maximum travel speed of my plasma on 1.3mm aluminium (optimum is 81"/min)]


The acceleration setting, I would not have a clue..... anyone any ideas?

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