1.2m x 0.6m plasma, <£2k questions!:

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04 Jun 2020 20:11 #170126 by WinstonTodge
Apologies for the beginner questions, but I'm speccing up a small 1.2m by 0.6m plasma table for ~£2k at the moment, and have a few questions after reading the (excellent) primer, watching all of the cymbal YouTube , watching the Making Stuff series on it and the 'Everything in One Place' thread by Tommylight...

My current plan is

  • A relatively cost effetive plasma (pilot arc start) off of ebay/amazon
  • LinuxCNC (OBVIOUSLY!)
  • Mesa 7i96
  • 15mm linear rails
  • 50 x 50mm TIGged steel box for the table
  • 50mm aluminium for the gantry (possibly some nice aluminium T slot extrusion borrowed from work)
  • Belts not rack (cost / weight / Tommy loves them, so they must be sensible, right?)
  • NEMA 23s all round
  • Not really sure on the stepper drivers?! Or sensible sources in the UK
  • Ball screw on the Z
  • THCAD - 10

The (random) questions (so far!)...

  1. What is the main advantage of interface card like the 7i96 over a parallel port and BOB / relays separately?
  2. What gauge/thickness of steel box section is sensible for the table?
  3. What stepper drivers are people using? TB6600 seem to be CHEAP, but what is a sensible compromise on cost / performance / reliability?
  4. In the primer there is a section on generating an ArcOK signal using THCAD, and if you don't have a CNC port on your cutter to provide this. What do people do normally? I was thinking of using the output of the THC-CAD and a simple compartor circuit to sense when the arc voltage has passed a certain value AFTER the pilot arc has fired. But I'm not really sure of the design intent of this circuit without really getting a scope out to identify when an Arc OK signal is valid.

Apologies again for the random questions, just formulating them as I go along... :)

I'm doing this as a means to design and make parts for my car (and engine swap!) addiction... See picture below of latest project!

Any help would be much appreciated!

Thanks all,

Chris.

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04 Jun 2020 20:34 - 04 Jun 2020 20:52 #170128 by thefabricator03
You had my attention at TIGed steel frame!

1: Its a modern card that uses a ethernet cable to connect to the computer. Parrellel port BoB's are cheap but most modern computers dont have a parrael port so you need to install a card. Ethernet ports are a dime a dozen.

2: For that size, really depends on the load you want to put on it. I would be happy with 2.5mm wall thickness but maybe you could get away with 2mm if it was just used for cutting sheet metal.

3: Not quite sure on stepper drivers. Just make sure they have all the signals you will need, drive enable, fault detection.etc.

4: Not 100% sure on that one.

Just make sure you have fun with the build. It will be a wild ride!
Last edit: 04 Jun 2020 20:52 by thefabricator03.
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04 Jun 2020 20:42 #170129 by WinstonTodge
Thanks for the input!

1. Does the extra cost of an 7i96 (and sourcing one in the UK) pay off versus a parallel port (I've got a load of old Dells hanging around at work with parallel ports) and BOB. i.e. what extra does off loading some of the work to the 7i86 give you over using the RT on LinuxCNC and a parallel port?

2. I think I'll go for 2.5mm as it's not a silly amount more than 2mm here.

Really looking forward to starting to compile bits! :)

Chris.

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04 Jun 2020 20:52 #170132 by thefabricator03
I think it depends on your goals. If money is the main deciding factor go with the Parrael BOB but if not I would go with a modern piece of hardware like the 7I96.

I live in Australia and I use a 7i76 on my plasma, with two THCADs. Shipping and currency conversion are a pain but at the end of the day it runs PlasmaC with no issues what so ever.

Also the 7i96 takes some of the burden of step generation away from the control PC. That means you can use just about any PC to run PlasmaC without worrying if its going to be able to generate steps quick enough.

With the frame, just make sure its straight before and after welding. With TIG it should not deform at all with welding but take your time to get it together well.


Also, what is that motor your pulling out of that front cut?
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04 Jun 2020 21:03 #170134 by WinstonTodge
Out of curiosity... Why would you need two THCADs on the same plasma table?

Does PlasmaC generate it's own ArcOK 'virtual' signal (I think I read that somewhere?! But may have imagined it)...?

I'm pretty sure I'm going to order a 7i96 anyway (just need to try to hide it from the wife, by doing some 'expensive' DIY on the house at the same time through PayPal...) but am quite curious about the benefits of off-loading the processing and some of the maths to something that doesn't seem (to me!) to be too processor intensive in the first place (in the grand schmem of things!).

Looking forward to starting the welding on the frame... Material going on order tomorrow(ish) I think.

It's an Audi TT 225 (VAG 1.8t) which I'm borrowing the whole lot out of, forging and big turboing (the standard K04-64 from E30 Golfs) to get a sensible, useable 350bhp / 350 lbft from, in the rear of the old (2006) Mk3 MR2 in the foreground! :)

Chris.
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04 Jun 2020 21:15 - 04 Jun 2020 21:34 #170136 by thefabricator03
I use one THCAD10 for THC and one THCAD5 for ohmic sensing (still have not set it up Lol but its wired in) linuxcnc.org/docs/devel/html/plasma/plas..._with_a_mesa_thcad_5

Yes PlasmaC does have a synthetic ARC OK signal, Looking in the docs for PlasmaC I have found this, linuxcnc.org/docs/devel/html/plasma/plas...ide.html#reed-arc-ok , it outlines using a reed relay to generate a real signal. Might be worth looking into.

Like I said, my 7i76 have not given me any grief what so ever. Definitely worth the money!. I really have a soft spot for things that just work.

Sounds like a interesting project. When your plasma table is done, custom engine mounts and cross members will easily be within reach without having to pay large amounts for them. Explain that to the wife, it will save you money!
Last edit: 04 Jun 2020 21:34 by thefabricator03.
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04 Jun 2020 21:18 #170138 by rodw
The 2 THCADs. One is for ohmic sensing. You need a seperate shield on your torch to do this. You may not have one. It has the advantage of freeing up a couple of input pins which is helpful on a 7i96 as it does not have many.

And yes, plasmac can generate an ArcOK signal from a voltage threshold. Its important to understand fully how to set it up as this seems to be a stumbling block for many users.

With a parallel port, Linuxcnc must generate the step pulses in memory. It does this on a seperate faster base thread. This requires a low latency PC and there is a limit to the speed it can do this. 50 kHz has been mentioned.

With a Mesa card, the stepper pulses are generated in hardware up to 10 Mhz. The base thread is no longer required so the latency requirements are relaxed which is a good thing with modern PCs which are laden with power saving latency creating features.
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04 Jun 2020 21:40 #170140 by WinstonTodge
Thanks both!

Is there a higher IO count MESA card, ethernet based but spindle control not being required (i.e. 7i76e) at around the same price as the 7i96?

I'll read up on PlasmaC more now, as I haven't really got into it yet. :)

Ta,

Chris.

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04 Jun 2020 21:44 #170141 by rodw

Thanks both!

Is there a higher IO count MESA card, ethernet based but spindle control not being required (i.e. 7i76e) at around the same price as the 7i96?

I'll read up on PlasmaC more now, as I haven't really got into it yet. :)

Ta,

Chris.


No. You could look at using a 7i92 and adding a parallel port BOB to it but you'd spend more on other hardware so there would be no net benefit.
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04 Jun 2020 21:49 #170142 by thefabricator03
I understand there is a bit of difference with the price, but for what you are getting I reckon the 7i76 is hard to beat. It really is a industrial piece of hardware with more I/O that I can use on my large machine.

The other option is trying to tie together limit switches and such to minimize the inputs needed for the 7i96.On a budget I am sure it could be done and work well, just needs a bit of planing.
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