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lockyg
19-12-2010, 12:12 PM
Hi all,

Geoff here, new member to the forum. I`m looking at designing and building a 2000 * 1000 cnc table to take my R-tech plasma cutter. Most of the design is vapourware in my head at present, but I have purchased Autohec`s 4 axis stepper drive pcb. Next stage will be stepper motors and PSU when time allows!

irving2008
19-12-2010, 02:59 PM
Hi Geoff and welcome to the site. We have a few members who have built or are building plasma tables of that sort of size, so should be some useful info for you. Look forward to your build log!

lockyg
04-07-2011, 10:13 PM
Ok, I now have the stepper motors, psu and driver board all wired up to the computer, running a demo version of Mach 3. I have control of the motors at the required speed. (stage one complete!) Next comes the rails and the linear drives - which is where I could do with some advice.

Currently thinking of using 20mm supported precision rails for x, and possibly y axis. Considering the part at the following link. My question is will the plasma dust cause problems with this rail? http://www.ebay.co.uk/itm/SBR20X2204-Precision-Supported-Rail-kit-2204mm-CNC-Part-/110705828120?pt=UK_BOI_Industrial_Automation_Contr ol_ET&hash=item19c694e118

The drive will be via Mod 1 rack, on both sides of the X axis. One single stepper motor for x, connected to both sides via a cross-shaft inside the gantry (probably box section) Motor will be geared down via pulleys and timing belt. Y axis also via Mod 1 rack and the same gearing. Again, can anyone advise how this will likely perform when subjected to plasma dust?

Z axis is likely to be bought in. Considering this one http://www.ebay.co.uk/itm/NEMA23-75mm-Z-Axis-CHROME-IGUS-Proxxon-CNC-Kit-/310328874362?pt=LH_DefaultDomain_3&hash=item48410ad57a. Will then add a floating head setup and control via Mach 3. Any comments / advise please from anyone who has done similar

Final thoughs for the moment are regarding limit and home switches. I want to use something reliable and with repeatable acurracy. Any suggestions?

Thanks in advance

Geoff

Tony Goodwin
10-07-2011, 01:34 AM
Hi Geoff, used to live in sunny sudders, If you are considering microswitches, most machines I have worked on have used a quality made switch say Honeywell, with at least a 1mill cycle life expectancy. Also belive it or not apart from bad setting the single cause of premature failure in these switches is the tendancy to fully compress them when they are unpacked. I have admonished many an apprentice for clicking a new switch in the hand. also limit and ot switches should be a roller type working from a cam with an actuation ramp length of at least 4 times the dia of the switch roller.
Guide protectors are available but expensive, the integral wipers on bearings cannot be relied upon permanently especially when passing the screw caps of the rails. One system I have used is to assert a low air pressure to each bearing therefore blowing out contamination, but these were relatively low traverse speeds and the particulate was brass, all the best Tony

Web Goblin
12-07-2011, 07:06 AM
Geoff,
for limit switches I would recommend proximity switches. Any dust created from plasma cutting would affect the bearings but you can get fabric bellow covers to protect them. Personally I would be more worried about the plasma fumes, they are lethal.
If you fit a decent extraction/filtration system this will also help to reduce the amount of dust escaping into the atmosphere.

Ian

lockyg
19-07-2011, 12:17 PM
Hi Geoff, used to live in sunny sudders, If you are considering microswitches, most machines I have worked on have used a quality made switch say Honeywell, with at least a 1mill cycle life expectancy. Also belive it or not apart from bad setting the single cause of premature failure in these switches is the tendancy to fully compress them when they are unpacked. I have admonished many an apprentice for clicking a new switch in the hand. also limit and ot switches should be a roller type working from a cam with an actuation ramp length of at least 4 times the dia of the switch roller.
Guide protectors are available but expensive, the integral wipers on bearings cannot be relied upon permanently especially when passing the screw caps of the rails. One system I have used is to assert a low air pressure to each bearing therefore blowing out contamination, but these were relatively low traverse speeds and the particulate was brass, all the best Tony

Hi Tony,

Thanks for that. Liking the idea of the air blast through the bearings. In you opinion which approach is less likely to have problems due to dust - the linear rails and bearings I have linked, or stub bearings running on cold rolled box section? (I`m not too concerned about the accuracy- it`s a plasma cutter afterall!).
I like the idea of buying in the linear rails and just bolting them in as opposed to building up carriages to run on box section. I don`t have a mill or lathe for accurate work, so any bearings on carriages would have to be adjustable (I can cut slots with my steelworker). Must admit the design hasn`t moved recently as I`ve been too busy with paying work - still thinking about it though

Cheers

Geoff

lockyg
19-07-2011, 12:22 PM
Hi Ian,

Thanks for the info - any chance of a link to somewhere that would supply the fabric bellows? As for extraction I am considering two different approaches. 1) pipe with holes around the perimeter of the work area and another hose attached at the cutting head, both attached to suction and filter. or 2) box the whole unit in the corner of the shop and extract to outside with a big fan, allowing it to overun before re-entering the work area. each approach will be accompanied by a water table to contain dross. What do you thinks?

Thanks

Geoff

Jonathan
20-07-2011, 01:10 PM
I think the hose attached at the cutting head should be sufficient on it's own. You need some sort of barrier round the cutter to stop the swarf flying too far away. A lot of pressure is required to get the very small (and dangerous) MDF dust.

Web Goblin
20-07-2011, 01:27 PM
Geoff,
Sorry I cant remember who does the bellows now. We had bought some stuff a while ago but I cant remember who we got it from.
As for the extraction system a hose connected to the cutting head will not provide adequate extraction of the dust and fumes created by plasma cutting. The majority of the dust and fumes will be under the plate. The gas pressure from the cutting torch will push most of the fumes down through the kerf cut in the plate. I think your best idea would be to box the machine up and extract from the box area under the cutting table. Remember your filter as well. I think plasma cutters have a legal obligation to have filters fitted to them, particularly dry cutting plasma systems but i'm not sure about this. If you are extracting large amounts of dust into the atmosphere I dont think your neighbours would be too impressed with this. Water under the cutting table does reduce the amount of dust made by a large factor so this would help. Remember you can also plasma cut under wate as well with the correct torch and consumables.

Ian

Jonathan
20-07-2011, 01:38 PM
Sorry I didn't realise it was a plasma cutter! Forget what I just said...

Web Goblin
20-07-2011, 02:08 PM
Geoff,
you could try this site for bellows.

http://www.par-group.co.uk/rubber-polyurethane/Sewn-Bellows.aspx

Regards

Ian