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Old 08-20-2016, 01:18 PM   #1
rodw
 
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Default Some people build Models, others build CNC PLASMA machines

This thread might take a fair while to develop. Its been 12 months in the making already. My Arduino Rotary table controller project got me interested in CNC but I did not want to do the obvious and convert my mill to CNC. I eventually got interested in CNC plasma cutting and I started about 12 months ago by cramming the electronics into an old Plasma machine case complete with a Torch height control. There are 3 layers of electronics in the case.



I got very disollusioned becasue my cheap plasma cutter died and the parallel port Break Out Board refused to talk to the PC.

So it sat on my desk for the next 10 months or so until one day I remembered how much time and effort went into it so I decided to kick start the project again. So in June, I purchased a 50 amp Everlast Plasma cutter with CNC interface and a CNC torch for it.

After hundreds of hours designing a machine, it is finally coming together. After machining 20 or so parts, finally, the Z axis is complete and I am sure it will work nicely.

Originally, I was going to make a ghetto build machine but since I got the idea, I've purchased an interest in a business that could use a plasma cutter so I am sparing no expense to do the job right. As I am surrounded by fabricators, the table will also be of commercial quality.

These are just quick pics taken with my phone, I will post up some better ones one day if I find the time to pohotograph it properly.





It has a 150mm ball screw which gives total travel of about 100mm. You can see that there are 2 linear rails. One is for the Z axis itself, and the other is a short floating section that senses the material.

Plasma is a bit different to other CNC machines as there are no cutting forces. Here is what happens at the start of a cut.
1. The torch rises until it trips the home sensor which you can see below the black bearing mount on the stepper side.
2. The torch hits the material and moves up along the floating rail. This trips the upper proximity sensor.
3. The torch height is adjusted for the switch hysterisis.
4. The torch raises to cutting height (around 250% above cutting height)
5. The controller fires the torch.
6. Once the Arc is established, the Plasma cutter sends an ARC OK signal back to the CNC controller
7. The torch is lowerd to the cutting height and off it goes.
8. The CNC controller monitors the arc voltage sent back though a voltage divider from the Plasma cutter as the voltage is a function of torch height.
9. The torch height is maintained constant based on this voltage as parts can tip up or warp while being cut.

The red micro switch is a limit switch so the ballscrew does not fall apart. In theory, it should never be tripped as the material sensor should always be triggered before it.

The other thing that could happen is the torch could hit something that could damage the machine. To protect against this, I've added a breakaway magnetic Torch mount.



This has 5 x 12.5mm dia magnets that are 3.5mm thick. These have been pressed into holes cut with a 12.5mm endmill so they are just proud of the mounting surface. They are secured by 3mm countersunk screws. The locating pins have been machined using my boring head ball turner documented elsewhere on this forum which are a press fit (helped with some retaining compound. This is the second iteration of the magnetic breakaway mount. Initially I wanted to use a pair of North and South oriented magnets so that one magnet was sitting below the surface and the other located into the recess. I found the magnets were not dimensionally accurate and also turned out to be a press fit. Breaking an M4 tap on th elast hole was the final straw but I decided the part needed a total redesign and went with a steel plate located by the pins.

If the torch ever falls off, it makes sens to stop the machine immediately so I've drilled and tapped the back side for another proximity sensor which will return an e-stop if it ever triggers.



The reason why the torch mount is a lot lower than the carriage is that I hope to be able to build a cover for the axis to keep out metal dust.

This machine will be controlled by LinuxCNC and I have purchased an Ethernet Mesanet 7i67E interface card and the matching MESA THCUD torch height controller daughter board. Unfortunately, the Mesa board is significantly larger than the Breakout board in the yellow case, so everything needs to be stripped and remounted into a 500mm x 500mm enclosure I've purchased.

Today, I managed to drill and tap 44 x 4 mm holes into a piece of Aluminium RHS on my little SX3 mill. This proved quite a mission as the X axis handle sits proud of the mounting table so the RHS had to hang off the table to get about 2mm clearance from the handle.



Its a bit hard to see but there is a length of 75mm x 50mm steel mounted to the table to act as a fence so I could slide the material along. I used a short piece of scrap linear guide when I had to reposition the material (table travel limited me to 7 holes at a time and there are 22 on each rail.). I've screwed this scrap down with 2 screws and located the undrilled hole with a close fitting punch. This worked pretty well but on the first side, I started at one end and was out a bit by the time I got to the other. On the second side, I started in the middle to halve any error.





The problem with this setup is that the holes have been referenced on 2 different sides so I don't really know exactly how far apart the rails are. That won't be hard to sort out.

I clamped one rail to the back fence when mounting it. The steel fence is dimensionally accurate, but I've forgotten the accuracy spec. The second rail is still loose and will be tightened up once I get a carriage mounted up.

I'll try and post up a few more photos tomorrow as I hope to get the Y axis close to finished. The plan is to have 2 linear rails on the front face of the RHS and a gear rack for the rack and pinion drive mounted underneath to shield it from dust as much as it can be. This is going to be a bit of a pain as its designed to be mounted from the rear into M6 holes in the rear of the rack. I'm going to have to drill access holes on the top surface to fix the rack. I'm glad thes holes are much further apart than the 60mm on the linear rails!


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Old 08-21-2016, 02:24 AM   #2
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Good luck with your build - I know how tricky they can turn out to be. Here's a pic of one I built a few years ago.


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Old 08-21-2016, 11:32 AM   #3
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Good luck with your build - I know how tricky they can turn out to be. Here's a pic of one I built a few years ago.
Cogsy, thanks for the encouragement. Hopefully all the bad luck got left behind last year in the yellow box. My table will be similar to yours.

Did you have to add gearing to your steppers for the rack and pinion or did you get away with direct drive? My pinion will give 30mm per revolution and I was hoping to get away with a direct drive.

I thought after I bought my linear rails that it probably only needed one based on their torque holding specs.

Anyway, I did not have as much time as I had hoped today but I wanted to get the rails in position and confirm they were parallel.



Here's what I ended up with.



The part connecting the linear carriages is just some scrap as I did not want to disturb my mill setup until I have mounted the gear rack.

I used transfer screws and a tap with a hammer to locate the holes. There is only 6 in the set but more than enough for this exercise.





Once this plate was nice and secure, I went along and tightened up the second rail one bolt hole at a time with the carriage positioned right beside it to set the width.

It turned out really well. So happy, absolutely no binding along the full length of travel (1240mm Y axis width).

So now all I have to do is to mount the gear rack, make the mounting plate to replace the temporary one, finalise the stepper motor pinion drive setup and mount the Z axis to have the Z and Y Axis done and dusted over the next week or 2.
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Old 08-22-2016, 01:27 PM   #4
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Rod,

That's an impressive project. For all-around work it looks like it's going to be a very useful tool. I'm looking forward to posts with some of the things you will make with it when it's done.

Thanks for posting.

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Old 08-23-2016, 07:25 AM   #5
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Did you have to add gearing to your steppers for the rack and pinion or did you get away with direct drive? My pinion will give 30mm per revolution and I was hoping to get away with a direct drive.
Both X & Y were direct drive via timing belts, no rack and pinion at all (although I'd have preferred it). Z was just direct drive ball screw. You probably know it already but you want pretty much the fastest travel you can get out of it in X & Y so try for direct drive if you can.

A nice blast shield around the torch nozzle really helps keep the grit out of the drive systems and allows you to watch the cut without going blind as well.

One of these days, when I have the time, money and somewhere to put it, I'll make a half-sheet one for home.
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Old 08-23-2016, 12:19 PM   #6
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Rod,

That's an impressive project. For all-around work it looks like it's going to be a very useful tool. I'm looking forward to posts with some of the things you will make with it when it's done.

Thanks for posting.

--ShopShoe

Thanks for the encouragement. I'm trying hard to make it commercial quality all the way.

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Originally Posted by Cogsy View Post
Both X & Y were direct drive via timing belts, no rack and pinion at all (although I'd have preferred it). Z was just direct drive ball screw. You probably know it already but you want pretty much the fastest travel you can get out of it in X & Y so try for direct drive if you can.

A nice blast shield around the torch nozzle really helps keep the grit out of the drive systems and allows you to watch the cut without going blind as well.

One of these days, when I have the time, money and somewhere to put it, I'll make a half-sheet one for home.
Cogsy, Thanks, you see so many with reduction gearing but the maths say to do it pretty direct. I did see it said the ideal speed was around 1" per rev so 30mm per rev is pretty close to that.

Z axis is running a 5mm pitch ball screw.

Been busy the last couple of days so no progress, I need to think out the Y axis wiring and stuff before I build too much more. Bought some more material today.
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Old 08-28-2016, 07:26 PM   #7
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I'll be following this thread. I want to build a combination cnc plasma cutter and cnc router next spring. I don't have room for one that could take a full sheet of metal so I'm planning on building mine with a cutting area of 4'x4' which is probably still too big for my small shop but I'd hate to go any smaller. Definitely be trying to understand your design to see if I can pickup any tips for when I start designing mine.
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Old 08-28-2016, 08:13 PM   #8
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I'll be following this thread. I want to build a combination cnc plasma cutter and cnc router next spring. I don't have room for one that could take a full sheet of metal so I'm planning on building mine with a cutting area of 4'x4' which is probably still too big for my small shop but I'd hate to go any smaller. Definitely be trying to understand your design to see if I can pickup any tips for when I start designing mine.

Not to throw water on the fire but trying to build a combo machine leaves you with a lot of trade offs that are less than ideal. Plasma machines really benefit from light weight designs that allow for high accelerations and high speeds at low costs. For a router rigidity is more important as you have to counter real reaction forces from the cutter plus surface finish quality increases with rigidity.

Im not saying it can't be done, just that you need to realize that if you design for one use you will loose for the other use. Of course your expectations are a factor here.

By the way I understand the frustration of a small shop! I'm stuck in a basement at the moment and need to work miracles just to make use of a bunch of rather compact metal and wood working tools. If you are already tight on space I'd suggest laying out the machine in a cad view of your work shop. Either that or build a CAD (cardboard aid design) template of the foot print and see how it will fit your shop. Actually a full so all card board template will highlight usability issues right away as you can walk around the "machine" to get a feel as to how convenient a real unit will be.

I only bring this up because siting a machine creates huge problems in small shops. For example I bought a small jointer as I don't expect to be doing massive wood working projects. Even so I ran into usability issues because even with a small jointer you need three side access.

By the way there is the option of placing a plasma unit outside. It would have the advantage of fire safety and you can build any size you want. Build the majority of the machines frame from aluminum and rust isn't an issue.
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Old 08-29-2016, 12:00 PM   #9
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Originally Posted by mjonkman View Post
I'll be following this thread. I want to build a combination cnc plasma cutter and cnc router next spring. I don't have room for one that could take a full sheet of metal so I'm planning on building mine with a cutting area of 4'x4' which is probably still too big for my small shop but I'd hate to go any smaller. Definitely be trying to understand your design to see if I can pickup any tips for when I start designing mine.
Further to Wizards comments, the difference between the two is that plasma is built for speed (up to 10 metres per minute) and the router is built for torque so it travels much slower. By comparison, a mate's CNC mill travels at 1.5 metres per minute while I'm aiming for 10 metres. He's still scratching his head about why I'm using rack and pinions when he'd use ballscrews. The difference is 30mm per rev on my rack v's 5mm per rev for a ballscrew.

Whilst a multi-purpose machine can be done, the difference is likely to become a compromise due to totally different design considerations. (light and fast v's slow and heavy).

Regarding full size sheets, I am hoping to have a simple pull out trestle on wheels at one end that will support a full sized sheet when required.

I did not get much done over last weekend other than make a shaft adapter to mount a pinion to a stepper. I needed to do that to work out some key dimensions.

After conferring with my CNC mate and mentor, I tried 2 different Y axis drive designs (top mount and bottom mounted racks) and was still not happy. I think I've worked out how to make a front mounted rack that sits centrally between the linear rails as the best option. There is so much to consider to ensure that limit switches and cables are mounted and routed nicely. If I'm lucky, I'll have it drawn up during the week ready for the weekend.
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Old 08-29-2016, 01:55 PM   #10
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If I do build it, it will be built primarily as a CNC router and secondarily to work with a plasma torch. Hadn't really considered the need for speed on the plasma side. Obviously a lot more research to do, just toying with ideas in my mind at the moment.

Mark


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