Plasma Cutter

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Rack Drive

Postby bdring » Wed Aug 01, 2012 2:22 pm

Here is a quick exploded view rendering of the rack drive. This is used on 3 out of the 4 motors and a very similar one is used on the Z. These were cut from UHMWPE, but they were designed so that they could be printed too. It can use NEMA17 or NEMA23 motors. I used smallish NEMA23's.

The ratio is 2.4:1 (20 to 48 tooth). The pinion is a 24 pitch 15 tooth spur gear.

The whole assembly rotates into the rack to set that distance.

rack_drive.jpg
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Re: Plasma Cutter

Postby Cre8ivdsgn » Wed Aug 01, 2012 3:18 pm

The pivot mechanism of your drive assembly - it looks as though you will adjust it so the pinion engages the rack and then screw it down. It does not look as though you are planning on spring loading the engagement. The loads are pretty light on this and so that will likely work well. I'm not sure how well this would hold up if you used this design on a router table. I just have no idea of how well the rack and pinion setup will wear.

Just to confirm: Mach3 through the parallel port? I just feel as though I am missing the boat and really everyone else seems to understand something else entirely! Along with a large table design, I am also working on retrofitting a Sharp knee mill for CNC operation and I've been wondering what control route to take.

Do you have an end goal in mind for the plasma table? Had you been thinking about making enclosure boxes or something similar that sent you down this road? I'm curious because i'm wondering how this fits in with the router you currently run!
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Re: Plasma Cutter

Postby bdring » Wed Aug 01, 2012 3:48 pm

The tensioning is not ideal, but works fine. I have some changes in mind, but I want some of the parts to be plasma cut, so I am holding off rev 2 until the machine can do it.

The tensioning of the belts took about three hands to do so I added 2 tapped holes through the sides of the UHMWPE (not shown in rendering). Bolts through the holes push the motor to tension the belt....it works great. You can't see it from the rendering, but the motor is in a pocket so the shaft can stick out enough.

I use a USB smoothstepper on my big router. It works best when you have a really high step rate (I have 28800 step/in) . I like it, but I have had some problems with it. If the computer cannot respond fast enough via USB, the smoothstepper locks up and gives a error about the USB speed. Mach3 has no idea where it stopped. You have to restart Mach3 to fix it. I bet the Ethernet version has the same issue. Get a fast computer and try not to watch YouTube while running a job :lol: Bottom Line: I like it for the router.

Obviously the parallel port is a dead end, but if you use special applications like close loop torch height control, that has to be programmed into the hardware of a device like a smoothstepper or it will not work.

I wanted to use an off the shelf solution to start with because I have never used a plasma. After that I have a group that wants to help do an open source replacement for everything.
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Belt Tensioner

Postby bdring » Wed Aug 01, 2012 5:32 pm

I added those bolts I use to tension the belt. The motor is attached firmly, but can still slide under pressure. The bolts are used to tension the belt.

rack_drive_tensioner.jpg


Belts are a lot more forgiving than racks. I was thinking about doing something similar with the rack using spring plungers instead of bolts. Some have good travel and can have a lot of force.

spr_plng.png
spr_plng.png (12.61 KiB) Viewed 23069 times
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Re: Plasma Cutter

Postby Cre8ivdsgn » Thu Aug 02, 2012 1:26 pm

Bart, let me preface these comments with "I am thinking out loud here simply because I am dealing with some of these design issues myself right now."

I can see where the motor could slide - especially since the motor mount is made from UHMW! The jacking screws are a good safeguard. I confess I am not a huge fan of introducing secondary operations on a part (the 90 degree tapped holes on the motor mount). You might have used an eccentric (no secondary operation), but on UHMW, even that might not be enough. The screws are likely most reliable.

I've been looking at Moore Gears and their rack supplies. Not listed is helical rack and pinion stuff, which is amazingly smooth. Because of the 30 degree helical engagement, you always have more than one tooth against the track. Its expensive though - at least as far as homebuilt stuff goes. For this machine design I am working on now, I think I will fall back to straight rack and pinion. I can go back to helical if I need to and can justify the expense.

I did have one goofy thought though. If you consider a drive mechanism for the rack (perhaps something similar to the unitized drive you have come up with, but that isn't important in this example). Now mount this to a flat plate made of thin G10 (or perhaps even the carbon fiber material I thought I saw on one of your designs) whose surface is parallel to the rack. Firmly bolt one end to the carriage and the other end to the drive mechanism and pick a position that forces the G10 to deflect a little. The rigidity of the G10 (given an approrpiate thickness) might be sufficient to keep the pinion and the rack in tight engagement without twisting and even 0.5mm deflection should be sufficent to keep the pinion engaged. At that little deflection even aluminum would work, I think. (I had part of this conversation with the folks at Moore - they said that if we could spring load the pinion's engagement with the rack, we would have the best chance of achieving the positional accuracy they could cut to.) On the other hand, this preloading might be a bit hard on the V wheels of the MakerSlide.

This is just a bit of thinking out loud.
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Re: Plasma Cutter

Postby bdring » Thu Aug 02, 2012 1:48 pm

I doubt the motor would slide after the bolts are tightened, but it was hard to use a screw driver and wrench while keeping the belt in tension, so I added the jacking bolts.

I really dislike secondary operations too. The holes location is not critical at all so I just hand drilled them without a press or vise. I don't really consider that a painful secondary operation. If the parts are printed, it is not an extra operation.

I have played around with some cam ideas for setting the rack engagement, but have not found anything I like. The distance is large because it is real handy to have the gear fully disengage. A cam with that range takes a lot of space. As long as I am going to all the trouble, I want to spring load it, which a cam can't do.

I finished all the home switch wiring last night. All the switches are wired in series on one input. It took a little RTFM to get the slave axis to play nice with one switch. It works and Mach3 can home it all on one circuit. I ran some G-Code too.

That is it until after Maker Faire. I have to do a little dis-assembly and flip it on it's back to get it into the van.
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Re: Plasma Cutter

Postby Cre8ivdsgn » Thu Aug 02, 2012 2:38 pm

Bart, I was thinking of using the cam in place of the belt tensioner screws. So you could add one more hole when you initially cut the UHMW material for a screw about which the eccentric could turn. Not necessarily for rack preloading....

I wish I could make it out to Evanston! I lived there for a few years - while in school and afterwords. For a while my better half and I had an apartment above a bread bakery on Central. Mornings were pure torture!

Have a good time!
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Re: Plasma Cutter

Postby Digitalmagic » Tue Aug 28, 2012 11:21 am

@Bart

i would like to apply the same anodizing/hardening process to my 1200mm Makerslide.
The goal is use steel V-wheel on it.

Did you just have them processed with a "vanilla" electrolytic anodization? Or a specific surface additive process hard coated them, like adding a layer?
In other words, which exact process is needed, and how does it cost?

Thanks!

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Hard Coating

Postby bdring » Tue Aug 28, 2012 12:08 pm

The process is known as hard coating. Only about 10%-20% of anodizers in my area do it. It requires a refrigerated tank and uses chemicals that have special disposal requirements.

You generally just ask for hard coating and specify the buildup (thickness). Maximum buildup is about 0.003" and is what I used. The places I have found have a minimum order of about $150 which would get me about 25 pieces.
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Re: Hard Coating

Postby Digitalmagic » Tue Aug 28, 2012 8:42 pm

bdring wrote:You generally just ask for hard coating and specify the buildup (thickness). Maximum buildup is about 0.003" and is what I used. The places I have found have a minimum order of about $150 which would get me about 25 pieces.

Bart, thanks! Am definitely interested in that.
Could you please tell me the company offering this? I already have a bunch of rails to submit.
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