Tube Coping motor setup

So I was cleaning the shop and wondering what to do with this 9x20 lathe I picked up for spare parts. And it occurred to me, that's practically all the parts a guy would need to cut tubing. The gears on my stepper motors even match the teeth on the lathe shaft. And so I dove in.

I saw the tube coping calculator and I get how the length of the curve wraps around the pipe so you just turn the pipe and move the gantry up and down (for your Y) to get the cut, but how would you set up the motor to turn the pipe so an inch drawn flat is an inch worth of turning? I hope that makes sense. I'm going to try to post a picture of how I'm going to mount the stepper motor to turn the lathe chuck, with the three different gears, I don't know how to figure out how to set the stepper motor up to turn everything correctly.

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My ad hoc plan is to mount the lathe head at the far end of the table in a way that can be easily mounted or taken down, hang some metal bearings from the gantry for the pipe to rest on just behind the torch head and go form there. All that's mechanics and details. I'm just clueless on how to set up the additional motor (and does it need to be the same size stepper as the other 4? Just wondering). Thanks for any help.

I used the same size motor as my other axes use, although I did not use the entire lathe reduction gearing that you depict. I did gear it to 4:1 and it works well on most tubing. Mine is wired such that I disable one axis (y in my case) and that axis becomes my radial axis. I use CNCwrapper for the CAM software to convert flat to radial g-code.

Another way is to realize the motor tuning (steps per unit) is based on the cutting surface (circumference) of the pipe you are cutting (PI times the diameter) and recalibrate your steps per unit for the rotary axis when you change pipe sizes. You can work out a simple formula based on the number of steps per one rotation and realize that is a constant in a formula that takes in the diameter and spits out the Steps per unit. I have had a Rotary Wizard application for several years for MACH3 that does all of the automatically. You do need to setup a home switch at zero degrees. If you use SheetCAM and are interested in a TRUE Pipe Fitting application that figures all of the copes and cuts for you on simple or complex multi-joint designs, than join my Support Forum and monitor the Special Announcements Forum. You will get first notice of new products and there will be an announcement soon for the Pipe Wizard.

TOMcaudle
www.Candcnc.com
301 Moved Permanently

Thank you. I think that's where I eventually ended up. I jogged the motor one complete rotation and noted that it came out to something like 5.23 inches. I took the steps per unit, multiplied it by 5.23 to get the number of steps in a complete revolution, then found the circumference of the pipe I was working with and divided to get steps per inch. Probably the long way around, but I got to the same place.

It took some tinkering to get it, but I'm pleased with it considering I've built it out of spare parts I already had, some scrap steel and $8 bucks worth of bolts from Lowes.

I ended up mounting it off the end of the table on a pivot so I could raise and lower it with a bolt for different size pipe.

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These are some random test cuts I made with it. Try not to judge my ability (or lack of) to weld thin-walled, galvanized conduit. It was just something cheap to cut up.

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I like it. There's a ton of things I've never made out of tubing because I hate notching pipe. And I hate those hole saw jigs. I really do. This was awesome.

http://youtu.be/9hT1cC0eSFs

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Originally Posted by drafterman

I used the same size motor as my other axes use, although I did not use the entire lathe reduction gearing that you depict. I did gear it to 4:1 and it works well on most tubing. Mine is wired such that I disable one axis (y in my case) and that axis becomes my radial axis. I use CNCwrapper for the CAM software to convert flat to radial g-code.

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I used the gears because it just happened to mesh with the gear that was already on my stepper. It was like providence. :-)

Are you grounding the tube through the spindle bearings?! I see no work lead connection to the tubing.

If this is true, your bearings will be ruined very very quickly. :eek:

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Originally Posted by keebler303

Are you grounding the tube through the spindle bearings?! I see no work lead connection to the tubing.

If this is true, your bearings will be ruined very very quickly. :eek:

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The ground was on the table. And the pipe is resting on a metal cradle I made for it to ride on, which is bolted to the table. I powder coated the individual parts of the lathe headstock and everything I used to mount it on... which I'm not saying electricity can't make it's way back to ground via that route, but it doesn't seem like it would be realistic. I hadn't thought of that, but everything at that end is basically wrapped in plastic.

It is a serious concern. If you didn't isolate it on purpose, it is grounded!

I'd recommend a more reliable grounding method that cannot include your bearings. Commercial solutions would use slip rings to ground the spindle.

What is the difference on how he is grounding the material to be cut ("in his cradle") and hooking up a ground lead to flat plate to be cut? What is the concern or other factors in rotary cutting. Any info is greatly appreciated.

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Originally Posted by brouser1

What is the difference on how he is grounding the material to be cut ("in his cradle") and hooking up a ground lead to flat plate to be cut? What is the concern or other factors in rotary cutting. Any info is greatly appreciated.

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I'm only guessing here, but I suspect the thinking is it could cause the bearings to arc as they turn, but I also suspect that would be much more of an issue with rotary welding than plasma cutting. We'll see. :)