looking for answers!

[redneck79]

I am planning on building plasma/oxy-acet table, and I have many questions.

1) My idea is to have both the plasma torch and the oxy torch on the z-axis together, so I dont have to change them out when I need to use one of them.
Can this be done, is there a way to simply reassign home for each torch in the software?

2) How exactly does the software control the oxy torch from cut to cut (turning it on/off)?

3) I am still planning now so I am not sure about size of motors concerning torque and speed? I would guess the gantry will weigh about 50-75lb.

4) I want to use rack and pinion, will I need gear reduction?

5) my design will be 4x8 cutting surface, but I want to be able to jog the gantry out of the way so I can load the material with a forklift.

I am sure I will have many more questions.
I would appreciate any advice!!!

[Torchhead]

Oxy cutting requires a whole set of different parameters. You have to use a solinode valve to turn on/off the cutting gas. You will also need an ignitor. The feedrate is much slower than plasma since it typically is used to cut much thicker material. The rigs I have seen the operator lifts the torch and lights it and lowers it to the cutting height. You need long pre-heat times but a CAM setup for the flame or plasma cutting like SheetCAM allows you to put preheat times in the tool parameters.

You may find the weight of your gantry to be more than you estimate if you are going to leave the oxy rig in place.

You can program in a tool offset so you could reset zero in the code easily to use the oxy.

All on/off functions are controlled in the g-code. You may need to modify the "Post" for the oxy so it turns on an off the gas solinoid using a different set of M codes. Normally M03 and M05 are used for Troch on and off but if you leave both mounted you should consider using the M03/05 for plasma.

Any kind of height control on oxy cutting gets really tricky. There is noting to measure but the physical height and schemes using proximity sensors are epensive and tricky to use (and easy to break). Takes the form of a ring around the tip. The good news is that the feedrates are low enough that the operator can hand adjust to gap but you need a rack type torch holder with a knob that can be used.

Once again you have two type of cutting with one set of motion control. You need slow steady moves for the oxy cutting and fast moves for plasma. Steppers have better torque at slow RPM but can run rougher at those speeds.

You probabaly would benefit from belt reduction for the Qxy speeds while direct drive is okay for the faster plasma speeds. If you direct drive you give up torque for speed. Torque is important in plasma where you need jackrabbit starts and turns and too litle torque means you get rounded corners and detail.

It's always a compromise to build a mulit-purpose machine!

[redneck79]

thanks for the info torchhead.

I understand what you are saying about torque vs. speed. The problem is I still dont know what size motors would be ideal for each application. Can you give me some sizes?
Something else that came to mind was a joystick, has anybody ever tried one from a video game console, could that work?

[mlaws1172]

Redneck
Tinman1133 from the CandCNC support formum (not sure of his name on this formum)
has done what you are talking about. I saw his setup and is is very well laid out and made. Maybe he can help.
mike laws

[redneck79]

thanks for the replies guys.

I really appreciate the wisdom that you guys are willing to give to anyone.

now for the next questions!!

1) is it possible to run the motors at different speeds for each of the above applications?

2) plas-cutter.com has the setup that I want to build, could one of you guys check it out and tell me what you think.

[Torchhead]

Consider a CNC machine like a car with one gear. You can run at really low speeds but make sure the drives are microstepping or it will vibrate and shake on really low speeds. Most oxy tables I have seen use gearhead reduction to get smooth slow speeds but then you have no high speeds for plasma. You are building a sports dumptruck. There will be compromises. The commercial oxy torches are big and bulky with manual height adjusters and other heavy components. Those are part of the mass you have to try and sling around at 200+ IPM while plasma cutting. At the very least I would consider fabricating a removable set of heads so you could switch them out.

You can get commercial gear reduction units and belt drives but you better be sitting down when you get the price. Best approach is to design them into your mounting. I am not a big fan of direct drive rack and pinion. You can get really impressive rapid moves but with virtually no torque. Your acceleration numbers will have to be turned down (longer acceleration curve) to keep from losing steps.

[redneck79]

Thanks Again For The Info Guys

By The Way Mike I Talked On The Phone With Tinman About His Machine. He Lives Not To Far From Me And Iam Going To Take A Look At His Setup Soon As Possible. I Think He Is Going To Be Able To Answer A Lot Of My Questions. I Will Keep You Posted!

[mlaws1172]

Glad to see someone else from up this way. Let me know what I can do to help.
mike

[redneck79]

yeah its me again,

hey guys I had an idea about the different speed problem. What if I were to use two sets of pulleys with one belt, the pulleys would be stacked kind of like a ten speed bike or a drill press, and when you want to go from one application to the other you could just move the belt over. you would probably have to have the motor on some kind of pivot like a car alternator, so you could get some slack in the belt. What do you think.

[Torchhead]

They would need to be toothed pulleys (no slippage allowed!). You would need to completely retune your axis with each belt setting. I guess you could have multiple profiles in MACH3 and just select the one with the right tuning. The Steps per Unit would be different as well as the velocity and acceleration settings. If you used sets that would give you the same approximate belt length and had a belt tensioning method I don't see why it wouldn't work. Might let you use smaller motors and drive components on a given table.