New Machine Build- TravsmaCut

[travmc1]

Hey everyone!
Here are some pics of my new build. Just would like to get some input from everyone. This is a 48X48 Plasma cutter and is going to be a water table with custom slats. I plan on using a Hypertherm 85 with it. Driven by a G540 w/620 ozin motors.

[Torchhead]

Better think about a way to keep the table from moving on those wheels. You will be amazed at the reactionary forces involved in slinging the gantry around at 100 + IPM and the Z back and forth (Newtons Third Law). You will find yourself chasing that table across the floor as it cuts. It will be a Self-Propelled CNC Plasma Cutter

I can't tell but I don't see any belt (speed) reduction. If not you can get astounding speeds (in the range of 1800 IPM) but you pay for that with having less TORQUE and resolution and less torque means less acceleration. While you can run the feedrate lower you don't gain any torque or resolution from that. Direct coupling of the motors to the spur (pinion) gears follows this: You get a speed gain of PI times the pinion diameter and a torque loss of the raw motor torque divided by the pinion radius. 620 motors at normal cut speeds will have about 400 oz-in or rotary torque. In any transmission you always trade speed for torque and resolution.

Here is the deal with acceleration. The ability of the mechanics to make rapid stops and starts (deceleration and acceleration) defines how close you can adhere to a toolpath (variance). Poor acceleration forces compromise and rounded corners and angles. The problem is amplified by the normal level of feedrates. The acceleration from 0 to 200 IPM is longer than from 0 to 50 (like on a router) so the toolpath is more effected at higher cutting speeds. Since acceleration = Force/mass changing either part of the equation creates a proportional change in the number across the equal sign.

One of the reasons I discourage the builds of mixed use (router/plasma) machines besides the obvious one of fire and wood chips, is that you have to compromise optimal performance unless you are willing to grossly over design. Sort of designing a Sports Dump Truck.

TOMcaudle
www.CandCNC.com

[travmc1]

Thanks for the reply Tom!
That is some great info! As for the wheels I am in the process of making some leveling jacks so when i put the table in place i could raise it a little and also level it. For the gear reduction what would you suggest? I have a 20* rack gear and a 15 tooth pinion gear. I could easily add belt drive gear reduction to my carriages. This is my first gear rack build and would like to get the most out of it. My plan is to have a well built table but low cost (the best of both worlds).

[travmc1]

Here are a couple more pics with the water pan done.

[Torchhead]

Originally Posted by travmc1 Thanks for the reply Tom! That is some great info! As for the wheels I am in the process of making some leveling jacks so when i put the table in place i could raise it a little and also level it. For the gear reduction what would you suggest? I have a 20* rack gear and a 15 tooth pinion gear. I could easily add belt drive gear reduction to my carriages. This is my first gear rack build and would like to get the most out of it. My plan is to have a well built table but low cost (the best of both worlds).

While you can use the number of teeth to figure a gears diameter it's just a lot easier to either look it up (pinion are sold in "DP" sizes (diameters). it is the precise diameter (centerline of the tooth contact circle). Just a quick measurement of the diameter works fine for the estimate formula:

"You get a speed gain of PI times the pinion diameter"

So a 1" DP gear will give you 3.1416 for a number (which is the actual circumference of the gear} . Round it off to 3. That indicates you need a belt reduction of 3:1 between the motor and pinion gear. While you can get into complex formulas that involve "reflected inertia" the rough method works pretty good since you have a pretty wide window of torque with a dual drive setup.

The numbers end up being a nice balance with steppers with plasma for getting good speeds coupled with good acceleration at the higher speeds. With steppers it is more difficult because you have a continuously changing torque curve so any calculation you do only gives you the optimum at a given speed.

Acceleration is all about time. It is the length of the curve that is important. (time it takes to go from the initial speed UP/DOWN the new speed) You have to set a target upper speed (feedrate) and you want to hit it in the shortest time possible. Obviously the curve is a lot longer if your target speed is 300 IPM than if it was 30. That is why one way to get sharp turns is to slow everything down, BUT in plasma cutting that has consequences. Some vendors have offered "corner slowdown" to combat the issue that they don't have the torque to get the level of acceleration. While it cures the corner issue it changes the dynamics of the cut to the point that you get wider plasma kerfs (changing the size of the part), and THC reactions to the resulting higher voltages , etc) Without dynamic anti-dive built into the hardware the torch will dive to the plate further changing the cut dynamics.

In one corner slowdown scheme ti turns off the THC until it makes the corner. All of that works to a level but it is a band-aid for a mechanical issue (lack of sufficient acceleration).

And you thought building and cutting with plasma (simple contours) was going to be easy!

TOMcaudle
www.CandCNC.com

[travmc1]

Thanks Tom!
So you are saying if the DP is .75 and the number would be 2.3562 round it to 2? So a 2:1 ratio would be good. did I figure that right?

[Torchhead]

Maybe I should have said " get within 10%". If you can hit it exact then that is better but the larger the number is the less significant the effect of the rounding. If you have a .75 Dp pinion the number is 2.356 and a range of 10% (+.23 to - .23 ) makes the numbers range from 2.1 to 2.6. I would see how close I could get to 2.35. Don't lose sleep over a 2:1 or a 2.5 :1 ratio.

[ward2699]

hey travmc1 where did you get the little rack and pinion for the z axis...im thinking about doing a floating head on mine....

[travmc1]

McMaster Carr

6325K63 pinion
6295K12 rack

[travmc1]

Originally Posted by Torchhead Maybe I should have said " get within 10%". If you can hit it exact then that is better but the larger the number is the less significant the effect of the rounding. If you have a .75 Dp pinion the number is 2.356 and a range of 10% (+.23 to - .23 ) makes the numbers range from 2.1 to 2.6. I would see how close I could get to 2.35. Don't lose sleep over a 2:1 or a 2.5 :1 ratio.

Tom
I could use a little advice on THC. I am new at this and was woundering on your products and how to set them up. I am in the process of purchasing a Hypertherm 85 with the machine torch. Should I get it with the serial port? Also need some help on setting up thr floating Z axis. Not sure what I do, is it just settings in Mach3.

[Torchhead]

Originally Posted by travmc1 Tom I could use a little advice on THC. I am new at this and was woundering on your products and how to set them up. I am in the process of purchasing a Hypertherm 85 with the machine torch. Should I get it with the serial port? Also need some help on setting up the floating Z axis. Not sure what I do, is it just settings in Mach3.

The serial port on the Hypertherm allows added automation options. It lets you set the cut current and air pressure (normally done manually with the knob on the plasma remotely. We have developed the HyT-Connect RS485 SIM Kit for our DTHCII based products that lets you take advantage of the new features. Do you need it? No, not to do normal plasma cutting, and the base interface kit we have to the 85 is just a single cable designed to plug into the CPC port and use the internal voltage divider that is part of the CPC option. Click, twist...done.

The only THC we sell with the one-cable connection and the option to use the RS485 serial port is our DTHCII product. It comes standard on all our BladeRunner Dragon-Cut controllers, all our larger Plazpak controllers and in our MP3000-DTHCII product. If you already have the driver and motors and plan to use MACH3 control software, then the MP3000-DTHCII is an advanced BoB with the DTHCII expansion option. It comes with a separate interface card for connection to most step & dir drives and it has a 25 pin header for a DB25 ribbon cable that will plug directly to a G540 motor driver. In essence the MP3000 box sits between the PC parallel port and the G540 Db25 input.

Here is a link to a PDF that shows the connection to a G540 for the MP3000-DTHCII

If you want some "light" reading download the DTHCII Manual from www.candcnc.com/manuals.htm. Ponder the setup and testing of the floating torch holder on page 48.

We also sell a LCTHC (Low cost THC) for the financially challenged but it does not have the Hypertherm direct connect options nor does it have most of the features of the DTHCII. It has a G540 Interface card option. It seems a waste to spend the money on a Hypertherm 85 and not take advantage of the easy hookup and advanced features the DTHCII will offer.

TOMcaudle
www.Candcnc.com

[travmc1]

Thanks Tom for the info. I will go to the links and do some reading. I got with the local district sales manager for Hypertherm yesterday. It turns out he lives real close to me so he loaned me his HP 85 for a week and I just used it tonight and it seams to work really good. I got my table up and running for now with the direct drive setup and have been cutting the slats for my table. It seems to run very smoothly. I still need to hook up my limits switches and figure out how to get the Z axis to do that thing were it finds the top of the metal before every cut. But you were right about how fast I can make the thing go and wants to take off out of my shop with the wheels. Can you send me the eact part number and the cost for what you think I need for for the THC If I get the HP 85 with just the cpc port? Thanks again for the help It sure has been very helpful.