CNC Plasma Table Speed Calculations

[Terry G]

Hello, I have been reading post and viewing pictures of CNC plasma tables for a few days now and am getting ready to build my own. I just picked up a Hypertherm 1000 cutter with a hand torch.

I currently own a smaller CNC taig mill which I have been using for a few years now. I have it setup so it is completely portable and self contained on a rollaway tool chest, with folding shelfs on either side, and a flip down LCD screen. PC and controller box are also located in the bottom drawer of the tool chest.

My plans for the CNC plasma table are to use my existing PC and electronics. I will most likely build around a 3 x 3 table that I can break down in a few minutes to store when not in use. Space is limited for me.

I was thinking of purchasing the exact servo motors I use on my CNC mill and woud like to run the calcuations I made by the group to make sure I haven't overlooked anything. I plan on using sprockets and chain with a jackshaft for at least the X and Y, and not sure on the Z yet. They are 30V DC servos with encoders running at 3000 RPM with 20 oz-in of torque and around 160 oz-in stall speed. Here is an example to make sure my calculations are correct:

To achieve 400ipm table movement I would use around a 1.5" dia. sprocket on the jackshaft. This would result in 4.71in/rev. So I would want the jackshaft to spin at 85 RPM. I would need a gear reduction of about 35:1. If the motor puts out 20 oz-in of torque with a 35:1 reduction, the torque would be around 700 oz-in. I know there is loss due to the reduction, and I don't know if I will find the right gearbox to reduce this amount. But do I have the general math correct? And would a servo motor of this caliber work on a smaller, lighter table as the one I want to design. My servo drivers are capable of 30VDC at 5A, so I could go with a bigger servo if necessary.

I appreciate whatever comments and suggestions anyone might have. Thanks for all the great info. I hope to share my completed system with the group shortly.

Terry

[Terry G]

After doing some research on gearboxes, I found they can be a bit expensive. I could probably use a dual chain and sprocket gear reduciton where I can go from the motor with a 10t to an intermediate shaft with a 60t, then from the intermediate shaft with another 10t to my jackshaft with a 10t to give me a 36:1 reduction. This is a lot of sprockets and chain and tensioners for each axis. Not very practical.

Since I am using the deskcnc servo and drivers on my mill, I am going to have to stick with a servo motor if I want to use my existing electronics. Are there servo motors that turn less RPM so I can do a realistic gear reduction with a single chain and 2 sprockets. I'll keep searching.

My only other option if I have to use servos would be to use a leadscrew of some type. Since my table is not going to be very big, these may not be as expensive. With a servo turning at 3000 rpm to achieve a around a 300ipm rate I would have to connect a 10tpi screw directly to the servo. But I don't think a 1/2-10 screw can spin safely at 3000rpm. My servos wouldn't even have the torque to turn them.

Hmmm. I might be missing something here. I'll have to go back and re-read a bunch of posts and figure out once agian what others are doing. I also want to be able to slow the machine down to 20-30ipm for cutting thicker materials. Though I don't mind changing a couple of sprockets or gears to achieve this.

Just wanted to share my thoughts in case anyone would like to comment.

[Terry G]

I'll just keep replying to my own posts. This information may be usefull to other people just learning.

I did overlook one small item. I was trying to make the servos run at 3000 rpm with a max of 300IPM. I forgot that I had complete control of the RPM of the servo through my software. So even at 3000rpm if I get like 700ipm, I can reduce the RPM of the servo and achieve exactly what I want. This way I can use a realistic gear reduction that will still give me decent torque. I will be designing my own 2 belt gear reduction to achieve around 15:1 reduction.

I'll build a prototype using one of my existing motors. Then I can measure the RPM's through the gear reduction, and make a jig to measure the torque on it.

[Torchhead]

10 to 15:1 reduction will work fine. Your max velocity should be a parameter you set in the control software. The DeskCNC system is interesting but you will have no option for integrated THC. Their pulsing card does not work with anything but their software. You choice for THC is to use a third party stand-alone system. Cheapest one out there that works is about 2000.00.

I know a lot of first time plasma guys don't quite understand the importance of Torch Height Control. Consider this: The cutting is done with the tip hovering about .062 above the material and moving at up to 300 IPM. Thick material 1/4" or >)and a really flat and level cutting surface can be cut with only minor problems but thinner material is not flat and if it is, will warp as it's being cut. If the tip touches the material or the pierce height is too high or too low you will get bad cuts and/or short consummables life.

Either consider a floating (spring downforce) and drag tip approach, or plan on using all of the money you are saving with the small motors and dual use computer to buy a stand-alone THC.

[Terry G]

Thanks for the info. Now I know what THC stands for! What you are saying makes a lot of sense. I fight with thin material all the time on my CNC mill. I was planning on making a floating torch holder with conveyor balls to set the heigth, similar to the one Weldtutor made. That seems like a pretty solid way to keep the torch at the right height. I guess the only drawback to this method is if the material has any imperfections in it causing the torch to move around a bit. Don't know if this is really a problem.

Thanks again.

[Torchhead]

Most people think plasma cutting is just pointing the torch at the metal and pulling the trigger):-)

To get consistantly good plasma cuts and not spend most of your time and money replacing tips and re-cutting parts you need to understand the correct cut sequence:

1. Piercing should be done at about 2X the recommend cut height. Lower than that and you get splash-back of molten metal into the tip....instant destruction if it hits right. So the sequence needs to include knowledge of where the top of the material is by either a probe or having the control software track all of the Z moves.

2. Pierce delay. Plasma cutters do not always fire instantly. If they are "cold" without purge air running, the delay can be a second or more. The thickness of the material also dictates the required delay. You can program in a generic delay to cover all circumstances but it will leave big old divits at the pierce. The better approach is to have a signal that will pause motion until there is a valid arc. (Arc Good)

3. Proper cut height. The cutting height is closer than pierce and is critical to maintaining a consistant cut with constant kerf width and minimum slag generation. A drag tip that is spring loaded will work well for larger non-complex cuts. For decorative type cutting with pieces that fall out, be prepared to have your drag tip drop into a void and drag your sheet around the table. Drag shields are a compromise and don't allow you to get the tip as low is required (.063 or less) to do qualtity cutting. Some tips like the Fine Cut for Hypertherm use gaps in the .040 or less range!

2. Smooth, constant motion. Any pauses in a cut leaves a "notch" in the cut wall.

3. Proper feedrate. Probably the most critial besides the cut height. Feedrate varies by material type and thickness ,the cut gap and the cut current. Feedrates for different material goes from 35 IPM to 350IPM. The thinner the material and the bigger the tip, the faster you have to go. A plasma flame is "hungry" it will reach out and cut metal out of its track if the primary cut path has been vaporized. It dances back and forth to the sides of the cut and continues to vaporize the material. The thinner the metal the faster this happens. At some point it can no longer jump the arc and flames out. Most plasmas will try to restart but only after a delay and only if there is metal to cut.

Being able to adjust all of the variables as you cut then retain those settings makes the process repeatable and helps you turn out consistant production.

If your intent is to cut out some occasional shapes and not count on the machine for production then don't waste your money on a THC. If the accuracy of the cut and consistant quality are important then your design needs to include a method of automated torch height control.

Tom Caudle
www.CandCNC.com

[Terry G]

As you might know, I am very new to the whole plasma cutting process. So this information is very usefull. I will most always be cutting small parts thats why I am building a smaller size table.

So it sounds like I will need a THC. I use my CNC mill for everything now which includes alot of 1/8" and 1/4" material. I would like to use the plasma cutter for these pieces and leave the cnc to do more intricate parts.

Looks like I will be studying a bit more on CNC plasma cutting before I start building anything. I want to make sure I build something that will suite my needs.

Thanks.

[Terry G]

You choice for THC is to use a third party stand-alone system. Cheapest one out there that works is about 2000.00.

I've been looking around and found the THC300 which runs under $400.00. After reading some more posts on this site I found you reference it a few times.

Is this a good lower cost alternative for a hobbiest as myself? I don't have a problem assembling or understanding electronics. I read through the TCH300 manual and it seems pretty straight forward. The biggest thing is learning the Mach3 software, and figuring out if it would work with my Deskcnc controller. Though I beleive you did mention that a THC system may not work with the Deskcnc controller.

Anyway, just something I came across.

Thanks.

[Terry G]

Torchhead,

I appreciate all your great advice. I have been researching on and off today about THC's, and re-read the THC300 manual to try and understand the whole process a little better.

I do most of my cuts in 1/8" and larger material, and sometimes may use 1/16" material. I would typically be cutting smaller pieces. From my experience on my CNC mill, a length of 10x4x1/8" flat stock usually has less than .010 to .015" of warpage. So could I setup my software with a fixed delay to pierce my workpiece at the proper piercing height (on the outside of the actual cut), then bring the torch to the recommended cut height, and proceed with my cuts. This would require small manipulatoin in my autocad drawings so I would never pierce the workpiece on an actual edge of my final piece.

So if I were cutting a straight line 10" long in 1/8" material with the proper IPM for my machine with my torch set to .063, would I see a big difference in the kerf size from a .010 to .015 change in torch height due to material warpage? If so, would you have any idea of what that would be?

I am not going to use this for any type of production use, and usually just to make parts for the custom projects I do. So I'm just trying to understand the actual gains I will get with a full THC setup over using what I already have.

Thanks.

[Torchhead]

The warpage comes form cutting with a 30,000 deg flame not from embedded material stress. I have seen 1/16 bow and warp up to 3/4" in 2 Ft if several cuts are made close to each other. The warpage does not effect the kerf width as much as cause head crashes and blown tips.

If you are going to use Autocad for generating the lead-ins and piercing than you might as well consider hand coding and modifying the g-code the get the pierce sequence and feedrates you want.

Since it's not for production I would go on with what you have planned and see if you can get it to work to your expectations. I sounds like this is a casual project and no big deal if it doesn't work the first time.

[Terry G]

Ah yes, the heat.

Right now I just use autocad to draw the shape and save it into DXF, and deskcnc to generate the g-code. And with a little massaging I get what I want. Deskcnc does offer some lead-in, lead-out options, maybe I could get those to work. Though drawing a lead-in, and lead-out line in autocad seems easier. Then in Deskcnc I just tell it where I want to start my cut and end my cut. Anyway, I'm not to worried about manipulating the software to do what I want to do.

This is a casual project, and I just wanted to make sure I was headed in the right direction. I can always design the table to accept a different type of torch holder that can be used with a THC in the future. It is good to hear that cnc plasma cutting can possibly be done without a THC. But is it going to be good enough for me? I'll soon find out.

Now I hope my timing belt gear reduction will work. I really have no idea as to what type of stress the timing belts can handle before skipping. I'll just have to experiment since every table is different.

Thanks again for you help.

[Torchhead]

Timing belts won't skip unless they are VERY loose OR you strip teeth off the belt (Not easy to do).

I did the initial design on the THC300 and it's a simple but effective THC with knobs for the height setting. It's a "dynamic" control meaning you have to set the controls as you cut. It comes from the reseller completely assembled and tested. It even has the interface card for the plasma unit and a Current Sense transformer to generate the Arc Good for controlled piercing. Coupled with MACH3 it offers a comprehensive THC solution with MACH being integrated with the control and active in the cutting process rather than stand-alone.

The DeskCNC pulse card will not work with MACH and you would use MACH3 as the controller software. You could continue to use their servo control cards and motors. There were some rumors that DeskCNC was going to make their new USB pulser interface card compatable with MACH3 as a plug-in but have yet to deliver that as a solution.

Tom Caudle
www.CandCNC.com