Hi guys,

Sorry if this has already been covered! I'm converting a cnc router table to a plasma table. The existing drive mechanism is nema 34 stepper to a geartrain, which drives a belt system on the axis, both x and y the same. My worries are that the gear system is highly geared down (33:1 ish)and speed is slow presumabley to cope with the torque needed to drive the router head into material. My initial thoughts are to do away with the gearing and replace with a belt drive. I'm just wondering what type of ratio i should be looking for on the pulleys. The only problem i see is the weight of the gantry on the x axis it weighs around about 30kg, but moves easily on the rails. Can anybody please give any advice or their experiences. The table is an old gerber router table.

Thanks,

2. Let me start by saying I'm no expert by any stretch of the imagination when it comes to cnc but I have been reading this board for a while and researching the plasma builds. From what I have read, a good ratio to aim for would be between 3:1 and 5:1 depending on how much definition you need. A good person to ask for a definative answer would be Tom at Candcnc.com. They make the electronics packages to cnc builds and conversions. Tom is a pretty helpful and knowledgable individual.

Thanks for the reply! That certainly seems like a realistic ratio, been looking into it today and i think maybe i would also benefit from higher rated motors, which might help to compensate for the removal of the gearing.

If anybody has any experience with using a similar setup i would be interested to hear what results you had in terms of definition and also what type of stepper motors have been used etc.

Thanks

4. Here is the technical answer:

A pinion gear has a step-up ratio equal to PI (3.1416) times the DP of the pinion. The DP is the diameter at the root of the teeth but for these calculations just measure the diameter.

What you want to do for a good balance of speed and torque/resolution for plasma you need to try and cancel out the step up ratio of the pinion by using belt reduction (step down ratio)

A 1" pinion has a ration of 1 X 3.1416 or roughly 3. A belt reduction of 3 then gets you back to the raw motor RPM = IPM of the load and you have the full motor torque and resolution. Add in a little more (say 4:1 or 5:1 and you slow it down even more but buy some more torque and resolution (steps per inch). If you are driving the gantry with just one motor then go for the larger ratio. If you are using a dual drive (motor on each side) then the 3:1 will work fine for gantry weights up to 30 - 40 KG.

If the pinons are bigger in diameter then you need to adjust the ration accordingly. A 2" pinion has a 6:1 step up so you start to get into needing bigger belt reduction ratios. About the biggest practical toothed pulley ratio you can get with a single step (two pulleys) in the 5:1 range.
BTW belt drive systems using a pulley to drive the belts uses the same math but it becomes the diameter of the final drive pulley.

At 33:1 to get 300 IPM you would need to spin the motors 3,000 RPM it a 1" pinion. If the pinion is bigger then your RPM would be lower to hit the same lateral speeds.

A stepper will spin (based on the drive voltage) from about 500 RPM to 1000 RPM. We use 600 RPM for 48V based units and 800 RPM for larger 65V based units as target numbers. So you have somewhere between 600 to 800 IPM top speed (rapids).

Your goal should be to operate the motor while cutting at no more than 50% of it's max RPM.

As you can see from the above numbers you trade speed for torque(acceleration) and resolution.

With steppers there is a diminishing set of returns. The faster you spin them the less torque they have so ratios too high that force you to spin the motors really fast don't gain you back a direct ratio of torque. You do get to keep the resolution.

33:1 on the existing table (?) would indicate you are cutting at really slow IPM rates OR the motors are servos which spin a lot faster than steppers (and don't lose torque with RPM). What are the maximum rapids you can hit now?

Steppers have come a long way in just the last 3 years. You can get smaller (23 frame) steppers that have over double the torque their older cousins had. Be careful though because you may not get as much upper RPM out of a higher torque motor. If you use the numbers I quoted for the steppers and work backwards so you can hit at least 200 IPM at the 50% RPM point you should have a nice cutting table.

Plasma is non-contact and you would think it needs very little torque but it does need good acceleration to cut sharp corners and tight radius turns while cutting at 100+ IPM so torque is needed to battle inertia and provide acceleration.

TOM caudle
www.CandCNC.com

5. I have just completed a belt drive machine that uses a 4:1 reduction. See my thread - a different machine - downdraft water table. I used a 10 tooth drive pulley with a 40 tooth final. Got the pulleys from econobelt. Good prices and good pulleys. The 4:1 reduction gives about .0005 per step using a gecko 540 with the microstepping. This resolution is much more than needed with plasma. One problem was that the drive pulley is aluminum while the final is steel. VERY heavy. The largest they make in aluminum is 30 tooth but this should be more than sufficient for plasma.

I don't know what speeds you want but I have cut at 150 IPM and could push it much faster if required.

Willy

6. I keep seeing statements like this....and I don't mean to knock the poster....however, the better the acceleration and the smoother the motion...the better cut quality you will recieve from a plasma.

Think of the plasma while it is cutting along on a piece of steel....it is producing very constant output power in killowatts while the machine is moving at a constant speed. When the motion de accelerates (such as into a 90 degree turn) it actually slows down....the plasma continues producing constant current, but as the machine motion slows, the cut kerf burn wider, the plasma power supply increases voltage, and the output kW increases. Kerf gets wider in corners and dross (low speed dross) forms on the bottom of the cut.

The above is solved with drives that have better accell / decell.....so that the speed during intricate contouring stays more constant. Expensive industrial machines that are designed for precision cutting with plasma (or laser....same thing occurs!) use inertia matched drives that are usually AC brushless servos coupled with low backlash planetary gearboxes to keep motor speed in peak efficiency ranges for the process. Those building machines with steppers should pay attention to maximum acceleration and minimum backlash in their designs as it does affect cut quality.

Smoothness of drives.....stepper drives tend to exhibit a slightly rougher cut edge with todays plasma systems as compared to most servo drives. Microstepping certainly minimizes this, and from my experiences belt drives seem to minimize the roughness as well....likely due to absorption of some of the cogging.

Just though I'd bring up the fact that smooth motion and excellent acceleration characteristics make a big difference with the plasma process.

Best regards, Jim Colt

"The 4:1 reduction gives about .0005 per step using a gecko 540 with the microstepping. This resolution is much more than needed with plasma. One problem was that the drive pulley is aluminum while the final is steel. VERY heavy. The largest they make in aluminum is 30 tooth but this should be more than sufficient for plasma."

7. I did not mean to imply that high resolution was bad but that in most cases it is not necessary. I look at a lot of the (home) machines that are offered commercially and they are using rack and pinion with no reduction. What is their movement per step ? It's probably in the range of .001 to .0015 and folks who have them are quite happy.

My other point was that at a 3:1 reduction, I could have gone with all aluminum pulleys and saved 8-10 lbs of weight off the gantry. My gantry weighs around 55 lbs so this would be a significant savings.

I agree that the smoother the movement and acceleration, the better the result but I don't think the difference between a 3:1 and 4:1 reduction will be that noticeable if at all. Just my 2 cents.

Willy

8. You are right on this type of machine. As a plasma guy though....when edges are not cut perfect, holes are out of round, and there is bad angularity and dross....the cutting process often gets blamed! In some cases it should be blamed.....but the majority these types of issues today (assuming you are using a good technology plasma) are caused by height control, motion, acceleration, speed changes and roughness or vibration in motion. Many of the edge variations we will see with plasma will not show up with routing or milling.....but with a very fast cutting thermal process they do.

Jim

9. You need to pick your motor and let us know the specs as well as the RPM it outputs. Then it can easily be calculated.

I'm not a plasma guy, but what kind of plasma do you have? I'm guessing the power output effects how fast you can cut, as well as what you plan on cutting. That question is better suited for people who own plasma machines.
However i can help you a bit with the gearing, might as well throw my 2 cents in with everyone elses.

(RPM / Gear Reduction) X (Pulley diameter x PI)

I.E.

1000RPM, 5:1, output pulley size 1", Pi=3.141 (for our purposes)

(1000 / 5) = 200RPM

(1" x 3.141) = 3.141"

200RPM x 3.141"= 628.31 Inches per minute.

Thats basically all you need. If the pulley is single or double flanged make sure you use the outer diameter of the actual gear teeth and not the larger flange diameter.
(If the gear ratio is 4:3 simply convert to decimal form 4/3=1.3333)

If you know the RPM your motor has, and the inches per minute you want, then you can easily figure out what gear ratio you need.
(Output Pulley diameter x RPM) / MPH X 366 (<== just a constant number)

1Mile per hour = 1056IPM
.75miler per hour= 792IPM
.5 mile per hour= 528IPM
.25 mile per hour= 264IPM
(must use decimal form if lower then 1MPH, dont use IPM or the formula wont work)
I.E.
1" face diameter output pulley x 1000RPM / 1MPH x 366 (constant)
= 2.73:1

So if i wanted my machine to be able to do 1056 IPM max rapids with a 1" diameter pulley attached to the stepper turning at 1000RPM you would need the other pulley to be 2.73 inches in effective diameter.

You can also use the number of teeth each pulley has to setup gear ratios.
say your output pulley has 10 teeth. you will need to find a pulley with 27 teeth to accomplish a 2.7:1 ratio.

This would net you 1163.55 IPM, slightly above your targeted goal of 1056.

Below is a gear pulley calculator. It has helped me a lot.

http://www.sdp-si.com/Cd/default.htm

Hope that was not to much, and it helps you out!

10. The majority of plasma cutting is under 200 inches per minute. Occasionally very thin materials are cut at up to 450 inches per minute....with loose tolerance expectations (26 ga galvanized for ductwork).

So I would shoot for 200 ipm, if you can go faster just for traversing from one part to the next...that can improve productivity.

Jim Colt

11. There is only one thing wrong with the numbers quoted (.0005 resolution). You cannot count microstepping as true resolution gain. It varies with speed and all Gecko drives have step Morphing (moves to full step mode at higher RPM) because you will lose upper torque at higher RPM if you leave microstepping in place.

To understand my statements you have to understand what microstepping is. You only have two coils in the motor. Microstepping is the process of making the rotor "hover" between poles by using opposing currents in the opposite winding. In essence you pulse it forward and backward at the same time to make it stop between poles. The faster the rotation the harder it is to hover. At some point it's just a waste of energy. The major advantage of micro stepping is it smooths out the rotation (at lower RPM) , decreases vibration and lost steps from motor resonance.

I know a lot of vendors quote resolution numbers using the 10X microstepping number but at typical plasma speeds it's not a true hard number.

Jim is correct that there are a lot of factors that contribute to smooth cuts especially on arcs and corners. One of the biggest is CV (constant velocity) where the cut line nodes are smoothed so you don't get jerky motion. What CV does is allow a certain deviation from the toolpath to smooth out the cut. The final attributes of the motion control and it's ability to accelerate and not mechanically flex and vibrate, predicts how close you can stay to the actual toolpath. The quality of the artwork going in, will effect how much CV has to "fix" to smooth the cut. That is why DXF files exported from some packages that do not do arcs (break all curves into lots of line segments) need a system with CV or they shake like dog crapping peach seeds!

In the end, the resolution of your drive needs to be about 10 times what you final target accuracy is. If you have a resolution of .001 (1000 steps per inch of linear motion) then all of the other errors (flex, backlash, etc) Multiply on top of that. It's true that air plasma has a flame that is "floppy" and holding + - .010 is a challenge but that error is Multipled by the other errors. So to say i don't need anything better than .10 inch accuracy is allowing for something like .025 deviation on the cut. You can see .025 stair steps on an arc cut.

Most plasma cutting is not that critical. If it's parts for welding together you can live with some pretty wide tolerances. If it's decorative, who knows what size it is supposed to be ? It will show up on smaller text where the eye is expecting a specific shape. Personally I think if you can build a table with at least .005 accuracy at the torch head you can cut just about anything you want.

One of the reasons I am a big proponent of 4 motor (dual gantry motor) drive systems is it resolves a lot of the issues. It prevents the racking and vibration you get from a single side drive and it doubles the available torque (acceleration) you have on the heaviest moving component. You can make the gantry lighter and that adds to the acceleration number. All of that for the cost of one motor motor and driver.

Tom caudle
www.CandCNC.com

12. Very well put Tom. I now understand the microstepping a bit better. I have worked with servos all of my career (I wasn't paying for them!) so I am not well informed with regards to stepper driven machines. I am thinking I'll have to build one this winter to do some comparative testing.

Jim

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