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question about velocity,acceleration, pulse rate

Posted: Thu Feb 23, 2017 4:55 pm
by Jtucker7000
I've seen several posts with questions about Velocity and Acceleration settings, but I'm still struggling on understanding what those numbers mean and what are realistic values. I'm using Mach4 but the mach3 settings seem similar. I have my table moving and cutting I'm just having issues with the cut having a slight wave to it. Its more pronounced on curves, and lead-in always leaves a dimple, its not terrible but not good enough. It was suggested that I needed to increase acceleration to fix this issue, or tighten the PID loop. I would like to understand what is a reasonable set of values and what they represent. Currently I have velocity at 300 and acceleration at 25 The pulse rate at 25000hz. My understanding is pulse rate dictates maximum acceleration? Velocity sets the maximum speed the table can operate at and the acceleration is the time it will take to reach the max velocity. If that's correct 25 inches per second squared seems fast but I'm guessing I'm misunderstanding what that means. I assume I could keep plugging in numbers until I get better or worse results but I'd prefer to understand what they mean and the relationship. Looking at what others use in a variety of applications is confusing I've seen acceleration numbers as low as 1 and as high as 4,000. I need a frame of reference to what is typical in plasma cutting. I assume table specs are important to help with my question and are listed below. Thanks in advance to anyone willing to attempt an explanation basic enough for me to understand.



4x4 table (plasmacam Retrofit)
Ke-ling 350 oz/in servo motors (hopefully an appropriate size oz/inch means nothing to me)
gecko 320x drives
Gear rack and belt reduction....I don't remember the ratio off hand but I can get it if its relative.

Re: question about velocity,acceleration, pulse rate

Posted: Thu Feb 23, 2017 5:40 pm
by motoguy
The super high numbers (4000) are usually metric (mm). The lower numbers are usually in inches. I work in inches.

According to Jim Colt and Tom at CandCNC, you really need to be at about 35 ips^2 (inches per second/per second) on plasma for good cut quality on details, corners, non-straight or long-curve pieces. If you have a machine capable of that, you should be able to handle most materials and jobs (as far as acceleration requirements are concerned). Acceleration is going to be related to the weight of your gantry, the rigidity of your setup, and the type and strength of motors you have (stepper/servo). A heavy gantry will need more powerful motors to accelerate/decelerate at the same rate as a lighter gantry. If your mechanical setup isn't rigid enough (material type, mechanical connection type, drive type, motion type), it will wobble/flex as it accelerates, which will show as wobbles in cuts, or lack of following the cut path. All these things interact.

You state that pulse width determines acceleration speed. Yes, and no. It's a factor. Acceleration is determined by many things, though. Your pulse width, your stepper rpm, your gear reduction (or whatever mechanical advantage is used transferring the RPM at the shaft of your motor to actual movement of the gantry), etc.

As far as the cut speed...I guess that's really dependent on what you're cutting. I think 325ipm is the fastest "book" cut speed given in my Hypertherm PM85 cut chart. That's basically 18ga and thinner, IIRC. My machine (Bulltear / Star Lab) has 500ipm rapids, and I cut at book speeds. I have the stepper motor system. The newer versions of my table (6'x12') are servo based, and have much faster rapid speeds (I think Matt said 1500ipm rapids).

Just a few things that came popping out of my brain.

Re: question about velocity,acceleration, pulse rate

Posted: Thu Feb 23, 2017 6:16 pm
by Jtucker7000
The fastest cut speed I've been using is 180 but most of the time I end up using much lower speeds to get better results. I'm cutting 14 gauge mostly and occasionally 1/4 mild steel. I used 300 for the velocity setting arbitrarily, what should that number represent? Maximum speed for rapids or max cutting speed?

Re: question about velocity,acceleration, pulse rate

Posted: Thu Feb 23, 2017 7:13 pm
by tcaudle
A = F/M (old physics formula) Acceleration =( Force/Mass) X efficiency

So what are the units" Well acceleration is a change in speed over time. It states that in the first unit if time (sec) it changes from 0 to the stated speed. After the first second it accelerates from the speed it is then to that much again and so forth. If you define a speed you want to hit you simply calculate how long that will take at the given acceleration rate. You can also express the numbers in "g's" or mili- g's . You can talk about acceleration on earth without using the magic number for acceleration of gravity of 32 Ft/sec^2 If you are sitting on a horizontal surface and the 'Mass" (weight of the gantry on earth) is not moving you have to apply a FORCE in order to accelerate it to a given vorticity at a set time period.

So far its just numbers. it simply says that for a given weight of gantry it takes a certain amount of force created from the torque of the motors and applied to the load though a transmission. Imagine two cars sitting at a stop light and both have 100 hp motors the same transmissions and both are in first gear. If one is a ponderous 3 ton 1973 Cadillac El Dorado and the other is a 1500 lb dune buggy which will reach 50 ft first? Which one can make a sharp turn and accelerate out of it quicker? Now put a 600 hp turbo charged engine in the Cadillac and do the same test.

So to sling a gantry around at high speeds you need a motor big enough keep the acceleration high enough. The heavier the gantry the bigger the motors you would need to hit the target acceleration. So the target of 35 iPS/sec says you can go from 0 to 35 ips in one sec. 35 IPS is 2100 IPM.
If you target speed is only 300 IPM than you hit that in .142 seconds . So how far have you traveled in .142 seconds ? The math gets a little hinkey because its during a move from 0 to 300 . but at 150 IPM as an average you cover about .355 inces

Acceleration is most important when you are changing directions (in any axis). An Arc is a change in directions in both axis. So why not just slow it down from 300 to 100 and give it more time to make the moves? Well you can but in plasma it changes how the arc cuts. The kerf gets wider , the voltage starts to climb and you start to see more slag. If you have a THC it reads the voltage for the height and it plunges the torch to try and "fix" the higher voltage. As you get below 20 IPS/sec acceleration it gets noticeable. At 10 ips/sec it makes cutting over about 80 to 100 IPM and doing tight turns or small arc or circles an ugly process.

One of the stats that a table provider should offer is the max acceleration. It should also list the max CUTTING SPEEDS (not rapids) and that should include stats to profile that with the fastest speed you can cut at with the THC tracking a 25% rise/fall from horizontal. 25% is a 1" rise in 4" inches or run. Unless the acceleration is high enough you may be able to cut at 300 ipm but you won't be able to turn a corner .

SO if you have plodded through this if you can come away with acceleration is tied to velocity and time and is defined by the amount of Force you have and the amount of weight you have to accelerate. I also hope you realize that the number you are looking for has been determined using trial and error over years. So to predict the max acceleration you can expect you have to have the Force ,which is how hard final drive can push on the load and is a function of everything from the moro torque , the motor torque at the RPM you need to get to the speed you want . the transmission ratios. Then you have to know the total weight and covert that to mass units by dividing by 32. The final piece of the equation is the efficiency which typically is about 25 to 30 %

See...its simple! :shock: