1. ## How fast do you accelerate your gantry?

I have designed a heavy aluminum gantry I might have to change. What do you accelerate your motors at? Based on my weight and motors. I will be accelerating up to 200 in/ min at around 30 in/ s/s. I will be plasma cutting with a hypertherm pm65 various gauges.

2. I was cutting at 75 - 100 IPM with a small chinese plasma cutter and at that time the acceleration was 25 in/s/s. When I changed over to a PM65, I increased the cutting speed to 250 IPM (16 GA) but didn't increase the acceleration. The corner cut were terrible. Most were large curves. Most of the advise I got here was to increase the acceleration as high as I could go even if I had to decrease the speed. My machine is maxed out at 225 IPM with 40 - 50 in/s/s (can't really tell the difference between the two). I tried to reach the recommendations in the Hypertherm manual for light gauge but the table wouldn't run that fast.

With that being said, I've designed a new gantry for my machine that will be about 40% of the weight of the current gantry. I am also changing the reduction ratio from 4:1 to 3:1 in order to increase the maximum speed for lighter gauge steel. The high reduction ratio gives much more accuracy than is needed for plasma.

I think each machine is different and has to be dialed in over time to get the best results.

Willy

3. Please don't get offended but are you sure you got your sums right? The units can be tricky.

Ignoring any rotational inertia, here's a quick example...

A 100 oz.in motor moving a 50 pound gantry 1" per rev...

Driving force = 100 x 6.28 / 1 = 628 oz = 39.25 pounds force

Convert the 50 pound gantry mass to slugs 50/32 = 1.56 slg

Theoretical max acceleration = 39.25/ 1.56 = 25 ft/s/s

4. I don't know where you got your numbers for your calculations. My gantry weighs 36 lbs and I have two 265 oz steppers driving it. The new gantry should weigh between 14 to 16 lbs (if my calculations are correct).

My current speed numbers (IPM) are from the Mach3 settings and timing the actual movements of the machine. I don't know if the actual acceleration is actually what the settings are but the results are satisfactory with Mach3 set at 50.

Willy

• Hpw far does the gantry move with one turn of the motor?

• So wouldn't 25 ft/s^2 = 300 in/s^2 ? For a 50 lb gantry with a 100 oz in motor that seems high. Do you have to use slugs when calculating acceleration? I understand slugs are used when calculating the force due to gravity.

• Originally Posted by will_eng
So wouldn't 25 ft/s^2 = 300 in/s^2 ? For a 50 lb gantry with a 100 oz in motor that seems high. Do you have to use slugs when calculating acceleration? I understand slugs are used when calculating the force due to gravity.
If you drop a 32 pound weight, it accelerates downwards at 32 ft/s/s due to a force of 32 pounds force.

force = mass x acceleration
32 lbf = 32 lbs * 32 ft/s/s doesn't work
32 lbf = 1 slug * 32 ft/s/s is okay

The Imperial unit of force is the poundal but slugs make easier maths

1024 pdl = 32 lbs * 32 ft/s/s

To convert the motor torque into a force you divide the distance travelled in to 2 pi.

If you put a one inch radius pulley on a 256 oz.in motor you would get 256 ounces of force on the belt and one turn would move you 6.28 inches.

If your gearing actually moves you x inches then the force is 256 * 6.28/x

I only know this stuff because I do ballistic tech support for living and I am forever being asked to explain where the magic number 450240 comes from, it's the number of grains in 2 slugs avoir dupois

• "If you drop a 32 pound weight, it accelerates downwards at 32 ft/s/s due to a force of 32 pounds force."

Acceleration due to gravity is a constant (relative), there is something wrong with your sentance above.

• Originally Posted by Todd Zuercher
"If you drop a 32 pound weight, it accelerates downwards at 32 ft/s/s due to a force of 32 pounds force."

Acceleration due to gravity is a constant (relative), there is something wrong with your sentance above.
I know it can be confusing, but to convert a mass in to a force you have to put it in an arbitrary gravitational field.

To convert kgf to Newtons you multiply by Earth gravity in m/s/s

It's converting mass to force that introduces gravity to the equasion.

Stepper motor suppliers will sell you motors rated in Nm but the humble poundal is ignored. Torque has been measured in ft.lbf for so long it is too late to change.

• I was just trying to say that your sentance could have been a little less confusing to a layperson (like me). If you would have just used a different arbitrary weight than 32 so that it didn't look like the weight was determining the acceleration to morons like me (who can't remember without looking it up that the approximate accel due to gravity is about 32ft/s^2).

Saying: "If you drop a 20 pound weight, it accelerates downwards at 32 ft/s/s due to the force of a 20 pound force." would have been clearer.

• I agree with the use of slugs in calculating acceleration. This is what I learned in school as well.

So for my gantry I am looking at the following.

2 x 565 oz-in motors = 35 lb-in x 2 = 70 lb-in
belt reduction = 1.5, pinion rad = .375"
Gantry mass = 200lb /32.2 = 6.25

So force i can make with my drive is:

70*1.5 =105lb-in /.375in = 280lbf

solving for acceleration:

280 lbf/6.25 slugs = 44.8 ft/s^2

here is where the numbers dont match what people are setting in mach 3 and getting results for plasma cutting.

I would think 44.8 ft/s^2 would - 537in/s^2

humm?

• Easier way to work it out...

If you drop it 200 lbf accelerates it at 32 ft/s/s

So 280 lbf accelerates it at 32 * 280/200 = 44.8 ft/s/s = 537 in/s/s

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