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Offline c30232

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x & Y speed requirements
« on: December 02, 2012, 04:27:55 PM »
What are the target x&y speeds for a plasma table?  I know . . . it depends.  But what is a ballpark top speed required in IPM for cutting light gauge sheet metal?  Maintaining a NEMA 23 stepper, what is the useful upper rpm limit?

Offline BR549

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Re: x & Y speed requirements
« Reply #1 on: December 02, 2012, 05:08:12 PM »
MAX 400+ IPM for XY.     THe important test of your system will be ACCELLERATION.  The faster the better or you may experience corner rounding.

ALSO the slower the accel the slower the machine will top out at when running.  In short segments the accel will limit how fast the machine will move. This will effect cut quality as the plasma is very dependant on constant vel for quality of cut.

With any normal size plasma table nema 23 will be tough to make work well. EVEN the Z will need to move very fast and accel very fast to properly allow the THC to keep up.

Just a thought, (;-) TP

Offline c30232

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Re: x & Y speed requirements
« Reply #2 on: December 02, 2012, 05:37:31 PM »
Thanks, I just started twisting a small NEMA 23 with Mach and a Gecko G540.  Very slow it is--thus causing the question.  I have in transit a 2x2 table with NEMA 23s but the manufacturer can’t or won’t provide the specs.  I will know when it gets here. The table looks okay otherwise.

Where would I start to calc a more proper sized stepper?  Hopefully, one that will work with the G540.  I understand the drivetrain math and just the theory of acceleration.  An acceleration number for Mach would be appreciated.  I am really lost on determining the RPM of a given stepper.

Offline BR549

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Re: x & Y speed requirements
« Reply #3 on: December 02, 2012, 07:55:50 PM »
Too late to worry about spec NOW that the machine is on its way. With  2x2 table and a lite gantry you may be fine.



(;-) TP

Offline c30232

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Re: x & Y speed requirements
« Reply #4 on: December 02, 2012, 08:18:02 PM »
We shall see on Tuesday!

Offline RICH

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Re: x & Y speed requirements
« Reply #5 on: December 03, 2012, 06:44:08 AM »
Acceleration is how fast an axis can go from no movement to some velocity in a time period. It is linear since it’s distance in a sec. It is also how fast it can increase from a current velocity to a higher velocity. The acceleration is determined by the ability of the motor to overcome the inertia of the axis. The axis usually changes circular motion to linear motion and as such it is made up of couplings, gears, belts, pulleys, a screw ( or rack and pinion), and the motor itself.
So to accelerate the motor it must have enough torque to overcome all the resistances which are trying to keep it from rotating.
A measure of that resistance is called inertia and all rotating components have it in motion and is based on their mass. Additionally other forces exist which add to the total force the motor must overcome ie; friction, efficiency of the components. You can increase the end user torque by gearing up or down, providing more energy to the system, or changing the system makeup ( different screws or pitch), etc.

The longer the time to accelerate to some velocity the flatter the angle will be in motor tuning. It takes a lot of  Force to move a big mass from standstill to a high velocity  in a short time period. The amount of torque / force available from the motor is not infinite
within the motors capability. It takes energy to rotate the motors shaft.

 That said , you will need to find an appropropiate accel value when tuning your motors.

RICH

Offline c30232

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Re: x & Y speed requirements
« Reply #6 on: December 03, 2012, 12:05:51 PM »
Thanks Rich.  I must be missing something.  As I said, I understand the drivetrain math and physics relating to acceleration. So whether the mass, friction, and resistance are large or small doesn't matter to the torch as long as the desired acceleration can be achieved.  What I don't know is how fast the torch should accelerate to provide a pleasing and eye-catching cut. Can you tell me from experience what the starting/target acceleration number should be for Mach while cutting thin sheet metal.  If I had that number then the math of acceleration would be of use. 

Offline BR549

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Re: x & Y speed requirements
« Reply #7 on: December 03, 2012, 02:33:48 PM »
20-35 IPS/S gives good cuts. The faster the better.  A lot depends on what the motor torque is and how much the gantry weighs VS overall gear ratio.

(;-)TP

Offline RICH

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Re: x & Y speed requirements
« Reply #8 on: December 03, 2012, 05:08:53 PM »
Quote
............doesn't matter to the torch as long as the desired acceleration can be achieved

So if it can't do what you want then what?

Don't know about using a cnc torch set up so use what others with experience  recommend.

Find the max velocity your machine can do with a low accel value and see where it starts skipping and back off by say 30% ( whatever), then start increasing the accel value at the 30%  velocity setting and see where it skips. That can be done quickly and will give you
a ball park figure for your machine.

There are programs that you can use to calculate but even then you still test.

Thus it would be acceptable to say the following :

In practical applications one defines an acceleration desired and finds what is required to achieve it ….or ….. accepts the value based on some other parameters. One could say that they are either designing a system to achieve the task or they accept their system “ as is “ and the acceleration is defined by what they have.

RICH

Offline ger21

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Re: x & Y speed requirements
« Reply #9 on: December 03, 2012, 05:15:33 PM »
Quote
Thanks, I just started twisting a small NEMA 23 with Mach and a Gecko G540.  Very slow it is--thus causing the question............................................................Where would I start to calc a more proper sized stepper?  Hopefully, one that will work with the G540.

What are the specs on the motors you have?

And what is your power supply voltage?

And how is the machine drives? Screw, or rack and pinion? If screws, what is the pitch? If r&p, what is the gearing and pinion size?
Gerry

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