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Mach Discussion => General Mach Discussion => Topic started by: c30232 on December 02, 2012, 04:27:55 PM

Title: x & Y speed requirements
Post by: c30232 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?
Title: Re: x & Y speed requirements
Post by: BR549 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
Title: Re: x & Y speed requirements
Post by: c30232 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.
Title: Re: x & Y speed requirements
Post by: BR549 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
Title: Re: x & Y speed requirements
Post by: c30232 on December 02, 2012, 08:18:02 PM
We shall see on Tuesday!
Title: Re: x & Y speed requirements
Post by: RICH 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

Title: Re: x & Y speed requirements
Post by: c30232 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. 
Title: Re: x & Y speed requirements
Post by: BR549 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
Title: Re: x & Y speed requirements
Post by: RICH 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
Title: Re: x & Y speed requirements
Post by: ger21 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?
Title: Re: x & Y speed requirements
Post by: RICH on December 03, 2012, 05:18:22 PM
Quote
what is the useful upper rpm limit

Velocity, accel, torque, rpm are all related thus each has a bearing on each other and is system related.
It all depends on the motor and other parts of the system. In the end  reliability / consistancy for what your doing is
just as important as.

Just want you to uderstand the whole picture.

RICH
Title: Re: x & Y speed requirements
Post by: c30232 on December 03, 2012, 06:24:25 PM
Gerr & Rich

Gerr,
The specs on the steppers remain top-secret.  The seller/builder can’t seem to get me the information nor can he provide a tracking number.  Gotta love eBay!  It is rumored that the machine will arrive Tuesday.  Hopefully the mystery will end there.  The picture of what I believe I bought shows rack and pinion on all three axis. No specs on their ratios either.  Arguably, it will have larger steppers than what we are messing with.  The power supply is 36V.

Rich,
You are correct we will do both, live with what we have or will soon have and define what we require then seek it out.  Remember the goal is to learn motion control to prep for a re-control.  I don’t really need a plasma table but it looked fun.

I have made a few attempts to re-control a router but bailed out from the frustration.  The last attempt was in 2002 with a brand name control that never worked.  In follow-up with the manufacturer a week ago I learned that:

1) The system was incomplete and therefore will never work

2) It was registered to a company that is not mine

3) The person who it is registered to is an employee of manufacturer that I bought it from

How’s that for a scam?  It also explains why support (the same employee) blamed us for its failure and, well, wasn't very supportive.

If I really have a need for a machine I just buy it.  In this case, while banging about the net, I saw a plasma table sans its control for sale.  I bought it.  May as well learn to control it, right?  I have a zippity-doo-dah-whiz-bang plasma cutter that has 5 minutes use on it. The cutter even has a machine torch and switch gear of some sort.  It all seemed like a marriage made in paradise.  All we need is some motion.
Title: Re: x & Y speed requirements
Post by: c30232 on December 03, 2012, 06:52:10 PM
Rich,

Thanks, I believe I get the larger picture.  I understand that there are numerous variables including ambient temperature.  I can see the top of the mountain from here I just need a little help finding the trailhead.  I think now we slap together what we have on the shelf and see what it’ll do.  From there we can throw some science and money at it.
Title: Re: x & Y speed requirements
Post by: ger21 on December 03, 2012, 07:03:57 PM
Quote
The specs on the steppers remain top-secret.  The seller/builder can’t seem to get me the information nor can he provide a tracking number.  Gotta love eBay!  It is rumored that the machine will arrive Tuesday.  Hopefully the mystery will end there.  The picture of what I believe I bought shows rack and pinion on all three axis. No specs on their ratios either.  Arguably, it will have larger steppers than what we are messing with.  The power supply is 36V.

In a lot of cases, changing to larger steppers can actually result in even lower speeds. Generally, as steppers get larger, their maximum speed decreases. That's why it's important to know everything about the drive system. Without knowing all the information, it's hard to determine where the problem lies. It could be the steppers, the drives, or the drive system (poor gearing). It could even be caused by misalignment, which can cause friction and binding.
Title: Re: x & Y speed requirements
Post by: BR549 on December 03, 2012, 07:38:29 PM
SHees I gave you  the important numbers target Vel and accel model numbers. Just apply those numbers against the weight the WEIGHT of your gantry.

To get best results you gear the axis for the ratios that give the best responce for accel/speed. It does no good to be able to rapid at 1000 if your accel never lets you get there cutting.

Acell in plasma IS the most important so you juggle gearing to get teh best torque at teh rpm range you plan to run in without stalling the motors

Next teh Z HAS to be fast accel/speed wise in order for the THC to keep up responce wise  to slow and you drag the tip a lot, too fast and you get into oscilations from the axis outrunning the thc and overshooting.