# Thread: Filling the machine velocity tab

1. ## Filling the machine velocity tab

Hi,

I would like to confirm some calculations of mine,to know if i am in the right path filling the machine velocities tab.I have servo motors with 1000 CPR encoders.The motors are rated at 2800 rpm.Let's do the math...To know how many steps per second are 2800 rpm, we have 2800/60=46.6 rounds per second.That means 46.6X1000cpr=46600 counts per second or steps per second.So the maximum velocity of my motors will be 46600 sps, right?
So why when i go to fill the tab "maximum velocity" and i put 46600 the graph has a large spike?(the manual of DeskCNC says to avoid a spike at the upper right of the graph).The only way to have a "small plateau" at the upper right is to fill the machine velocity tab with 4000 sps.That means that the maximum velocity of my motor will be 4000/1000=4 rps,4X60=240rpm?that doesn't seem right
Why can you have 46600 acceleration and only 4000 max velocity???
How about the start velocity what is this and why the value is so low???
Any ideas here?

Panos

Hi,

So many people use the DeskCNC program.None of them has ever calibrated the machine?

Hmmm...

3. Try this

http://www.imsrv.com/deskcnc/setup/

Fred Smith - IMService

4. I tried these values on one of my machine, and they work..( Mill) but you may have to reduce the sps a little if the servos fault on G00.
Acceleration Tab ( Machine set up)
acceleration = 4500000

accel scale = 30

Max Velocity = 45000

start velocity = 1500

This will give you 2700 rpm max, you can up it, but I have found that getting to the upper rpm limits will fault the servos, depending on the machine dynamics.

The time of accel with this set up is .286, and unless you have a real heavy machine ,should be enough. ( untill you change the Max velocity (sps) in the axis set up page you will get a warning "Max Velocities do not match "

AXIS SET UP PAGE

change the the Max velocity (sps) to 45000 on all axis used.(See comments below)

I assume from your math that you have figued the steps per inch ok ?

You can also do a "custom Accel graph" by going to set up, and the last one listed is "modify accel graph"..I'm sure after you look at it, you will be able to configure a graph the fits your machine.Takes awhile, but I changed mine for the Lathe project, made it real smooth.

After you get past the accel stuff, the system works real good..I dinked with the values 2 days before understanding what was needed on the 1st machine.

You can raise or lower the velocities as needed by the machine...The machine dynamics are all different, so to have an absolute number will not work.Typically, you would want to slow the Z axis way down as they usually are kinda short..just change the max vel(sps) in the axis set up page., Certinly the Y axis would react different than the longer ? X axis, so it might take some time to make your machine work right for you.

5. ## A little bit more???to get going...

Thank you for the interest!

I thoughted that i was alone on that.I already read the posts and i will take them into account.I will have to make some experiments with the servos.

Is it true that by executing G00 commands(by trial and error and changing the values) you can find the
optimum speed for your servos?Do you have,anyone, gecko drives?During the tests of the servos, the limit pot should be in full scale?
Example:My servos are 3.5Amps rated.I should keep the limit pot near this value or should i put the pot to 10Amps during the test.
I know that when i put the servos on the machine,i should make the final adjustments because of the moving masses of the axes.The reason not puting the servos at this time on my machine is that i want to know their behavior and how to reach their rated rpm,before i scale it down to suit the machine.

I look forward for a reply...
Panos

6. I would contact Gecko and ask them their reccomendations with the servos you have.

I always start with a conservative set up,then find the best setting for your machine..You do not want the machine to shake and shudder every time you have a direction change, or stopping or starting. If it is a new machine, there may be more issues than maxium IPM. Depending on the reduction to the ball shaft and the lead, full IPM might be way more than the machine will operate comfortably at.

You should have the ability to adjust the limit and gain with the servo amps installed on the machine.

The most important thing is to have fun..!

7. ## Seems to work!!!

Thank you very much!

The settings that you proposed work great.I have 2650 rpm(measured with my tachometer).I was a little nervous at start with the acceleration value that went to high,but i read in the DeskCNC manual that at high SPS values the acceleration should be way to high.So now i have a starting point and i can experiment with the acceleration graph and the scale.Trial and error(always on the safe side) should reveal the real intentions of the motors and the axes.

Thanks,
Panos Labrakis

8. That top number in the Velocity tab is not the actual number for max accel..actually the instructions say to put in the max.sps, an accel scale and a start sps , then some number up top and start adding zeros. The number on the top seems to be just a kicker number that sets the accel curve.If your tach is correct, your just almost at max. sps..

What is fun is to map your own accel curve after you get your machine up and running reliable , your back lash set etc.

9. ## Can i get more?

Hi again,

I have some questions about DeskCNC.What the "buffer output" does?
What means "Meter in backlash comp" and why when i disabled it the backlash steps started to work as they should be?And one last question.Backlash compensation works also when you jog the machine?By the way i have 20 steps backlash compensation in the Z axis(so i can have repeatability).This value considered to be high?

Panos

10. Sorry, missed this post then had a little stay in the hospital, will answer tomorrow morning.

11. ## Setting accelerations in DeskCNC

Hi Panos,

While you are able to set the accelerations within DeskCNC it is much easier to do it with a spreadsheet like Excel. Open any one of the existing .acc files and you will see that it is entered in plain text. Use the spreadsheet to calculate the in-betweens given starting and ending speeds, there is also an 'S' curve acceleration tool available, mail me and I will send it to you.

Regards
Greg

12. ## Father time

Sorry to hear that you spent some time in hospital.I dont like hospitals because they remind us that health is fragile!!!I hope that all went good...
Life is short...sniff, OOh father time you re so ruthless...............

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