This is one of the thinks, which i couldn't find solution for.
I did some research, people found some workarounds but i wasn't entirely happy about it.
When i clicked on the help it gave me some idea of what needs to be done.
This setup is for standard rotary axis which came with my device (on picture)
For the Circle pulse you will need Steps per revolution and Gear Ratio
For Diameter you will need Diameter of your 4th axis roller
For length of your drawing in RD works you will need circumference of your work piece
..so lets have a look at it in detail:
Step 1: Steps Per Revolution
First think you need to find out the steps per revolution of your motor driver. Thats the amount of pulses the controller needs to send to motor driver for motor to complete one rotation.
Open your (turned off) laser cutter and check the driver name and rotation settings.
The photo is not the best as the driver name is behind the cables but it is: 2M542, and the rotation setting of the DIP switch were like on the image table attached.
Rotation setting are the 8,7,6,5 position of the switch, in green on image.
You can find manual for your driver which will be somewhere on internet, this is for 2M542 : http://www.hi-end.ro/pdf/motoare/CMP5042.pdf, but i would say the drivers have some standards and it is probably the same. (but rather check)
In the documentation i found a match for the switch configuration of my driver and it was set to 5000 pulses/rev. (see image)
Step 2: Gear Ratio
You will also need the gear ratio of your roller.
You can measure the sizes of the gear or simply count the teeth.
In my case motor had smaller wheel with 20 teeth and roller had 32 teeth.
so the ratio is 32/20 = 1.6
This means the motor must rotate 1.6 revolution to rotate the roller 1 full revolution.
Step 3: Circle Pulse
So 5000 steps must be sent to motor driver for the motor to rotate 1 full revolution
5000 steps *1.6ratio = 8000 Circle Pulse
8000 steps to turn your roller one full revolution.
Step 4: Diameter (mm)
This is simply the diameter of your roller which in my case is around 2.5 cm
Diameter (mm) = 25
Step 5: Drawing in RD Works
The length of your drawing (y axis) in RD works will be: the circumference of your work piece
in my case
6.2cm * PI= 19.48cm (194.8mm) long.
Example image from RD works shows setup for 20mm wide 194.8mm long square.
So only think you will ever need in the future is calculating the circumference for your work piece, the rest of the data stays the same.
Note: it might be possible it will not be 100% accurate as the work piece might slip if it rotates too quickly or it is simply too slippery material or you measured its diameter not precisely enough or the roller is actually not 100% 2.5cm as it is kind of bumpy. So you might slow down speed and for the diameter size, you can tune it by changing the Diameter setting or even better as it is probably more precise approach the number of steps per rotation (circle pulse).
You might also look at the enable feeding option to extend the rotation of the y axis if your machine doesnt want to rotate the work piece as the origin might be rotated already too far.
Look at the video of the setup from the RD works image to see how it worked.
Step 6: Aligning Workpiece
This is not related to this instructable but it is just another use for the cross pointer from my other instructable.
You can nicely align the work piece, center it and find the edge.