[stucaruk]

Greetings,

I've been installing, retrofitting, and servicing CNC plasma systems for years and noticed that few companies, or individuals actually know how to size a drive system appropriately. Typically they appear to just guess, or make logical assumptions based on what has worked in the past. It seems strange that in todays marketplace, major companies are still pushing good old fashioned DC drives using resolver sync to keep 2 drives powering say the rail axis, square.

There are loads of really good AC servo brushless drives out there that use electronic gearing and I'd like to use this on a retrofit project I'm planning, rather than a traditional CNC Plasma system industry standard type drive.

I see some companies are now using Yaskawa Drives and I'm frankly amazed at how teeny tiny the drives are to whip these machines around at 1000 IPM. I'm not surprised though when I go to fix them to see the geraboxes torn apart. Usually the input shaft to the planetary gear reducer just gets ripped apart. Clearly the engineers calculated the inertial loads to accelerate and decelerate the gantry (one would hope) but forgot to compute what happens in an E-Stop situation when the gantry stops without the normal decel rate.

I have yet to see a drive sizing guide that takes into account dual drives either, so I'm looking for a bit of advise from someone knowledgeable in this area.

Essentially I want to size the drive and gearbox's for a gantry that weighs around 4000 pounds. It rolls on wheels down the rail axis using a drive on either side of the gantry in a master/slave relationship. The cross axis only moves about a 100 pound load, and will use a single drive. Both axis must be able to hit 1000 IPM in rapid, and contour around 2 - 300 IPM depending on whether we are cutting with gas or plasma.

It's easy to compute the gearbox ratios, by knowing the max travel speed and the rated speed of the servo motor. I can cheat the speed if needed by adjusting the pinion size. From the manufacturers tables, I can get data for the weight of the motor and drives as well as the inertia.

What concerns me is how one deals with the torque requirements to move the gantry down the rails and the master station along the gantry.

How do you estimate the torque required to move the axis on a single and dual drive system? How do you calculate the inertia of the mass that has to move?

How do you make allowances for the gear rack and the reduction gearbox?

My tendancy is to oversize the drive, since it will last longer, although I'm aware that going to big actually causes instability issues, and increased operating cost as well.

Any ideas would be helpful. Thanks,

Stu

[jimcolt]

Stu,

You are on the right track for plasma performance when talking about AC brushless servos and planetary/low backlash gearboxes. Most new industrial quality (read expensive!) plasma cnc machines are equipped today with AV brushless drives and good quality gearboxes....as well as linear recirculating way bearings....and precision racks or helical geared racks. In order to take advantage of todays high definition class plasma systems....drives have to be mechanically tight and have the ability to accelerate and deaccelerate quickly and smoothly......any imperfections will show in the cut.

I would suggest working with cnc control and drive manufacturers that work primarily in the Plasma CNC machine business. Yaskawa is a good choice......Hypertherm Automation produces some very high end cnc control and drive packages and can help offer advice for upgrading drives on older cutting machines.

Jim Colt