machine tool simultaneous axes control

[BobM3]

I watch some of the machine tools in our shop and am amazed at how tight control loops must be to perform some of the operations. Rigid tapping requires the spindle and the vertical stroke to be in sync very tightly. Even more impressive would have to be a gear hobber where 2 spindles have to be tightly in sync with each other as well as the moving table.

Are there any good books out there (or links to good technical papers) that talk about control of muliple axes and how well it can be done? I tried GEFanuc's web site but couldn't find any good technical info.

 

[itsmoked]

Probably not as that is some hard fought engineering stuff that will likely be held close to the chest of all the makers.

I agree though amazing stuff. I was looking at some Utube stuff where they were cutting hex flats on a rotating piece of stock using a rotary cutter. As the two met in tight synchronizm the flats just rapidly appeared with everything rotating rapidly. Really amazing.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MikeHalloran]

The spindles may be mechanically synchronized. Rotary broaching is old tech on screw machines. Screw machines are old tech; they predate electric motors by decades. They are "programmed" with cams and with specialized tooling like this:

http://www.championscrew.com/slater.htm

;---

Rigid tapping is pretty cool. Thread milling is pretty cool, too, and much faster.




Mike Halloran
Pembroke Pines, FL, USA

 

[cswilson]

It's fun and amazing stuff (and it's my field), but the basic concepts are pretty simple. There are really two parts to it.

The first is the multi-axis trajectory generation. The controller must take a basic move description (usually in "G-codes" for CNC controller), generate equations of commanded motion for each axis from the description, then solve these equations at high rates for each axis for each small increment along the path.

The second part is servo loop closure. Each servo cycle, the controller takes the commanded position for each axis, compares it to the actual position read from the feedback sensor, and computes how hard to drive the motor to try to push the error to zero.

As I said, the concepts are simple, but the details are not. itsmoked is correct that a lot of these details are held close to the chest. There is a lot of academic coverage of the servo loop part, but very little on trajectory generation. I had to go back to first principles to derive everything there when I entered the field.

Some things that seem tricky are not so difficult once you start to think about them. For example, the equations of motion for rigid tapping are really the same as for a diagonal line in a plane.

Curt Wilson
Delta Tau Data Systems

 

[blacksea]

Some motion controllers have special functions for few axes sinchronization - lok for "electronic gearing" for any amount of axes or "gantry" for two axis.

 

[richs]

Hiya-

There is a "hobby" user's group on yahoo that might be of
interest to you.

http://groups.yahoo.com/group/E-LeadScrew

From their "splash page":

Description
"ELS" - Electronic Lead Screw - is an alternative to full CNC for simple bench lathes - especially those lathes which do not have a "Quick Change Gear Box". For such lathes, it would be very useful to have a cheap method of sychronizing spindle speed to lead screw speed. If this can be realized electronically, then all threading can be done without the necessity of changing manual gears. Also, all saddle traverse speed ratios become possible.

The aim of this group is to develop a simple, cost-effective electronic and mechanical system for implementing ELS - which in theory would be cheaper than CNC, and which can be ported to hobby-class bench lathes.

You have to sign up to get on the list. I've just started
looking at it, and they seem like nice folks.

Hope this helps.

Cheers,

Rich S.