Lathe modernization concept 1

[Z0RAN]

Hi! I am really delighted to join your engineering community!

Generally, yet I’m a post-graduate at Ivanovo State Power University in Russia. I’ve got a scientific task to solve the following problem. We have a simple 2-axis turret lathe with CNC – absolutely typical one: “X” and “Z” axes plus Spindle. All the motors are induction asynchronous ones.

The spindle gear-box includes 2 ranges of reduction and, accordingly, contains BACKLASHES (non-linear component); moreover, the spindle motor is connected to the shaft via belt transmission that is an ELASTIC component. The problem is to transform the velocity-controlled spindle into an additional position-controlled axis.

We can see 3 basic ways of the spindle control:
1. Across elasticity and backlash by taking velocity data from the spindle motor and position from the spindle shaft. Here, the position control quality seems to be poor.
2. By using another induction motor connected directly to the spindle shaft past elasticity and backlash. Here, we will lose on output load and the needed motor must be powerful and therefore of special design.
3. Trying to combine the two variants. Velocity-controlled powerful spindle motor plus position-controlled shaft motor. This way looks most interesting, intelligent, scientific and prospective.

The problem seems to be standard, so my interest is: perhaps, you would point out some solutions that already exist. Maybe, you will supply me with certain links on that subject or share your experience with me.

Hopefully, somebody replies my questions! Thank you!

 

[jbartos]

Suggestion: There are similar products available on the market. Try to learn from those and their schematics. Visit

http://www.broadbentstanley.co.uk/cncflatbed.html

http://www.rmt.net/tenco/ver/

etc. for more info

 

[EdDanzer]

Typically this is called the C axis. The Wasino LJ63M we had used a precision worm gear with servo motor that locked to the spindle when positioning needed to be done. This positioning is at low speeds and high torque if milling is to be done.

 

[EnglishMuffin]

There are many solutions to this problem from an OEM perspective. But since you are apparently modifying an existing machine, it is impossible to say what the simplest and best solution would be, without more detailed information. One solution would be to use a new main drive motor having an optional low speed mode especially made for machine tool "C axis" applications. Such motors are available from Indramat, GE Fanuc and Siemens, among others. To handle the backlash problem, one feasible method that I am aware of is to provide an additional gear which, during the "C axis" positioning mode, can be brought into engagement with a spindle mounted gear. The additional gear must be connected to a servo motor such that it can exert a constant torque which opposes the main drive motor torque, thus eliminating the backlash. It is also possible that the additional gear and servo motor could remain permanently engaged. You might with luck be able to mount the gear pair and servo motor externally on the lathe head at the rear of the spindle. I suspect that the drive belt stiffness would not be a major issue, but if it were, you might be able to use wider and stiffer belts with kevlar cords. Toothed belts tend to be noisy.

 

[EnglishMuffin]

On thinking about it further, a simple solution might be to mount a brushless ring torque motor on the rear of the spindle. No mechanical engagement/disengagement would be required and there would be no backlash. Etel make such a motor for machine tool use, and they are compatible with Siemens and Indramat drives. A 385mm OD x 265mm ID Etel motor, for example, can develop 468 Nm. See:

http://www.etel.ch/rotary/default.asp

 

[EdDanzer]

The simplest solution would be to take a 4th axis for a milling machine and connect it to the spindle via a air cylinder actuated curvic coupling.

 

[jbartos]

Suggestion: Visit

http://www.mecha.osakafu-u.ac.jp/~syslab/teachers/sugimura/9802.PDF

for more info