View Full Version : Travel Locks with CNC

Paul Alciatore
11-06-2009, 12:33 AM
This question was in my mind about 10 years ago when I was just starting to get into machining in a serious manner and now that I am in the early stages of thinking about converting a machine or two to CNC it is back.

I know from experience that if I want to do the most accurate work on almost any machine I have ever used, that it is a definite advantage to lock down any axis that is not required to move in the current cut. A lathe has a carriage lock. And you can lock the cross slide and compound with the adjustment screws. A milling machine has locks on the ways in addition to the adjustment screws. I have eliminated problems while making more than one part by just locking them down except when moving that axis.

So, my question is, do any CNC systems make allowances for locking the axies when they are not moving? Many CNC systems or software talk about how many axies they can control: two, three, four, etc. Would I need to think in terms of additional axies to gain "lock down" control? Or would that just be a simple ON/OFF output associated with an axis?

Or is there some reason why this is not done?

11-06-2009, 12:58 AM
No, CNC machines do not lock any of their axis. Except Z sometimes, but it is a brake on the motor to keep the head from falling with the power off. Locking down a manual machine is mostly to counteract backlash on a mills screws and on the lathes handwheel gearing. CNC machine dont need this since they have very low to no backlash ballscrews or other methods of drive.

Converting to CNC is a waste of time. Find a used machine with a toast control. They go for near scrap and you will be miles ahead. And a CNC machine, in general, is useless as a manual machine once its converted. By the time you do the conversion correctly (ie, ballscrews) the machine will be very difficult to use manually. Ballscrews are to easy to back drive. And ACMEs have too much backlash.

It takes a lot of time and a good chunk of change to do a CNC properly. I still recommend servos over steppers, especially now that cncdrives.com has come out with dc servo drives cheaper than a gecko stepper drive.

11-06-2009, 12:59 AM
A commercial CNC machine has effectively zero backlash; for such operations such as drilling on a mill or facing on the lathe, this simply isn't needed since the motors actively control position even at zero speed. For a homebrew rig, air operated axis locks might help - but I'd be inclined to address the real problem - backlash between the motors and the machine motion.

If the motors & encoders or stepper torque is sufficient to do accurate profiling, the machine is plenty stiff enough for single axis operations.

- Bart

Jim Shaper
11-06-2009, 01:03 AM
There's no point to locking the axes on cnc.

The servo motors used are capable of holding the axial location of the shaft, so the ball screws are held by the motor itself when motion is not commanded.

This also means you need to scrap all the lead screws on your manual machine to make way for ball screws which lack backlash. It's not often cost effective to convert a manual machine, when you could start with a fully functional OLD cnc machine and upfit the control and servo's to wind up with a better end product for less money.

My nakamura was 4K and far better a turning center than any lathe I could assemble for that much money. The control on it is ancient, but for turning you really don't need a whole lot of controller to do complex work. At some point, I'm liable to retrofit it with a better control, but I haven't gotten there yet. Hell, my voest was 4500 and didn't even have a dro...