DRO/CNC retrofit question

[Seb]

Disclaimer: I'm new to this.

I think I want a mill that can be operated in CNC mode and manual mode. For manual mode I think I want DRO.

It seems like the most common way of converting a manual mill to CNC uses open-loop control, where the computer gets no feedback on how the table and quill are actually moving. I understand that by tuning the system properly (motor size and power supply size vs material and feed & speed, etc) you can nearly eliminate error due to lost steps, so in practice this open-loop design works well. By using dual-shaft motors, you also retain retain the option of manual control.

But since the system lacks feedback, you don't get DRO capability. Separate DRO systems are available for retrofit, but I don't think they integrate with the open-loop CNC retrofit.

It seems preferable to me to have a single-retrofit closed-loop system, where the machine movement feedback is available to both the computer (to avoid lost steps during CNC operation) and the humans (in the form of DRO).

Is this line of thought on track?

If so, the next step is: what are my options for closed loop CNC/DRO? The two options I know of are encoders on the motor shafts, or some kind of caliper-like rack-and-pinion setup mounted on the table and the mill chassis/frame.

Are there any other options?

What are the tradeoffs between these choices?

Thanks for helping a poor noob....

[Al_The_Man]

If you went the low cost way of Mach for e.g. for the CNC, one method is to use steppers, no feed back would be required for the drive, but if encoders were fitted for DRO use, you could implement two parallel ports, one for Mach and one for the DOS DRO from http://www.lindsayengraving.com/othe...rests/dro.html (see arts link lower on the site).
If you used servo's, they would be slightly better to use in manual, I would think, due to no indent causing manual move error, the only thing here is, you would have to split the encoder signal via a buffer to both the drive and the Para. port.
Al.

[tai42]

One thing to keep in mind if you switch out the acme screws for ball screws is that ball screws are much more efficient. That means that the motors are used as much for holding as for moving. If you keep the acme screws, then you may use a dual-shaft motor to keep hand wheels. However, you will need stronger motors to overcome the inherent inefficiency of acme screws, and you will most likely have more accuracy issues. (Ball screws is the way to go, in my opinion.)

One solution is to get a machine with a very nice DRO and a machine that gets converted to CNC. This is the option I took. I have a IH mill with DRO/power feed that I use as a manual mill, and an X2 for CNC since I have yet to need more than the working area of the X2 for something that "requires" CNC operation, and I prefer to do most things on the manual mill. Of course, I also have a wife that was very understanding about me needing 2 mills.

The other option is to convert the mill to CNC with ball screws (the right way ), and still use the software to do the "manual" milling through the use of a jog wheel and pendant. This isn't quite as convenient as a manual mill, as you only have one "hand wheel" and you use a button to switch between the X, Y, and Z. You also loose the "feel" of machining, so a little more care needs to be taken. However, it has the advantage in that you can still use steppers and run the system "open loop" because the software will keep track of where you are and the motors stay powered on to hold and move each axis. Also you can use servos and not have to worry about running the encoders back to the computer and still let the servo loop handle everything.

-Bob