4th axis indexer anyone?

[Mike Heidrick]

Any of you boys run a 4th axis index table?? I just retrofitted an old 175lb Moog 8" with a servo and 2000ppr encoder for the mill. These things are super heavy.

[William Adams]

I've got making one for my hobby-level CNC mill in the planning stages --- how necessary is an encoder? That's for providing feedback on the position, right?

[John McClanahan]

William, the encoder is necessary with a servo motor. For a stepper system, the stepper driver would need to support an encoder. (at least with Mach 3)

John

[Mike Heidrick]

I've got making one for my hobby-level CNC mill in the planning stages --- how necessary is an encoder? That's for providing feedback on the position, right?

The encoder gives you closed loop verification you motor turned to where it was told to go. With a stepper only you are hoping it went there. Steppers can run encoders too but you will have to get creative to wire in the encoder with a typical stepper controller setup. A Servo driver setup typically includes the capacity to wire the encoders to the controller. On my mill I run a Ajax Centroid AllinOneDC controller setup for DC servos/encoders. I added a 4th axis for the rotary table.

you will alos want a way to home the table. Proximity switch is what I am figuring out now.

[stan kern]

i added the 4th axis and then used my rockwell lathe at the front end of the cnc ,takes about 20 min to install the indexer
cnc manufacturer quoted me a big bill ,did it myself for under 900.00 excluding the lathe(had it)

http://www.youtube.com/watch?v=jbnsWgiJX7E

[Art Mann]

I've got making one for my hobby-level CNC mill in the planning stages --- how necessary is an encoder? That's for providing feedback on the position, right?

I have a Camaster CNC router that does not have positional feedback and will run continuously for hours without losing a step in X, Y or Z. I would think that using stepper motors for rotation only would be trivial by comparison. I am not saying that closed loop feedback isn't better; I am just saying that it isn't necessary, especially for hobby applications. Apparently, most manufacturers of small machines think the same way.

[Gary Campbell]

William...
The guys are correct. For a stepper system, encoders, or closed loop as they are often called are simply a higher priced option. On a servo system they are virtually required. Both read shaft position and report to the drive. If there is a discrepancy, the drive signals the control OS and action is halted, possibly saving "the job". Most good encoders use 4000 pulses per motor revolution, and therefore are capable of reporting an error of 1" divided by 4000 or two and a half ten thousandths of an inch. This level of reporting is usually adjustable. In some cases companies set their drives adjusted to "more than an inch, for more than a second", which I feel negates any value gained by the extra cost of the closed loop system. There is however, some gain as the drive will use max power to try and regain position. That said, "the job" has already been ruined. Like Mike alludes to; I will choose strong motors without closed loop over weak motors with closed loop, but neither is as reliable as a strong motor with an encoder. Providing that the encoder is allowed to perform its intended function without electronic neutering.

All that is required for closed loop operation of steppers is to have a compatible drive. The alarms may or may not be wired and may or may not be programmed to "report" to the control OS. Note: Most stepper or servo drives with encoders require a computer connection to program them prior to running. This is not a difficult process, but does involve paying attention in order to get expected results.

There is some new servo technology out there at present. Here is an example of a servo that is programmed to spin at preset rpm increments. I plan on using this to "spin" a lathe headstock along with a reduced 1200 ozin stepper for indexing type operations. https://www.youtube.com/watch?v=HRC816hozOU