Using non-Alpha steppers with PRT-Alpha

[richards]

It's a shame to have the possibility of adding two more stepper motors to a PRT-Alpha and to put it off because of the price of the Oriental Motor Alpha stepper/driver package.

If anyone is interested, send me a private email message and I'll help you properly add standard Gecko G203v stepper drivers and Oriental Motor PL296A2A-SGxx geared stepper motors (for use as a 2nd Z-axis and/or a motor to drive an indexer) on your PRT-Alpha. (You'll still use the Alpha motors on your X-axis, Y-axis and 1st Z-axis.)

miker@xmission.com

[dubliner]

Hi Mike, I presume theres an appreciable difference in price then?

[richards]

Neville, depending on where you buy the parts and exactly which components you select, there can be a very significant difference in price. The Gecko G203v and a PK296A2A-SGxx motor (list price) is 31% of the price of a AS98AA-T7.2 (list price).

The Alpha motors are great motors that perform flawlessly. They have never let me down since July, 2004, when I bought my PRT-Alpha; however, not every application requires an Alpha motor.

Last Spring and early Summer, I had a contract to cut a lot of 32" round table tops. Each top took about two minutes maximum to cut on the Shopbot, but the cutting time was less than 25% of the total time it took to sand and edge each piece. As an experiment, I built a rotary table for the Shopbot that rotated the work so that the cutter stayed at the same X/Y coordinates. Except for one shallow divot on each piece that I had to hand sand, the table tops were perfect as they came off the Shopbot. That was one "invention" that occurred after the fact. The job ended before I had a chance to enjoy the rotary table.

Many of us could use a 2nd Z-axis for drilling. Many of us would like to add an indexer to our machines. Most of us have the ability to add a geared motor to a Harbor Freight mini-lathe or to build a multi-stage belt drive to drive that lathe; but most of us are too frugal to add an Alpha motor/driver for those applications. So, the do-it-yourselfer could add an indexer, or a 2nd Z-axis, or a rotary table, or a horizontal drill, or a rotary panel saw, or a tilting table, or anything else that the imagination could conceive of without breaking the bank.

[dubliner]

Well thank you for your elaboration Mike, I have thought of adding an indexer if only to "mess with" so price would definitely be a factor there. Your posts always shed a great amount of light on any topic you introduce - thank you very much. Neville

[jhicks]

But that solution does not address the root cause of the cut quality related to X&Y instability? If you are going to use the CNC as a fixed router table and spin the piece, that's going backward. May as well get a router table pivot or band saw and spin the stock across the bit or blade no?
Mike in another post you say the machine performed flawlessly but clearly this is not flawless performance.
What am I missing here?

[richards]

Jerry,

I've posted photos of the cut quality from time to time. The original 1:1 Alpha motor showed significant chatter. The posts after adding belt drives and after adding the upgrade 7.2:1 motors showed significantly less chatter; however, there is still some chatter. I attribute that to the general construction of the Shopbot PRT-Alpha. It DOES flex. That is inherent in the design and has nothing to do with the ability of the motors to position the cutter.

Doing the math, an Alpha motor geared 7.2:1 with a 30-tooth spur gear should give 0.000654 inches per step. (1.5 inch pitch diameter X pi / 1000 steps per revolution / 7.2 gear ratio). Clearly the "roughness" that I get has nothing to do with the stepper motor's ability to precisely move; but, it does have something to do with the inherent flex in the machine.

Think about the process of cutting a large circle with a CNC router. Both the X-Axis and the Y-Axis have to move. Depending on the forces present, each of those axes might flex a little during their movement. I did use the word "little". The chatter is about 0.050" or less. Enough that it can't be ignored. Enough that it has to be sanded away for some customers.

Now, think about the process of cutting a large circle using a rotary table. Only the table moves. If the table is properly built and uses quality bearings, there will be no flex and the cut will be optimal. That was the case with my experimental rotary table. The little nick I got was caused by moving the X-axis towards the material for the final full-depth pass. The X-axis flexed just enough to leave a "divot" on that one spot of the edge.

Now, as to whether it is a step backwards to use a rotary table instead of using a CNC router's inherent ability to move all 3-axes independently, I say that fine-tuning a process to meet the total needs of a customer is always a step forward. It is not cost effective to add a rotary table to do one piece, but it is also not cost effective to needlessly hand-sand hundreds of pieces when a rotary table could eliminate the need to hand-sand.

Maybe I should use another word instead of "flawlessly". To me a machine performs "flawlessly" when it has no unexpected glitches. The chatter caused by the PRT-Alpha is expected. It is part of the design of the machine. It is going to happen whenever I run more than one axis. It is NOT a haphazard occurrence that sometimes happens and sometimes does not happen. It ALWAYS happens whenever I cut certain patterns. So, to me, the PRT-Alpha performs flawlessly whenever it does the expected job without additional problems occurring.

I might use the same terminology when talking about my Dodge pick-up truck. It gets me from Salt Lake to Springville everyday flawlessly when I make the trip without a flat tire or without a break-down. Making that same trip in a Lincoln might give a better quality ride, but, barring that flat tire or a mechanical breakdown, both vehicles would perform their job flawlessly if they got me from Salt Lake to Springville.