PRSalpha vs. PRSstandard

[sciencekinetics]

Hello,
I have been researching CNC machines and ready to take the plunge and make the investment. Shopbot is at the top of my list but I having difficulty with one variable. Will PRSstandard meet my needs or do I need to go with PRSalpha? From what I understand, the primary advantage of PRSalpha is speed. For my application, speed is not at all important. Accuracy is of slight importance, however from the specs I've viewed, there is not a significant difference between the two versions. The material I will be cutting is primarily plastic. 2D and 2.5D only. I am considering the 48" x 96" platform. Therefore, from what I understand, PRSstandard should be perfectly acceptable. Am I missing out on some other factors to consider? Thank you to anyone who can offer some advice!!

[david_gardner]

The simplest way I can answer this is that the standard can lose steps and the alphas don't. Speed is the next biggest difference. I went through these same questions before I bought mine. I ended up with a used alpha with a spindle and if I had it to do over I would get the same again.

[erik_f]

I bought my standard for cost reasons. I just couldn't justify the extra expense for what I do with my machine. Besides if things are really going that well in the future you could upgrade to the Alpha. Working within the limits of the machine I have never had a problem with missed steps. The only time I have had stepping problems is when I've done something bone headed or the machine was out of adjustment. Both these conditions would cause poor cuts and problems on an Alpha as well. Both machines should work equally well in most situations, although the Alpha would be better suited for high speed, high production. I find that even though my machine can cut at 5ips that most of the time I'm cutting around 1-1.5ips anyway. I would say if you can justify going with an Alpha it is the SB's top of the line cats meow...but there are plenty of SB standard owners out there who, if they are like me, don't feel like they are missing out on anything either. They are both great machines. You have to think about the future too...if you are making money with your CNC and if you may end up using it for things you didn't intend to use it for.

[tkovacs]

My experience is the same as Erik's. I have a standard and working within the machines parameters it has be perfectly accurate as far as I can tell. Of course I would love to have an alpha but I used some of the saved $$ to add a spindle an I have not regretted the decision. My cut speed is typically ~2 ips, jog speed ~4 ips.

[toych]

I agree with Erik and Terry.

I used the price difference to purchase a Standard with a spindle and it has been the right choice (for me) so far.

You always have the option to upgrade if you find your needs exceed the limits of your machine.

The only second thought I had is table size. I somtimes find I could have used a 60" wide table (if I had a bigger shop) to handle larger sheet goods but the 48x96 handles 90% or better of what I do.

Regardless of which you chose your new Shopbot will deliver exceptional bang for the buck.

[tkovacs]

A correction - my xy jog speed is set to 8 ips

[richards]

I have a PRT-Alpha 120X60, but I would not hesitate to buy a PRS-Standard. The PRS-Alpha and the PRS-Standard use the same mechanical components (as far as I've been told). That means that the difference is the electronics.

The Alpha motors are excellent, but they are still stepper motors. They will loose steps and they will cause a fault.

The Gecko G203v, used in the 4G board, can drive motors up to 7A, which includes the Oriental Motor PK299-F4.5 motor. That motor, when wired bipolar parallel is rated at 800 oz*in of torque. Multiply that by 3.6:1 if you attach a belt-drive and you'll have an incredible 2,880 o*in of torque - or 180 lb*in of torque. That is 100 lb*in GREATER than the Alpha motors.

With a 3.6:1 belt-drive, you will also have the same excellent resolution as the 7.2:1 Alpha motors, since they require 1,000 steps per shaft revolution compared to the 2,000 steps required by a Gecko.

Cost-wise, four PK299-F4.5 motor and home-made belt-driven gear boxes will cost just about the same as ONE Alpha motor/driver.

So, if you're mechanically inclined, you could add four high torque stepper motors with belt-drive gear boxes for about $1,000. You would probably have to upgrade the PRS-Standard's power supply, but that would only cost about $150 for a 50V 600W unit.

The Alpha series machines have other nice electronic features, but we're not dealing with rocket science. If I can build an electronic package to control a Shopbot, anyone can.

[bcondon]

I went the same way as David. I started thinking that the standard was the way to go and went to class. Because I could be machining some large items like Kayak parts, a miss step gets expensive quick. At the class, I changed my mind.

The first piece of info is that I continue to be thrilled with the machine and all the people associated with it (which includes the company, the teachers, the entire support staff and of course, this forum).

IF you decide to go with the standard, you can upgrade from the standard to the alpha for $6000. You then turn in your standard parts for a $1000 return, so for about $1000 more (The price difference up front was $4000) , you can delay your decision to see if your system works well for you (This data was from Dianne when I attended the class)

So, you have options, options, options which shows that the Shopbot crew really understands their market.

Good luck with your decision.

[myxpykalix]

Jeff,
One thing you will quickly learn is that the primary reason for which you buy a bot sometimes gets altered and someone comes to you with a project totally unlike what you envison yourself doing. So for that reason I felt like the better course was to buy the most accurate best unit (at the time) so I wouldn't have to spend the money later because we all know prices go UP not DOWN.
So my recommendation is, if you have the money, get the better unit because in the long run you save money and when you need the upgrade down the line you may not have the money to buy it.

[richards]

There seems to be a misunderstanding about what happens when the PRx-Alpha machines go into "Alpha Mode". The Alpha step motor/driver package has a built in encoder that is monitored by the stepper driver.

Here is a greatly simplified explanation of what takes place inside the Alpha driver unit: As each step pulse is received by the driver, the driver increments a counter. As each encoder pulse is received (showing that the shaft on the motor has actually turned), the count is decremented. If that count starts to increment, the driver knows that the motor is not keeping up properly, so, if possible, it does its "alpha thing" and catches up. If it can't catch up, it sends a fault signal to the Shopbot controller card and the machine shuts down.

That's very good in theory, but it causes problems if more than one motor is active. There is no feed-back connection between motors, so one motor has no way of knowing that it needs to compensate for another motor that is struggling to run at the programmed speed. The result is often a ruined cut - and you'll never know that you have a problem unless you carefully examine each and every part (which we should all do anyway for routine quality control, but hey, I'm lazy and I only want to sample a few parts out of every batch).

The "Alpha mode delay" is factory set for about 1/2-second. That means that the Alpha motor/driver has about 1/2-second to catch-up before it sends out a fault signal. If you're cutting at 6-ips, that means that an axis could fall behind THREE INCHES before the fault signal is sent - and, if the motor/driver recovers successfully, you might not even know that a motor had slowed down during the cut.

Believe me when I say that I've learned to live within the capabilities of my PRT-Alpha (which means that I don't push the machine hard enough so that it ever enters the dreaded "Alpha Mode").

On the other hand, when a "standard" motor looses steps, you'll hear it - loud and clear. It doesn't hurt the motor, but it sounds awful. The cure is to slow things down a little so that the cutter cuts its way through the material instead of trying to force the cutter to take bigger bites than it should.

By the way, I am totally satisfied with my PRT-Alpha. When I had a chance to upgrade the motors from the AS911 non-geared motors to the AS98 geared motors, I bought the upgrade. The machine has worked wonderfully since July 2004. I've hardly done any maintenance outside of a little greasing and a little cleaning. The only part that has failed was one of the proximity sensors. That's just about perfect performance and much better than I had expected. One of these days, when an Alpha motor/driver finally fails, I'll replace all the steppers and all the drivers with standard steppers that have belt-drives attached. I ran the original motors through a belt-drive transmission and had excellent results, so I have complete confidence in going that route - when the time comes.