PRS Evolution Question

[Mark Farris]

Hello,
I currently have a 2003 PRT 9648 which has been serving me well. I have been a cabinetmaker and architectural woodworker most of my life and having the bot has made my life a lot easier and more fun. I have read most of the threads and archives concerning maintenence and upgrading these wonderful machines. I have recently been approched by some new clients interested in multiple medical and sports related products in acrylic and other plastics which appear could be very profitable. I have been researching whether to upgrade the PRT or buy a newer PRS. After adding up all the upgrades to the PRT that I would want (which is everyone there is), I have decided it is time to move up to a PRS. I understand the differences between a standard and alpha, but I have a couple questions which will help me decide which direction to go.
1) Could someone tell me what year the PRS and the new gantry design were introduced?
I know that the basic machine design is the same for both the standard and alpha, and that the difference is in the controls, drivers, and motors.
2) From the time the PRS was introduced what advances have been made that would make a newer model better. For instance is a 2009 PRS Alpha as good / accurate / fast as a 2012 providing it has been properly maintained and cared for? (I realize a new bot would be just that "new" and have a warranty) The same for question a PRS standard.
I am simply trying to make an informed decision whether to buy new or one that is a few years old. The obvious answer is that if money is no object then buy new, but if money is no object then I want to be your best friend. Thank you for a wonderful forum and your time and knowlege.

Sincerely,
Mark Farris

[garyc]

Mark...
I will make a stab at this.

1) 2007 (I bought my first that year)

2) There are constant minor upgrades, too many to list here.
a) Major upgrades in 2008 and 2011 that to the knowledgable would affect both cut quality and resale value
b) up to a point they are equal. However, the newer models have structural advantages that will allow them to outperform even the 2009 models in high load cutting operations. The older the model, the more the difference. The ShopBot Tools have been constantly evolving into stronger, faster, more accurate machines.

[richards]

Don't be afraid to buy a PRS-Standard. They are very good, very reliable machines. I have closely followed the components used in both the PRS-Alpha and the PRS-Standard, and I would not hesitate to buy the PRS-Standard unless I was absolutely certain tha I needed the fastest possible cut times. (Even then, I would probably buy a machine without steppers and install my own stepper motors and stepper drivers.)

Every stepper motor can only be pushed "so far". Know the limitations of the stepper motors, the stepper drivers, and that pulse generator, and you'll do just fine.

Unless you're a cabinet maker who is pushing our the maximum number of parts every day, a PRS-Standard will "probably" meet your needs. I owned a PRT-Alpha and soon learned that the "Alpha" feature was just an advertising "gimmick". There was no correlation between axes when one axis went into "alpha mode". I was left with "divots" on parts cut when the "alpha" mode activated. It didn't take long to realize that slowing the machine down kept the "alpha mode" from activating. After I learned to "slow the machine down" so that I didn't push the stepper motors harder than they were designed to move, the "divots" disappeared.

Know you machine. Know how fast it can cut in each material. Stay without the safe cutting speeds and you'll do just fine.

[donek]

Keep the old one, buy a new one. Having two is great.

[Mark Farris]

Thanks Guys, I appreciate the quick responses.

Gary, You pretty much confirmed what I thought. Structurally I have seen a lot of the changes, I just wasn't sure what changes had been made to the steppers, drivers, and controls. I'm assuming the major changes in 2008 and 2011 included changes to these areas. I know this isn't your area but do you have a rough idea (won't hold you to it) what it costs for a full upgrade from standard to Alpha or would you rather I contact Shopbot directly?

Mike, you bring up interesting points about speed that I have wondered about when considering an alpha. Since I am currently cutting a variety of products and my future interests are all over the place, I need to take a good look at the speeds I will need to cut at. Are you saying that when cutting at lower feed speeds there isn't much difference between a standard and alpha, and it is only when you reach speeds greater than the standards ability that the alpha "kicks in". I realize there are other differences such as the "closed loop" steppers in the alpha which affect accuracy and cut quality, but strictly talking speed. I'm afraid my weakness is in the complete understanding of the control components, steppers, and drivers. I am confident Shopbot will provide me with a system that will work for me, I need to focus on production. Some day I will corner one of you guys over a few cups of coffee to explain it to me. I read about it, but it doesn't sink in.

Sean, I would love to keep it. Money, space, and the possibility of divorce papers will force me to sell.

Mark

[steve_g]

Mark

Let me make a comment here... when alpha mode is entered it's due to catching up missed steps. The part is already likely ruined. where it really shines is in step and repeat operations! I had a product that involved lots of small ABS parts... I had files that step and repeated through a matrix of parts and bit changes. If one part got loose and jammed up the router, alpha mode would kick in, one part was ruined but all remaining parts were still cut correctly.

SG

[richards]

Mark,

Steve explained the advantages of Alpha motors very clearly.

A stepper motor is normally used in an open-loop system. That means that the controller sends pulses to the stepper driver and the stepper driver causes the motor's shaft to rotate one "step" for each pulse. In an open-loop system, there is no feed-back from the motor to indicate whether the shaft actually turned. The PRS-Standard uses an open-loop stepper system. Open-loop works very well as long as you do not expect the motors to handle a load greater than their torque capacity. So, you run some tests in the material that you're going to cut. You increase the cut speed until the motor looses steps. You'll be able to hear a grinding or popping sound. (It doesn't hurt the motor. Nothing has been broken. The magnetic field inside the motor has just been disrupted.) When you know the speed that causes problems, back off that speed by about 25% and you'll be safe.

The Alpha motors have built in encoders that keep track of the motor shaft's rotation. If the shaft "stalls", the driver "knows" how many steps were missed and attempts to catch up. The length of time that the driver has to catch up is a setting that you can change. I believe factory settings allow 1/2 second. If the driver cannot catch up within that time limit, the driver sends a fault signal to the controller and the cut is stopped. However, if the driver can make the motor "catch up", the cut continues.

Because there is no feedback between motors, having one motor stall for 1/2 second and having the other motors continue as if nothing happened, can cause "divots". If you're cutting at 5" per second, the stalled axis can be 2-1/2" out of sync before the driver faults. But, as Steve said, if you're cutting a bunch of parts out of a sheet of material, only one part will be wasted and the cutting will go on. In that same situation, if the PRS-Standard faulted, you would have to manually reset the X/Y/Z position to continue the cut. Depending on how many parts you're cutting out of a sheet, that may or may not be a problem.

[garyc]

Mark...
Get that info direct from ShopBot.

I agree somewhat in principle with what Steve and Mike say, but add the following caveat: If you are buying the machine for a commercial endeavor, i.e. you intend to make a living with it, BUY THE ALPHA!! PERIOD.

There has been a lot said about alpha and standard and "I can cut the same stuff only slower, etc" That statement appears only to be true to those that have never owned the alpha. Double the speed, 4 times the power and 50% higher resolution are the main reasons to me. I have only faulted (alpha mode) one time in thousands of cut files, and that was a user error with a collision involved. They are both great machines, but each has its place. Personally I dont believe I would engage in a for profit venture with the lesser powered of two options.

[Mark Farris]

Steve and Mike,
Thanks for the explanation, again very good information that I did not previously know.

I think Gary's last post ends this discussion. As one who has spent years building up my tool arsenal to commercial levels, it became clear that the modest amount of money invested in getting the alpha vs potential profits seems kinda petty. Don't get me wrong, money is money, but the old adage that you have to spend money to make money is probably in play for me on this one. Guess the wife won't get that vacation this year either.

Thanks again for your time and it's always nice to learn something new.

Mark