View Full Version : G4's serious limitation

10-07-2006, 10:28 AM
After the somewhat lengthy discusions on other threads during the last few days about altering the PRT to use higher powered electronics, I had egg on my face when I went to the Shopbot's main website and saw that they had published a lot of information about their G4. (I'll have to admit that I usually only visit the main website when I check for new software updates.) The G4 looked impressive until I read this:

"Note that outputs from this board are limited to 2A/phase—just right for your existing PRT motors."

Why would anyone cripple a brand new design by limiting the current to 2amps when the Geckos can handle 7amps? Has anyone even looked at the current requirements on Oriental Motor's website for their PK296 and PK299 motors?

In my most humble opinion, anyone who is serious about getting the most out of the Shopbot should, at the very least, add the G202 stepper drivers and then replace the little stock geared motors with motors that are big enough to do the job properly - and then gear those motors down to get the torque necessary to make the machine perform properly. The stock PK296A1A-SG3.6 motor is limited to 320 oz*in of torque. That figure is clearly stated in Oriental Motor's literature. Any of the big brothers to that motor, the PK299-02AA, PK299-03AA or PK299-F4.5A, when geared down, will do the job properly - thank you very much. You'll note that I did not list the PK299-01AA motor. In order to get full speed and torque from that motor, you would have to feed it 150 VDC (25 X 6V) and the G202 is limited to 80VDC . Remember, a stepper motor's torque is determined by the current (amps) that the motor draws and speed is determined by the available voltage. Motors that draw 3 to 6 amps when connected to power supplies giving 50 to 70 VDC have adequate torque and adequate speed - comparing very favorably to the Alpha's AS911 motor and driver.

The only thing limiting the G202's current is the power supply and heat. You can build or buy a larger power supply at moderate cost and you can add heat sinks to the G202 drivers to allow them to handle higher current. Computer CPU heat sinks that cost about $15 each at your local computer store fit the G202 almost perfectly - or you can have your local machine shop machine an aluminum plate that is at least 1/2-inch thick to let the G202 do a proper job.

10-07-2006, 11:40 AM
As stated above:
The stock PK296A1A-SG3.6 motor is limited to 320 oz*in of torque.

All motors on my PRT came with PK296A1A-SG7.2 which are not mentioned above. I am curious to get a sense of where my motors would fit into the equation. At this point I am ready to get the G4, new pinions, and beef up the table. I figure than the total investment would be worthwhile. If my motors are a serious limitation I would have to hold off awhile. If you have any comment I would love to hear it.

Thanks, Robert

10-07-2006, 11:42 AM
From a mechanical viewpoint, those Gecko's are fairy heavy and I'd be nervous of the way they are attached to the "motherboard". Under static conditons probably okay, but don't put the control box down hard. During shipping it is probably safer if the Gecko's are unplugged.

10-07-2006, 12:34 PM
The PK296A1A-SG7.2 gear motor is limited to about 45 lb*in holding torque (720 oz*in) that drops very quickly to about 240 oz*in at 100 rpm and to about 80 oz*in at 200 rpm.

I don't know the number of teeth on your spur gear, but using an approximate 4-inches of linear travel per spur gear rotation, 200 / 60 = 13.3 inches per second jog speed, which is about where I run my Alpha with a 3:1 gearbox.

To everyone else, I want to clear up one thing very quickly. If you're happy with your PRT now, except that you want faster speed and smoother cuts, you should be VERY happy with the G4 and your existing motors. The G4 or the Agek will greatly enhance your stock PRT. However, if adding the G4 is just the first step towards modifying your machine so that it performs in every respect like an Alpha, you'll have to give serious consideration about changing out the motors.

Also, I agree with Gerald's comments about the mechanical fastening of the G202s to the controller board. From the Shopbot website, I can't tell if there is some kind of angle plate behind the drivers to help stabilize them (to keep them from 'shaking and dancing' - electical connections don't like vibration). However, it would be a simple thing to spend $10 at your local hardware store to buy a strip of metal and then build some kind of bracket. The photos that Dirk has posted on his website of his Agek show that his G202s are mounted in the more traditional manner to a piece of aluminum that would act as both a more secure mounting block and as a heat sink.

10-07-2006, 02:50 PM

I have 25 tooth pinions and I often cut at 2-3" 1/4- 3/8" deep and jog at 3.5" with no problems. I am trying to get a reasonable idea of potential cut and jog speeds. And of course increased resolution would be my biggest need.

Brady Watson
10-07-2006, 03:20 PM
The G4 is as Mike states, "If you're happy with your PRT now, except that you want faster speed and smoother cuts, you should be VERY happy with the G4 and your existing motors" This should cover most people who own PRTs.

If you want to run motors greater than 2A/phase then ShopBot has you covered with a full Alpha 'Development Board' I believe this is (#15305) which would have accomodations for Geckos or any other stepper/servo driver that uses step & direction signals. If you scroll down the 4G page, you will see a link to the Developer page. (http://www.shopbottools.com/developers.htm) It may not be clear in the write-up that you can run Geckos @ 7A with the developer's board, but it will. Not sure when they become available for purchase.


10-07-2006, 05:47 PM

I must be awfully dense today because the prices listed don't follow my line of thinking. Let's see if I understand. For someone upgrading a PRT with the G4 option, the initial cost would be about $795 (after returning the original board) if part #15304 is used. Sometime down the road, that 'botter gets power hungry and decides to buy new steppers to get Alpha performance. From Shopbot he can buy an upgrade that includes four Alpha stepper motors and the Alpha stepper drives to run those motors for an additional $2,800 (which is way below the prices shown on Oriental Motor's website - give a big thumbs up to Shopbot for passing along the discount). The 'botter would lose one-half the resolution that that he had just gotten used to with the Geckos (2,000 steps per rev. from the Geckos vs 1,000 steps per rev for the Alpha - switchable between 500 and 1,000 steps per rev). And he would end up with four G202 drives that he had just bought as part of the G4 upgrade (about $400 for the four G202 drives). But getting the additional muscle because of the Alpha motors might just be worth it. Going that route, the 'botter would have Alpha performance with Alpha drivers and Alpha motors for about $3,600 plus other parts and pieces that may or may not be required. (I really hope that I've understood the projected pricing and that I'm relaying correct ball-park prices.) So far, everything sounds reasonable. Assuming a non-geared motor spinning a 1-inch pitch diameter spur gear and a top pulse rate from the G4 being about 30,000 pps, that machine would have way more speed than could ever be used - score another point for Shopbot. So, at this point the 'botter has spent about $3,600 or so and has a machine that is functionally the same as an Alpha - at least where motor torque and cutting/jogging speeds are concerned and the resolution is as good as any other Alpha that Shopbot sells.

Here's the part that I don't understand. That same 'botter could puy the G4 board with four G202 drivers from Shopbot for $795 (after trade in of his old board), pay full price to Oriental Motor for four PK299-02AA steppers ($206 each) and install a 70VDC power supply (PMDX-135-8020 for $119 + Avel Y236801 toroid from Parts Express for $75) and end up with a machine that had twice the resolution as an Alpha with the same speed and torque as an Alpha for $1,813 total or a savings of $1,782. In my book getting something with the same torque, same speed and twice the resolution at half-price beats paying twice as much for something with one-half the resolution. I'm afraid that I'd have to take aways the thumbs up and extra point that I just gave Shopbot.

Sorry, I just found an error in my math. I forgot to add in about $100 for heat sinks (which would not be necessary if PK299-02AA motors were selected, but I'm going to include the $100 for the heat sinks anyway).

I afraid, Brady, that unless I read the Shopbot website incorrectly, that I'm going to have to give you a friendly poke in the ribs and admit that you just about pulled one over on me.

10-07-2006, 06:06 PM

Let's do the math on your situation. You have motors geared to 7.2:1 and a 25-tooth spur gear. I'll have to assume that you're using a 20-pitch rack, which is what I have with my Alpha. With a 20-pitch rack, a 20-tooth gear has a pitch-diameter of 1-inch, so a 25-tooth gear would have a pitch-diameter of 25/20 or 1.25-inches, which means that the 25-tooth gear would travel 1.25 inches X pi per revolution, ending up with about 3.925 inches of travel per revolution. However, each revolution of the spur gear would require 7.2 revolutions of the motor's shaft, because of the gearbox. And each revolution of the motor's shaft would require 2,000 pulses from the G4 board. So, doing the math to move 3.925 inches gives us 2,000 X 7.2 or 14,400 pulses. The G4 can produce about 30,000 pulses per second, so, disregarding anything that would keep your machine from using the G4's top pulse output, you would be able to jog at about 8-ips.

As far as resolution goes, I haven't really studied your setup, but I think that your factory setup requires 400 steps per shaft revolution. The G4/G202 requires 2,000, so your resolution with the G4 would be 5X better. (I'll have to admit that something doesn't look right with that figure, but I don't have the specs to know if it is correct or not.)

This is just guessing on my part, but I would assume that you could just about double your current speeds - if your machine is aligned properly. As always, your mileage may vary - considerably.

10-07-2006, 06:49 PM
Harold Webber changed his motors over to the ones you currently have on your machine. You may want to contact him. I think you will be very pleased using those motors. I'm thinking if you are getting standard speeds on your machine your box may be set up in 1/2 step. If thats the case you will have 5 times the resolution you have now using Geckos. You should get a top speed around 8 inches a sec and possibly 10 to 11 with a serial card and the faster controller software.
If the faster speeds are important to you it may require a higher voltage supply.

10-07-2006, 06:56 PM
Thanks Mike, Brady, Gerald, others

I still need to process how the math works but now I have a reference point that I believe can sink in. When you blokes have these exchanges on this topic and THAT! other one it's pretty impressive and I realize how much there is to yet learn. I've gone back over the information in your posts sometimes I think a dim picture is emerging or at least some of the ideas will part my hair as they whiz by. I have always wanted to learn more about electronics and you guys have been my inspiration to at least try. I would probably settle for learning just enough to be dangerous.

All of this has me thinking that it would be worth while to uprade to G4's , get new pinions, and beef up table etc. I could enjoy some improved performance until I begin plans for a new machine like the Mechmate. It also seems that more options for the control box and motor setups will be appearing soon. I have loved having my PRT it has created many oppertunities for me and I have a perfect dedicated function for it when I am able to add a second machine. When all is said and done and I don't sing well but it is has been fun for me to part of the SB community.
Thanks to all, Robert

10-07-2006, 07:39 PM

Dirk made a very valid point about voltage and speed. Remember, current (amps) determines the amount of torque that the motor produces and voltage determines the motor's possible speed. Gecko G202 drives use a resistor to limit the current that the G202 passes to the motor that you or Shopbot or whoever sets up your machine installs onto the screw terminals on the G202. Those current limiting resistors are common 1/4 or 1/2 watt resistors that you can get at Radio Shack for a few cents each. Mariss (at Gecko) has a simple math formula to determine which resistor to use. The values are not critical. Standard values have worked fine for me, meaning that you don't have to worry what would happen if the math said you needed a resistor value that Radio Shack didn't have. You just get the closest match and away you go.

The voltage is a little more difficult for most people to understand. They get confused when they realize that a 24 volt transformer really gives about 34 volts DC (after rectification through a diode bridge and ripple reduction via a large capacitor - which is handled perfectly by the PMXD-135-8020 power board - or something similar). The thing to remember is that you multiply an AC transformer's voltage by 1.414. If the product of that multiplication is between 10X the stepper's rated voltage and 25X the stepper's rated voltage, and if the current produced by that power supply equals the current requirements of all the steppers added together, you'll have a workable power supply. (Don't forget, the G202 only works with voltages between 24VDC and 80VDC.) If you need all the speed that you can get, just make sure that the power supply generates voltage that is 20X to 25X the stepper's rated voltage, but be aware that the higher the voltage the hotter the stepper will run - and a stepper can get HOT, too hot to touch. Be careful!

Personally, I run my 3-volt steppers at about 70 volts. I use a 50-volt transformer that produces about 70 volts after rectification and smoothing.

If you're just a little confused about how/why a power supply that's rated at 50 volts produces 70 volts - just realize that it is magic. Actually, it's because the voltage read and reported by most voltage meters report the root-mean-square of the voltage, not the peak-to-peak reading of the voltage. You can see that for yourself by hooking up an oscilloscope to a 120VAC line in your house or shop. What you'll see on the oscilloscope's screen is a sine wave that is about 170 volts peak-to-peak. There you go, the magic is no longer magic. Don't worry about it; let the power company do your worrying.

Another thing that is interesting (to me), but not at all important, is to understand basically why a stepper motor that is rated at 3V can be run at 70V. It's all due to the way that a stepper motor's drive works (the fabulous G202 driver). The G202 is designed to force as much voltage through the motor as it can until the windings in the motor are at rated voltage. What happens is that the G202 gives full voltage to the motor each time it steps, for a very brief time, and then immediately cuts that voltage down to the motor's rated voltage. That's called a chopper circuit. The bi-product of that chopping action is heat, both in the driver and in the motor. The heat in the driver is removed by using a heat sink attached to the driver. The heat in the motor is simply radiated into the surrounding air. That's why a stepper motor can get awfully hot and that's also the reason that the G202 requires that the available voltage to the motor NOT be more than 25X the motor's rated voltage.

There you have it, another long winded explaination that is of no interest to anyone but me. Just follow the guidelines found in the G202's instruction sheet (that you can download from www.geckodrive.com (http://www.geckodrive.com)) and don't worry - or you'll turn into a white-haired wrinkled old man like me.

10-07-2006, 08:34 PM
Word of caution.

If you use an oscilloscope to measure line voltage be sure to use an isolating transformer between the scope and the mains or use a handheld battery unit.

One side of the scope probe is connected to ground and connecting to the wrong side of the powerpoint can be a rather startling experience.

For more details see

10-07-2006, 08:46 PM
Dick, you're right about that. I use a 'cheater plug' on my oscilloscope's power cord. A 'cheater plug' is the adapter you buy at the hardware store to plug a grounded plug (three-prong) into a non-grounded (two-prong) receptacle. With the 'cheater plug' in place, I have to connect the 'scope's DC 'ground' connection to the AC Neutral conductor and then the 'scope's probe is connected to the AC line or hot conductor. BE CAREFUL! 120 Volts can kill you.

10-07-2006, 09:03 PM
"BE CAREFUL! 120 Volts can kill you."
Actually no it can't... but .03 Amps can. Sorry, I know what you meant.

Reading the thread, I'm sure you guys are up to speed and all... but some of the stuff you describe has great potential to get you killed.

If you don't understand how to properly connect test equipment and you find yourself reading steps like "connect probe A to the ground source and make sure earth ground is..." and you THINK you know what that means... don't do it until you KNOW what it means or find someone else.

Remember, these motors are generators. I had a guy near about kill himself arting around with slewing motors on a large dish. When the motor is turning for any reason, current is flowing.

Just thought I'd interject a word of caution.


10-08-2006, 02:21 AM
Mike, don't forget the cost of upgrading your building's power supply and airconditioning when going to that power-hungry Alpha.

10-09-2006, 05:28 PM
Belated thanks to Dirk & Mike for the helpful information provided about my particular set up and the important considerations of voltage and torgue. Dirk's post slipped in while I was putting my last post together.

I had figured that were going to be differences in the AGek and G4, now I hope that I have a better realization of it.

Would it be safe to say that the (light duty) G4 and PRT will run the socks off of table with a single router do light work?

And would it also be safe to say that the
(heavy duty) AGek with more input and output options more amps etc. would be a must for a spindle, indexer, cutting cabinets parts etc.?

Any comments?

A final thought. It seems to me that depending on ones needs the price difference could be made up in far less than the earnings of one production day.

Thanks, Robert

10-09-2006, 08:25 PM

The way I see it, there are several things going on simultaniously in the Shopbot community. The first thing is that almost everyone agrees that it's time to update the PR and PRT with more modern electronics that give much better resolution and higher speeds. That movement can be subdivided into three camps: 1). Shopbot with the G4 that basically seems to offer a very simple update to enable you to use your existing motors and your existing machine if you replace your controller card to the new G4. Simple. Economical. No hassle. 2). Dirk's Agek does everything that the G4 does plus it seems to me that it would be much easier to use as a basis to build upon when the time came to use more powerful motors to make the PR or PRT functionally the same as an Alpha. Again, Dirk's solution is economical, robust, expandable to keep up with your future needs and is simple to install. 3). The Ascension for those who want to use G-code. It does basically what Dirk's box does, but is totally based on G-code - meaning that you won't be able to use Shopbot code to make your parts. It can be expanded and upgraded as your needs change. It also is inexpensive, reliable, and robust.

The second thing is that Alpha users are asking for less chatter and better resolution. There option is to either buy a Shopbot supplied gearbox equipt Alpha motor or build/buy their own gearboxes. Brady has stated that Shopbot offers several different gearbox ratios on new Alpha - I assume at a higher price. In any case, the Alpha users are not left out in the cold.

So, in one way or another, everyone's needs will be met.

If I were a PRT user or a PR user and wanted to keep using Shopbot code, I would buy Dirk's box to keep from being limited in the future when I wanted to beef up the machine by adding some high current motors. If I knew that the standard PRT motors would work for everything that I planned to use the Shopbot for, I'd buy the G4 directly from Shopbot. (Just a note here: On the Shopbot site, they pretty well state that their G4 is a perfect match for the 1-amp motors found on the PRT and PR machines. I would take them at their word and only use the G4 if I planned on sticking with 1-amp motors. It seems foolish to modify the G4 board when Dirk has a board that doesn't need to be modified.)

If I were a PRT user and thought that G-code would be better suited to control my machine, I would buy the Ascension box or build something similar using standard parts from the manufacturers that supply boards to the Gecko oriented crowd.

If I were an Alpha user (which I am), I would build or buy a gearbox to get smoother cuts without chatter (which I did).

Choices, choices, choices. I don't think that you could make a bad choice - as long as you were firm in your choice of software, Shopbot or G-code.

Also, as you mentioned, there really isn't much price difference between the various offerings (except for the Alpha upgrade which has no prices listed - to my knowledge). I would think that if you based your decision totally on price, that it would be better to wait just a little until you had saved up a little more money so that price would not be the main criteria. All of the offerings are very economical - in my opinion.

10-09-2006, 10:05 PM
Mike, I just returned from the Kansas Camp and I saw that the Mach3 software would convert any Shopbot file to G-Code. So if someone switches to the Ascension box they will not have to redo their existing files. I also understand that Shopbot offers a converter from G-Code to Shopbot code. I also, as you know, have an Alpha and have been watching very closely what you have been doing with your tool. I don’t think that I would have the patients to go through all the upgrade development that you have taken on. My hat is off to you! You’re a great asset to the CNC community.

10-10-2006, 12:13 PM
OK, so lets say I have an alpha and I am not happy with the chatter that I am getting. I would like a much smoother cut and don't particularly care about the speed.

So you're saying SB has a gear drive option that I can order to modify my X,Y and Z motors? Or are you saying that I have to replace all my motors (again)?

It seems like building my own 3:1 belt drive reducer might be preferable to replacing all the motors. The pictures on Gerald's website were pretty impressive.......... There was also no change in software and was other than the belt drive changes, just a stock alpha.

Hummmmmmmmmmmmmm Food for thought!


10-10-2006, 02:41 PM
I'm as much in the dark about the gearbox option that Shopbot is offering as everyone else. Judging from the amount of email that I've received, there is a lot of interest in that option. All I know is what I've read from Brady Watson's posts on the forum. Brady or Ted Hall could give you a definitive answer about the upgrade options.

When I started looking around for gearbox options, I didn't even think to ask Shopbot if they might be offering an upgrade. I went straight to Oriental Motor and asked them for a part number for gearboxes for my Alpha motors. They told me that the Alpha motors could either be ordered with or without gearboxes, but that gearboxes couldn't be added to a non-geared Alpha motor as an upgrade. After talking with Oriental Motor, I called a gearbox supplier that they had suggested. That supplier had gearboxes, but they were too expensive for my tastes. (Little did I know that building several versions my own belt-driven gearbox would cost more - a lot more - than buying bolt-on gearboxes.)

10-11-2006, 04:05 PM
In another post, I said that EZ-router had belt-driven gearboxes for $175 each. I should have dug through my notes and not trusted my menory. The price quote from them was $275 each.