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  #1  
Old Mon 15 June 2009, 13:20
Kenrbass
Just call me: Ken
 
Richmond Tx
United States of America
Shopbot prt advice - Richmond, TX, USA

I am running a shopbot prt standard (2004) and thinking of either building a mechmate gantry for it, or just building a new machine. My cuts are ok, but there is to much flex going on and feathered edges on my cuts. I am not reallyl looking for more speed, just better quality.

I have an extra shopbot controller board and am wondering if the regular oriental motors with the 7.2 gearbox will work on the bot boards ???

I have both electronics and welding background, so I'm not concerned about the actual build. I am just wondering if a "bot/mate" would be workable.

Thanks for any help you may be able to offer.

Kenneth
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  #2  
Old Mon 15 June 2009, 21:33
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Ken, that is very much where the MechMate project started . . . . . we had a ShopBot that was too flexible and too rough.

Look at changing only the gantry and y-car to solve the flexibility issues, and then add gearing/belt reductions for the motors. Changing the whole motor is the more economical thing to do (unless you enjoy building belt drives).

How much of your current electronics will be compatible with standard stepper motors is best to listen to Mike Richards who will hopefully chip in here.
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  #3  
Old Mon 15 June 2009, 22:47
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Ken and Gerald,

I read Ken's post earlier today and almost responded, but decided to think for awhile before answering.

Shopbot developed several different controller cards. The only two that I've had direct experience with are the PRT-Alpha controller card that came with my machine (2004) and the 4g upgrade card that Shopbot offered a few years ago. From what I've read, the pre-4g PRT machines were furnished with several different controllers.

So, starting with the basics, the driver you need depends on the motor. You specified the Oriental Motor 7.2:1 geared motor; however, Oriental Motor offers two different models, the PK296A1A-SG7.2 and the PK296A2A-SG7.2. The A1A motor draws about 1A of current and the A2A draws up to 3A of current. The A1A motor has high inductance and the A2A motor has low inductance. My testing shows that a motor with low inductance is preferred unless your machine will be used mostly for low speed/high torque cutting. My choice is the A2A motor.

As far as I have read, the pre-4g PRT controllers were furnished with low current stepper drivers to match the motors that Shopbot ordered from Oriental Motor (which were basically a 4-wire version of the PK296A1A-SG3.6 motor). That means that your controller might not be able to handle the higher current demands of an A2A motor.

So, if I had an extra PRT controller card, I would find out if the card supplied Step/Direction signals or if it supplied CW/CCW signals. If it supplied Step/Direction signals, I would build an interface circuit (if necessary) to drive Gecko G203v stepper drivers. In other words, I would turn the controller into something very similar to the 4g board that Shopbot sells. However, the 4g card has some serious limitations (no heat sinks, limited drive current, overheating - to name the most common), so I could mount the G203v drives on a proper heat sink and run cables to each G203v. Finally, I would build or buy a 35VDC to 40VDC power supply. The price for four G203v drivers, a power supply and the other parts and pieces would cost between $750 and $1,000, depending on how elaborate your interface becomes.

A Gecko G540, power supply and Mach3 license would cost less. Going with the G203v drivers, Mach3 and a PMDX-122 would only be about $250 more than the cost of adding the stepper drivers and power supply to a Shopbot controller.

Designing a process control computer like the Shopbot controller board often requires some "adjustments" to normal process control computer design. That's a delicate way of saying that the designer did not fully understand what he was doing when he designed the I/O portion of some of the controller cards. For instance, my PRT-Alpha controller card has 5VDC connections for proximity sensors. Most proximity sensors are not rated to run at 5VDC - including those sold by Shopbot ($175 for two sensors and two very simple mounting brackets). None of the inputs or outputs have optical isolation, which leads to serious electrical noise problems. (Visit the Shopbot forum and you'll see posts - almost daily - where someone asks for help in solving unexplained movement or lack of movement. Most of those problems are caused by electrical noise.)

If you modify/build an interface so that you can use Gecko G203v drivers with a Shopbot controller card. Go the extra mile and add opto-isolation to each input and to each output. The Gecko drivers have built-in opto-isolation, so you won't need to add isolation to the step/direction signals.

It really comes down to your choice of software. If you need Shopbot code, you'll need to find a way to use a Shopbot controller (the Shopbot V20x series of boards is a possibility if you can/want to design a complete interface board). If you are willing to switch to G-code, you'll need to look at Gerald's design. Vectric's VCarve software will generate either Shopbot code or G-code. Shopbot's SB3 code has some nifty instructions to control a lot of I/O; however, I've had some extensive telephone conversations with some "power" users who have added a lot of I/O driven devices to their machines. Even with proper electrical grounding and proper opto-isolation, those "power" users have had some serious problems with unexplained glitches. Those of us who have been in the process computer business know that even well written software has to sit on top of Windows software. Sometimes the Windows software has to do its 'housekeeping' chores and signals are delayed or missed. What I'm saying is that SB3 software has lots of promise, but Mach3 software is probably the better choice.

Last edited by Richards; Mon 15 June 2009 at 22:51..
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  #4  
Old Tue 16 June 2009, 06:59
Kenrbass
Just call me: Ken
 
Richmond Tx
United States of America
Thanks for your thoughts Mike. I have the original version 3 board taken out of the machine at the moment and have one of the first (repaired) 4G boards running it now.

With having the extra board, I was thinking of building a whole new machine around the 4g board and then putting the old board back in the bot and selling it.

I would like to stay with the bot code for now. They have an option for using ecabinets for cutting and I already use ecabinets for desings, but have not purchased the shopbot link yet. If I go with G code I will loose that option to run a program I am familar with. Currently I run Vcarve pro for all my cutting needs though.

My controller has a 48 volt power supply and I kept that when I put the 4g board in. I didn't care for the way the geckos were just hanging on the side of the board, but it is working fine for now.

I can say that the motors are pretty hot to the touch compared to the old board, but it is much smoother in it's movements.

The current bot motors on it are 4.39 ohms, 2 phase,and 0.5 degree step, but that does not tell me inductance.

With the four wire motor, I am thinking it has power and ground to two of them and they reverse polarity of the other two to change direction. If it just used positive or negative pulses on one of the wires for movement, then I would think it would need some circuits in the motors, and as best I can tell it's a pretty basic motor.

I believe most of my issues are the light, bolt together gantries, but if I build half of a machine, I was considering going all the way.

Kenneth
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  #5  
Old Tue 16 June 2009, 08:39
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Ken, sounds like you already have 3.6:1 geared motors run by Gecko drives, and you won't get real cut quality improvements in that department. You could stiffen up your gantry and car without too much effort . . . . one of the key things to fix is the hold-down system for the y-car.

I just have this nagging feeling that building a whole new machine is not going to give you the huge improvements that you need, unless your current machine is worn out, in which case a little effort on the current machine is going to make a real difference.
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  #6  
Old Tue 16 June 2009, 09:39
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Ken, I agree with Gerald. Bracing your existing machine will give you about all that you can expect mechanically from this class of machine. A good friend built a really heavy-duty CNC machine in Boise, Idaho. That machine weighs several thousand pounds. He discovered that the PK299-F4.5 motors, when geared 4:1, were too small to handle the weight of the gantry. He had to go with the larger PK2913-F4.0 motors.

According to my data sheets, you have motors equal to the PK296A1A-SGxx motors, with the wiring options limited to bipolar series. The A2A motors are rated at 0.96 ohms bipolar series and 0.48 ohms unipolar (half-coil).

Your paragraph starting with "With a four wire motor, " has me a little worried. If power is applied directly from the power supply to the stepper motor, the stepper motor will be ruined. Power is applied to the Gecko G203v stepper driver and then the G203v applies power to the stepper motor in a specialized way that causes the motor to turn. The stepper motor has two coils, A and B. The stepper driver connects directly to the motor's coils. For each step pulse received, the stepper driver outputs a specific pattern to the four wires connecting it to the motor, which causes the motor to turn its shaft 1/2000th of a revolution per step pulse.

The 48VDC power supply is not optimum for the motors that you have. Using the formula that Mariss gave us, 32 X SQRT(30.8) = 177VDC. The Gecko G203v can only handle voltages up to 80VDC. If the motor had the standard six leads of other PK296A1A-SGxx motors, you could wire it half-coil, which would have 1/4 the inductance, i.e. 32 X SQRT(7.7) = 88VDC. In that case, a 70V to 80V power supply would give those motors acceptable performance.

If you replaced that power supply with one giving 70V to 80V, your existing 4-wire motors would run hotter, but they would also give your better performance. CAUTION: Shopbot supplied some 4g boards with Gecko G202 stepper drivers. On those stepper drivers, Shopbot switched an internal jumper to defeat the auto-current reduction mode. That would cause the stepper drivers to overheat if you use a 70-80V power supply. As far as I know, the G203v stepper drivers that Shopbot supplies with its 4g controller are Geckodrive factory default units - with auto current reduction built-in. There was some talk of getting a special model G203v from Gecko drive with an internal resistor changed to allow the G203v to better match the higher inductance of the Shopbot supplied motors. A special model stepper driver MIGHT also have the auto-current reduction feature turned off.
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  #7  
Old Tue 16 June 2009, 09:43
Kenrbass
Just call me: Ken
 
Richmond Tx
United States of America
By the Y car hold down do you mean a bearing held by a spring mechanism like the motor is on the other side?

I think the gantry is the weak link, but instead of welding on this one, I was thinking of building one of your designs.

Then I got to thinking that with an extra board, maybe I would just build the whole thing.

It is not worn out, just loose. I think the steady gantry without the slop will be enough to make me happy.

I just made a deal to purchase 3 slightly used geared motors (identical to what I have) for 100 each, so I guess I'm jumping in. lol

Kenneth
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  #8  
Old Tue 16 June 2009, 09:47
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Welcome Ken,
Watch out it is addictive.
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  #9  
Old Tue 16 June 2009, 09:53
Kenrbass
Just call me: Ken
 
Richmond Tx
United States of America
Mike, I kind of agree that bracing my machine will be the most help I can get on this type of machine. I can move the y gantry about 1/8 of an inch side to side on the oposite end that does not have a motor when the motors are locked in. I think this is my biggest issue.

If I buiild a new machine with the three motors I am buying, would you suggest the PK296A1A-SGxx for the fourth ( maybe Z) motor?

Kenneth
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  #10  
Old Tue 16 June 2009, 10:00
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Ken, you are plenty welcome to build a totally new machine, there is a lot of fun to be had in that as well! (But I just thought to make it clear that it isn't totally necessary to discard the whole ShopBot system)
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  #11  
Old Tue 16 June 2009, 10:06
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Have a look at Scott's hold-down here:
http://www.mechmate.com/Forum/messages/10/440.html

And his car stiffener plate here:
http://www.mechmate.com/Forum/messages/10/1251.html

The gantry needs 4 small brackets/links, but I am still searching for a pic. (From under the unistrut ends to a point near the wheel)
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  #12  
Old Tue 16 June 2009, 10:36
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Ken,

Although it's best to keep all motors the same so that all motors can run off the same power supply, personally, I would sell the A1A motors and get some A2A motors and some G203v stepper drivers for the A2A motors. The four-wire motor that Shopbot sold is just not a good match for any stepper controller that I've ever tried.

(I have four PH299-01 motors that have even higher inductance than the PK296A1A motors. Those PH299-01 motors have never been used, and most likely, they never will be used. The inductance makes them run rough with every controller that I've tried. A 'snake-oil' salesman sold me the motors for use with a Vexta stepper controller. He assured me that because the motors were rated 12V and the Vexta stepper driver was rated for 12V motors, that they would be a perfect match. When I called Oriental Motor and complained about the rough motion, the technician told me how lucky I was that my only complaint was a rough running motor. That's when I learned my first lesson about choosing stepper motors and stepper motor drivers. The OM technician assured me that although the Vexta driver could not be used with the PH299-01 motors, that it would be a perfect match for some 23-frame size motors. He was right. I learned a $600 lesson - and I still have the motors sitting on the shelf.)
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  #13  
Old Wed 17 June 2009, 07:08
Kenrbass
Just call me: Ken
 
Richmond Tx
United States of America
I am going to go ahead build a new gantry for the bot, but with a lot of thougths borrowed from the repaired versions you showed me Gerald.

I am still purchasing the other three geared bot motors and want to add a good z motor for my new bot/mate. It sounds like the pk296a1a-sg7.2 bipolar series will be a decent match for the old bot controller board. I will start gathering parts for this project.

Thanks for the hellp and advise.

Kenneth
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