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  #1  
Old Sat 10 November 2007, 11:23
mtgstuber
Just call me: Michael
 
Newman Lake, WA
United States of America
Upgrading a ShopBot PRT - Newman Lake, WA

Hi folks,

I have a vintage 2000 ShopBot PRT w/ PK296 3.6 stepper motors on the X and Y, and a 7.2 on the Z. I'm interested in doing some upgrades on the unit to improve the quality of the cut and the speed of operations. Originally I had thought about buying an Ascension, but they seem to have disappeared. I'm not impressed with the ShopBot Control software, so moving to the new control box doesn't do much for me either. So, while I really don't need another project (ask my wife), I'm now thinking about building my own.

I've poked around the different forums and reviewed the plans (nice work by the way, thanks!). I suspect what I'm looking for is in the forums somewhere, but there's a lot to wade through. I get the impression that fundamentally I need a set of Gecko 203Vs, a PDM 2212, a copy of Mach, and a power supply, as well as bunch of connectors, switches, and time.

My questions are thus:
(1) Beyond the control box, what things should I consider doing to tighten/stiffen/improve my rails/gantry/etc. I built my own steel table out of C channel, and my gantry has the older design with the steel angle bolted to unistrut to stiffen it.

(2) I've come across various drawings for the control box on the site. Is there a "current" or "best" drawing? (I couldn't find a proper schematic that describes the geckos or the PDM in the PDFs, but maybe I went too fast and missed it.)

(3) In line with #2, is there a "parts list" for the control box?

(4) What silly assumptions am I making about this that I haven't realized? Any tips to prevent me from doing something bone-headed would sure be appreciated.

Feel free to refer me to other parts of the site. Thanks!
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  #2  
Old Sat 10 November 2007, 11:58
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
As far as the electronics go, if I were thinking of modifying a PRT, I would get the electronic parts and 'bread-board' everything on the kitchen table, just like Gerald and others have done. By doing that, you will be able to verify that everything works and you'll be able to test various layouts and designs before building the final version.

Look carefully at the various photos here on the Mechmate Forum to get an idea of what you will need. The major components will include:

4 each Gecko G203v stepper drivers (or G202 if you prefer)
1 each PMDX-122 break-out-board (or another brand if you prefer)
1 each 35V to 70V, 10A to 15A power supply (depending on how you wire the stepper motors.

Besides that, you'll need terminal blocks, DIN rail, industrial switches, contactors, ferrules, etc.

Pay very close attention to Gerald's recommendations, including his recommendation to use crimp terminals. It costs a little more and it takes a little more time, but you won't be wasting time and effort tracing down errors as time goes on. If you follow Gerald's advice, you'll end up with a top notch controller.

Finally, if you build a prototype controller you'll be able to install and learn Mach 3 before 'crunch' time. That will make the transition between Shopbot SB code and G-code much easier.
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  #3  
Old Sat 10 November 2007, 12:08
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
mtgstuber (is that what people really call you?) welcome!

For the y-car, Have a look at the last two threads of this page in the old forum. (I think the linked drawings are gone)

On your gantry, you could weld a brace from near each wheel to the bottom of the unistrut : small bit of flat bar 1" wide 2" long if I remember correctly. (If you have a "racking" problem, you can weld in bigger gussets).

Sorry, the control box here is not so detailed. A couple of people have built their own successfully, but no two of them are alike. I don't think it is difficult to build a successful box, but you need to arm yourself with some knowledge of the basics and the risks.
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  #4  
Old Sat 10 November 2007, 13:58
mtgstuber
Just call me: Michael
 
Newman Lake, WA
United States of America
Actually people call me Michael. I suppose I should update my profile.

Thanks for the advice. I'm particularly liking the plate added to the existing Y carriage in 2nd to last thread. Guess it's time to finally buy that welder my wife wants. (You just gotta love a woman that wants a welder.)

I'd like to drill in a little on the command that Richards made about 35 to 70 based on wiring. I've wired houses and soldered motherboards, but I've never wired up a set of stepper motors. I'm assuming there are probably some trade-offs between different configurations.


According to the G203V manual:
The choice of power supply voltage depends on the required high-speed performance from the motor; doubling the voltage doubles the motorís high-speed power. In all cases the power supply voltage should be no less than 4 times or no more than 20 times the motorís rated voltage. The motor may not run as smoothly as possible if the power supply voltage is less than 4 times or more than 20 times the motorís rated voltage. A power supply voltage greater than 20 times the motorís rated voltage may overheat and damage the motor. Motor winding inductance
should be 500 uH or greater.
I've been looking at the Oriental Motors Site, and their specs on my PK296A1A-SG3.6 motors. http://www.orientalmotor.com/product...XTA/StPk29.pdf

The current seems straight forward, sort of. The catalog lists the motor as 1 Amp, while motor says 1.5A. I'm at a loss to determine what the motors rated voltage is, such that I can determine what voltage supply I need. The catalog lists the voltage as 4.4 volts. But I'm not sure what that means in terms of a maximum.

Is there a "wiring stepper motors for newbies" page / post someone can recommend?

On a related note: Is it silly for me to be sticking with my current motors? Looking at the specs it seems the gear heads are prone to a lot of backlash. Would I do better to change to something else? Any recommendations?

Part of what I'm trying to figure out is could I just use a PMDX 8020 or 5020 with these motors, or would I hose them up?
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  #5  
Old Sat 10 November 2007, 15:48
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by mtgstuber View Post
On a related note: Is it silly for me to be sticking with my current motors? Looking at the specs it seems the gear heads are prone to a lot of backlash. Would I do better to change to something else? Any recommendations?
Michael, that's a new one on me. I think many ShopBotters suspect the gearheads for their jitters because of potential backlash, but I can't think of one post where anyone has actually proved a backlash problem. To the contrary, everyone raves about how smoothly those geared motors cut . . . . . . when run on proper micro-stepping drives like the Gecko, G4, Ascension, Bob Campbell, etc.

The other Mike will surely have more to say on the motor voltages, but you could do more homework over here.
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  #6  
Old Sat 10 November 2007, 20:28
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Michael.

"The PK296A1A in series connection does not match to the G203V drive because the inductance is too high. If this motor/drive combination is used, it must be wired half-coil. The better motor choice for the G203V drive is the PK296A2A motor with gearbox)"

Mariss helped me verify for my system that 56 Volts would be an ideal match (actually something close to 49.732 Volts) for proper cornering and speed based on the PK296a2a-sg7.2

Mariss at Gecko was extremely helpful in verifying what Gerald, Mike Richards and others have already pointed out on the suggested geared stepper to match the 203V drive- it all came down to motor efficiency, cornering speed, proper voltage/output speed/torque......
based on my machine set up of 20pitch/20 tooth pinion/7.2 gearbox/56 volt supply.


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  #7  
Old Sat 10 November 2007, 21:45
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
I had to take a few hours off to till up the garden. We're probably having our last comfortable autumn day before the cold weather sets in and the thought of looking at weeds sticking up through frozen ground was more than I could handle. Running the tiller for 1/2 hour was more than my old body could take so I spent a few hours catching my breath.

Now, on to the mystery of stepper motors and stepper motor drivers. I think that stepper motor manufacturers and stepper driver manufacturers must have formed a secret society years ago whose goal was to make everyone else think that stepper motor products were complicated. In large part, they've succeeded with their goal. The data sheets are overly complex - mostly because the manufacturer doesn't include wiring diagrams or sample connection circuits to show the different ways that a stepper motor and a stepper motor driver can be used.

We need to always remember that most stepper motors and stepper motor drivers are used in some kind of process-control application. When used in a process-control application, a stepper motor normally does just one thing over and over. That means that the process-control designer needs to have some flexibility in connecting a stepper motor, otherwise he would have to specify many different stepper motors for every process-controller, rather than just using the same model motor in differing ways.

So, with that little bit of background, think of a six-wire stepper motor as two motors in one. One motor is designed for highest torque at low speeds. That motor uses full-coil connections, usually known as Bipolar Series connections. The other motor is designed for slightly less torque at much higher speeds. That motor uses half-coil connections. On the data sheet, there are several items that help us know how to configure a stepper driver to get the best performance out of the stepper motor. Only two of those data items are necessary if we use Gecko stepper drivers. We need to know the Amps rating and the Inductance rating. Because a six-wire motor is two motors in one, there will be two Amps listings and two Inductance listings. One listing will be for the motor when it is connected using full-coil connections. The other listing will be for the motor when it is connected using half-coil connections. The confusing part of all of this is that most manufacturers print the half-coil ratings directly on the motor's spec plate, without specifying that the printed rating is the motor's rating when it is connected using the half-coil connection method.

Now that we know that we really have two motors in one, we have to decide which of those two motors we're going to use. I'll make it really simple. If you're using Oriental Motor stepper motors and if you're using Gecko G203v stepper drivers, use the half-coil connection method. That means that you'll be using the Black wire and the Yellow wire for the A coil and that you'll be using the Red wire and the White wire for the B coil. (If you want, you can substitute the Green wire for the Black wire and you can substitute the Blue wire for the Red wire. The motor will still run.) Using the half-coil wiring method means that the motor will have just as much torque at 500 RPM as a full-coil connection gives at 100 RPM. On the other hand, if you're going to be using your machine at slow speeds and need to have the absolutely highest possible torque at slow speeds, then wire the motor using the Bilevel Series wiring method (full-coil).

Once you've decided on your connection method, half-coil or full-coil, then look at the data chart to see how much current the motor can safely pull. The PK296A1A-SG3.6 motor is rated to pull 1A when connected full-coil and 1.5A when connected half-coil. Once you know the current rating of the motor, then use Gecko's formula to determine which current limiting resistor to use. The formula is: 47 X I / (7 -1) = resistance needed X 1000, where I = Amp rating. A full-coil wired motor would require: 47 X 1 / (7 - 1) = 7.8 and 7.8 X 1000 = 7.8K resistance. So, you would use a resistor that had the nearest standard value to 7.8K, which would normally be a 8.2K 1/4W resistor. A half-coil wired motor would require: 47 X 1.5 / (7 - 1.5) = 12.8 and 12.8 X 1000 = 12.8K resistance. So, you would use a resistor that had the nearest standard value to 12.8K, which would normally be a 10K and a 2.2K 1/4W resistor connected in series. Using the correct resistor will keep the motor from overheating while allowing the motor to work as hard as it was designed to work.

The last thing that we need to determine is the voltage of the power supply. Mariss at Geckodrive published a nifty formula that determines the MAXIMUM voltage that can safely be used with a good quality motor (and the Oriental Motor stepper motors are very good quality motors). The voltage must be LESS than 1000 * SQRT(inductance). You must also remember that a Gecko G203v stepper driver has a MAXIMUM rating of 80V. The PK296A1A-SG3.6 motor is rated at 30.8mH when it is connected using the full-coil method, so 1000 * SQRT( 0.0308) = 175V. Since the G203v is limited to 80V, you must use a power supply rated at no more than 80V. (I've set the upper limit to 70V because I know that my local power company sometimes delivers 125V power.) Look again at the computed voltage for that motor when the motor is connected using the full-coil method. 80V is less than 50% of the voltage that the PK296A1A-SG3.6 motor could handle, so if you wire that motor using the full-coil connection method, you're going to be throwing away half of the motor's potential. On the other hand, if you wire the motor using the half-coil wiring connection method, then 1000 X SQRT(0.0077) = 87.7V. In that case, you could use a 70V or even an 80V power supply without burning up the motor. I don't like excessive heat, so I would use a 35V to 70V power supply with that motor when it is wired half-coil. Most likely, I would use a 50V power supply as a good compromise between excessive heat with slightly limited speed. (The voltage of the power supply has the greatest effect on the motor's top speed. The higher the voltage, the faster the motor will spin. But, the faster the motor spins, the lower the motor's torque. So, getting high speed out of a stepper may not be practical.)

Now that we've explored some of the secrets of stepper motors and stepper motor controllers, let's specify a power supply and current limit resistor for a Mechmate that uses four PK296A1A-SG3.6 motors with the motors being connected using the half-coil connection method. Four times 1.5A = 6A, so I would use a power supply that delivers at least 6A at 50V. (Even a small power supply from AnTek would put out at least 10A, so I would buy a 10A power supply to let the motors pull just as much current as they need without overheating the power supply.) I would install a 12K current limiting resistor on each G203v stepper driver to allow the motors to pull about 1.5A. That's all that there is to it.

As to the question about the gear heads having too much backlash: I haven't talked to anyone who has experienced a backlash problem with Oriental Motor PK296A1A-SGxx motors. Mach 3 has backlash compensation built in, but I don't think that you would ever have to use it.

As to the question about the PMDX-135: One of my test benches uses a 50V toroidal transformer connected to a PMDX-135-8020. Sometimes I wire the transformer to output 25V, other times I wire the transformer to output 50V. After rectification and filtering through the PMDX-135, I get 35VDC or 70VDC. If I need a lower voltage, I use a Variac to reduce the voltage going to the toroidal transformer. The PMDX-135-8020 works perfectly, but an AnTek power supply that is ready to use costs about the same as just the PMDX-135, so, in the future, I'll most likely just buy an AnTek power supply. I saw two AnTek power supplies at a Shopbot Camp a few weeks ago. The AnTek is well built.

As to the question about 4X to 25X rated voltage: Before Mariss released his new formula for computing the MAXIMUM voltage for a stepper motor, he published the 4X to 25X rule. That rule works very well. I usually picked 10X to 15X as my target voltage and the motors ran very well with only moderate heat. By the way, 3.3V X 15 = 49.5V.
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  #8  
Old Sat 10 November 2007, 22:50
mtgstuber
Just call me: Michael
 
Newman Lake, WA
United States of America
I guess I need to call Gecko and see what they recommend. I've had no objections to the motors I have, I just noted that the gear head motors (according to the oriental motor catalog) have a lot of backlash relative to other units.

Believe me, I'm happy not to spend $600 (or more) on new motors.

In the other reading I've done, it seems perfectly reasonable, if not desirable to run the motors I half-coil.
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  #9  
Old Sat 10 November 2007, 23:36
driller
Just call me:
 
Quote:
Originally Posted by Richards View Post
<snip>
Now that we've explored some of the secrets of stepper motors and stepper motor controllers, let's specify a power supply and current limit resistor for a Mechmate that uses four PK296A1A-SG3.6 motors with the motors being connected using the half-coil connection method. Four times 1.5A = 6A, so I would use a power supply that delivers at least 6A at 50V. (Even a small power supply from AnTek would put out at least 10A, so I would buy a 10A power supply to let the motors pull just as much current as they need without overheating the power supply.) I would install a 12K current limiting resistor on each G203v stepper driver to allow the motors to pull about 1.5A. That's all that there is to it.
I'll not comment on a few of the assumptions, but the motor selection is dependent on voltage and inductance. The A1A version is 30.8hM series connected. The A2A version is only 6mh series connected and would be the proper choice for series wiring and a Gecko.

http://www.orientalmotor.com/product...XTA/StPk29.pdf

The power supply selection is simple. 4 motors at 1.5A = 6A times 50 volts, one gets 300 watts. So, 300 watts is the base result of the calculation.

ALL calculations come back to watts for wiring sizes, fuse sizes, circuit breaker sizes and the like.

Mariss has recommended that only 66% of the result of the calculation is required to run the motor. 66% of 300 watts is roughly 200 watts. I think he has supplied over 200,000 Gecko's and has never reported a failure due to a power supply that followed this rule.

Most people like to add safety factors, so adding a 50% safety factor, one would multiply 200 watts times 1.5 to get 300 watts as a power supply that is 50% more than required.

500 watts is 250% oversized. Since the motors cannot use anything more than the 200 watts, it really does not matter a whole lot if one goes wild and gets something even larger.

Motor protection is another calculation. Motor protection should not be more than 125% over the motor rating. A transformer larger than that will be outside of any motor protection circuitry.

Since 6 amps at 50 volts is a DC power rating, the transformer would need to supply 37 volts AC at 300 watts. 300/37= 8 amps, so a 10 amp 37volt power supply would be a perfect match.

As I mentioned, everything comes back to watts.

6 amps at 50 volts DC = 6x50=300 watts
300 / 50 = 6 amps

37Volts AC = 50 volts DC
300 watts / 37 = 8 amps

300 watts at 110 volts AC = 3 amps

300 watts / 230 VAC = 1.3 amps

So, depending where you need to put a fuse or a breaker for that 300 watts, the amps will be dependent on voltage.

I have seen some people ask about sizing wires from the breaker to the control panel and about sizing breakers at the wall and in the control panel. Comming back to watts reveals the answer to that question.


Quote:
Originally Posted by mtgstuber View Post
I guess I need to call Gecko and see what they recommend. I've had no objections to the motors I have, I just noted that the gear head motors (according to the oriental motor catalog) have a lot of backlash relative to other units.

Believe me, I'm happy not to spend $600 (or more) on new motors.

In the other reading I've done, it seems perfectly reasonable, if not desirable to run the motors I half-coil.
Do some math on the step distance on the backlash. I think you'll find it might be almost too small to measure at the work.

As for motor wiring, the reason there are so many options is because they are ALL correct and all are perfectly reasonable for some application. Each application has it's best choice.

Steppers are constant power motors. they have the same power at low speed as at high speed. however, the torque is speed dependent. Higher torque at low speed to almost no torque at high speed.

If you have motors, use them. There should be no reason to swap them out unless you find the backlash is not acceptable.

Dave
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  #10  
Old Sun 11 November 2007, 02:27
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Michael, I believe the PRT contains a 48V 300W (or 300VA) power supply which you can start off with. But I see that ShopBot have now dropped this style of power supply for the "Buddy" and are also using an un-regulated tor.trans. and rectifier like us. Mike says to aim for 50V which is conservative for heat - I like heat, so I would aim for 75V, which may bring some extra speed/torque.

Anyway, we could get buried in the details; To sum up, get yourself 4x geckos, 1x pmdx-122 and for the rest use what you have. Stiffen up the Y-car with the plate, ditch the existing "hold-down" rollers (move 2 of them to the z-axis), add a spring-loaded bearing/roller opposite the y-motor, weld the legs of the gantry to the unistrut. With minimal expense you should have a vastly better machine.
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  #11  
Old Sun 11 November 2007, 07:15
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
I think that we still have some confusion about driving stepper motors. If voltage is compared to water pressure and current is compared to water volume, then it is easier to visualise how electricity works. In the case of a chopper drive for a stepper motor (such as a Gecko G20x stepper driver), the desire is to force as much current into the motor's coils as quickly as possible. So the higher the pressure (voltage) the faster the stepper driver can force electricity into the motors coils. However, once the motor's coils are full of electricity (this is really an extreme oversimplification, but I hope that the visual image is still working), the pressure (voltage) is turned way down. In order for the valve mechanism to work, the stepper driver requires a minimum amount of voltage. Mariss states that the range of voltages that can be used with a G203v stepper driver is between 15V and 80V. He also states that the range of voltages that can be used with a G202 is between 24V and 80V. That does NOT mean that a minimum of 15V or 24V is constantly fed to the motor. It only means that the stepper driver requires that minimum voltage so that the internal valve or gates can function properly.

When we have a range of voltages to choose from, all of which will work, how do we choose the proper voltage? That depends on how we're going to use the motor. In my case, I mount the motor on a test bench and play with a standard 70V power supply, a Variac (to adjust the voltage going to the power supply), and an ordinary fish scale (spring type used to keep fishermen honest when they're talking about the 'big' fish they catch). I attach one end of a small cord to the fish scale, wrap several non-overlapping turns of cord around the motor's shaft and hold the other end of the cord in my hand. Then I let Mach 3 spin the motor at a desired step rate. When the motor is at speed, I pull on the cord as I watch the fish scale. At a certain point, the motor is going to stop - suddenly. Then I use a little math to determine how many oz*in of torque the motor produced before stopping (loosing steps). I do that at various RPMs, until I hit 500 RPM or 600 RPM. That's as high as I need to go for my application.

After I have performed the torque tests, I have a pretty good idea of how that particular stepper motor is going to perform for me. It becomes very evident that the motor's ability to perform hard work rapidly diminishes as its speed increases.

Finally, I calculate how fast the motor is going to move the CNC router's axis at various RPMs. For instance, a PK296A1A-SG3.6 motor with a 30-tooth spur gear (about 1.5-inch pitch diameter) will move an axis about 2.2-ips at 100 RPM, 4.3-ips at 200 RPM, 6.5-ips at 300 RPM, 8.7-ips at 400 RPM and 10.9-ips at 500 RPM. If I'm going to be doing mostly 3D work, then I pay particular attention to the 100 RPM range because ramping will probably keep the axis from ever going faster than 2-ips. On the other hand, if I'm using the machine mostly as a saw to cut long passes at high speed, I look at the 300 RPM to 400 RPM stats to see how quickly I can make a cut. (I also carefully examine data from a chip load calculator to see what type of cutter I'll have to use and how fast I'll have to spin that cutter to make a cut. If the chip load calculator shows that 6-ips is the ideal feed speed for my material and my cutter, then I don't worry about pushing the axis faster than 6-ips.)

Finally, after doing some testing and making some calculations, I know the best voltage for my application. What I'm trying to say is this: If a 35V power supply lets me run an axis at 6-ips with really good torque and at 12-ips at marginal torque AND if I'm going to use tooling that will limit my cutting speeds to 6-ips, then that 35V power supply is all that I'm going to need to do the job because it would allow me to cut at 6-ips and jog at 10 or 12-ips. On the other hand, if my tests show that a 50V or a 75V power supply is needed to produce enough torque to do the job, then I use the higher voltage power supply. If a 75V power supply won't do the job, then I have to choose a different motor or different gearing.

What do you do if you don't have the luxury of a test bench with a Variac and enough spare time to play with motors? One simple approach is to buy the highest voltage power supply that you can safely use. If you take that approach, then you'll know from the start that there is nothing that you could have done differently to get more speed or torque out of those motors. That approach works and that is the approach that I would take if I didn't have the luxury of a test bench. The only drawback to using that approach is that you may have to deal with extra heat from the motors. Most likely the motors will have no trouble handling the heat. You or your workers will learn, in a hurry, to not touch the motors after they've been running all day.

Fuses are another point of confusion. Simply stated, most fast acting fuses that you buy at the local hardware store are rated to BLOW at the fuse's current rating. They can and do blow at current ratings 25% less than the fuse's current rating. So, when I size a fuse, I pick one that has a current rating at least 125% of the working current of the circuit BUT I'm careful not to pick a fuse that is much higher than 125% of the fuse's rating. I want to make certain that the fuse will handle normal current without blowing but that it will blow when that current is exceeded. Then, I make sure that the wiring is rated to handle at least 150% of the current that will pass through it at the circuit's rated voltage. That's how I prevent fires in electronic circuits.
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  #12  
Old Sun 11 November 2007, 10:44
mtgstuber
Just call me: Michael
 
Newman Lake, WA
United States of America
Gerald, Mike, thank you. I have a much better idea of what I need to do with the control box. Seems like it's time to spend some money, and litter the kitchen table with stuff.

As Gerald suggested, tearing apart my ShopBot control box, I find that I have a 48V 6.25A supply, so I'll hold off on buying a supply for the moment. (As I think to myself, duh, why didn't I take this apart earlier)

A question about the current set resistor: Am I correct that this is wired in series with the motor? Neither the Gecko documentation nor the Oriental Motors documentation seems to say, though perhaps I missed it.

Two more questions:
Quote:
ditch the existing "hold-down" rollers (move 2 of them to the z-axis), add a spring-loaded bearing/roller opposite the y-motor, weld the legs of the gantry to the unistrut. With minimal expense you should have a vastly better machine.
Is there any specific reasoning for the size of the bearing used for the roller hold down? In reviewing the drawings, I'm thinking I could simply use a bearing I already have and design around it.

With respect to move 2 of the V-bearings over to the Z axis, I'm curious as to why. Don't get me wrong, I tend to be the sort that if something requires 4 screws I'll use 8, that said, I'm wondering what mechanical advantage adding the additional v-bearings provides.

One last note: Due to starting a reply, heading out for a few hours, and not refreshing the page prior to posting, I ended up saying, "I guess I need to call Gecko" right after Mike's very clear, very helpful explanation regarding the motors. While I certainly need to call them to order things, I think Mike's explanation provided what I needed. My apologies for appearing boorish. I'll hit the refresh button next time.
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  #13  
Old Sun 11 November 2007, 10:58
gmessler
Just call me: Greg #15
 
Chicago IL
United States of America
Hi Michael,

The current set resistor goes between terminal 11 and 12 of the Gecko.

Good luck
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  #14  
Old Sun 11 November 2007, 11:21
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
"Is there any specific reasoning for the size of the bearing used for the roller hold down? In reviewing the drawings, I'm thinking I could simply use a bearing I already have and design around it."

Any old ball bearing would mostly do. Let's say between 1 and 2 inches OD. The bigger the OD, the less the rolling surface resistance (under the raw painted angle iron), but it could start looking ugly.

"With respect to move 2 of the V-bearings over to the Z axis, I'm curious as to why. Don't get me wrong, I tend to be the sort that if something requires 4 screws I'll use 8, that said, I'm wondering what mechanical advantage adding the additional v-bearings provides."

The bottom pair of bearings of the z-slide carry the most load, and they tend to wear a hollow in the slide over the short working range. We often found these rollers to need re-adjustment to compensate for wear. Doubling the number of rollers reduces the loads by half. When our old ShopBot got jittery it was very often because the z-slide had developed play at this point. The double rollers there helped a lot to firm it up.
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