#47 is making dust - Macon, MS
 

HI, I may or may not have introduced myself, I really can't remember. I have been lurking here for several years. I own a sign company/fab shop in Mississippi.

About a year and a half ago I was getting pretty serious about building a MM, then I found a used PRT Shopbot with Colombo spindle for sale. The man must have needed to sell because I got it for a song. I still want to build a MM because I believe it is a better machine (less give and flex in the frame) then my SB.

Now for my question.

This SB was built in 2004. I cannot move it in any Axis over 60 IPM without it loosing steps. They (Shopbot) have a new control board that I can buy and install that is suppose to give me about 200 IPM. It is $1400 dollars. I see that some of you are getting much faster travel then that. Could I build up a control box like the MM to run my SB. I no doubt would not be able to use the SB software but I am not real attached to that anyway. I am attaching a picture of one of my stepper motors. They are not Gecko's so I am hoping someone here can tell me if they will work with the Gecko Drives.

The other advantage to building this controller box would be that when I build my MM it will already have that part done.

Thanks for your help.
Blake

http://picasaweb.google.com/lh/photo...eat=directlink

Hi Blake

We had one of those motors on the Z-axis of our first MechMate, running with our "homemade" control box. So, the short answer to your question is: yes, a DIY control box will work with those motors.

However, those motors want 80V to get a decent performance out of them (ShopBot only gave them 48V) whereas we generally use motors that run at a lower voltage. If you want to build a control box for your current SB and later want to use that box for a MM, you will either need to:
- change the transformer to reduce the voltage, or
- buy motors for the MM that are suited to 80V, or
- move your SB motors to the MM and scrap what's left of the SB.

From a purely economic standpoint, it would make more sense to build a MechMate and sell the shopbot.

A serviceable professional grade MechMate can be built for $4500, including a chinese spindle. I built mine for about half that, and I complain because I can only cut at 240 ipm. :o

Around here, ShopBot PRT's sell for much more than that.

Put the chinese spindle on the bot and keep the Columbo, if you prefer.

Do you feel like if I built a control box like the mechmate only with a 80V output would I achieve decent speed before loosing steps?

What would be the reason SB only supplied those motors with 48 volts?

I am not ready to drop $800 bucks on new steppers but neither am I willing to build this box to work with the ones I have and not get a substancial speed increase.

Thanks

At that time, the motor supplier indicated the voltage to use. Also, the "drives" used at that time were only half-stepping. A lot has changed since those motors were first used by SB nine years ago.

I am not making any promises regarding the new speeds that you will get (I havn't made any promises for MM performance, ever) and there is not much experience here with those particular motors. However, a self-built controller (using typical MM components and Mach3) will be technically as good as any controller that SB could sell you now - not better, nor worse. Your decision is whether you are willing to invest the time and go through the learning curve, plus spend about $1000 on components for the controller. Or, you can buy a ready-made SB controller, or you can buy various Mach3 based controllers from a couple of vendors (but these vendors need to be warned on the voltage).

Would that 80v be on the outside edge of the Gecko 203 operational window? Would it be better to run them at say 75 volts to be able to use the Gecko Drives?

Mike, I have a bit of a problem with this 75V versus 80V argument for geckodrives....

The geckodrive designer will tell us that he has already applied a safety factor when he chose to spec the drive at 80V, knowing that the internal components components can handle well over 80V. Why do so many people feel compelled to add another measly 5V safety margin? We are not dealing with a finicky device that absolutely dies at 1V (or even 10V) above 80V. If the safe voltage for a gecko is 75V then it is the gecko designer's job to tell us to use 75V - but he tells us 80V. Let's leave it at that.

Good point. I guess maximum operating volatge is within the specs.

http://www.geckodrive.com/upload/G203V-REV-7-MANUAL.pdf

I just know that making sure these power supplies are putting out the true voltage they are rated at can sometimes be an issue, more so if your building your own. But I think current would be more damaging than a little extra voltage.

But like you say - The Geckos are such good drivers all that has been thought of by Marcus

Mariss has mentioned the voltage parameters a couple of times (example) and it goes as follows: "Our 7A / 80VDC drives are designed to operate at 100VDC and don't come apart until 116VDC". In other words, he has already applied a 20 to 36 Volt safety margin, yet there is the common (mis)conception that one needs to use 75V to avoid exceeding the 80V.

With the tolerances Mariss has on his drives, you will have no trouble operating your supply at 80V, not worrying about high or low line and output fluctuations of your supply. :)

Well... you talked me into it - Macon, MS
 

It didn't take much convincing tho:)

So without further ado... The journey begins.

No, no, no. Much too organized!:D Welcome to the funhouse.

you should have taken the red pill:D

Welcome aboard !!!

Welcome Blake!

Here is to the journey Blake, I am starting mine this week also.

That sounds great Dave! I wish we were closer. I have been lurking here for a long time. I was almost ready to get started a year and a half ago, then I found a used Shopbot. I bought that and have used it till now. I was thinking of upgrading it with some MM components but when I thought it over it made more sense to sell it and build a complete new MM. Well it sold within one day of advertising IT! Shopbots really do have a great resale. I hope that MMs will be that way one day too.

I am going to be ordering some of my control components soon. Is the general consensus that the G540 performs as well as using the breakout board, g203V drivers combination?

Also going to be ordering my laser cut parts. I have read posts where you cannot get them from Alabama, and posts where you can. Can you or can you not at this time? I am around 150 miles from Birmingham and would love to get them from the boys in Alabama if possible.

Blake

Good luck with your build.

Blake,
I ordered mine from Metalhead, I believe he is in Alabama. Not sure how to post a link on here yet, just search for him.

G540 decision
 

I for one can attest to the simplicity of the G540 installation if you want to save space in the box and can either solder DB9 connectors or are willing to buy the screw-type adapters. Once I got mine up and running (all the problems were of my own making and could have been avoided with a little more reading) it has been very reliable and robust (also there are no heat problems). It is also somewhat cheaper than the BoB/203 combo.

Best of luck on your build.

Joe

I am here. Just PM me for the parts.

As the original advocate for the G540 solution, I'll of course say it performs just fine. The caveat is that you have to match the motors and gearing (or belts) carefully. The OM 7.2s work just fine, and the equivalent stepper motor plus 4:1 belt transmission should too, although I can only vouch for the OMs personally.

As I can't seem to avoid analogies, the G540 solution is a moderate size four cylinder engine. You have to match the rest of the car to the engine. The BoB/203 combo is a big block 6 cylinder; it'll drive anything.

Brad,

I don't understand why the G540 with OM 7.2's would be any less powerful than 203's/BoB with OM 7.2's since you can operate the OM 7.2's with the same voltage and current settings in either case. Can you enlighten me?

I'm curious since I'm planning my next MechMate.

John, the OM 7.2s won't be less powerful on the G540. However, if you wanted to drive a different stepper motor that needed more current or voltage, the G540 wouldn't work as well as the 203s; it would starve the motor (at best), or fail to work.

As it is, the OM 7.2s are a nice match for the machine, and they run lovely with a G540. Sorry for the confusion.

Thanks Brad.

I'm exploring options such as using the G540 (or G250 plus BoB) or other drivers and belt reduction. So I'm trying to understand the trade-offs of the various approaches.

John, I don't think it's a tradeoff between the G540 and belt reduction; the tradeoff is essentially motor size. With the G540, there is one fairly specific, single stack Nema34 motor size that works well (inductance under 4mh, amperage 3A or less). That motor size is available from multiple suppliers, with or without a gearbox. (But I need someone else to confirm they've actually used something other than the OMs.).

That motor size is widely believed to need some mechanical advantage to work well on the MM and have adequate torque. So you need either a gearbox, or a belt drive; it isn't suitable for direct drive.

Because gearing or belting down the motor requires more step pulses for a given distance, you will be reducing the maximum speed a given PC can provide. Generally, this speed is above what you will need to cut, but might slightly limit your rapid positioning speeds.

So the compromise you make with the G250 based (incl G540) drives is that you use a small(er) motor, have to reduce it, and limit your top speed. However, arguably this is the normal MM configuration; using larger direct drive motors is rare. The gearing/belting does also increase the theoretical accuracy of your machine.

Thanks for the explanation Brad.

A couple of questions.

Since the maximum voltage the G540 can support is 50 volts, wouldn't the maximum inductance be about 2.5 mH? (Mariss' formula for calculating maximum volts being: square root of inductance multiplied by 32).

Also why limit the current to 3 Amps? The G540 is "engineered" to support 3.5 Amps. Do you feel you should not use the full range of G540 capability?

John, you've caught me sleepy in the morning working from memory, I should have grabbed the spec sheet:

"1) MOTORS: One to four step motors are required. All NEMA-17, most NEMA-23 and a few NEMA-34 motors are acceptable. The motors preferably should be square in cross-section, not round. The motors can be 4, 6 or 8-wire motors. 5-wire motors cannot be used with the G540. Choose a motor that has a rated current of 3.5A or less. Choose a motor that has a rated winding inductance of 2.5mH to 3mH if maximum power output (>100W mechanical) is a requirement. Never use a power supply voltage greater than 32 times the square-root of the motor inductance expressed in milli-Henries (mH)."

Note that it can be difficult to find a NEMA34 at 2.5mH, so you end up with a compromise, trying to get as close as you can, but running over. You fail to run the motor at it's maximum theoretical output, but with the mechanical reduction you still have more torque than you need.

The numbers in my head come from the OM gearmotors, which are at 3A (I think).

Brad & John, realise that this is Blake's thread and you probably won't find these posts again. ;)