PRT standard shopbot needs advise

[David Iannone]

Hi everyone, I have a PRT standard. I am thinking of upgrading to the 4G board, or upgrading to alpha. The alpha is closed loop, but I don't know how that will benifit me, and not sure what closed loop is. Every once in a while I lose some steps in a big job. Not a big deal though because every long job I run I drill a hole at 1,1 after I zero the table in case of this. I stop, rezero, and restart a few lines before the mistake. This only happens once every few months, so is something I have learned to live with. Here are my questions:

1. If upgrade to alpha, will the alpha pause if a step is missed, or do the alpha tables just not loose steps?

2. 4G upgrade looks good to me, but what I read so far on the forum about this board confuses me a little. It sound like the PRT motors aren't made for the higher speeds. The resolution is what I am curious about also. I do small engraved rowmark nameplates, as well as regular carved signs, will the 4G board resolution be tighter on fine engraving than my PRT or an alpha table?

I run a porter cable router head, and relize if I upgrade to 4G or Alpha I will still not reach cut speeds of a spindle. I am looking for Higher jog speeds #1, and possibly more reliablity of no loss steps with the alpha #2. I genrally cut 1/2" wood, 1.5" cedar and signfoam. My cut speeds have always been .5 - .7 inchs/sec. Production wise I'm fine with these speeds. But I have just stared doing more textured backrounds in V-Carve, and looking into cuting more 3-D signs on the bot. My last signfoam job I cut out the texture portion at cut speed 3.5 inchs/sec and z speed at 1.5 inchs/sec. It took 12 hours (no lost steps). If I had the 4G board could I cut that same job out in 6 hours? I am steering most of my signs now to be carved out of signfoam.

I'm not sure which upgrade to do. Money is not the most important matter. Anyone with 4G upgrade please advise if anyone has suffered loss steps, alpha botters, do you loss steps?
Thanks,
Dave

[richards]

David,
The 'closed loop' function on the Alpha is misunderstood by many of us. On my Alpha, when something happens and one of the four motors slows and starts to stall (missed step syndrome), the Alpha motor tries to regain its position. Sometimes it can't and it stalls, causing a fault that stops the machine. Sometimes it can fix things on its own - which usually causes another problem, especially when one of the X-axis motors takes corrective action. The result is what I call a 'divot' in the cut. If you're critical, you'll see the 'divot' and usually throw out the part. If you're not so critical, or if the part is something that you can sand a little, you'll be alright.

Personally, I think that it would be better to slow things down just a little so that the stepper motor doesn't stall. In the long run, you'll get more consistant parts and none of the problems of trying to rezero and then restart a cut.

[rhfurniture]

David, I am in a similar position as yourself. I am not really interested in higher speeds, as I only have a benchtop and a 2hp router (though to jog at twice the speed would look cool), but if the 4G would give a better finish to the cut at my current low speeds then I would go for it. Mathematically, increasing the resolution from 0.03mm (published spec pre 4g) to 0.012mm (1/4 stepping to 1/10 stepping) isn't going to affect my accuracy, or I suspect in itself, the cut finish - mechanical issues, cutter sharpness etc will have far more effect, and shopbot themselves publish the overall accuracy as being 1/10th of this figure (0.3mm). Most of the "chatter" pictures posted on this forum seem to be of a much higher magnitude than resolution based jaggedness, and personally I get a pretty good finish unless I push it cutting birch ply, when it can start to look "scaly", - I am sure this is mechanical, not a resolution issue.
However, I think that the 4G could have positive affect on finish due to it's driving the motors more smoothly, and as I have not yet tried it out, this is all pure spectulation - I would love someone who has one to confirm it. Up to now most talk is of speed gains, not of finish improvement (though most say it sounds smoother). If I move my shopbot at different speeds and at different angles in XY, then I get a different degrees of juddering at the peripherals of the structure. If running the steppers "smoother" reduces this, then I suspect the finish quality goes up. Does it?

[patricktoomey]

I have the Ascension 1000 controller but I believe the 4G controller is supposed to be about the same resolution. If that's the case then it makes a big difference over the stock PRT controller. The cuts are definitely cleaner and require no or very little sanding. My chatter marks went away except when I push it too hard. The speed increase has been great, I now jog at up to 12 to 16ips and cut at usually twice my previous speeds but it's the resolution increase that sold me on the change.

[joenagel]

Here is a photo of some Corian cut with a 4G board. This is a 4' Circle in 1/2 material. This photo shows the roughest part of the cut. Other spots on the circle are smoother.
The speed is nice on this board, but the really great thing is how smooth it cuts.

Sorry I don't have any before pictures, and there is no way I'm putting my old board in to run samples.
193

[David Iannone]

Joe,
That cut looks Great. Are you using a porter cable, or spindle? Also what cut speeds, and bit did you use on the picture above. Have you suffered any lost steps since upgrading to 4G?
Thanks,
Dave

[David Iannone]

Mike,
So in your opition, the alpha wouldn't lose a step? It would either fix the problem itself, or pause the machine, and I could get the line number it stopped on?
Thanks,
Dave

[joenagel]

Dave,

I'm running a Milwaukee router at 10,000 rpm, feed speed at 1.5 IPS with a 3/8" downcut spiral bit (just what I had laying around).

With the beta version of software I can jog at 12-14 IPS without missing a step. I have not missed any steps while cutting as long as I don't go too fast and exceed the power of the stepper motors. I have done some 3D work at 8 IPS with great results!

Joe

[David Iannone]

Thanks Joe, I am leaning toward the 4G upgrade I think.

[richards]

David,
The Oriental Motor Alpha stepper drivers are designed to self-correct whenever possible. However, when the stalling problem is automatically fixed, the part my be ruined, depending on whether the move was a single-axis move (good) or a multi-axis move (possibly bad). Multi-axis moves, when one motor self-corrected have caused me problems, sometimes severe enough to ruin the part.

Frankly, I don't remember exactly what happens when the Alpha stalls. I believe it is the same as if the e-stop switch were pushed, but it hasn't happened in several months. My procedure was to re-zero the machine on all three axes using the Z-zero plate and the proximity switches and then manually edit the cut file to get me as near as possible to the place where the fault occurred.

Just so nobody gets the wrong impression. My Alpha has only faulted a few times attributable to normal operating conditions since I bought it in the summer of 2004. The first few times was my inexperience with feed speeds and the limitations of the PC7518 router that I used back then. Most of the faults occurred when I was having problems with a wireless ethernet card. (At the time, those problems were thought -incorrectly- to be caused by the Colombo Spindle.) Because the steppers would behave erratically, there were plenty of faults and lots of ruined parts. Since removing the wireless card and after learning to use more modest feed rates, there hasn't been a fault. In fact, thinking back, I can't remember the last time that the drives faulted. Since, adding gearboxes to the stepper motors, I just don't have torque problems anymore.

[David Iannone]

Mike,
Thanks for the info. As I understand it, the alpha shouldn't lose steps just jogging I guess. In the past with my PRT Standard, but not very often, I have lost steps jogging. It is so rare that I don't worry about trying to figure out why. The last time it happened was probably 8 months ago. I jog my PRT at 3.5 IPS. Possibly I shouldn't jog that fast with PRT? Just something that I always worry about on long runs.

[richards]

David,
I've never checked the accuracy of my Alpha's jogs, but, since the parts were accurately cut, and jogs were used to position the cutter, indirect evidence would show that the jogs were accurate, even at 30-ips.

If I had a PRT, I would consider buying either the G4 or Dirk's AGek. Either upgrade would greatly enhance both your machine's performance and your cut quality.

[David Iannone]

Thanks Mike, wow 30-ips. I can't imagine that fast movement. I think I am going to upgrade to G4 though. If I can reliably double my speed I have now I will be good.