What about really ancient machines?

[jseiler]

I have a pr32. (Yes, feel my pain for a second). I went into this machine knowing exactly what I was getting...an old, relatively easy to maintain machine that would fit in my garage and not drive me into the poorhouse. It has 5tpi ball screws and they work (even though they are a little bent from previous owner abuse), and unistrut that needs to be rotated (grooved abit, about 20-30mil). Its only 12V...I've not had time to take it down from being available for cutting for a 24-28V upgrade.

Its great and its awful all at once. When its tuned up, it cuts at .7 in/s reliably and jogs at .9in/s, with its stock pk266-02a motors. When I'm scared, it take it back to .5 in/s. I run a pc890 variable speed.

Since I'm going to have my machine apart completelly anyway to rotate the uni, replace wiring (another joy of having a cnc jalopy), would an old pr32 with stock motors really benefit from the G4 upgrade? Would I have to consider a motor upgrade as well? Or should I just do the 24 upgrade and save my money toward a more modern machine? Do I have a recent enough control box to qualify for the "old board return"?

I'm I gonna get my $800-1000 out of this upgrade, or feel screwed that I wasted my effort? I like the machine, I'm not likely to sell it.

Decisions, decisions...any thoughts?

[scott_smith]

“Feel your pain”….. Heck no….. you got a ShopBot!

This is just a personal opinion, but I wouldn’t go with the G4 upgrade with that machine. Do upgrade to the 24v supply though. On my PR that I recently sold I was getting PRT speeds out of it but the rigidity of the gantries were limiting the cut speed, not the electronics. If you are rebuilding the machine anyway, I’d look into changing the configuration of the rollers to mimic the PRT setup. There are other threads with good pictures on that subject.

[Brady Watson]

There's no need to go to the 24v supply if you decide to do the 4G upgrade. Since you have a PR, you need to buy the whole controlbox, which has a PS in it already. This will really speed up the PR, but the real benefit is smoothness.

I would definately look to see what others have done with thier PRs and upgrade and stiffen the mechanicals. It would not cost a lot of money to convert the unility strut 'rails' to a BishopWisecarver hardened rail (probably under $100 for the entire machine). Then the v-rollers will run smoothly and you will never have to deal with that 45° issue.

Replace the screw. I don't believe that it is a ballscrew. I am pretty sure that it is Acme thread. That can be replaced inexpensively as well. I'm sure that ShopBot can suggest where to get the screw & nut for your machine or even sell you one.

With some steel gussets and other improvements, I don't see why your machine couldn't be upgraded to give more reliability and performance. If money is your concern, then this lets you upgrade a little at a time. The $ you spend on the 4G can be recapped later if you sell the machine and buy a PRT that needs an upgrade. Just swap boxes.

-B

[jseiler]

Brady,

It is ball screw. It has recirc ball nuts to match. I have considered replacing the bent ones with cheap acmes and delrin nuts, but I'm not sure that would really be an upgrade. The screws are out maybe .125 over length. Its visible, but it doesn't seem to bind. Its probably beating up the screws and nuts, but they are "trashed" so I don't really care so much.

I went and found pix of some of the machines other people have upgraded with hardened rail, but I don't think the machine is worth that time and effort, when I can easily rotate or swap the rails to get clean running surfaces. Its got thousands of hours on it and the grooves are still quite shallow.

I did have an email with Ted a while back (he was the only one that seemed to remember even making a pr32...hehe). He didn't remember making a 12V pr32.

As for cutting smoothness, its pretty great. I have zero trouble with chatter and other ills I read about here. Probably also due to cutting speed as well.

I already have the 24v supply, so I'll probably do that. Got any suggestions on how to get to those 3/4" bolts that are buried inside the c of the c channel? thinwall socket??? I couldn't get a wrench in there.

John

[dirk]

John, I'm pretty sure these are the same screws that came with the machine

Here's the ballnut

http://www1.mscdirect.com/CGI/NNSRIT?PARTPG=IMLMKD&PMPXNO=1878226

Here's the ballscrew

http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=1878213&PMT4NO=14177825

Dirk

[jseiler]

Dirk, they look about right. Thanks.

[jseiler]

OK, I redid the power supply, 28V for the x and y axis, 12V for z didn't have easily upgraded resistor (do it later maybe). Its about 50-100% faster. I replaced the resistors to 680ohms in the control box.

Now the dumb question....what's the biggest motor I can reasonably run on a 28V bot and what resistor do I need for limiting?

I'm also considering making this a 4x4. Anyone heard of a ball screw pr48 with sliding table?

[richards]

John, the speed of a stepper motor is more affected by the voltage of the power supply. The torque of a stepper motor is more affected by the current (amps). I can easily run a PK299-F4.5 on a 28VDC supply, but it likes to have 6 amps when wired parallel. That motor is very similar to the Alpha's AS911 motor in speed and torque.

[jseiler]

The driver looks like it will handle 3A. It looks like I can change the limiting resistor and allow more current. I have read all the threads about motor upgrade, but I haven't seen how to calculate current flow of the drivers from input voltage to get the right resistor and if its safe to drive these old boxes harder than 2A. I guess if all else fails, I can experiment and if I kill a driver, its only $13.

This increase will allow me more choice in motors. I'm really drooling over the idea of making this pr32 into a moving table pr48, if I have to replace the screws anyway and I really don't think pk266-02a's are up to the challenge, but I'm about to find out (in baby steps).

I have noticed that the torque at the screws, even at 1.5in/s with the new alston supply is very much improved. I can stop the screw with my hands, but it takes a lot of effort. With the 12V pc supply used, I could easily stop the screws even at .5in/s. This makes me think that the pc supply was current starving the motors. The torque should have been about the same.

In the meantime, I'm ordering one screw, one ballnut and expanding to 32x48. If this doesn't work out, I can always recut the screw and go back to pr32 world. If it does work ok and I can use 3A motors (or any suggestions on 2a motors with more than 120in-oz torque at 2A), that would be my next move (and deciding whether I want the rail/r&P upgrade)

The speed/torque increase has really amplified how bad my x screw is. The y is in pretty good shape, really (shielded from dust, I suppose).

If all works out, I may have a try at Gerald's cold rolled rails.