I bought a used Shopbot PR 120 model about four years ago, and just the last year decided to upgrade it. Here are a couple pics and some info that may help others with older shopbots thinking about upgrading.
First, my Shopbot had lots of things going for it before I upgraded. I have a steel table and very nicely installed X rails and racks, a level and square table with a four zone vacuum system.
The problems with the old PR were that it was slow, went out of square easily, had constant problems with Z axis loosing steps and diving unexpectedly. It also was limited to DOS based control software from Shopbot. Even with that, I was able to use the PR in daily small scale production in my woodshop.
The upgrade: I decided to upgrade the controller to an Ascension 1000 from Custom CNC. This controller uses Mach 3 software, a windows based and very flexible control program with a great support network. I also decided to replace the unistrut gantry and the Z axis with improved components, and to install Bishop Wisecarve hardened steel rails.
I first make a wooden mockup of the gantry and Z axis so that I could size the components I needed to order. I also worked out how much Z travel I wanted, and how to configure the new rails, wheels and motors, etc. The mockup was a big help.
Then I ordered the Bishop Wisecarver rails for both X and Y axis. And I found a beefy linear actuator on Ebay to use as a Z axis. I also ordered and received the Ascension control box which included Mach 3 software and was preconfigured for my shopbot motors.
First, I took the big leap and dismantled the PR. I removed the gantry and Z axis and the controller and power supply. To make sure I was starting from a level base, I carefully leveled the table with a spirit level. I made sure the rails I was going to build on were square and level as close as I could get them.
First, I positioned the new X rails with clamps. Then I carefully tapped and drilled holes to mount the rails to the exsiting PR unistrut. I used a simple carpenter's square to set the height, and double checked for level with a laser level. I actually screwed up the first rail pretty bad, and had to remount it to get it right. No problem, just shift it to one side, and try again.
Then I assembled the 8020 aluminum gantry, and positioned on the existing PR X rails. With this in place, I was able to find the right position for my new rails and wheels. I made brackets from aluminum angle iron, which I attached to the extruded alum gantry ends. I used some parts from the Shopbot PR, but found that it would be easier to use new wheels from BWC that had mounting studs attached. Viola! The new gantry was rolling!
I was worried that the gantry would be too massive, but after comparing it to my PR gantry, its not really much heavier than the PR gantry was. I checked with a fish scale- the kind with a hook at one end that you weigh you catch with. The force needed to initiate motion on the gantry was just over 5lbs. Nothing to worry about.
After this, I made some mounts for the linear actuator that would be my Z axis. I figured out how to mount the motor and got my shaft coupler to line up correctly. I then used 6" aluminum C channel to mount the router to the linear axis carriage. This gave me about 10" of Z clearance, and was stiff and light. The Z axis was really overkill, but I am looking forward to mounting a spindle, so I wanted to err on the side of overbuilt.
After this, the task was to install limit switches, wiring and accessories. This went very quickly, since the aluminum extrusion makes it very easy to mount stuff. I used the PR's cable flex thing to carry the cables for now. I also used the old PR dust collection shoe, modified so that I can mount it to both the moving Z axis, or to the fixed Y carriage.
Finally, I wired the Ascension controller, which consisted of swapping out the Shopbot PR connectors for supplied wago connectors. Allen at Custom CNC gave instructions, and I was able to get help from the forum at Shopbot for all of this.
Finally I got to turn on the beast. After checking and rechecking connections, fired it up and jogged the gantry around. I had some problems at first with intermittent connections on the motors that caused lots of grief. After some testing and stomping my feet, I figured out which connectors were goofy, and fixed them.
With the machine up and running, I ran some tests and setup. I found speeds that I could reliably jog and cut at, and set the Mach software there. I calibrated the axis for distance and acceleration, and tweaked the constant velocity settings to minimize jerk.
I'm very pleased with the outcome overall. I'd like to improve the mechanicals- maybe upgrade the BWC rails to HiWin bearings at some point. I have a great machine now for less than half what I would expect to pay for a new shopbot, and I have learned a great deal about CNC routers in the process.
Thanks for listening. I hope this helps someone considering an upgrade.