Marc Gingras published a new build:

Read more about this build...

 

Hi Dave,
I found the C-Beam fence compares really well with off the shelf products that retail here for well over 500$ that do not have automatic positioning "potential". It should cost under $200 to build it with off the shelf Openbuilds components eg. 500mm x 500mm C-Beam fence & rails (3 c-Beams, 2 c-beam gantry kits + odds and ends). The fence holds square without the positioning option if it rides free (ie. is not tied to a screw or a belt). The toggle clamps one at each are secure (500lbs per clamp). It will not move during a rip, I suspect the beam would bend before it slips. I found that adding the "positioning" optional lead screws and/or motors actually complicate alignment.
I just started the "optional" motorized positioning option part last week. I am new to robotics/cnc/motorized positioning and am pretty rusty/incompetent with programming in general. I have bought most of the parts (smoothieboard/ power supplies/ stepper motors/controllers) and hoping to test out the bugs with bCNC on my windows tower before deciding on what I will be using in the sawdust. As an aside, I though it was going to be an easy first step.. it is not. Installing bCNC on a win7 64bit non-root drive along with it's recommended ancillary programs (python, numpy, scipy, Visual studio, matplot, pillow, pyserial and opencv) has been a frustrating exercise to say the least. I ended up loading everything into Anaconda, all except for bCNC which I have not been able to load/recognize as of yet This is starting to remind me of the stone-age when I was trying to get selectrics to kern & snake with rs232C's and CP/M kludges. Anyhow...
The fence is part of a much larger build which is far from complete and is a "design as I go" project. The "optionaI" screws on the rails that you see in the pictures were chosen for build symmetry and don't add strength. The fence sits on a floating zero clearance work table (work in progress). I am using lead screws for strength on the table, since I was concerned that the table might fly out if I used belts and they snapped during a sloppy rip. I secure the work piece to the table and slide the work table over the blade.
I am not familiar with rotary encoders.
Motorized positioning for a table saw fence & table ... overkill perhaps, ... if things go as expected with minor modifications the fence with positioning should also be able to double as a sphinx platform.
Hope this helps
marc

 

To secure the fence to the side bars at the correct stop distance I used two Samona 80827 500 lbs toggle clamp I had lying around. They sell for under 10$ and happen to be pin compatible with C-Beams. Samona 50827, Vertical Toggle Clamp 500 lb - 50827

 

Hi Dave,
Just an after though on my previous answer... You could reduce costs further by replacing both gantries with Robert Nee's Y Axis trolley build. The costs would fall by another $100 and basically be the cost of C-beams.