When I first started milling, I put the workpiece right up against a bolt with a washer attached then tightened down the bolt which caused the washer to hold down the piece. This works, but it tends to bow the piece a bit since it's clamping right on the edge and it forces you to use a workpiece of a set size to get it near the holes or you have to twist the piece which generally wastes space when you're cutting out square stuff. I went exploring to my local Rockler store and purchased some hold down clamps which give you a greater radius from the threaded hole to hold the piece down and allows you to hold the piece away from the edge. This has worked pretty well for quite some time but I recently ran into a limitation due to the way I created the grid of threaded holes.
To create the holes I used the CNC machine itself. This means each hole is within the cutting area of the machine (the 3D volume where the machine can guide the tool). This limits your hold down ability because there is quite a bit of space outside of the cutting area that you could otherwise use with a clamp. This means I need to redesign the tabletop to enable clamping outside of the cutting area. I could just fire some more holes into those untapped areas (pun!) but I've grown tired of the limitations of the threaded hole system. The holes are tapped MDF which tends to deform with use and with changes in humidity so some holes are really tight, others hardly hold at all. It's also takes a long time to reposition a clamp to a new hole. Not a deal breaker, but annoying. There's also the risk of using a screw that's too long which can extend too far past the bottom of the table and hit the gantry as it moves up and down the X-axis. Thankfully there's a better alternative.
During the aforementioned trip to Rockler, I took a look at their CNC machine called the CNC Shark. It's a pretty slick setup but what I most enjoyed was the hold down system. Rockler makes a product called the Universal T-Track which is a metal track that you can secure hold downs. It accepts a few standard bolt sizes as well as the bolts that came with my other hold down clamps. The benefits are numerous. No more fiddling with allen wrenches to adjust the hold downs, easy repositioning and no risk in hitting the gantry under the table.
My initial plan was to take off the top board of the table (it's currently 2*3/4" MDF boards) and replace it with another 3/4" MDF board with slots cut in it for the T-Tracks. This would require me to buy a 3/4" routing bit and to remove the router from the CNC machine, neither of which I want to do. I happened to read a post while searching for information about the T-Tracks where someone suggested mounting the track to a 3/4" MDF board and then putting 1/2" MDF boards between the tracks. The end result would be the same and I'd be able to replace just the top boards if they get messed up for whatever reason. This is ideal since it can be done with tools that I already have.
So I fired up my solid modeler of choice and came up with the following.
I've elicited the help of a coworker with a table saw to cut the top pieces precisely. In the mean time I took apart the current table to get measurements and to show the construction.
Here we see the lower of the two slabs of MDF that make up the old table top. You can see the heads of the 4 bolts that hold the table down to the machine's frame and the grid of threaded holes that are also in the top piece. On the sides I attached some angled steel to keep the MDF straight. This actually works really well as the MDF was ever so slightly bent before the addition. In the next shot, we see the machine without a table.
This gives a great view of how the machine works. Without the table you can see the rails for the X-axis and the anti-backlash nut that rides on the threaded rod which moves the X-axis.
Now I need to construct the bottom MDF piece of the new table so I'm ready to throw everything together when the top pieces arrive.
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