Master rail guide; limit switches
5 Bears Home Homebrew CNC bench mill

As the mill comes together, I have to machine or fabricate a number of ancillary devices, like limit switch brackets. Easy to say, a bit harder to do well!

First, let me discuss mounting THK linear bearing blocks and rails. The THK catalogue has much excellent info on this. Suffice it to say that doing the job correctly takes more work and care than one might imagine. The rail-mounting surfaces must be absolutely flat, and the rails installed parallel to one another, to better than .001" per foot, or accuracy will suffer. Normally, one rail is designated the master, and its installation forms a datum from which the other rail (and any attached hardware) must be installed. Likewise, one set of blocks is designated master, usually the blocks on the master rail.

Look at the drawing... note that the master rail is set firmly against a precision edge, usually machined directly into the mounting table. The other rail is free to be adjusted within the limits of the mounting bolts. This is necessary so that the slave rail may be adjusted to perfect parallelism with the master. Also, the table mounted on the blocks also has a machined datum. If we first mount the master rail, pressing it against the machine datum, then mount the work surface, pressing it too to the left, we have by definition fixed the mounting base and the work table in truth to each other. The slave rail can be adjusted as needed to create stress-free, parallel motion. I hope this makes sense!


Fortal aluminum machines beautifully. Even with normal (not roughing) cutters, the chip is usually discontinuous.
In light of the above, I decided to machine an aluminum block which would serve as both a master rail datum edge, and a limit-switch bracket. An 8" piece of 7075 (Fortal) aluminum, roughly 2.5" X 1.5" in section, was hogged as shown with a 3/4" end mill. The upper surface in this photo will be the datum edge, and will press against the master rail when mounted on the Bosch 4590H extrusion. This will be clear in a moment.

The Lovejoy face mill goes to work, producing a truly flat surface.

After roughing, the datum edge was cleaned up with a finisher end mill, and the surface which mates to the structural extrusion was milled flat.

In place on the extrusion, its action is apparent. It will be mounted with 2 ea. 1/4"-20 SHCS to the T-slots in the extrusion.

I began the mental gymnastics of designing limit switch mounts on this device. After a bit of thought, I realized that there was room inside the base, and keeping the wires and switches away from the swarf was a good idea. So I now have a big, heavy, 7075 aluminum straight-edge. It'll work fine in that role. Talk about overbuilt, though.

Y-axis limit switches.

My goal with the switches was to make adjustments simple and painless. I cannibalized the bracket shown here from my scrapbox. It didn't include the end mounts, which were engineered to mate with the extrusions, again with the T-slots. The T-slots allow longitudinal adjustment of the entire assembly
Working drawing of the bracket's end mounts. Four of these were milled from aluminum, although only two are needed. The other two are saved for CNC mill MK2.
The drawing called for 1" X 0.250" thick stock. All I had was 1/2" thick stock so that is what I used. One switch (with roller trip lever) is mounted with 2 ea. 4-40 screws.

No, the hole in that nut is not off center... really, it's the photo...
The important stuff is on the back! I detest trying to hold tiny nuts in awkward locations for tightening of the screw, so I made some very tiny 4-40 T-nuts, which ride in the slots. To adjust the location of the switches, all I have to do is slightly loosen the screws, slide the switch, then tighten. The baby T-nuts work great.

Mounted inside the base of the mill. The location is directly to the right of the ballnut and ballnut bracket. A round trip device will be screwed into the nut bracket to trigger the limit switches.

Boring stuff? Overkill? Maybe, but quality accoutrements make a tool like this a pleasure to operate.