[momus_cnc]

As has been mentioned in a few other threads, some people have had problems with the carriage shifting diagonally during use. So far the solution to this has been using 5/16” grade 8 bolts to replace the four 1/4-20 studs that clamp the carriage together. There are a few threads that discuss this change. This greatly increases the available clamping force and I have not heard of anyone having any problems after upgrading to the 5/16 bolts. However, I've been working on an upgrade for the next release of the plans that will provide a positive connection between the parts, and not rely entirely on frictional forces. I've attached new drawings of these parts for use on existing machines (plan version 1.2 and older.) I'm looking for some comments and feedback before going ahead and integrating this change into the next release.

The upgrade involves fabricating five new parts: new left, right, and lower carriage blocks, and two new lower spacers. The lower block is slightly taller, to allow for the insertion of four roll pins, which positively lock it to the left and right blocks, as well as the spacers. The additional height (1/2”) may reduce your Z axis travel slightly. On the forthcoming version 2.0 the Z axis will be redesigned slightly to compensate for this, so it will actually have more usable Z travel than version 1.2. Version 2.0 will also have slightly modified bearing blocks, so that the spacers will be .75” thick instead of the odd size.

The new lower block as it is shown here is slightly more complex to fabricate than the old version (I'm contemplating simplifying it, see the end of this description), but it makes carriage assembly much, much easier. No more trying to hold 8 parts in perfect alignment with clamps while tightening the nuts. This design also makes these 5 parts essentially permanently attached to each other, but again, makes overall assembly much easier.

Photo 1- The new parts. The 3 parts at the top center are the same as the old ones. The other 5 are new.



Photo 2- I began carriage assembly with the 3 parts shown.



Photo 3- With an accurate square, scribe guide lines that correspond to where the edges of the spacers will be when the parts are assembled.



Photo 4- Clamp the 3 parts together with 5/16” bolts. Use the scribed lines as guides to keep the left and lower blocks square to each other. Double check that they are square to each other and fully tighten the bolts. Triple check that left and lower blocks are square to each other.




Photo 5- Flip the assembly over and clamp it into a vise on the drill press table. Check again that the parts are square to each other. Insert a 3/16” drill bit into the chuck. Lower the quill next to the parts to make sure that the drill press spindle is parallel to the edges of the parts. You'll be drilling through the thickness of all 3 parts, so the drill needs to go as straight through as possible. Carefully align the drill bit with the existing holes of the lower carriage block and drill down through the other two parts. Align the bit carefully so that you don't ream out the existing holes.



Photo 6- Take the parts out of the vise and disassemble them. Mark them first so that they can be put back together with the spacer in the exact same orientation. Carefully clean the parts of any oils, cutting fluids, etc.. With a hammer, drive two 3/16” x 2” roll pins down through the 3 parts to assemble.



Photo 7- Repeat the procedure with the right side parts. For accurate alignment, use the square as well as taking multiple measurements between the left and right side carriage blocks to make sure they are exactly parallel.



Photo 8- With no bolts installed, the assembly should be very rigid.



Photo 9- Install the remaining carriage parts. Install 5/16” grade 8 bolts and fully tighten.


The entire assembly that is seen in the last photo only took me a couple of hours to fabricate. That includes fabricating all of the parts shown, as well as the final drilling and installing the roll pins. I did use the Momus CNC machine to mill the corner notches and 3/8” holes out of the lower block, but otherwise all was done by hand. I haven't installed the new carriage on the machine yet, as I'm constructing an entirely new machine to test out the version 2.0 plans.

I was a little worried that this design would require lots of precision to assemble successfully, but it all went together quite easily. I actually built two of them to test the procedure. Getting the parts aligned before drilling was actually very easy and quick, since you are only dealing with a few parts at a time. Much easier than the old method of carriage assembly. And once it is assembled, those parts are aligned forever.

I've been debating on whether the new lower block should have the notches at the corner as shown here, or if it should be a simple rectangular bar that is 1.25” over its entire length. Cutting the notches makes it look a little more elegant, and it keeps the bearing bolts a consistent size. Not putting the notches would make the part easier to fabricate, and would only require the use of 2 longer bolts.

Thoughts, opinions, comments?

-Bob