A slight change to my MPCNC feet design was in order.
MPCNC feet are used to attach the machine to the work table. As originally designed, the machine sits on short vertical pipes, and the foot is screwed into the table using wood screws. This seems to work fine. But I wanted an method to attach the MPCNC feet that was a bit stronger and more flexible than wood screws.
Mike came up with an idea for adjustable feet using a turnbuckle and threaded rod. I may adopt this approach in the future, but I wanted to start with the idea of a 5/16 bolt through the foot and table top. Fortunately, he provides a foot model with a center hole for a bolt, rather than using small wood screws to attach the foot.
The problem with using a bolt inside a plastic foot and piece of conduit is: how do you hold it firmly in place when you are tightening a nut onto the bolt at the other end? You cannot access the head of the bolt.
I solved this challenge by designing and printing a plastic bolt holder that fits snugly inside the conduit.
The bolt head holder for the MPCNC feet is shown in yellow ABS plastic above. It is a roughly 20 mm cylinder about 8 mm tall. The top 5 mm is a hex shaped extrusion that fits the head of the bolt. The lower 3 mm is essentially a washer. The bolt goes into the bolt head holder, and the bolt head holder fits snugly into the interior of the conduit. It’s all “press fit”.
When all is said and done, each foot will be attached by a 5/16 bolt through the table top.
MPCNC Feet – Calibration Helps
Two types of calibration are important. The first is 3D printer calibration. Getting “press fit” to work (for the bolt head, bolt diameter and conduit inner diameter) requires accurate 3D printing. Typically, within 0.2 – 0.4 mm. I designed the part with an extra 0.2 mm. I ended up extending this slightly after the first test print proved slightly off. The second try resulted in a perfect snug fit for the MPCNC feet – no sanding or filing required.
The second calibration requirement is the ability to square the MPCNC frame. Wood screws are unforgiving, and my friend Gord pointed out that imperfections in the plywood layers of a table top sometimes result in a less-than-accurate fit. So, the table will have 5/16 holes for the bolts. But two of the four holes will be slightly enlarged in diameter. This should allow me to move the bolts to different positions during the squaring process. Then, compression will be used to hold the MPCNC feet in the right place to achieve square.
Incidentally, the blue plastic feet required a bit of inner diameter trimming to get the conduit to fit properly. This was done by lightly sanding the interior walls with medium Emery cloth wrapped around my finger.
You are turning into a machinist already and without the tools; amazing.
One little consideration. Snug fits may not be snug when it comes to tightening the assembly to the table. You may want to pin the bolt holder insert by drilling for a #6 by 1/2 inch self tapping machine screw through the slot in the foot and then through the conduit wall and into the yellow insert. Alternatively, place a washer or two that is just big enough to slide over the threads of the bolt before tightening. This will compress the bottom of the insert and in turn will increase its diameter a bit for a tighter fit. Good luck.