Can I make my posture better? #
We got nice, 1/2" 4x8 plywood this week, so I wanted to make something useful for my room. My parents always complain about my bad posture, and I found these kneeling chairs that are supposed to keep your posture upright and are ergonomic.
I found some dimensions on a post in pinterest and madea sketch based on those dimensions.
I added splines around it and filetted angles to make the structure more strong (allegedly?) and also to make the design more visually appealing. I also added notches into the wood for mounting beams. I made a “dog bone” joint on the connecting beams for a press fit.
CNC and Group Assignment #
This week, we used the ShopBot CNC router to cut our pieces. We used 1/4" two flute endmills, one for the initial downcut, which prevents delamination of the top layer, and then an upcut. I made my gcode using Fusion CAM.
For the group assignment, we reviewed the safety training for using the ShopBot, and the differences between an upcut and a downcut endmill. Downcuts push chips into the cut, so the flutes are aligned with the axis of rotation, while upcuts push chips out of the cut, so their flutes are aligned opposite the axis of rotation. We started with a downcut to have a nice finish, then an upcut to remove the rest of the wood material.
I made a few initial mistakes, one was not zeroing x correctly, and realizing that the endmill would run off of the board. This was only the downcut pass, so I could still salvage the wood, but there was already grooves in the wood from the initial pass. I re-zeroed the y-axis correctly then it cut fine.
For the upcut, I realized that I did not select the bottom contour in the Gcode initially, so it did not cut all the way through. I reselected the bottom contours and generated a new toolpath, which worked very well.
After cutting, I was able to assemble the “shell” of the chair, and the press fit joints fit well.
I then cut the “seats” for the knees and the actual seat. I used the “pocket” toolpath generator in fusion to make 0.5" pockets to slide the seats on top of the wood. The chair is assembled only with press fits and slots, no glue or fasteners!
After a bit of sanding and assembly, here is the final chair!
I have tried sitting on it and it is quite comfortable! The only issue is that it is a bit wobbly, so I need to add more constraints.
In fact my dad is so interested in the chair that he really wants one. I would definitely build this again out of 3/4" plywood so my Dad who is heavier can sit on it, and redesign the notches and mounting so it’s not wobbly for him. And since everything is flat, I can easily ship it to him for him to assemble, and it would make a fun holiday gift!
Reflections: (or, what I would do different for my Dad’s chair) #
- Always double check the zeroing and make sure that the toolpath is within the stock limits!
- Always double check the feeds and speeds on your toolpath, and simulation is a very helpful tool for that!
- Add more notches to the main side panels for mounting instead of the sliding slots for more stability
- Add notches and mounting panels to the long bottom section to reduce wobbliness.
- Check for symmetry (I realized I forgot to fillet one of the corners on the chair faces because I added them after the mirror)