Week 7 Computer Controlled Machining

Summary

This week we learned about computer controlled machining using the shopbot in the Fab Lab. I designed a joint clearance test for our group assignment and then designed a simple clothes or blanket bar in Solidworks using pressfit slots.

Assignments

Group assignment

Do your lab's safety training Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths for your machine

This is the link for the group assignment.

I worked on a joint clearance assessment tool to determine the width of the press fit joints that I used in my individual assignment.

Individual assignment

Make (design+mill+assemble) something big (~meter-scale)

The plywood we are using for this assignment is 12mm per the spec. For the first joint clearance test i designed comb that went from 12.3 to 12.05 with 0.05 difference between the slots. and with this I was not able to get a solid press fit. I then redesigned it and and narrowed the slots to start at 12.1 with 0.1 between the slots.

Then I was able to achieve a good press fit with 12.1 and a solid press fit at 12.0. There was a slight difference in thickness between the two pieces of plywood, the first was 11.8 mm vs the second vs 11.9. This indicates that do the joint clearance test it is best not only to use the same type of material but to do it on the exact piece of plywood that you will be using.

I made a simple combination of three parts in Solidworks with one rectangular piece with two slots and two square base pieces with one slot each. I then made two identical rectangular piece with a little "shelf" that fits into the slots for a press fit. The idea is that you could use this to throw a blanket over it for storage or some clothes that you might wear again later.

Here are some images of the drawings. I made them parametric so that I could easily change dimensions. I also learned from Svavar to make sure I fix the different parts of the drawings to each other so that when I change the dimensions they keep the relative relationship to each others. Otherwise a slot that was previously centered will be offset to the right or left once you make a part longer or broader. Part one is 1m long to meet the criteria of the assignment.

Once I made the drawings which required several revisions and some help from Svavar on how to make them correctly I exported the files to a DXF and then imported this into the Vcarve software and laid it out. After running the joint clearance test I adjusted the width to 12mm for an optimal pressfit. I also added dogbones to the corners of the slots and the the end of part 2 using the fillet tool.

Then it was time to create the toolpaths. I created two toolpaths one for the slots and one for the outline of each part. When I then exported them I forgot to check the save all visible toolpaths checkbox so I only got the outline. I used profile toolpaths as I wanted to cut all the way through the material. The thickness was set at 12mm and the cutting depths at 12.2mm.

When we ran the Shopbot we then noticed that it was only cutting the outline and not the slots.

I then went back to the computer to save the second toolpath for the slots and then I got this result.

I removed the parts using a hammer and wood cutting tool and then sanded the edges and inside the holes. I was then able to pressfit the different parts together and assemble the clothes bar. It is a little unstable so it would probably benefit from an additional stabilization bar and perhaps larger supports. In my intial design the supports were 25x25 cm but I reduced it to 15x15 which on second thought was probably worse for stability.
The final result is at the top of this page.