Week 7 Computed Controlled Machining

Week Objectives:

1. Make Something Big (Chair or Furniture)

2. Group Project: Complete Lab Safety Training

3. Group Project: Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths for your machine

Truss Stool

This week, I wanted to design and make a stool that uses the method of joints. The method of joints is a way to analyze trusses on statically determinate bridges like a Pratt Truss Bridge. A bridge is statically determinate if 2j=m+3. J= number of joints and M= number of members. If the 2j is greater than m+3 the structure is unstable and if 2j is less than m+3, the structure is statically indeterminate, and the method of joints will not work.

Fusion Design

First, I created a thickness parameter. Wood's thickness fluctuates with humidity and temperature so it is important to be able to easily change thickness in fusion.

I measured the wood to be 0.718 inches thick, so I set that value equal to thickness.

I designed the rest of the truss stool. I made sure to keep it in a sketch so I can save the sketch itself as a DXF file then import it into Aspire.

After I designed the truss stool, I wanted to do some calculations using the method of joints to determine how safe my stool is. I decided on a weight of about roughly 200lbs to be applied to the top of my stool. I multiplied that weight by 9.81 to get newtons, then divided the weight by 3 because the stool is supported by 3 different trusses.

Above are the rest of my calculations

Member	Calculated Load	Load Type	Maximum Load	Factor of Safety
AD	357.46N	Compression	4400N	12.3
AB	129.96N	Tension	15000N	115.4
DB	0N	N/A	4200N	N/A
BC	129.96N	Tension	15000N	115.4
DC	356.46N	Compression	4400N	12.3

I created this table showing my results and comparing them to each materials maximum strength. Members AD and BC were overengineered and I could definitely make them smaller to save material costs, however in this application I'm not worried about saving material costs. A bridge is a strong as its weakest member so my overall factor of safety is 12.3 which is very safe.

I used this website to convert this table which I originally made in google docs into markdown format, so I could paste it in here.

I saved the fusion sketch as a DXF file then imported it into Aspire to generate the toolpaths.

**Click here to download my fusion file

I recieved a 4x8 foot piece of wood. I measured the thickness and it came out to be 0.718 inches. Using the shopbot, I made sure to use not offset and makee the z zero position on the machine bed.

I decided to use 3 different toolpaths: inside profile, outside profile, and tab pockets.

Below are the settings for each toolpath

Outside Profile

For this toolpath, I used a 1/4 flat bit. I also made sure to add tabs so that my pieces dont fly out of the machine.

Inside Profile

For this toolpath, I used the same bit and again added tabs. The process is very similar for these two toolpaths.

Pockets

For this toolpath, I used a pocket toolpath instead of profile because I wanted to cut an area instead of a line. I made sure the pocket cut 0.718 inches down to match the thickness of the material.

After I calculated all the toolpaths, I viewed each one to see how they all looked.

Here are toolpaths after I simulated them. I downloaded the vectors to be sent to the shopbot.

My File can be downloaded by clicking here

Shop Bot

I took my material and used a plastic nail gun to nail my piece to the board. The plastic nails are very strong in linear strength and if the bit hits them, nothing bad happens. The nail gun process was very easy, just hook it up to a compressor, load the nails, then pull the trigger to shoot a nail out.

The shop bot uses its own software which is very easy to move. As far as Settings go, I used the default settings in the software.

I decided to do an aircut first to see if my toolpaths were correct. I jogged the Z axis to 3 inches by doing Z3 in the terminal, then I zerod the Z axis by doing ZZ. Now The Toolhead is 3 inches above the the machine bed, however it thinks its at 0.

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I ran the aircut and my file seemed good.

Before I ran this though, I decided to cut a couple of pockets with different clearances to find the best fitting pocket size. I decided that a pocket with a positive 0.005 inch clearance fit best.

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This is my cut running. In the video it is cutting the pockets, but in the background you can see it cut some of the other parts.

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These are all my pieces below after the cut. Before this though, I cut the tabs off of each piece with a oscillating blade.

Each piece looks correct and the tabs were removed with ease.

This is the finished stool. Because I tested the fit of the pockets, I was able to get a press fit stool.

This is me sitting on the stool. It handles my weight (~140lbs) with ease and can definitely handle more weight.

Link to Group Project

Reflection

This week was not my favorite, the week was pretty time consuming, I ran into some problems, and the ShopBot is a little bit scary. There were two problems I ran into. When I went to run my cut, the tool plunged down into the material. I had to hit the emergency stop button. It turns out this wasn't a problem with my file or anything on my part, this had happened the day before. The second problem was a broken bit. Because the tool had been plunged into the bed it became very weak. While I was cutting, it randomly broke. I had to wait for Mr. Budzichowski to come on his day off so I could get a 1/4 bit back on the machine. If it wasn't for him, this week would have been much harder.