I spent two evenings working on this next part. The goal was to remove the bottom bracket nut from the chain side of the bike. Since I was unsuccessful using conventional tools, I decided to make a wrench that would fit snugly onto the nut and allow turning with a 1/2" drive breaker bar.
When I'm making anything in my home shop, I like to have a drawing similar to this. There are definitely WAY too many dimensions on there. But its better to have more than I need rather than waste time making multiple trips back to my computer, which is at the opposite end of the house on another floor. You can see I've also added dimensions showing the 1/4" end mill I plan to use and the exact movements I'll be making. This removes any guesswork or figuring while at the machine and lets me just use the numbers.
It's nice to use up material you never thought you'd use but have been keeping in "just in case". The four holes in this piece were existing and won't get in the way. The first step was to give a light spray with flat black BBQ paint in lieu of layout dye. A quick blowdry and we're good to start laying it out.
The setup for machining is as follows:
- Rotary table mounted on 3x 1-2-3 blocks to raise it to where the quill of my machine can reach it (no knee or column adjustment)
- Sacrificial aluminum plate underneath part to be machined
- Clamps with t-nuts
- Spindle centered to rotary table and part's large hole
After aligning the centerline of the part with the machine, I noted the reading on the rotary table. This can't be zero'ed so I did the math to figure out the angle at either end of the cut. The angle written on the table is "zero".
I don't generally use layout lines to cut, rather as a double check to prevent me from doing something careless. In this case I've moved the machine's table and the rotary table to where I think the end of the cut will be and just scratched the surface with the end mill. It looks pretty darn close to me.
So I take the first cut using only the rotary table to feed. I took it an about 3 passes with the last one using the sharper top end of the cutter to clean up the walls.
Well if that doesn't make you happy, I don't know what will
Would you believe I didn't plan this? It's true. Like the first slot, I plunged through for each end before cutting the slot. The result made me smile.
It looks like there are a lot of burrs being raised along the edge of the cut but it's actually pretty nice for gummy P&O sheet steel. The burrs can pretty much be brushed away by hand. I continued cutting each arced slot then the line that joins them. The last bit of the cut is always a bit iffy and you need to be careful with what happens to the slug that falls out!
With the large hole complete I rotated the part back to the initial "0" position and moved over to the small square hole. I took several plunge cuts to get rid of the majority of the material before moving to a smaller end mill to get into the corners.
Unfortunately one of the table locks came loose during one of my plunge cuts and the table started move. This caused a bit of cutting beyond the layout line but it won't affect the function of the tool.
Finally I went around the outside with a 1/8" carbide end mill running my machine flat out at a less than impressive 3000 RPM. It did the trick and most importantly my 1/2" drive breaker bar is a perfect fit without too much slop.
My original plan was to saw cut and grind the outside shape. There's really no point in wasting the time machining it. With the inside done though, I was eager to try it out. Again, a perfect fit. The stock I hadn't removed yet got in the way a little but not so much that I couldn't remove the nut.
The tool has served it's purpose; probably once and for all since the new bottom bracket will use a different tool. That didn't stop me from a quick trip over to the bandsaw to hack away some of the excess material. I'll clean it up if I ever need to use if again.... that is, if I haven't thrown it out before then. ;)
