For the past many days, I’ve been working on crossing out the Great Wheel. Of course, I start with the hardest one! It has 8 tapered spokes, plus lots of little doodads in amongst its spokes, making it a nice, challenging place to start. One of the issues is the bottom spoke (as shown in this picture anyway), where the spoke stops and then opens into a curved window. This window allows a pin from the maintaining ratchet to pass through. We’ll talk more about that later. But for now, I just need to make that window. Additionally, there’s an attachment point for a different pin on the right spoke, which is another spoke anomaly.
I usually just do some trig and figure out the location for each corner of the spoke, calculate the x-y coordinates for each one, then drill the holes on the mill using the DRO. But this often leads to some little errors when trying to connect the dots on the rotary table.
So this time, I decided to figure out how to drill the holes on the rotary table for one spoke, then rotate to the next spoke and do the same. This seemed like it might minimize the x-y to polar coordinate mismatch issue I’ve had in the past.
To this end, I created a spreadsheet that lets me enter the number of spokes I want, the inner rim diameter, the outer hub diameter, the width of the spokes at the top and bottom, PLUS the diameter of end mill I’ll be using to mill out the spokes. With this input, it calculates the angle to set the RT for each spoke side, and the X and Y coordinates for the top and bottom holes. What could be better, right?
Spreadsheets are wonderful things. They do exactly what you tell them to. Unfortunately, I calculated all my angles backward (sign error) so the left side spoke became the right side, and the spokes were all numbered backwards! No problem. That’s what Sharpies are for, right? So I renumbered the spokes and moved some of the ‘special features’ around to adjust for my oversight.
The spreadsheet in the middle is for the main 8 spokes, with the two anomalous spokes having yellow highlights so that I (hopefully) notice that they are different. Some of them are now crossed out and moved to a different spoke due to my inverted angle issue.
The spreadsheet on the right is for the little bits of spokeage that connect that middle bottom spoke with the window to the hub (see the above picture). I calculated the locations of those as if they were just straight spokes (which they are) but just short. And I will only use two of them (but not two consecutive ones
). If anyone’s actually interested, I’d be happy to explain further. But I’m sure others would figure this in a different way. That just happens to be how I did it.
I blued up one side of the great wheel and centered it on the rotary table. Then I proceeded to use the info in my spreadsheets to layout the spoke locations. While not a strictly necessary step, this gave me a LOT more confidence in my numbers. And, it is also what helped me discover my angle math error. So I’m super glad I took the time to do this step!
Next, I went around spotting and drilling 1/8" holes at each corner of each spoke, or spoke ‘feature’ (adjust the angle of the RT, spot two holes, drill two holes, etc.) At one point, I had the bright idea to just use the center cutting end mill to make the holes, thinking it would save me a tool change (having to spot, then drill). However, this did not go so well. You can pick out a couple of holes that look over diameter. Those are the ones I tried just using the mill. I think the issue was that I had to have the end mill sticking so far out of the chuck (to reach past the hold-downs) that it would flex some, especially when starting the hole. This wasn’t good, so I quickly went back to using a twist drill.
I also started to cut the spokes out with the end mill, but then remembered that my official order of operations was to 1) make the holes, 2) pan out the recess in the wheel, then to finally cut the spokes. I liked this order of operations because I would only have to cut spokes that were half as thick, which seemed like a significant plus.
It was at this point that my stomach dropped… Because I realized that in all my angle sign changing and spoke re-arranging, I’d ended up getting the special features of the spokes 180 degrees out of phase! Oops! (Just a sign error, right?) I did not relish the idea of starting the whole gear over yet again (remember, it took me FOUR times to get this one made!) You’d think I could just flip it over, but the gear is NOT symmetrical. It has the recess on one side, then the two anomalous features are 90 degrees apart.
After some pondering, I came up with a (hopefully) cunning plan that would allow me to potentially save all the work I’d done so far. I was already committed to the locations of each special spoke. But I had NOT committed which side of the gear was front or back, so I actually COULD flip it over, if I changed my order of operations. My original plan was to drill the corners, pan out the recess first, then cut the spokes - all in one setup. But if I were willing to risk another setup and recess the OTHER side, then all the spoke features on the wheel would be in the correct place.
To minimize the number of re-registration steps on the wheel, I decided to cut the spokes on this side now, as it is currently positioned. That should keep the spokes aligned well with the holes I just drilled. My only issue would be when turning it over to pan out the recess, I will likely get a bit of a disconnect between the recessed inner rim and the inner rim cut on the spoke side. I’d hoped to avoid that with my original plan, but decided that a little bit of fettling work would be a small price to pay for not scraping the part.
The wheel is 3/16” wide. The recess will be 3/32” deep, so the spokes only need to be cut 3/32”. So, I started cutting the spokes 3/32” deep on this side. I started with the outer diameter, taking special care around that anomalous spoke with the wide window in it (top of the picture here).
Then I did the bottom of the spokes around the hub, and the lower part of the window portion of that split windowed spoke.
Then I cut all the left and right sides of the spokes, including the sides of the spoke with the window. I took each cut very carefully, double-checking the location and end points of THAT spoke in particular. I really didn’t want to screw things up!
With all the spokes outlined, I needed to make the ‘round’ gizmo around the spoke with the pin. To do this, I moved the gear so that the pin hole was centered on the RT. Originally, this would have been my last step, after panning out the recess and cutting the spokes. But in my new, somewhat less ideal order of ops, I’m doing it now, before flipping it over to cut the recess.
And here you go, the round thingy is cut. One of the oversized holes is right there. It looks really bad
, and it’s not ideal, but I don’t think it’s QUITE as bad as this pic makes it look. Guess we’ll see when I go to do the final shaping.
Flipping the part over and recentering it on the RT, I finally panned out the 3/32” recess in the wheel. I did this with a 3/8” end mill, and it actually went quite well. There’s a little discontinuity around the rim, which I knew there would be. But I think it is recoverable. (I hope!)
Here’s the great wheel, still unfettled, but showing that through all my machinations, I did end up getting the features all cut as designed. There is a little extra pip on the bottom side of the spoke on the right. That is because I somehow didn’t cut that spoke all the way down to 3/32”. I must have gotten lost in my process because it still had a good 50 thou of metal there. But a file can fix that issue.
I didn’t get around to cleaning up the wheel. So, we’ll see if it can truly be salvaged. I’m quite optimistic it can, but we won’t know for sure till I get that job done. But that’ll be next shop time, probably early next week. We’ll see. Being retired is fun, but I don’t always get as much shop time as you’d think!
Thanks for looking in. And if you made it this far, you deserve a gold star!
Kim
