Legs Finished!

I did the final things on the list today to the legs. Yesterday I had dabbed a bit of touch up body filler--the Bondo kind in a tube, it's petroleum based, so it dries with air contact-into a lot of little flaws revealed by the first coat of primer. Today it just took a few minutes to sand those down flush with some 320 and 400 grit paper. Then I vacuumed the legs thoroughly and took them outside for another coat of primer. The temp was probably around 50 degrees, which is just at the bottom of the operating range for that spray paint. But primer is pretty forgiving. The second coat went on without any real problems and these are ready to go now:


This is the whole package for the builder who commissioned these from me. I'd probably put a coat of black paint on the hubs if they were going in my droid. I'm happy with the results:







The only remaining flaws at this point are very small and will be covered by paint. The next steps would be to add a few more coats of primer with some very light sanding--maybe with 00 or 0000 steel wool in between--then several light coats of a gloss white with some light sanding in between the early ones.

What did I learn this time through:
I got the high end baltic birch 12 ply plywood. It's very good, stable, and straight. But the outermost layer, the layer that will end up showing to the eye, is fragile. Home Depot and Lowe's usually have some good 12 ply cabinet grade plywood, and I have found that the outer layer on that stuff is a bit thicker and a bit more sturdy. I'd go with that next time. I'd also cut a 3" circle out of the insider layer of the legs at the top where the shoulder hub is going to sit, although the router method I used yesterday wasn't too bad. The router method for cutting the pockets out under the shoulders is a very clever idea I got from Mike Senna through Victor Franco. The best way was to have a couple, but not all, of the layers glued, and then rout out the pockets with a template from the inside. Each leg weighs several pounds. I think that next time, I could rout out a lot of material on the insides of the pieces without compromising the strength at all. The ankles could have a lot of interior material removed, the channel for the wiring could be wider, and the shoulders could be more hollow. That could save a pound or two per leg, and that would add up. The modification to the center leg tip to make a better connection to the foot shell is an improvement. Next time, if I was setting up a fixed three legged droid, I think I'll work out the plans to do that on all three legs--building the ankle locks into the design.

Legs 8

I'm working on the final list of things to do on the legs. Today I got the circular pockets cut for the shoulder hubs. The diameter of the hubs is 3.35" or so (I can't remember--check the plans), so I need a way to cut a clean perfect circle into the center of the shoulder. The best method I've found for small diameter circles like this is the router table. I've made an extra top for the table and after putting in a relatively sharp groove cutting bit, I've drilled out a pin at the exact radius away from the FAR SIDE of the router bit. If you measure to the near side of the bit, your hole will come out too big. Here's what I mean:


I only bring up the mistake because I did this once and ruined a pair of horseshoes that I had put a lot of work into.

I have a plunge attachment for this router but I don't like it. I just drill a small pilot hole into the center of the shoulder and then I work that hole down onto the pin sticking up in the picture. To start, I set the router height very low and turn on the router and ease the piece down onto it. Then I gently rotate the whole leg around that axis point at the pin and the bit does all of the work. The danger here is that a dull bit, or a mistake will rip out a nasty section in the beautiful perfect legs that I've worked on so hard. Routers operate at very high speeds so the slightest mistakes invariably spin into big gouges that tear across the face of your work piece. I did several test runs with this set up before actually putting one of the legs on here. Once the groove is started, I change the depth on the bit until it has cut to the 1/2" or so depth I'm shooting. This is a tricky operation, and the details of how you do it will vary depending on your tools. There are some very nice circle jigs for routers out there, but I don't think many of them will scale all the way down to a 3.35" diameter. Another option would be to use a collar and a circle template. Here's the results of the first circle that I get:

The cut is clean and perfectly round. So I'm happy. Once I got it set up, I ran all four legs through.

For the shoulder hubs to sit down into the legs, there needs to be another smaller circular pocket down inside of this one. Check the plans for the shoulder hub. You'll see that it has a stepped profile. But I've got an empty chamber in there that I left for the wiring. You can see that I already made a round hole in that chamber, but I wasn't planning ahead very well. The bit I used to round out the top of the hollow chamber in the legs was a 2 1/8" door knob hole cutter. But I need a 2.5" diameter hole here. So I've got a problem. Now that the plug is popped out, there's no material for the pin to ride in for the router jig:



That is, I set my router circle cutter to the 2.5" diameter and started cutting, but when I got down to this depth, the piece that the router pin was sticking into popped loose because there's nothing holding it in. In the picture you can see the diameter of the hole that I need. It's just a bit bigger than the one that is in there. I hope that makes sense. So what I'm going to do is put a 2.5" hole saw bit on the drill press. Then I'll get this piece centered just right and clamped down under the drill press bit and I'll cut the large inner circle down to the right depth that way. Then I think I'll be able to pop that plug out with a chisel. I'll show more pictures when I get to it. But here's a really valuable tip. When you are doing the initial cutting out and you get to the middle layer of plywood that has the channel down the middle for wiring:



Make this circle that's at the top of the channel 3" in diameter. That way you won't have to do this extra work when you go to cut the pockets for the shoulder hub. Make sense?

The one other thing I did yesterday was glue and clamp the curved ankle details to the outer legs:



This is a pretty easy step. I put a couple of pins into the leg and then drill the curved pieces to accept them. And when I say "pins," all I mean is that I am taking a small finish nail and cutting it off with some pliers, then I'm sticking that down into the work piece. The pins keep the work pieces from drifting out of alignment when they are under the clamping pressure. As before I put enough glue evenly spread around both gluing surfaces to ooze out of the sides all the way around. That was all the gaps will be filled for finishing and painting work and I'll have a good bond. Resist the temptation to clamp these too hard. The MDF is prone to get dents and it doesn't take much pressure to make a really strong bond.

Outer Legs, stage 4

I'm going through some complicated ordering of cuts and gluing stages in order to accomplish a couple of things. I want to be able to finish sand all inside cut edges with the belt sander or maybe the orbital sander before assembly. I also want to glue multiple pieces together where possible when they are all going to be cut to the same dimension. If they are glued first, the cutting second insures that they are all aligned. But both of those goals can't be met with every part of this. There are three big pieces that are glued together that make up the outer legs. The outer most one is cut down to a point and goes down into the slot on the foot shells. The inner two are cut off higher so they clear the tops of the foot shells. Here I've glued the two inboard pieces and then cut them off at their shorter length.



Then I sand that edge clean. Next I glue the longer outer piece onto those. This piece closes up the passage inside for the wiring. Next, I set up the table saw to make most of the cuts down the blank. You can start to see the shape of the leg come out here (no, the saw isn't running here).


Cutting them this way helps me get a straight, consistent cut. Since all of my blanks are the same size, I can set the saw up once and then do all of them quickly. Just flip the piece over to do the other side. Before making these cuts, I scraped the excess glue off of the sides from yesterday's glue ups. Otherwise, the glue will push the piece out from the fence and mess up the cut. I slide a chisel down the glue drips when they were about 2 hours old. They were dry enough then to come off, but soft enough to be pliable. Then I hit the side with the belt sander a bit before this cut. I also marked the point where the leading edge of the saw blade underneath would come to and made sure not to cut too far into this piece. Very important. I'll finish this cut--since the table saw leaves a curved cut in there--on the bandsaw.

Next I cut the angles under the shoulders with the bandsaw. I've got a wide blade with a pretty high tooth count on there right now, so cuts like this are straight and pretty clean. The edges that are left will need some filing, and then some cleaning up with an orbital sander.



Next I've roughed out the curve across the top of the shoulders on the bandsaw, and then I take it to the belt sander again for cleaning up. Starting too look more like a leg.




Next up, it's time to deal with those frustrating under shoulder pockets. On my first set of legs I did not find a really efficient way to do these. Victor Franco, with Mike Senna's help, has worked out a trick by routing out these pockets oversized and then lining them with thin MDF. His results are nice once all the joints and cuts are filled and sanded. I'm not going to go that far. 90% of the pocket cuts will be covered by the little greeblies that fit in here. But I do think I'll copy part of their method to rout them out. So I've made a template on 1/8" MDF for cutting. If you haven't done it, take your R2 plans to kinkos on a jump drive and get them printed up full size. I had an extra full size leg blueprint for this:



I sprayed some spray on adhesive to the MDF and to the back of the paper and then just smoothed it on. Then I cut it out on the bandsaw. I've cut the pocket profile slightly large--about 1/16" over--so that I can use the template collar on my router for this. The next stage will be tricky. I think what I am going to do is this. I've got the two big inboard pieces of the legs glued together for the both sets of legs. I think I'll screw the third outboard piece onto those and then cut them all into the shape like the one above. Then I'll take the outboard piece off and cut these pockets with the router. Then I'll put the outboard piece back on with glue. We'll see what sort of results I can get inside the pockets this way. I think it will be clean enough to require a minimum of additional work. But that's all for tomorrow.

Once those pockets are cut, I can glue the final two pieces on the inside and outside of the ankles and finish them out. That will be tricky too. But I'll be able to explain with pictures better when I'm there.