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 9: Finishing touches

Several small things finished up in the last few days on the legs. First, I needed to cut the second 2.5+" pocket in the bottom of the shoulder hub pocket. After fooling around a bit, I remembered my technique from before. The easiest way to do this is to set a router bit down to the right depth plus a bit, and then just freehand the inner circle out. There's not much you can mess up, and the pocket won't show, so the freehand line won't be a problem.



I'm all about fast and efficient, and this was both. The results:



I think the plans call for 1.34" depth on the shoulder hub. I cut this to 1.4" or so. There will be some variation on the resin or the aluminum shoulder hubs that the club produces. I couldn't get mine off my droid to check. But I think that the resin hubs will fit, or if they don't, the backs can be trimmed and sanded a bit. And there should be clearance here for the aluminum hubs too. If not, a bit of dremeling could bring the levels down for a good fit. The hubs should fit flush to the face of the body of the leg.

Then I spent some time trimming, sanding and cleaning up the under shoulder detail pockets:



After painting, the wood grain and some chiseling will show inside of these if someone was really looking for it, but it would be hard to spot. I think if a builder wants the inside behind the greeblie to have a really clean look, a piece of styrene, maybe .020, cut to fit and glued back there would polish it just fine. That would take 5 minutes.

And now after some finish sanding down to 320 grit, and some filling in a few odd spots, these are ready for a coat of primer:



The primer makes any chips, dings, gouges, or gaps that I have missed so far immediately obvious. So I'll let this dry over night, fill some remaining spots, do a light finish sanding, and then one more coat of primer and these are ready to ship out.

I've completed that process on the center leg and they are ready to go out.





Any remaining wood grain will be covered by paint. At this point on my droid's legs, I layered on several coats of primer, sanding lightly between each coat with 320 or 400 grit, and then I built up several layers of gloss white, sanding between the early ones. The layers and sanding will progressively fill and cover any remaining micro wood grain texture to the point that the finished surface will be complete glassy and smooth. But I'm leaving that to the guys who bought these from me. Final steps tomorrow, and then shipping early next week.

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.

Legs 6

I got the rest of the main bodies on the legs glued up today and sanded the curves on the top. Next up, I went to work on the curved ankle details. You will remember the big 1.5" thick blank that I made a few posts ago for this purpose. I've got 4 pieces to cut for the two center legs and 4 pieces for the 4 outer legs. Check the plans. The dimensions are slightly different.

First, after marking the curves on the ends, I used the bandsaw to rough in the curve cut all the way down the blank. This will save me a lot of sanding later.



These cuts take some tricky set up and dangerous maneuvering on the bandsaw:



Then some sanding on the belt sander got me to here:


I'm applying soft, even pressure along the full length of the piece and rolling it constantly back and forth on the belt until I get close to the profile of the curve I'm after.

I moved over to the table saw and cut four of these to 4" and four of them to 4.1" as per the plans, giving me 8 semi circle plugs. Next a diagonal slice needs to be trimmed off of them at 35%. I set the table on my bandsaw to 35%, adjust the fence and play around with the fit a bit, then I can run these through and slice it off:



Once that's set up, it's easy to do all 8:



The pieces that go on the outer ankles have a little recessed rectangular pocket in them. I have a trick for making those. The pocket is .25" tall, and .06" deep. I set up the router table with a .25" groove bit and the router fence 1.3" inches away (per plans):




Then I run all four of the outer pieces across the router to make a groove in them:






Then I cut a little strip of MDF to fit tightly into this slot, but a bit thicker. Next I cut some strips, and glue them back into the sides of the groove leaving the rectangular pocket at the correct dimensions.



I used a paint brush to put just the right amount of glue in there. I want the glue to ooze out just a bit and fill up any gaps between the base and the strip.
Once these dry, I can put them onto the belt sander and bring them down flush with the base piece. They may take a tiny bit of filler, but they'll be ready for paint and I have perfect rectangular pockets now exactly to the blueprints.




Now that all of these are shaped, I can start mounting them onto the legs. First, the center legs:



I use enough glue to ooze out of the sides a bit and fill in any gaps, and I purposely made the curved piece a tiny bit wide. Once these are dry, I can return to the belt sander and bring them down flush with the base. Once I get the ankle pieces glued onto the legs, I'll put the slotted curved pieces on them too.

Center Leg

The center leg is assembled with two layers of 7/16" plywood for the central piece, sandwiched by pieces of 11/16" on either side. The center piece is the bit that sticks down into the slot in the foot shell. That slot is 1" thick. With glue and paint the two plies of 7/16" is just right. So I cut and laminated those together. The screws will be hidden by the outer pieces:


And then I cut the outer pieces. Here I've screwed them together and put the curve line on them to be cut out. By screwing them together and cutting them at the same time I save time and I get the curve on both of them exactly the same.




I have found that for curves with a small radius like this (anything less than 9"), I get the best results by roughing in the cut with the bandsaw and then finishing it on the belt sander:






I got that belt sander from Grizzly Tools when I was building kitchen cabinets and furniture more and had a lot of sanding. It is huge--the sanding area is 6" x 48", I think. And it tilts up with this table on the side for doing right angles. It cost a bundle at the time, but it's been worth every penny. It's a 220 volt machine which required some creative wiring in the shop, but the motor doesn't bog down under any load that I've put to it. The results here are fine:




The curve cut has exposed a couple of small voids in the plies, so bit of body filler putty and another light sanding and they are ready to go:




Since these edges will be up against a larger piece, it won't be possible to sand them or finish them effectively once the center leg is put together, so I save myself a lot of hassle by getting them paint ready before they go together:




The whole stack of all four pieces will look like this:



And then screwed and glued together with another run on the belt sander to get all four edges clean and flush:




You can see the layout marks on a design modification I've made on the R2 plans here. I've sloped one side of the pivot point back at an angle so that it will all rest flush into the slot on the foot shell. The one on the right has the line for the cut that the plans call for. The pointed design on the real R2 isn't very good for several reasons. First, there's a lot of weight and torque that gets put on a relatively small area here. This front foot shells will be leading into cracks in the sidewalk and all kinds of rough terrain. Leaving more material here makes the joint stronger. Second, if you watch the discussions on the club board, everyone is having problems with their foot shells popping up and shifting around. So it's common for builders to go back and add ankle locks to keep the foot shell in position. This is a much sturdier, more effective ankle lock. And with the full length of that section resting down into the slot on the foot shell, the weight is distributed better. The alteration will be visible from the back if someone got down close to look at it and knew what the real one looks like. But the advantages here outweigh the disadvantage. And here's one more shot from the side. You can see that the lines are clean, straight and smooth. All done with power tools--no long hours scrubbing with sandpaper by hand.