The claws on the B9 Robot are a very distinctive feature, one that needs to be done just right so everything looks good.

 

But claws are not simple, they have several different radii, they taper down from 1” thick to about 5/8” thick and they have a tricky bend to them.

 

So far there are generally two ways to make claws:

 

• Casting. Make a mold, pour in suitable plastic or molten metal, let harden/cool.
• Shaping. Take a chunk of raw material, carve & cut till done.

Casting needs an investment of time, material and labor to create a mold. Of course once the mold is done parts can be made in quantity quickly and efficiently. But maybe not the thing to do if you want to make just one set for yourself.

 

Shaping, be it with tools like a band saw, belt sander, milling machine or pocket knife, needs lots of time as well as thick raw material, probably 2 1/2” thick. So unless the material is wood this will be expensive. Yes, two pieces of thinner material could be connected (glued, screwed, welded) but this is also expensive and time consuming.

 

The new trick here is to use a very well known fabrication method but apply it in a slightly different way. Collar and vent ribs are made by heating up square plastic rods and bending them in a fixture. Claws can be made in much the same way.

 

Its quite simple, build a wooden form, heat up some plastic in an oven, squeeze the now warm (and soft) plastic to shape. Now instead of needing raw material that is 2 1/2” thick (expensive and not very available) 1” thick material can be used!!!

 

There are many plastics that could be used here. ABS, PVC and Nylon come to mind, they are all thermoplastics and can be formed by heat. PVC can be hard to paint and Nylon is a bit expensive (can be double that of ABS). ABS is cheap, strong, light, has a low forming temperature and will accept paint. Many car bumpers are made of ABS. So ABS it is.

 

Key here is to use a simple mold to get as many of the desired features as possible made during the heat forming. Most critical is that the outer bend of each claw is about 25 degrees (with a radius!!) and the inner bend is sharp and about 20 degrees. This gives the claws that nice taper. And this is where the simple wooden form is used. You will need 2 wood 4x4's that are each 14" long. Use the drawing below to complete the mold. 

 

Note: All of the measurements and dimensions were gleaned from existing drawings or interpreted from photographs. I do not imply that they are any more or less correct then previously published data.

Make sure to get the 1/2" radius right, first draw the straight lines to the dimensions on the drawing. Only then draw in the radius.

 

The better this mold is made the better the parts that come from it will be. Take your time, do a good job, make your cuts smooth and straight. In any case this should not take long with a band saw. It is a very good idea to use some sort of alignment method between each part. I used ground down lag bolts.

The raw material is ABS plastic available from McMaster-Carr in 24” x 3" x 1" size, their part number 8712K63. Buy 2 and cut into 11" long lengths for 4 blanks, altogether this will be enough to make claws for 1 Robot.

 

Pre heat the oven to about 325 deg, place the ABS on some sort of flat sheet (I used aluminum) and put it in the oven. Don’t just put it on a wire rack, it will flow around the wires forming an extremely ugly and unusable mess...  It should take 35 minutes for the plastic to soften. Yes, there will be some smell, do this in the garage and/or when your spouse is away...

 

When the time is right take the plastic out of the oven (remember to use suitable gloves), quickly place it in the wooden form and then squeeze the form together. Some sort of machinery will be needed to do the squeezing, A couple of c-clamps worked for me. Do all this as fast as possible, the plastic cools quickly and gets harder to form! But if at first you do not succeed try again! Back in the oven for 15 minutes or so and give it another try. One part I did took three tries...

 

And then, and this is an important part, leave this clamped for at least 15 minutes to cool and harden.

 

 If you worked quickly and carefully you will now have some material that is about 1" thick on one end, has the correct 20 deg and 25 deg angle, an inside corner and an outside radius not to mention the nice taper.

 

Make 4 of these. They should be all more or less identical.

 

 

 

 

Cutting the shank is fairly easy, A couple of small radii and then all straight cuts. Follow the drawing below:

Next do the claw end. This is not as simple, two large radii to cut. Note that the outer radius does not show a proper location as the fold line messes up the geometry, its best to make a paper template and lay it out to fit at the tip (4" from the fold line) and at the other end. For the inner hole I used a 3 1/4" hole saw. Do not try this with a hand drill!!!  You must use a drill press or a milling machine! If you don't have a drill press this can be done with a normal saw, it will take longer and may not be as accurate. Follow this drawing:

Last part. There is a funny angle in the sides of the claws. Its funny cause it gradually changes the cross section from the tip where it is rectangular, to the base where it is sort of a trapizoid. I did this in two steps. First with the shank flat on the table of the belt sander I sanded the edge till the top was even with the bottom. This continues to around 90 deg, the mid point. Then I blended so that the cross section gradually changed from the trapiziod to a rectangle at the claw tip.