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Cross Crocodile

The next toy I built from David Wakefield's book, was the Cross Crocodile.  The basic design is almost identical to the Hungry Hippo.

Wood

I had some more of the Brazilian Cherry to use up, so I used this wood for this model also.  This is a very dense wood, much denser than red oak.

Google Sketchup

I entered the patterns into Sketchup.

How it Works

This Sketchup view shows how this toy works, with the 3/8" diameter dowel in each of the front wheels to raise the mouth.

Compound Scroll Sawing the Main Crocodile Body

This design borrows a standard technique used in the scroll sawing world, called compound scroll sawing.

You put a pattern on 2 sides of the wood block, then saw out side #1 and then side #2. You may have to tape the waste pieces from side #1 back in place to provide support for the 2nd cut.


I usually make 2 paper patterns and glue them onto the wood block.  Since I may make more of these someday, I made plywood patterns and penciled the pattern on the 2 sides.

I used the band saw with 1/8" wide blade to cut out the crocodile body.

Rotating Mouth Assembly

I used a fixture, as recommended by David Wakefield, to locate the pieces that make up the rotating mouth assembly. I sanded off all the wood burn marks from band sawing. I had to use the Dremel drum sander and hand file to get at some places.



Error in the Book Pattern :(

I have been doing woodworking on and off again for almost 40 years. By now, I should assume any pattern will have some errors.

After I completed the rotating mouth assembly, I assembled up the whole crocodile before final glue-up.  One big problem arose, the mouth of the crocodile would not close!!!

I finally went back to my Google Sketchup model and tried rotating the mouth assembly, and I found it would not close either!!   An my Sketchup model matches the book pattern exactly.


The little piece that connects the two sides of the mouth, has the wrong geometry on the left hand side shown above.  It is too big, and won't allow the mouth to close.  Since I already had the 3 piece mouth assembly glued up, I used the Dremel 1/2" diameter drum sander and hand file to remove material from the little cross piece.  Once I reduced the size of the cross-piece so the mouth would close, then it would open and close properly using the 3/8" diameter short dowel in the wheel.

I am a little disappointed in Wakefield's design, in that the mouth doesn't open further.  It would take me a while to figure out how to change the original design, so I did not pursue it further at this time.  The higher the mouth  opens, the bigger the sound affect when it closes.

If you are going to build this model, send me an email, and I will send you the correct design for the little piece.  Or you could not glue up the 3 pieces that make up the rotating mouth, hold them with a clamp, and assemble it to the body. Using your eyeball, you can keep removing material until the mouth fully closes. After you have the correct geometry on the little piece, then glue up the 3 piece assembly.



Finished Cross Crocodile

YouTube Video

You can use this link to watch a video of this model in action.


Sketchup Warehouse

You can use this link to download a model of my design.


Closing Thoughts on This Project

My grandkids all love this toy.  Someday, I would like to modify the design so the mouth opens even further that it does.


Working on an improved design

I want the crocodile to open its mouth wider. There are 2 basic ways to do this.


1. Change the geometry of the mouth so it opens further.

2. Use a bigger diameter wheel so the driving pin is further away from the center of the wheel


Using Sketchup, I added some material to the mouth so it would open further. Unfortunately, I could only gain about 4 degrees more opening using this method.


So, I then increased the wheel diameter from 1.75 to 2.0 inches, and combined it with the improved mouth design........to give a net increase of about 12 degrees!!


Material Selection

I decided to make the new Crocodile from red oak, the wheels are made from maple.


Bandsawing the Body

I used the smallest blade I have, an 1/8" wide blade. I sawed the smallest thickness first which is the front view of the body, taped the pieces together, then sawed from the top view. Had to clean up burn marks on drill press  using 60 grit 1" spindel sanding drum, then 220 grit.


Gluing up Mouth

I drilled one hole in each of the side pieces, then used this drill bit, 7/32", as guide to keep them aligned when I clamped them up.  This worked great !


Wheels

I can make wheels, so they are so cheap, I usually buy them.  I did not have any 2 inch diameter wheels in stock, so I decided to make 4 for this project. My process was:


1. Plane a piece of 3/4" maple down to 3/8" thickness

2. Mark 2" circle and center using plastic template

3. Drill 3/8" center

4. Scroll saw 2" diameter slightly bigger than penciled pattern to leave some stock to clean up on lathe.

5. Put 3/8" steel bolt in lathe chuck, bolt wheel blank on to lathe

6. Clean up OD lathe, round over edge of wheel, using round edge tool, make indent in side of wheel, sand


About Done

One Slight Hickup

As I was trying out the model before final finishing and glue up, I cut the two dowels, 3/8", drive dowels, too long. They need to be extend less than the 3/8" thickness of the sides of the mouth, or they will hit the body. 


Another Problem

Sometimes the mouth will not close because of too much friction between mouth and inside of wheel, can happen on either side.  Don't remember this problem when I built the first one. I used mineral oil on the cherry of that one, maybe the mineral oil reduced the friction enough........because it never misses a beat when you push it across the rug or carpet??


I figured it out. These toys look very simple, but they are really a nice balance of features to make it all work right!!   On the original Wakefield design, the 2 drive dowels rub on the body as the wheel rotates, preventing any friction between the wheel and mouth.  When I moved the dowel out from the center of the wheel to increase mouth opening angle, the drive dowel goes below the body causing an interference versus the guiding function.  I simply added a stub piece of 3/16" dowel close to the axle, and it keeps the wheels in the correct position all the time :)



Ready to Finish

I used red, oil based Cherry stain on the oak body and mouth, then Gloss Polyurethane.


Sketchup Warehouse

You can use this link to download a copy of this design.


YouTube Video

You can use this link to watch a video of the model in action.


Finished Project

Closing Thoughts on this project

It was harder to modify this model to gets its mouth to open further than I thought it would be. These toys look simple, but they are sensitive to design changes. Hopefully the grand-children will have fun with this model.