Gun Carriage Assembly Fixture
As they say, "necessity is the mother of invention"
How it all came about. My website partner, Scott Bushnell is working on the Sergal/Mantua "Sovereign of the Seas". The scale is 1:78. He asked me to make the Gun Carriages for him because he was unhappy with his attempts. They are to be made of walnut, and will be painted red. Note: Please keep in mind that this Fixture could be made to suit any scale/model. References at the bottom of the article page.
Scale is 1:78
Cheeks are .085" thick by .329" wide/high by 1.048" long.
There are two sizes of axle trees because the gun carriage is narrower at the front than at the rear.
The front axle tree is .085" thick by .197" wide/high by .450" long.
The rear axle tree is .085" thick by .197" wide/high by .508" long.
The dimensions for the locations of the axle trees and cheeks were taken from the plans supplied with the kit.
Make an accurate layout, to scale, of the overhead view of the Gun Carriage in a large format. This is useful because it pinpoints possible trouble spots when making the Fixture.
I used the multiple of 14.
The 1.048" length of the cheek became 14.672"
The .450" width of the axle trees at the front of the Gun Carriage became 6.300"
The .508" width of the axle trees at the rear of the Gun Carriage became 7.112"
The angle, when calculated, of the sides of the cheeks is one degree and thirty five minutes. I arrived at this number by applying the trig formulas using the .450" and .508" and 1.048" dimensions of the respective parts. This helped when planning the location of the containing dowel pins for the outboard sides of the cheeks.
Cutting and preparing the parts for the carriages:
Axle trees and the cheeks:
Cut some blank strips for the T-shaped axle trees with a Preac Table saw. They were made to the following dimensions from the plans that Scott calls "The Stupid Italian Plans":
Front axle trees - strips were cut to .450" wide by 6" long out of some 6mm stock
Rear axle trees - strips were cut to 508" wide by 6" long out of the same 6mm stock. Note: I had to make about 24 carriages so cut plenty of extra material to cover setups, goofs, etc.
After the cutting of the blank strips, reduce down from 6mm (.236") to .197" using a Byrnes Thickness Sander.
The cheeks are .085" thick by .329" high by 1.048" long. Note: I did not have stock thick enough to make the .329" height. So, I glued up two pieces at a suitable joint to make up the required dimension. Plus some allowance for the thickness sanding after. The axle trees are also .085" thick by .197" by .450" (front axle trees) and .508" (rear axle trees) respectively.
The T-shapes are milled in a vertical Sherline milling machine.
The cheek profile is milled in the same milling machine, but used as a shaper. Clamp a precision parallel to the table aligned with the table travel (X axis). Push the blank strips through, against the parallel fence while having a piece of spring steel bear against the edge.
Slice all the blank strips to the .085" thickness.
Use the Preac for the axle trees. Since the Preac can only cut about 1/4" thick, use the Byrnes saw for the cheek blank strips. Note: Use a special thin kerf blade to do this.....020" thick. Works fine.
Making the Gun Carriage Assembly Fixture:
Use the Sherline lathe to turn a piece of nylon to 1 1/2" diameter (Nylon was chosen for the fixture because the glue will not adhere to it). Face it off, and leave it in the chuck.
Put the chuck with the nylon on the rotary table on the Sherline milling machine. Make sure to align the center of the rotary table with the center of the mill spindle.
Do the math and center drill and ream for a total of 18 (eighteen) .0625" holes for Stainless Steel (stainless won't corrode over time) dowel pins. These hole locations cause the axle trees and cheeks to be in the correct relationship. As you can see in the photos.
Twelve Stainless Steel Dowel Pins to hold the axle trees and cheeks in location for Assembly:
Now, to make a Gun Carriage:
So, the procedure is to take a "set" of two axle trees .450" wide, two axle trees .508" wide, and two cheeks. Note. Since the gun carriage is narrower at the front, I calculated the angle and came up with one degree and thirty five minutes. When applied to the thickness of the axle trees where they contact the angled cheeks, there is a taper of .0022" along the .085" thickness. A small amount, but I did "sand" it in. No unsightly gaps for me! Better glue job that way....
One cheek is placed against the containing outboard pins.
The axle trees are placed between the pins to locate them. Use a squared off ice cream stick to gently nudge the axle trees into place against the cheek.
Axle trees in position in Assembly Fixture:
Using an artist's brush, apply glue to the vertical surface of the axle trees and the underside of the inverted T.
The second cheek is then put inside the outboard locating pins. Note. A really good fixture, which I like to think this is, also acts as a filter to exclude bad parts. If they are too thick, they will not go into the space between the pins. If they are too small, they are already sitting in my reject bin, having been weeded out as I made them. I use a drop dial indicator to measure the .085" thickness:
Cheeks added to Assembly Fixture:
After the glue is applied, use a small C clamp to hold the assembly together:
Two plastic extraction forks in foreground:
After the glue is set, remove the finished Gun carriage. To do so safely, make two plastic extraction forks....using the principle of the inclined plane. Note. Anoint the pins with a mold release (thin grease) to prevent the glue from sticking to the pins.
If anyone wants, I can make a sketch with dimensions that I used for my Gun Carriage Assembly Fixture. At the present time, I am researching for the best glue to use....
Once I find that, I will make a few test assemblies to prove out the Fixture, and then it will be "flank speed ahead"! :)
Jerry G (Glickstein)
Gun Carriage Assembly Fixture
Byrnes Model Machines
Ship Modeler's Desktop