How to make a Bolt Tool
In order to polish the head of a flat top or cheese head screw, you'll need a special tool
commonly called a bolt tool or frog polisher. This tool comes in different styles and sizes,
but the one we're making is mainly for polishing screw heads ranging from 12 BA (1.4 mm)
to 8 BA (2.2 mm). With a bit of adjustment, it can also polish sizes from about 1 mm to 3
mm.
To use the bolt tool, first, you'll need to have hardened and tempered screws. Then, you
clamp the screw with a clamp bar onto the bolt tool so that it sits securely in a "V" notch.
Next, adjust the two screws forming the adjustable feet so that the flat head of the screw
lies flat on the surface formed by the screw itself and the adjustable feet. After that, rub the
tool on progressively finer abrasive material fixed to a flat plate until you achieve a mirror
finish on the screw head.
Here's what you'll need to make the bolt tool:
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Brass sheet, 3 mm thick, at least 50 mm x 25 mm
Round brass rod, 6 mm in diameter, at least 20 mm long
Brass hexagon with sides measuring 8 mm across, at least 20 mm long
Silver steel rods, one with a diameter of 3 mm and one with a diameter of 6 mm,
each at least 50 mm long
The lengths mentioned include allowances for holding the materials in a lathe for turning.
The hexagonal brass section with sides measuring 8 mm across is similar to 8 mm, suitable
for an 8 mm or a 2 BA Spanner.
You won't need a special tool like a knurling machine, as we won't be tightening the
locknuts with much force. However, if you prefer, you can make the locknuts from 10 mm
diameter round bar and knurl the surface, though knurling is a machining process not
covered in these instructions.
You'll also need to get commercially made screws: 2 screws measuring 10 BA x 3/8 inch,
and 3 screws measuring 8 BA x 1/2 inch. These screws will be used during the
manufacturing process of the bolt tool. If you can't find these screws, you'll need to make
one additional screw of each size, along with a short length of rod threaded 8 BA for 3 mm
at one end.
Illustrations, measurements, and guidance
When applicable, the illustrations are shown using first angle orthographic projection.
Allowances
The allowances to be used for the measurements in this task are detailed in the table
below. For the screws, the allowances should aim for the best possible fit with their
corresponding parts.
Material Inspection
Prior to beginning the task, it's important to measure the dimensions of the brass section to
ensure you have the right sizes and enough of them.
Step-by-step process and initial actions
Before beginning any work, carefully review the drawings and plan out the sequence of
tasks. First, it's important to ensure that the two holes in the clamp bar are perfectly
centered on the 3 mm thick plate and aligned with the threaded holes in the body.
Instead of marking and drilling the parts separately, it's easier to mark and drill the holes
before cutting the clamp bar from the body. Also, it's crucial to make the "V"-grooves before
riveting the bushes in place, as doing so afterward may make it harder to hold the body
securely in place without causing damage.
Start by finishing the 50 mm long brass plate to precisely 25 mm wide and filing one end
square, making sure the edges are perfectly square and flat. Then, mark the center for each
hole at the squared end, ensuring they're both perfectly centered on the thickness. Use a
center punch to mark the centers and inspect them with a 3x loupe for accuracy. Clamp
the plate to an angle plate and adjust it so that it's perfectly upright on the drilling machine
table.
Drill two holes to a depth of 15 mm using a 1.4 mm diameter drill bit, then enlarge the holes
to a depth of 2 mm using a 1.7 mm drill bit. Next, partially thread the holes while the clamp
and body are still connected, ensuring precise alignment for later assembly. Carefully
thread the holes using a taper tap until they reach a depth of 9 to 10 mm.
Complete the marking out by scribing lines at specific distances from the squared end,
indicating where to cut off the clamp bar from the body and where to position the holes for
the bushes. Cut off the clamp bar between the scribed lines and finish the ends with a file,
ensuring they're flat and square. Mark one end of each adjacent surface with a small center
punch mark for future orientation.
To flatten the 3 mm edges, rub the plate along a square metal bar placed on 320 grade wet
and dry paper until they're perfectly flat. De-burr the holes slightly. Then, complete the
threading of the 10 BA holes in the body using taper, second, and bottoming taps
sequentially.
Assemble the clamp bar to the plate using two commercial 10 BA x 3/8 inch steel cheese
head screws to check for alignment. Cut the plate body to length and finish the edges with
the clamp bar in position using the square bar as a guide. Finally, center punch and drill 4
mm diameter holes in the body for the bushes, lightly countersinking the underside in
preparation for riveting.
Cutting the “V” grooves
To make sure the screws we're polishing stay upright, we need to create some grooves on
the edge of the body. The drawing shows the sizes of the three screws mentioned earlier.
Although specific dimensions aren't provided on the drawing, it's easier to file the grooves
to the right depth using the shanks of three twist drills as a guide. You'll know you've
reached the correct depth when a straightedge, representing the clamp bar, just traps the
drill shank in its respective groove.
The depth of each groove is calculated using a formula: Depth of vee = 0.5 x screw diameter
x (1 + √2) - 0.25 mm. These grooves are made with a square file, ensuring it stays perfectly
straight and at a 45-degree angle to the edge of the plate for a symmetrical V-groove. For
the larger grooves, it may help to use a piercing saw to remove most of the metal first. Then,
finish the surfaces of the V-grooves with fine flat needle files.
Next, we can finish the upper face of the plate and clamp by bolting the two components
together using two 10 BA commercial screws. Use 320-grade wet and dry paper attached to
a flat surface as before. While the clamp is in place, you can also finish the edges so that
the clamp matches the plate exactly in size. However, leave the lower face unfinished for
now; it will be finished after the bushes are riveted to the plate.