FORGEthe milling table and the collar, and then milled the
small keyways into the bar stock, repeatedly indexing
the part by resetting the piece to the next face of the
octagonal collar.GETTING IN A SPIN
Another workshop item that is a midpoint tool (and
not as costly as a dividing head) is a rotary table. A
rotary table is a milling accessory that is primarily
designed for machining arc paths on the manual
milling machine. However, as it can rotate a workpiece
through a known number of degrees, it can also beDividing heads are the professional tool used in
machine shops to perform dividing and indexing
operations. They are an assembly that can rotate
a piece of work and use dividing plates, which are
metal plates with lots of circles of holes accurately
created. These plates allow the work to be indexed
through very precise angles. The workpiece is
rotated, and the division is secured by a locking pin
being inserted into the correct hole on the dividing
plate. The dividing plates are swappable, and plates
can be created to allow dividing heads to create
virtually any number of desired divisions.A COMMERCIAL SOLUTION
For the X and Y co-ordinates, we use the formula
seen in Figure 2, where ‘R’ is the radius of the circle,
‘H’ is the hole number, and ‘N’ is the total number of
holes. It can be quite tricky if you are out of practice
to put this into a calculator or a spreadsheet, so it’s
worth double-checking that you get the right result
for the first hole. The X co-ordinate of ‘H1’ should
be the same figure as the radius of the circle, and
the Y co-ordinate should be zero. Using the formula
will only return positive values, so it’s important to
note that some of the answers are going to actually
be negative co-ordinates when you use them in the
real world. It can be useful to roughly draw the hole
locations as you work them out so that you can tell
when co-ordinates are going to flip to being negative.
Continuing and solving all the X and Y co-ordinates
gives all the information needed to create a circle
of holes.
While a spin indexer might give us more flexibility,
for creating simple divisions, a 3D printer can be
extremely useful to make an object we can use as the
reference for indexing. In Figure 3,we have used a
3D-printed octagonal part to create a collar for some
12 mm brass bar stock. We quickly designed and
printed the octagonal collar with the inner diameter
being a press fit onto the brass bar stock. Using
the faces of the printed octagon, we can rotate the
circular bar eight times and perform an operation on
each face. In this instance, we have used a precision
vice to clamp the workpiece. We set the position with
the 3D-printed collar using a small riser block between
R
H1
H2
H3
H4
H5
H6
H7
X = R Cos
(H-1) x360
N
Y = R Sin
(H-1) x360
N
R
H1
H2
H3
H4
H5
H6
H7
X = R Cos
(H-1) x360
N
Y = R Sin
(H-1) x360
N
Figure 2
Using these
equations, we can
work out the X and Y
co-ordinates of the
centre of any number
of holes around a
circular pathIt can be useful to roughly
draw the hole locations as
you work them out”
”