DIYbanter - View Single Post - layout question

wrote:
I need some guidance on figuring out what the bolt hole co-ordinates
are to build a drive flange. The hub I need to bolt to has three
holes, evenly spaced with a center to center dimension of 3.607. How
do I calculate what the bolt hole x-y co-ordinates are to drill this
when held in the mill vice? thanks in advance, ron

Hi Ron,

To some extent I'm figuring this out as I go along, so if anyone can
think of a better method, please say.

I'm assuming you have the round, flanged component ready made. You'd be
better off holding the component in a three or four jaw chuck clamped to
the mill table rather than a vice. I'm also assuming you have a DRO on
your mill, but if you don't it just requires a little extra mental
arithmetic.

Once you have the component clamped to the table, you need to centre it
under the spindle. Get one of those adaptors which enable you to fix a
dial test indicator to the spindle. Bring the test indicator into
contact with the component and slowly turn the spindle. Adjust the
position of the table until the reading on the indicator doesn't change.
The centre of your round component is now directly under the spindle.
Make this position the origin (0.000, 0.000) on your DRO.

Now you need to find the radius, r, at which you will drill the holes:

r = 3.607/(2*sin60) = 2.083"

The first hole may be drilled at (0.000, -2.083).

The other holes will then be positioned at (-r*sin60, r*cos60) and
(r*sin60, r*cos60). Evaluating these gives (-1.804, 1.042) and (1.804,
1.042).

If you don't have a chuck which can be clamped to the mill table, and
you're making the component from scratch, you could start with a
rectangular block of material, drill the holes (including a central hole
to align the shaft), then cut out a rough circle and turn it to size on
a lathe. But this presents its own problems.

I also doubt your hole positions need to be accurate to 0.001". Crude as
it sounds, you might get away with marking the positions using a scriber
and centre punch, then lining up the drill by eye.

Hope this helps,

Chris