G’day All- Santa brought me a nice Foredom H15 Hammer Handpiece for
Christmas. Nice tool, fun to play with. However, the manufacturer
recommends it is to be used only at speeds of 5,000 rpm or less. The
Foredom motor I have is the jeweler standard model with variable
speed up to 20,000 rpm. To keep the speed down when using the hammer
handpiece, I put a piece of cardboard or wood under the foot pedal
so the pedal can’t be fully depressed and the motor runs more slowly.
However, I have no real idea if I am within the 5,000 rpm
recommended speed so that the handpiece does not overheat and come to
an early demise. Any ideas how to estimate speed in rpm or other ways
to keep speed down when using the hammer handpiece without the
expense of buying a low speed Foredom motor?
Well, if you’ve got a 20K motor, and you need to stay under 5K, no
faster than quarter speed.
Silliness to the side, the rule with hammer handpieces is slow.
Especially as you’re learning, you want to be able to hear (and
control) each strike.
So it should be ticking over like ‘tick-tick-tick’, rather than
"burrrrrrrrr". If you can’t hear each individual strike, you’re
going too fast. If it gets anywhere close, you’re probably going
faster than you can control anyway. When you get better, you can
speed up, but not at the beginning. (That’s for bezel closing-if
you’re using it for diamond stippling, a slow ‘burrrrrr’ is OK. You
don’t need to hear each strike for stippling, but you really want to
make sure you’ve got control if you’re using it to bash on a stone
seat.)
The ‘old school’ trick for keeping them slow was to use the old CC
motors that had the reduction gear on the back. Gave more power, and
got the spindle speed down to where you almost couldn’t over-rev
them. Haven’t seen one of those (besides mine) in years, but they’re
nice if you can find one that still runs. They probably float across
ebay every so often, and probably not for that much.
Method 1. Buy or borrow a rev counter as used by metal workers. You
can get small, hand held, units very cheaply.
Method 2. Hold a thin round rod in the chuck of the handpiece,
attach a known length of fine thread (cotton?) to it, and attach a
small weight to the other end. Operate the motor and time how long it
takes to wind the thread up. Measure the thickness of the rod and do
the calculation.
Method 3. Make a disk with concentric bands of equally spaced radial
lines or spots. Different number of spots per band. Spin the disk
with the handpiece and observe the disk with a mains operated lamp.
Adjust the speed until one of the bands of spots or lines appears
stationary. Do the calculations.