Question:
I'm not a total novice when it comes to metal, but I'm darn close. I'm
definately totally new to machining and could use a bit of advice in
selecting a metal AND its dimension for an appilcation.
I am modifying a solenoid from a pull-type to a push-type.
The solenoid (Ledex-Dormeyer #3000-M-1) is going to be used for an
unconventional application: the motion of ther plunger/core will used for
its percussive force to strike a stationary metal plate. The solenoid is a
massive laminated pull-type unit and in order to have the solenoid strike
a surface, I need to drill through the frame and into the plunger, then
insert a stainless steel rod...this is a common modification and many push
type solenoids on the market are fabricated in this manner. The rod
becomes an extension of the solenoid's plunger and is used to actuate a
switch or mechanism: in my case the rod has a small larger diameter
'hammer' section on its business end, and this will allow the solenoid to
'hammer' into metal plate. The solenoid generates a maximum of 32 ft/lbs
of force and my application demands that the plunger/rod transfer as much
percussive force as possible. I would also like to keep the design as
simple as possible.
Answer:
Here is where I'm at:
I'm new to machining metal and am using a university's metal shop, where
the technician has taken the time to instruct me on basic lathe and mill
functions.
My plan was to use 303 stainless steel. The reason for the 303 is that the
rod material must be non-magnetic (so it doesn't alter the magnetic
properties of the solenoid...flux paths, etc.) and is also relatively easy
to machine.
I started with 1/2" rod and turned most of it down to exactly 3/16". Total
length of the finished rod is 3-1/2". A small 3/8" section at the end of
the rod was left at 1/2" diameter...this latter section formed a small
'hammer-type' head and would function as the actual impact surface. The
opposite 3/16" end of the stainless steel rod was lightly hammered into a
matched hole drilled through the solenoid frame [the latter 1/2" thick]
and then drilled 7/8" into the end of the plunger.
Total length of unsupported SS rod extending from the solenoid is 3"
(2-5/8" of 3/16" dia. and 3/8" of 1/2" dia. rod).
TEST RESULTS*:
I set the solenoid up to energize on a vertical axis and to strike a metal
plate at the point at which the plunger is fully energized/seated...the
point of maximum force. Everything worked well, but after approx. a dozen
energizing cycles, the stainless steel rod started binding. Upon
examination I noticed a slight bend to the rod. It had obviously mildly
deformed under the force and was binding in the feed hole in the
solenoid's frame.
So, I now need to redesign the dimensions of the modified rod section in
order to withstand the 32ft/lbs of force (go thicker than 3/16" dia...but
how thick?), or go with an even stronger non-magnetic metal, or both. I
prefer the former solution, but I also have to keep in mind that the
thicker the rod gets, the heavier it gets, and thus the impact force
diminishes.
As has been said, anyone can design something strong enough, but it takes
mastery to design it only as strong as it needs to be.
*NOTES:
a) When I bought the stainless steel rod, the gent that sold it to me
wasn't sure if it was 303 or 304.
b)The tech who helped me used a power hacksaw to cut the finished section
from the rod while still on the lathe. He lost control of the saw after it
cut through the rod and the saw went down hard, slightly bending the 3/16"
dia. rod. I gently bent it back. This may have had some effect on the
strength of the rod.
I would suggest using aluminum rod of the largest practical diameter
for the device. Try for one of the harder alloys, 6061,6063,
(www.onlinemetals.com) If you can accommodate say 3/4" diameter then
use that, machine a smaller diameter at the plunger end to press fit,
cross drill and pin (spring-pin) into the plunger, drill a hole in the
other end and make a steel top-hat shaped button and cross drill and
pin that in place. Light strong and non-magnetic. An alternative to
the top-hat could be a larger diameter with a hole that the rod goes
into then cross drill and pin for an even large contact area.
