THIS IS A SIMPLE LOOKING PROJECT, BUT ENDED UP BEING QUITE CHALLENGING. I ULTIMATELY PLAN ON USING THIS PROCESS TO CREATE A PAIR OF DICE WITH PRESS FIT DOTS AS SHOWN IN THE CAD MODEL AND DRAWING.
TO CREATE SMOOTH DOTS, CUT SECTIONS OF BRASS ROD WERE PRESSED INTO HOLES IN THE ALUMINUM BEFORE THE ENTIRE SURFACE WAS MILLED, REMOVING THE PROTRUDING ROD AND FINISHING THE ALUMINUM. BECAUSE OF THIS, THE PRESS FITS HAD TO BE TIGHT ENOUGH TO ALLOW FOR CUTTING FORCES TO CUT BOTH METALS CLEANLY WITHOUT SHIFTING.
AFTER LOTS OF EXPERIMENTATION AND CAREFUL MEASUREMENTS WITH AN OPTICAL COMPARATOR AND MICROMETERS, I DETERMINED THAT THE PRESS FIT HAD TO HAVE AN INTERFERENCE OF BETWEEN -0.0002 AND -0.0008 IN TO BOTH STAND UP TO CUTTING FORCES AND BE EASY ENOUGH TO PRESS IN.
THROUGH MANY ITERATIONS, I DEVELOPED A PROCESS USING DELICATE REAMING, HAND SANDING OF THE SPINNING ROD ON A CUSTOM REST, AND CAREFUL MEASUREMENT TO SUCCESSFULLY CREATE REPEATABLE PRESS FITS WHICH FULFILLED ALL OF MY REQUIREMENTS. I ALSO DESIGNED AND FABRICATED A CUSTOM FINISHING JIG TO HOLD THE CUT ROD FOR DETAILED CHAMFERING AND SANDING, ALLOWING FAR SMALLER ROD SECTIONS TO BE USED FOR EACH DOT WHICH MADE THE DIFFICULT TO FABRICATE TIGHT TOLERANCE ROD SECTIONS GO MUCH FURTHER.
THE PRECISION REQUIRED FOR THIS PROJECT PUSHED THE LIMITS OF THE MACHINE TOOLS AND MEASUREMENT TECHNIQUES AVAILABLE TO ME, SO IT REQUIRED SOME OUTSIDE THE BOX THINKING. FOR INSTANCE, I FAILED TO DEVELOP A PROCESS THAT COULD BRING THE ROD INTO TOLERANCE OVER ITS ENTIRE LENGTH (OR EVEN OVER A FEW INCHES,) BUT COULD GET IT CLOSE ENOUGH THAT I COULD USE THE OPTICAL COMPARATOR TO FIND AND CUT OUT SPECIFIC SMALL SECTIONS THAT HAPPENED TO BE WITHIN TOLERANCE. THIS MEANT THAT ONLY ABOUT 10% OF THE ROD WAS USABLE, BUT SINCE 10% OF THE ROD WAS FAR MORE THAN ENOUGH MATERIAL THAN I NEEDED, THIS WAS AN ACCEPTABLE SOLUTION.