Gears for the Orrery

Jan 26, 2017

The earth gears both need 39 teeth. The purchased #3 (35-54) gear cutter was used. (DP = 30: PA = 14.5°) The process began with two brass blanks, 1/4" X 1.5" X 1.625". The center was marked on both blanks, drilled to the C drill bit and reamed with a 1/4" reamer. Both blanks were placed on the arbor back to back. Their diameter was reduced to 1.367". The arbor with the blanks still affixed was moved to the Sherline gear cutting head via the 3-jaw chuck. The cutter was touched to the top of the blank and lowered 0.110" followed by an additional drop of 0.684" to center it relative to the blank. The teeth were cut in increments of 0.010" except for the last three cuts at depths of 0.065", 0.070" and 0.072". The teeth were cleaned up with a few light touches with a 3-corner needle file. The edges were slightly chamfered with a file in the lathe.

This required about six minutes per tooth!! The #2, #3, #4, & #7 cutters were purchased from RDG Tools UK.

The 27 tooth Mars gear was prepared next. A square of 1/4" thick brass was reduced on the lathe to 0.967" after reaming the center hole to 1/4" and placing the blank on the arbor. The chuck was moved to the indexing attachment and 27 teeth were cut with the #3 cutter!!! Wrong cutter, but the fit was fine! May redo this gear at some point.

Next up was the 48 tooth Venus driver gear. A 1/4" X 2" X 2" brass bar was cut to a 1 3/4" square with the hacksaw. This was drilled (C) and reamed on center to 1/4". It was placed on the arbor in the 3-jaw chuck and reduced to 1.667". The chuck was moved to the indexing attachment and the #3 cutter was centered vertically on the blank. The cutting depth was increased with cuts at 0.020", 0.035", 0.050", 0.060", 0.070", and 0.072". Cutting speed was about 760 rpm. The gear was transferred back to the lathe and deburred with light touches of the 3-corner needle file.

The Venus gear had to be remade. (2/2/18) There were a number of spots that continued to catch even after significant filing. The new gear worked smoothly without any modification. The new Sherline Dividing Head Calculator was used to determine the offsets for making the cuts.

The 51 tooth Mars gear was made similarly to the Venus gear above, except it began with a 2" square of 1/4" brass and reduced to 1.767" after reaming the 1/4" hole in the center. After cutting the 51 teeth it was cleaned up a bit with 150-grit sandpaper. The Venus 30 teeth gear was made from a 1.25" square of 1/4" brass and reduced on the arbor to 1.067". The 30 teeth were cut with the #4 cutter.

The Mercury 15 teeth gear needs an arbor with a smaller back support (~0.45") and positions the gear further from the chuck to avoid interference with the cutter. This arbor was made from a 2 1/4" length of 1/2" steel round bar. It was faced and center drilled on one end and this end was reduced to 0.375" for a length of 1.5". The bar was flipped end-for-end in the chuck and the opposite end was faced and center drilled. The bar was reduced to 0.450" for the remaining 3/4". 1/2" was further reduced to 0.249". A 1/4" length was threaded after chamfering 1/4-28. A short spacer was made to move the nut farther from the blank. A nut was made from a 3/8" piece of steel hex by drilling and tapping 1/4-28. With the arbor in hand a 5/16" thick slice of 3/4" brass was drilled and reamed 1/4". It was reduced to 0.567" and both sides were chamfered. The 15 teeth were cut with the #7 cutter. The 63 teeth Mercury gear was made similarly to those above after more brass was purchased. All of the gears worked well except for the Mercury pair where there was some binding in a particular spot on the large gear. These few teeth were filed until the fit was acceptable.