Wednesday, May 4, 2016

Replacement Gear for 1974 Okuma Lathe

Making a 50-tooth replacement gear for a 1974 Okuma metal lathe.

Difficulty Level (Easy, Medium, Hard, Insane):

A gentlemen from PEI had recently aquired an old 70's Okuma, high-end Japanese metal lathe. Unfortunately, one of the gear train gears was on its way out and instead of waiting until it broke, he asked me if I could make him a new one (plus a few spares).

Originally, I was going to make the gear out of delrin, but after some further discussion, he told me that he wanted to fortify the keyway and have a brass core instead.

The process was fairly simple, as I had all the tools necessary. First, I had to cut and turn the brass cores to 1.750" diameter, with a 25mm bore and a 7mm by 3.5mm keyway broached. Then I had to mount the cores on an arbor and knurl the outside with my Home-Made Knurling Tool so that when I pressed the cores into the delrin gear rim with my bench-top vise, the straight splines would lock the core to the delrin and prevent any slipping.

After the cores were done, I cut up a 6" chunck of delrin on my Metal Band Saw and turned it down to a bit over 3", bored the hole to 1.740" (10 thou smaller than the diameter of the brass cores) and pressed the cores into the delrin rims.

After the gear blanks were all done, I re-mounted them on the same arbor, turned the delrin to exactly 78mm and cut the gears on my lathe/mill with the Dividing Head I had built a few months back and a gear cutter that was made out of a 1/4" square HSS bit and hand-ground to match the original gear.

All-in-all, the process took about 15-20 hours but I certainly learned a lot and got a knurling tool out of it...


Cutting the delrin on my band saw
The cut up chunk of delrin
Starting to turn the delrin
The strings were just flying off the piece
Almost done roughing the delrin
The rough-turned chunk of delrin
Another view
Drilling the hole
The drilled hole in the delrin
Grinding my gear cutter to match the original gear
A quick AutoCAD drawing to perfectly radially align the tool bit 
Close-up through the loupe
Another view of the hand-ground cutter
Another view
Cutting up a brass rod to 3"
Turning and center drilling the brass
Getting ready to turn and bore the brass
Turning the brass to about 1.750" diameter
Broaching the brass cores
The broached cores
Another view
Turning an arbor to size
Test fitting the brass cores
Cutting the thread
The finished arbor
The cores mounted on the arbor
Getting ready to knurl the cores
Another view
Yet another view
Half-way done the knurling
Knurling the re-mounted cores
The finished parts, ready to assemble

The finished gear blank
Mounting the gear blanks on the arbor
Turning the gear blanks to a final size of 78mm
Aligning the dividing head parallel to bed ways
Cutting the gear teeth
The finished gears
Another view
The finished gears
Close-up of the gear
Another view
The 3 new gears with the original one on top
Metal lathe & accessories
Dividing head
Metal band saw
Diamond sharpening tool
Hand-ground involute gear cutter

4" of 5" delrin
3" of 2.500" brass

15 hrs

They turned out really nice.


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