Saturday, October 15, 2022

Metric Gear Set

While most people would 3D print a set of metric transposing change gears to be able to cut metric threads, I've had this weird sense of betrayal, because original gears were metal, not plastic.

I've been watching for 16 pitch (DP) metric thread cutting gears for some time, and never seen them.  However, someone posted a combination gear for a really old Sheldon lathe, and one of those combination gears was a 127 tooth gear - the expensive, elusive change gear that most people can only dream of.  This was absolutely what I was looking for.  I bought the gears, and promptly disassembled it.

First, it was perfect.  It sat on a boss that was 1.25" in diameter, and had a set screw to keep it bound to that boss.  The other end of the boss was another gear for the Sheldon, but I could not use that - the boss was too long, and too integral to that gear.

I needed to make one gear - a 100-tooth gear.  The 16DP change gear for 100 teeth is 6.375" in diameter for the blank.  I had an 8" round piece of aluminum that was 1/2" across the teeth (standard dimension for these South Bend gears).

Unfortunately, I had another project going on in the lathe that I could not interrupt.  So, everything needed to be cut on the mill, except for the initial drilling for a 1/4"-20 set screw like on the 127-tooth gear (that was done on the drill press).

Note that I story-boarded the part.  I essentially mapped everything out in sharpie on the piece of aluminum, instructions, and even the order of operations.  (You can see the story boarding when I'm cutting the gear teeth, later on, but it is there).

I used my rotary table, and a 1/4" end mill.  I got everything written out, and drilled some holes for a hold-down, then locked it to the small rotary table.  Essentially, I milled a round blank on the small Harbor Freight mini mill.

The boring of the blank was the next operation.  While there, I milled off a little boss where this piece of aluminum had been separated.  It had been separated by as cut off tool, initially, but that didn't get all the way through, so they had to finish the cut off with a saw.  Anyway, this gave me my blank.

The next operation was to set this up in the dividing head.  The dividing head is a 40:1 ratio, so I went with a 15 hole plate, and then keyed off of every 6 holes.


The gear was rough, but passable (except for the one tooth that I missed on).  Yes, you can now see the story-boarding.

Anyway, that was enough for one night, so I opted to wait until the cool morning, when I needed to be quiet while people slept.  I used a file to remove the burrs for all of the teeth, and had a good, functional gear ready for the sleeve.

I cut the sleeve out of steel.  It had a 1.25" outer diameter, a hole along that 1.25" line, and a 0.749" inner diameter (I measured the original 80-tooth idler gear for that one).  First, the hole for the 1/4"-20 tap had to be cut (I used a #7 drill bit, this was ductile iron, though swarf came off like it was a mild steel of some sort), and then the part was put into the lathe and bored, then turned down.

It was then taken to have both gears put on, the holes lined up, and then tapped.

The result is a 127-tooth/100-tooth compound gear that should be one step closer to a full metric set.

Now, for that 1.5" metric thread I need to cut, I should be able to put a 48-tooth stud gear on, with the 64-tooth screw gear, the 127 tooth inboard to the headstock, and put the gear box plungers into C-1. However, on doing so, it came out as about a 1.25mm thread pitch.  So, I adjusted to a 13-pitch instead of 16-pitch, and had it close enough for the task I was in need of, a double-ended 22mm-1.5 male-male adapter for two ends of lathe handles to become one :


And the results were, it was still slightly off.  I've done some math on it, and I should have gone with 14-pitch instead.  But, that slight error still got the two bars to mate together close enough to be stable.

Woohoo!  Metric threading is now possible!  I picked up a metric threading chart for South Bend 8 pitch lead screws, and then needed to start amassing a gear set for the other pieces.  I need the following in 16 diametral pitch, 5/8" bore, and 1/2" wide :

  • 48 tooth for 6mm, 3mm, 1.5mm, 1.2mm, 0.75mm, 0.6mm, and 0.3mm pitches
  • 44 tooth for 5.5mm, 2.75mm, 1.1mm, and 0.55mm pitches
  • 40 tooth for 5mm, 2.5mm, and 1.25mm pitches
  • 36 tooth for 4.5mm, 2.25mm, 0.9mm, and 0.45mm pitches
  • 32 tooth for 4mm, 2mm, 1mm, 0.8mm, 0.5mm, 0.4mm, 0.25mm, and 0.2mm pitches
  • 28 tooth for 3.5mm, 1.75mm, 1.4mm, 0.7mm, and 0.35mm
  • 26 tooth for 1.3mm, and 0.65mm pitches

So, I started machining some gear blanks.  I did manage to find a gear that was 26 tooth.  I did manage to use the Junior gears I had on hand to cover the 48, 40, and 32 tooth gears.  That opened up the options I needed for a few 1.25mm, 1mm, and 1.5mm pitches for those wood lathe handles.


I'm almost in business!  I just need gears!

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