SwarfRat Enterprises, L.L.C.


 Quick-Change Gearbox

for the

Chinese 7x10 (or longer) mini lathe

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Here's one of my best projects.  It was a good mental workout, as all good projects should be, but best of all, it's been the most practical addition to my lathe.

I was really sick of changing gears each time I wanted to cut a different thread, and then switch to a finish cut.  (I started to miss my BIG machines!)

So, I set out to see if I could build a quick-change box as a

'proof-of-concept' project.  If it seemed like it would work, I'd plan on building a really neat finished version.


Since metal is expensive, I prefer to make my first attempts

using 'virtual' material.

I've saved a fortune this way...  :-)

There were several design goals, but one important one was finished size.  This is a mini-lathe, after all, and I did want to keep it that way.  The CAD program allowed me to explore different configurations, until I had one that used the smallest

amount of space.

As you can see, it's pretty simple inside. 

Since the motor on the 7x10 mini lathe is rather short on horsepower, I needed a transmission system that would absorb as little power as possible, yet offer the ability to change gears with a simple movement of a lever or two.  This led me through several different systems, before I settled on this one.  I had several ideas where I could have reduced the internal power losses by a good percentage, but each of those methods either offered less convenience, used up much more bench space, or were two-to-three times more costly in parts.

The carrier on the right controls the coarse range, the left carrier the fine.


Since I didn't know how many iterations I'd end up making of this beastie, I decided to have the blanks for the gear carriers sand cast.

These are a couple of the wood patterns I made.  I made a dozen, all together.  Since the foundry I was using charges by the pour, I'd at least get quite a few for my money!

Even so, I really need to build my own mini-foundry pretty soon...




Here is a pair of the castings ready for machining.



Here's the casting after boring and fitting with a roller bearing.

Here's a view of the outboard end.  I've left the shafts a bit on the long side, for some future improvements I have in mind. 

But mainly, you can see the cheapie gearmotor here, and its switch.  Inside the box I have the standard 'finish-cut' gears installed out of the way of the other change gears.  When the gearmotor is engaged and started, it engages these 80 tooth gears to move the carriage at an extremely slow 5/8" per minute.  This is independent of the spindle speed, so that depending on your material and type of tool, you can achieve some really nice mirror-finish cuts!

I've been able to cut threads into any material I've tried so far, namely hot and cold rolled steel, O1 tool steel and, of course, aluminum and brass.  It's handled the loads nicely, and I've yet to stall the motor out or slip a gear.

With the current setup, I can cut 12, 14, 16, 18, 20, 22, 24 &26 TPI with the coarse range engaged, and 24, 28, 32, 36, 40, 44, 48 & 52 TPI with the fine. It also has the standard 'finish cut' that the machine came with (something like 254 TPI, if I remember correctly) and, as I mentioned above, the 5/8" per minute feed rate for finish cuts.

I've used this box now for over a year, and except for stripping a gearmotor (I forgot to disengage the fine feed lever - a really dumb mistake) it's worked wonderfully.

This was a great project!

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