Miniature 4 mm ratchet wrench

[mklotz]

These inexpensive 4 mm precision screwdriver kits, e.g....

https://www.amazon.com/Syntus-Precis...11&s=hi&sr=1-9

are great when working with miniature equipment. A bit for virtually every screw type (with the possible exception of JIS Pozidrive) is included along with metric sockets in the 2.5 - 5.5 mm range. All these bits have a 4 mm hex shank that fits the included handle and extensions therefor.

The one thing that isn't included is a 4 mm ratchet wrench, a tool that allows access to fasteners in locations that prohibit use of the screwdriver format handles. Commercial 4 mm ratchets are almost impossible to find. The only one I could locate is this...

https://www.amazon.com/Lisle-14540-H...8191086&sr=8-3

gadget for adjusting Ford headlights. Its head is rather bulbous and the ratchet isn't reversible; to reverse the action one must remove the bit and reinsert in the other direction - not a deal killer but not ideal given that there are better solutions.

Chris Rueby, one of the genius-grade modelmakers over on MEM, designed and fabricated a scratch-built 4mm ratchet...

https://www.modelenginemaker.com/ind...topicseen.html

George Britnell, another genius modelmaker, made his own version...

https://www.modelenginemaker.com/ind...html#msg239454

I didn't want to take the time to do a complete build as they did, but still wanted to have the capability for the rare occasions where it might be needed.

This style offset screwdriver...

https://www.amazon.com/MulWark-Profi...191343&sr=8-20

has the advantage of a small, thin head as well as a switch to reverse direction. I have one I picked up cheaply at HF a long time ago.

The problem is that it takes only a 1/4" hex, not 4 mm. An adapter is needed. Wiha makes a 1/4" to 4 mm adapter, e.g. ...

https://www.amazon.com/Wiha-75802-In...8958965&sr=8-1

but its length nullifies the advantage of the thin offset screwdriver head...









I have some 1/4" hex steel but carving a 4mm hex into it would test my patience. This is a really low torque application so a simple hole with a setscrew acting on the 4mm hex flats would work so I made this...



A half inch length of hex has a turned down section (to slip between two retaining tabs on the wrench) which has a cross-drilled and tapped 0-80 hole for a setscrew to lock the bit in place. The hex is drilled through with a #16 drill (0.177" ~= 4.5 mm) that is a slip fit on the 4 mm hex shanks on the various bits and sockets.

Fully assembled, it looks like this...



and can fit into a crevice less than an inch wide.

I abandoned the idea of a 0-80 setscrew early on; they're just too tiny. A 0-80 SHCS works much better; it can be held on the end of a 0.050" Allen wrench for mounting.

Yes, this is all rather fiddly but it will only be used very rarely, if at all, so I'm willing to live with it.