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.

[NortonDommi]

Chapman make a very good gunsmith driver kit that has one of the nicest wrenches I know of and it is rated at 200 pounds/inch. Bits have a collar on them to stop them being pushed through the head.

[mklotz]

When I was making the adapter for 4mm bits, I noticed that the headroom needed to use the wrench was limited by the length of the bit. If the bits were shorter, they could be inserted into a smaller free space above the fastener. Not a real problem; when necessary, simply cut the 4mm bit shorter and gain a little clearance.

Then it occurred to me that the same thought applies to the 1/4" bits commonly used with this style wrench. I was ready to get out the disk grinder and make a set of amputated bits when it occurred to me that I couldn't be the first person to notice the occasional need of short bits. A quick check on Amazon proved this true...

https://www.amazon.com/dp/B07YCP213C...roduct_details

This set is shown in the photo along with a short 3/16" hex bit compared with its normal sized brother.

Normal size length = 0.98"
Shortie length = 0.59

Headroom gained = 0.98 - 0.59 = 0.39"

Something to keep in mind if you do a lot of work in close quarters.

[mklotz]

Just for completion of this topic, I ran across this four way ratchet...

https://www.amazon.com/dp/B09GY3Z1MN...roduct_details

on Amazon. It accommodates 4,5,6, and 8 mm shanks. It's a bit taller than my solution above but has the advantage of not requiring any construction effort and working with other hex sizes.

[mklotz]

I discovered that the 0-80 hex head screw mentioned in my original post, while a workable substitute for a 0-80 setscrew, got in the way on some of my smaller models. Not many folks have 0-80 setscrews on hand but I've got them in several lengths and metals. The one I selected, shown in the middle of the penny, is 0.064" in length and 0.060" in diameter. Below it is shown the 0.028" Allen wrench used to tighten it. [Aside: How in blazes do they make the 0.028" hex hole in the setscrew?]



This photo shows the setscrew installed in the adapter and the machine screw it replaces.

[Frank S]

Marv; are you sure that is not a 3" diameter Penny with a 1/4-20 setscrew in the picture.? It would be much easier to fake a picture than imagine making a .028" hex hole in a screw less than a 1/16 of an inch in diameter. But I have worked with 0-80 screws in the past so I am aware of their size. Can't say as I have ever needed a 0-80 grub screw though.