A T-handle for any hex key, and the key sizes behind it

11 October 2026

geometrybasics

An L-shaped hex key is cheap, accurate and miserable to hold. The short arm is 20.5 mm long on a 2.5 mm key. The last turn of a stuck screw goes into the side of your finger. A T-handle fixes that, and a hex key is a standard part, so the size stamped on it is enough to print one that fits. That is the whole idea of the Hex Key Handle Generator. Pick the size, pick a T-handle, a screwdriver grip or a lever sleeve, and download the STL.

What the size on the key tells you

The number on a metric key is its width across the flats, in millimetres. ISO 2936 fixes that width and both arm lengths for every size. Here are the standard-series numbers the generator starts from, long arm × short arm, with the cap screw each key usually turns (ISO 4762):

• 1.5 mm: 46.5 × 15.5 mm
• 2 mm: 52 × 18 mm
• 2.5 mm: 58.5 × 20.5 mm, M3
• 3 mm: 66 × 23 mm, M4
• 4 mm: 74 × 29 mm, M5
• 5 mm: 85 × 33 mm, M6
• 6 mm: 96 × 38 mm, M8
• 8 mm: 108 × 44 mm, M10
• 10 mm: 122 × 50 mm, M12

Long-series keys keep the short arm and stretch the long one, to 120 mm on a 5 mm key, and inch keys follow ASME B18.3 from 1/16 to 3/8 inch with the same idea and different numbers. A 3/16 key, for example, is 4.76 mm across the flats with arms of up to 72.2 and 26.2 mm.

Both standards give a range, not a single length. The generator uses the maximum, so a key at the top of its tolerance still fits and a shorter one simply sits a millimetre deeper. Some sets use their own lengths. Measure both arms with a caliper and type them in.

How the key sits in the handle

The long arm runs through a hex channel in the shaft and the short arm lies in a groove along the crossbar. The channel only centres the key. Torque goes through the walls of the groove, so the handle works without a tight fit, and those walls are why the notes ask for four perimeters or more.

The groove is as wide as the key is across its corners, plus the clearance, 0.15 mm per side by default. That way the short arm drops in whichever way the key happens to be rotated. Where the two arms meet, a real key is bent, not cut, and ASME puts the bend radius at about the key size. A sharp inside corner in the handle would hold the bend off its seat, so the generator cuts a 45° relief there, 1.2 times the key size.

Four ways keep the key from falling out. A snug channel, two small lips at the groove mouth that the short arm snaps past, a 6 × 3 mm magnet under the short arm, or an M3 screw driven in from the side with a nut in a slot. The size is engraved on top of the handle or inlaid in a second color.

About 35% of the long arm stays out of the handle by default, 30 mm on a 5 mm key. Enough for a counterbore. Less sticking out turns stiffer, more reaches deeper, and the field next to it says how far you can go before the handle has no body left.

Printing it

The T-handle and the screwdriver grip print standing on the face the key goes into. That puts the hex channel vertical, where a printer draws a hexagon cleanly, and the groove is open to the bed, so nothing hangs. The crossbar has a chamfer of up to 2 mm on that face. On the bed it stops the first layer from flaring out, and in use it is the edge under your palm. The lever sleeve prints lying down with its channel corner up, a hexagon whose roof is two 60° faces, which also needs no supports.

If you are unsure about your printer, print the fit-test ring first. It is 6 mm of the same channel and takes a few minutes. Key will not go in? Add 0.05 mm of clearance.

The extension that could not reach

My first plan had a third tool, an extension: a tube on the short arm to reach screws at the bottom of a deep hole. It could never work. The long arm is the one that goes into a deep screw, and a tube on the short arm only makes the key longer sideways. So the third tool became a lever sleeve. The long arm stays in the screw and the sleeve slides onto the short arm. With a 120 mm sleeve, a 6 mm key goes from a 38 mm lever to 128 mm, which is what a stuck screw needs.

Testing caught two more. Round handles were first built as a cylinder trimmed by a cone, and after an STL reload, the way a slicer loads it, the mesh tore where the two met. Now every round body is one revolved profile, and the fluted grip is that profile pressed in at each flute. Then the browser test of the page found the server accepting a lever sleeve with both parts unticked, a request for nothing. The page refused it. The server did not, until I moved that check to the top.

I have not tested every size against a real key yet. If one fits loose or tight, the report button on the generator page sends me the exact settings you used.