How to stop Robertson screw bits from slipping

Robertson square-drive screws are one of those wonderful little inventions (Canadian, of course) that just work perfectly, every time. They self-centre, they don't strip their heads, they grip the screw at any angle, and they don't cam-out.

If your first thought, upon reading that, is "No they don't, the little @#$%ers always spin and slip and strip the heads", your problem is not with the Robertson design- it's with incorrectly made driver bits.

The square Robertson bit is supposed to have a very slight taper, which matches the slightly tapered socket in the head of the screw. But some driver bits (mostly imported from a certain large country around 120 degrees east longitude) are inexplicably made with too large a taper, so they can only penetrate a millimetre or so into the socket. Unless you put half your body weight on the drill, they slip out and spin, shredding both bit and socket.

If your bits do that, a few minutes with a file or grinder should put them right. Just file down the flats of the square bit so that its taper matches that of the screw heads and can penetrate the full 4-5 mm into the socket....

And the modified bit will grip the screw, won't slip, will last much longer, and can be power driven with one hand and minimal pressure.





I was wondering what the hell was wrong. I just got a set of square drive screws and the bit was jumping all around in there. One thing that I thought was extremely funny was that the box of screws I bought came with a "free bit." Well that free bit was an S01 and the screws are clearly S02 in size. Fortunately I had some other bits hanging around but I'm on my way to file the taper right now since they aren't right anyway.


Glad I could be of some help

Matthew's picture

I wasted quite a few hours over the years before figuring this out. Never assume that the supplier did it right!

Add new comment