Thread: Countersink screws

Originally Posted by vGolfer

For a bit of a novice, could someone explain the theory of countersinking screws. I understand it is to make the screw flush with the material the screw is going into but not sure how the drilling etc works.

I have a thin piece of metal that is probably 5mm thick - is it possible to countersink a screw into a piece of metal this thin?

Shouldn't have any problems with a good coutersinking bit. There are washers around that allow the use of countersinking screws but they sit high and defeat the flush finish.

You can get combination bits with the countersink included ie.......

http://www.timbecon.com.au/products/...its-362_0.aspx

depends on the size of the screw head.

There are also handheld countersinking tools, but they generally don't produce as good a finish as the one Benny has guided you to. On the other hand, they are cheaper

When people make their own handtools, they often use screws in that, countersunk, then file the part that is still sticking up to produce both a flat finish, and one that doesn't still show the phillips or flathead cuts in the top of the screw - basically just leaving a round circle of brass that when sanded almost disappears into the parent metal.

Thanks for the info.

What would be the minimum thickness of a piece of steel to countersink a screw in?

Depends how much force the remaining head of the screw has to endure.

And how accurately you can machine a hole to match the core diameter of the screw.

Sorry, but it is the same question as "how long is a piece of string"

You can, sort of, do the countersink with a big enough HSS drill. It isn't quite the right shape for the job and it won't be a good idea in thin plate.... but it can work sometimes. Careful you don't go to deep now!

Won't be pretty, though because, if memory serves, the screws have an 82deg angle whereas the drill bit (and quite a few co-called countersinks havs a 90deg cutting angle)

How thick a piece of metal? It varies - if you get a good metal countersink, as opposed to a wood countersink (I think you want to countersink a sheet of metal) you should measure how deep the head of the screw is - that is the minimum thickness that you can countersink & have the sheet held rigidly in place by the screw shank.

If the metal is thicker than the head depth, then you have an even stronger join. If the sheet is thinner, then you have a couple of options - the simplest is to drill the countersink after the pilot hole has been made and just continue it into the underlying layer of metal - if the joint is placed under a big load the hole may rip out.

Another one that I discovered, more or less by accident is to countersink the piece underneath, then punch the pre-drilled sheet on top down into the divot & then put the fastener in. This can be stronger than drilling a big divot out of the outer sheet - I have used this and a similar method for creating flush rivetted joins in sheet metal that needed to be quite resistant to tearing.