I want to drill holes in MDF of varying thicknesses, laid out to sub millimetre precision. In other words, I need the positioning of the center of the holes to be accurate to within a half millimetre (0.5mm or 0.02 inch) or better.
My problem is that I can't decide whether MDF is considered a wood or a grain free composite material. So would it be best to use brad point wood bits with centre punched holes as in woodwork, or HSS bits with pilot holes as in metalwork? Or anything else?
The brad point is the way to go.
Whenever you are making precisely located holes you should always use a bushing or jig of some type if you can.
Of course, making jigs is time consuming. One quick and dirty alternative is to use what is called a "tap guide" which is a block of steel with standard sized holes in it. Get a set of transfer punches and put the fitting punch in the hole you want in the guide and move it around until the punch settles into the center punch divot. Clamp the tap guide to the work and drill.
I want to drill holes in MDF of varying thicknesses, laid out to sub millimetre precision. In other words, I need the positioning of the center of the holes to be accurate to within a half millimetre (0.5mm) or better.
That's mostly a matter of accurate layout and a starter mark/hole (a single push from an awl can be sufficient) than a call for one particular style of bit.
So with regard to your question:
So would it be best to use brad point wood bits with centre punched holes as in woodwork, or HSS bits with pilot holes as in metalwork?
Yes :-)
All bits that you could use, from basic twist bits through brad-point to auger can achieve the required accuracy.
I'd add both flat/spade bits and Forstner bits to that list as well, but only if you can use a drill stand or drill press. It's possible to control both with a power drill used freehand, but it requires experience and maybe a bit of luck and as such I would not recommend it where accuracy is paramount.
My problem is that I can't decide whether MDF is considered a wood or a grain free composite material.
It's closer to the second option here. It's not quite grain free, see previous Question Does MDF have a grain direction?, however it's close enough that it doesn't matter much in practice.
I think you would be safe with forsner bits or HSS brad points.
Both have the advantage to cut from the outside so as long as you start slow you can reduce tear-out to near zero. Some people use spade bits but they would not be my first choice.
If accuracy is important make sure that you clamp down your work and use a drill press. If you only have a hand held drill then follow some of the tips you see in this question to increase your accuracy:
How do I ensure my drill is perfectly vertical before cutting a hole in my desk?
None of the answer provide a satisfactory answer as far as my amateur experience is concerned. I am fitting handles to new kithen cupboards and drawers, where there was no predrilling or marking by the manufacturer. I started off with Irwin jetpoint bits. I think you wold call them bradpoints as they have a spike in the centre of the bit. It went well but after only 6 holes the bit became blunt and burned. The most successful was my HSS bits which I bought to use for drilling through metal. These I would use with a narrower pilot hole as without the spike the bit could wander even with a bradawl induced starter point.
