Boring/drilling a conical hole

Thanks John, Yes I had step drilled it. The Tufnol jig just wasnt up to it tho. I will get the grinder onto the D bit. I had made the thickness a few thou less than half. I was just a bit worried that the overall thickness at the tip was a bit lacking to stand up to cutting. Putting it in the tailstock sounds a good idea, - escaped me entirely. Cheers Nick

It's got to be thicker. Sorry if I told you something wrong.

No problem. It also has less radius so less torsional force (yea, not linear).

Nick

Well done, Nick. Brilliant effort! We want to know your marks for CDT in due course! --

Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"

The "thickness" of a D bit tends to be regarded as magic number to be carefully adhered to without paying much attention to the reason for " a few thou (or 1/40ths of a mm!) over half diameter"

The "thickness" is only critical if drilling a flat bottomed blind hole. In this case, if the "thickness" is less than half dia, a pip of dia twice the undersize will be left at the centre.

A suitable over half diameter "thickness" can produce a true flat bottomed hole but at the expense of degraded cutting action.The main part of the cutting face is OK but there is NO cutting action at the dead centre of the hole end face and also up to a dia of twice the "thickness" oversize.

This small central area is removed very inefficiently by a mixture of crushing and abrasion.

This problem is eliminated if the D bit is used to clean up a pilot hole. In this case the precise "thickness" is no longer important and the cutting action is excellent.

An over or under "thickness" D bit can still be persuaded to produce a flat bottomed hole. In this case, first drill the pilot hole to the full depth with a standard twist drill. The chisel edge of the twist drill must be wider than the D bit "thickness" under or oversize.

Jim

I may have missed something here but isn't what is required here is a conical hole. So the cutter needs to cut on the side of the tool as well as the end. I thought that making the tool just over 50% thick prevented it from cutting on the side. So in order for it to cut it should be slightly less than 50%.

My comments concerned the basic use of a D bit for drilling blind or cored holes. In this case the D bit is cutting quite efficiently acting as a single flute end mill ground with close to zero top rake and a front clearance angle determined by the angle ground on the end face.

A taper ground D bit cutting the walls of a conical hole is a different case. It is cutting pretty inefficiently because, whatever "thickness" is chosen, as far as the side walls are concerned, it is trying to cut with ZERO front clearance.

Since it's not a blindhole there's no problem with a central pip and side cutting is essential so there is everything to be gained by your choice of below 50% thickness.

Jim