The size and shape of drill holes

I think we're slightly at cross purposes here... yes your D bit from the viewpoint of how far across the rod it's been ground flat would be a tad less than half.. but i was thinking in terms of the tip needing clearance (as opposed to flat across the end) so to flatten the end of a blind hole it has to be able to traverse between center and circumference???

pgk

pgk,

It's the end that cuts. If you leave less than half diameter the side can cut and result in an oversize hole. The reason for leaving it slightly over half diameter is to prevent the side cutting. For a full exposé of how they work and how to make them see G. H. Thomas' book "The Model Engineers Workshop Manual".

Russell

Many years ago there was an article in the ME discussing D bits by Roland Harman, from memory It was a comprehensive account of making and using said tools.

Emgee

If you want a reamer for a through hole machine a slanting flat on the end of a piece of silver steel and harden it then stone the flat. I don't know how it works, but it does

When someone asks about tool makers reamers, I pull this out of my album. If you want to ream a hole in brass, or aluminium these made of steel can do the job unhardened.

Ian S C

That remains a very useful sketch, Ian

Great to see that the digital version hasn't got grubby and faded with use.

MichaelG.

Actually - that sounds you're drilling it perfectly and the dowels are fitting perfectly! Sounds to me like you're getting a vacuum lock. Solution: (i) if the workpiece will stand it, a tiny vent hole all the way through; (ii) a tiny hole through the dowel pin; or (iii) use a removable pin with either a vent flat or a spiral groove (have a look, for example, at the range of removable dowel pins in the MSC/J&L catalogue).

Dave

A lt of info in this thread, thanks

Russel: Gotcha re D-bits.. I admit was still thinking in terms of reaming the hole diameter as well as the base.

Re: Toolmakers reamer. I can see that a ideal for holes in thin sheet or to 'true' up a pilot hole but a single long bevel will tend to push the tool laterally? The principle of a horseshoe nail is a bevel to direct it away from the white line (sensitive hoof layer). Also note that orthopaedic pins have three short bevels to a point and are used to drill the hole. Living bone is quite elastic and that pin will drive straight and the hole be tight enough for the pin to stay held in situ. (note for situations where the bone end it's being buried in isn't firm enough they are also available with a thread behind the point)

pgk

Hi Guys,

At one time I used to use "D" bits to drill 3/32" holes in brass 6" deep. Needs a lot of pecking and blowing out !

I stopped using a "D" bit as a drill, when Dormer made me a very long 5/64" drill specially for this job. I first drilled the hole with the new drill and then followed with the "D" bit.

I'm about to make an 8mm D-bit to finish to size a long hole in a part. I've thus far step drilled in from both ends (having very carefully centred the work each time). It's been a long time since I've done anything like this, as I [now] have a good collection of reamers but not as it happened an 8mm one (but several suitable lengths of 8mm silver steel). Anyway - I had a look earlier this week at what the 'Gurus' advise.

The simplest approach is that from Ian Bradley - who basically advises the 'halving' to be 1/2 Diameter + 5 thou - with the cut-out being 3-4 diameters deep. He advises a 10 degree front and end clearance. His article showed a 'commercial' D-bit that had short 'flats' for side relief, as well as cutting fluid channels provided - but Ian stated that he didn't really see the need for the side relief.

Emgee - I also looked at the Harman article and his approach was quite a bit more complicated - basically stating that side relief was required and suggesting an eccentric machining method to achieve it. The second part of his article then described a D-bit 'milling' tool (used to make tapered holes) and his logic for a) using an eccentric cutter and b) the angle at which the cutting edge should be made relative to the eccentric made a bit more sense to me (than his D-bit did). So whilst I might use Harmans approach to make a custom tapered milling bit if I ever needed to - I'm going to stick to the Bradley approach for my D-'drill' - although I did briefly think about adding a little (flat) side relief, although I know a D-bit cuts on the front edge.

Ian Bradley also advises that the D-bit must be withdrawn frequently (to clear debris) and the body must be smooth and kept well lubricated (to ease rubbing). This 8mm one is a bit longer (and larger) than I've tried before, so it will be an interesting exercise. Fingers crossed...

Regards,

IanT

One old design for a drill for accurate holes in hard materials is to put three flats on a rod to give a triangular shape with three short lands. Put a triangular point on the end as a relatively blunt pyramid, and if large enough notch all three cutting edges.

I haven't tried this.

Neil