D-bits

I seem to have varying success with d-bits (all sub 6mm) especially on mild steel. I recently made a 3mm one as per this pattern http://www.machineconcepts.co.uk/smallpipes/d_drill_drawing.jpg (but without the scalloped out bit) rather than the usual 'flat' ended ones. It seemed to cut ok but the hole was a tad undersize - I think the relieved edge may have strayed over the centre line (when viewing the flat surface from above). The thickness to the filed flat was within 1 to 2 thou across its width/length so I don't think that was to blame.

I used the lowest speed on my lathe but when I tried a fraction higher speed in the drill press later as an experiment the tool jammed in the work.I haven't tried using a d-bit by hand but I can't imagine it would be very easy to keep it straight... maybe I should put it in the tail-stock chuck and rotate the work by hand while pushing the tailstock bodily along?

Also, I've seen some home-made d-bits that have almost no relief and others that have 45 deg or more. Does anyone have any advice on this - I need small, sometimes blind/flat-bottomed, holes in the 3-6mm range.

Cheers

Paul

The pattern linked to doesn't look like any D-bit I've ever known? In this picture, far right, is a home made D-bit:

The flat section is just that, flat, and a thou or two over half diameter. The end is left at right angles to the shaft. There is a secondary relief on the end to prevent rubbing, and there was a smaller thru hole at the bottom so I didn't need the complete area to be machined anyway. The relief was filed by hand to no particular angle. Granted this D-bit (7/16" diameter) was used on gunmetal, not steel.

If I'm not too fussed about the bottom being exactly flat I use a slotdrill in the chuck, rather than muck about with D-bits. I made the one above because it wasn't worth buying a new 7/16" slotdrill for a one off, and the bargain Dormer brand one I 'bought' on Ebay never turned up.

Andrew

For a D-bit to cut exactly to size the cutting end must be left slightly over half diameter. I usually leave it about 2 to 4 thou over half dia. for small diameters up to about 10 thou over for half inch and over.

Andrew's photo shows the correct shape.

Russell.

Look up the youtube video by Clickspring in his 'spare parts' section.

The d bit style in the first post are great for hard woods , i use them in my work, but to cut metal you need different ones.

Thanks for the input - the type Andrew shows is nearest to what I've used in the past although I had the relief going almost all the way across the front edge rather than half way. I'll give it a try using this pattern and seek out the Youtube clip.

I think the tool briefly shown at 4:20 in this video is what I need: https://www.youtube.com/watch?v=jlNxvnOxMCE

The same as Andrew's but with an added scooped out part... although how it would work with steel I shall find out!

I use Andrew's style and they always seem to work.

Important that the cutting edge (which is the END not the sides!) reaches past the middle if you want it to cut flat bottomed holes.

Neil

Well that worked great . I made a 3mm version, the smallest I'm likely to need - I had to use a magnifier to see what I was doing on the business end though! Using 48 rpm in the lathe and a #32 drill for the initial holes, I made a through hole and a blind hole in two pieces of steel and the fit of a separate piece of 3mm tool steel is excellent; one piece will sit on the rod while it's being held vertically without sliding down, and pulling the rod out of the blind hole gives a satisfying 'pop'. Whether or not the hole is flat bottomed it's hard to say - the bottom of a 3mm hole half inch deep is difficult to see!

The video suggests what I did in training. Very slightly below centre for the flat but that was for a flat bottom for cap head screws. No need for relief when the end is being ground just do it very very slightly out of square. The other aspect would be more tricky now. Add a bit of rock when the the flat on the d is filed. There is no way I can file that flat now so I just mill them flat. I'd add too much rock naturally.

One I remember was to provide 1/64 clearance pocket for something like 3/4 dia headed cap screws. The flat was about 0.002" under centre. Once it was shown to work on mild steel it went straight into the instructors pocket. I'm sure he wanted it for his own use. One day he said we had better do D bits and picked me. Probably because I am 6ft plus and the vices were too low for easy flat filing. I can't remove metal that quickly with a file now either. It only took a couple of mins. Takes longer to mill one.

I've made a couple at home and they have usually worked ok. Only problem has been not letting red heat work it's way right through before quenching or too high a cutting speed or feed. They need to be fed slowly and also be kept cutting for things like cap head screw pockets. Hole size wise they aren't that much worse than a reamer if everything is square on etc. Not used them for that. My recollection is reamer + 5/10ths max D bit plus 1 1/2 thou max.

John

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All my D bits are cut to centre line. The outside is clearance to within about 0.05mm of cleaning up the cutting edge and backed off. Depending on what I am doing, the back off is anywhere from 10 deg to 25 deg. When used in a mill, hold the tool as close as possible to the shortest amount to do the job. I have used D bits for a variety of work. From a precision tapered reamer effectively, to a combination drill and boring tool. Or as the new cutter companies sell a MDT, multi direction tool. I do use a D bit grinder for making them and take from 10 mins to 3 hrs depending on accuracy and geometry etc. I make most from micro grain carbide.

Neil

This advice was really helpful thanks. I recently bought a D bit from a well known supplier but it had a flat end so just rubbed on the work piece. I've ground an angle as described here and it worked well. Before and after photos here. The part is the regulator valve for my Box Hill loco.