D bits.. which part to angle?

[paul]

I'm following instructions in Stan Bray's "Making Simple Model Steam Engines" to make some D bits, but it's not clear which bit needs chamfering. It says "..one end is filed flat to half the diameter of the steel, and the front of that is tapered by about 10 degrees". The question is where is the "front"?.

Here's a very quick 3d model - am I chamfering...

A - the side edge
B - the corners where the flat meets the curve
C - the untouched curved edge at the end
D - the straight egde at the end

and "ten degrees" from where?

There are two photos in the book that "make it clear" but I'm damned if either looks clear to me! Sorry Stan.

Thanks in advance!

[alanstepney]

The front is the flat end that meets the metal first.
I reckon 10 degrees is a bit much, but it will work OK.

Another way to visualise it, is to think of it as a cutting edge, a chisel, for example, and imagine how it is going to cut the metal.

[paul]

Thanks Alan, so it's edges C and D. But is it 10 degrees from the horizontal in the above image or 10 degrees from vertical?

[chris vine]

Hi Paul,

It is 5 or 10 degrees back from straight across!!!

By the way, I find D bits are extremely difficult to get to work really well, I find that they bind when I get to the the full diameter part in the work, they seem to bind.....

Chris.

[paul]

I must be on a dense day... is that A or B in the image below?

[lancelot]

Hello Paul,
Look at the last image that you have just drawn and forget about ''A'' or ''B'' lines, where the three red lines intersect on your drawing take as the datum point to drop a line ''DOWN'' and to the rear of the tool at approx 5 degrees, grind to that line. so you are really giving clearance from cutting edge ''D'' back to ''C'' in your 3 D drawing. I hope that you can follow this.
All the best for now,
John.

[paul]

Thanks lancelot that's clear - I see what Alan was saying now about a chisel!

[alanstepney]

I LOVE D bits!

I back off the front edge by about 1-3 thou (ignoring the angle), make the actual horizontal flat with two angles, the front (cutting) one being at 90 degrees, to the midpoint, and beyond that, slight clearance, and also reduce the diameter by a thou or so, from just behind the actual cutting flat, to reduce friction on the shaft in the 'ole.

(Not sure if that makes sense, but I know what I mean. lol)

I find that provided they are kept sharp, they work well, cut cleanly and last for years so one eventually builds up sets of all sizes.

[havoc]

I'm not very knowledgable about tools, so forgive this probably stupid question.

I understand the way a D bit looks and works. But is this better than a drill? I mean, will this allow me to make better/rounder/straighter holes on the lathe?

Right now I drill holes, unless they are so large I can put an bit through it. This starts at about 8-10mm. So, if I would make such a D bit of 4mm, would it enable me to drill nice straight holes of that diameter?

[paul]

I've just made 2! now to try 'em out :-)

[lancelot]

Hello Paul,
I hope they work for you,
Alan gave a good description of the way that one is made, the ''Binding'' that Chris drew attention to, can be over come by liberal use of ''Candle ''grease lubricant.
Havoc asks ''how it works'', I would say that for the ''metal worker'' it is just a finishing ''drill reamer'' i/e in that it gives a flat bottomed finished to size hole, that in most cases is dead accurate. Some one will posssibly correct me in this, but I believe from some research I did that the original use for ''D'' bits, reamers, was to drill the very long holes in ''Bag pipe'' wind pipes, I note in ''Engineers Workshop '' mag. , a letter relating to the ''boring '' of gun barrels using a ''D'' bit reamer, it must have been quite long compared to the few inches or so that we as modellers use, how did he control straightness? let alone lubrication.
All the best for now,
John.

[steammadman]

Don't forget to feed the "D" bit into a pre-drilled hole up-side down,or
with the flat to the bottom

[steammadman]

Don't forget to feed the "D" bit into a pre-drilled hole up-side down,or
with the flat to the bottom

[lancelot]

Hello Steammadman,
Bearing in mind that I am just after a bottle of vintage ''vino'' and a large plate of ''ribs and onions''....WHY?
John.

[paul]

The good news is that the 5mm D-bit I made seems to work fine - with a little pressure and some 3-in-1 it cuts nicely (thats the only lubricant I have apart from WD40!). The downside is that the safety valve I'm using it on is not working properly, it seems to let air through regardless of how it is adjusted while testing... I think my ball-bearing isn't seating properly...bum!

[SteveW]

Guys,

As I understand it 'D' bit reamer fits in the gap left by not having a conventional reamer to do the job. However, their real benefit was to produce a square bottomed accurate blind hole. Ordinary hand reamers have a starting taper at the 'sharp end' and only really suited to through holes.

The hard bit is getting the 'exactly' half 'D' section. A long while back I saw a simple solution to this one: Simply grind the end of the embryo reamer to a 30 degree wedge (or chisel) shape and hone it (think of a chisel made of round stock). This way there will always be a bit of the 'D' edge at the half thickness point.

I recall the other point with 'D' bits is not to expect to take more than a few thous off. Quite how many thous equates to 'only a few' I never found out.

And not forgetting the hardening and tempering bit else the edge won't last long. The real trick is not getting a bend on the quench cycle. A trick here (I recall) is to use the drill press (maybe running) to drop the thing vertically into the quench liquid.

[alanstepney]

Paul, glad your D bit worked. It is always nice when things go as planned (hoped).

As for safety valves, the edge of the seat should be sharp, then get a ball, same size as the one you plan to use, place it in hole, put piece of brass rod on top, and gently "biff" with hammer.
That should take away any burrs, and give you an edge that seals.

Then fit the correct ball, and, bingo.

One thing I do suggest, is try to get hold of a load of Model Engineer's from the 1940's and 50's.
Read the articles of that era, particularly those by LBSC.
There is a wealth of information there, particularly for the novice, and much / most still applies today.

[paul]

Alan, I think the problem is that the hole that the ball covers is slightly off centre. Rather than using the D bit to make the end completely square I left a bit of a 'countersink' where the original bit had drilled so the ball would drop into that - it seems to do that, but I think the hole below is not central in this countersunk part.

[chameleonrob]

according to K. N. Harris in his boiler book a counter sunk hole is the worst way of getting a good seal, a flat face is ok and a reverse counter sunk hole is best. as far as I'm aware the smaller the contact area between the seat and the ball the better, and less chance of getting a bit of grub preventing the ball from seating properly.

[paul]

Hmmm, maybe i'll remake the body of the valve - I can hardly bear to look at that off-centre hole anyway!