G'day Chris here and welcome back to Clickspring.
As essential as these commercially produced cutters are, there is another type of cutter
that can often prove to be equally indispensable.
The shop made D Bit.
Now the term "D Bit" has a fairly broad meaning, and can be used to describe a wide range of
cutters with a D shaped cross section,
but in this video I'm going to make a milling cutter to form a V shaped groove in brass,
so the geometry will need to be something like this.
Starting with this length of drill rod, the first step was to face it to length.
The cutter I'm making has an included angle of 92 degrees, and I do need it to be dead on,
so rather than rely on the lathe protractor, I'm using a dial indicator to get it exactly right.
The better the surface finish on the cutting edge, the better the finish the cutter will leave on the part,
so I'm using a shop made emery stick to give it a mirror finish before proceeding to the next step.
Click on the link if you'd like to see a video about making this sort of re-usable emery stick.
To get the D shaped cross section the cutter needs to be milled down to just short of half way.
I remove the bulk of the stock using an end mill like this, and then rotate the work through 90 degrees
to finish the rest of the stock removal, and create a recess behind the flat.
This recess becomes quite helpful when sharpening the cutter, which I'll show you later.
With the flat just short of the half way mark for sharpening, the bulk of the metal removal is complete.
A little bit of time on the coarse stone brings it even closer to the centerline, and also
removes the burrs and tools marks in preparation for hardening.
A coating of boric acid and denatured alcohol, along with soft iron wire can help to reduce
the oxidation scale that would normally build up during the heating process.
I used some emery paper to clean up the surfaces, and a quick test with an old file confirms
the part is glass hard.
At this point it's much too brittle to be of any use, and needs to be tempered.
Ideally I'd use a heat treating oven for this, but I don't have one, so instead I use the
oxide colors as a guide.
I heat it gently from the shank end, and let the heat slowly walk up the length of the cutter.
The oxide colors do roughly indicate the surface temperature,
but I give it plenty of time for the heat to pass into the interior
and transform the structure of the metal.
Emery sticks can again be used to brighten up the surface,
and the cutter is ready to be sharpened, using the coarse medium and fine oilstones.
And you can see the recess ensures that the cutter naturally sits flat on the stones,
so that the entire surface is being sharpened parallel to the axis of the cutter.
After sharpening, the flat ends up just a few hundredths of a mm short of the centerline,
at which point the cutter is complete and ready for use.
Of course, there are some limitations when using this type of cutter.
For one thing, quench hardened carbon steel can't compete with carbide or HSS,
so I find its best to not be too ambitious with the depth of cut and feed.
And to be technically correct, a side cutting D Bit really should have clearance angles
behind the cutting lip, much like an engraving cutter.
But it's difficult to form the relief angles for this sort of profile without a tool and cutter grinder,
and in practice I've found that if I keep the feed rate modest, I can get away with
leaving them out, without too much penalty.
For other shapes, putting in the relief angles can be quite straight forward;
it really depends on how the cutting edge will be made to move through the metal.
D Bits are an incredibly versatile and effective type of cutter,
and there are many ways to use them beyond what I've shown in this video.
They're particularly well suited to drilling very deep, close tolerance holes, and can
be tailored to suit a specific requirement,
for example in this case a square bottomed to the hole.
They're a great way to solve those unusual cutting problems that crop up from time to time,
that might otherwise require spending money, on an expensive cutter.
Thanks for watching, I'll see you later.
If you enjoyed this video, here's another one I think you'll like.
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Thanks again for watching, I'll catch you on the next video.