Making A D Bit Single Flute Milling Cutter, by Clickspring
The 'D' Bit is a piece of old school shop tech that I absolutely love to use. They're perfect for profile milling cutters, countersinks etc. as demonstrated in this video. But they are particularly well suited to drilling very deep, straight holes. They are incredibly accurate, and can leave a surface finish that approaches what you expect from a HSS reamer - And that's from a shop made drill!
This video is related to: "Spare Parts #9 - Making A Rectangular Bluing Tray": https://youtu.be/uST7iJgC_gs
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"Solidworks 2013 Bible": http://amzn.to/2FObS1D
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Generic Dial Indicator 0.001": http://amzn.to/2FOFTyF
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Norton 1-by-2-by-8-Inch Fine/Coarse India Combination Oilstone, Red: http://amzn.to/2tTEPb0
Saint Gobain (Norton) - 4 Arkansas Stones + case: http://amzn.to/2HCOAMX
Kaowool 24" X 12" X 1" 2400 F Ceramic Fiber Insulation: http://amzn.to/2pfsM3d
00:17 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.
00:32 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, using a dial indicator to get it exactly right.
00:50 The better the surface finish on the cutting edge, the better the finish the cutter will leave on the part.
01:27 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.
01:40 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.
02:01 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.
02:23 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.
02:36 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.
02:47 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.
03:01 Emery sticks can again be used to brighte up the surface, and the cutter is ready to be sharpened, using the coarse medium and fine oilstones.
03:12 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.
03:33 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.
03:42 And to be technically correct, a side cutting D Bit really should have clearance angles behind the cutting lip, much like an engraving cutter.
03:49 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.
04:17 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.
04:25 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.
"The Model Engineers Workshop Manual" pg 94
George H Thomas
The many forum posts on the subject of D Bits
by John "Bogstandard" Moore
Making A D Bit Single Flute Milling Cutter, by Clickspring.