Making a tapered D bit

[Brian Rupnow]

This morning I decided to try my hand at making a tapered D bit.--This is a direct follow up to my post about making a tapered reamer.  As in all things, I started out by modeling what I wanted to build, and creating a detail drawing of it.

[Brian Rupnow]

One of the things I have learned.--Every time I cut a piece of drill rod with my band-saw, it costs me $50 for a new band-saw blade. so--Out to my big ugly old reciprocating hacksaw that I built 40 years ago to cut off a 4" length of 1/2" diameter drill rod.

[Brian Rupnow]

Then into the lathe, face both ends, swing the compound rest around to 8 degrees and cut the taper.

[Brian Rupnow]

Now it doesn't show up very good in this picture, but after cutting the taper, I turned a 3/8" length of shaft down to 7/16" diameter right adjacent to the taper. Why did I do that?--Because I don't absolutely trust the vernier on the mill quill downfeed, and I wanted to be able to have a surface I could set the micrometer against to measure how close my cut was getting to the centerline when I got set up in the milling machine. I couldn't think of a good way to do that without having this small turned down area.

[Brian Rupnow]

So, here we are in the milling machine, milled down to .002" shy of the centerline. I used a 7/16" diameter endmill and cut with the end of the mill down to the depth I needed to go. Then I used a 3/8" ball nosed end mill to put the radius in the heel of the cut. I dressed the newly milled flat area with a medium grit flat diamond file. (one of a set of 4 cheapies that I bought from BusyBee Tools .

[Brian Rupnow]

After heating the new tool to an orange/red with my acetylene torch and dunking it into a can of water, I dressed the flat side with a few more stroked of the diamond file (I'm not really sure it was necessary, but I did it anyways). Then I set up a scrap piece of aluminum in the lathe and drilled a 0.195" hole in the end of it. That is the bore through the carburetors I want to build.

[Brian Rupnow]

The new D-bit works very well!!! It cuts smoothly and leaves a nice finish on the tapered hole it cuts in the aluminum. I did not relieve the non cutting side of the new D bit--I left it full size, reasoning that it would prevent the D bit from wanting to be pushed away from the side that was doing the cutting and possibly break the bit.

[Jasonb]

Good to see it worked OK for you Brian, they are handy tools and not too hard to knock up and a lot cheaper than those tapered mills you were looking at.

J

[Brian Rupnow]

So---The making of the D bit was successful, and worked great. I am not certain, but I THINK I have to grind an angle on the very nose of the taper. Now we get to the sad part of the story. Although the operation was a success, the patient died!!! The protractor on my compound rest is one of those "By Guess or by God" kind of things. It is not engraved very well, and difficult to see. I estimated where 8 degrees would be, and used my cad program to determine that if the taper extended back .999" from the end of the material, it would leave me with a .188" diameter at the very tip, which would fit nicely into a pre-bored 0.195" hole. Damn Damn Damn--I didn't check that before I hardened everything. The small end of the taper turned out to be .215" diameter, and it leaves a shoulder at the end of the tapered hole--Exactly what I wanted to avoid!!!  I have one trick left. I have made up an aluminum bracket which clamps in my toolpost to hold my heavy duty pneumatic die grinder. I may put an abrasive cut-off wheel in it and try to turn the taper down a bit more until the small end reaches the diameter I want. Fortunately I haven't changed the angle setting on my compound rest. At any rate, it has been a great learning experience.---Brian

[Coopertje]

Too bad Brian. Chuck a piece of stock and turn it down a little so you are sure it is straight. Put a dial indicator against it and use the tangent function to calculate how much the dial indicator should change when you move the compound slide over a predefined distance. This method is simple and gives you a very accurate angle.

Regards Jeroen

[Brian Rupnow]

WELL!!! That went amazingly well!! I have a whole flock of small, unidentified grinding wheels mounted on 1/4" shanks that I have had, well, forever. I mounted my pneumatic grinder in a home made aluminum clamp and mounted it in my lathe toolpost, and with the lathe turning at 970 RPM I took multiple cuts .001" deep until the small end of the taper measured 0.188" diameter just as I wanted. The angle of the taper is not terribly important. It probably isn't exactly 16 degrees included angle, but it is certainly close enough for a carburetor air horn. Not wanting to start crowing about it too early (in case my "external grinding" had somehow rendered the tool inoperable), I again dressed the flat side a bit with a fine flat diamond file, then flipped my "test piece" with the .195" hole through it over and proceeded to put in a new taper. The nose of the tool fits INTO the hole now, cuts a lovely taper, and doesn't leave any "register" at the bottom of the taper.---Hooray for my side!!!

[bp]

Excellent, good result.
For more confusion I have created tapers for the same item, using basically the same process.  Mine differs only in that the reamer smallest diameter is made significantly smaller than the venturi drill size, then in use the tapered reamer is simply fed in to the appropriate depth, which seems pretty much the same as your second (recovery) method anyway!!
cheers
Bill