An Aside Making Counterbores

TinyMill Part 6

Over the course of designing some more parts for TinyMill, I realized that I needed to counterbore some holes. I could easily fake it with an end mill or drill bit, but one of my design goals was to not have any kludges. I could buy a counterbore, but thats $30 for a set that I might use one of every (not) so often. Or I could buy some drill rod and make my own. Guess which option I chose.

Before I start, I need to give credit where its due. I first learned that I could make counterbores on the HMEM forums, and then learned how to from Deans excellent page. Seriously. Dean did a really good job of walking through how to make a counterbore. So Im not going to make a tutorial. These are just a few selected photos of the process, and some notes. So lets begin.


This is a length of 3/8 O-1 Drill rod. A 36 length cost me about $3 (shipped!) from Enco. Its about 5 long. The counterbore Im making is for a M4 SHCS, so it has to be turned down to 8.25mm.


A sharp carbide cutter works wonders on tool steel. I got to about 0.0005 over my final diameter, then brought it down with some 220 and 600 grit sandpaper (with oil) on a parallel to keep everything nice and flat.


Repeat that for the pilot and you have a nice cutter blank.


Lacking any method of indexing, I had to make a fixture for cutting the flutes. It would have been nice to use a piece of hex stock (to get a 6-flute cutter), but I didnt have any. So a piece of 7/8 square 12L14 was pressed into service. It was drilled for the cutter, and had a set screw added in the top.

I didnt take any pictures of milling the flutes. Thats a lie. The milling fixture was just so in the way of the light that none of them were any good at all.

On Deans page, he mills the reliefs for the ends of the flutes. I had just indicated my vise in square, and didnt really feel like canting it off at an angle, so I did all the reliefs (very, very carefully) with a fine file. It worked well.


A final shot, of the heat treating. It went fairly well, although pure propane lacks a bit in the heat department and my flame was a bit on the small side of what I would have liked. A quench in motor oil and then a quick tempering brought the hardness to somewhere around 50 rockwell. Plenty good.

So now I know how to makeĀ  counterbores. Its pretty good, and I have to make a couple more, but now Im a bit addicted. Uh-oh.

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