Chris has a website as well (www.clickspringprojects.com) and a Patreon channel where I help support Chris' work and as a result gain access to videos and tips not available to the general public. You can do it for as little as a dollar a month - quite a deal.
I decided that a project on Chris' website called a "Screwhead Holding Tool" would be a nice one to have, and push my skill level that tiny notch forward. Chris didn't post a detailed plan, just photos, so part of the exercise is figuring out how to make it. Since mine is still in progress, I'll borrow a finished picture from Chris' site to show what it should look like:
The idea is that you have multiple bushings (the brass bit around the screw) sized for various screw types. You insert the screw into the appropriate bushing, the bushing into the hex holder, tighten the holder onto the shaft, and then put the shaft in the lathe to allow precise shortening and/or modification of the screw. The hex is a standard 3/4" nut, and the shaft is made from 3/4" threaded rod.
I gathered up the nut and rod from my local Lowes store, and started with careful measurements to try to decide where to put the holes. Here's my usual crude drawing with numbers from those measurements:
I then decided on a setup in my mill vise that would allow me to drill one side, and then reliably rotate the nut to the next hole location. Real machinists have fancy "stops" for their mill vises to reset a workpiece to the same position, but mine (made from a bent bolt and an angle plate as a holder) works pretty well too. I have the nut on a stack of precision parallels, so I can reset it at the same height every time at a right angle to the drill bit.
My recently installed DRO (blogged as Part 1, Part 2) made it easy to align the drill bit to the spot I had chosen in my drawing:
Then, I used a technique I've seen Chris use - I temporarily bonded the top plate to the nut using super glue (cyanoacrylate). That would ensure that the holes matched precisely between top and nut. I made the top from 3/16" thick brass, just because that's what I had around. I made it oversized so I could precisely trim it later. Because of my careful setup, I could then return the workpiece to the mill and trust that the drill bit was still aligned properly.
I carefully drilled the three holes using a #43 bit, which is the "tap size" bit for the #4-40 screws I was using. Then, I used a larger bit to drill clearance holes just through the brass. Now for the scary part - tapping those tiny holes. I used what is (actually, was... keep reading!) my usual procedure, which is to chuck the tap in the mill's drill chuck, and turn it by hand for a couple of full turns to start the thread, and then use a tap handle to complete the threads:
The first one worked great, and then... snap! I broke the tap. It happened while I had the tap in the drill chuck. Turning by hand against the friction of the mill's motor, I couldn't feel that the tap was in trouble. Using my brain for once, I still rotated the piece and drilled the third clearance hole. I then could remove and reuse the brass, knowing the holes would align with another nut drilled with the same setup. I used a torch to heat the brass until the glue joint failed, and then cleaned it up. I didn't have enough hands for a picture of that!
That tap breakage started me down a path of research and experimentation that really changed my way of tapping. In the first place there is a quantity called "thread depth" in tapped holes that basically determines how tight the screw or bolt will fit in the hole. "100% thread" means that the thread on the bolt will go all the way to the bottom of the thread in the hole. That's not commonly used because the tiniest error in making the bolt or threaded hole will cause it to bind. The standard tap drill size (that #43 drill for my 4-40 screws) is for a 70% thread.
Turns out that 70% is the right value for soft materials like aluminum or brass, but for steels like that hard nut I was drilling, I should use a 55% thread instead. That's a #41 bit, a difference of 0.070". In machinist land, 70 thousandths is a lot! It also turns out that the tap drill chart in the Android phone app I use, Drill Bit Chart Pro, even had that information if I had only known to look for it.
The other issue was how to hold the tap straight while starting the hole, while still being able to feel what's going on with the tap. I came up with a solution that I think is pretty good! I already had a set of Hanson tap sockets designed to be used with a quarter-inch drive ratchet. I reasoned that I could use a hex adapter in the socket and hold it in the drill chuck with a sliding fit, like this:
It worked great! With that small tap, I could turn the socket with fingertip pressure, and easily tell when the tap felt like it was binding. Being hyper-cautious (after all, I had just replaced a $6 tap), I would only turn a half-turn before backing up to break the chip, and every second full turn I fully extracted the tap and cleaned both the tap and hole, and re-lubricated the tap with Tap Magic. Three tapped holes, no drama! And I tapped them pretty deep, 1/2", because all I had was 1/2" long screws and I didn't yet have a working Screwhead Holding Tool to shorten them. See why I wanted to make one?
There was only one thing I had to do to make this possible on my small mill. A hex adapter is too tall to fit easily in the clearance available. Fortunately, I had two hex adapters, so I modified the lower one in the picture for dedicated tapping use:
Of course, since I'm a beginner, it never occurred to me that those hex adapters are hardened until I dulled a very nice bandsaw blade. Rats. I had another blade in stock - many thanks to Phil, who had given it to me. I was able to grind the adapter in two and then dress the ground end by spinning it in a hand drill against a running belt sander.
I assembled my previously-drilled brass top to the nut, trimmed it close on the bandsaw, and then carefully milled it flush. A quick trip to the belt sander to break the sharp edges, and here's where we are for now:
Three things are immediately obvious:
1. Huh. Cleaning up the edges milled into my screws. Not a huge problem, but Chris' isn't like that. Look back at the first picture - I missed that he drilled at the points, not on the center of the flats. More meat there. I'll re-measure, re-draw and make a new setup on the mill to match that on the next one.
2. 3/16" (0.1875") is way too thick for the top, because it will unnecessarily limit the length I can shorten a screw. 1/16" (0.0625") would probably be fine, although I'm considering milling my stock to 0.100" just to make the math easy when I'm deciding how much to take off. If I carefully make the bushings so that the shoulder is always 0.100" also, I would know that 0.200" of the screw is behind the face of the fixture.
3. Notice how the corners of the hex in Chris' example have a chamfer that matches the original chamfer on the nut. That's cool. I have a vague mental image of how I might set up the lathe to accomplish that, and I'll experiment on this initial piece to see if I can get it right.
More to come!
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