Today was a good day. It was warm enough to work in my own shop using my own machines and it felt good. I wanted to start making the front bar micrometer as it is one of the more difficult items. Everything has to be perfect to be sure to achieve the accuracy of a good micrometer. I decided to use the Quorn micrometer setup as it is simple and much easier. The Bonelle micrometer is way over engineered. There are three main parts to the micrometer. The barrel , the shaft , and the thimble. I am adding a ball bearing to the setup on each end to reduce friction and make the micrometer smoother.
These are the three main parts of the micrometer. I didn't take a lot of machining photos because most of it was simple turning and facing. All the dimensions are held to .0005" for the part surfaces that "fit" to one another. I want to have an accurate and smooth micrometer, so careful attention here will get me there. The first part machined was the barrel. It was simple turn and face but the size is held to 1" =/- .005". I tapped it with a good sharp tap. That way I can make the threaded shaft to fit and have a class 2 or preferably class 3 fit on the threads. I also turned a .040" boss on the end of the barrel to register in the base . This was held to a really close fit also.
The second part to make is the threaded shaft for the micrometer. The cross drill jig I built from plans (I got from Jim B.) came in real handy here. I turned the shaft, the step for the thimble, and threaded the shaft in one setup to keep everything exactly concentric. This is important to the smooth operation of the micrometer. Next using the drill jig, I made a cross hole .189" diameter in the step. The diameter of this step is within a couple thousandths of the diameter of the barrel. This is because they both have to fit smoothly into the thimble and still turn freely. The cross hole will contain two 3/16" ball bearings separated by a spring. They will ride in a "detent" groove machined in the thimble to keep it in place and provide tight friction so the thimble can be zeroed but still rotates when the knob is turned. At final assembly I will assess whether or not a lock screw is needed on the thimble. If the design works right , it won't be needed.
I used and adjustable die to thread the shaft instead of single pointing the threads. I did this because it was easier and I can still get a good fit on the threads by making small adjustments to the die until it is a perfect fit. It worked well. I would say I got between a class two and class three fit. That means the micrometer will be very accurate and smooth turning. I chose 1/2-20 threads. This will give me .050" per turn of the micrometer. There will be about .600" travel. A lot of travel is not needed as it is for minor adjustments to grind the tool.
This is the threaded shaft completed. after the threading it was turned around in the chuck and the knob was knurled and faced to .375" width. Here you can see the .189" cross hole in the .250" step. the knob has been polished to a nice shine to look "pretty".
This photo is the micrometer parts assembled. You can see the register boss here to keep it aligned in the base. There are no markings on it yet. This will be done later in the build. Hopefully, I remember to document it.
This is the complete micrometer. I still have to drill the mounting holes in the barrel. This is next on my list.
I was asked about my choice of paint for finish. Many years ago ( as a teenager) I built a lot of electronic devices and ham radio equipment. I used a gray wrinkle finish on my things. It is a really cool looking finish and very durable. It is the same finish Kennedy tool boxes had on them. I always baked the part or used a heat lamp to "bake" the finish as it gives a more pronounced wrinkle and looks better. The paint came in brown, black, red, and gray. I have not seen it in years but Lowe's used to carry it. I went shopping last night and could not find it anywhere
anic:. Everyone knew what I was talking about ,but did not know if it was still available or where to get it. After a search on the web, I found it at the most unlikely place .... AUTOZONE! I'm glad I found it as I had my heart set on a black wrinkle finish. Yea , I may be a little :nuts:
For all those following this little adventure, I usually come in the house and set down to the computer about 8:00 or 9:00 pm EST
