[Part 6] High Precision Fabrication

Now is the time to do some real machining. Some of the components I designed require very high precision. The shear pin design for the navigation pole, tolerance larger than 0.01'' will surely result in a wobbly pole. Anyhow, but first I'm going to do a quick walkthrough of how I made the control hub.

This is a housing for the display module and control switches. I made a linear slot in the middle of it. A 4-40 1-1/2 brass flat-head screw will go through this slot and get screwed into the navigation pole (through the handle pipe). It is hard to explain this assembly scheme, so just bear with me until I show you a picture. 

I made a slot on one side to house a linear potentiometer. This potentiometer will be used to control feedback gain. I also made a hole for a potentiometer pot.

The diameter of this potentiometer hole is slightly smaller than the diameter of the pot (by 0.002'' to be exact). The purpose is to pressure fit the pot so that no screws are necessary to mount it. The smaller hole size makes it impossible to put the pot in with bare hands due to the tolerance. So this is done by two small clamps.

I'm using a piece of paper and duct tape to protect the surface finish of the case. This is a pressure fit so once the pot is in, it will be permanent. 

This is just another picture showing how I'm doing this.

Aside from the potentiometer hole, I made another hole on the other side of the control hub box for a 4-disc tumbler. The tumbler has a pre-made thread going around it. The only problem was the thread was in SI unit system (sigh). It took me a long time to figure out the closest pitch in royal units. I'm using 1/2'' 20 thread on the hole.

This is my tumbler switch for ignition purpose. The optional RFID ignition module board will be embedded inside the control hub.

This is my handle bar pipe. I cut it into 15'' long and made a tap hole and a square groove exactly in the middle. The tap hole is for the 4-40 1-1/2'' brass screw and the slot is to make a passage for two 10-pin ribbon cables.

This is one end of the pipe. Always deburr when cutting a pipe. In this case, this is absolutely mandatory because it becomes impossible to slide rubber grips in due to friction from jagged edge.

When the control hub is mounted on top of the pole, it looks like this. You can see the brass screw going through the slot I made. The threads of the brass screw engages a tap hole made on the handle bar. So as I screw it in, a compressive force developed between the hub and handle holds the whole geometry to the pole pipe.

There is another size 10 stainless screw on the navigation pole. This additional screw provides a good fixation to the handle bar along with the brass screw. 

This is the screw I'm talking about. Holes for the handle bar is 0.002'' larger in diameter than the bar itself. So it is almost a perfect fit as can be seen in the picture. The whole construction is very sturdy and reliable. The control hub is placed at an angle to give out a correct viewing angle. One thing I have to say is, the last three pictures so far were taken before I made linear slots and two holes for the potentiometers and the tumbler. So  they don't show up there (the pictures are not in chronological order!).

Now moving on to the shear pins. As I said before, the position of shear pins has to be dead accurate to prevent wobbly pole. I used a digital ruler on my lathe which can measure things upto 0.0005'' precision. So in principle I should be able to get a pretty decent tolerance on this. The plan here is simple. Make four holes for the pins so that when they are engaged, the surface of shear pins contact the inner surface of the triangles perfectly, therefore fixing the pole to the base without a single screw.

One advantage of this design is that the length of the pole can be adjusted easily. I just have to remove the pins and adjust the length and put shear pins back into appropriate holes. For this purpose, I made another set of four extra holes 0.50'' offset from the other holes. The diameter of holes is 0.125'' identical to my stainless shear pins (If you notice there are four more holes in a row which I didn't mention. They were made for a testing purpose and will not be used). I'm making my own shear pins out of a stainless welding rod which has a pretty accurate diameter. 

 Shear pins were cut into identical length, one side was deburred and the other side was bent 90 degrees. When taking out the pins, one has to grab this bent side and pull.

 Four pins are engaged.

The same rectangular pipe which was used to make the triangles makes a perfect contact with the pins. No wobblying or whatsoever. In actual mounting, the triangles mate with the pins from both sides of the pole, so they won't slide out.

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