Oct 17, 2016

Kinematic Coupling Pen Fabrication

This is the Kinematic Coupling Pen! I'm showing the finished photo first, after three coats of walnut oil to bring out the cocobolo sheen. The metal bits are steel, and the pen cartridge is the same as the one in my brass pen

This completes my 2.750 Kinematic Coupling Lab, and here is my writeup for the class. The following blog post goes more into detail about fabrication and test setup, and is slightly more handwavey about analysis. It otherwise has the same info. 

Fabrication of the kinematic coupling started with steel round stock turned to a final outer diameter and a 6mm clearance hole for the pen cartridge. I bandsawed the shaft into halves after turning diameters to ensure they were a matched set, then faced them flat. 

Both the socket half and the grooved half used the indexing head on the mill to align features at 120deg angles. The 45deg V- grooves were made with a small endmill followed by a 90deg countersink (start with the endmill for stress-relief!), and the hemisphere sockets were made with a 1/8" ball-endmill.


I then turned some alignment offsets in both halves, but didn't really plan how I was going to cut my parts off the excess stock. Sloppy bandsawing and attempts at salvaging parts left my alignment offsets significantly shorter than I had hoped and my actual kinematic coupling (KC for now on) parts really dinged-up with tool marks.

I cleaned up by first press-fitting a 1/4" dowel into the 6mm hole, then gently filing away the tool marks by hand. This means the parts aren't perfectly circular anymore, but you wouldn't notice unless you spun them at high speeds. Kinematic coupling part of the lab now completed; moving on to the pen itself!

The wood phase of this project took place at the Hobby Shop, because I needed its wood lathe, dust collection system, and expert advice. 

Expert advice: Hayami is an awesome guy. I show up with a block of pain-in-the-ass wood, and he showed me everything I needed to complete my pen. He even set up an additional orientation period when I completely forgot to show up to the first one.

Dust collection system: Fun fact, cocobolo dust is super allergenic. You are highly likely to develop skin rashes from prolonged contact, and it's toxic if you breath it in. The wood itself is fine, but the dust is yikesy stuff. Hobby shop gave me a personal respirator mask, and I ran a vacuum at every machine station I touched. I'm pretty happy to say that I haven't developed an allergy to the stuff yet, because I still have plenty of pieces left.

But let's talk about the wood lathe. I was pretty happy to finish the actual kinematic coupling part early on, because then I had an excuse to spend lots of time (2 weeks) learning woodturning. My 2x4 scrap of cocobolo got bandsawed to two rough rectangular pen-blanks, then I found the rough centers and drilled a 1/4" through-hole.

The 1/4" through-hole served two purposes: one, I needed a through-hole for the pen cartridge; two, the Hobby Shop pen-turning kit consists of a 1/4" bolt and a thumb-nut that clamps the workpiece to the lathe.

The woodturning process has three parts. First, the pen blank is made cylindrical and roughly even using a scallop-shaped rough gouge. Then, a hemisphere-shaped gouge turns the proper outer diameter and shapes contours. Finally, a parting tool forms tongues for alignment with the pen tips and cuts my parts off the original stock.

After the wooden parts were done, it was back to MITERS for the steel pen-tips. The tapered front was made using similar methods as the brass pen, except this time I didn't bother dealing with cutting threads in steel. Drilling tiny through holes was bad enough (I broke 1 drill bit and 1 centerdrill in the process, but 3rd try is the charm!)

Once all the components were complete, they were glued together with 30min epoxy and brought to 2.750 show and tell the next day! Following that, I sanded all the components' edges flush and cleaned off all remaining epoxy residue. Officially finished!

Of course, there's still science to be done. I evaluated the kinematic-coupling part of the pen with two tests: a repeatability test and a stiffness measurement. In both cases, I wanted to better simulate a freely-waving pen solely held in place with magnets, which meant I had to avoid standard laser pointers (since I'd be doubling the weight of my pen!) My test setup consisted of a disassembled laser diode taped to the top of the pen in an attempt to add as little weight as possible.

     An aside about this laser diode - it's a really fantastic 400nm blue experimental laser diode
     Bayley let me borrow so that I can eventually make a shiny knurled case for it for science, 
     where the normal application is for something like ridiculously-specific X-ray equipment.
     But more importantly, the laser diode emits a very beautiful blue hue.

Given my contact surfaces of steel and neodymium alloy, I expected both high stiffness and high repeatability, so my test setups required magnifiying small errors over long distances - Abbe Error!

image credit: Matt Rosario

Basically Abbe error (sine error) takes advantage of magnifying angular error over distance to make observing tiny errors easier. In my case, that meant clamping my pen in the mill and observing laser pointer errors 24.5 feet away (giving me a magnification factor of 840:1)

I got an average error of 8μm, which was the expected order of magnitude for a steel-steel manually-machined object. I could have probably cut down on error if I had invested more effort than just taping the laser pointer on, or if I had been more careful machining the ball sockets in the kinematic coupling (one is noticeably shallower than the others)

Setting up laser diode for repeatability test

Laser pointer is blue dot in center of blue circle
For measuring stiffness, I used the same laser diode and attached it mostly parallel to the bisector of the magnet-triangle, and pointed it at a wall 15' away (528 magnification factor). Then I pushed on it perpendicularly with a force sensor (approx. 1N) and measured laser error on the wall.

I got an average deflection of 3.5μm after 5 trials, which resulted in a measured stiffness of 0.30 N/μm. This was twice the stiffness predicted from the kinematic coupling spreadsheet (0.14 N/μm), but is pretty close. Discrepancies could come from many sources, ranging from assumptions in the spreadsheet to again user error with the laser pointer, so it's unclear how closely my pen follows the theoretical model.

Finally, I've been doing some user testing over the past week. It writes! Unfortunately, due to the size of the kinematic coupling it has the ergonomics of an expo marker. The size of the pen threw me off initially but I got used to it eventually.

Additionally, the magnets used in this pen are not so strong that they perfectly resist my hand, so the back of the pen wobbles a bit if I write too quickly or squeeze the joint. This means my hand is located further down the pen than it normally would be when writing with an expo marker or a different pen.

I also experienced the fun thing where the tip of my pen cartridge is also steel and will therefore adhere to magnets, so I have to take care when removing the ink cartridges. The pen cartridge-tip gets press-fit in the pen, and the first time I tried pulling it out I accidentally separated the ink reservoir from the steel tip. Ink got everywhere! So, in order to remove ink cartridges I have to tap the pen-tip with the steel back of the pen.

A happy side-effect of the pen-tip getting press-fit within the pen is that the pen-tip stays put under normal writing conditions, even though there is space in the back of the pen. That means I don't need to make a cap for this pen, and can instead tap the writing-tip inside the pen-body if I want to store it in my bag.

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