Thursday, April 11, 2013

Manual Helical Mill Turn

We had an interesting job come through the shop the other day. We have been working with a physicist who is taking magnetic measurements of the magnetic field quality of one of our large superconducting dipole magnets. He measures and maps the field generated by the magnet when the magnet is operational.

This entails that his magnetic measuring probe must move up and down through the bore of the magnet that is cooled to 4.2 Kelvin. He uses an interesting anti cryostat device inserted down the bore of the large magnet. And inside this his probe rotates, and is moved up and down axially through the bore of the magnet to map the magnetic field. The purpose of the anti cryostat is to keep the measuring probe at room temperature when it is inserted in the super cold bore of the magnet.

The problem we were asked to solve was a failure of a plastic part at the end of the magnetic measuring probe that couples to the rotational motor. During the test run this part somehow got warm, and the small set screws holding the coupling to the probe collapsed the probe end. There are very small instrumentation wires that pass through the center of the coupling and the probe end that really don 't like to get twisted around.

The part that had failed had some semi-complex geometry so the way they were manufactured was by the 3D printer or FDM process. This was fine for previous tests and the parts performed well. On this particular test there was a heating problem that caused the FDM parts to soften and fail. The first step was to make the probe end out of something a little more durable. We had a good sample and a drawing so I gave the job to one of the new technicians to fabricate. We chose PEEK for the probe end material. This is a tough strong high performance plastic that has a wide operational temperature and excellent machinability.

While the probe end was being fabricated I had another discussion with the scientist and we decided that the failure was most likely from eddy current heating of the metal bearing spacer that happens to ride on the probe end. With this realization we decided to try to eliminate as much metal from this particular area as possible.

One of the parts that needed to be made of some non metallic material was a small helical shaft coupling. This type of coupling is used because of its zero backlash ability. The magnetic measurements are correlated to the probes position inside the magnet with rotary and linear encoders so a map of the field can be determined for that particular magnet configuration.
Our task was to duplicate this coupling in PEEK as quickly as we could, with stuff we had around the shop. There is limited measuring time and the cost per hour of the large magnet test is quite high so speed was important. I scrounged some tooling together and formed a plan of attack.

This is just the kind of job many machinists love to do. A tricky job with a minimum of detailed constraints, and the freedom to do the job anyway you can. The only thing that machinists generally don't like is the time constraint. When I was learning this stuff I would have liked to sink my teeth into a job like this every day of the week. In fact, I have never done this type of operation before. Sure, I've cut lots of helices and all manner of threads, but I never had to make a coupling like this. Honestly I really wanted to do this one myself but now its more important for me to allow other folks to have the experience and successes in the trade.

Part of the tooling we needed was a special holder to fit in the toolpost of the lathe. I made a quick hand sketch and gave the job to the other new technician I'm looking after. If this was going to work we would do it as a team. The tool block was needed to hold the hand piece of a Foredom tool

The general plan of attack was to use a thin saw blade mounted on the tool post to cut the helical groove in the coupling. I had a couple of small saws with arbors that would fit in the Foredom tool.
The blade width with was pretty close to the cuts on the sample. Definitely close enough for this operation. So both technicians, Matt and Nick were busy prepping the tooling and coupling blanks at this point.
There was a little more to the setup than mounting the slitting saw in the lathe. We wanted to match the pitch and number of flexures on the sample coupling as closely as we could with the tooling we put together. A quick calculation gave the helix angle of the "thread" we were going to cut. The pitch on the coupling was 12.5 TPI which is a bit of an oddball. We went with 12 TPI because this would thicken the flexure membrane a little and the Monarch could do this pitch.
 From this calculation we get the helix angle of the thread which we use to set the compound rest and the tool holder.
We also had to pitch the toolholder at the same angle since the tool we were using extended below the machine center line. If you look at the picture above showing the hand piece clamp you will see a set screw in the tongue of the holder. This was used to tip the tool in the Aloris holder to the helix angle. 
The idea was to use the lathe as a synchronization and holding tool as opposed to a machine tool. This little Monarch 10EE has a nice DRO so we could control the Z axis position well. We set the threading levers to our pitch (12 TPI) and engaged the half nut. We then ran the saw up to the face of the part by hand with the threading lever engaged and picked off the Z position.
The saw was set on centerline because I couldn't think of a reason why is should be anywhere else.
The first pass was to see how things went. PEEK is easy to get hot so we had the koolmist setup going. The saw blade would be passing through quite a bit of material so I was worried about overheating the material.
Here Nick has done a few passes along the helix. We used the DRO to reference the start and stop positions of the groove. When the end was reached he backed out the compound that was set at the helix angle and then just hand reversed the spindle by hand to get into position for the next pass. There was a fair amount of backlash in the system but with the DRO it was easy to get back to the start.  Nick and Matt cut the helix on their own. I had to go to a meeting right around the time they were cutting the helix. There was enough time for me to try my hand at one pass. So I did get to actually try it. Thanks Nick and Matt for giving me a go at it.
The results speak for themselves. Not bad for throwing some stuff together in a few hours. So with some Yankee ingenuity and some curious and willing teammates you can get a tricky job done and have some fun too. I know I still feel satisfied and proud even when I didn't turn the cranks myself. It would have taken me three times as long if I had to do all of the work myself. This is an excellent example of how team work can pay off.

We shot some video of the operation which I will put up on my YouTube channel in the next couple of days.

Thanks for looking.


  1. 12.5 tpi is closer to a 2mm thread! 25.4÷12.5 = 2.032. not weird at all! I know you like to swap back and forth between the systems, 12.5 is close to the magic 12.7 is how I spotted it!
    I'm enjoying your blog!
    Regards, Matthew

  2. Hi Matthew,

    It never even occurred to me that the helix might be a metric pitch. Thanks for the comment.