Saturday, June 18, 2011

Cutting an Internal Channel

I stopped making casters for the Takahashi EM400 because they were too much work to make.  I had a set for the Takahashi EM-400 80% complete a while back, when I decided life was too short to make these.  A customer in Belgium really wants a set--so here's a chance to turn what would otherwise be junk that I could not sell into some profit.

The reason the set for the Takahashi EM-400 was so much work is that the tripod legs on the Takahashi have a beautiful finish, and an early customer asked me to use a brass compression ring, rather than have the bolts that lock the casters in place directly attacking the legs.

A brass compression ring, if you are not familiar with it, is a common method of applying pressure to an eyepiece barrel that applies the pressure evenly over the entire barrel of the eyepiece, instead of applying it with one or two screws.  Screws mark the surface.  Instead, the screws press against compression ring, which squeezes against the eyepiece barrel.  The same amount of pressure is applied--but across a much larger area.

The way you make a brass compression ring is to excavate a channel on the inside of the bore, bend a piece of brass (in this case, about .050" thick) into a curve, and slip it into the channel.  Cutting the interior channel, however, is not easy.  You can use a boring bit...but that makes only the end of the bored section square.  You can use a reverse rake boring bit to make the start of the channel square.  Switching back and forth, however, is annoying.  If only you could cut them both at once!

My solution was to take a 1/4" cutting tool, which would ordinarily only have a single cutting point, and make it a 1/4" cutting tool.  Now I start the lathe, advance in to where the channel starts, move the tool out until I cut a 1/4" wide channel, then advance the carriage 1/4".  Now I have a 1/2" wide channel, with square edges on both sides.

The downside of having a blade instead of a point is that you have to cut much more slowly--and this works in acetal, but might not work in aluminum or steel at all.  (I have not tried it.)  Still, it solved my problem.

Bending the brass to fit, however, is a struggle.  My solution was that the interior channel was about 2.5" ID, so I took a piece of 2.5" acetal scrap, and used it as a mandrel around which to bend the brass in a vise.  This did the job for two of the three pieces, but for the third, I needed to apply clamps to bend the brass to fit entirely inside the channel.


  1. The solution to bending the ring is simple, don't bend it. cut it from a piece of brass tubing then slot it. a piece of 2-1/2" (2-7/8" o.d.) sched 10 pipe with a slot to allow it to fit in the groove should be about right. if you cut the slot at an angle, then you will have 360 degrees of clamping force. another way to skin the cat is to use a piece of solid rectangular bar, bore a hole slightly larger than the tripod leg off-center of the bar such that the centerline is equidistant to three of the four sides, and saw a slot along the fourth side, drill clearance holes on one side and tap the other and use screws to pull the slot closed, gripping the tripod leg much better than set screws and a compression ring would.

  2. Cutting a piece of tubing, then slotting it--an EXCELLENT suggestion!