Some People Have All The Fun

I mentioned my friend Chris - the Dental Tech in my previous post about my career in electronics. I didn't mention him by name then: but I don't think he'll mind having his name mentioned now.  I knew he would be able to help me out with the tools required to replace broken rubber drive bands in the tight spaces which exist on compact cassette decks.  Only a few days ago I ordered the bands from Wagner Electronics in Sydney.  And then, yesterday afternoon,  I fortuitously ran into Chris in one of the shopping malls while he was buying his young daughter a 'product' from Wendy's.  I mentioned the need for tools.  He said he would arrange something and that he would be in his Lab on Saturday morning.

Chris and I go back a long way, but we haven't seen each other to talk to privately for over a year.  So, as there was no one else in the Lab, and the Dental Surgery next door was closed we were able to "catch up" as he worked.

He was making crowns, by the lost wax method, out of a gold alloy. I knew about the lost wax method of making sculptures - but had never really understood it.  I hadn't realized it was used to make crowns for teeth. 

It is fine work modelling the wax on top of an impression taken from the patients ailing tooth.  The work involves building up what is missing from the tooth and making sure that the resulting crown will fit neatly with the tooth above or below it.  For that reason an impression is taken from the full set (if there remains a full set!) of the upper and lower teeth of each patient. The desk was strewn with molded pairs of Halloween chompers joined by a plastic hinge, allowing him to build up the wax on the tooth, using a tiny spatula heated on a Bunsen Burner.  The hinge allowed him to continually check that the dental occlusion was OK, as if the teeth were fitting together with the mouth closed.

The wax molds of the potential crown were then fitted with a 'sprue', a thin tube of wax which would create a channel for the gold alloy to flow through. [You've probably seen little bits of round plastic projecting from injection molded plastic objects.  It is usually pretty obvious that the round plastic projection is a left over from where hot plastic was injected. Today, I learned that that projection is the remains of a 'sprue' - at least in Dental Lab talk. Maybe you could check out what the plastics people call it and give me a buzz?] 

Once the sprue was attached, the wax molds were removed from the stump on which they were fashioned and put in a special container, with the sprue facing upwards.  They were then invested, or encased, with a quick setting 'cement' made up from a very fine powder and some liquid chemicals.  The cement mold surrounding the inner wax mold was placed in a furnace at 866°C  which is hot enough for the wax to get lost, and if that temperature is maintained, it is also hot enough to allow the gold to flow down the sprue when the time comes.  Chris measured the correct amount of gold required, by multiplying the weight of the wax, 0.5 gram, by 15.2 which was the specific gravity of the gold alloy he was using.  I assisted with this on a calculator.  He needed 7.6 grams of gold and measured some tiny ingots out and put them in a crucible, similar to that on the right. The crucible itself had already been heated to 866°C in the furnace before Chris took it out and placed it on its holder on a large specially designed centrifuge. He then attacked the gold in the crucible with a blow torch driven by a mixture of acetylene and propane. The gold melted, suddenly became liquid, and emitted a firey orange glow.  Chris then took the mold from the furnace and placed it on the centrifuge behind the crucible.  Initially I did not realize that there was a hole in the rear of the crucible corresponding to an opening in the top of the cement mold. Chris set the centrifuge in motion. The crucible and mold spun. I watched the firey glow move around in a circle about three times before the gold suddenly vanished. It had moved through the hole in the rear wall of the crucible and disappeared into the hollow space in the cement where the wax had been. Centrifugal force would lead to displacement of any air in the irregularly shaped spaces inside the cement mold. I danced around with joy saying I hadn't had so much fun in a long time, while Chris told me that he would have to use a sandblaster to get the cement off the gold crowns inside the mold. But he would do that next week, and I was welcome to come back to watch. 

Chris presented me with a supply of discarded dental probes which he had obtained from the Dental Surgery next door to his Lab.  He uses the same probes himself as sculpting tools.  He was happy to help me manufacture them into the shape I would need to replace rubber bands in cassette machines. I'll need a hook at one end and a U-shaped pusher at the other.  He said the blow torch he used to melt the gold was good at softening up the tips of the probes and he has a special set of pliers with which to bend the hot metal into the shapes I need.  "Why not come back and watch me sandblast the cement away from the gold crowns, and we could bend the probes at the same time." 

As I said: Some people have all the fun.

Comments

[Maree]

Boy, you certainly are a good student. You could probably teach this subject now?

[Jude]

Most interesting. Now we know why those crowns are so darned expensive! Pleased to see the blog up and running again ... and Kim's name popping up again too.