Making it out of lead! How?

[graham-xrf]

This is related to the XRF (X-ray Fluorescence) alloys identification project "Needing more than a spark test?" We have come to a point where it seems clear a sensor assembly concept using lead shielding is looking to be the one we need. I have tentatively have asked this sometime before, but I forgot when.

There are problems in even holding lead in a chuck. My guess is, do it carefully. Probably one has to melt some lead into the bottom of an ex-Red Bull can, to make a round lump to start with. The part has to be reasonably precision, such that the ex-smoke detector radioactive sources can be pressed into position, and stay there. No glue, loctite, nor any other substances other than the lead must be there. The gadget identifies elements, and the presence of other stuff just causes confusion.

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I start with wondering what shape the cutting tool should be. The diameter of the whole assembly is about 40mm (around 1.5 inches)
Any advice on making this would be appreciated.

 

[addertooth]

Get a copper plumbing end-cap. Pre-heat the copper and flux the inside of it. Then pour your lead in it. The copper will act to give the whole assembly more rigidity, and make it less likely to bend in the chuck. It will act as a foundation.

 

[Nutfarmer]

Would it be easier to make a mold out of aluminium and cast the part ? The bores that hold the radio active material could finished by swaging with punch.

 

[Lo-Fi]

Yep, cast it. We make special profile lead gaskets all the time at the railway using two piece steel molds. Or you can make silicone molds quite effectively that will cast lead with surprising precision. Great candidate for 3D printed pattern.

 

[Bi11Hudson]

Most of the stuff I cast is much smaller than what you have in mind. 1-1/2 inch is pretty big for what I do. That said, most of my work is cast simply because lead is difficult to machine. Even with a hardener, Antimony, alloyed in it's a PITA. On other parts, I need multiples and machining once to make a dozen pieces is obvious.

I use aluminium for molds because lead doesn't bond at the temperatures where I cast. There are fishing gear and bullet molds that differ, some steel some carbide(?). But for what I do, aluminium gives good results. And easy to work.

I often must put a lot more thought into making the mold than I would for making the part itself. But it does pay off in the long run. And the mold goes on a shelf for future use.(?) The part pictured above would be a fairly simple mold to make. Likely a little deburring, but with lead no big deal.

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[WobblyHand]

Lost wax casting, perhaps?

For a single use, make the wax piece out of file-a-wax rods, turn on lathe, then machine pockets. Add a sprue, or whatever the technical term is, coat with investment casting ceramic, pour your lead, let it cool, crack off the ceramic. Mind you, haven't done it, but as I understand it, this is a common practice.

For multiple use, lost wax, could make a rubber mold of the machined wax piece, then cast wax positives using the mold. Beats me what tolerances one could hold, but might be fun trying.

The way the divots for the sources are in your drawing might make it hard to extract from a mold, so either lost wax would be good, or modify the drawing to allow extraction direct from the mold.

 

[RJSakowski]

This is related to the XRF (X-ray Fluorescence) alloys identification project "Needing more than a spark test?" We have come to a point where it seems clear a sensor assembly concept using lead shielding is looking to be the one we need. I have tentatively have asked this sometime before, but I forgot when.

There are problems in even holding lead in a chuck. My guess is, do it carefully. Probably one has to melt some lead into the bottom of an ex-Red Bull can, to make a round lump to start with. The part has to be reasonably precision, such that the ex-smoke detector radioactive sources can be pressed into position, and stay there. No glue, loctite, nor any other substances other than the lead must be there. The gadget identifies elements, and the presence of other stuff just causes confusion.

View attachment 391332 . View attachment 391333

I start with wondering what shape the cutting tool should be. The diameter of the whole assembly is about 40mm (around 1.5 inches)
Any advice on making this would be appreciated.

I wouldn't worry about using cyanoacrylate adhesive for fixing the sources. It is composed of hydrogen, carbon, nitrogen, and oxygen, all low Z elements which shouldn't interfere with your spectrum. Additionally, any emission would be shielded from your detector by the lead. I would avoid quick setting epoxy as it contains sulfur.

If you don't want to use an adhesive, you can make your pockets a little deeper and lightly swage the rim of the pocket to contain the source.

Casting the part seems like a good way to go. Either directly machine the mold or make a master for a subsequent silicone mold. If using silicone, an aluminum outer shell will help to provide the necessary rigidity for the mold.

Good luck with the part. I am eagerly waiting to see your final results.

 

[graham-xrf]

Would it be easier to make a mold out of aluminium and cast the part ? The bores that hold the radio active material could finished by swaging with punch.

Yes, I had thought of trying for a casting of some sort, and then doing some finishing by hand.
I just needed to get some reliable low-down on whether lead is even machinable at all!

The smoke-detector radioactive sources are pretty much harmless when shipped in their metal chamber assembly, but what we are doing is stripping them down and then pressing the final coated metal strip disc to sit in the hole in the lead shield. I can punch-peen the edges to get it to stay, but here, we would be hitting on it real real close.

The thread is enormous - a log of our discussions, and I would not inflict the whole thing on anybody to take in one hit. For those new to this endeavor, here are some pictures from earlier. I filed off the swaged-over edges, and pressed the source out of the contact ring.

 

[graham-xrf]

Lost wax casting, perhaps?

For a single use, make the wax piece out of file-a-wax rods, turn on lathe, then machine pockets. Add a sprue, or whatever the technical term is, coat with investment casting ceramic, pour your lead, let it cool, crack off the ceramic. Mind you, haven't done it, but as I understand it, this is a common practice.

For multiple use, lost wax, could make a rubber mold of the machined wax piece, then cast wax positives using the mold. Beats me what tolerances one could hold, but might be fun trying.

The way the divots for the sources are in your drawing might make it hard to extract from a mold, so either lost wax would be good, or modify the drawing to allow extraction direct from the mold.

We have considered the sources facing forward, so easier to make geometry. It is wasteful of radiation we need just getting absorbed in shielding. The angled irradiation increases the number of hits. This is a back-scatter scheme. The higher energy gamma rays enters the sample, and some of then get lucky and hit an atom of sample. That hit makes it "glow x-rays". Of the X-rays generated, going all over the place, some fraction are in the direction of the detector.

Multiple use? We do hope so! If we get it really low cost, and it works, some HM members might want to get one together. If they can machine their own somehow, all we need distribute is the drawing.

 

[graham-xrf]

I wouldn't worry about using cyanoacrylate adhesive for fixing the sources. It is composed of hydrogen, carbon, nitrogen, and oxygen, all low Z elements which shouldn't interfere with your spectrum. Additionally, any emission would be shielded from your detector by the lead. I would avoid quick setting epoxy as it contains sulfur.

If you don't want to use an adhesive, you can make your pockets a little deeper and lightly swage the rim of the pocket to contain the source.

Casting the part seems like a good way to go. Either directly machine the mold or make a master for a subsequent silicone mold. If using silicone, an aluminum outer shell will help to provide the necessary rigidity for the mold.

Good luck with the part. I am eagerly waiting to see your final results.

OK - looking at the energies, the detector cannot see any element of number less than 11 (Sodium 1.04keV), and to be honest, it would be a struggle to see 12 (Magnesium 1.25keV). That said, I have tried to get the gain and resolution good enough to try for Magnesium, Aluminium, Silicon, Phosphorous, Sulfur and higher, but one may have to wait some time to get sufficient counts.

There is a whole lot we don't know. Most of the 60keV gamma photons from the Am241 source might go through aluminum without stopping!