Machinability of Pewter

[rcoope]

We are thinking about tin or tin alloys for an electron beam shielding application for radiotherapy. It could potentially replace Cerrobend (bismuth/lead alloy), which would in particular, enable us to cast squares of ~1cm thickness, and then waterjet specifically shaped cutouts. All the material could then be re-melted for ongoing use, which is how it's currently done with Cerrobend. Currently, positives of the the cutouts are shaped on a CNC foam cutter and then the Cerrobend is cast around them, which is a more labourious workflow and there's a room full of bismuth/lead. You obviously don't want that near your waterjet tank lest you turn the whole thing into toxic waste. It turns out that tin has potentially good absorbtion characteristics in the energy range of interest. We're going to get ourselves a sample to fool with but I was wondering if anyone had experience machining either pure tin or pewter (Sn/Sb/C), which is harder. There's some applications where one might want to actually machine it to fit in an apparatus, but I'm mostly curious about how it will waterjet vs say steel or aluminum. Steel, which you could use in this cutout application, is about 2.5 times tougher to waterjet than aluminum for example. Worse, Iron doesn't have great absorbtion characteristics where we need it so you'd have to make a steel plate much thicker so waterjetting would get that much slower and more expensive. I'm hoping pewter is closer to aluminum in waterjet toughness factor and we can use thinner sheets. In case anyone's interested, here's some absorption data from https://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html The upward spikes are where incident X-rays excite the atoms' various inner electron orbitals. You can see here there's a region above 30keV where tin is way better than Iron.

 

[technocrat]

No tips and have not tried, but pewter is regularly machined for jewellery and table ware. Pewter often has some bismuth as part of the alloy.

 

[swarfless]

Since you are into melting for recycling would a lost wax process work for you? Fashion 'part' in wax then use that in the usual 'lost wax' process. Too much dicking around? Very flexible tho'. Suspect shrinkage might be the killer but don't know.

 

[implmex]

Hi Robin:
I suspect you're going to have to find out the old fashioned empirical way and just try it out.

My guess is it'll cut just fine on the waterjet...my only experience with pewter is with normal chipmaking machines (actually a dental handpiece and a carbide burr) and I found it, as expected, to be a bit gummy, but not bad at all.

Can you score some of the family heirlooms and sacrifice them in the interests of science?

Of course, what would be REALLY cool is if you could find a way to 3D print them and thereby justify a Rapidia...but I'm fantasizing here.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com

 

[Joe Gwinn]

Would a Babbitt bearing alloy work?

https://en.wikipedia.org/wiki/Babbitt_(alloy)

 

[rcoope]

Thanks guys, we will find out and let you know. These are pretty boring parts, like 12cm squares 1cm thick with something resembling a circle or ellipse similar shape cut out of the middle. The sides of the cast material are always straight. The current process is kind of a very simple lost foam process, but you just pull the foam plug out and toss it. Waterjetting will allow them to retire the CNC foam cutter and generally reorganize who's doing the process to make it more efficient within the larger organization.

 

[Milland]

Before you get too enamored of waterjet cutting pewter, you may want to check local laws on whether the swarf would be considered hazardous. Some alloys contain a tiny amount of lead, even if they claim to be lead-free. It's worth confirming the other constituents won't be a problem.

 

[Bill D]

I think pewter is probably like brass. Lots of slightly different alloys, slightly different machining abilities, slightly different uses. Hot rolled, cold rolled, stress relieved etc.,
I bet you will need to be fairly specific when ordering.
Bill D

 

[Peter Colman]

Have I got this right, I think we are looking at a 120 mm square plate 10 mm thick with a shaped hole in it,
if so why not die cast the plate with an undersize hole and then broach the hole to size, the casting could be done in a steel tool with interchangeable cores for the holes. To improve quality the tool could be fitted with a tubular extension as a feeder and then the column of metal hit with a punch as it cools.
Dependant on tolerances possibly a cast only finish may be adequate.

 

[implmex]

Hi Peter:
These are custom radiation shields for cancer therapy...every one is different.
So every one has to be a custom.
You have to be able to make them fast.
You have to be able to chuck them back in the melting pot when the patient is done with them.
They don't have to be super accurate.

Cheers
Marcus
www.implant-mechanix.com
www.vancouverwireedm.com

 

[gustafson]

Tell em to gun all this electron crap, protons are better

 

[Milland]

Tell em to gun all this electron crap, protons are better

I'm partial to quarks and leptons, but that's 'cause I'm a rebel...

 

[Bill D]

Tell em to gun all this electron crap, protons are better

I would assume electrons are much easier to steer at sharp angles in short distances. So smaller machines to better fit inside a room rather then a full warehouse size experimental chamber.
Further I would assume the lighter mass electrons would be generated at lower input power needs. Bigger accelerators need their own power substations. The big cyclotron at LRL had something like 2,000 horsepower of steering magnets alone.
BIll D

 

[memphisjed]

If you cast them why waterjet afterwards? You can cast into wood molds for one and done customs. 3d print and use green sand- arbor press for squeezer. You do not even have to discard flash layer so you can use 100 percent finishing sand.
I have never milled pewter or 99.9 tin. Wax files work well for rough shaping, pewter being really gummy. Both drill fine.
Pewter is hard (relative) after casting- it work softens. It fabricated like a dream. One of the cooler metals to play with.

 

[rcoope]

Most radio therapy is actually X-ray on Linacs, but there are some uses for electron beams and indeed you can get both out of a medical Linac. A Linac contains entire linear accelerator for electrons including a thermonic source, acceleration chamber with an on-board RF source to run it, and a bunch of steering magnets. All of that is packaged up in a multiaxis moving structure with a multi-axis patient couch so it's at least a five axis setup. It's easy to steer electrons because they are charged and don't weigh much. To make X-rays, you just have to aim the electrons at the target and then have them hit a metal film (I've been trying to get them to tell me what kind of metal, but not sure yet). When the high energy electrons hit the metal atoms, it excites the lower orbitals, due to William Roentgens's famous Bremsstrahlung or braking radiation, effect. The X-rays fly out like any photons but you can collimate the beam with various chunks of tungsten so you get the beam shape you want to treat the patient. Linacs are amazing machines and there's a lot more to it, but one of the cool things is what you end up with is a shapeable beam of X-rays, or electrons, and the idea is to plan the movement and shape of the beam such that the dose adds up the most in the volume of interest, which is where the tumour is. The process is exactly CAM, only the tool is the shapeable beam and the workpiece is the patient.
Protons are a different thing, you need a cyclotron or synchrotron, yes you can get the protons to decay at a set distance into the body. I'm not clear there is actually much evidence that its better. My colleagues were doing it for ocular therapy and I had led the development of a digital workflow to make bass disks with a similar patient-specific cutout to the larger ones were talking about here. There, we took coordinates from the planning software, did some coordinate transformations and formatting to get a DXF, and then either CNC milled or waterjetted the cutouts from pre-made brass blanks. That pipeline got paused for some reason but they're doing those treatments with brachytherapy instead.
The reason to get rid of the casting with Cerrobend is it makes the workflow nicer where the people planning the treatment can exactly specify the shape of the cutout device, and then it can go to manufacturing without staff having to make guesses and eyeball the mould setup. There's some other reasons, such as changing the staff who would be doing this, rationalizing the equipment and space involved, and getting rid of some mildly toxic liquid metal. Of course this presupposes your hospital has a waterjet cutter, but what self respecting hospital wouldn't have such equipment?
Anyway, don't worry, we're getting some lead free pewter and will do some tests and we'll see what happens. More in due course....

 

[gustafson]

I would assume electrons are much easier to steer at sharp angles in short distances. So smaller machines to better fit inside a room rather then a full warehouse size experimental chamber.
Further I would assume the lighter mass electrons would be generated at lower input power needs. Bigger accelerators need their own power substations. The big cyclotron at LRL had something like 2,000 horsepower of steering magnets alone.
BIll D

They have been doing protons for 50 years. You don't need a warehouse sized accelerator, although 35 years ago a 100 inch accelerator took up a small house.

Proton Therapy for Cancer Treatment | Massachusetts General Hospital

Learn about proton therapy for cancer treatment. Mass General has an unmatched history in treating both benign and malignant tumors with proton therapy.

www.massgeneral.org

protons No entrance dose, no exit dose. 40 years ago they could do 3 mm accuracy