Fitting Copper Liner

I fit the copper to the plastic cavity today and the seams look excellent:

I am super happy having gone with computer-aided printing and cutting because the accuracy is unmatched not to mention the final product looks way cool!  I am using the mismatched top and bottom plastic pieces first because I want to see how badly the plastic melts when brazing the copper lining.  I should have time tomorrow afternoon to make an attempt, and living in a cold climate will be useful because I can do the brazing in the garage where it is about minus five degrees.

For the vacuum chamber, a friend who does glassblowing (Silica is his gaming handle which is fitting, eh?) for the University of Calgary suggested using a Helium leak detector, specifically a Varian 979.

Searching through the UofA’s inventory management system, it turns out they have one:

Property Tag Classification Description Manufacturer/Make/Model Custodian Name Status
443300 DETECTOR,VACUUM LEAK VARIAN 979 HELIUM CHEMISTRY ACTIVE

I guess we will be taking a trip to the chemistry department.
I finally won a digital vacuum gauge off eBay ($200CND shipped) and it comes with a Pirani sensor that can go from atmosphere down to one micron.

Copper Cut and Vacuum Chamber

The copper got cut yesterday and the waterjet sure does a beautiful job:

To keep the copper from flexing while being cut, we nailed it to 1/2″ plywood and it helped transport the pieces afterwards.

Here is a clip from the documentary footage (Brian is checking the clearance before starting):

This week I plan to test fit everything together and get the probes started.  The probes are the only outstanding part to finish and I have an idea of how I can construct them, although I have yet to solve where to source the dielectric.

I spent today at the university working on the vacuum chamber:

End cap

Side portal with an acrylic window and o-ring:

Top portal (need more bolts):

The seals all look solid because I can see the o-rings pressed against the acrylic, but I won’t know until the first pump down.

Underside strut:

I still have one portal to bolt on and the front door, and then everything is ready for the first test.  Finding leaks is going to be tricky though – soap? Sound?

3D Printed Part Analyzed

Waterjet cutting of the copper got delayed until next week as the vendor is busy. The parts on the left will be cut from the thinner 12oz copper (0.0162″) and the circles will be cut from 32oz (0.0431″) copper.

I have 4′ by 8′ sheets of both the 12oz and 32oz copper.

I spent yesterday at the university trying to bolt the vacuum chamber together, but as usual, there were surprises.  I need bolts with a different pitch to fasten on the ports, and after a trip to Home Depot, I am pretty sure I have the right size now.  The new drill bits work excellent on the acrylic.  I have a length of 2″ aluminum square tubing which will go from the center underside of the top shelf to bottom shelf which should increase the maximum weight of the top shelf considerably.  Having measured everything at the university, I will be in the garage this weekend, plasma cutting it to size.

I got the 3D printed parts from Motive delivered by courier yesterday and they look awesome!

Because of problems with the machine, there are enough for almost two cavities (which I paid an extra $200 for, although I think it barely covered their cost)

Having looked at the surface, some spots can be bumpy and I am glad now with the plan to line the inside with copper.  The precision is quite impressive though, for example, below is a rendered shot compared with the printed part:

As you can see, in the rendered version I filleted the edges of the bolt blocks and the similarity is impressive.  The picture also shows there can be some surface irregularities, but they are superficial.

 

3D Printed Cavity Finished!

Will at Motive got back to me with great news on the 3D printing front – the cavity is finished:

(The black stuff which seems to be dripping from the part is support material which is dissolved away.)  Once the cap for the top is printed, everything will be shipped up to me this week.

I also got the drill bits for the acrylic yesterday and will be up at the university sometime this week to move the construction of the vacuum chamber forward.

The copper lining will be waterjet cut in order to make sure the accuracy is as close as possible and I have Superior Waterjet lined up to do the job this week.  I have been working these past few days to make sure my pattern (shown below) is workable and still need to lay out two more pieces.  The great news about having the plastic to insert the copper lining into is that everything can be unbolted and taken apart, for example if we want to insert a superconducting lining, etc.

3D Printing Mess & Options

The first attempt at 3D printing the part produced this:

All I can say is – awesome!  It is really neat to see something that was only a sparkle in your eye be turned into a hard touchable object!  Apparently, there is a problem with the machine printing objects over 8″ and the printing head bumped the part, strewing the printing material everywhere.  The work around for the moment is to split the cone in-half, which is what will run this weekend, and glue the results together.

Besides a buildup of TOF work, I spent the last few days looking at electroplating solutions and I am convinced they will not work.  There are three problems:

  1. Cost – upwards of $500, which is expensive considering I can buy a thick 4′ by 8′ (yes foot) sheet of pure copper for $360
  2. Electroplated Thickness – typically only a couple microns, where a thousand microns is 1 mm.  Unfortunately, because we are using high power microwave, it isn’t so much penetration of the microwave energy as, it is momentary heating.  I can just see a bit of heat bubbling the plastic and electroplated surface, which will then arc doing something like this.
  3. Adhesion – getting the copper layer to stick, or plate evenly sounds very difficult.  Part of the problem is that for electroplating, it needs about an amp per square inch, which in our case is a lot of amps given the inside volume of the part.  Too many amps leads to unpredictable results.

I spent many hours reading through forums dedicated to electroplating to get a sense of the difficulties involved and it sounds cumbersome, for example, here. Other people have been thinking of plating 3D printed products too but haven’t reported back any successes.  I also looked at using copper leaf which is actually pretty inexpensive, but probably too thin and the surface can be tricky to get flat.

Once the plastic part shows up, I am going insert a copper lining into it, which means I am back to square one with seams.  However, the external plastic will act like a mold, and as long as I can cut the copper closely enough, I should be able to glue it in place, the braze the seams

I built a plastic model to get an idea of what the seams will be like:

The great news is that

  • I figured out how to calculate the angles and heights I need to draw out a pattern for the “frustum” cone.  Creating a frustum cone pattern is pretty easy actually.
  • I learned Rhino 3D has an “unroll” function where I can take the cone and roll it out in preparation for water jet cutting.

    My plan is to cut two cavities at the same time by putting two sheets of copper together and waterjet cut them at the same time.

The plastic model makes something apparent – the angle at which the frustum cone joins with the long tube section is pretty extreme and is going to be difficult to join properly.

Motive and 3D Printing

The last four days have been intense as I worked to get a printable design to Motive so they could get started.  As it turns out they had the plastic I wanted (red!) already in the machine and they had time to do it this weekend, hence the expediency.

As shown below, I went through four iterations in 24 hours, the first three drawn from scratch:

The first design was because I envisioned their FDM machine using powder, but it is an additive process which means it builds free standing structures.  Design #1 would have required a lot of dissolvable supporting plastic inside and would have increased the cost greatly.  Hence, designs #2 and onward are designed with as little support as possible and everything higher up on the model is supported by something below.  The third iteration fixed a few problems and shows that instead of creating a large cap, I am going to cut out a piece of the pure copper sheet metal I have and bolt that to the large end.  The narrow end cap has that strange square structure jutting up because that is where the tuning mechanism is to be mounted, shown below:

The final cost was $840 (GST included) which is very good considering alternatives and the technical help (we exchanged over thirty emails in a 78 hour period).  The part is now being printed as I type this and baring any unforeseen events, it will take a full three days for it to finish.

While I was drawing up the models, I also had a concurrent conversation going on with an electroplating place on the West Coast called Pacific Plating.  I have been lucky enough to deal with the head guy who apparently likes challenges, and he is the reason for revision #4 of the model above.  If you look closely it has extra protrusions, the top two for hooks to hang the part in the electroplating bath and two at the bottom to put weights on it keep it from floating.  Pacific Plating specializes in plating plastic and he suggested that I send them samples of the ABS covered with the copper conductive paint on the surfaces I want coated.  They can then test each sample for a different time interval, and I can decide which is best, or if the process works at all.  The model is designed to handle upwards of 0.45 mm of electroplated copper around the narrow end, which is a significant amount to electroplate.

The copper does not have to be thick because most of the energy is centered in the middle of the end pieces (assuming the TE0,1,n mode), which is where thick pure copper plates will reside.   One benefit of copper plated plastic will be weight, which given this cavity is to be hung from a pendulum, will make any force that more apparent.

Before we push a kilowatt of power into the 3D printed cavity, I am going to built a second test cavity using the wood molds I have mentioned previously.  The idea is to strap pieces of copper over the mold, brazing them, then repeating for the other side.  Then the two sides can be bolted together after the probes have been mounted.

With the handmade mold, we can make sure there is no arcing between the probes and other unforeseen events before attempting it with the more expensive but more accurate printed cavity.  I plan to use the cracked mold I already made to test the idea, and then I will have to put the second piece of wood on the lathe.  The good news is I have lots of copper.

My To Do list now consists of:

  • Purchase the copper conductive paint
  • Get the ABS plastic samples painted and sent to Pacific Plating
  • Finish the vacuum chamber once the drill bits for the acrylic get in.
  • Get the copper only test cavity built.

Plastic Cavity Electroplated?

Dr. Karumudi and Kevin, I want your opinions on something – Do you think a plastic cavity electroplated with copper will work?  Here is why I ask:

  1. Cost –Motive Industries in Calgary is a company that designs and builds concept cars and they recently purchased an FDM machine.  They are willing to print the part in ABS plastic (specs attached) for just over $800 for the entire cavity.  I would then spray the plastic with a conductive paint, then have it electroplated with a kit like this.
  2. Microwave Skin Effect – Because of the high frequency nature of the microwaves, they only need 1 millimeter or less, right?

My only concern is heat – if the cavity gets hot, it will melt the plastic.  However this might not be a problem because:

  1. If the plastic melts or browns, we can tell where the cavity is getting hot.
  2. We can make sure the electroplating is extra thick, say a couple mm, and even if the plastic melts or burns away, the copper will still hold it’s shape.

The plan is to split the cavity in two, add bolt holes around the outside, mounts for the probes and for the tuning mechanism.  Here is the basic model I am starting with:

Here are the benefits of using a 3D printing method over brazing copper sheeting together:

  • Accuracy – easily within 0.010″ or better
  • Reproducible – We can offer to build these cavities for other people who may want to duplicate our results
  • Fast – The mounts for the probes and the tuning mechanism at the back will be built at the same time as the cavity is printed
  • Weight – a plastic cavity plus copper plating will weigh less then a pure copper cavity

Using Motive’s FDM machine is by far the cheapest method of fabrication I have found besides trying to braze copper sheeting together myself.  They have also been very responsive.

I have also been contacting vendors all morning about copper plating the plastic because if it is inexpensive enough, I would rather pay someone else to do it.

Copper Spinning Rejected

I spent Monday trying to get a plastic model built around the wooden mold (in preparation for the copper), but ran into significant problems.

As shown, the mold has since crack significantly, making the circumference much larger then originally planned.   Even with the cracks, it proved difficult to get my test plastic around the mold without creating a large number of seams.  The seams are a problem because they are not smooth and would need to be brazed.  I have, for the meantime, rejected using the mold to strap the copper around.

As I mentioned previously, paying someone else to do the metal spinning is too expensive, and I put more thought into doing it myself, but decided again that too, for a number of reasons:

  • I don’t have the right tools (I would need pretty much everything on that page)
  • I need a lathe with a larger space between the spindle (the part that spins on either side of the piece) and the bed (the part that holds both side together).  The largest piece of copper I can fit onto the my current lathe is 11″ in diameter which is not enough copper to push over a 15″ long mandrel.  Good metal spinning lathe are worth thousands of dollars.
  • After talking to a supplier, it sounds like the mandrel I have created is not a hard enough wood, nor is it centered properly.

I have since been investigating using “3D Printing” technology which uses metal powders, and adhesives to first make a “green” object, which is then baked, and then infused with bronze.  I have sent in a request for quote to Prometal (US based) and Shapeways (UK based) and I am hoping to get the cavity made for under $2K CND.  The beauty of using 3D printing technology is that I can design the cavity in two parts including all the mounts necessary for the tuning plate and the two probes in Rhino3D (which I already own) and then have it printed with 0.1mm accuracy!  I am also looking into getting a much cheaper plastic model “3D printed” and then electroplated with copper.

I spent the afternoon at the university running more simulations – I currently have a problem where if I run a simulation with the highest number of tetrahedrals possible (~600K), I get different results then if I run it with fewer tetrahedrals (~300K to ~445K).  The results are different enough that it looks like the TE0,1 mode is resonating at a different frequency.  With the ~600K model, I tried using the tuning plate at the back of the cavity to bring the TE0,1,n mode back to resonate at 2.45Ghz, but with ~600K tetrahedrals, it takes five to six hours just to run one simulation.  I then make a small change to the position of the tuning plate and retry the simulation but the length of time for the iterations makes the method unfeasible.  My analysis is that the higher number of tetrahedrals changes the results because the resonant frequency of the TE0,1,n mode is sensitive to the accuracy with which all the round parts are meshed at.  Everything in this model is round, from the probes to the tuning plate to the entire cavity itself, which makes an accurate meshing very difficult.  However, I am confident the model will work, but it will need a tuning mechanism to make the TE0,1 mode resonant at 2.45Ghz and I am going ahead with the design.  (This could be my biggest mistake yet, if the cavity costs $2K to get built and it doesn’t resonate as expected)

I did get the new L gasket for the vacuum chamber and I was very happy to see that it fit:

Now I am waiting for:

  • Vacuum gauge – I unfortunately lost my bid and will have to find another one.
  • Drill bits for the acrylic – they should be here within a week.