DaveDC
Active member
Parts made of rubber eventually die. Anyone who has worked on an old car or piece of vintage gear knows this. Depending on the rubber formulation, it will either turn rock hard, become a sticky goo, or disintegrate into dust. The foam on my Crown SASS did the latter.
The SASS stereo microphone consists of an ABS plastic baffle with two PZM capsules mounted to it. Between the capsules is a foam barrier, and this is what disintegrated on my mic. The foam lining of the mic’s hard case also disintegrated. Replacement foam is unobtainable, so I decided to remanufacture all new foam pieces.
Note: the methods I used to design and shape the foam reflect the unique equipment and resources I have on hand, as well as my own design and fabrication style. These methods may be inappropriate for some people, but they can be easily adapted to suit your own style and resources.
The first thing I did was identify the type of foam I would need for the new pieces so I could order it online. I used a firm called “The Foam Factory” (no relation, just a customer) and ordered a 72” x 40” ‘half sheet’ of 1.5” Solid Charcoal Regular Foam. Delivered, it cost about USD 40.
Next, I removed the old foam from the mic (it just pulled out), cleaned everything, took careful measurements of the mic, then created a 3D model of the mic in SolidWorks.
To get used to the fabrication process, I decided to do the less critical case foam first. Since my sheets were 1.5” thick and the case depth was about 9.75”, I calculated that I would need six 1.5” layers plus one 0.75” layer total. I used SolidWorks to model each layer using the mic model to determine the shape of each layer’s cutout. I also made additional cutouts for the various mic accessories like the original case had.
When I was happy with the design, I created drawings of the layers to act as templates, then printed them to PDF at 1:1 scale. Acrobat is pretty good at maintaining dimensionally-correct drawings that print accurately to size. Because most of the cutout features were the same for the different layers, I ended up needing only two templates for the case.
I then printed the PDFs on letter-sized sheets and pieced them together to make larger paper templates true to the size of the case. Using double-stick tape, I fastened these paper templates to sheets of 1mm aluminum that had been cut slightly oversize of the inner dimensions of the case. The oversizing was to ensure the foam fit snugly inside the case. I then transferred the paper template pattern to the sheet metal by center punching the cutout vertices and connecting the center punch marks with scribe lines. I cut out the features using a scroll saw, and filed the edges smooth.
I then made a hot wire foam cutter. It used 0.5mm nichrome wire I got from Amazon. I happened to have a granite work table made from a kitchen countertop leftover from a neighbor’s remodel. That countertop also happened to have a 1/2” hole bored through near one edge (I told you I had unique resources.) But any smooth surface would have worked. I was able to machine a wire guide by turning and drilling an appropriate aluminum sleeve on the lathe. The guide’s OD was a light press fit to the hole in the table, and guide’s center hole was drilled to 0.6mm to be a close fit to the wire. The closer the fit between the guide hole and the wire, the better the guide will be able to keep the wire precisely in place.
Using a thick wood block as a spacer, I clamped a wooden 2x2 to the tabletop so that its tip was positioned exactly over the wire guide. I put a nail in the end of the 2x2, fastened the end of a piece of nichrome wire to it, passed the wire over the 2x2's end, and threaded it down through the guide to the area beneath the table. I then fastened a 5 pound sash weight to the other end of the wire to keep it taut. The 2x2 was long enough to give plenty of space for the template to maneuver between the wire and the spacer block.
Using a pair of alligator clip leads, I connected the upper and lower ends of the wire to a bench power supply. I set the supply to constant-current mode, adjusted the voltage to maximum, and set the current to about 4 amps. The nice thing about using a constant current is that both the length of wire and the resistance of the clip leads don’t matter. The wire reaches a good cutting temperature after a few seconds. Different foams will probably need different temperatures, so experimentation is warranted. But I found that the exact temperature was not very critical.
After making some practice cuts with the hot wire to dial in the current, I attached the metal templates to the foam using double-stick tape. I found Scotch 410M tape worked very well because it released easily. It was then a simple task of guiding the wire around the outer edge of the template, and around the perimeter of each cutout. Rinse and repeat for each layer. The photo below shows the hot wire cutter and one of the case templates before it was taped to the foam.
As a last step, I used 3M Super 77 spray adhesive to glue the layers together. There are other adhesives that probably outgas less, but this is what I had at hand. I found it necessary to make some cardboard pieces to cover the cutouts to prevent the adhesive from getting down inside them. I was careful not to get any adhesive on any of the surfaces that would be visible in the final product. I then pushed the foam into the case, and that part was complete.
I then started on the mic’s acoustic baffle. It was made from a stack of layers too, but the layers had to first be brought to the desired thickness; 1.5” was too thick for several of them. I sized the sheet thickness by first cutting some spacers to the desired thickness. I then clamped nichrome wire to the table atop these spacers, which set the wire to the correct height evenly across the table. I dropped one wire end over the table edge and hung the weight on it to tension the wire as before. The nichrome expands when it gets hot, so it will sag if the tension is not maintained somehow. You could use a spring for this, but I found that using a weight was much easier. BTW, if you plan ahead, you can order the foam in the thicknesses you need for each layer. That would save you from having to size them yourself.
I cut all of the pieces for the baffle using the same aluminum template technique, then glued them with Super 77. The final assembly was fine and would work perfectly, but the original foam piece had nice rounded edges that my new replacement did not. That made my piece look unprofessional. So I made an edge radius cutting tool, again using a hot wire cutter.
The radius cutting was probably the hardest part of the project. The idea was to make a fixture to hold a nichrome wire that has been pre-bent to the desired radius that the foam could then be slid past. This is like a woodworking router table, but with a hot wire cutter. I made the fixture from wood and sheet metal and it worked okay, but if I do it again, I think I would find a better way to do this. The fixture is shown below and has a vertical aluminum plate and a horizontal aluminum plate. The radius wire is fastened to each plate with flat head screws. The two aluminum pieces were energized by the power supply, then the foam was slid past the wire. Kapton tape allowed the foam to slide better.
After the edges were radiused, I installed the foam into them mic. I was very satisfied with the final result.
The SASS stereo microphone consists of an ABS plastic baffle with two PZM capsules mounted to it. Between the capsules is a foam barrier, and this is what disintegrated on my mic. The foam lining of the mic’s hard case also disintegrated. Replacement foam is unobtainable, so I decided to remanufacture all new foam pieces.
Note: the methods I used to design and shape the foam reflect the unique equipment and resources I have on hand, as well as my own design and fabrication style. These methods may be inappropriate for some people, but they can be easily adapted to suit your own style and resources.
The first thing I did was identify the type of foam I would need for the new pieces so I could order it online. I used a firm called “The Foam Factory” (no relation, just a customer) and ordered a 72” x 40” ‘half sheet’ of 1.5” Solid Charcoal Regular Foam. Delivered, it cost about USD 40.
Next, I removed the old foam from the mic (it just pulled out), cleaned everything, took careful measurements of the mic, then created a 3D model of the mic in SolidWorks.
To get used to the fabrication process, I decided to do the less critical case foam first. Since my sheets were 1.5” thick and the case depth was about 9.75”, I calculated that I would need six 1.5” layers plus one 0.75” layer total. I used SolidWorks to model each layer using the mic model to determine the shape of each layer’s cutout. I also made additional cutouts for the various mic accessories like the original case had.
When I was happy with the design, I created drawings of the layers to act as templates, then printed them to PDF at 1:1 scale. Acrobat is pretty good at maintaining dimensionally-correct drawings that print accurately to size. Because most of the cutout features were the same for the different layers, I ended up needing only two templates for the case.
I then printed the PDFs on letter-sized sheets and pieced them together to make larger paper templates true to the size of the case. Using double-stick tape, I fastened these paper templates to sheets of 1mm aluminum that had been cut slightly oversize of the inner dimensions of the case. The oversizing was to ensure the foam fit snugly inside the case. I then transferred the paper template pattern to the sheet metal by center punching the cutout vertices and connecting the center punch marks with scribe lines. I cut out the features using a scroll saw, and filed the edges smooth.
I then made a hot wire foam cutter. It used 0.5mm nichrome wire I got from Amazon. I happened to have a granite work table made from a kitchen countertop leftover from a neighbor’s remodel. That countertop also happened to have a 1/2” hole bored through near one edge (I told you I had unique resources.) But any smooth surface would have worked. I was able to machine a wire guide by turning and drilling an appropriate aluminum sleeve on the lathe. The guide’s OD was a light press fit to the hole in the table, and guide’s center hole was drilled to 0.6mm to be a close fit to the wire. The closer the fit between the guide hole and the wire, the better the guide will be able to keep the wire precisely in place.
Using a thick wood block as a spacer, I clamped a wooden 2x2 to the tabletop so that its tip was positioned exactly over the wire guide. I put a nail in the end of the 2x2, fastened the end of a piece of nichrome wire to it, passed the wire over the 2x2's end, and threaded it down through the guide to the area beneath the table. I then fastened a 5 pound sash weight to the other end of the wire to keep it taut. The 2x2 was long enough to give plenty of space for the template to maneuver between the wire and the spacer block.
Using a pair of alligator clip leads, I connected the upper and lower ends of the wire to a bench power supply. I set the supply to constant-current mode, adjusted the voltage to maximum, and set the current to about 4 amps. The nice thing about using a constant current is that both the length of wire and the resistance of the clip leads don’t matter. The wire reaches a good cutting temperature after a few seconds. Different foams will probably need different temperatures, so experimentation is warranted. But I found that the exact temperature was not very critical.
After making some practice cuts with the hot wire to dial in the current, I attached the metal templates to the foam using double-stick tape. I found Scotch 410M tape worked very well because it released easily. It was then a simple task of guiding the wire around the outer edge of the template, and around the perimeter of each cutout. Rinse and repeat for each layer. The photo below shows the hot wire cutter and one of the case templates before it was taped to the foam.
As a last step, I used 3M Super 77 spray adhesive to glue the layers together. There are other adhesives that probably outgas less, but this is what I had at hand. I found it necessary to make some cardboard pieces to cover the cutouts to prevent the adhesive from getting down inside them. I was careful not to get any adhesive on any of the surfaces that would be visible in the final product. I then pushed the foam into the case, and that part was complete.
I then started on the mic’s acoustic baffle. It was made from a stack of layers too, but the layers had to first be brought to the desired thickness; 1.5” was too thick for several of them. I sized the sheet thickness by first cutting some spacers to the desired thickness. I then clamped nichrome wire to the table atop these spacers, which set the wire to the correct height evenly across the table. I dropped one wire end over the table edge and hung the weight on it to tension the wire as before. The nichrome expands when it gets hot, so it will sag if the tension is not maintained somehow. You could use a spring for this, but I found that using a weight was much easier. BTW, if you plan ahead, you can order the foam in the thicknesses you need for each layer. That would save you from having to size them yourself.
I cut all of the pieces for the baffle using the same aluminum template technique, then glued them with Super 77. The final assembly was fine and would work perfectly, but the original foam piece had nice rounded edges that my new replacement did not. That made my piece look unprofessional. So I made an edge radius cutting tool, again using a hot wire cutter.
The radius cutting was probably the hardest part of the project. The idea was to make a fixture to hold a nichrome wire that has been pre-bent to the desired radius that the foam could then be slid past. This is like a woodworking router table, but with a hot wire cutter. I made the fixture from wood and sheet metal and it worked okay, but if I do it again, I think I would find a better way to do this. The fixture is shown below and has a vertical aluminum plate and a horizontal aluminum plate. The radius wire is fastened to each plate with flat head screws. The two aluminum pieces were energized by the power supply, then the foam was slid past the wire. Kapton tape allowed the foam to slide better.
After the edges were radiused, I installed the foam into them mic. I was very satisfied with the final result.
