Machining XPS Foam Patterns

I remarked in previous threads regarding Owens Corning Foamular 150 being IMO the best trade of low density, strength, machinability, and ability to achieve fine surface finishes with common abrasive paper…..amazingly so in fact. It’s also very economical. Foamular 150 is 1.3 lbs/ft3 and available in the US home store retailers. Here is a little more detail on some machining tricks. I have an overarm router that I made a number of years ago and it remains as one of my favorite shop machines and builds I've ever done. Though most do not have an overarm router, much of this can be done on a conventional table router. There are also many pin router conversions and motorized “Router Lifts” that provide an active Z-axis so if you have a table router, you can easily convert. Just search “Inverted Pin Router” and “Router Lift” or look in the popular woodworking magazines and supply companies.


I use carbide woodworking bits at typical wood working or higher speeds. I'm sure high speed steel would be just fine as well. XPS is homogenous material and very free machining. With the right feed rate it machines very cleanly. It can be secured to jigs and fixtures with coarse drywall screws and/or with packing or two-sided tape.


After machining, the parts can be glued together and sanded. I find starting with just few swipes worn out 220 grit makes a big improvement and the foam can actually be worked to 300-400 grit. Wax can be used for filleting and details. Parafin waxes can be thinned or many just buy a toilet wax ring seal which is workable at room temperature. Afterward a single or double coat of thinned dryall mud will produce the same quality finish achieved on the foam pattern.


Here’s a more detailed description of how the tubular foam patterns used in the link below were tooled and made.

http://forums.thehomefoundry.org/index.php?threads/tubular-lost-foam.11/


First a template is made, attached it to some MDF, and a bearing guided ½” diameter router bit is used to cut a groove down the centerline of the tube. Then the pattern is flipped over and did the same only mirror image.





Another piece of MDF is attached and a 1 3/8” diameter round nose bit installed in my over arm router, centered over the guide pin. The depth of cut is set, the guide pin rides in the slot, and cuts the cavity that can hold a half round section of the tube, then the same was done in the mirror imaged part.





A pattern was made using the same tube centerline, attached with a few coarse thread drywall screws, and a bearing guided round over bit was used in the overarm positioned to cut a solid half tube elbow. Then the pattern was flipped over to make the mirror image part.





Now, these solid half tube elbows fit snuggly in the MDF holding/guide jigs in the previous pictures. For good measure the perimeter was secured with clear packing tape.







Next, it’s back to the overarm router. This time a 7/8 diameter round nose bit was installed, set to depth, and the guide pin guides this bit down the centerline of each foam pattern making uniform ¼” wall half tube elbows. The OD round over bit is also shown.







On the first part, the ½-tube elbows were taped together, cut to length on a band saw, touched up with a belt sander, and then a spindle sander was used to cut the coped intersecting joint. A forstner bit was used with some speed to cut the intersecting hole. A foam brush was used to apply shellac to the joints and the pieces taped together with small pieces of clear packing tape. On the subsequent four patterns, I learned that if I applied several layers of shellac, I could dry each coat with the heat gun in 15-20 seconds, then the second or third coat activated the previous coats and acted like contact cement so I could just stick them together, but some places still needed a little tape. When the joints are dry they are quite sturdy.







For the hose barb bead, I had all sorts of elaborate thoughts about how to machine them integral to the tube and then came to my senses. I used the smallest round over bit I had and cut a bull nose on each side of a small foam plank and then cut the half round bead off with a straight edge and razor to make beading stock. I wrapped them around my finger to pre-form them, wrapped them around the tube, cut them in place, then stuck them on with shellac and a small piece of tape across the joint. You want the shellac to be a little wet with these so you can move them around a bit and position them accurately after they are applied.





For the thermostat housings, I just drilled holes with forstner bits, shaped the perimeter on the belt sander, and used a bearing guided round over bit. I think I had four hours into making the jigs and first foam pattern and then 2 hours total for the other 4 foam pattern copies. Subsequently I refined the design of the thermostat housing and developed a few additional jigs to assist.

Best,
Kelly

 

Here are the jigs and a short video for the mock engine block described in the link below

http://forums.thehomefoundry.org/in...ted-closed-loose-sand-coring-in-lost-foam.12/








Best,
Kelly

 

…..and here are the jigs for the finned cylinder head discussed here


http://forums.thehomefoundry.org/index.php?threads/high-aspect-ratio-features-and-deep-draws.13/


I set it up to use that finger joint bit in the foreground so I could cut three fingers at a time. The finger joint bit cuts tapered fins and I scaled the piece down to match the fin proportions. The bore is now 1 ¼”, the cylinder wall a little over an 1/8”, and the annular fins 3/8” deep tapering from 1/8”at the root to 1/16” at the tip. I can make three foamies in 15 minutes from 2”thick foam, boring a hole with a forstener bit, and mounting them on the jig.




Here’s a few of the first batch. I made a Siamese twin just for grins.





Here’s a close up as machined.





Here's a short video of the router jig for the finned cylinders in action.





There are a half dozen more videos of machining a similar but more detailed foam pattern for a chainsaw cylinder and head here.

http://forums.thehomefoundry.org/index.php?threads/chainsaw-cylinder-head.268/#post-4714

Best,
Kelly

 

Working with foam has been fun but can make a bit of a mess….and it even gets worse than the picture below.



So with the foam work I’ve done and plan to do I decided to get serious about dust control. In addition to the subject of this thread, this involved a hot wire build. The hot wire is to rough out foam pattern blanks and resize foam board with no dust. That build is here:

http://forums.thehomefoundry.org/index.php?threads/hot-wire-bild.132/


This thread is about a dust collection system for my overarm router and some upgrades to that machine. So here’s the result so far. I’m yet to build the dust hood and will be adding that in the near future. For the dust collection system, I made a cart for my dust collector that stacked the bag over the vacuum blower. The system as purchased had the bag house mounted beside the blower on a cart. This cut the footprint in half. I wont go into that too much other than it’s on wheels so I can wheel it to other machines. It centrally connects to a collection plenum mounted on the over arm router via a 5” flexible duct.



I also added another live router head on the left mast. There was a power molding feeder hanging there but since I hadn’t used it for 5 years I figured it was safe to remove it. I now have four live heads, two above and two below the table. The main advantage of this is I will be able to do between two and four set ups and this will speed foam pattern cutting.


So here’s the dust collection plenum. The table is 58” wide. My sheet metal shear and brake is only 36” wide so I had to do it in sections and weld it together. It has vanes that equally partition the 5” inlet across the table.





Here’s the plenum with the lid installed. It’s hard to see but there is an L-shaped dam stored on the center of the plenum. It can be installed in front of the plenum inlet on the side of the table not being used which intensifies the air velocity in the remaining active table surface. It is magnetically held in place.



It works well but there is still some airborne dust so I’m going to make a hood which should really keep the vast majority of dust confined within the work area and make clean up very short and easy task.

My 30 year collection of router bits. There are more specialty joinery and cabinetry bits buried in the drawers.



Stay tuned.

Best,
Kelly

 

Finished up the dust hood for my over arm routers. The top frame mounts on the two machine masts and rotate upward and out of the way. The rear film wall of the hood attaches at the bottom with magnetic strip so it can just be pulled away to access the rear of the machine. The sides are acrylic, hinged, and also secured with magnetic tape for easy removal.



Best,
Kelly

 

I added a few more accessories to my overarm router this weekend. Using the machine to make foam patterns often involves frequent starts and stops to make set up adjustments or change cutting bits. I frequently mount and unmount foam patterns from templates and since adding the dust hood, the dust collector is constantly running. In use, I typically sit on a high stool in front of the machine and found myself getting up and down to turn the dust collector on and off. The Dust Collector has a mechanical latching on/off switch. So I made an automatic starter that senses current draw from a running router and then energizes a contactor to start the dust collector.

There are two separate circuits; a 120VAC for the routers and circuit and 240VAC for the dust collector. The 120VAC router circuit is just a pass through so current can be sensed so it is essentially just a four-outlet terminal strip. The 240VAC for the dust collector is also just a pass through to incorporate the contactor so the dust collector can be started from sensed current. In practice, I just plug the routers and the dust collector into the autostart box and this allows me to unplug and move the dust collector to other equipment. I also added a remote switch to the autostart box that that allows the dust collector to be turned on without a router running for cleanup. The power switch on the autostart box only energizes the sensing circuit and power for the contactor coil.

There is small 24vdc switching power supply I bought on eBay ($7) along with the current sensing module ($12). I used an extra solid state relay I had to power the contactor coil which was a spare left over from my electric furnace build. The PS and the coil use 120VAC input. I had originally used one leg of the 240VAC circuit to power the contactor coil but when the dust collector motor started either the inductive load or corresponding voltage drop cause the contactor to flutter…..so I re-wired the control circuit to tap the 120VAC router circuit and this solved the problem.






When doing pattern guided work, I like to keep both hands on the templates/patterns. I already had foot pedal switches for the overarm router heads but not the under-table heads unless I crawled around back to switch which router was plugged into the foot switch. Many years ago when I made the overarm routers, I made satellite foot switch housings (castings of course!) and decided it was time to put one together for the under-table router heads. It can sit on the ground or be parked between the other pedals for use. I toggle the power switch on the router heads to the off position when not in use or changing bits to avoid inadvertent starting from the pedals. Together these two features had the added benefit of organizing the previous rat's nest of power cords.





Best,
Kelly

 

Thanks -3. The dust hood, autostart, and foot switch will definitely make using the machine more enjoyable and productive but it did take longer than I envisioned. It's one of those type of projects that after completion you're happy to have done it, but may have thought twice before you started if you knew what is was going to take to complete it.

Best,
Kelly