[Courtesy of Ed Berris (ed "at" skykingrcproducts.com), May 1999]
Like many other soaring junkies, some of the members of the Minnesota Radio Controlled Soaring Society wanted a foam cutter similar to the well respected FeatherCut® system but did not want to invest the funds to purchase a commercial unit.
After researching what was available we decided on a cutter that was compact and easily stored or transported. That ruled out designs such as the Bill Evans "Man Alone Cutter" (the originator of hands off cutting machines) which required the construction of a cutting table and a drop arm that was attached permanently to the table. While the Evans design is in some ways superior to ours for production work it was more complicated and took up more space than most of us wanted to invest in.
The MRCSS System
Click on each of these small thumbnails to see drawings and pictures of the cutter design and how to cut constant and tapered chord panels.
The heart of our system is a readily available aluminum extrusion sold under the name 80/20. It is available in any length at a very nominal cost. A 60" length, called for to build our cutter, costs about $20.00 delivered to your door. That includes the cost of the extrusion, the cutting fee and shipping charges. Naturally, if a club ordered several pieces the cost per piece would be less.
80/20 extrusions come in a few different sizes. We selected the one inch square material sold as 10/10 extrusion. This extrusion has track slots running along the length of all four sides. These slots allow the rollers to slide to any position for easy adjustment of the roller positions.
Our system uses four nylon rollers with axle bushings and bolts which may be purchased from MRCSS for a cost of $5.00 including mailing costs anywhere in the lower 48 states OR you can simply go to the hardware store and find something similar in the department that sells screen door repair parts. You'll also need four 1/4" wing nuts for this part of the assembly.
There are four aluminum parts cut from 1/16" thick x 1" bar stock. Each of the four pieces is 4" long. Two of these parts are brackets to clamp the extrusion to the edge of your work bench or cutting table. The other two are used to suspend the swing arm which will pull the cutting wire.
The swing arm can be made from wood, steel or aluminum. We chose either steel or aluminum. A 60" piece of 1/4" thick by 1/2" stock will do nicely. If you decide to use wood you will need to increase the size of the stock. I cannot make a suggestion, however because I have not used wood for this part.
You'll need some small alligator clips and nylon string (we used winch line but you could use Dacron fishing line as well) which is attached to the bow cutting wire and the swing arm to pull the cutting wire.
Lastly you'll need some spring clips, the kind used to hold a stack of paper together. These spring clips are used to attach the pull strings to the swing arm. I'm sure some inventive sole can dream up a more elegant device but this does work and it is inexpensive. You'll see them in the photos of the cutter and immediately recognize them better than I can describe in this written description.
You'll also want a weight to attach to the swing arm. This weight helps provide the pulling power to draw the cutting wire through your foam core. I made one by cutting a length of the cardboard tube found on the inside of UltraCote. I placed a large eye bolt in the center and poured lead shot and epoxy around it. I opened the eye bolt to create a hook which is hung from one of the spring clips attached to the swing arm.
You'll need to make at least one bow and perhaps more. A 32" length is a good starting size. We made ours from 1" square aluminum tube which was purchased at Home Depot. You'll also need some 1/4" piano wire for the bow legs, a small tail wheel and wire tail wheel assembly. Sullivan or Dubro make these but you could also bend your own. The tail wheel is used to support the bow as it makes it's cutting pass.
Some people prefer to suspend their bow from the ceiling using a thin bungee material. I have found the tail wheel works just fine contrary to the rantings of some who insist that suspending the bow is the only Correct way to cut cores. But, If you will be cutting cores that have a root dimension that is deeper than the throat depth of your bow, suspending the bow will allow you to cut most any size root dimension.
In conclusion...... We have over a dozen of these cutters in our club. This system has always produced exceptional quality cores. It is easy to build and use, folds up for storage or transport to another site and was inexpensive compared to the commercial cutters. It is also very sturdy. If you choose to build one and need additional information I'll do my best to help.
Parts List and Sources
10/10 extrusion. 60" pieces. Contact 80/20 corporation for the name and phone number of a local distributor. 80/20, Inc. - 1701-T S. 400 E. - Columbia City, IN 46725- 8753 USA - Tel: 219-248-8030 Fax: 219-248-8029
4 each 1/16 x 1" x 4" aluminum bar stock for (2) clamping arms and (2) to suspend the swing arm from the 10/10 extrusion. You'll need to drill the clamping arms on one end. Measure 1/2" from the end and drill a hole in the center of this line. Repeat this on the two pieces that will be used to mount the swing arm. On these pieces you'll need to drill both ends.
Nylon Rollers - you'll need four 1" rollers and the correct size machine screws to mount them to the track system. These are available from most building centers.
Binder Clips - get two medium sized clips from any office supply store.
Alligator Clips - get four small clips. Two will be used to attach the power supply leads to your bow and the other two will be used to attach the pull strings to your bow.
A word about using alligator clips for your power leads. The best connection IS NOT made with alligator clips. A hard wire connection to your bow cutting wire is the best, however, the alligator clip attachment is more convenient and in actual practice it has not caused any problems. Naturally, if a clip came loose during the cutting process you would lose the core and have to start over. You decide which method you prefer now that you know the down side of using alligator clips. As for me......I'll keep using the clips.
Power Supply Requirements
The Soaring bulletin board has been filled with comments about building a power supply. Our club built about a dozen supplies which have worked out quite well and like the foam cutting tool it was simple and inexpensive.
The heart of the system is a 28 Volt, 5 amp transformer and Light Dimmer Switch. Yes, I do know that a dimmer switch is not as desirable as a Variac but it does work. If you can afford the Variac I would suggest this combination. The Variac provides much finer resolution in setting cutting temperatures. Frankly, I use both set ups and am happy with my dimmer power supply.
Since there are already several diagrams and building instructions for a power supply on the net I won't waste the bandwidth.
If you cannot find a source for transformers or variacs in your area there is a good surplus electric source in Minneapolis called AEI Electronics. (612) 338-1016. I believe they have a minimum order amount of $25.00. The transformers are under $15.00 but if you order wire to connect your bow to your power supply and perhaps some other parts you will easily make up the minimum. I use "Test Lead" wire to connect my bow to my power supply. It is very flexible and has a tough rubber insulation. You might also want to order some banana jacks and plugs while you are at it. They make for a nice connection at your power supply.
The absolute best wire for cutting cores is called Rene. It can be difficult to find and is expensive. If you have a company that makes white bead foam in your area, call them. They use this wire on their foam cutters. It is a springy wire that is extremely tough, can be heated red hot without breaking and produces beautiful cuts. If you can get twenty or thirty feet it will last a long, long time.
Rene is available online from Sky King RC Products.
Realizing that you might not be able to find a source for Rene, another good wire is Aircraft Safety Wire. You can get various gauges but .020 works nicely. This is a stainless steel wire like fishing leader but comes in spools at a much more reasonable cost. A wholesale fastener supply company should be able to supply you. If you cannot find any in your area, try one of the smaller airport hangers. If they don't have it, they know where to buy it.
Nychrome is not worth wasting your time with.