Most of the information I've ever seen on foam cutting is guessed or summized from past experience. For folks who do a lot of foam cutting, they have figured out what works. Unfortunately, the casual or infrequent cutter will be left to recreate that information for themselves or will have to be satisfied with less than perfect results.
To take some of the guess work out of setting up a new bow and cut parameters, I created an Excel spreadsheet to calculate the parameters. This is a simple application of a catenary solution for cutting wire sag. A catenary is a fancy term for the droop you see in overhead power lines and suspension bridge cables. The spreadsheet calculates the cutting wire sag in the foam using the bow pull force and cutting wire tension. One thing that I didn't incorporate into the spreadsheet is a calculation of how much airfoil error is created during a cut from the sag. This is directly related to how much curvature the airfoil surface has. I will let some of the other smart guys on the list (Drela, Garton, etc) figure out how to quantify the curvature for a particular airfoil and the relationship to cutting error. A lot of bow designs lack the simple ability to measure wire tension. Heck, most bows can't even maintain a consistent tension in the cutting wire going from power off to power on! I've included in the spreadsheet a drawing of a simple bow that includes the ability to maintain fairly constant wire tension as well as the ability to measure the wire tension. The spreadsheet is only a rudementary tool, but it should be helpful in setting up a foam cutting session. There is still some ramping up that will be necessary to fine tune cutting wire current, pull force, and bow tension, but this spreadsheet does provide an indication of what they can tweak to produce better cuts. Cutting speed is the focus of the original post. I think this tool should make it possible to move away from cutting speed to more accurate and meaningful parameters. I have forwarded the spreadsheet to the Charles River RC website for posting. Hopefully it will be available in short order. The spreadsheet includes the cutting calculator, some cutting tips, and a drawing of a recommended bow design. Drop me a line if you just can't wait to get the spreadsheet off the Charles River website. Here's to more fun! Martin Brungard Tallahassee, FL "Meandering to a different drummer" _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
