View Full Version : Need Help! Constant Power W/ Variable Wire Length

07-05-2010, 08:42 AM
Hi Folks,
I have an interesting problem to solve, and I am not sure how to do it. I am building a large Foam Hotwire, ~7' X and 4.5' Y travel with about 6' wide wire length when both wire trucks are at the same XY location. What I am trying to cut with this 4-axis cutter are wing cores and molds. The wings could be highly swept or have a strong taper. I believe this drives me to needing a fairly variable wire length. In an extreme case, I could see where having 1 truck all the way to the left and top, and one all the way to the right and bottom of their travels, making the long diagonal of the cube.

I intend on making a pulley that will hold the extra wire along with a constant force spring (Like a tape measure) to keep wire tension. I will use a tensioner system to keep the wire in contact with the electrical leads to keep the part of the wire in the cutting zone live, while the pulley will be electrically dead.

What I am worried about is, if the wire can grow to almost twice its length over the course of travel, the wire resistance will increase, and consequently, the heat generated in the wire will be changing. I believe this will cause un-even cutting sizes as the temperature changes.

Does anyone have a simple solution to this problem? I am thinking some sort of resistance measuring and voltage system that will regulate the power to keep constant temperature.

Am I just way off base and making this too complicated? Has anyone ever desired to have the ability to stretch the wire along the diagonal like that, or is that found to not be important?

Thank you for your advice and input?


07-09-2010, 12:49 PM
Are you still looking for a solution? This isn't that hard a problem really.

07-30-2010, 07:58 PM
post the solution for the other nubs here then! I'd be interested myself

07-31-2010, 03:46 PM
The PC controlling the machine can work out the "heated" length of the wire from XYZ using pythag theorem.

Then if the wire is 20% longer than the minimum length just feed 20% more power to it at that point in time.

Like Zenock said it's pretty straightforward, provided the PC software has this feature.

If it doesn't, you could put a sensor on your take up spool so it knows the length of "heated" wire, then use an external device, either software or hardware that feeds an adjusted amount of power into the heated section of the wire.

(edit)I just thought of this, a third (mechanical) option would be to keep the wire always at its max (diagonal) length like some type of bow-string. The mechanicals could be designed to move the "bow" and the wire power could be set to a fixed power as its heated length never changes.

07-31-2010, 11:39 PM
cool thanks!

08-26-2010, 10:37 AM
Or feed it with a constant current supply.

08-28-2010, 07:08 AM
Constant current won't equal constant power because the foam cools the wire and lowers its resistance. The result with constant current is that when it goes into the foam the power drops a lot and you will get a bad cut.

Regulating power itself means that when it enters the foam, the current is increased so it still delivers the same heating power.

09-11-2010, 09:20 PM
Hi Zenock,
What is the solution you are implying with that last sentence? My current plans are still a 4-axis system, with a wire extension mechanism (Probably pulley system and hanging weights on each end of the wire for tension with extra wire draped over vertical axis mechanism) and I keep seeing some mention of certain boards that modulate wire heat. Can I drive this type of board with mach 3, or better yet, how would a system like that work, and what components have others had experience with? Thanks!


Are you still looking for a solution? This isn't that hard a problem really.

09-12-2010, 08:57 AM
Here's a concept of where I am trying to go with my 4-axis system. I want to cut conical fuselage cores, and highly tapered wings. This is where the variable wire length comes in, when the cone angle of the fuselage is high, like near the nose cone.

Scott Lister
10-18-2010, 12:54 PM
Most software has an entry for 2 different wire kerfs when cutting tapers. You will need too calibrate you kerf for the current and speed you are using.


11-25-2010, 02:59 AM
Hi All
I am new to CNC foam cutting, the Machine is up and running and now I want to start cutting R.C. aircraft taper fuselages, where do I go from here? I am a firm believer in KISS (keep it simple stupid) I have no CAD experience, how should I start learning this, which program is the most user friendly to learn on and still functional enough to reach my goal.
Kind Regards