Thread: Question Hot Wire Foam cutter power supply View Single Post
Sep 06, 2012, 08:19 AM
low tech high tech
Southern Vermont
Joined Feb 2007
3,029 Posts
Quote:
 Originally Posted by richie967 I downloaded your Spreadsheet calculator and put in these settings: 1200mm desired wire length 12v (power supply does 0-15v, but seems people use 12 so went with this.) 3 amps. 0.5 watts per inch (went in the middle, the guide in the spreadhseet says that 0.4-0.6 is desireable) It says the best wire diameter to use for this is 0.52mm, but the closed i could find is 0.6mm, will that be okay? The Calculator says that I need a wire with a resistivity of 700 - how do i find out if the wire I have is that spec? Rich
Okay just a few notes on using the calculator.

In General: you enter figures in the white blanks, and read calculated results in the yellow blanks. Click anywhere in the spreadsheet to recalculate results after changing something.

1.) Always put in the actual heated cutting length of your wire, not the overall bow length. You want to put in 1 meter (40 inches) if that is your heated length, not 1200 mm. Unless that is what the actual heated length is. The heated length is the distance between electrical contacts.

2.) Put in the rated maximum nominal voltage of your power supply unless you have some specific reason not to. It's okay that you put in 12 V but it will be better to put in 15 V. (note: voltages over 35 are dangerous never use any power supply with higher voltage outputs for wire cutting.)

3.) Put in the maximum rated current available from your power supply. In this case it is 15 amps. The calculator won't force you to use all 15 amps, it will simply check to make sure that the wire you choose won't exceed the rated output of your power supply. That's the only reason it asks for this figure.

4.) I usually cut at .4 to .6 watts per inch, but personal preferences vary. People have reported cutting at 1.5 watts per inch and running the wire at red heat. Personally I can't imagine this. I don't like the idea of running that hot, and their wire probably doesn't last too long! But to give yourself some adjustment range, you might choose say 0.6.

5.) You didn't mention the adjustment figure called "Setting" on the spreadsheet. What this means is that for the cutting heat you have chosen, where on your power supply's settings should this "normal" cutting heat be? In the middle, or closer to the maximum setting?

In other words, if you have a knob to adjust your supply, at what position of the knob do you want your "normal" cutting heat (chosen in the last step) to be? If you are concerned about safety -- put this fairly high, like say 80% or more. By setting this high, you can't dial in much more heat by accident than the usual cutting heat.

Fixed power supplies can't adjust so they should set this at 100% unless there is a specific reason not to.

5.) Resistivity is something you fill in. There is no suggested single figure for this. It merely means how much resistance a material has for its thickness in wire form. In other words an insulator, like plastic, has more resistivity than a metal like copper. And stainless steel has more resistivity than regular steel. There is a short table of resistivity included on the spreadsheet for reference purposes (the grayed area). If you are using stainless steel wire you enter 700. But that is just for stainless. All of these figures were derrived from actual user input, So they are practical figures. Unfortunately we don't have a value for nichrome in the table yet. But we can approximate that from your specs.

Things like "guitar string" are difficult to pin down, because strings may be made from different materials. I think we got one data point for guitar string, and that one appeared to rank somewhere around steel's resistivity (low) , rather than stainless steel (higher). Anyway, I'll give you an estimate for nichrome shortly, from your data of the wire you bought and you can try that in the calculator.

phew, enough for one post........

Below is a picture of the spreadsheet as an example. It does not have your figures in it. It's just an example of what it looks like for people who are trying to follow along here.

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Last edited by vtdiy; Sep 06, 2012 at 08:45 AM.