Wire expansion calculation

[tltaylor]

How much strech can I expect out of 8 foot of nichrome wire when it gets hot? I want to make sure I have enough spring to keep the wire tight. How do you calculate the size of electrical transformer needed to heat this wire as well? Sorry for the basic questions. I am running some experiments for a custom application at work in the next few days. I've got an 8' bow made that will ride on some guides. I've got a transformer coming in friday, but can't remember how big it was. Thanks for the help.

trent

[NC Cams]

Depends on following:

1. Exact resistance per foot,

2. Amount of current being drawn,

3. Is current PWM'd or linearly applied,

4. Amount of temp rise verus how much is lost via heat transfer in cutting process versus regained in free air.

5. Coefficent of thermal expansion of the material.

1, 2, 3 affect 4. Once you have 4, you can do some math after you confirm 5.

Don't have, can't find what's needed???

Resort to cut & try or try to find someone who's done it already.

[martinw]

Originally Posted by NC Cams Don't have, can't find what's needed??? Resort to cut & try or try to find someone who's done it already.

Dear NC Cams,

You are right on the button.

A long time ago we had to cut slabs of 4 inch thick Styrofoam into 12" radius curves. We made a bow with a six inch throat and found a ten amp transformer. I think the nichrome wire was about 1/32 " diameter or a bit more ( 1mm ). It was a total disaster.... the wire stretched horribly and the cuts were totally useless. At the time, we concluded that the temperature of the wire, when cutting, was vastly more than the temperature it would be in free air because the wire was effectively embedded in an extremely good insulator, Styrofoam. This would lead to massive elongation of the wire due to its coefficient of thermal expansion, and hence loss of wire tension. It seems glaringly obvious in retrospect, but we were new to this.

We never tried hot wire cutting again, so we are obviously not in any way experts. All I can say is that,if you are cutting a thick insulating material, you should "suck and see". Eight foot is an awfully long distance if the coefficient of thermal expansion sees a really big temperature rise.

This is all obvious with the benefit of hindsight!

My best guess would be that you might need a wire the diameter of a rebar, and a nuclear power station at your disposal.

IMHO

Best wishes

Martin

[vulcom1]

If you are cutting using a bow or straight wire you will have to experiment with your power and tension. Ten amps will probably be to much and I am betting 5 or 6 will be closer. As for stretch you have to keep it tight so quite a bit of tension is better unless you break the line. If cutting cnc when set right your wire will melt the foam before touching it. If cutting by hand and the wire drags you are going to fast. Take some scraps and play to get your settings. Once you have them write it down for future referance.
When done a couple of projects I want to build my own cutter. I have been doing it by hand since the 70's.
John

[NC Cams]

Something to consider in solving the problem:

Maintaining a constant current in a wire of changing temperature is not something that is easy to do with a simple rheostat - you need something more sophisticated.

I suspect that wire resistance will change as temp changes as will the wire length. If this is true, it becomes even harder to keep current constant as temp and resistance changes.

Then there is the issue of constant current. You can generate constant current with a PWM current source or a linear current source.

A rudimentary linear one is here:

http://www.vision.net.au/~timotsc/plugdrvr.pdf

The PWM circuit has a bit more charm in my opinion for a number of reasons not the least of which is all the heat is dissipated in the wire, not in a power transistor.

THe following is a link to a simple 555 based PWM circuit:

http://www.webx.dk/rc/glow/glow.htm

Check out the link below for the magic chip that will do a PWM albeit with a bit more finess:

http://www.microsemi.com/datasheets/SG1524.pdf

The SG3524 is cheap and readily available (less than $1 each if I recall). There are several model airplane glow plug drivers listed on GOOGLE that use this IC.

We did one, however, some time ago that is much simpler. I'll try to find it and provide a schematic.

Essentially, it is the 3524 without the inductor that used a fet instead of a darlington. Ran cool and trouble free once you got the resistance values figured out.

[HuFlungDung]

FWIW, I built a hot wire cutter for a guy once, to slice the end off of 11.5" wide foam blocks, 3.75" thick. It was an interrupted cut, so to speak, as he was cutting through 30 holes, 6mm diameter.

Anywho, I used a weight to tension the wire, because this was a chop-saw sort of operation. As I recall, I used an insulated guide at the far end of the wire. The wire guide itself was a piece of 1/8" soft copper tubing, which served as the electrical contact. I bent the tube with about a 2" radius spanning 90 degrees of arc, so the weight just hung below this tube. Worked like a charm, so he said.

[martinw]

Dear All,

As my post #3 says, I really am no expert.. in fact quite the opposite as I gave up on hot wire foam cutting after a bad experience mainly due a complete lack of knowledge and expertise.

I am still curious though.

The original question was about an eight foot throat of wire.

I would be grateful if anybody has any comments on the following.

If the cuts are straight, the workpiece will tend to keep the wire in line during the cut.

If the cuts are radiused, the wire will tend to bend laterally into some kind of compound curve because the wire tension cannot be infinite.

How do people get round this?

Genuinely curious,

Best wishes

Martin

[vulcom1]

Martin,
When cutting by hand you need templates to run your wire on. Long cuts is better done by 2 people and mark your templates about every 1/2" around the outside. Have one person call the numbers and the other person adjust to his speed on the numbers. Keep a good tension on the wire. I use a 1/2" in coil spring about 1/16" wire 2" long. Adjust your current setting on a piece of scrap. Anchor your foam down so it does not move or bow. I use a piece of 3/4" plywood on the top that is smaller than the foam so it does not interfere with the cut.
I am going to build the cnc version so you could probably disregard the templates and numbers. You still need the rest. As for line stretching with heat. Yes it does but that is where the spring comes in. It doesn't know any better and adjusts adjust on it's own. Don't need any formulas for what I do.
John

[NC Cams]

When you assume things, you make a a$$ out of me and you.

Don't assume that the wire will remain the same temp from end to end.

Why? It is altogether possible for the wire to vary slightly in gage and resistance from one section to another. Thus it will heat unevenly. This could cause it to wander a bit.

If the current is not constant, the wire will not stay even temp.

If taughtness is not sufficient, it will not cut straight.

If infeed speed/force is not the same, the cut can wander.

if the wire temp is not kept relatively constant, the melt path will vary and the cut will be irregular.

This is why a PWM style power source would be my preference over a linear one. The PWM style puts more current into the wire during the ON phase (actually 100% of what it can carry). At 20-30khz (easily done with a SG3524), you are really keeping wire hot and as constant a current as you'd ever hope. Keeping the wire current constant should do a lot to facilitate quality cuts.

Yes, if you keep the wire taught, theoretically, the cut will be perfectly straight. However, even when you cut bread with a knife by hand, is the cut perfectly straight (and the knife is infinitely more rigid/stiff that the bread)??? Technique does come into play at some point....

A bunch of little flat cuts will make a decenlty smooth radius as will a templated cut. The reason why the stuff looks so good when it is done is that the surface was probably ground smooth AFTER the cutting was done to hog off the material.

[vulcom1]

I did not assume anything. Just stating what I do over the years. If everything is set right and the speed is okay the wire does not even touch the foam. As for finishing you do not need to grind for a finish. We fine sand to take the glaze off so the glue gets a better penetration. The full scale home built boys do the same and it is not complicated. Practice is where it is at.
John

[NC Cams]

VULCOM1: my reply re: "assume" was to MARTINW - your post slid in between.

Only trying to point out ways for him to avoid problems with regard to maintaining consistant wire temp so as to help make cutting easier, especially for a newby.

I meant no offense or criticism of anyone who's having success with their hot wire cutting prodecure.

EDIT: by the way, there was a product out there called an Accucharger 1 and 10. IT was a PWM constant current battery charger that ran off of a 12v car battery. IT was adjustable from under 1 to about 5 for the model 1 and 1 to about 10 amps for the model 10.

The internal design would be such that it wouln't know the difference between charging nicads or heating wire - in fact, the callibration process used nichrome wire for the load to callibrate the out put. If you can find one that works, it would make a great power source for a wire cutter.

IF you can one, period, I used to service them as it was a long ago project that was a technical success but a business failure. END EDIT

[samco]

interesting.

I can only go from my very limited experience. I cut some wings out of some 2 inch pink foam. They where about 3 ft long with a metal "guide" the shape of the wing on each end of the foam.

I ended up using .042 welding wire streatched between the headstock and tailstock of a 8ft bed laith. This was about 10 years ago so it may be a little rusty. I just tried to take the slack out of the hot wire by moving the tail stock back (that didn't work)

I ended up puting a spring at one end - put quite a bit of tension on it and applied the power (from a 0-30 volt large power supply). I increased the current until the spring had lost a bit of tension and it cut beautifully.

sam

(just thinking out loud. I don't really know if it is relevent)