Saturday, March 1, 2014

Large Multicutter Design

Another fairly common question, large multi cutter design

I'm planning to build a prototype for a large foam block multi-wire slicer.
1. I will need 5-8 wires
2. The length of the NiChr are going to be 8-9 feet in length
3. I will be cutting different densities of foam, 1lb to 3lb
4. I will be using 220V power

What do yo recommend in this situation?

With a cutter that large, there are going to be a few difficulties.
1. Power Supply
2. Wire Tension

1. Power Supply

The size and power of your power supply will solely depend upon the wire you choose and the overall resistance of your circuit. Over those lengths, I'm not sure that NiChrome wire is going to be your best option. You may want to consider stainless steel wire.

The biggest factor with resistance and necessary amperage to heat your wire, is how you lay out your design. If you go with stainless steel wire, chances are you're going to want to set up your wires in circuit series, rather than parallel. But if you use NiChrome wire, you'll probably want to use a combination of series and parallel to achieve the best combination of voltage and amperage requirements. 

This is discussed in further detail on the website, and should help explain that recommendation a bit more.

Don't even both trying to find a DC power supply. For something like this an AC Step Down Transformer is going to be your cheapest option for overall output power. If you're using 220v power, I'd assume you're not in the US? If so, I do not have a ton of international options for power supplies. But feel free to scan through my power supply section for some US suppliers that can ship to you.

2. Wire Tension

For lengths over 24" I typically recommend using 24 gauge NiChrome wire. But if you're looking at 8-9 feet, you're going to need something quite strong that will respond to both the pressure of cutting as well as the linear applied wire tension. I still think stainless steel wire is going to be your best option, but I do not have an idea of wire size. Good news is stainless steel wire is cheap, so buy a few rolls and see what gauge wire works best for your design. Keeping in mind that wire gauge and length have a bearing on circuit resistance, and require amperage to heat. You're going to want to find the smallest gauge wire, that can hold up to cutting pressure. Probably 24-18 gauge.

Next, I'd recommend designing your cutter so that at one end you have a rigid extension spring and at the other end some type of winding device. Even if that winding device is just an eye bolt you turn to increase wire tension. Keep in mind that as a wire is heated it will expand, and with expansion comes a saggy wire. If you're cutting a grid out of large foam billets, the last thing you want is wavy lines. I suppose it all depends on your application, but you're going to want wire tension. I have actually seen a design were someone used a FULLY threaded pipe, from end to end. And all they had to do was twist the entire pipe, and it increased resistance on all the wires. It was basically a huge, large diameter machine screw at that point. Simplistic design, just requires you to know someone who can thread pipe. 

I’m thinking of using the Transformer from MastechPowerSupply.

Volteq 5KW Transformer Variac 5000VA 0-250V 220V Input, with maximum current output of 20amps (@250V)
or do think I can get away with….
Volteq 2KW Transformer Variac 2000VA 0-250V 220V Input, with maximum current output of 8amps (@250V)

Not just because I'm American.. but I'd have to say "bigger is better." When it comes to non hobby large scale cutters, especially unique set ups, you do NOT want to be 100% ready to go and have an inadequate power supply.  With the 5kW unit, you have the same voltage regulation on the dial but have 2.5 times the power. Using the 5kW model, you'll have the best power supply, just in case you need to wire in series on the fly...

Just remember… Greater than 45 dry volts will penetrate human skin. If you're moving 5kW across something like that… there is a pretty high risk of shock. Just be incredibly careful. 

I’m also thinking of having them wired is Parallel setup, just so I can hook them up on two shafts, and have adjustability on the thickness of the slices, just by moving the Springs up and down along the shafts.

I love the idea, and would love to see it in action. Send some pictures when you get it set up. 

Just remember your electrical properties when placing a circuit in parallel. An easy way to think of it is like this….

Placing four wire in series, is like making one wire four times as long

Placing four wires in parallel is like making one wire four times as thick

When you have a long wire or wires placed in series, the resistance dramatically increases, the voltage required to heat dramatically increases, but the amperage pretty much stays the same. 

When you have a larger gauge wire or have wires placed in parallel, the resistance dramatically decreases, the required voltage decreases but the necessary amperage to heat the wires dramatically increases. 

I hope that wasn't confusing. 

Long story short… depending on how this pans out, you may end up needing a combination of series and parallel to find the right combination of circuit resistance, voltage and amperage. You may even consider having switches to "activate" a cutting line. You'll want something rated at 20amp max, which is usually a house circuit switch.

The majority of the time, I’ll be using one wire, but I want to make sure that I can do 5-7 wires without any issues.
With the Transformers I have listed above, what do you think there limits are as far as the number of 8-foot wires they can handle?  We’ve been using two sizes of wire, 0.010” and 0.014”, in 8-foot lengths for many of our cuts, usually lettering and shapes, so we need smaller wire gauge.  Do you think that Stainless Steel is better to use that NiChrome?… with the wire gauges we are using, 30 AWG and 27 AWG, which one would be best for heat and length? Right now we have it automated to cut at around 45in/min on 1 lb density foam.

Well… There are a few ways to answer. 

For lengths greater than 2-3' I typically tell people to shy away from NiChrome. Its a good wire, don't get me wrong. But often the resistance is too high at those lengths. Where as, for a small cutter <2' I exclusively recommend NiCHrome wire and NOT stainless steel because NiCr's resistance is higher and makes power supply selection easier. So both are good options, it just depends on your application. 

BUT… JUST started to carry 27 gauge Rene wire…. I think you might like this… Rene wire is considered a super alloy, and virtually indestructible. It is an "up and comer" in the foam cutting world, and a lot of guys use it to cut large (8') foam wings. You might like it a lot, I know Gary Jacobs really likes it. Personally, I am looking into using it for my combination foam cutter as a good straight wire cutter. 

27 gauge Rene wire has similar electrical properties to 28 gauge NiChrome wire, so you can use that reference for calculations. As you would imagine, high wire resistance will be counteracted when placed in parallel. 

Just for kicks, lets see what we can do with a little math. 

A single 8' length of rene (34ohms) would take about 150 volts to heat to 1800° and only draw about 4.5 amps from your power supply.  If you put six 8' lengths of rene wire in parallel (5.667 ohms), my guess would be it's similar to 20 gauge based on length and resistance. Here's the difference… you'd  require about 80 volts to heat to 1800° but drawing close to 15 amps from the power supply. 

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