Automating an Acrylic Line Bender With Arduino

Why build a Line Bender?

Why not? It's a fabulous tool that gives you enormous power over a wonderful material.

Sheet acrylic (one brand name is Plexiglas) comes in a wide array of incredible colors and patterns. Acrylic can be transparent, translucent, or opaque. It is super-easy to laser cut and engrave, making it an ideal material for an amazing array of beautiful and creative projects. I mean...how can you resist something that looks like stained glass, or powder-coated metal, or lipstick red, but doesn't break easily and is pretty accessible cost-wise?

We use acrylic to manufacture our company's electronics-mounting system...but discovered, after much frustration, that there was an art to bending and shaping acrylic.

It should be simple. Right?

1. Heat it.
2. Shape it while hot.
3. Hold it there and let it cool.

Done; see? Easy? Not.

We started out with vacuum forming. We successfully got the material to take the shape of the form, but the form had to be absolutely flawless; and it required us to make a custom form for even the smallest change. Tedious and frustrating.

Hey! What about line bending? Much better. Why?

• The bend is clean and elegant.
• Line bending doesn't "transmit" flaws on the surface of the mold to the final product (unlike vacuum forming).
• The angle of the bend can easily be adjusted.
• One line bender can create many different items (unlike vacuum forming).

Automate the process with an Arduino controller. Why?

• Reduces material waste.
• Reduces time and frustration.
• Ensures beautiful, repeatable bends every time.

This project is especially great for Maker Spaces or small fab shops. We use several of them to manufacture our company's product, but we have made a few projects for around the house. And now that our Maker friends know about this, we bend the occasional item for them as well.

Here are the four most important things that we will cover in this Instructable:

1. Clamp -- Position and hold the workpiece firmly in exactly the right place on your heating rig.
2. Heat -- Apply a known, consistent and controlled amount of heat to a well-defined area for a precise amount of time.
3. Bend -- At the precise moment, turn off the heat, then quickly and decisively bend the workpiece to exactly the desired angle.
4. Hold -- Hold the workpiece in exactly this position for another precise amount of time until the acrylic freezes into its new shape.

Ready? Let's get started!

For the line bender chassis:

• Nichrome 60 heater wire
• Tension spring
• 3/4" Cabinet-grade "Blondewood" plywood--the more plys, the better
• 2x4 scraps
• Vise-Grip 18SP welding clamps or similar drill press clamps
• 4" strap hinges (heavy-duty, tight-pin hinges)
• 5/8" x 1/2" x 1/16" thick aluminum U-channel (Hillman Group Part Number 11380)
• 12VDC Power supply capable of supplying at least 15A continuously; more capacity is better
• Timer
• Magnetic catches (optional) or Bungee cords
• Various screws and other hardware for assembly

For the automated controller:

• Arduino Mega 2560
• 5V DC 4-channel relay module capable of controlling at least 10A DC current at 12V
• 3W 8 ohm mini speaker
• 1602 LCD/keypad shield for Arduino
• Screw terminal Arduino shield
• Terminal block
• 5V DC Battery/Power Supply (cell phone charger)

SOURCES

• Electronics: Digi-Key Electronics and Amazon
• Nichrome wire: Jacobs Online
• Hardware and wood: Ace Hardware, Lowe's or Home Depot
• Acrylic sheet (wide range of colors in less than 4' x 8' sheets): EStreetPlastics.com

This project assumes that you will make the size decisions based on your needs, and that you have sufficient woodworking experience to handle everything safely and correctly. Therefore: I'm not going to provide precise measurements for the chassis, nor will I describe how to use common wood working tools and principles. I can point out that my line bender has an 18.5" wide table, which gives me a usable bend length of just over 16", and that has worked well for me. The length of the bend dictates the power requirements and the gauge of nichrome wire you use, so I would strongly suggest not going much bigger until you have some experience with a bender about this size.

My image shows a double-sided line bender...which is what I needed for production. A single-sided line bender is sufficient for Makers where production time is not a critical factor.

I recommend you read through everything and lay out your design on paper BEFORE starting to build. It will save you time, material, and revisions.

Support platform:

Lay out and assemble a support platform using plywood and short pieces of 2x4. Your platform needs to fully support the base, the folding arm, the channel, and the hardware to manage the nichrome wire. Be sure that your support platform gives you sufficient clearance underneath for the clamp you intend to use.

Base, channel, and folding arms:

Lay out and cut the base and the folding arm. The length of the aluminum channel for your nichrome wire is one of the critical dimensions; be sure you make it long enough to handle the size of acrylic you intend to bend. However....and this is a big "however"...at the same time, you want to keep the nichrome wire (and therefore the channel) as short as possible, to minimize power requirements. I'll get into the intricacies of the channel, the nichrome wire, and the power requirements in Step 2.

The base needs to be large enough for the largest piece of acrylic you envision working with. The folding arm needs to support the heated acrylic and the non-heated acrylic that extends past the bend. Since the largest portion of your project should always be clamped to the base, the folding arm does not (usually) require more than 4 to 5 inches of width.

Heavy-duty, tight-pin hinges with plastic spacers keep the hinge from being sloppy, so the hinge stays consistent in the bend. I hacksaw off the straps because they only need to be about an inch past the pin, not the full length. That still leaves you two screws in each leaf of each hinge.

Folding arm stops:

The stops can be a simple or as sophisticated as you want. The important factors in the folding arms are twofold:

1. The folding arm must be able to move decisively into the correct position as soon as the acrylic has been heated for the ideal amount of time.
2. Once in the correct position, the folding arm must hold that position long enough to allow the melted acrylic to freeze in that shape.

If your folding arms don't move quickly or hold the right position, your acrylic may droop into the wrong shape, or it may induce stresses in the material--leading it to break more easily.

As you can see in the photo, I put a magnetic catch on the stop itself. (The catch is optional; I have also used bungee cords.) But notice that I don't fasten the stop itself to the base because I want it to be adjustable. Instead, I fastened a chunk of 2x4 to the base as a brace. That allows me to use a C-clamp to easily change and set the angle of the folding arm.

The clamp and the clamp pad:

I use Vise-Grip 18SP welding clamps to hold the workpiece. Other approaches can work just as well. Whatever you do, make and use a clamp pad of the appropriate size to protect the workpiece, and to spread the load along the heating line.

This is the part where you will need to do the most testing to optimize for the thickness and color of the acrylic you want to use.

Think of it like cooking on a grill: too hot and the outside of your meat gets black, while the inside might still be “frozen.”

What you want is a slow, even heating that gets the acrylic soft enough to make a good bend, avoids carrying material stresses into the bend (by forcing a bend in material that is not fully softened), but you also want a process that isn’t so hot that the acrylic starts to bubble/boil--or worse, catch on fire!

It’s a fine line to walk…and it may take a number of test runs before you get the magic combination for your application and the thickness and color of the acrylic you are working with.

With that in mind, let's talk about how to build the heating element.

I have found that 6 to 6.5 watts per linear inch of nichrome wire is the sweet spot for 1/8" (3mm) acrylic sheet. At this rate, it takes about 2 to 3 minutes to heat up a bend in this thickness. This is a good power level to let the heat soak through the acrylic without boiling it on one side.

Your total power requirement will depend on the length of nichrome wire conducting current in your linebender--that is, the length between the alligator clips in your design.

My line benders typically have about 19.5" of heated wire, which has let me do everything I've needed, so my instructions will be based on this design. If you vary the length of your heating element significantly (either longer or shorter), be sure to adjust the gauge of your nichrome and/or the voltage you are applying to the wire appropriately so you end up close to the target 6 watts/inch.

This is Ohm's Law applied. Nichrome has a known resistance per foot. If we are using a 12-volt DC fixed-voltage power supply (the cheapest, most readily available type), the voltage is known. Work backwards from the equation for power to determine the resistance you need to get the right heat output for your length of wire.

Let's say we have a 20" wire and we want ~6 watts/inch, which gives us a total power output of 20 x 6 = 120 watts.

Since Power = Voltage x Current, that means Current = Power/Voltage. We know that we have 12V with our power supply, so we divide 120 watts by 12V to find that we'll be moving 10A of current through 20" of nichrome.

Now we can apply Ohm's Law to determine the gauge of wire we need. Since Voltage = Current x Resistance, we can find the total resistance in this circuit from Resistance = Voltage/Current. A 20" length of wire needs to have 12V/10A = 1.2 Ohms of total resistance to give us the heat output we need.

Divide 1.2 Ohms by 20 inches to find the resistance per inch, which is 0.06 Ohms/inch. Now you can select the proper gauge of nichrome wire based on this value. Because Jacobs Online classes wire resistance by the foot, we multiply by 12 to find that we need about 0.72 Ohms/foot.

Looking at the Jacobs Online website, we find that 20-gauge wire is 0.66 Ohms/foot and 21-gauge wire is 0.83 Ohms/foot. I suggest getting a small quantity of three or four successive gauges so you can experiment and get your rig dialed in the way you like it. The 20-gauge in this situation will give you about 130W of heat; the 21-gauge about 104W. I'd go for the higher heat.

Get all your nichrome wire from a single source; this ensures consistent stair-stepping of the resistance between gauges. I like Jacobs Online, mostly because they can give you all the gauges you want, in order, without skipping gauges.

Insert the aluminum channel between the base and the folding arm. Notice that the hinge is centered over the channel. I use 5/8" wide x 1/2" tall aluminum channel from Lowe's, but you can find similar hardware at most building supply stores. The aluminum channel only needs to run under the heated part of the wire. In my case, for a 19.5" wire, I used a piece of aluminum as wide as my table, which is 18.5". Drill some holes in the center of the channel to screw it down. Be careful not to let the screw heads touch the wire, or you will create a short circuit. I suggest countersinking the screw holes a bit and using flat-head screws to keep a very low profile.

Install a support screw at each end to hold the nichrome wire up and help stabilize it. Use a screw designed for straight-blade screwdrivers and turn the slot in the head to align with the axis of the wire. Just lay the wire in the slot. Place the screws far enough outboard on each end to leave room for your alligator clips, which are attached to the nichrome towards the inside of the screws. Install the screws so that the wire is centered in the aluminum channel, and about 3/8" below the lower surface of the acrylic.

Make sure to use a tension spring on one end of the nichrome wire.

IMPORTANT: The nichrome expands so much when it heats that you need the spring to keep it tight; without the spring, the wire will droop or rise to touch your workpiece. In my build, a 19"-long nichrome wire grows over 1/4" when heated. I used a Century C-33 (7/16" OD x 1-1/2" Length x 0.050" Wire Dia) spring. If your spring is too small or too weak, you'll end up with an out-of-control hot nichrome wire.

Now let's talk about the power supply. At 12V DC, you typically need 10-15 amps per wire, depending on the length of your wire. Use a power supply capable of supplying much more than the upper limit of your calculations; the power supply will last a lot longer. There are a number of 30A power supplies on Amazon; here's just one.

Make up two power leads. They need to be long enough to run from the power supply to the two ends of your nichrome wire. Use beefy stranded wire (The Engineering Toolbox suggests 12-gauge wire for the leads in this application) with large alligator clips to connect to the nichrome wire. I crimp and solder the alligator clips to my leads to make sure I'm getting a good connection. You don't have to use a spade terminal on the power-supply end of your leads, but it makes for a better connection.

Attach the first power lead to the nichrome wire inside of the support screw, leaving enough space in front of the support screw for the wire to expand. If you are too close to the support screw, you defeat the purpose of the tension spring. The second power lead goes on the other (stationary) end of the wire, also to the inside of the screw.

Now is a great time to hook everything up and put power to your creation for a smoke test.

• Connect one of your power leads to a positive terminal on the power supply and one to a negative terminal.
• Attach the alligator clip on a power lead to one end of the nichrome wire (inside the support screw) and the other alligator clip to the other end of the nichrome wire.
• Connect AC power to your 12VDC power supply per the instructions for the power supply. Do NOT plug in your supply line until everything is properly connected and you have verified the circuit.

If everything is correct, when you plug in the power supply, the nichrome should heat up to a nice orange glow, the spring should contract, and nothing should catch on fire. Congratulations!

The timer is the most important part of your line bender! The time is critical when heating because ACRYLIC WILL CATCH FIRE if it gets too hot! And good timing is also important to successful cooling. In short, for the success of your project, and the safety of your workspace and anyone around you, the timer is critical.

For my early test runs I used the timer on my phone. It is precise, but using the phone meant that I was constantly looking and running when the alert sounded. And for the 2-4 minutes in between, I was bored and annoyed while I waited.

I needed a reliable timer that would not only pay attention to the timing, but also switch off the power (preventing fires and ruined projects) and give me a definitive audio alert when it was time to close the folding arm, and when it was safe to remove the cooled project

Note: If you decide not to automate your line bender, you'll want to add a switch to your power supply so you can turn the system on and off without having to unplug it from the wall.

See Step 3 for the controller.

The material below is a summary of the two Controller videos included here.

Where could I find a reliable controller? I needed a:

• Timer - adjustable countdown clock for both the heating and cooling cycles.
• Switch - turns on the power for the heating cycle, and turns it off automatically when the cycle is complete. Prevents fires and ruined materials by "overcooking."
• Audio alerts - now I simply listen for the tones to close and open the folding arms.
• User interface - allows me to adjust the heat cycle timing for different thicknesses and colors of acrylic.

Arduino to the rescue!

Here is a repeat of the materials list for the controller.

• Arduino Mega 2560 - clock, control - the heart of the project.
• 5V DC 4-channel relay module - switch DC power on and off automatically
• 3W 8 Ohm mini speaker - audio alerts
• 1602 LCD/keypad shield - user interface
• Screw terminal Arduino shield - more robust attachment points for wiring to IO pins
• Terminal block - attach and detach heavy wires easily
• Battery (cell phone charger) - power for controller

Building the controller:

I'm assembling this controller on one of our company's WorkBench Project Development Kits (PDKs). They are perfect for accelerating and protecting projects like this. The entire electronics mounting product line can be found at PhaseDock.com. We use this controller to make a wide array of acrylic-based products. Automation was very important to our production operations.

The heart of the project is the Arduino Mega. You can use an Uno, but I like the extra space and additional I/O. Attach the screw terminal shield to the Arduino Mega, then align the 1602 LCD/keypad shield into the screw terminal shield. I use the A0 pin as a good reference point to ensure alignment.

The next component is the relay module. See the video as I point out the various components of the relay module, such as the optical coupler, the LED indicators, and the control pins. Be sure to use the normally-open terminals for your load wires. (In the video I said to use the normally-closed terminals; that is incorrect.)

Wire the Arduino to the relay module. Only three wires are needed:

• 5 volt power - which is at the top of the right-most set of pins on the Arduino and at the close end of the relay pins. [3:33, Part 3].
• Ground - which is at bottom of the Arduino and at the far side of our relay module.
• Relay signal - we are using the right-most relay, which is the right-most control pin. I programmed it to use pin 31 of the Arduino.

That's all there is to that.

This is where the terminal block comes in. In the video I had already attached pigtails to the normally-open screw terminals of one relay on the relay module. Connect the other end of these pigtails to two adjacent terminals on the terminal block. Eventually you will connect one of the 12-gauge power leads that attach to your nichrome wire to a corresponding screw on the terminal block. The other screw on the terminal block connects to the positive ("hot") 12VDC output from your power supply. Note that you don't need a terminal block, but it makes for more secure connections overall, especially if you disconnect your leads to the nichrome wire frequently.

Now mount the speaker. It comes pre-wired and will be attached to the screw terminal shield. There are only two wires. Ground is the second to last wire on the Arduino. The best wire on the Arduino for sound is D3.

Last, but not least, some power for your Arduino. In the video I'm using a simple 5VDC cell phone charger/battery to the USB port on the Arduino, which are readily available. Use a short USB cable if you take this route.

You can also use your 12VDC power supply to power your Arduino. If you do, I suggest using the 2.1mm center-positive barrel jack socket on the Arduino, which has reverse-polarity protection built in. Observe proper polarity when wiring up the barrel jack.

When I push the start button on the battery, the LCD screen will light up, the speaker will sound the fanfare to let us know it is booting up and starting properly. Then the controller will go into "wait" mode for further instructions.

I will cover the Arduino programming instructions in Step 4.

The video has much more detail, but I will summarize key concepts and steps below.

Program Concept 1: Heating and Cooling Protocols

Heat times depend on the color of acrylic, so I created two different heating/cooling cycle times the user can choose from.

In our testing, two color groups were sufficient for our needs:

• BLK (Black) - used for all dark colors
• CLR (Clear) - used for light and transparent colors

The TST (Test) cycle is much shorter than the others. I used that protocol to debug the program.

Program Concept 2: Typical Heating & Cooling Times

For each Protocol, the program stores and executes two back-to-back timing cycles; one to heat the Plexiglas and one to cool it. Depending on color and thickness, the heating cycle runs between 1:45 to over 3:00 minutes. (Everything we describe here applies to 1/8" acrylic. If you are using a thinner or thicker material, you will have to adjust accordingly.)

The cooling cycle is about 2:00 for all colors.

TST is a lot shorter. Just over 30 seconds to heat, and about 15 seconds for cool. It lets me verify that critical program events are happening in the right sequence and at the right time.

Program Concept 3: Storing Cycle Times

Cycle times are stored in the EEPROM of the Arduino, which allows it to retain the memory even when the Arduino is powered off.

We also have a function in the program (SET) that allows the user to change the cycle times in the protocol. Changes are automatically saved when you leave the SET function.

Program Concept 4: Two Simultaneous Timers

The program runs a count-DOWN timer on the left, showing the time remaining in that cycle. And a count-UP timer on the right which shows the overall elapsed time since the start of a given protocol.

Program Concept 5: Audio Alerts

The audio alerts let the user know that something is about to happen.

• Bootup Melody: Four-note melody (fanfare) lets you know the program is booting properly.
• Cycle Start: Two-note melody lets you know the controller heard you and has initiated a cycle.
• 30-Second Warning: Three high-pitched beeps sound at 30 seconds before the end of a cycle.
• 10-Second Warning: Two high-pitched beeps sound at 10 seconds before the end of a cycle to make sure you are ready to raise or lower the folding arm.
• End of Heating Cycle Countdown: At 3-2-1 of the heating cycle, four ascending tones alert you to raise the folding arm.
• End of Cooling Cycle Countdown: At 3-2-1 of the cooling cycle, four descending tones let you know your workpiece is cool enough to remove safely from the line bender.

With that introduction, see the video to run the program. [3:26 - 6:26 of Automated Linebender Part 4: Code]

The top level menu shows three options: RUN, SEL (Select), and SET.

• RUN initiates the active heating and cooling protocol.
• SEL chooses the protocol that the other two options act on.
• SET allows you to adjust the heating and cooling cycle times of the active protocol.

Before you can run or set anything you have to select the active protocol or you get an error. The active protocol is shown in the upper right of the 1602 screen. Three dashes indicate that nothing is currently chosen. The start-up zeros out any previously used protocols to ensure that you actively select the protocol you want.

Video demonstration of how to set the minutes and seconds for both the heating and the cooling cycle times. [See 6:27 - 7:51 of Automated Linebender Part 4: Code]

CODE REVIEW

We have posted the code to our website with some strong disclaimers:

• I did not write the menu-management code. It was scraped from various websites, so use it at your own discretion.
• The code I did write was not designed for public consumption, so documentation is sparse.
• I can't help you modify or adapt the code to different purposes.
• In short, there is no support offered or planned for this code.

You can download the free code by registering at https://phasedock.com/automated-linebender/

A custom library that I used for the speaker output is included. Other libraries are easily found online.

Libraries used include:

• AnalogMultiButton.h - Keypad button management
• LiquidCrystal.h - LCD output
• avr/eeprom.h - Read & write to EEPROM
• stdio.h - General purpose IO
• AudioTools.h (custom) - Sound generation
• Pitches.h - Sound generation

We use the line bender extensively to build product for our business. The WorkBench Bases have two bends. The Covers have four bends. It's proven to be a highly repeatable process, and delivers elegant, clean bends every time.

In addition to acrylic line bending, I can see this simple controller being enhanced or extended by:

• Controlling other components via the relay module. For example, you could add pneumatic solenoids, motors, or any other high-current component.
• Incorporating feedback sensors to create closed-loop systems. Sensors might include temperature, pressure, proximity, motion, and so on.
• Using it to control other equipment. Reflow ovens, kilns, presses, or dedicated processing machinery were a few ideas I had. Perhaps you will have others.

If you do take this further, kindly share it with the larger community of Makers and Engineers. I'd love to hear where this could go!

See Part Five: Tips and Tricks. for a few more helps to improve your line bender build.

Thanks and good luck with the project!