This post is a set of instructions for creating a utility knife guide, for making fast, straight, precise cuts in corrugated cardboard, using a utility knife. This guide only cuts cardboard to 4″ widths, but you can easily modify it for other widths. It uses about $7 in parts, and takes about 15 minutes to construct.
Background.
I’m in the process of making a do-it-yourself floor-to-chair aid for wheelchair users. It’s a broad, shallow set of lightweight portable steps made from cardboard, hardboard, and tape.
For that project, I need to cut a lot of corrugated cardboard to exactly 4″ widths. You can do that with a tape measure and straightedge, but that’s tedious and imprecise. I found that my cuts varied just enough to be a problem when I assembled the stairs.
I really want some sort of “knife guide”, in the same sense that you might use a circular-saw guide or a jigsaw guide. Some device to ensure that the cuts are perfectly straight and at the exact right distance.
I couldn’t find one. I could not find a commercial product designed to be a guide for cutting with a utility knife. For a price, you can get a picture-framer’s matt cutter. But, aside from cost, most of those won’t handle thick corrugated. And I really didn’t want to shell out $120 for a matt cutter that I would probably only use once.
Hence, this cheap little cutting jig. As constructed, it only does what I need it to do (cut 4″ widths of corrugated cardboard). But it does that well. Even if I never use it again, the fifteen minutes I spent making this will be more than paid back in time savings on the project I’ll be using it for.
Materials:
- One 7′ piece of aluminum window screen framing, $4.38 at Home Depot.
- One pack of four screen frame corners, $2.28 at Home Depot.
- A small amount of epoxy glue, such as JB Quick Weld (OR a small tube of gel-type superglue.)
- A small amount of tape (masking tape, packing tape, duct tape — it doesn’t matter).
- Two quarters or two large washers.
Tools:
- Hacksaw or close-quarters hacksaw.
- Materials for mixing a small amount of epoxy (piece of cardboard, Popsicle stick).
- Two utility knife blades (for setting the spacing of the guide).
Instructions.
1: Cut the aluminum frame into lengths
- 3 pieces 24″ long.
- 2 pieces 4″ long
- 2 pieces 2″ long
The only critical dimension is the 4″ length. Those pieces set the size of the pieces of cardboard you will eventually cut. If you want a different size of cardboard, modify the length of those pieces accordingly.
Note that, as sized, this used the entire 7′ piece of aluminum screen frame. So if you want a deeper cut, you’ll have to make a shorter device, or use an additional piece of screen frame.
Take care to remove any bits of metal that protrude once you have these cut. Those can keep this from fitting together well.
2: Dry-fit the pieces as shown.
Let me make up some names here. The two long pieces at the bottom form the blade guide for the utility knife blade. The long piece at the top is the cardboard stop, to hold the cardboard in place.
When you epoxy this, the two 2″ aluminum pieces will sit atop the cardboard stop. Thus giving room underneath for the cardboard.
The assembly of the corners is almost-but-not-quite idiot-proof. The plastic corners only fit into the aluminum screen frame one way. That way requires that the spline channel (the open U-shaped channel on the screen frame) face the inside of the rectangle. But you do have your choice of assembling this with the spline channel facing up or facing down. I’ve done mine with the spline channel facing down.
The short sides of the rectangle determine the size of the cardboard pieces to be cut.
3: Line it up and epoxy it.
Take a couple of utility knife blades and some tape, and use them to set the spacing on the knife guide. Put the base of the blade between the two pieces of the guide, and use the tape to hold those pieces in place. Leave enough room so that you can epoxy the washer (shown) onto the end of the knife guide.
Mix up a bit of epoxy. I used all of this, but I could have used less.
Dab some on both ends of the blade slot. Put a washer on top, and push down to seat it. Try to keep the epoxy off the sides of the window frame, so that you have the option of changing those out for some other size at a later time. Below left to right: Tape, blade, epoxy-and-washer, side of frame.
Note: You are supposed to do all kinds of surface prep before you do metal-to-metal gluing. If you are into that and/or routinely follow the manufacturer’s directions, then do that. I skipped that and got away with it. YMMV.
Place epoxy on both ends of the cardboard stop. Place the 2″ pieces of window frame on top of the epoxy, and press down to seat. Carefully align that so that those 2″ pieces are perfectly vertically aligned with the cardboard stop. Below is the left short piece, weighted down by a tape measure. The edge of the U-shaped spline channel is directly over the spline channel underneath.
Maybe weight down those corners a bit. Then let the epoxy set. Here’s the device, corners weighted by a roll of tape and a tape measure.
4: Use it.
When the epoxy is set, remove the weighs, remove utility blades, strip off the tape.
Place this on top of the cardboard to be cut. Align a factory edge of the cardboard piece with the inside of the cardboard stop. (This is now oriented 180 degrees from prior pictures, so the stop is at the bottom and blade guide is at the top.)
Place your hand on the cardboard stop and cardboard (not shown, as I am holding the camera). Run the knife down the knife guide.
The result is a nice straight edge, 4″ from the original factory edge.
If you want some length other than 4″, replace the 4″ sides with your preferred length. (Bearing on mind that you may need to buy more screen frame to do that.) I could not figure out how to make a continuously-variable blade guide without making this a lot more complicated and/or requiring me to drill precisely spaced holes in the aluminum.
I hope it goes without saying that this device won’t take a lot of abuse. Given that you spent under $5 for a 7′ piece of rigid aluminum, that seems fair. The material is definitely thicker than a beer can. It is plenty strong enough and rigid enough to provide a framework for straight cuts. But it’s definitely a hang-on-the-wall kind of tool, not a toss-it-in-the-toolbox kind of tool.
