Picture of Cardboard folder/creaser
I like to make stuff from cardboard. from cutting down a big box to a smaller one for shipping, to making props, storage solutions etc.
The hard part with corrugated cardboard is in bending it nicely. someone else has shown how to cut the inside ply and corrugations to make a neat bend, but I have a problem with that method. #1 it's weak, and #2 Have you ever seen a commercially made box made that way? 
I used to try holding a yardstick down on it and bending up that way, but it was very unpredictable and often I got poor results. At some point I took a straight board and cut a bevel on one edge to crease the bend. it worked sorta. since the pressure was spread over a large area, I had to really lean on it hard. the sharp square end made it hard too extend the crease if a long fold was needed.  
Recently, I had an epiphany! If a made the creaser curved like a pizza knife, it would apply the pressure to a small area at a time and it would require a lot less force to do the job. Also, if I needed a long fold, the curve would let me ease out of and back into the work for a smoother transition.
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Step 1: Lay it out

Picture of lay it out
I used a scrap of MDF 1x6 trim board. Mostly because  it's what I had and it's easy to work. also it's stable and wont cup or twist.any wood would work if it is flat. solid wood is stronger and hardwood probably would last longer.  
I cut a piece about 18" long and about 4 1/2" wide.
To mark the curve, I put a small nail about 2" down from one edge, one on each end of my piece.
I then used a thin strip of wood about 1/4" thick and 3/4"wide as a spline and layed it against the two nails then bowed it to the curve I wanted and marked it with a pencil.
neffk18 days ago

I like making stuff out of cardboard, too. I've tried using a straight edge and creasing with a bone folder. Usually it just cuts through if I'm going with the ribs. This creaser is just what I need.

Toga_Dan4 months ago

good idea. I've sometimes folded cardboard over a 2x4 or similar lumber, but dont think result was too pretty.

TrophyJoe1 year ago
I too enjoy making things from cardboard (free is good). I've always used a creasing tool and straight edge. That is hard work. Your tool is much better. Thanks for sharing this.
This is a great idea. I bet with a little practice you could even score shallow curves with it.
heathbar64 (author)  ArtisanEclectic1 year ago
Yes, I suppose you could, but you would have a fun time trying to bend a curve.
heathbar64 (author) 2 years ago
I am surprised and gratified that this project has been featured on the homepage and apparently appreciated by all of you. It was such a simple project that I almost didn't post it. Seems to have filled a need, so I'm happy that I did go ahead and publish it.
It's often the simplest Instructables that are appreciated the most.

I nearly didn't post one because I thought that, while the idea was innovative, the process was too simple.

That one got featured too. :)
indeed! the simple, ingenious ideas are the ones that blow minds, make people slap their heads, are the ones that make instructables legendary.
chewybb2 years ago
It was over 20 years ago and they were made by the in-house engineers, but I remember them basically being a piece of steel approx 3mm thick, cut into the shape of a handle with a round end. The steel would then be rounded off on the edges to avoid piercing the cardboard too easily.

Another method that could work well would be a small wheel with rounded edges attached to a handle.
chewybb2 years ago
you're short of time/ materials, use a coin with a smooth edge (2p ideal in uk) with a straight edge. A small amount of pressure with the coin will be enough to "dent" the cardboard before folding. NB. Much lighter pressure required when creasing in the direction of the flutes.

I used to work for a very large packaging company in Europe, making sample packaging from 8' x 4' triple fluted sheets, and a simple coin worked wonders when the usual tools went a wandering!

This looks a great tool for creasing a lot of cardboard though, and looks like it won't leave slight marks on the cardboard that an old coin often does.
heathbar64 (author)  chewybb2 years ago
thanks for the tip. So what is the "proper" tool for this like? since making this I've heard of several ways others have done it.
WWC2 years ago
WOW! After seeing this i feal like i just learned the magicians secret. Really nice to know this. I have been trying to figure out what to use for storage boxes and i think this will help. Thanks
buteomont2 years ago
Awesome! I'll be making one of these for sure!
paganwonder2 years ago
Simple, effective...a perfect tool! Very well done!
msraynsford2 years ago
That's a great idea, well done, I really wish I'd thought of it while I was making the Northackton spider
Prfesser2 years ago
Neat idea! Since I'm lazy, I might try gluing a length of solid bare wire, maybe 14 gauge, to the curve, instead of forming the "v". The wire pressing into the cardboard won't make quite as sharp of a groove, though.
katzsplat2 years ago
You helped me solve a problem I didn't even know I had, thanks! I'll be making this soon.
heathbar64 (author)  katzsplat2 years ago
Love it when that happens! Aint this site great!!