Best way to cut very soft 4mm (1/8") poplar plywood sheets with hand tools

Question

Sometimes I need to cut sheets (cca 50x80cm = 20x32") of soft 4mm (1/8") poplar plywood into smaller rectangular pieces and I find it surprisingly difficult to do with hand tools - it's too soft and thin for most saws, but it's too thick for a knife.

Currently, for most cuts I do:

1. shallow line with a marking knife using ruler or a square
2. follow the line "freehand" with chip carving knife to make it deeper
3. repeat heavier cuts with chip carving knife until it's done (it usualy takes 3-5 cuts)

But it's very tedious process which also leads to heavy damage to cutting surface and I have to be very careful to stay in the marked line while applying quite a lot of pressure on the knife. Also the knife must be VERY sharp, otherwise I get jagged edge with chipped out pieces.

For smaller pieces needing more precise cuts I use small 24 tpi saw (Zona). Which is also slow and usable only for really short cuts (no more than 15cm / 6").

I can't use heavier saws because anything less than 24tpi destroys the edge badly.

Is there a better way/tool to do it? (I'm only interested in hand tools)

Answer 1

I thought I remembered something from Colin Knecht (WoodWorkWeb) on this and here it is, Woodworking with Thin Plywood, like Baltic Birch Plywoods. The relevant portion is right at the beginning, with a tip not just on how to cut it with a knife¹ but also on a good way to back the cut, although if you have a decent cutting mat that should be fine too. I've cut 3mm / 18" hardboard (much harder to cut than ply) on a cutting mat many times without causing undue damage, although there is inevitably some wear and tear.

An alternative method that may also work nicely with your material: scribe-and-snap². See it in action here, Cut thin plywood without saw and chippings Be conscious of the orientation of your plies here. Note that Colin's tip about which way to orient the plywood cutting surface underneath isn't relevant here, since you don't cut right through.

Note: when cutting flat material with a knife it is the point that does almost all of the work. With a standard Stanley knife/utility knife you can greatly extend the useful cutting life of every blade by filing, grinding or honing the tip back a small amount each time it has become blunt. I've extended the life of single blades for more than a year using this simple trick. Here's approximately what they looked like after many months of 'resharpening':

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¹ He's using a standard utility knife but a boxcutter/Olfa knife should work as well, and more easily allows for a fresh cutting point to be available after the material inevitably blunts the tip.

² If your thin plywood is more than 3-ply this method won't be quite as effective as shown, but could still work well enough that it's the method you'll end up using.

Answer 2

Here are a couple ideas that might help:

1. Get a mat cutter that makes vertical cuts, such as a Logan 350-1. These normally use razor blades or sometimes utility knife blades. 4 mm plywood is probably a little heavier than what they're meant for, but a half-decent one would still help you make a nice straight cut several times on exactly the same line. You'll probably go through a lot of blades, but that's OK, they're very inexpensive.

2. It sounds like edge quality is very important to you, so consider cutting your pieces just a bit larger than you want and then sand the edges to get to the final size. You could make a long, narrow sanding block to help you keep the edges very straight. A roll of 1"-wide, 150-grit sanding cloth would be a good choice for this application: it'll leave a smooth edge but still cut quickly enough to get the job done in no time.