Saw blade for thin plywood?

[millhavenguy]

Howdy all,

Here I sit with my plywood (4 & 6mm) and an old dull blade! What type of blade is best for cutting this thin Plywood so I'll get the right one the first time.

Thanks, David

[b.o.a.t.]

Is it a hand saw or power saw of some sort ?

[millhavenguy]

It's a 5 1/2 " battery powered Ryobi.

[Boatmik]

Just choose any blade for wood or wood/plastic that is not superfine. It will work OK.

There is a trick about direction of cut when going at an angle to the timber grain - one side will cut clean and the other side will chip ... I will write a bit about that this evening unless one of the knowledgeable bunch here beats me to it!!!

MIK

[PAR]

Blades are sorted by type and tooth count. You'll want a fairly high tooth count, but not the highest, as Michael mentioned.

Typically the blade rotates up, through the material. This leaves a clean edge underneath the cut, but rips out the grain on top. This is worse with some species of plywood then others. To eliminate this annoyance, use a piece of sacrificial plywood on top of the piece you're cutting. This keeps the grain pinned down as the blade rotates through it and it doesn't rip out.

[blackhole]

for that size blade I am going to say somewhere around 30 to 40 teeth

there is a theory of using masking tape to prevent/minimise tearout

The festool people say to set the blade to about an eighth of an inch and score the cut line then cut as normal. You could do the same with a sharp knife.

[BobL]

there is a theory of using masking tape to prevent/minimise tearout


yep it sort of works but a couple of tips.
- don't leave it on for longer than is necessary or press down any more than necessary otherwise when you go to take it off it may still tear out. You do need to press down somewhat otherwise it doesn't work. Necessary is determined by trial and error.
- If you want to minimize tearout when removing the tape use a hot air gun or hair dryer on low and gently warm the tape and it will come off much easier

[bateau]

For a 5 1/2 blade you shouldn't have more than 20 teeth for ripping. For ripping you want to have not too many teeth. For crosscutting hardwoods you want lots of teeth. Seeing as you are using a battery powered circ saw, and cutting plywood that is quite thin and made of mahogany, it will chip on the 'up' side almost no matter what you can do. Masking tape won't work at all here, and only rarely will it work to prevent chipping anyway.

You will have better luck against the chipping by scoring your cut line with a knife.

In fact, for you 4mm stuff, you can easily cut right through it with a sharp utility knife. Easy easy easy - and less dust and noise than a saw too. 6mm will take some patience but can be done too, but might need cleaning up with a block plane. Mahogany is pretty soft.

Another option would be to score the plywood itself with the saw blade - just lower it enough to cut through your veneer on the first pass, and for the second lower it the rest of the way. However, this really only works when using a straight edge as a guide, so the saw is held in the same place for each cut.

[PAR]

Scoring the plywood along the cut line works sometimes, but if you wander off the line (easy enough to do) then you're out of luck. Tape works sometimes too. Both of these techniques seem to work better on some species then others.

The sacrificial piece of wood on top of the cut, works every time, assuming you have firm contact between the two.

[Boatmik]

OK...

Now .. I wasn't talking about the top and bottom of the sheet. The top will tear out more for sure.

But an interesting thing happens when you are cutting at and angle to the plywood grain - ie at an angle to the longest axis of the sheet.

The grain will tear out on one side of the cut only (eg the left side) ... the other
side will tear out either not at all or much less. So if it chips out on the side you want to keep much more then you should be cutting back in the other direction.

The reason is the tearing out happens in front of the blade and follows the direction of the grain. If the grain goes into the panel you want to keep it will be chipped. If the grain goes into the waste ... then that side will chip.

Cool eh!

Also ... different teeth for ripping and crosscutting. In general this is a true statement ... but about the only place I have ever seen people bother to change over is in sawmills where there are saws set up to do both jobs permanently.

If you have heaps of timber to cut it would very much be worth changing over. But also you need to think how you will actually use the tool.

A saw bench will usually be required to rip.
A radial arm saw will usually be used for crosscut
A hand circular would normally crosscut in most trades, but some use them for roughing out planks in which case they will normally rip.

Think it through and it becomes quite a useful tip too.

MIK

[Theodor]

The sacrificial piece of wood on top of the cut, works every time, assuming you have firm contact between the two.

But how do you see your lines under a sacrificial piece of wood, especially if you're cutting along bendy lines?

[b.o.a.t.]

David
It won't help with your saw, but my deWalt jigsaw comes with very close-fitting clear
plastic shoe inserts around the blade. Near zero tearing with one fitted. Without it,
only a *very* sharp blade used *very* lightly seems to avoid tearing meranti BS1088.

I would expect with the circular saw that (comparing it with my Ryobi 7.5inch), if you can
++ set it to only *just* cut the depth of material, and
++ use a really sharp cross-cut blade, with maybe 3 - 4 teeth per inch and
++ use it lightly & slowly,
you should get a relatively clean cut in most thin plywoods.
Only way to eliminate tearing is to do the sacrificial layer on top routine.
Mark your form on the sacrificial sheet & clamp it over the real one.
Then go for it as hard as you like!
cheers
Alan J

[Boatmik]

Howdy All,

Don't be too worried about this David, Just cut it, plane it, glue it. if there is any edge chipping it will usually be hidden by some other bit of structure.

MIK

[PAR]

Your cut line has to be drawn on the sacrificial piece, which is a pain.

If you're doing a glued seam boat and the edge you're cutting is eventually going to be under a fillet or tape (or both), then some grain rip out is a good thing, as it will help "key" the fillet or epoxy to the piece, making things marginaly stronger.

You could also go to the trouble of removing the teeth "set" on the blade, which will reduce rip out. I have several circular saw blades with modified teeth. A few that have tapered teeth to cut wedge grooves, others with more set worked into them so I have a bigger kerf and a couple with no set, which I use on thin materials, including aluminum sheet.

It's a lot of trouble removing the tooth set on a saw blade, so make sure this is what you want, as the blade will be useless for anything but light duty afterwards.

[millhavenguy]

After reading through my OZ PDR plans, the part where Michael reccomends coating the ply with 3 coats of epoxy, I'm fairly confident that the blade (5 1/2", 24 tooth, carbide tipped, thin blade) I've got will do a fine job. Thanks for all the hints mates!

[Boatmik]

I'd take a slightly different tack from PAR (hoping to end up picking a bit of a lift that will allow me to cover him next time we cross - its the only way to keep a hotshot like him from getting a big lead) ...

But ... the comment about chipping out from the sawing is true, usually it will be glued or taped or rounded ... so not usually much of a problem in the end.

But I would probably refrain from saying it improves the bond ... then everyone will be looking for jigsaw blades that do a good job of chipping!

Rather than being equally worried now about finding blades that do a minimum!

In the end cutting is only a small part of the reason for ending up with a good result.

Best wishes
MIK