What's the best blade for cutting 3/8 ply?

[PeterSibley]

I'm going to be starting on my 18 foot canoe yawl moderately soon!

I expect to be ripping planks with my 7 1/4" Makita ..... now I'm pretty inexperienced with plywood or at least treating it gently. With this job I don't want to splinter the edges and would like everything to be neat and crisp !

What are the panel's recommendations for the right 7 1/4" blade and maybe any tips on the correct care and feeding of 3/8" marine ply .

[David G]

Peter,

First - THE INQUISITION!!!!

What sort of plywood? Meranti takes more effort to keep from splintering than occoume.

I find a good jigsaw to be better than a good skilsaw for cutting out plywood planking... are you set on the skilsaw?

What boat/how much planking?

What's your cutting setup? Will you be cutting on a sacrificial egg-crate grid? On a sheet of styrofoam? draped across a pair of wobbly sawhorses? What?

[TerryLL]

Peter,

A thin-kerf, 40-tooth, carbide-tipped blade will do a nice job. The best way to get a crisp edge right on the line is to cut just a bit wide and finish the edge with a low-angle block plane honed wicked sharp.

If this is an epoxy ply boat destined to be painted, then you need not worry too much about every little nick and ding. Thickened epoxy cures a wide assortment of errors, and paint hides all.

[PeterSibley]

Peter,

First - THE INQUISITION!!!!

What sort of plywood? Meranti takes more effort to keep from splintering than occoume.

Australian hoop pine ply, good quality if a little dense. Not overly given to splits , better than meranti in that respect.

I find a good jigsaw to be better than a good skilsaw for cutting out plywood planking... are you set on the skilsaw?

I'm MUCH better at using a Makita circular than a jigsaw but take your point . I have a Makita jigsaw but it lives at a friend's house . I could find it if necessary !

What boat/how much planking?

A Selway Fisher JIM. An 18 foot canoe yawl, 7 planks per side .... so a fair bit of cutting .

What's your cutting setup? Will you be cutting on a sacrificial egg-crate grid? On a sheet of styrofoam? draped across a pair of wobbly sawhorses? What?

No setup as yet as I seldom cut ply. Usually if I need a lot ripped I take it to the local kitchen shop and the guy rips it on his $70 K computerised marvel. That's not an option this time.

Some form of sacrificial bench will be built and suggestions are welcome .

[Gib Etheridge]

I've never had 100% success with eliminating blow outs, but have found that the more teeth the better. Use a thin kerf, like a Freud Diablo 40 tooth, and cut slowly.

It also helps a lot to set the blade depth as shallow as possible, maybe only 1/8" out on the back side. That way it's not trying to push splinters of at such a steep angle relative to the face of the work.

Also, with a skilly you should put the good face down. With planking both faces are the good faces, but often enough one will be hidden or beveled after cutting.

I've tried protecting the edge with masking tape, it never helped very much. The tape can pull off splinters where there weren't any to begin with.

Here's something that just occurred to me. If your'e going to epoxy the face of the ply later it would make it less likely to blow out if you epoxy the full sheets before cutting. Maybe it would even be appropriate to glass them if that's in the plans for later.

Cutting a bit oversized and planing to the line is an absolute necessity for me.

[PeterSibley]

Peter,

A thin-kerf, 40-tooth, carbide-tipped blade will do a nice job. The best way to get a crisp edge right on the line is to cut just a bit wide and finish the edge with a low-angle block plane honed wicked sharp.

If this is an epoxy ply boat destined to be painted, then you need not worry too much about every little nick and ding. Thickened epoxy cures a wide assortment of errors, and paint hides all.

Thanks Terry, that does sounds reassuring ! I think I could clean up an edge with a pattern bit quite well too.

[PeterSibley]

I've never had 100% success with eliminating blow outs, but have found that the more teeth the better. Use a thin kerf, like a Freud Diablo 40 tooth, and cut slowly.

It also helps a lot to set the blade depth as shallow as possible, maybe only 1/8" out on the back side. That way it's not trying to push splinters of at such a steep angle relative to the face of the work.

Also, with a skilly you should put the good face down. With planking both faces are the good faces, but often enough one will be hidden or beveled after cutting.

I've tried protecting the edge with masking tape, it never helped very much. The tape can pull off splinters where there weren't any to begin with.

Here's something that just occurred to me. If your'e going to epoxy the face of the ply later it would make it less likely to blow out if you epoxy the full sheets before cutting. Maybe it would even be appropriate to glass them if that's in the plans for later.

Cutting a bit oversized and planing to the line is an absolute necessity for me.

Thanks ...a good reminder. Cutting solid wood I tend to use full depth all the time.

[David G]

Here's one version of a shop-built cutting grid:

http://f1.grp.yahoofs.com/v1/kKh3T48...ttingTable.jpg

[PeterSibley]

My ! Isn't that a neat idea ! I'll need something to scarf up 20 foot long sections on though and that's going to be space consuming. I'm assuming people glue up ply into a long enough section to get out planks full length ?

[TerryLL]

Some form of sacrificial bench will be built and suggestions are welcome .

Peter ,

This cutting table works pretty well for ripping 4 X 8 sheet stock, and can be quickly tucked away in a corner when not in use.




For scarfing the sections together, and laying out the final plank shape and doing the final cut of the full plank, you will want a long cut table, about 4 X 20. A good example shown here:
http://bdobrowocy.blogspot.com/2010_08_01_archive.html

[PeterSibley]

Terry , that was a good link ..I'll read and learn .

[chas]

Peter /

As a wood butcher I have cut up acres of fir ply, occasionally to what might be classified as a finer tolerance than good enough and sometimes curved as well. I would suggest using your utility knife as a marking implement, substituting for the pencil. Cut deep enough to sever the outside skin, or thereabouts. If I am cutting sheets while I'm standing on them and bent over, shortness of breath and poor eyesight require that I dress them up with my electric plane and a sanding block with a 100 grit belt.

If I have a bench available and can get close to the work, the block finishes it in short order. Plywood tells you in short order if you cut too close to the scribed line. You'll find that the saw depth of 1-1.5" is more stable for cutting shallow curves in 3/8" ply rather than this ..."It also helps a lot to set the blade depth as shallow as possible, maybe only 1/8" out on the back side." The backside will be a cleaner cut with the increased depth. The extra stability allows you to cut closer to the cut line.

Most of this is a matter of familiarity, however. The 7-1/4 Makita and their 3-1/4 plane are my bread and butter tools, right after the broom! Lol. / Jim

[Lewisboater]

I second the utility knife...(only because Chas beat me to it)...and I usually cut ply on a piece of sacrificial foam. I use shallow blade depth too which helps in cutting curves and lessening tearout but using the knife really makes things clean.

[Gib Etheridge]

And I'll third the utility knife, it works very well. I forgot all about that.

[JimConlin]

If I have a lot of cuts, I sometimes sacrifice a sheet of 3/4" ply on sawhorses as a cutting table. Set depth to just 1/8" over.
The sheet of foam (on a ply table) is a good idea.

[PeterSibley]

OK, the knife seems like a REALLY good idea ! I wish I had thought of it .I can get sheets of wrapper mdf ...fiberboard for free, which will make a good disposable bench .

The blade for my Makita ? A 40 tooth? Just ask for a ply cutter ?

Yeah Chas, I'm reasonably handy with a 7 1/4" makita , it's been my #1 trade tool for years .

[Lewisboater]

One thing about using a utility knife... When cutting along the grain watch which way the grain wavers and cut in the direction that will pull the blade away from your good piece and into the scrap. That way if the blade follows a particularly firm bit of grain the cut is directed into the scrap and not into your working piece. You can then work at cutting the tougher part and continue on your merry way. A few light strokes is usually the way to go.
One reason I use the foam is that it does not wear the blade whereas any sacrificial wood product is also contributing to blade wear and reduced life. The foam is also grippy...it holds the piece firmly in place with no slippage.

[Mrleft8]

There are special blades with very little set to the teeth expressly made for very clean cuts in plywood. They are not good for general framing cuts, because they take a really long time to use, but are excellent for fine cuts in expensive panels.

[David G]

There are special blades with very little set to the teeth expressly made for very clean cuts in plywood. They are not good for general framing cuts, because they take a really long time to use, but are excellent for fine cuts in expensive panels.

Those are excellent for straight cuts. However - one of the things that makes cutting curves with a skilsaw possible is the tooth set.

Also - someone mentioned depth of cut. My experience is that anything more than just piercing the panel completely also makes swinging those curves harder.

[johnno]

Peter, a few days ago I 'discovered' a Makita jigsaw (4340CT) that I'd brought back from the shack to use here, and forgotten. Previously I'd been using the Makita 4327 that lives here normally. The difference between the two is remarkable. I'm not sure what's in your treasure trove of tools there but if you don't have a really good one, I'd recommend getting one. This one has an orbital action if you want, splinter-free attachments, guides, etc. Suddenly with the good Makita, I'm using it more often, whereas I tended just to use the lesser one only for cutting large holes, and used the circular saw for large cuts in plywood. I wasn't much of a fan of jigsaws before but now I am changing my opinion.

[chas]

Peter /

How many planks in the boat you're building, and what is the depth of the curve you are cutting in a twenty foot plank. Sounds like an unwieldly brat in 3/8 ply. / Jim

[PeterSibley]

Noted John ...my current jigsaw is unmentionable ! (Ryobi and cheap ... perhaps because I have an excellent bandsaw .) I think I might have access to that model Makita if I can track down the bloke who borrowed it !!

Chas ...7 planks per side and I'll get by.I'm rather used to solving puzzles by myself .

and I've just ordered a 60 tooth 7 1/4" blade for the Makita circular . I'll run a few test cuts when it arrives .

[keyhavenpotterer]

Beyond the blade, same as with normal wood, if I cut 'out of the grain' (top/ bottom laminate on plywood sheet) on the side of the piece I'm keeping, it gives a smoother splinter free finish.

Ed

[RFNK]

This is a good thread Peter! I've already picked up about three great tips from it! But I'd recommend against using mdf for your sacrificial cutting board. The stuff's very toxic and renders blades blunt very quickly. One sheet of crap ply from Bunnings would only set you back about $30 and it'd be enough to cut all your planks on with the recommended shallow blade setting.
Rick

[PeterSibley]

I was think of using mdf for moulds too Rick ... any reason not to? If free from the kitchen manufacturing shops in town and I hate wasting good material on eventual throw aways .

[Mrleft8]

Ah.... And if you make a sacrificial sub-base out of Masonite or better yet, clear plexiglass, for your saw shoe, and plunge the blade through that you'll get a "zero clearance" plate that'll reduce tear out significantly. The plexi is nice because you can see where you're cutting, but the Masonite cuts better.

[WX]

Peter I have a quite nice Makita Jigsaw you can borrow. Epoxying the sheets before you cut is not a bad idea. If nothing else it keeps them clean.

[chuckt]

Gib said tape didn't work for him but I used painters tape when ripping meranti. I thought it helped a lot.

[WX]

Meranti is pretty bad for splinters. With a Jigsaw I think the more teeth the finer the kerf.

[PeterSibley]

Ah.... And if you make a sacrificial sub-base out of Masonite or better yet, clear plexiglass, for your saw shoe, and plunge the blade through that you'll get a "zero clearance" plate that'll reduce tear out significantly. The plexi is nice because you can see where you're cutting, but the Masonite cuts better.

Noted !

[PeterSibley]

Peter I have a quite nice Makita Jigsaw you can borrow. Epoxying the sheets before you cut is not a bad idea. If nothing else it keeps them clean.

Not yet but I may take you up on that Gary .

[RFNK]

I was think of using mdf for moulds too Rick ... any reason not to? If free from the kitchen manufacturing shops in town and I hate wasting good material on eventual throw aways .

I don't think so. I just think it's good to avoid making dust out of it. It's good for templates etc. so should be good for moulds I think.

Rick

[Ron Williamson]

Modern MDF isn't really that bad.They are controlling the content to be in compliance with CARB standards.
Plenty of real wood is worse.

The thing you need to check on a jigsaw is the stroke length.
Cheapie homeowner ones are 11/16".
Good ones are 1".
R

[Gib Etheridge]

Ah.... And if you make a sacrificial sub-base out of Masonite or better yet, clear plexiglass, for your saw shoe, and plunge the blade through that you'll get a "zero clearance" plate that'll reduce tear out significantly. The plexi is nice because you can see where you're cutting, but the Masonite cuts better.

Ah, nice one Lefty! I'm going to remember this, I hope.

[andrewpatrol]

Pete, If you havnt got the room, scarf your planks on the boat, that way you get the angles right.

[RFNK]

Modern MDF isn't really that bad.They are controlling the content to be in compliance with CARB standards.
Plenty of real wood is worse.

The thing you need to check on a jigsaw is the stroke length.
Cheapie homeowner ones are 11/16".
Good ones are 1".
R

The use of formaldehyde in mdf is falling so it's possible that it's not quite as toxic as it used to be. However, the new stuff, like the old stuff, is still a very dense pack of randomly aligned, very fine particles. So the new stuff, like the old stuff, still turns into very large quantities of very fine dust. Much more than any plywood or any other timber that I've used.

Rick

[seo]

Often when ripping out planks it's essential to use a batten to get a fair curve. I also use a batten as a guide for my saw, whether it's a skilsaw or jigsaw. That was I do the careful work with the batten, where if I screw it up I can do it over. When I've got it right, I can pay full attention to running the saw, where if you screw it up you can ruin the plank or lose a finger. Most of my planking has been done in the boatyard, or in a storage shed, without long tables, or good light, etc.

[chuckt]

Good suggestion on the sacrificial plate. I had forgotten all about that trick. It works