Finally got my marine plywood today, and sure hate to start carving on it by hand.  This plywood cost lots of $$$ and I don't want to mess it up. Is there any way to get a precise cut?

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Check this out. This is a great way to build a cheap scarfing jig and can be customized to any scale (1/10, 1/12, etc...)

http://www.oneoceankayaks.com/stitchglue/plyshophtm/scarfjig2.htm

Some MDF and a Skil saw and you're ready to go. Depending on blade width you can do quite a few sheets.
I have made the same type of jig as the one in the web page.

I was not all that happy with the result and have gone back to using a hand plane. I work in a shop all day with jointers, planers, tables saws etc etc. I still have all my fingers. I didn't like the feel of 3 inches of blade in a cut. A mistake with a blade guard wedged open is not good. Be careful with this type of cut and have a good hold on the saw. The jig is now in the corner of the shop collecting dust.

It can be done safely if one does not get careless. I actually like doing it with a hand plane but, I wouldn't want to do it all day long.

I share a shop with a fellow who runs a frames worm drive saw every day with the guard off and with the plate set to full depth. He has been doing it for years. We only get one mistake with this setup. I will not touch that saw.
West Marine sells a tool called the "Scarfer". It is a huge time saver and cuts a clean straight line. It is basicly just a couple of pieces of angled metal that you bolt to your skill saw. After you get your plywood clamped to your table you just run your saw down the edge and your done. Whatever way you end up doing it, just make sure you practice on a piece of cheap plywood first so you get it perfect on the expensive stuff. You can buy that scarfer off west marines website for like 60 bucks.
This is very helpful! This is exactly what I was looking for instead of building the jig I posted for Riverman. This site is just great!

I bought the West Marine scarfer and was disappointed to find it only cuts an 8:1 ratio in a 1/4 inch panel. West Marine says this is a strong enough, but I'm using Roger's book to guide me through some Don Hill plans, and I'm of the conviction that this joint needs the 12:1 ratio. Can anyone offer any guidance on this?

Disregard this, I just Randy's comment about the ratio, 12:1 with a planer it is--since I don't have a CNC router.

Dave Z covered it nicely here: Scarfing Plywood There are a couple kinds of jigs you can buy or make, but I didn't see one that appealed to me - maybe if I were building a number of boats. I suspect the hand plane method is easier to get precise (or at least less likely to screw up), just takes a little patience and elbow grease. Make sure your plane is sharp, and resharpen during your planing - meranti in particular dulls edges quickly. I use honing film on a piece of marble tile (or glass) to touch up the blade.
Michael
Sounds familiar, I asked this question myself several months ago and got a lot of responses and encouragement, go to the forums search and type in scarf joint or scarf jig and you will see many discussions also try byyb.org/forum/index.php?topic=931.0 This one looked the most precise to me. I wish I could tell you how good it works but since purchasing all my materials and the equipment I was lacking for my build time has been on back order with no e.t.a. in sight. Good luck and show us pics
Gary
Practice, then practice some more. Scarf some 1/4 inch lath, then practice on 1/4 scrap plywood - especially if you are going to use a plane (which is my preference). A critical issue in any method is having the end of the board well supported. If you use a scarfing jig (for example the West scarfer on a circular saw) and the plywood flexes you will not have an even scarf. Once you get the hang of it you will be looking around to see what needs to be scarfed!
Apparently, that West scarfer is not used on 1/2" plywood... everything I read shows up to 3/8. I need 1/4" and 1/2" joints.
you can use the scarfer on half inch, you just have to sand off a little edge that is left after you cut it with the saw, doing it this way saves you a lot of time and gives you a smooth scarf
Riverman, take a look at ML Web;s recent set of pictures he posted on his personal page. Initial quick material removal followed be accurate fine tuning.

Rick Newman
One way you might get a little more "depth" to the cut using a scarfing jig is to see if your circular saw can be modified with a little filing/grinding to accept a full 8' blade rather than the usual 7 1/4". I have done it with a Craftsman saw. The guard has to be removed to make it work. This probably will give me some grief comments from the forum as per safety- but I still have all my fingers. The plane is much more work- and as previously stated Merranti eats the hell out of the blades. Fortunately my kids bought me a Tormec Grinder and that saves a lot of time.
Good luck

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