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cross cutting plywood square

charlie44732377's picture

 


This is my biggest problem. If the 4x8 sheet comes out of square, one must somehow make a cross cut square to the long edge so that the fence can then be used as a guide to run this cross cut edge against to cut smaller sizes that are also square. How to make a true square cross cut without some kind of sliding table rig that one can hold the long edge against and make the square crosscut. A good attachment like this for a table saw runs around $1000. Too much for my budget. Actually all I really need is a way to make a square line on the plywood, then I don't mind using a straight edge and a circular saw. I should say that I don't believe a framing square or a sheetrock square or the 3 4 5 method are really accurate enough

 Looking up "squaring plywood" on the FWW website gave no results. Curious! It's such a basic and presumably oft-encountered situation for anyone dealing with plywood.
 

Charlie 

BruceS's picture

3.4.5 (post #165776, reply #1 of 10)

How can the 3.4.5 method not be accurate ?  Use a good sharp marking knife,  same as you would for dovetail layout.  Make the saw cuts with a Festool TS55 and the rail.

Work Safe,  Count to 10 when your done for the day !!

Bruce S. 

 

SteveSchoene's picture

Festool makes the saw, the (post #165776, reply #2 of 10)

Festool makes the saw, the guide, and now has a squaring devise for positioning the guide on the wood.    All really good stuff, but also out of budget. 

And, yes, 3-4-5 must define a right angle to what ever degree of precision you can measure and mark the lengths, it's not an approximation. 

Test your finish on scrap, FIRST, or risk having to scrap your finish.

woodhacker's picture

3,4,5 or diagonals (post #165776, reply #3 of 10)

The 3,4,5 method certainly should give you what you're after, remember that you can use feet as a unit of measure, not just inches.

Another approach would be to take diagonal measurments of the panel


Either of these methods should allow you to score a line more accurately than you'll be able to position your straight edge to the line.

oldusty's picture

Diagonals may be even but the (post #165776, reply #5 of 10)

Diagonals may be even but the piece may still be out of square , it may be a trapazoid .

          been there      dusty

oldusty's picture

Charlie ,          It's not (post #165776, reply #4 of 10)

Charlie ,

         It's not rocket science , a framing or the longer drywall square will be fine once you connect the lines .

       You know it is out of square because you prolly used a framing square to tell you .

    Why not cut to the line then crosscut the first cut off a tad longish and then go back and trim the original cut end .

         When I check a corner in a room for square I use the 3 4 5 method , 3 feet one way 4 feet the other way should leave 5 feet between the marks , if you get 60 1/2" the corner is open and wider then 45° , if you get 59 1/2" then the corner is closed or less then 45° . This method is a quick teach on what the corner is doing it is not dead on accurate but more of an indication. Using the 345 on a sheet of plywood will not aid in making it square to my knowledge and if it is out of square then what ?

             regards   dusty

      

     

Jigs-n-fixtures's picture

Make a cutoff jig that is square (post #165776, reply #6 of 10)

Make a cutoff board/guide for a skill saw.  It is just a straight 1X3 glued and nailed to a strip of plywood that is wide enough that you can trim it to match the cut of your skill saw.  It needs to be about 60-inches long.  Make up the guide, and install a high quality blade with the most teeth you can find, and use that to trim the width. 

Once you have it built, install another piece of straight 1X3 on it at 90-degrees, with one screw so that you have a Tee square, that is still not fixed so it can be adjusted.  Install the 1X3 far enough in from the end that the front of the blade is a little behind the face of the Tee, so you can set the saw on it.

Get it as close to 90-degrees as you can, and clamp it tight with a small clamp.  Make a test cut and adjust as needed untill you have it as suare as you want it.  Then with out releasing the clamp add a couple of more screws, to lock it in place.  And, then remove the clamp and put in two more screws so the blade and tee are locked into place. 

Drill a hole in it so that you can hang it up so it doesn't get knoced around. 

Simple tip:  Put a 1-inch piece of foam on your work table, set the depth of cut on your saw to about 1/8-inch deeper thatn the thickness of your plywood, and then make your  cuts on top of a nice solid table.  This helps keep things true, and gives you a nice zero clearance "insert" on the bottom of the cut.  Remember the good side goes down.  The foam will last a year or so before you get so many cuts it is too funky to use. 

sapwood's picture

I have built a whole bunch of (post #165776, reply #7 of 10)

I have built a whole bunch of custom cabinets and furniture out of plywood never having a big slider saw. First off, it is seldom that one needs the entire 48" width, so it's better to start by ripping the sheet. If you have to crosscut, do it "long" for later trimming. By ripping first you'll have narrow pieces that may fit into your cross-cut sled. If they don't and you can't make one large enough, then use the 3,4, 5 method or a really good square. I've got a t-square by Bridge City Tools that is amazingly accurate. In any case, the narrower pieces will minimize your error in layout.

Now, if you simply must accurately cross-cut the entire width the only method I know of that is reliable is the 345 method. I have always found it best to use a long ruler... I've got a 6' one that is both accurate and inexpensive. You could even make one out of wood that was dedicated to this purpose.

Once your line is marked out, use your Festool if you got one, your skill saw with guide, or a router fitted with a guide bearing. All of these methods are really rather slow but doable. Accurate jigs will speed things up a great deal. 

DonStephan's picture

Don't think of the geometric (post #165776, reply #9 of 10)

Don't think of the geometric layout in terms of feet, but instead using inches or centimeters.  Using a Stanley or other retracting tape measure, start from the 10" mark for consistency.  Come in an inch or so from the end, make a starting mark on the edge and swing an arc (3 * 14 =) 42" across the width.  Measure (4 * 14 =) 56" along the length and make a mark.  From this second mark, swing an arc (5 * 14 =) 70" to cross the first arc.  Connect the starting mark with the intersection of the arcs.  A Festool 4' guide and saw is excellent for cutting along this line to make the end perpendicular to the incorporated long edge.

As long as the measurements are accurately marked and the line cut, the results should be extremely accurate.  If both long edges are irregular, trim one straight before beginning.

If this is a regular need, you can build using 3/4" baltic birch a very accurate right triangle 54" on a side and use as a saw guide, avoid laying out a triangle on every sheet.

VAM's picture

Just Use Your Compass (post #165776, reply #8 of 10)

This method is quick and can be used to bisect a line or to draw a perpendicular to a line.  Just need a compass and a straightedge

http://www.mathopenref.com/constperplinepoint.html

forrestb's picture

or, build a longer fence (post #165776, reply #10 of 10)

I built this fence for my ShopSmith to cut 4x8 ply by myself.  It requires 2 more stands than shown for big ply (the pic is ripping long 2x4s) for proper in/outfeed support.  It gives me a square cut on the long side of the ply every time and is very safe to use since it is easy to hold the ply edge against the fence.

You really need a jointer/planer for machining the 2x4 long support to assure a very straight edge against which to ride.  A well seasoned 2x4 will minimize any seasonal variations.

Of course, if you can afford a Festool 55 and extra guide rail you don't need something like this.

 Forrest 

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