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Discussion Starter · #1 ·
The problem: Starting with a sheet of plywood, I need to cut a full length piece with two perfectly straight and parallel edges.

Sounds easy right... that's what table saws are for? Ah, but there's no table saw available... The challenge is to do this with no power tools other than a cordless circular saw, a cordless drill, and a router. And just to make it interesting, the longest available straightedge that I trust is only 36".

Additional context... When I say "perfectly" straight, that begs questions about acceptable tolerances. Think tool-worthy tolerances rather than furniture or construction quality. The pieces I'm cutting will become references for other projects and/or pieces of jigs/tools.


The crazy solution I'm considering...

First, either trust a factory edge, or (my choice) make one edge straight as follows...
  • When oriented right, my Bosch RA1054 edge guide looks remarkably like a jointer, so add a thin piece of laminate to the "outfeed" side of the guide using double stick tape.
  • Put a 1/2" spiral bit in the router
  • Align the router bit with the shimmed outfeed side
  • Use the router like a jointer - making a few passes along the length of the plywood.
  • Although the edge guide's "infeed" and "outfeed" fences are only 5" long each, we're starting with a factory edge, and a few passes "should" leave that edge as straight as possible within the tool constraints.

Second, make a cut, perfectly parallel to the reference edge...
  • Remove the laminate shim (see step 1) from the edge guide
  • Insert a 1/4" spiral bit (or perhaps 1/8"?)
  • Set the edge guide to cut at the right width
  • Then make a series of increasing depth cuts to cut off the piece - referencing from the previously straightened edge.


The questions I'm asking... since ya'll have more router hours than I have router seconds...
  1. Is there any reason this can't or shouldn't work? Any safety concerns?
  2. What changes would you make to the above procedure to improve safety, precision, or anything else?
  3. Is there an entirely different procedure you'd recommend? (Like buying a proper straightedge and using it as a saw guide instead of using a router for a saw.)
  4. What should I be asking that I'm not asking? (ie the things I don't know that I don't know.)
 

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time to get you a quality clamp on straight edge...
 

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Re the router edge guide;
Ashley, I don't have mine handy, but I think there's screw holes in the two flat metal edge guides (either side of the bit cutout).
You could mount aluminum extrusions on either side to extend the length of your straight references...say maybe a foot either side.
I had wooden extensions on my old Craftsman edge guide for years.

Forgot to mention, my old wood shop teacher warned us to not ever trust the factory edges on plywood. Mind you technology has changed dramatically in the last 50 years. No idea whether modern plywood is guaranteed perfectly square and straight...
 

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I would imagine you could always go to a local woodworking shop, and have them do it, if nothing else. I'd likely just use the edge off of a sheet of plywood.
 
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I think the factory edge is pretty straight. Now, the question is how do you get the inside edge equally straight? Well, you have two strait factory edges. Buy some excellent ply to start with, not the cheap stuff that will splinter on you. If you're using a circular saw, work face down. Place painter's tape along the cut line, preferably on both sides. Now, cut the sheet down the middle as straight as you can. Now you have two straight edges to work with. Cut a couple of wooden blocks the width of the final straight edge you want, adding the offset for the saw blade. My 18v saw is offset by 1 1/8 th inch, so if I want a 6 inch wide strip, I'd cut the squares 7 1/8th inch.

Lay the blocks onto the edge of the bottom piece, butt the straight edge of the other half up against the blocks. That's going to give you a good cut line. Clamp it together and make the cut being very careful to make the cut straight. Draw arrows toward the factory edge, otherwise you'll forget.

Now, you have two straight edges. Use them to trap the saw's base and your cuts will be very straight. Cut as many straight edges as you want. Do trap the base, it is really a challenge to cut a truly straight edge without trapping the saw.

Now I have a track saw, but this is how I have done it before the that, using my nice little DW 18v saw. I have an 8 ft aluminum L beam that cuts a fairly straight line, but it is not a perfect straight edge.

Last thing, if you can get a premium saw blade, full kerf if possible, or if your saw will accept it. Hope this helps.
 

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Ashley,

You didn't mention if you own/use a square or a T square to check how straight the edge is. That would be my first stop. I have a 4ft T square that I use for drywall marking and cutting, and also to check squareness on large sheet goods. I ran a business several years ago and our neighbour was a wholesale sheet goods vendor that had a wide variety of sheet goods. During a discussion with the manager, he stated that while the sheet goods manufacturers AIM for perfect 90 degree square edges, reality is they cannot guarantee it. A perfect example is some sheet goods come in 49'' x 97'' to allow for both slight damage caused by handling and also for out of squareness. I check this when it is to be used for furniture or jigs etc... and to be honest, I have not come across a sheet that was enough out of square to merit an adjustment cut. In my humble opinion, using a circular saw or a router are both possible, the choice is which one are you more comfortable and confident during use.

Dan
 

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My suggestion above will work about as well with a router. My only concern is that you used the word "perfectly" and "tool worthy." We work with wood, which moves, twists, is under unseen tension, and variances in straightness and thickness, wierd grain patterns. Prefect can't exist in woodworking. A precision straight edge is expensive. A 96 inch precision straight edge is a deal breaker for most of us. You'll find that even your carefully cut ply straight edge will make it almost impossible to draw a perfectly straignt line with a pencil. But for jigs and guides for woodworking, it will be good enough. I visited Laguna a few years ago and in the process they brought out their macined straight edge, which they kept carefully packaged to prevent dings. I hate to think what they paid for those four foot straight edges.

To tell the truth, that's my main reason for buying the track saw. Although I could cut a nice edge with my table saw, hoisting and feeding a 4x8 sheet of 3/4 ply through the saw is just not going to happen anymore. I got a Triton track saw--great, straight, no-tearout cuts, middle price range. Add a premium full kerf blade, a fine lead mechanical pencil for marking, and you have the right tools for the job.
 

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You could buy the plywood sheet at one of the big box stores and have them make the cut for you on their panel cutting saw. I don't know what the tolerances are for the saw, but all the cuts that I've had them do seemed to be pretty accurate/straight.
 

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Discussion Starter · #9 · (Edited)
Hmmm, it seems we're split on whether the factory edge is a trustworthy straight edge. If we assume it is, then Tom's technique is perfect... simple, obvious in retrospect, but I was stuck at the incorrect mental assumption that cut #1 had to be perfect. By allowing cut#1 to be imperfect, I get two straight edges from the outside edges of a single sheet and from there everything falls into place. (Thanks Tom for a nice whack on the side of the head!)

If we don't assume the factory edge is perfect, then Dan's suggestion (use aluminum extrusions to lengthen the edge guide) improves my router based jointing approach and gets me the two straight edges needed to use Tom's technique. And, it allows me to use the saw to saw and drop that second step with the router-as-saw.

Stick and Dan, I like your suggestions for straight edges as I trust them more than a factory edge, but I'm not aware of either solution in lengths sufficient for ripping an 8' sheet. I have a ten or twelve foot length of 2" angle iron for this purpose, but it's in a storage unit a couple states away.

Unless someone else pulls another rabbit out of their hat, I'll probably do two things...
  1. Use a tight string to be picky about the edge when selecting plywood... then use Tom's technique... then test my newly cut edges against each other to see how good they are. I'm OCD tempted to use feeler gauges, but they're in the same storage unit, so an index card will probably be the gauge.
  2. If the results are too sloppy, and if I believe the factory edge is the issue rather than my technique, then I'll order some extrusions (like this or this or this) to extend my edge guide, and use the router to get two straight edges before using Tom's technique to get the parallel edge.

Tom, you have my number with that track saw. Although I'm not ready to buy one yet, my first use for a long straight edge is turning my saw into a DIY tracksaw so this problem is solved well enough to stop missing my table saw. I don't really want to replace it since I have at least one more move in my near future.
 

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Ashley; it doesn't necessarily have to be metal extensions, it's just that would be my preference...no flexing. Aluminum is lighter but steel would also be fine, except that would add a bit of unwanted weight.
Wood works just fine... a couple of 3/4" x 1" strips of Birch or whatever you have.
 

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I agree on fitting two sheets together on edge to test for straightness. I used an aluminum L shaped angle iron for awhile, but it flexed a little with the saw against it. Never thought to test it with a string or thread.

Take a strong friend along with you to wrestle the sheet goods. You'll want to pick some a few sheets down the stack to avoid damaged or warped sheets. The concrete floors are reasonably flat so shouldn't be a problem. Given your storage and shop limitations, I doubt you'll be using these for long.

Get a 4x8 sheet of insulating foam while you're there. Makes cutting easier when the ply is sitting on the foam, laid out on the garage floor.

I find it fun to think these things through.
 

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You could buy the plywood sheet at one of the big box stores and have them make the cut for you on their panel cutting saw. I don't know what the tolerances are for the saw, but all the cuts that I've had them do seemed to be pretty accurate/straight.

Chuck, that sure isn't true where I live. You are lucky my friend.
 

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Ashley; it doesn't necessarily have to be metal extensions, it's just that would be my preference...no flexing. Aluminum is lighter but steel would also be fine, except that would add a bit of unwanted weight.
Wood works just fine... a couple of 3/4" x 1" strips of Birch or whatever you have.
Back when I was making solid surface tops, I had a straight edge that was made from a piece of 3/8" thick x 5" wide aluminum flat - sneaked it in the back door at work and they kissed the one edge flat to within less than .005" on the big mill. Sure wish I still had that.

You can check a straight edge with a string line, or with a laser if you have one. The straight edge doesn't have to be super stiff/heavy - I've seen a metal stud recommended for this use - all you need to do is clamp a couple of ply strips to the material being cut to butt against the back of the straight edge and prevent deflection - I've used 3" wide strips of plywood, long enough to run from the opposite edge of the sheet and butt up against the back of the straight edge. It goes without saying that the blade being used should be sharp, and the work area set up so that the cut can be made in one nice even pass, nice even feed rate, no stops and starts.
 

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The method for checking squareness of a sheet:
Measure the length of each side. If parallel sides are not equal in length then the sheet is not square.
If parallel sides are the same lengths measure across the diagonals. If the diagonals are equal then the sheet is square.
 
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I would recommend getting a good straight edge rather than reinventing the mouse trap...routers are not good at cutting like a saw is intended.
 
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This is my version of a "track saw". An 8 foot length of 1x2 poplar on a 1/2" piece of birch ply. I attached the poplar on the ply, leaving a little wider than the base of my circ saw, then using the poplar as a straight edge, cut the ply to the exact with of my saw base to the blade. Clamp the straight edge to the sheet goods on the cut line and rip away. The guide is marked with the name of the saw used to make it. I've got one in a 4 foot length as well.
 

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Discussion Starter · #20 ·
I find it fun to think these things through.

Yes, what an adventure! That's the other reason I'm not in a rush to replace my table saw - I'm exploring how much I can bootstrap with the minimal tools I have on hand. I enjoy figuring out what can be done, what sequence is necessary/optimal, and how to maximize precision. That makes up for my withdrawals from the amazing shop I used to use at the Dallas makerspace.
 
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