I have a board about 1.5" thick and 12" wide and 8 feet long.

I need to cut the board diagonally and want the cut to be as straight as possible. (I am cutting the board diagonally. This does not involve a bevel cut.)

enter image description here

How can I do that?

I have a circular saw and a small portable table saw. I can buy equipment, but nothing large. I cannot buy a giant table saw with an 8' bed, for example.

(I do not think using outriggers with the portable table saw will work, because I have tried that before and it is not feasible to keep a board straight while feeding it 8 feet onto saw horses.)

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    The "at an angle" part of your question relates to the diagonal cut, not an angled cut (with tilted saw)? --- Also: is a sacrifical board below the one you want to cut unfeasible for some reason? otherwise you would put one below, have the circular saw on a depth that cuts just the board on top, and then use a ruler (or long board) to guide the saw – bukwyrm Apr 27 '18 at 12:09
  • @bukwyrm I have added a diagram to make it obvious. I do not have an eight foot long bench, so I cannot make a long fence. – Tyler Durden Apr 27 '18 at 12:23
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    Sawhorses with a board on top to form a makeshift table, then your green board on top, and a straight aluminum ruler (up to 3m, in most hardware stores) fastened with clamps to the green board parallel to the cut with an offset as big as your circular saw is wide? Or is your circular saw fastened to a saw-table? Like so: images-na.ssl-images-amazon.com/images/I/… – bukwyrm Apr 27 '18 at 12:27
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    @bukwyrm: This should be an answer. - Suggestion, additional, a straight board will work as well as a metal ruler and is a lot cheaper. – A. I. Breveleri Apr 27 '18 at 12:32
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    Possible duplicate of How to rip unparallel on table saw? – isherwood Apr 27 '18 at 13:04

Sawhorses with a board on top to form a makeshift table, then your green board on top, and a straight aluminum ruler (up to 3m, in most hardware stores) fastened with clamps to the green board parallel to the cut with an offset as big as your circular saw is wide? Or is your circular saw fastened to a saw-table?

Like so. image: amazon.com

Alternately use a very straight board clamped to the work as a guide for the saw.

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  • This is what I've always done. You can use anything for the guide - doesn't have to be as fancy as an aluminum ruler. Could be a scrap piece of plywood or any other lumber that's straight. Clamp the guide board to your workpiece and run the circular saw along it. – dwizum Apr 27 '18 at 16:58
  • Any tips for beginning and finishing cuts with a guide like this? I find it difficult to get/keep the saw aligned properly when the bulk of it is off either end of the work piece. – Adrian McCarthy Apr 27 '18 at 20:23
  • This is how I do it. The machine-made edge of a sheet of plywood is a great straight-edge; it will be straighter than anything in the shop. – Tony Ennis Apr 28 '18 at 2:08

A great thing to have on hand is a zero-tolerance circular saw guide. You can make one (I have multiples in different lengths) with a couple of boards you can get at a big box stores, some glue and a few screws (optional).

Get a length of MDF board. Something like this:

enter image description here

You want this to be dead straight so you'll want to buy it in this form. I would go with the wider pieces but it depends on what you need. Don't get it too thick or it will limit the depth of your cut.

Then take a piece of thin plywood that is as long as your MDF board and at least as wide as the MDF plus the with of your saw base. Glue the MDF to the plywood like shown in the following picture from this article with instructions:

enter image description here

The article suggests different materials but the MDF plus plywood worked for me. Then take your saw and put the base up against the MDF fence and cut the wide side of the guide. Now you have a zero-tolerance jig that you can clamp down and know that your cut will be dead straight and exactly on the edge of the guide.

In your specific situation, I would mark the line you want and then take the guide and line up the edge of the guide parallel with your marking. Place the guide on the side of the piece you wish to keep if possible. If you are forced to put the guide on the waste side of the cut, make sure you consider the kerf of your blade. Clamp in place (I like quick-clamps for this) and you can't miss.

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    @JimmyJames if the plank is 8 feet the cut will be longer than 8 feet. Not much longer - only 0.74 inches longer if an internet calculator is to be believed. – stannius Apr 27 '18 at 19:11
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    @JimmyJames since it's only an extra inch, an 8 foot jig should be "close enough" to cut the extra inch just based on the fact that the base plate of the saw is more than a couple inches long. – stannius Apr 27 '18 at 19:16
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    @TylerDurden Since you are cutting a board, you should be able to manage with 2 or 3 saw horses. The board will support itself. The guide lays on top of the board you are cutting and is clamped to it. The one thing that might pose a challenge is the depth of cut. You say 1.5 in the question but 1.75 here. You could go with a plywood fence like the article and use a thinner plywood base. You just want to use a really clean straight side for the fence. – JimmyJames Apr 27 '18 at 19:16
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    @JimmyJames would OP need some sacrificial wood standoffs between the horses and the work piece? Usually I clamp the work piece off the edge of the saw horses, but, that seems like it would be difficult here. – stannius Apr 27 '18 at 19:18
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    @stannius I consider my horses sacrificial since they are just some 2X4s and metal clamps I bought 14 years ago. If you care about your horses, yes, that's a good point. – JimmyJames Apr 27 '18 at 19:22

I had a bunch of super-oblique cuts to do like that. I did this:

  1. Mark the board the way you want it cut, clearly (not pencil on pine, but more like Sharpie, use pencil to get the line adjusted.)

  2. Optional: Attach the board to a piece of scrap, such that the straight edge of the scrap is perfectly parallel to the line you want to cut.

  3. Take it to the lumberyard and have them cut it for you on their big table saw for $1.

Big-box stores in malls with large, well-lit parking lots and neon signs are not lumberyards. Nor would I trust any of their $9/hour clerks to make such a cut properly if they had the saw, which they do not. A lumberyard has a small parking lot where you're 30 feet from their front door, a small shop with yet most of the selection of a big-box, and a huge backlot loaded with saws and every kind of lumber. Prices are competitive.

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    Hah, I was once at a big box store and I needed two five foot shelves, so I figured I'd just ask them to cut me one ten foot shelf in half rather than buying two six foot shelves and cutting them myself. The employee standing by the saw, which was spotless, said sorry, the saw is closed. I asked the manager if it was the policy of the store to "close the saw" before the store closed, who said, "no, but I know who told you that: the guy who just cleaned the saw". Why you would clean a saw before closing time is beyond me, unless you like cleaning it twice. – Eric Lippert Apr 27 '18 at 16:57
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    I have had varied experiences with the big box stores. Sometimes the saw is not available and nobody even knows why. But there are other times where they say "We can't do precision cuts, so you take what you get" - and then proceed to measure down to 1/8" and do a fantastic job, and often not even bother to charge ("4 cuts free and I got 7 cuts" -> "don't worry about it". – manassehkatz-Moving 2 Codidact Apr 27 '18 at 18:16
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    As far as I know the big box stores are only set up to do 90 degree cuts. They wouldn't be any more set up to do an angled cut than OP. – stannius Apr 27 '18 at 19:13

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