circular saw jig question

[barryvabeach]

lielec11,  if you run into the situation again, or don't have a square face, you can use geometry to help.  First, assume the panel is 3 foot wide by 6 foot long  ( because you already cut off a piece.)   measure up 1 1/2 inches up at both ends on the 6 foot side, and draw a line that is exactly parallel to the bottom edge of the 6 foot edge that is 1 1/2 inches up from that edge.  ( I picked 1 1/2 inches at random, use any distance you want, so long as it is exactly parallel to the bottom edge)   Then use a compass,  or even a stick with a nail in middle and a hole in one end for a pencil, to draw a perpendicular line. http://www.mathopenref.com/constperpendray.html


This is another way to do it, just sets the line a little farther in from the edge   http://www.mathopenref.com/constperplinepoint.html

The math teacher that originally taught you geometry is slapping their head right now saying I told them they would use this later in life, and no one believed me.

[Mike in kc]

I measure as Joe mentioned, and for my straight edge, I just cut a one foot wide slice off a sheet of 3/4" and use that for one of my straight edges.   I'm space challenged  so I use the table saw top to break down sheet goods.    In your current case, just pic an edge that is straight, as "reference" edge and measure from that, or use as the base for your square.

Obviously the "sheet goods" are missing, but it lays on the stickers, and the straight edge lays on top secured with a bar clamp at each end, and I use my little 6" saw boss to break it down.  



I also use them on the work bench.

[lielec11]

You can use the square to sort of locate where you want to place your cleat. but allow plenty of excess ply past where that saws blade will cut to.  Also allow plenty of ply on the other side of the cleat if you plan to use clamps to hold the jig in place. That off side doesn't need to be straight BTW, just ample space for clamps, so they don't interfere with the saws progress as it goes down the jig. The advantage of extra material on the cutting side of the jig is, even if your straight edge turns out to be not so straight after all, when you cut the excess off the ply off it won't matter, that outer edge will be exactly the same distance from your cleat, which is the functional working part of this jig. Crooked isn't in the equation, just an equal distance from cleat to edge.



Two other things I do using that type of jig when cutting plywood that you want to be CRISP on it's edges.

1) put down a thick piece of that styrofoam, insulation foam board under your ply, the blade will bury into that and any bottom side tear out will occur on the foam.

2) On the top side figure out where your cut will be, and end to end cover the ply with a band of painters tape, you will cut through the tape, but it makes for something like a ZCI on your table saw.

Both sides will have crispy edges. Do a trial run before you actually cut, and make sure you understand the dynamics of pushing the saw forward, while also holding it tight to your cleat. The number 1 complaint I have heard is that evidently some folks have a hard time holding the saw to the guide cleat???

Joe has a carpenters background, and so do I. I think we take sawing with a circular saw for granted maybe?

I actually saw #1 elsewhere and purchased a 4x8 sheet of interior foam insulation for this purpose. It has paid for itself already.

lielec11,  if you run into the situation again, or don't have a square face, you can use geometry to help.  First, assume the panel is 3 foot wide by 6 foot long  ( because you already cut off a piece.)   measure up 1 1/2 inches up at both ends on the 6 foot side, and draw a line that is exactly parallel to the bottom edge of the 6 foot edge that is 1 1/2 inches up from that edge.  ( I picked 1 1/2 inches at random, use any distance you want, so long as it is exactly parallel to the bottom edge)   Then use a compass,  or even a stick with a nail in middle and a hole in one end for a pencil, to draw a perpendicular line. http://www.mathopenref.com/constperpendray.html


This is another way to do it, just sets the line a little farther in from the edge   http://www.mathopenref.com/constperplinepoint.html

The math teacher that originally taught you geometry is slapping their head right now saying I told them they would use this later in life, and no one believed me.

As an engineer this speaks to me, I was thinking of geometrical ways to do this and immediately thought of using trig and the angles but realized it would be very hard to measure the angel that is only 1/8" out of square. I'll give this a try just for fun. Thanks!

[AgGEM]

Don't beat yourself up. We were all there once---and sometimes we even go back!

And some of us dwell there....speaking for myself.

[handi]

I have a full sized Unisaw and all the room to dice up full sheets on it, but I don't do it. It is simply not safe and not accurate, you still need to shave off the factory edge. So I cut up full sheets with a circular saw too.

The jig shown is quick and easy to make. MicroJig gives away plans I made for them for making on that uses their Dovetail clamps to hold any piece that fits under the jig.

Kreg makes a very good circular saw guide system now that is pretty reasonably priced.

And if you want to get really fancy, I sell a set of plans for making a robust guide rail system for both your circular saw and router that can handle over 100"

[srv52761]

My personal experience... this is a jig.  Don't try to make do or "fix" it to save a few dollars.  Look at the pic of Jgrout's jig.  It is very well used.  

You will be using this thing a lot.  Spend time to make it as accurate and easy to use as possible.  Several posts have given advice on features to include.  Consider these carefully.  If your present jig does not incorporate these, consider starting anew.  Others may chime in with their suggestions. Consider them.  Remember, these people have been doing this for awhile, and have already learned some from their mistakes.  Take advantage of their experience.


My contribution:  My jig, similar to Jgrout's,
On the wide side, the jig was trimmed with the circular saw to be used after the cleat was installed (I used a 6" straight-edged particleboard shelf  from Menards as the fence to insure straight edges on both sides) with the base shoe supported by the hardboard.  This makes the jig saw specific, but not blade specific as long as the jig rests on the good side of the cut and not the waste side.


On the short side I turned the saw so the edge side was against the fence.  This allows me to use the guide with either side of my saw.  This (side of the jig) is both saw and blade specific.  But that is ok, I picked up a number of the same blades for just that reason.

[lielec11]

The jig wasn't the issue was much as my ignorance is making something square. My jig is accurate and I made a new one 8ft long for ripping larger piece of ply.

[handi]

The jig wasn't the issue was much as my ignorance is making something square. My jig is accurate and I made a new one 8ft long for ripping larger piece of ply.

Ok, cool. We can help with that.

First, you simply cannot guarantee a square cut off a factory edge of a plywood sheet. The first step is to rip one edge off in a straight line. I find this easiest using a guide rail. I typically remove about 1/4".

Now you have one true edge, you can lay a square on it and reset your guide to that square. Simple.
If you do not have a good square, invest in one. For handling sheets like this, a farmer's square will not only help you set your guide accurately, it will also allow for checking your parts. 

Buying a square, you largely get what you pay for. Accuracy is the only factor.

Once the sheet is in manageable pieces, it can be taken to the table saw where you can refine the sections to what you need. 

If the parts need to be truly square, I will cut them slightly oversized by hand then use the TS to cut them to final size, square and true.

[Steve N]

For a circular saw jig the absolute most accurate way to get your "good edge" is as I had previously described. Allow for more material that the edge of your blade to the cut side of the guide cleat. Hold the saw tight to the cleat and make the cut right through the jig. Your distance from the cleat to the edge is always going to be exactly the same IF you hold the saw tight to the cleat. You don't need to measure, you don't need to lay a square on anything, you just need to cut it tight to the guide. If the entire jig ends up looking like an triangle it matters not, as long as the distance from the cleat to the edge of the cut is exactly the same the entire length of your jig.