Measuring for equal cuts

OliLt

Member
I am trying to rip 4 X 8 plywood sheets into 3 equal parts, roughly 16" minus the width of the kerf. What math do I need to apply to get the distance between the blade and the fence. So far, my results are close but the last piece is under size. The kerf of the blade is 1/8". I can accurately measure to a 32nd. Somehow I am missing something. Thanks.
 
If you're using a usual spring-wound tape measure, also remember that the tips of those usually have a slotted end on the rivets to help account for tip width, kerf, etc. Also...check the plywood, across each end and in the middle. Unless it's furniture-grade ply, there will likely be some variance sheet to sheet.
 
I agree with evilboweivel's answer. That's what you can shoot for in a perfect world.

But if getting them all the same is more important, go with 15 7/8.
 
I'm not sure what kind of plywood you're using, but sometimes plywood is slightly undersized to allow for a small gap between sheets when used as sheeting.

If getting them exactly the same is more important than getting the absolute widest piece, I'd just set the saw for 15 3/4" or so and rip the last piece as well.
 
Okay, everybody is throwing answers at you, but not answering the question you asked. You asked for the math you would need to get the correct measurement. To get three pieces you have to make two cuts. Each cut produces a 1/8" kerf, so subtract 1/4" from the actual width of the plywood, and divide that figure by 3. For instance, if the plywood is a true 48" wide, you would subtract 1/4" and get 47-3/4". Divided by three that's 15.917", which is about 15-29/32", which is about 1/100" (0.01") off. That's as close as you can get with a measurement accuracy of 1/32".

I would have guessed that your error was something most of us do all the time---forget that cutting something into three pieces takes two cuts, not three. It's a dandy theory, but it doesn't work. If you calculate for three kerfs instead of two, your last piece comes out wider than the first two, not narrower.

Stan
 
If you expect to make two cuts and get three identical pieces, it's not going to happen. You have to set the fence slightly under your calculated width, then rip three times. If you've set it right, the last rip will remove maybe a sixteenth of an inch. I usually set my saw for 15-7/8 inches.
 
Measure the plywood and mark it in exact one thirds. Set the fence to cut exactly On the line. Make first cut then flip the plywood end for end. Make both cuts using the factory edge against the fence.
 
> Measure the plywood and mark it in exact one thirds. Set the fence to cut exactly On the line. Make first cut then flip the plywood end for end. Make both cuts using the factory edge against the fence.

Won't work. Middle piece gets cut twice, so it ends up smaller.
 
OK I am not a cabinet maker but just kinda like a wood worker wannabe, but to 1/32" over the length of a sheet? on a standard table saw? with the standard issue type fence? and hand fed? Really?? Not on my saw with me running it.
To answer your question; You need to realize that two of your pieces are cut on one side and one is cut on both sides. Thus your saw kerf needs to be such that 2/3 of it is out of the end sheets and 1/3 each side of the middle sheet, then they will be equal size. (my) real world is you aint going to be with in a 32 anyway, on a table saw, with standard fence, and hand fed,,,
 
Guess I should have added the math for your fence off set is as follows. Actual sheet width (which aint 48" by the way) divided by 3 and then minus 1/3 of actual saw kerf. This is your fence to fence side edge of blade measurement.
 
Hi, I would mark plywood at 16 " from each side. Set fence so kerf is centered on mark. After first cut use factory edge against fence for second cut. Then cut all 3 to gether so there will be minimum waste. No figuring out. Ed Will Oliver BC
 
First measure the sheet and see if it is truly 48". Then if your blade is removing 1/8" per cut subtract 1/4" and divide by three. Problem is 47 3/4" doesn't divide by three very good. It comes out between 15 29/32" and 15 59/64".
 
I'd set the fence to 15 7/8" and cut all three to that width. You waste an 1/8" somewhere but I wouldn't lose sleep over it.
 
Set inside of sawblade 15 7/8 inches from fence. Run panel through three times, same direction same side.
 
Hi, I would mark plywood at 16 " from each side. Set fence so kerf is centered on mark. After first cut use factory edge against fence for second cut. Then cut all 3 to gether so there will be minimum waste. No figuring out. Ed Will Oliver BC
 
Don't know where you get your plywood but any I've bought recently has been reasonably square and spot-on dimensionally.

Don't know what exactly you're doing but it is definitely possible to rip a sheet of plywood into three equal strips with only two cuts. I do it with an old handheld circular saw and a Kreg saw guide all the time.
 
> ...it is definitely possible to rip a sheet of plywood into three equal strips with only two cuts.

It depends on your definition of "equal". If you mean within a sixteen of an inch, sure, it can be done. If you mean close enough that the difference isn't discernible to the naked eye when you put the pieces end-to-end, that's nearly impossible.
 
> I would mark plywood at 16 " from each side. Set fence so kerf is centered on mark. After first cut use factory edge against fence for second cut. Then cut all 3 to gether so there will be minimum waste.

Let's work that out, assuming a 48 inch wide sheet and an 1/8 inch kerf: After you make your first two cuts, you have the two edge sheets, each 15-15/16 inches wide, and the middle sheet, which will be 15-7/8 inches wide. In order to rip the sheets a third time, you have to reset the fence to the width of the narrowest sheet, 15-7/8 inches. Why wouldn't you just set it to 15-7/8 to begin with, and save yourself the aggravation of re-ripping the edge sheets?
 
Set the distance between the fence and the inside of the blade to 15 3/4". Cut the sheet three times. The end result will be three pieces the same width. You will have a small drop but if you are worried about 1/32"......

If the project will require more than one full sheet to be cut down the saw will be set for the next sheet and varying sheet width will not affect the out come (assuming all sheets are wider than 45 5/8").
 

For something as long as a sheet of plywood your blade has to be very very true to your table top or your kerf can be significantly wider than the width of the blade.
 

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