Fingers

There is more than one way to do it...but spme that will guarentee results.  One of the first question anybody here will want to know before they can answer your question is "what do you have to work with"?  Are you working with festool stuff....together with a table saw.....do you have an equipped shop?   Let me know what you are working with and I will try to guide you.

mwhafner hit on my first thought, which is to get accurate diagonals you must first start with parallel cuts.  If you're out of parallel to start with, square will never come.  There are a number of jigs and techniques that members here have come up with to ensure parallel cuts, then to ensure square cuts.

Here's a thread that talks about parallel cuts, including my jigs:
http://festoolownersgroup.com/index.php?topic=475.0

Here's a thread on how I make very square cuts:
http://festoolownersgroup.com/index.php?topic=2674.0

By using these methods, my accuracy is consistently less than 1/32" delta between diagonals on cabinet size panels.  I'm sure others will chime in with their methods and jigs that are equally accurate.

These panels started out as 4X8 sheets which Home Depot ripped to 2X8 to ease handling.  At home I used my TS55 and (2) 55 inch guide rails to trim them to 23 1/2 inches  They appear to be parallel along the long edges.  I then took my roofing square to verify the factory short edge was 90 degrees to the long edge and it seems to be.  I measured 30" from the short end and layed out my cut.  Lined up one saw guide and made the cut.  Checked the diagonals and they are not equal.

At this point I have to question the accuracy of my roofing square, my technique, and my TS55/guide rail combination's accuracy.  I've used the a square + b square= c square method and it proves I am not square.  I do not have an MFT so I must rely on some other method to establish a 90 degree corner from which I can work off of.

I do have a pretty good table saw and fence which I could use to make sure I have parallel sides.  But that doesn't help getting a 90 degree adjacent side.  I don't know if a speed square would be accurate over a distance of 20-48 inches.  What other methods do you recommend to get it dead on?

Here's another thread containing many ideas how to align MFT fence and Guide Rail to get square cuts.  Square MFT.  My personal favorite is to use a large reference square to set the MFT fence square to the Guide Rail.  I use an 18 inch precision carpenter's triangle from Woodpeckers.   This same tool can also be used to lay a pencil line where you intend to cross cut and thus to confirm alignment of your planned cross cut before it is made.  In this photo, the precision triangle is the gold colored item which I am using to confirm that the DeWalt T-square accessory is ninety degrees to my Guide Rail.  .

To get parallel strips that you will later cross cut into shorter lengths, there are also many ideas on FOG.  I prefer use of stops or a story stick.  Jerry Works manual on uses of a Festool MFS shows how to make and use such stops for accurate, repeatable parallel cuts.  I made a much less expensive version.

To use these shop-made stops, first lay the Guide Rail on the sheet to be cut and rip a straight edge.  Then make a pair of pencil marks at the desired strip cut width using a shop rule or tape measure.  Then set the Guide Rail with its splinter strip on the pencil marks.  Then set both shop-made stops to the required width setting, with both stops being placed on the Guide Rail next to each other.  Then relocate one of the preset stops adjacent to the other end of the Guide Rail, reset the Guide Rail, and make the rip cut.  You can now rip as many pieces as you need using your preset stops and all will have parallel sides and be of identical width.

Note that many roofing or framing squares are not square.  They may be close from the factory, but not precisely square.  You can check any square to determine if it is truly square by using it to draw a pencil line that is presumably perpendicular to your reference (base) edge with the handle of the square extending to your left, then flipping the square 180 degrees so its handle is extending to your right and drawing another line on top of or very close to the first line you drew.  If the lines are not coincident or parallel, your square is not square.

Dave R.

Thank you to everyone who has posted suggestions to create square panels.  Andrew, what you have shown is exactly what I was looking for to use arcs to get it accurate. Dave, I may invest in one of those tri squares you have as well.  When funds permit, I think I will go for an MFT and use its capabilities to get square, repeatable cuts without all of the fuss required with just the detached rail method I am currently using.

I would bet that the framing square is not exactly square and the ripped edge form HD is not exactly straight either...so, you are seeing a combination of the errors when you try to cut your 90 degree square cut...

Best,
Todd

My Dad taught me to peen an out-of-square framing square with a scratch-awl to get it back to square.

I have his old framing square and it has several dimples at the elbow.


Tom

Every time I try to peen the things back to square I can get either the inside or outside back to square, but not both.

Hi,
 i heard about a simple rule (pythagore) to remind: the 3,4,5 rule

square(3?+4?) = 5

The diagonal is 5.

Adjust with a factor, for exemple here: 6

exemple: sqrt((3*6)^2+(4*6)^2)

This way : 18"x24" diagonal is 30"

Hope it helps.

pci.

I think the hardest thing to make is actually a perfectly spherical solid wood ball.

1/64" is more than adequate for sure. I can spit on the wood and get it to move that much.

I do not get that I use my HUGE dividers and the arc method for tons of stuff in the shop.

The way templates by numbers uses a trammel I do not see how it could not work as the points are fixed on the trammel.