3-4-5 90-degree formula

[Howard Rosenberg]

Hi all -

I'm starting a dado router jig tomorrow night.

The idea is to glue and screw a fence to a 1/4" piece of plywood and trim the plywood so the edge is parallel to the fence.

So far, so good.....

But I need to mount another fence underneath the plywood to register the jig against the edge of the board - perfectly 90 degrees to the board's edge.

What's the best method?

Is holding a 12" square against the edge of the plywood enough?
I'll clamp it and then finnesse it....
But if the intended dimensions will be approximately 48" long for the fence and the plywood will be less than three inches projecting from the fence, is there an easy-to-use/understand variation on the 3-4-5 formula to make sure the lower board edge registration fence is 100% perpendicular to the upper fence that'll have the router riding against it?

Hope this was clear - I've got to start this jig tomorrow night...

All help will be appreciated.

Thank you.
Howard

[Pat Monahan]

Howard
If you want to use a square, I would go bigger than 12" to get a more accurate measure, assuming your square really is square. Just MHO. If you want to use the 3-4-5 formula (aka Pythagorean Theorem (sp?)), assuming your fence attached to the plywood is 48", ignore the width of the plywood. Place the fence perpendicular to a straight board or any straight edge and clamp in place. Measure from where the fence meets the straight edge to thirty-six inches along the straight edge and make a mark. Measure from that last mark to the far end of the 48 inch fence. When this measurement is 60 inches, your fences are square. Line up your second fence flush with the straight edge and attach to the first fence. If you need more help, just holler.
Pat

[Bob Worrel Jr.]

Howard

Any variation of 3-4-5 will work.
6-8-10
15-20-25
30-40-50

Hope that helps

[Bob Hovde]

You can use any numbers. From the point where the boards meet at 90 deg, measure the same distance left and right on (or parallel to) the first board. The distance from these marks to the end of the second board need to be the same.

Bob

[Ted Shrader]

Bob -

I just noticed you tag line "Spinning is good on a lathe, not good in a Miata." LOL :D :) Is that based on practical experience?

Regards,
Ted

[Chris Padilla]

Howard

Any variation of 3-4-5 will work.
6-8-10
15-20-25
30-40-50

Hope that helps
Basically, any division or multiplication of the infamous 3-4-5 will work. So, you could use 1.5-2-2.5 as well. Those dimensions might be easier on you....

[JayStPeter]

It seems that every time I make a jig that must be square, the best way always comes back to trial and error. I have taken to using angle aluminum as my fence on most jigs, so I drill a bunch of holes and make one of 'em oversized. I put one screw in a normal hole and one in the the oversized using a speed square for initial reference. I can then loosen the screw in the oversized hole and retighted until it's right, then zip in the rest of the screws. You can use pencil marks next to the Al for reference to see how far you're moving the fence each time. It's a little painful, but only once.

Jay

[Jerry Olexa]

You may have to play around w it but the number formula works!! On bigger outdoor projects I've even built makeshift large wooden squares using this formula to insure proper alignment. You'll make it work! G Luck..

[Bob Hovde]

Bob -

I just noticed you tag line "Spinning is good on a lathe, not good in a Miata." LOL :D :) Is that based on practical experience?

Regards,
TedUnfortuately, this is the voice of experience.

Bob