Can anyone explain this?!

[brownmike]

I'm trying to glue up a 24x36 panel out of 1x8s that have been ripped down to 6 inches wide. When I assemble the boards there is a small but significant gap in the middle of each seam (wide enough that I couldn't over come it with clamps). I layed the edge of each board on a flat surface and sure enought, each of the boards is slightly hour glass shaped.

When I ripped the boards I took 1/4 inch off of one side, flipped the board then ripped it to width. The blade is a WWII and the fence and blade were just trued up. The 1x8s were pine (S4S) and reasonably straight and unbowed. I can't figure out what in my setup or technique would cause this "hourglassing".

But wait, there's more . . . a this will probably sound even stranger. Not having a planer and only having 4 boards that needed to be trued, I figured I could set up the router and a straight bit to accomplish the same thing. I clamped down the subject board and used a 8ft Tru-Grip as my straight edge. I took .030 off of the edge of the boards, then layed out the boards again. Now the gap between the center 2 boards (previously about .040) was now double that! The Tru-Grip is not bowed and I kept the point of contact between the router base and the Tru-Grip at the same position for the entire cut. Also the gap can be seen from top and bottom.

The first issue confused me . . . the 2nd is shaking my faith in geometry.

Anyone have an idea why this is happening?

Thanks, Mike

[David]

[e-mail] wrote: Now the gap between the center 2 boards

Even though the TG seems not to be bowed, is there any chance it is giving in the middle under pressure to keep the router against it? That's the only explanation I can think of.

Dave

[Chris Friesen]

I've run into this on my saw as well, but had assumed it was something wrong with my saw (ancient Rockwell 9").

Maybe the force pushing the router against the straight edge was enough to bow it slightly in the middle?

Chris

[Joe Barta]

MY faith in geometry and other assorted laws of the universe remain fully intact. My faith that your setups are as straight as you think they are... that's what is shaking for me.

[Toller]

The first is obvious. A table saw cut is only as straight as the side against the fence. You think a 2x4 come straight to 0.040"? You have to joint one side and only then cut the second side.

The second; well I don't know what your tool is, but obviously it either isn't straight or is getting deformed in use.

Tools don't work by themselves; you have to use them properly.

[WillR]

Assume that you mis-fed...

As the board started you had the tail swung out. Then as reach the end=20 of the cut you push tight to the fence.

Flip the board over, and follow the fence more tightly -- hourglass.

It happens.

--=20 Will R. Jewel Boxes and Wood Art

power of accurate observation is commonly called cynicism by those=20 who have not got it.=94 George Bernard Shaw

[Leon]

Snip

The 1x8s were pine (S4S) and reasonably straight and unbowed.

There is your problem. Reasonably straight is not straight. The nonstraight edge follows the fence and wanders back and forth as it does.

I can't figure out what in my setup or technique

If your router base is round and you let it turn just a little bit during the pass you are probably not getting a straight pass.

If you can manage, clamp the Tru-grip to the top of the board over hanging the fence side of the board. Let the true grip run against the rip fence. If the Tru-Grip and your fence are truly flat, you should have success.

[Swingman]

To do this correctly you need to joint one edge, then rip the opposite edge.

If you did the above and were still having a problem, I would say check the face of the fence first, although it could be a combination of factors.

But, IMO, you are in effect chasing your tail by not having a known reference edge to start with.

As far as the router issue goes, concentricity of the router bit with the base is the likely culprit. Best to do this with a router mounted on a table with a split fence so that the outfeed fence can be adjusted for the amount of cut you take. This also takes possible lack of concentricity out of the equation.

Suggestion: take one of your S4S boards and lay it on the same flat surface. Note the amount of/lack of bow (your hourglass). Then using that edge against your fence, rip and lay the freshly cut edge against the same surface.

If the "reference" edge was flat and the second cut has the "hourglass", you most likely have an alignment problem with the fence/saw/blade, or a combination thereof.

If it's the same, you now have an excellent excuse to get yourself a jointer, or a jointer plane.

[brownmike]

Thanks for all the replies.

A little more info . . .

the Tru-Grip is a 6"x.5" extruded pieced of aluminum. Not much chance it deformed, but just to be sure, I checked it against 2 other straight edges. There were no gaps between the two.

I kept the router base against the Tru-Grip at the same position (no rotation).

I considered that I might be feeding the board in a little askew, but I'm using a Dewalt 746 with a 30" table (front to back). The boards in question were only 35" long, so about 18" was engaged with the fence at the beginning of the feed. I used my hand as a feathering board.

New info . . . .

The TS was still setup, so with the saw off I pushed a board through. As expected during the first third of the board, the blades just barely touched the board and the blade starts to rotate. During the middle third the gap gradually get bigger, maximizing at the center. The gap gradually gets smaller during the final third (same as 1st third).

If I angled the board in during the rip, I can't see how the cut would be perfectly hourglass shaped. . . maybe bowed, but not nicely symmetrical.

With a fairly straight piece of lumber (esp. one 35" long), shouldn't I be able to cut one flat side, then flip and rip without issue?

I probably am chasing my tail . . . maybe Santa has room in the sleigh for a jointer.

Thanks again, Mike

[Joe Barta]

Possibly, but if you clamp a piece of wood to it at both ends, the wood may deform as you route it. Try it again and clamp in the middle as well?

[Robatoy]

My hunch is that your hunch is right.

The router itself will exert some pressure towards the fence which will deflect ANY fence somewhat. I always clamp a cleat at the centre of my long fences in order to stop that from happening.... or at least reduce that flexing. Even then, I make a 'whisper pass' as a final clean-up so the router doesn't have to work that hard.

[JJ]

Check your fence face to make sure IT is flat. If it happens to be concave relative to the blade, you would probably get what you are getting. Leading edge of the board pressed against the fence near the center, where the blade starts to cut, would start out wide, then as it gets to the trailing edge of the fence, the center of the board would come closer to the blade. Then as the trailing end of the board follows the curve of the fence near the middle, it gets wide again. Hourglass.

-jj

[brownmike]

Yeah, right . . . . a brand new fence is warped . . . . (as I run out to the garage to check).

Well, the fence is flat . . . unless the height adjustment butterfly knobs are snugged up . . . then the fence deflects at the knob locations . . . . about the same offset I am seeing on the boards. The Dewalt 746 has 3 knobs to lock down the height of the fence (1 at each end and 1 in the middle). Each of the knobs exhibits the deflection behavoir. If the knobs are just barely snug, no problem . . . another half turn and deflection occurs. Apparently I had the center knob tightened more than the outer ones.

Can't wait to rip some wood to see if this is the root cause.

Great suggestion.

Mike

[Swingman]

LOL ... first time you heard it, eh?

[Doug Miller]

NO! With a "fairly straight" board as your reference edge, you can't do any better than to get "fairly straight" results. If you want "perfectly straight" results, you need to start with a perfectly straight board.

[Larry Blanchard]

As others have said, "fairly straight" just doesn't cut it! And your straightedge is deforming under pressure. Clamp it in the middle or even use four clamps.

Best of all - buy a jointer.

[JJ]

Let us know ASAP if this was the problem. I wanna make sure physics and geometry still work OK, or if we need to turn in a problem report. :)

-jj

[brownmike]

With the fence straight I was able to rip boards that were straight enough to biscuit and glue up. Yeah!

I called DeWalt about this. They have a shim kit that is supposed to solve the problem and they're shipping it for free. Hopefully this will solve the problem, otherwise I'll be taking DeWalt up on their 90 day guarantee.

What I don't understand is that the DW746 has been on the market for several years and the problem is fairly egregious, yet DeWalt didn't make the shims a part of the standard design?! I can't be the only one who has experienced a bowed fence.

Mike