Compression bit issue

[Cherryville Chuck]

My kids are replacing their counter tops with butcher block slabs made from Hevea (rubberwood). They come about 74" long by 25 1/4 wide. That required me to splice two together for the sink run. The ends are fairly square but the factory cuts are a little rough and the edges rounded a bit which required me to trim them to get a flush tight fit. The edges weren't dead straight either but that isn't a big issue.

My idea was to lay the slabs out on my workbench and a workmate, level them to each other. Clamp a string line to the far corners and line the middle corners to the string and with about a 3/8" gap between the slabs run a 1/2" compression bit between them. First try I came close but I didn't have the slabs clamped tight enough to my bench and they moved slightly when I leaned over to finish the far end of the cut. Second try failed when the bit was struggling to make the cut (brand new Whiteside comp bit).

When I stopped to see what was the matter I found that the bit had sucked the router down, even with the column lock tightened, and I was slicing a groove into the top of my workbench. I'd heard of up spirals doing that before but I hadn't heard of a compression bit doing it.I switched to a straight bit for the third try and that one worked.

The one picture shows why I like pipe clamps. With couplers you can make them as long as you want. I have these two out to about 11 feet. I cut grooves in the ends of the slabs and added splines for alignment and to strengthen the joint.

[sunnybob]

I've had several fails with routers in that kind of situation. I've even had bit AND collet get pulled down from the router. In your case I would have lined them up touching and used a circular or plunge saw to straighten them.

[Cherryville Chuck]

I've had several fails with routers in that kind of situation. I've even had bit AND collet get pulled down from the router. In your case I would have lined them up touching and used a circular or plunge saw to straighten them.

I recall the late Pat Warner talking about the problem too and that he wasn't overly fond of using spirals because of it. With a compression bit you're cutting both ways so I thought it would equal out the pull. I don't have a good quality circ saw, just a construction grade cheapie which has managed to do a good enough job that I didn't need to replace it. It usually leaves kerf marks. I got a perfect joint using the router method although it doesn't look it because the hevea is so light colored.

[Biagio]

Charles, I would also have thought the up and down forces would balance out.
Were the opposing spirals equidistantly positioned in the wood?

[Stick486]

Instead of the compression bit how about a pilot panel bit?...
or use your router as a joiner...

R4 JOINER SUBSTITUTE.pdf

the movement of compression bits is because the bit is not horizontally centered on the material being cut... Center of bit - to center of material... if the bit is off center it will pull in direction of the greater amount of material being cut..

[Stick486]

there's nothing stopping you from using a dado jig to flush cut your panels using a router..
a straight bit would be the bit of choice here...
.

exact width dado lig - TOP VIEW.jpg

EXACT WIDTH DADO JIG - UNDERSIDE.jpg

EXACT WIDTH DADO JIG - BUSHING.jpg

[Cherryville Chuck]

Charles, I would also have thought the up and down forces would balance out.
Were the opposing spirals equidistantly positioned in the wood?

No. The bottom up spiral on a 1/2" bit is 3/8 to 1/2 (10-12mm) without going out to measure it. The rest is down spiral and the butcher blocks were 1.5" thick. No doubt being between two slabs and cutting both at once made a difference too.

[Cherryville Chuck]

Instead of the compression bit how about a pilot panel bit?...
or use your router as a joiner...
R4 JOINER SUBSTITUTE.pdf

the movement of compression bits is because the bit is not horizontally centered on the material being cut... Center of bit - to center of material... if the bit is off center it will pull in direction of the greater amount of material being cut..

Essentially, that is what I did. Although the edges of the slabs were not dead straight as a string line showed us. The ends seemed to be square, as best we could tell, but they had to be perfectly square and smooth for a seamless joint. By clamping a string line on the end corners and lining up the corners at the joint I could guarantee we had an edge straight enough to line up against the wall behind the cupboard. I just clamped a straight edge to one slab and jointed both at the same time. If the straight edge wasn't perfectly square, which is something I couldn't guarantee because the edges weren't dead straight, it wasn't going to matter because the other slab was going to be the exact mirror image of the one with the straight edge.

It would have worked on the first try had I done a better job of clamping the slabs to my bench. It might have worked on the second try had I noticed that the bit had moved. It did work the third try after I switched to a long straight bit. I got a little fuzzing on the edges with it that the compression bit didn't get but that disappeared when I sanded the joint area smooth.