Dado routing question

[jarhead]

I have to route a dado (on a router table) wider than my biggest router bit. I remember reading somewhere than there is a right way of accomplishing this task - the problem is that I don't recall how. Do you first router the "front" of the dado, move the fence back, and router the rest or vise-versa?

[MikeMcCoy]

jarhead - I'm no expert but I never move the fence if I can avoid it, especially if I have several cuts to make. I position the fence for the final cut and then clamp a spacer to the fence. I then make my first cut(s), then remove the spacer and make my final cut. Hope that makes sense.

[stoli]

Assuming you have a table mounted router, if you decrease the distance between the bit and router for the second pass, I think you are creating a trapped cut. The outside of the bit is in the previously cut dado, and the near side of the bit is in contact with the material, which is against the fence. Since the bit is moving CCW, this will end up pulling the material away from you and launch it across the shop. Instead, if you increase the distance from bit to fence, the outside of the bit is doing the work, and is resisting your pushing motion. This is the safer mode.

To address Mike's point (if you have to make more than one of these large dados): position the fence once and make a dado on all the pieces, then move the fence further from the bit and enlarge the dado on all the pieces. This way, all the dados should be exactly the same size and in the same place, relative to the end against the fence.

-=[doug]=-

[LarryG]

I agree with Mike about using spacers rather than moving the fence ... except, ironically, for routing dados. More on that in a minute.

To cut a dado in two passes: set up your fence so that the edge of the dado nearest the fence will be exactly where you want it. Make your first pass, using the normal (for table routing) right-to-left feed direction. Move the fence BACK as required and make the second pass using the same right-to-left feed direction.

If you do use a spacer, do it exactly as Mike says: make the first pass with the spacer in place, then remove the spacer (which has the same effect as moving the fence back) and make the second pass. Again, feed right-to-left for both passes.

Using either method, you'll be feeding against the bit's rotation (i.e., NOT making a climb cut) for both passes.

Regarding the use of spacers ... they work well when you need, say, a series of slots some uniform distance apart; you cut a bunch of spacers to that width, rout the first slot with all of them in place, and proceed to remove one spacer at a time as you cut each slot. For a dado, though, the spacer's width has to equal the dado's width less the bit diameter, and to me it takes way too much trial-and-error time to cut a spacer to the very precise width required. I find it easier to just keep nudging the fence and making test cuts on scrap until I get the width dialed in.

[jarhead]

quote:Originally posted by LarryG

...To cut a dado in two passes: set up your fence so that the edge of the dado nearest the fence will be exactly where you want it. Make your first pass, using the normal (for table routing) right-to-left feed direction. Move the fence BACK as required and make the second pass using the same right-to-left feed direction...

Larry,
That is exactly how I made the dado last night sneaking up on the second pass. I was not sure if that was the right or safest way of doing it. Nevertheless, I should have asked if I was not 100% sure. [B)]

quote:Originally posted by stoli

Assuming you have a table mounted router, if you decrease the distance between the bit and router for the second pass, I think you are creating a trapped cut. The outside of the bit is in the previously cut dado, and the near side of the bit is in contact with the material, which is against the fence. Since the bit is moving CCW, this will end up pulling the material away from you and launch it across the shop.

Doug,
Thanks for explaining what the ramifications are making the cut incorrectly.

[boblon]

I have an Incra fence for my router set-up, so I've not worried about moving the fence back and forth, but I must admit I haven't conciously thought about the way the bit contacts the wood. I haven't had the fence that long, but this was a great topic to make me aware that you must always THINK the cut through before actually making the cut. Thanks for that.

BTW, the fine adjustment on the Incra is supposed to be 1/1000th of an inch and recently I had a chance to find out if its true. I had veneered up a panel for the bottom of a box I'm making. This made the thickness anything but standard. I took a straight bit whose diameter was less than the thickness of this panel. I made a test dado, measured the panel, measured the dado and the difference was 90/1000. So, I did 90 clicks of the dial, ran the test piece through again and the panel fit perfectly. Love them fancy fences (they are expensive tho).

Anyhow, thanks for the heads up.
BobL.

[crokett]

quote:Originally posted by stoli

Since the bit is moving CCW, this will end up pulling the material away from you and launch it across the shop. Instead, if you increase the distance from bit to fence, the outside of the bit is doing the work, and is resisting your pushing motion. This is the safer mode.

This is true. My old shop had a dent in the wall and I ended up with a chipped router bit from this exact situation. On a handheld router this would be called backrouting, which while it gives a smoother cut does require extra caution and care.