When routing dovetails in Cherry or White Oak and using a Rockler dovetail jig, I get some tearout on some pins and tails. I'm moving the router at a slow speed. What am I doing wrong or is this common? Thanks for any help!
Careful climb cutting (right to left) of each pin and tail will help significantly to avoid chip out.
Don't try to cut the entire slot. Just climb cut the first 1/8" or so (keep it less than 1/2 of the bit's cutter blade diameter). Then follow by a second cut deeper and counter clockwise into the jig along the the right side guide, across to the left guide and then back out. Do this two step cut for each pin or tail slot that you cut and I'm quite certain that you will see a significant improvement.
Sacrificial strips clamped to both sides of each board as it is cut will also help, but in some cases, when using dovetail jigs, it is not very practical.
The chipping occurs as the tooth of the router bit is exiting the board surface. Climb cutting (moving the router in the opposite direction) will remove the surface wood at the usual exit point before the bit can grab and remove a big piece at this point if cutting in the usual direction, and result in chip out. Once the light climb cut pass is done, the remaining surface wood around the cut area will help prevent further tendency to chip out around the edge of the cut. If you closely watch the Leigh Dovetail jig demo below you will see that they do this climb cut and then go back and remove the rest of the cut in the regular way.
This is actually a video for the Leigh FMT, but it shows climb cutting (Clockwise direction) at 2.00 minutes into it as the first cut of a tenon being made. Then the cut is repeated, but in the (counter clockwise) normal cutting direction to reduce the tenon size to it's final dimension. I'm showing you this video because it's easier to see how little chipping occurs when they are climb cutting, but it's necessary to remove only a little material from the board surface during this climb cutting, because taking a bigger bite will cause the router bit to dig in and pull the router out of your hands. They then show going back and making the second pass to finish the cut, moving the router in the normal direction, where it doesn't try to pull away.
An older video for the D4R Dovetail jig showed this same climb cutting technique being used to cut dovetails and pins, but their latest D4R video that's now on YouTube seems to have left this scene out. Regardless, it is still the best way that I know of to reduce the chip out if you can't use a sacrificial piece.
These are full thru dovetails and you start with the dovetail bit then rotate the template and cut the pins with a straight bit. The finished dovetail is as close to perfect as I can do. The slight amount of chip out really takes away from an almost perfect fit! The are new Rockler bits. Maybe I'll try another brand of bits?
Chip out occurs where the cutter blade is pulling out of the wood, because the wood fibers adjacent to the cut aren't being held up close to the cutting point to keep them from being lifted and breaking out. A zero clearance insert or sacrificial strip holds these wood fibers in place right up to the cutting point. Climb cutting removes the wood where the blade is entering, and there is no wood being removed at the point where the blade exits the wood, so there is no chip out at the exit point.
You need to closely study the direction of the cutting in relation to the wood fibers and come up with a way that supports and holds the wood fibers right up close to the blade exit point, or do the cutting in a way that causes the cutting to occur at the point where the blade is entering the wood. Just a shallow climb cut before switching direction and completing the cut in the normal direction will make a huge difference in your chip out problem because the blade will be exiting the cut where there is no wood fibers to chip out. Whether you go with a sacrificial strip or with climb cutting your best chance of minimizing chip out is to use one of these two methods. A good sharp bit also helps, and some woods will chip easier than others, so choice of wood and grain within the choice also affects chip out.
Even though you are only cutting between the fingers of your dovetail jig, at the point where the bit just begins to touch the wood, the fingers of the jig are wide enough for you to do a shallow and short climb cut. Once done, you can go back and plunge the bit into the wood and follow the side fingers to complete the pin or dovetail without getting significant chip out.
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