# Tear out prevention with Leigh dovetail jig



## altenewbie (Jan 14, 2018)

I'm just beginning to use a Leigh Super 18 dovetail jig. I'm having the same problems as others regarding tear out. I appreciate the suggestions of scoring the back sides of the boards as well as using some sort of tape. The backer board makes sense but, I don't think I understand the use of a backer board completely.

First, there is a support board placed horizontally on the jig and this supports the fingers. Is this also the board that is used as a backer board? If so, the pass through cuts would be made into the side grain. If not, I would assume that a different board would be placed with the end grain backing up the tail or pin board. In either case, once the cut is made through to the backer board, that backer board can no longer literally back up the tail or pin board because it has already been cut into. I am a newbie at this so, please forgive what probably sounds like a dumb question.

Right now, I'm using pine to practice. Perhaps some of the tear out is due to the softness. In practice, I get very little tear out when routing the dovetails, even with no backer board. However, there is considerable tear out at the bottom of the tail sockets on the pin boards. I noticed that Leigh has recommended an up cut spiral bit for use on the pin boards. Maybe that will solve some of the problem. I will appreciate any advice anyone has to offer. Thanks.


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## LesB (Dec 21, 2008)

The backer board is necessary to prevent tear outs. I use a piece of MDF most of the time and just cut off the end when I need a new surface to cut into. Also the way you cut into the wood can help. Make a shallow scoring cut on the first pass and as you finish the cut go from right to left or against the rotation of the bit. 
The biggest problem is when I use my 690 Porter Cable router the motor rotates in the base for height adjustment. If the guide that is mounted on the base plate is not precisely centered to begin with (it almost never is) when you change cutter bits from dove tail to straight bit and rotate the base to adjust the cutting height an eccentricity is created and the subsequent pin cut will be slightly off. This can occur on other routers if the guide is not securely locked down or somehow move a little. This is usually less than a mm but it can make for a bad fit.


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## altenewbie (Jan 14, 2018)

Thanks for your response, Les. At the moment, I have a call into one of the techs at Leigh to ask about the possibility of using MDF or, perhaps, two 8" boards as backers for my 14" wide blanket chest sides.

My router is also a Porter Cable 690 and I have been researching a foolproof method of assuring that the base plate is centered. If I could guarantee centering, I don't think the lowering/raising of the straight bit would be a problem. I've seen the Jasper universal base plate with centering pin as well as a Bosch centering cone. I will ask the Leigh tech his opinion on these issues and get back to you with his response. Again, thanks for the reply.

Ken


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## LesB (Dec 21, 2008)

Ken,
If you can put the straight bit in without rotating the motor in the base the alignment to the center of the dove tail, the straight bit and the cuts will remain the same so the cuts should match; as long as the guide is not loose in the base plate. The problem occurs when you change the orientation of the motor and base by rotating them, even a little.


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