In search of flat doors

Other Versions
Spanish Even slightly inaccurate machining can produce frame-and-panel doors which are less than flat. June 6, 2001

Question
I am making paneled doors with M&T construction. When dry assembled, I notice that some are not flat (I test by balancing them on their edges and sighting down them). The stock is flat and the joints are accurately cut, but obviously, something is wrong. How do you achieve perfect flatness?

Forum Responses
Do you use "set-up blocks" when setting up your knives? There could be inconsistent cutting by a fraction, which mis-aligns the joint. For example: the tenon on one end of the rail could be very slightly different from that on the other end, if you had different set-up times. I like to do all the stiles with the same set-up, then using one of them as a set-up block, align the knives in the spindle and make sure it turns freely without scraping. Accurate set-up should take the wind out.

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My experience is that winding doors are usually caused by a twist in the panels. Place your panels on an absolutely flat surface such as a cast iron table saw top and see if they rock a bit. If so, that is your problem.

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Perfect flatness is an unachievable goal. But we can dream and try. If your machined joints slip together flatly and finely, then the only trouble can be in your clamping bed. Lay a straightedge across corner to corner, side to side, and edge to edge, until you are fully confident all clamping points are co-planar.