How to Make a Full Radius Arch Top Cab Door
1- Make pie segments with grain oriented with your radius. Glue these together into a slightly oversized half round (I prefer loose tenon joinery here because you can cut your pie sections close together from a single vertical grain board and, when rejoining them, get easy, continuous circular grain flow that machines well and matches the color of your stiles). Then machine the final dimensions out of the half round stock and join to stiles (I prefer mortise in the rails and vertically oriented blind tenons in the stiles.)
2 - I have some customers who insist on a single piece top rail, horizontally oriented like the rest of the rails. For these I just cope and tenon a huge top rail a bit wider than your radius, say 19" for a 36" roundtop, making sure to thoroughly saturate the copes with glue and get lots of squeeze-out when assembling. After curing you can cut out the radius.
Couple problems with this method, though. The spot where the top rail starts to blend into the stile is a risky cut. Even on a CNC machine, those tiny end grain tips might blow out, or look different on one side than the other. Note this is, even after glued, much easier than using a sacrificial stile machining it alone. Even if they come out perfect, that end grain turns out darker than the adjacent stile when finishing. You also end up with more exposed end grain on the top of the door. It is also not as strong. Furthermore, since you're cutting out the radius after the door's been assembled, you'll be limited to panels held in with stops. The fact is, stops glued and shot in fail way more often than not. The raised moulding or stops buckle and move easily when the panel moves, which is not only unsightly, but there goes the weather-tight integrity of the door. I see these doors everywhere I go, not just at work, but even in places like a doctor's office, a very climate controlled environment.
I have one customer who requires some tricky joinery that is a hybrid of the above two methods - basically making two horizontally oriented top rails with a bridle joint right in the center, allowing you to field panels when assembling. It would be a viable method for your cabinet doors.
It is extremely rare to get blowouts machining in a circle on pie segments, and that alone is the overwhelming reason I prefer it.
From contributor E:
I think you're on the right track with the sacrificial stiles, but maybe the tape is not holding the blow-out blocks tight enough to the cope cut. Try using an MDF jig under the rail and blocks, and screwing the blocks down to the jig, after clamping tight to the rail blank. Orient the grain in the blocks so that it is parallel to the grain in the rail stock. Take several steps in your cut (1/4" depths), and then clean up with a .06" pass. I think this will come out clean. It may take a little more setup time, but consider the time to re-do broken pieces.
I have also done segmented arches like contributor D recommends, cutting out the arch and the end cuts on the CNC (make the end cuts first, a little bit long, then do the arch cuts). Then make a matching MDF sled to cope the ends on the shaper, with back-up blocks behind the cuts.
From contributor C:
The answer is hinted at within your question. We have 2 sets of stile and rail cutters, exactly the same profile. One is run face down, counter-clockwise. The other is run face down and it is a clockwise cutter. This way, the critical cut is approached from each end. The first cut is done on the clockwise cutter to the center of the arch. The next cut is done from the other end with the counter-clockwise to the center. Very few defects occur when extreme arches and elaborate curves on door rails are done this way.
From contributor R:
We cut the rails to length and make them oversized on the width. Tenon one end, run the profile, then tenon the other end. If part breaks off during the process, before the final tenon cut, we can trim the width and rerun it, but that doesn't happen very often. This is with shapers, a panel crafter and slide.
From the original questioner:
Thanks for your responses. I like contributor C's method the best and will invest in another reverse cutter soon. I only had one door to make this time, and I ended up assembling the cabinet door with the tenons cut and glued, and then carefully used the CNC to cut the arch, matching the depth of cut to the stiles. It worked but was very nerve racking and took quite a bit of time. Next time I will be ready with the 2 cutter system.
From contributor T:
I work old school: A half inch thick, net size template to include the radius top rail and stiles, is prepared. This can be cut on the CNC. I then glue up the rough pieces based on color and grain with only the sap-side being the face. I fast cut on a bandsaw within 1/16 of the finish size. I attach the template, which has aluminum tape applied to both sides, and then shape the interior sticking and outer profile with a free bearing collet. I remove the tape and finish shape for a clean and defect-free edge. The stiles are left 1" long for a blowout free composite. This is a modified method from having worked large exterior doors. The results are fantastic. If you have multiple doors with various radii, this method allows for the most efficient process. If you're set up with a double spindle shaper, you and an assistant can produce each door frame in as little as 30 minutes.
Special Note: The top rail is comprised of 2 pieces of wood: shoulder-locked and then shoulder-locked to the adjoining stiles. This will prevent separation.
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