Machining Curved Crown Moulding
From contributor C:
Assuming you don't have a CNC router, there are a couple of ways to handle it. Probably the easiest is to outsource to someone who does curved mouldings as their business. Or you can try to match it with a flexi-moulding - there was a discussion about it recently.
Until getting a CNC, I handled it as contributor C is suggesting on a lathe. I built a corrugated back knife holder with a shaft on it that I could mount in the tool rest base, and a leadscrew to feed the carriage with the knife into the work. After roughing the face angle onto the blank, I could then feed the knife into the face and cut an exact match profile to the linear moulding I had run as I was using the same knife I had used in the moulder. I don't think I ever tried it other than on the face mill side (Delta DL40 lathe).
Of course this has limitations as far as sizes. I don't think I ever did anything over 3' diameter, mostly because I never had a need, and over 4-4.5" face is a lot of torque, but I think I ran a 5.25" once or twice, slowly. A good turner with a standalone tool rest and steady hand might be able to make up to whatever size the lathe will spin. Then it's just a matter of cutting out the section you need. Even now with the CNC, the profile may need to be split in order to machine it and then joined to form the full piece.
From contributor B:
The best method for making this is going to depend upon the radius. We run all our curved crown that has a 12" or larger radius on our tilting moulder. For profiles over about a 7" to 7 1/2" face size we will run them in upper and lower sections, treating them as separate crown mouldings. Then we will glue the two layers together to make the single large moulding after both sections are profiled. We will try to make this break at an inside corner of the profile but this is not necessary.
For crowns with a radius smaller than 12" we will either use our 3D software on the CNC or send the project to a lathe specialty shop. I've run curved crown mouldings on our shaper in the past. However, after one knife breaking incident, I was done with that. Talk about a knot in the pit of your stomach... very scary. However, I'll take a knot in my stomach over a piece of steel any day!
From contributor L:
We run them on a very heavy tilting shaper. A sub table has the curve CNC routed into it as a full circle for parts up to about 5' in diameter. The CNC router then cuts a matching MDF carrier to match the curve and the projected profile. The glued up blank is screwed to the carrier and the back face is run on a straight spindle shaper so that it fits the backup blocks on the carrier that will run on the tilt shaper carrier board. We mount a power feed and remove all but two wheels so that it can feed (down) on the backup block. No way I would try this without a power feed and a very trapped setup. But, as setup, it runs very smoothly with no vibration (the shaper weighs 2050 lbs.) Wax everything, move the spindle to an angle that will just kiss the crown on the first pass. The molder head is almost fully enclosed with a guard. The carrier and blank are fed around and around the track about 5 passes (changing the tilt each pass), with the spindle being brought to 38 degrees for the last pass where we make a change of only 1 degree (the profile is run upside down). It works better than I had expected with very few blowouts and an exact match to the molding that comes off the molder (run the same head on the molder).
We've run them on the CNC router and it works okay, but there is more sanding required and some blending where it joins the straight parts. We bought the heavy tilt shaper just to run curved crown, so it is left set up for that. We run both inside and outside curves on it much the same way, just have to move the power feed to a different location and change the enclosure around the head. We have a couple of items that we sell that use the same curve every time so it was worth the setup/learning curve. The knives need to go back to the grinder fairly often to keep them as sharp as possible since you are cutting against the grain part of the time. Given a little practice and some fooling around you can do a lot with a shaper and a feed. For one-offs that are not a true radius, we run them on the router - usually set up just before we go to dinner and let it run since it takes quite a bit of time to do the very fine passes for the last cut. I'll come back to the shop later in the evening to check on it and make sure everything is shut down okay. If we had a 5 axis machine it would be a lot faster.
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