TEAR OUT

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Question: What can I do about tearout? I'd really like to use my router more but I'm discouraged by so much random tearout (chipout). Do you have any advice on how to control it?

I believe more than 95% of tearout (chipout) produced by routers can be eliminated. In fact most of the routing I do is "tearout-free" and the little that I do encounter is usually traceable to crummy wood, (invisible checks, poor drying schedules, wild reversing grain, etc.), not to technique or tooling. Let me list some ways for you to get better results. 1. Dimension and prepare your stock well. Try and waste away cup, crook, and bow as much as possible, as each of these defects can present the stock to the cutter out of square or at some unintended angle. Most cutters are designed to attack wood at 90 and if the stock is dimensionally compromised it can be fed poorly (on the router table) or the router can rock as it travels down the work causing tearout. 2. Regulate the depth of cut. A good baseline for the maximum depth of cut on good wood (using 1-1/2 H.P. or more) is about 3/8" x 3/8". Depths greater than that will often tearout. If you think you can take that much wood (or more) in a single pass, take a few swipes on some scrap stock of the same origin first. If you are tearing out try and do the cut in stages. I usually use two routers with similar cutters to do 2-stage cutting. Say I want a 3/8" x 1/2" rabbet. I might cut more than half of the profile with 1/4" rabbeter set to a 3/8 vertical depth of cut. This may tearout but the finish cutter will often erase the tearout produced on the first cut. Light cuts substantially reduce tearout. 3. Use sharp cutters. Sharp cutters tearout far less than dull ones and antikick back tools tearout less than standard tool bit designs. Cutters with some shear angle also reduce tearout but may I caution you against the aggressive use of spiral ground cutters. Deep side cutting with spiral ground cutters can unexpectedly lift the router or force the work out of your hands on the router table. On-shear cutters are just as good and a lot safer. Incidentally a lot of tools bits are designed to cut on the bottom of their flutes as well as on their sides, (e.g. rabbeters, slotters, straights, dovetail bits). The sides of the flutes have had a lot more technology in their grinding procedures than the ends of the flute and consequently tearout is often worse on the bottom of the cut. This is especially true on resharpening as the bottom of the flutes are rarely touched on the regrind. Because of this phenomena, if I can, I'll rout the cut on the router table to full vertical depth but only a fraction of the side cut will be taken for each pass. This maneuver minimizes the end cut but takes full advantage of the more efficient cutting design of the side of the flute. 4. Control your feed rate. A fast feed rate, even though the cutter can handle it, can aggravate tearout. There is an optimum feed rate for any cutter/router combination. Try and figure out what it is on scrap. Too slow and you'll burn wood too fast and you may chip out. Experiment with the climb cut. The climb cut can virtually eliminate tearout but always at some risk. Climb cutting is that practice of routing in the direction of the cutter rotation, (right to left on the outside of stock with the hand router and feeding stock left to right on the router table). Climb cutting is less efficient than anticlimb cutting and therefore takes more energy. That coupled with tendency of the work or router to self feed can lead to an unexpected loss of control with the hand router or the work being pulled from your hands on the router table - both potential disasters. Very light cuts are permissible but if "light cuts" turns into everyday heavy hogging you should consider using the router table exclusively and only with a power feeder. For a further discussion on tearout please see the authors' "Getting The Very Best From Your Router", a Betterway Book, Chapters 5, 8 and 10. 1. Finally, buy good stock. Straight grained, well seasoned material will provide you substantial immunity to tearout. Crummy bent up stock with ever changing grain patterns can be extraordinarily difficult to rout. Knotty stuff with interlocking or rowed grain and pronounced figure (birdseye or quilted maple e.g.) can be quite striking but not much fun to rout. Use only the sharpest of tooling to rout figured material. I use all of the aforementioned techniques to rout without tearout and usually I'm successful. Practice as much as you can on scrap material as the key to good cuttings is experience. If I have to have a flawless cut I always commit the entire procedure to a test or two even though I've been doing it for 25 years. Article first appeared in Q&A, Woodwork magazine, Issue No. 50, April 1998.

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Copyright © 2009 Pat Warner
Last modified: Thu Sep 1 08:02:25 PDT 2005