Binding while ripping and Riving Knife Drag
We have read any number of posts on this and other sites about problems encountered when ripping lumber and the search for solutions.
The most probable cause is case hardening of the lumber. Red Oak is often steam dried. Rushing the drying cycle, and/or having insufficient ventilation between materials in the kiln, among other reasons, can cause tension in the lumber. European Beech, Cherry and Black Walnut are other species that are often steam dried for color. Steam darkens the sapwood.
Wood dries from the surface first. The new growth dries faster than the heart. If the material is dried too quickly the wood shrinks too much at the surface and slower in the center, creating compression stress in the still damp interior.
In addition the natural tendency of the material to warp to the new growth side can create unusual tensions that are released in unexpected ways when ripped or resawn. The illustration in fig. 1 shows the natural warpage in a typical piece of lumber.
The two most common results of tension release in lumber when ripped are shown in fig. 2 and fig. 3.
In fig. 2 case hardened material tends to pull apart as it is ripped (most common). A bow is created and the very act of pushing the material against the fence will create pressure and friction on a riving knife. In fig. 3, called reverse case hardening the material bows inward and can bind against the blade causing
Kerf burning as the material is ripped. As the material is ripped it can pull away from the fence causing side pressure on the blade and unequal pressure against the riving knife.
When tensions of this type are encountered, make the rip with a minimum of two or three passes for optimum results. Set the blade at approximately 1/3 the thickness and rip the board. If it feels like it is dragging during the first pass, flip the board end for end and make second pass on the other side before raising the blade to cut entirely through the material.
If a glue line rip finish is what is desired, then first rip the material a bit wider and then resize it to the final size. Glue line ripping is a mill term and operation. Glue line ripping machines typically have motors in the 10hp – 50hp range with power driven chains to keep the board both straight and moving through the machine.
Aside from the binding factors in ripping lumber that is case hardened, the potential for kick back can be very high and potentially very dangerous if not lethal.
Understand that the average table saw has an arbor rotation speed of 3750 rpm, with some saws turning at up to 6000 rpm. With a 10” blade running at 3750 rpm, the distance traveled in 1 second is approximately 0.031 miles per second. Even with just a 1 hp. motor, binding the blade usually won’t cause it to stop. Think of the force behind a 3-5 hp. motor. As tempting as it sometimes is to use our body mass to aid in pushing boards through a table saw, this is an unsafe practice. Care should be taken to stand beside the material or use an anti-kickback device.
I have watched 3’ projectiles hurled thirty five feet across the shop. Several years ago I bound up a red oak board on a 3hp. saw that kicked back, hitting me in the chest and knocking me to the floor. The shock was great enough to paralyze my diaphragm, stopping both my breathing and my heart beating for almost a minute.
After almost 50 years of woodworking, I knew better but got in a rush. Think of the momentum of 15 lbs. of board traveling at 1/32 of a mile per second. How many galloping horses can you stop with your bare hands?
Some of our work can be seen at: www.newguildworkshop.com
Anti kick-back solutions and a retro-fit Riving Knife can be seen on our as yet unpublished website at:
Look under the Products section: NG-3 Stock Support Rollers
-- elcw and bdk