How to Avoid Tear-Out

Have you ever made a cross-cut on your table saw only to have it splinter? Or perhaps you're putting the final edge on a table top and a chunk breaks away on the final pass? Knowing when tear-out can occur is the first step in understanding how to control it. In this post I'll discuss some of the strategies you can use to avoid this problem in the future.

Tear-out typically occurs when cutting across the grain (e.g. crosscuts, dados, drilling holes, etc.). When the blade or bit exits the board, it can cause some of the wood fibers, which are all lined up in the same direction, to tear away. We can use this knowledge to avoid the tear-out in the first place.

Sharp Tools

The easiest remedy for tear-out is to make sure that your tools are nice and sharp. A sharp tool is much more likely to cut a wood fiber than try to push it out of the way. Think of what would happen if you were to hit a piece of grass with a sword versus a baseball bat. Obviously, the sharp sword will cut the grass, whereas the bat will just knock it out of the way.

Backing Board

In our analogy of cutting the piece of grass, even the sharp sword is likely to bend the piece of grass before the cut is complete, possibly breaking it in an unexpected place. However, if you put the piece of grass on a cutting board, the strand is supported while you make the cut.

In much the same way, a backing board of some kind provides support to the wood fibers as the bit or blade exits the wood, allowing the fibers to be cut cleanly. How this support is provided depends on the tool being used.

Table Saw

On a table saw, the blade cuts down and towards you as you move the wood through the blade. This means that support is needed on the surface of the table and along the trailing edge of the board.

Zero Clearance Table Saw Insert To support the wood as the blade exits the bottom of the board, use a zero clearance throat plate. You can buy these, ready-made for your saw, or make your own out of some scrap plywood. As the name suggests, the first time you use the insert, you raise the blade through the insert, creating a slot that is exactly with width of the blade. When wood is passed over the blade, the fibers are now supported by the throat insert as the blade exits the bottom of the board.

As the blade exits the trailing edge of the board, support is provided by adding a sacrificial fence to your miter gauge. This is nothing more than a piece of scrap wood or plywood, taller than the height of the blade, that is screwed or clamped to the miter gauge. As the cut is made, a slot is made in the backer board, allowing the blade to pass through but providing support to the wood fibers on either side of the cut.

A panel cutting jig, combines these strategies into one unit. The base of the jig provides support similar to the zero-clearance insert and the rear fence of the jig provides the same support as a sacrificial fence attached to your miter gauge.

Miter Saw

A miter saw is similar to a table saw, but in reverse. The blade exits the wood along the table and the fence. The strategies are almost identical to that of the table saw.

To support the wood along the table, make a custom throat plate for your miter saw. Alternately, place a piece of 1/8 or 1/4 inch hardboard or plywood on the table of the miter saw.

To support the wood along the fence, add an auxiliary fence. Again, 1/8 or 1/4 inch hardboard or plywood is a good choice here. Use something cheap since you will likely be replacing it on a fairly regular basis.

Router

When using a router, the forces applied to the wood are only in one direction: perpendicular to the axis of the bit. This means that tear-out can occur in two ways when using a router. When routing along the long edge of a board, tear-out can occur every time the cutter exits the edge of the board. When cutting perpendicular to the grain, tear-out occurs as the bit reaches the edge of the board.

If you are using a router table, tear-out that occurs when routing along the grain can be controlled by adding a zero-clearance insert to your fence. The bit is then raised into the insert or the fence is rotated into the bit, creating an opening that exactly matches the profile of the bit. This provides the support needed to keep the wood fibers from breaking and tearing away from the board.

When routing across the grain, support is provided by adding a scrap to the point where the bit will exit the board. On a router table, this takes the form of a push block. The block is used to push the board through the bit, but it also provides the same support provided by the sacrificial miter gauge fence on your table saw. For hand-held routing, the support is provided by clamping a piece of scrap to the board where the bit will exit the cut.

Drilling

Whether you're using a drill press or a hand held drill, the strategy for controlling tear-out is the same, place a piece of scrap against the board where the bit will exit the wood. On a drill press, this can mean a replaceable insert or just a scrap piece of wood under the work. For a hand held drill, this might mean clamping a scrap under the board on your bench (which also protects your bench).

Order of Operations

Sometimes, the order you make your cuts makes it irrelevant whether you get tear-out or not. If you order your operations correctly, any tear-out that does occur will be removed by a subsequent operation.

For example, imagine you're making a raised panel or putting a profile on all four sides of a table top with your router. If you make the cross-grain cuts first, any tear-out that occurs will be removed when you cut the profile on the long grain sides.

If you are cutting dados across the grain, you might make the board extra wide. Any tear-out that occurs while milling the dados will be removed when you rip the board to it's final width.

Plywood

The veneer on plywood is usually quite thin and very prone to tear-out. While this can often be dealt with using the same measures as for solid wood, there are some additional strategies to keep in mind when working with plywood and other sheet goods.

Sharp Tools
I mentioned this earlier, but it bears repeating: Use a sharp blade or bit. This is especially important with plywood and other sheet goods. The veneer has very little support and will tear or chip easily if the blade or bit is not sharp.
Lots of Teeth
The more teeth a blade has, the less likely it is to chip or splinter the plywood veneer. Manufacturers even make blades specifically for cutting plywood. It is not unreasonable for a table saw plywood blade to have upwards of 100(?) teeth or more!
Scoring
If all of the other methods discussed here still don't prevent the tear-out, it is sometimes useful to make a scoring cut with before making the real cut. To do this on the table saw, raise the blade only 1/32 to 1/16 of an inch above the table and make one pass, cutting a shallow dado in the plywood. Then raise the blade to the normal height and make another pass.
Some high end table saws include the option of adding a scoring blade, which allows you to make the scoring cut and regular cut in one pass. This is a special blade that is really two saw blades, one in front of the other. The leading blade scores the plywood while the rear blade makes the cut as usual

Tear out is a normal part of woodworking, but if you anticipate when it might occur, you can control and avoid it. What strategies do you use to avoid tear-out?

1 comment:

  1. The more teeth a blade has, the less likely it is to chip or splinter the plywood veneer.
    CNC Mill

    ReplyDelete