When was the last time you checked to see if your throat plate was level with the saw's table? This often overlooked adjustment can cause all sorts of trouble and much time is often wasted looking elsewhere for the problem.
When your workpiece hangs up on the front of the table saw insert, it's pretty obvious that the insert needs adjustment. What's not so easily noticed is when the back of the insert is raised slightly, causing the workpiece to lift a small amount as it passes over this high spot.
The shop made insert is adjusted from the bottom and the stock insert from the top.
This of course will affect your depth of cut when making non through cuts such as dados and rabbets. As the workpiece starts to lift off the table, the depth of cut decreases.
An insert that is tilted toward either side of the table can affect the squareness of the cut, sometimes even causing the workpiece to wander away from the fence.
Many cases of kickback can probably be traced back to cutting short, narrow workpieces with a badly warped or misaligned table saw insert. While some shop made wooden inserts with a slight warp can be repaired by running them over a jointer, severly warped ones should be discarded. Metal inserts that can't be twisted flat should also be tossed.
Whether you're using an original equipment throat plate or a shop made zero clearance table saw insert, taking the time to properly level it will improve the accuracy of your work and lead to safer working conditions.
Many woodworkers adjust the throat plate so it sits just below the surface of the table. I like to adjust mine exactly even with the table's surface. That's what it's there for, to serve as an extension of the table around the saw's blade.
Leveling an insert by adjusting the set screws with an allen wrench. If your insert is held to the table with a screw, back it off before making adjustments.
Before attempting to adjust your insert, remove it completely and clean the area where the insert sits of any sawdust chips before reinstalling.
If any of the adjustment screws are a loose fit, they'll likely vibrate out of adjustment. Use a little teflon tape or silicon sealant around the threads.
To adjust an insert with set screws accessible from the top, back off the screws so there's a gap between your straight edge and the insert.
Starting at one corner, adjust the screw so it lifts the straight edge slightly and then back it off till it just touches the table. Repeat for the other corners and then go around one more time. The insert should be level with the straight edge along it's entire length.
Shop made inserts with screws on the bottom take a bit more time because they have to be removed each time you want to tweak a screw, but the end result should be the same.
If your insert has no means of adjustment, it will need to be shimmed with layers of tape, either on the insert itself or on the flange it sits on. On some table saws, it may be possible to drill and tap holes in the flanges where the insert sits. Allen screws can then be used to provide adjustment.
If your throat plate adjustment was really out to lunch, you'll probably see a significant improvement in cut quality once it's leveled out with the table. A quick check whenever you remove and reinstall the table saw insert will help keep it that way.