Perfect Edge Joints
A 6-step tune-up sets your jointer straight.
By Dave Munkittrick
Jointers are simple machines with few moving parts, but the two beds, the fence and the cutterhead all have to be in alignment for a jointer to function properly. Few things are more frustrating or more common than problems with jointers. This is especially true when you’re trying to get straight, square edges on your boards. I’ve come up with a six-step tune-up that should set your jointer straight. It’s easy to do and will only take an hour or two, depending on how many problems you unearth.
Jointers are supposed to cut straight, square edges, but all too often, they leave a sniped or a bowed edge (see “Common Problems,” page 2). Snipe results whenever the top of the outfeed table dips below the knife’s top cutting arc. A bow cut results whenever the outfeed table rises above the cutting arc. A cutterhead that’s not parallel to the outfeed table, or tables that are not parallel to each other, will make it impossible to get the table height set just right for all fence settings.
Our tune-up will help you identify and correct four common jointer problems:
Problem #1: A table surface that’s not flat.
Problem #2: Tables that are not parallel to each other across their widths.
Problem #3: Tables that are not parallel to each other along their lengths.
Problem #4: A cutterhead that’s not set parallel to the tables.
Click any image to view a larger version.
Common jointer problems result in a sniped or bowed edge. Adjusting the outfeed table height usually cures the problem. However, if both tables and the cutterhead are not in perfect alignment, the problem will return when you move the fence. This tune-up procedure takes care of all the possible misalignments that can cause jointer problems.
The Right Stuff
You will need a few tools to perform this tune-up: A good straightedge, a set of feeler gauges and machinist’s metal shims are must-haves for this job. For some steps, a dial indicator is easier to use than a straightedge.
The straightedge, shims and feeler gauge run about $105 total. The optional dial indicator with a magnetic base and extension arms adds another $40 and is well worth the cost. All these tools can also be used to set and tune-up other shop equipment and to check your own work for flatness (see Sources, below).
A precision straightedge is essential. You can perform all the tune-up steps using this 50" precision straightedge that costs $79. Unlike inexpensive straightedges, this one has a precisionground edge with a tolerance of .003" along its entire length. Such a good straightedge is not cheap, but it’s a good investment for your shop.
A dial indicator with magnetic base and arm can’t be beat for tool setups. A number of these six tune-up procedures are best done using a dial indicator. Like the straightedge, this tool is also useful for other machine setups.
A feeler gauge set is used in tandem with a straightedge to measure very small gaps. If the straightedge reveals a gap, you can measure that gap by finding the feeler gauge that fits under the straightedge.
Metal shims align jointer parts. Variety packs are convenient and easy to use. A strip of aluminum cut from a soda can is a quick substitute for a .005" shim. That coupled with some .001" shim stock should cover all your tune-up needs. Shims can be stacked to create any desired thickness.
Fine Tuning Your Jointer
Step 1: Check For Flat Tables and Fence
Check each table and the fence for flatness (Photo 1). The accuracy of later measurements depends on flat tables. Measure for dips or a droop using the straightedge held parallel to the table bed. Then, hold the straightedge diagonally across the table to check for twist. The good news is that finding twist or dips in the table is highly unlikely. The bad news is that if you do find things out of whack, you can’t do much about it. In extreme cases, a messed-up fence or table may be reground at a machine shop. You’ll have to weigh the cost in time and money against simply buying a new jointer. If your jointer is under warranty, talk to the manufacturer.
Step 2: Align Tables
It’s not unusual for the two tables to be out of parallel across their widths (Photo 2). It’s easiest to check the tables for parallelism with a dial indicator (Photo 3). You can also do the check with a straightedge. Hold the straightedge down on the middle of the infeed table so it extends over the outfeed table. Set the infeed table to the exact same height as the outfeed table. Slide the straightedge over to the fence side of the table and use feeler gauges to check for gaps. Repeat with the straightedge on the user side of the table.
Align your tables by shimming the outfeed table. Loosen the outfeed table’s gib nuts and lift the table so you can insert metal shims on the side of the table that’s low (Photo 4). Shim the outfeed table only because it is moved very little and the shims are less likely to shift during table adjustments.
Recheck the tables and make any necessary shim adjustments until the tables measure in exact alignment.
Step 3: Fix Sagging Tables
Tables can also be out of alignment along their lengths (Photo 5). Use the straightedge to see whether the table end dips below the infeed table (Photo 6). Correct a dip by adding shims to the top or bottom of both gib ways on the outfeed table (Photo 7). Retighten the gib nuts and check the tables again. Make any necessary adjustments until the tables lie in the exact same plane.
Step 4: Level the Cutterhead with the Tables
Now that the tables are parallel to each other along their lengths and widths, it’s time to make sure the cutterhead is parallel to the tables. If the cutterhead is not level with the tables, your cut will be heavier on one side of the table than on the other. Jackscrew cutterheads allow you to set the knives to compensate for this; spring-loaded knives or a segmented carbide insert cutterhead do not. The fix for this problem is so simple that I recommend leveling your cutterhead no matter what type of knife holder you have.
Use a dial indicator or straightedge to check cutterhead alignment (Photo 8). If the cutterhead is off, measure the exact amount on the low side. This equals the size of shim you’ll need to raise the cutterhead (Photos 9 and 10).
Step 5: Set Proper Knife Height
To minimize kickback hazards, jointer knives should not project more than .020" from the cutterhead. (Owners of spring-loaded cutterheads have a knife-setting gauge that automatically sets the proper knife projection.) A potential hazard exists with knives set parallel to the outfeed table: It’s easy to unintentionally set the knives so they project too far.
A dial indicator is the best instrument for checking knife projection (Photo 11), but you can make do with a straightedge and feeler gauge.
Step 6: Set Proper Outfeed Table Height
Your knives should be set so that the very top of the cutting arc, also referred to as top dead center, is the same height as your outfeed table. We used the straightedge to accomplish this task (Photo 12), but a dial indicator is another option. To do this, set the dial indicator on the outfeed table and zero it. Then set the plunger over the cutterhead with the body of the indicator on the outfeed table. Rock the cutterhead back and forth; the indicator should hit zero as its highest mark. Check this at several points along the width of the table. Repeat for all three sets of knives. straightedge no gaps outfeed table knife at top dead center
Edge-joint a couple of boards to test your jointer (Photos 13 and 14). In practice, it often takes a little tweaking of the outfeed table height to get it just right. Often the table ends up set .001 or .002" below the cutterhead. Now your jointer is ready to go and should create perfect edge joints every time.
1. To start your tune-up, check each table for flatness. Lay the straightedge on a table and use the feeler gauge to check for gaps. A gap of .003" or less is acceptable.
2. Infeed and outfeed tables that are not in the same plane across their widths need to be made coplanar. (With any luck, your tables aren’t this bad.)
3. Check the tables for parallelism across their widths. Bridge the dial indicator from the center of the infeed table to the center of the outfeed table and zero it. Slide the indicator across the width of the table to measure any difference in height.
4. Bring the tables into alignment by inserting metal shims on the low side of the outfeed table. Choose a shim thickness equal to the amount your table was off. Loosen the gib nuts and lift the table while you insert the shim or shims. Then retighten the gib nuts.
5. Tables can be out of parallel along their lengths. Typically, the tables sag on the ends. This is especially true on older jointers that have worn gib ways.
6. Check for table sag by holding a straightedge tight against the infeed table. Set the infeed table height so the straightedge just contacts the outfeed table. Then use a feeler gauge to determine the amount of dip or rise in your outfeed table.
7. Correct a sagging table by shimming the bottom end of the two dovetailed gib ways on the outfeed table. A table that dips toward the cutterhead would be shimmed at the top end of the gib ways.
8. Check that the cutterhead is parallel with the tables. Rotate the cutterhead so the knives are below the table. Clamp a guide board parallel to the cutterhead. Set the dial indicator against the guide board so the plunger contacts the cutterhead. Zero your dial indicator; then slide it back and forth.
9. I removed the cutterhead here to illustrate how it is mounted. Two threaded rods attached to pillow blocks run through holes in the base and are held in place by a nut and a washer. Place shims between the pillow block and the jointer bed casting.
10. The cutterhead is easy to shim. Remove the drive belt and loosen the bolts that hold the cutterhead in place. Lift the low end of the cutterhead and insert shims under the pillow block. Then retighten the bolts.
11. Proper knife projection increases jointer safety by limiting the cut’s aggressiveness. Use a dial indicator set to zero on the cutterhead. With your hand on the pulley, rotate the cutterhead backward. As the knife rides under the dial indicator, it should read no more than .020".
12. To set the outfeed table height, place a straightedge on the outfeed table so it projects over the cutterhead. Rotate the cutterhead backward and raise or lower the table until the knife barely kisses the straightedge when it’s at top dead center.
13. Fine-tune the outfeed table height by edge-jointing a couple of boards that are narrower than the fence height and no longer than the infeed table.
14. Put the newly jointed edges together and hold the joint up to a light source. No light leaks indicate a jointer that’s perfectly tuned. If you are getting a snipe at the end of your cut, raise the outfeed table a bit. If the jointer puts a concave edge on your board, lower the table. Repeat the process with the fence set at the far edges of the table. The results should be the same, and that should put a smile on your face.
Note: Product availability and prices are subject to change.
Lee Valley Tools, leevalley.com, 800-871-8158, 50" aluminum straightedge, #05N63.05; Dial indicator and magnetic base and arm, 88N31.20; Feeler gauges, #86K99.01; Brass sampler, 6-1/2" x 6", .001 to .010 thickness, #27K07.50.
This story originally appeared in American Woodworker August 2006, issue #123.