10 Woodworking Rules of Thumb and Exceptions
1. Rule of thirds for joinery
This rule states that for housed joints, such as mortises and tenons, the thickness of each component should be a third of the overall thickness. For example, if using 3/4″ thick stock, the tenon and each mortise cheek should be 1/4″ thick. This is to ensure that each component is not weakened more than others. In the case of the mortise and tenon joint, a 1/2″ thick tenon is definitely stronger than a 1/4″ one, but if this means reducing the cheek thickness to 1/8″ in creating a stronger tenon, we have weakened another part and will result in a joint that is weaker overall.
When would you want to deviate from this rule? If two parts being joined are of uneven thickness, the rule of thirds may apply to one but not the other. For example, I might cut a 1/2″ thick tenon on a 3/4″ thick table apron and cut the matching 1/2″ wide mortise into a 2″ leg. Another instance to ignore the rule is when joining materials with dissimilar strengths. If using a hardwood spline to join two MDF parts, use a tenon less than one-third the thickness of the particleboard because the hardwood tenon is very strong and the particleboard is comparatively weak.
In the end, the goal is to create a strong joint by ensuring that each part is equally strong.
Rule of Thirds
While the 'rule of thirds' often makes sense to adhere to, when members of a different thickness are to be joined, it's more important to concentrate on making the joint as strong as possible.
Mitre Gauge Danger
Using a mitre gauge and rip fence at the same time can result in an airborne workpiece if the offcut gets trapped between the fence and the rip fence and it's not being held in place. Sliding the rip fence behind the blade, or using a thick enough stop block clamped to the fence, are both workarounds to the rule (Photo by Chris Wong).
Proportions are Important
A workpiece is usually longer than it is wide, and if crosscut with a rip fence, can send the workpiece flying. When the workpiece is actually wider than it is long, or isn't too far off from being evenly proportioned, the likelihood of an accident is extremely low (Photo by Chris Wong).
Moving the workpiece in the same direction the cutter is spinning has caused many problems to unsuspecting woodworkers. Taking off a very small amount of material per pass, or using a power feeder to feed the workpiece, is much safer and also has some other advantages.
A solid wood panel which is glued into a groove will very likely cause the panel to split. Adding a dab of glue to only the center of the groove will stop the panel from rattling or shifting, yet will allow the panel to grow and shrink in width.
Many situations call for a healthy predrill diameter in order to stop splitting. There are also times when drilling too large of a predrill hole will reduce the screw's ability to hold onto the material (Photo by Chris Wong).
The Perfect Angle
What angle your edge needs depends on a lot of factors. While 25°-30° is a good middle ground, there are times when a steeper or shallower angle might work best (Photo by Chris Wong).
Sneak Up to a Perfect Fit
There are many ways to fine tune the amount of wood to remove to get the perfect fit. One approach is to use a wood block clamped to a fence with a screw in its end. The screw can be turned slightly to adjust the amount of material being removed.
A Moxon vise is a great way get a closer look at your work. There are many other ways to adjusting the position and height of a workpiece. The right decision depends on your specific situation.
Against the Grain
As long as your tools are sharp, working across the grain will shear the fibres of the wood, reducing tear out (Photo by Chris Wong).
2. Never use a table saw miter gauge and rip fence for the same cut
This rule exists to help prevent kickback. If your miter gauge is to the left of the blade, and you’re using the rip fence to the right of the blade as a stop, once the workpiece is cut through, the offcut will be trapped between the blade and the fence and could easily get thrown backwards by the back teeth of the saw blade.
Using the rip fence as a stop for repeat crosscuts done with a miter gauge is logical, and there is a safe way to do it. The key is to ensure the offcut is free to move away from the blade after the cut is made. This can be achieved with a block of wood clamped to the rip fence (just subtract the width of the wood from your fence reading), or by sliding the fence back on a European-style fence (e.g. Unifence).
If at any point you are not controlling a piece of material trapped between the rip fence and blade, it is a kickback hazard. A short fence or stop block needs to be set back far enough that by the time the material is separated, it is beyond the fence or stop block.
3. Never use the rip fence for crosscuts on a table saw
This rule is also about preventing kickback. The less material registered against the rip fence and the further away from the blade the fence is positioned, the greater the risk. If the workpiece rotates away from the fence, kickback is probable.
This rule assumes that the material length (the dimension in which the grain runs) is always longer than the width (the dimension in which the grain runs across). I feel it’s okay to crosscut a board which is close to square or wider than long with regards to grain.
4. Always feed against the rotation of the cutter
Another kickback rule. Feeding with the rotation of the cutter can be very dangerous, as it is harder to maintain control and can lead to a workpiece suddenly becoming a projectile. There is the risk of damage or injury caused by the flying workpiece, and to your hands that used to be holding the wood if they are near or moving towards the cutter, or even if your hands get in the way of the quickly moving workpiece.
Cutting with the rotation of the cutter, or climb cutting as it’s known, is a useful technique that can be safely done with a handheld router – in fact, I teach climb cutting in my entry-level router seminars. Always take light cuts, and be aware of knots or other trouble areas that might be harder or more likely to grab and cause a catch. Start with a smaller diameter bit if you haven’t tried the technique.
Climb cutting is also acceptable when using a power feeder or other equipment that controls the movement of both the workpiece and cutter, such as a CNC router.
5. Never glue in a floating panel
This rule also applies to breadboard ends, and other cross-grain assemblies. A frame and panel door relies on a four-piece frame to minimize any changes in size. The panel sits in a groove and is given room to expand and contract freely within the frame. Generally, floating panels are made of flat sawn solid wood with grain running vertically that could expand and contract horizontally by 1/4″ or more seasonally across the grain. Conversely, the rails, with grain running horizontally, may not register a measurable change in dimension horizontally. Running a bead of glue down the groove of the rail during assembly would result in a broken panel because the panel will want to either expand or contract and the rail will not. Eventually, the panel will split.
One annoying thing about frame and panel doors is that sometimes the panels shrink so much that they rattle. There are a number of solutions, but the simplest is to apply a small amount of glue in the rail’s groove at a single point at each end. This is usually right in the center of a door panel, but may be at one end of a breadboard end. The glue will keep the panel from rattling, and still allow horizontal expansion. If you use sheet goods for your panels that are dimensionally stable, you can apply glue all around the frame. This can substantially increase the strength of the door.
6. Pre-drill for screws with drill bit equal to root
The root of a screw is what you have left if you remove the threads. Using too large of a drill bit to predrill reduces the amount of material for the screw threads to engage, lessening the screw’s holding power.
For hard woods, this is a good rule. Drilling a smaller hole, or not drilling one at all, will increase the chance of the wood splitting when driving the screw. However, for softer woods which compress, choosing a drill bit 1/64″ or 1/32″ smaller is a good idea. Some screws have a self-drilling tip (look for a section of missing threads) that are capable of being driven in some situations without a pilot hole. Remember that a workpiece is more likely to split if it is narrow, very dense, has compromised grain strength (e.g. an existing split or other defect), if the screw is near an end, or multiple screws are going in the same line of grain. When in doubt, test, or pre-drill.
7. Sharpen your blades at 25°-30°
A bevel angle for a chisel produces an edge sharp enough to cut well and is also strong and durable enough not to need sharpening too frequently. In a bench plane, this angle range affords enough clearance for springback so the plane can cut effectively, and a lower angle would affect edge retention without a proportionate benefit elsewhere since the effective cutting angle isn’t being changed.
For general use chisels, 25°-30° is fine. However, chisels for fine work such as paring can be sharpened at a much lower angle such as 20°, or even 15°. A blade sharpened this way cuts with noticeably less effort and easily shaves end grain. However, the thin edge is more fragile and more likely to require sharpening sooner. For tough tasks such as chopping mortises, an angle 30° or higher will ensure the edge is durable enough to drive with a mallet to cut many mortises.
When sharpening bevel-up planes, such as block planes and low-angle planes, the bevel on the blade affects the effective cutting angle, so must be taken into account when choosing a cutting angle. Lower angles require less effort to push through a cut, but higher angles can produce cleaner cuts in difficult grains.
8. Measure twice, cut once
Perhaps the most well-known rule of thumb, this one suggests that you double check your measurement before cutting.
When the highest level of precision is required, I would instead suggest measuring twice and cutting two or more times. If using a machine, set up and cut a test piece (preferably of the same material as the actual part). Check the fit of the test piece and, if required, make adjustments and cut another test piece. Once satisfied with the fit, cut the actual part. If using hand tools, cut it once oversized, then trim it to fit with a plane or chisel, pausing frequently to check the fit.
When exactness is not required, there may not be a need to measure at all, let alone a second time.
9. Your workbench should be at the same height as the palm of your hand
This is a time-tested height for using hand planes and doing other handwork at the bench. For me, this measurement is 31″.
However, depending on what kind of work you do, and what tools you use and your personal preferences, this could vary quite a bit. When using a router, you may prefer to work at a much greater height, with the bench top up around your elbows (around 43″ for me). Without such a bench, I find I am often crouching to get a better angle to see the bit.
The Moxon vise and bench-on-bench concepts have gained popularity in recent years. They effectively increase the height of your bench by 8″ or more and reduce stooping when cutting joinery by hand and doing other intricate tasks. As an experiment, I used a 42″ tall workbench for a year and I quite liked it. It was, however, difficult to use when planing large workpieces as my reach was reduced. When doing detail work at the tall bench, I tended to sit on a stool at the bench.
And, of course, if you sit on the ground when working, as is traditional for the Japanese, the work surface can be much lower.
Don’t forget to consider the typical range of workpiece heights when you choose your workbench height, since most work happens not directly on the workbench top, but on top of the material on the bench. Build a shorter bench if you work with a lot of 8″ thick timbers.
10. Always work with the grain [Photo 10]
Working with the grain usually produces the smoothest surface when cutting. When abrading the surface, the scratches in-line with the grain are more difficult to differentiate from wood grain and therefore less visible.
Working across the grain has its benefits, too. Stock removal with a belt sander or hand plane is significantly quicker when working perpendicular to the grain direction. Some highly figured woods – especially those with reversing grain – can be smoothed most successfully without tearout with a finely set plane working across the grain. Shear-cut planing heads work in a similar manner and as a result can produce a smooth surface regardless of figure or direction of grain.