Chapter • 7
Adding Accuracy
BY BILL HYLTON
When you are using power tools for cutting parts and joinery, accuracy has less to do with laying out individual workpieces and more to do with mastering setups.
I have a simple routine. Whether I’m cutting parts or joints for a project, the routine helps me achieve good results with a minimum of fuss:
• Set the cutter and the guide device.
• Make a test cut and measure.
• Tweak the setup.
Do the initial setup carefully, keeping a few guidelines in mind that will simplify the inevitable tweaks. But don’t get nuts about it. It’s easy to waste time trying to get a perfect setup. For the initial setup, close is enough.
The test cut is your best means of evaluating the setup and making effective adjustments. Practice does make you better at this. Your “eye” improves, and your hands become more deft. Once you understand the routine, the whole process becomes second nature.
OK, What are the Guidelines?
• Start BIG. If you are cutting parts, for example, you want your test cut to leave the part oversized so you don’t waste material. Once it is too short or too narrow, you can’t go back. You’ve got to start over.
• Account for the “backlash” that’s inherent in adjusters. Backlash is excessive play between adjacent moveable parts in a mechanism. Many of us are familiar with the looseness in a table saw’s blade-height adjustment. Crank the blade up, then lower it. As you reverse the direction, there are several degrees of free movement before you feel the resistance of the gears meshing. That slack is the backlash and it is a problem.
A reference block and shims allow controlled tweaks to both fences and stops. The reference block can be almost any scrap. Suitable shims range from playing cards and scraps of plastic laminate to precision-thickness gauge blocks (the brass bars in the photo) and feeler gauges.
The way to deal with it is to stage the setup so the adjuster is moving against the pressure that results when the tool is turned on and the cut begins. If you neglect to lock the crank on the table saw, the blade is going to slowly drift down. Make the initial setting and your tweaks by elevating the blade. Begin the setup with the cutter below the saw’s table. Raise it for the initial test cut, but deliberately leave it low. You want to creep up on the setting.
Remember that backlash isn’t a problem only on the table saw. It affects the adjustments on routers, router lifts, edge guides, radial-arm saws, planers and many other tools.
• Use stops wherever you can. Stops do more than arrest movement. They establish limits. When you use a stop for an operation, you have a good means to ensure consistent, accurate results. You also have a good base from which to tweak your setup.
Initial Setup
Initial setup can be done with a rule. Here are some examples:
• Almost every table saw’s rip fence has a scale giving you a reading on the distance from the fence to the blade. But you can’t use it for every setup. If you are setting up for the shoulder cut of a rabbet, you need to include the blade in the measurement. The scale, though, doesn’t give you that.
• For crosscuts, whether on the table saw or a power miter saw, use a rule butted against a tooth (not the plate) of the blade to set a stop (see photo at right). A tape measure is usually on my belt, right at hand for all sorts of measuring. But for this task, I use a steel rule, whether a 6” pocket rule, a yardstick or something in between.
• Set a router table fence with a square. Hold the head against the fence with the rule extending just above the bit. Measure either to the cutting edges or to the axis of the bit.
When possible, eschew measurement entirely and use a setup gauge. The obvious example here is using an existing part to set a cutter height, fence position or stop so you can make a duplicate.
Machinists use a multipurpose gauge system for accurate setups. A steel one-two-three block is precisely 1” thick, 2” wide and 3” long. You can make your own using a stable hardwood. Setup gauges are slivers of steel, aluminum or brass in precise thicknesses.
Want to rip a 3”-wide stile? Butt the end of the one-two-three block against a tooth of the saw blade, then slide the rip fence against the other end. Want the stile 2 5⁄16” wide? Turn the block so its 2” side is against the blade and add a ¼”-thick gauge and a 
”-thick gauge.
When you are making a two-part joint (tongue-and-groove, cope-and-stick and half-lap are prime examples), use the first half of the joint to reset the cutter height or fence position for cutting the mating part
The Test Cut
All this gets you close, but where accuracy is essential, you must check things with a test cut. Follow-up tweaks are usually needed.
Don’t make the test cut in just any old scrap from the offcuts bin. If you are simply checking the width of a rip cut or the length of a crosscut, by all means use scrap. But think it through. If you’re testing a joinery cut, the actual girth of the working stock is of critical importance. Yes, using a real workpiece can be risky. As you plan a project, you can foresee this situation, so make it a point at the outset to prepare extra pieces for testing setups.
Evaluate the resulting cut. If it is a joint, see if the mating parts fit together properly. If it is a part, see if it fits the assembly. If measurement is required, use a precision tool. Your trusty tape measure isn’t it. In many instances, dial (or digital) calipers are.
With a standard 6” caliper, you can measure inside and outside dimensions, as well as depths. You can measure the thickness of a piece of plywood or a scrap of veneer, the width of a dado, the depth of a rabbet. You can make the measurements quickly, and with unmatched accuracy.
You can set the bit height using a rule but sometimes using a workpiece is more accurate. Here a mortising bit is being set to cut tenons. Whether you use a rule or a mortised workpiece, leave the bit just a bit low. Cut a test tenon and fit it to the mortise. Fine-tune the bit height based on the fit.
Use a steel rule to set a stop for crosscuts on the table saw. Butt the rule against a tooth (not the plate), align the block over the target graduation, and clamp the block to the fence.
To accurately set a router table fence for a precision test cut, use a machinist’s small adjustable square. Rotate the bit to align the cutting edges, then get the rule as close as possible to the appropriate edge.
Gauge blocks can be used to set a fence in relation to a cutter. Here the gauge is a scrap of ¼" hardboard and the tool is a mortiser. Just hold the gauge against the fence and slide the fence into contact with the chisel.
Sawing a rabbet in two passes? Forget the fence’s scale when setting up the shoulder cut; you have to measure from the outside of the blade. Use a scrap of the stock that will be housed in the rabbet. Your touch will tell you when the scrap’s face is flush with the blade’s cutting tips.
When readjusting the blade for the bottom cut, a shoulder-cut sample is going to be the most accurate gauge. Raise the blade so its teeth just skim the shoulder; you don’t want a ridge of waste where the shoulder meets the bottom.
Machinists use dial calipers to measure in thousandths. Fractional dial calipers do the conversion from decimals to more familiar fractions for you. If you can measure a part or a cut with the calipers, and you have the means to move the cutter, fence, or stop a precise distance, you can tweak setups efficiently.
Tweaking Your Setup
In many woodshops, a gentle bump with a fist or the heel of a hand on a fence is what passes for a tweak to a setup. If that works for you, fine. I do it too sometimes, but the results are seldom really satisfactory. Your aim is precision. Having done the initial setup so it would be tweak-able, and having made and precisely measured your test cut, you should follow through with the same mind-set.
At this point, the question is: What’s the difference between the cut you want and the cut you have? Is the cut you have too deep or too shallow? Too wide or too narrow? Is the piece too long or too short? The answers tell you how much and in what direction to adjust your setup.
Most table saw rip fences (and some commercial router table fences) have a scale to aid in positioning. The scale works for adjustments of 
” but precise movements smaller than that are iffy.
One helpful trick for fence adjustments is to scribe a pencil line on the tabletop along the fence during the initial setup. This line gives you a way of assessing the movement. If you move the fence away from the line, you may be able to actually measure the gap. The risk in moving the fence toward the pencil line is that once the line disappears under the fence, you have no way of determining how far past the line the fence has moved.
On a router table, you can often put geometry to work for you. Did you know that if you move only one end of the fence, the distance you move it at the edge of the table is halved at the middle of the table? That means swinging one end of the fence ” moves it only ” at the bit.
When you must machine both halves of a joint, use the first part to set up for cutting the mate. Set up for cutting a tongue by aligning the slot cutter against a sample of the grooved stock. “Prove” your setup with a sample cut before cutting all the work.
The Reference-block-and-shim System
Let’s say you want to move the fence ” closer to the cutter. First place a block of wood against the back of the fence and clamp the block to the table. Now unlock the fence and move it far enough to drop a ”-thick shim between it and the block. Slide the fence back, pinching the shim between it and the block. Re-lock the fence.
If you need to move the fence away from the cutter, the tweak is just as simple. First pinch the shim between fence and block as you first position and clamp the block to the table. Then unlock the fence, remove the shim, shift the fence directly against the block and re-lock the fence.
Depending on how far the fence must be moved, you can use scraps of MDF as shims at one extreme and feeler gauges at the other extreme. The machinist’s setup gauges I mentioned earlier are perfect for this. By combining the various thicknesses, you can range from ” through a full inch by 16ths. For smaller increments, I use feeler gauges, playing or business cards, alone or in combination with other shims.
Use a reference block and shims to move a router-table fence closer to the bit. Unlock the fence and slip your shims between the fence and reference block. Push the fence tight against the shims and the block and re-lock it. The movement at the bit will be half the thickness of the shims.
Use dial calipers to measure a test cut. The tool’s jaws accurately capture the measurement, and it displays the dimension in increments as fine as one thousandth inch (.001). Fractional dial calipers eliminate the need to convert from decimals.
I use this block-and-shim approach primarily for adjusting the fences on my router tables, but it works equally well with a few jigs and fixtures I use with portable routers, and with the table saw’s rip fence.
A variation is a block with a machine screw threaded through it. Clamp the block with the screw against the fence. Turning the screw one way pushes the unlocked fence toward the cutter, while backing the screw away from the fence allows it to be pushed back away from the cutter.
Using a commonplace ⅜”-16 bolt or screw in your block provides a measured adjustment – ” per full turn. I use a cap screw and turn it with an Allen wrench. You can make adjustments of 1⁄128” pretty easily. It’s ⅛ of a turn – just 45°.