Precision tool setup directly affects the quality of every cut and joint in woodworking and construction. When setting router bit heights, calibrating saw fences, or checking miter saw stops, the measuring tools you use determine how accurately your machine performs. Brass setup bars have served as the standard reference tool for these tasks for decades. Magnetic shims, made from brightly colored plastic with embedded magnets and thin steel spacers, offer a different approach that can speed up repetitive alignment work. Much like plastic hinge shims used for quick door adjustments, these workshop shims prioritize convenience and visibility without sacrificing the core function of establishing reliable reference thicknesses.
Comparing Magnetic Shims with Brass Setup Bars
Brass setup bars are precision-ground metal blocks sold in sets covering common thicknesses such as 1/16, 1/8, 1/4, 1/2, and 1 inch. They are durable, resist wear, and provide a known reference dimension that does not compress or deform under normal shop use. The main drawback is that brass bars can slide around during setup. They often need to be held in place manually while you check clearances between the bit and the bar. A dropped brass bar on a concrete floor can become dinged or burred at the edge, reducing its accuracy for future measurements and requiring refinishing to restore the reference surface.
Magnetic shims approach the same task from a different angle. The primary shims are made from impact-resistant red plastic with magnets embedded into the body. Thin steel spacer plates complete each set. The magnets let the shims stick together, to the steel spacers, or to any ferrous machine surface such as a cast iron saw table or router table plate. This self-sticking property eliminates the need to hold shims in place during alignment checks. The plastic construction also means the shims will not damage or dull cutting edges if accidentally contacted during setup, as long as the steel spacers and magnets stay clear of the cutting path.
Visibility and Identification Advantages
The bright red color of magnetic shims makes them easy to spot against cast iron tool tops and dark machine surfaces. Brass bars, with their metallic gold tone, can blend into wood shavings and shadows on the bench. The shim edges also feature rounded corners that serve double duty as radius templates. Each shim includes reference curves for 1/8, 1/4, 1/2, and 3/4 inch radii, and an inner notch provides a 1/4 inch inside radius reference. This multi-function design reduces the number of separate layout tools needed at the setup station.
Using Magnetic Shims for Router Table Setup
Setting router bit height is one of the most frequent setup tasks in any woodshop. The process requires positioning the bit so its cutting edge sits at an exact distance above the table surface, typically matching the thickness of the material being routed. Any error here produces a miscut that wastes material and time.
Bit Height Adjustment Workflow
With brass setup bars, setting a bit to 3/4 inch height requires stacking a 1/4 inch bar on top of a 1/2 inch bar, then using a third bar to bridge across the top of the bit and the stack. This three-bar method works but can be fiddly when the bars shift during positioning or when you need to hold them steady with one hand while adjusting the router lift with the other. Magnetic shims simplify this to a single operation. For a 3/4 inch setup, stack seven 1/8 inch red shims next to the bit, then slide the seventh shim over the bit to check whether the cutting edge matches the stack height. The magnets hold the stack together so it does not tip or scatter during the check. Miter saw dust collection accessories follow a similar design philosophy of reducing setup friction through thoughtful engineering, and magnetic shims apply that same principle to the alignment process.
Checking Bit Height with Stacked Shims
To verify bit height, lower the router bit until it just touches the top of the shim stack, then lock the lift mechanism. Run a test piece across the bit and measure the resulting cut depth with calipers. Adjust in small increments until the cut depth matches the target. Repeat the process whenever you change bits, since different bit designs seat at different depths in the collet.
One practical sequence for router table bit height setup:
- Install the bit in the collet and tighten to the manufacturer’s specification
- Raise the bit until it sits slightly below the target height
- Build a magnetic shim stack matching the desired cut depth
- Place the stack next to the bit on the router table surface
- Raise the bit until the cutting edge contacts the top shim
- Lock the lift mechanism and verify with a test cut
- Adjust in small increments and retest until the cut is accurate
Table Saw and Miter Saw Calibration Techniques
Table saw and miter saw setups require a different type of alignment check. Here the goal is not just height but parallelism between the blade and the fence or miter slot. Magnetic shims serve as quick reference gauges for these checks because they stay in place while you make adjustments.
Blade Height and Fence Alignment
To check table saw blade height, lower the blade below the table, stack magnetic shims to the desired cutting height on the table surface next to the blade slot, then raise the blade until its highest tooth contacts the stack. The magnetic hold keeps the stack from shifting as you raise the blade. For fence alignment, place a single shim between the fence and the blade at the front of the blade, then repeat at the rear. An identical gap at both positions confirms the fence is parallel to the blade. Any difference indicates the fence needs adjustment.
Miter Saw Stop Repeatability
Miter saws used for repetitive crosscutting depend on consistent stop positioning. Magnetic shims can act as sacrificial setup gauges placed against the stop block. When cutting multiple pieces to the same length, stick a stack of shims to the saw fence at the stop position to verify each piece seats at the same location. The magnetic grip holds the stack on steel fence faces without requiring clamps or tape.
Understanding Shim Thickness Combinations and Stacking
Magnetic shim sets come in two configurations: imperial and metric. Understanding the available thicknesses and how they combine determines what range of setup heights you can achieve.
Imperial vs Metric Configurations
The imperial set contains eight 1/8 inch red plastic shims and two 1/16 inch steel spacers. The metric set contains six 2 mm red plastic shims and two 1 mm steel spacers. Neither set uses color coding, so mixing the two sets requires careful separation to avoid contaminating your reference stacks.
| Set Type | Plastic Shims | Steel Spacers | Total Thickness Range |
|---|---|---|---|
| Imperial | 8 x 1/8″ (0.125″) | 2 x 1/16″ (0.0625″) | 1/8″ to 1-1/8″ |
| Metric | 6 x 2 mm | 2 x 1 mm | 2 mm to 14 mm |
Calculating Total Stack Height
Total stack height equals the number of shims multiplied by the individual shim thickness, plus any steel spacers in the stack. For the imperial set, seven 1/8 inch shims give a stack of 7/8 inch. Adding one 1/16 inch spacer brings the total to 15/16 inch. The ability to add or remove individual shims in 1/8 inch increments lets you dial in common workshop dimensions quickly without reaching for a separate set of setup bars.
Material Properties and Shop Durability
Plastic vs Brass in Workshop Conditions
The switch from brass to plastic raises legitimate questions about durability and long-term accuracy. Brass setup bars are machined to tight tolerances and hold those tolerances for years under normal use. They are resistant to most shop chemicals and can be cleaned with solvents without damage. The main failure mode is mechanical damage from dropping on hard floors, which can create burrs that throw off reference measurements.
Plastic magnetic shims handle drops better than brass because the material absorbs impact rather than deforming. The embedded magnets add weight and the plastic body resists cracking from short falls. However, plastic is more susceptible to wear from abrasive dust and swarf that settles on tool surfaces. Router tables, in particular, generate fine dust that can act as sandpaper between the shim and the tool surface over months of use.
The shim-to-shim consistency within a single set matters for reliable stacking. If one shim measures 0.124 inch and another measures 0.126 inch, stacking seven of them produces a cumulative error of 0.014 inch. That is enough to produce a visible misfit in joinery. For rough carpentry and framing, that tolerance is acceptable. For fine woodworking and cabinetmaking, it warrants verification with calipers on the assembled stack before making critical cuts. Checking the stack height before each setup ensures the reference dimension matches what you expect, regardless of how the individual shims have worn.
Heat exposure is another consideration. Leaving magnetic shims on a sunny windowsill near the shop or storing them near a dust collector motor that runs hot can soften the plastic over time, altering the reference thickness. Storing them in a cool, dark drawer when not in use preserves their dimensional stability.
For most workshop applications, a combination approach works best. Keep a set of brass setup bars for critical joinery where tolerance matters in the thousandths of an inch. Use magnetic shims for repetitive production setups where speed matters and the acceptable tolerance is around 1/64 inch. The two tools complement rather than replace each other, and having both available expands your options for different types of work.