Build a Precision Workbench Jig Using Metric Fasteners

If you’ve ever tried to line up a hole on a piece of metal and ended up with a crooked mess, you know why a solid jig matters. A good jig takes the guesswork out of repeatable work and saves you from the endless cycle of re‑drilling, re‑tapping, and sighing. Today I’m sharing a step‑by‑step guide to build a metric‑based workbench jig that’s sturdy enough for a shop full of projects yet simple enough for a weekend DIYer.

Why a Metric Jig?

Most of us in the engineering world grew up with metric fasteners – M4, M6, M8, and so on. Using imperial‑sized hardware on a metric design is like trying to fit a square peg in a round hole; the tolerances just don’t line up. A metric jig lets you keep everything in the same system, which means less conversion math, fewer stripped threads, and a cleaner final product. Plus, it’s a great excuse to dust off that set of metric bolts you bought years ago and never used.

Materials and Tools List

Below is everything you’ll need. I kept the list short on purpose – you probably already have most of these items in your garage.

Fasteners

M8 x 30 mm socket head cap screws – 8 pcs (for the main frame)
M6 x 20 mm socket head cap screws – 12 pcs (for the clamps)
M4 x 12 mm pan head screws – 6 pcs (for the guide pins)
M5 washers – 20 pcs
M6 lock nuts – 12 pcs
M8 nyloc nuts – 8 pcs

Raw Materials

2 × 50 mm × 600 mm square aluminum extrusion (6063-T5, pre‑drilled)
1 × 30 mm × 400 mm aluminum plate (for the base)
2 × 25 mm × 200 mm steel blocks (for the stop pins)

Tools

Metric drill press with 4 mm, 6 mm, and 8 mm bits
Tap set for M4, M6, and M8 (hand tap or power tap)
Hex key set (4 mm, 5 mm, 6 mm)
Torque wrench (10–30 Nm range)
Center punch
Caliper (digital preferred)
Bench vise
Marker (fine tip)
Safety glasses (don’t skip these)

Step 1 – Layout the Base Plate

Place the 400 mm aluminum plate on a flat surface. Using a ruler, mark the center line lengthwise. This line will be your reference for all hole placements.
From the left edge, measure 50 mm and make a small dent with the center punch. This is where the first M8 hole will go. Repeat every 100 mm along the center line – you’ll end up with four evenly spaced holes.
Drill the four 8 mm holes using the drill press. Keep the drill perpendicular; a tilted hole will cause the extrusion to wobble later.

Pro tip: I always drill a tiny pilot hole (4 mm) first. It guides the larger bit and reduces the chance of wandering.

Step 2 – Prepare the Extrusions

Take the two 600 mm extrusions and lay them side by side on the base plate, aligning the drilled holes with the marks you just made.
Insert an M8 x 30 mm socket head cap screw through each hole, then add a washer and a nyloc nut on the underside. Tighten with the hex key until you feel resistance, then give it a final turn with the torque wrench set to 20 Nm.
The extrusions should now sit flush on the plate, forming a sturdy “U” shape. This will be the backbone of the jig.

Step 3 – Add the Clamping Arms

The clamping arms let you hold a workpiece in place while you drill or tap. They’re simple, but a little thought goes a long way.

Cut two 150 mm lengths from the leftover extrusion (or use a spare piece). These will become the arms.
On each arm, mark a spot 30 mm from the end – this is where the M6 clamp screw will go. Drill a 6 mm hole, then tap it to M6 using the tap set.
Attach the arms to the main frame with M6 x 20 mm screws, washers, and lock nuts. Position them so the arms swing inward, forming a V‑shaped cradle. Adjust the angle until the cradle can hold a 50 mm wide piece without squeezing it.

Step 4 – Install Guide Pins

Guide pins are the secret sauce that makes repeatable hole placement possible. They act like a railroad track for your drill bit.

Take the two steel blocks and drill a 4 mm hole in the center of each. Tap both to M4.
Mount the blocks on the base plate, one on each side of the central line, about 150 mm apart. Secure them with M4 pan head screws and washers.
Insert a hardened steel rod (M4) into each block. These rods will sit just above the work surface and guide the drill bit.

Step 5 – Test the Jig

Before you start a real project, give the jig a quick trial run.

Place a scrap piece of aluminum (or wood) in the cradle.
Align the drill bit with the guide pins and lower it until the bit just touches the workpiece.
Drill a pilot hole using a 4 mm bit. If the hole lands where you expect, you’re good to go. If not, loosen the clamp screws a bit and re‑align.

Tips for Long‑Term Use

Lubricate the threads on the M8 and M6 screws with a drop of light oil. It prevents galling and makes future adjustments smoother.
Check for wear on the guide pins every few months. A worn pin can cause the drill to drift.
Store the jig in a dry place. Aluminum can corrode if it sits in a damp environment for too long.

Why I Like This Design

I built this jig for a project that required drilling a series of 10 mm holes in a 500 mm long aluminum bar. Using a hand‑held drill, I would have taken at least an hour and probably ended up with a few mis‑aligned holes. With the jig, the whole job took 15 minutes, and every hole was spot on. The metric fasteners kept the whole system tidy, and the ability to swap out the clamping arms means I can adapt the jig for different workpiece sizes without rebuilding from scratch.

If you’re looking for a reliable, repeatable way to handle metric projects, give this jig a try. It’s a small investment of time and a few fasteners, but the payoff is a cleaner shop and fewer wasted parts.