A fixture plate is a base that locates and holds a workpiece for repeated identical operations. For drilling, a well-laid-out plate means every part comes off with identical hole positions. A poorly-laid-out plate creates part-to-part variation that you discover after the first batch is drilled and sent to QC. Getting the layout right before you build is much cheaper than rebuilding it after.
The fixture design follows the part. You're not fitting the part into a generic fixture — you're building the fixture around the part's specific geometry and the holes you need to drill. Start by identifying the datum surfaces — the faces or edges that define where the part "is" in space. Everything else is located relative to those datums.
The most common datums for prismatic parts (blocks, plates, brackets) are two perpendicular edges and one flat face. The part rests on the flat face, butts against one edge stop, and butts against a second perpendicular stop. Three points of contact — one flat and two edges — fully constrain the part in the three most important dimensions without over-constraining it.
Holes too close to a workpiece edge create thin walls that deflect, crack, or leave insufficient material for thread engagement. The standard rule for drilled holes: minimum edge distance of 1.5x hole diameter, preferably 2x. For tapped holes, the requirement is more critical because the thread needs material to grip — minimum 2x hole diameter to any edge, 3x preferred in load-bearing joints.
On your fixture design, mark the edge distance exclusion zones before plotting hole locations. If a required hole location falls inside an exclusion zone, the design or the part geometry needs to change — not the rule.
When two holes are too close, the second drill breaks into the void of the first during cutting. This sudden loss of support causes the drill to deflect and the hole to go off-location. Minimum center-to-center distance between two holes: 3x the larger hole diameter. In soft materials with very sharp drills, you can go slightly tighter, but 3x is the safe starting point for most shop work.
Hole patterns drilled in sequence — as opposed to all at once with a multi-spindle head — also need to consider chip clearance. Chips from one hole can fall into the adjacent hole cavity and interfere with the next cut. Orient your drilling sequence so chips are moving away from already-drilled holes, or cover finished holes between operations in critical applications.
Locating pins (round and diamond) engage holes or edges in the workpiece to position it against the datums. Round pins locate in one plane; diamond (relieved) pins locate in a second plane. Using one round and one diamond pin is standard for two-pin location — it tolerates minor variation in hole spacing between pins while still constraining the part accurately.
Clamp positions are determined after locating positions — you clamp against the locating forces, not into them. If the part is being pushed against an X-stop and a Y-stop by clamps, the clamps should push in the X and Y directions toward the stops. Clamping away from a locator lifts the part off it, which defeats the fixture's purpose.
Clamp position relative to the drilling operation matters too. Clamps should be as close to the drilling location as practical without interfering with the drill or the chips. A part that is clamped 6 inches from where you're drilling can still deflect between the clamp and the drill. Bracket or strap clamps can often be placed much closer to the hole location than a standard bar clamp.
Fixtures for drilling need chip clearance underneath. If a through-hole drill exits the bottom of the part and there's no clearance, the drill buries into the fixture plate and ruins the drill. Either the fixture plate needs a relief pocket under each hole location, or the workpiece needs to be raised on parallels above the plate surface.
Relief pockets are more elegant — they keep the part flat on the fixture without shimming, and they can be sized precisely so the exit burr has somewhere to go. The pocket should be 1.5–2x the hole diameter in both directions and at least 0.10" deep to clear the drill point as it exits.
Before drilling the first production hole, verify the fixture using a reference part or a dedicated check piece. Load the part, locate against all stops, clamp, then measure from the spindle center to the intended hole locations. If any location is off, find the error in the fixture before it's in 50 parts. Mark the check part at each hole location with a sharp scribe, then unload and measure the marks against the print. The check takes 10 minutes. Reworking a batch takes much longer.
SEND US YOUR DRILLS →