Door Sensor Magnet Bracket Sheet Metal: How a Small Part Decides Whether a Door Reads Closed

• 1
  Why the Gap Between Magnet and Reed Switch Is the Whole Design
• 2
  Gauge, Bend Radius, and Hole Placement on a Magnet Bracket Sheet Metal Part
• 3
  Picking Material for Indoor, Outdoor, and Washdown Doors
• 4
  How Tolerance Stack Moves the Magnet Off Center
• 5
  Plating, Coating, and the 0.02 mm That Changes Your Fit
• 6
  What to Measure Before a Bracket Ships
• 7
  Sheet Metal vs CNC vs 3D Printing for a Magnet Bracket

A door sensor is two parts that never touch. A reed switch or Hall-effect sensor sits in the frame; a permanent magnet sits on the moving leaf, held by a bracket. The switch closes when the magnetic field at the sensor crosses a threshold. That threshold is set by the gap, the magnet grade, and the angle between the magnet's axis and the switch. Change any one and the trigger distance moves.

This is why the bracket is not a cosmetic housing. It is the mechanical link that keeps the magnetic axis pointing where the circuit needs it, year after year. A bracket that bends 0.5 mm under a slammed door can push a sensor from reliable switching into intermittent behavior.

Most industrial doors specify a switching gap somewhere between 8 mm and 25 mm, depending on magnet size. The bracket does not set that gap directly, but it controls the two variables that drift: position and angle. Position drift comes from hole clearance and bend angle error. Angle drift comes from twist in the bracket, usually after forming or after over-torqued screws.

Sheet metal suits this job because it gives stiffness in the plane that matters, low mass, and a shape that can be formed in one press cycle. The trade-off is that every bend introduces a small error, and those errors add up along the bracket.

• 1
  Gap sets sensitivityA 2 mm gap change can move the trigger point noticeably on small magnets.
• 2
  Angle sets repeatabilityA few degrees of tilt reduces field strength at the sensor.
• 3
  Stiffness sets lifeThin, unsupported flanges flex under vibration and door slam.

Start with gauge. For a bracket that only holds a 6 mm × 3 mm disc magnet, 0.8 mm to 1.2 mm cold-rolled steel or 1.0 mm to 1.5 mm 304 stainless is usually enough. Going thicker does not buy much alignment accuracy; it adds press tonnage and makes tight bend radii harder to hold. Going thinner saves mass but the flat area around the magnet pocket starts to oil-can.

Bend radius needs to respect the material. A common shop floor rule is a minimum inside radius equal to the sheet thickness for mild steel, and 1× to 1.5× thickness for 304 stainless, which work-hardens. Below that, the outer surface can crack. For a 1.0 mm 304 bracket, plan on a 1.0 mm to 1.5 mm inside radius and design the flange length accordingly.

Keep bends at least 2× the thickness away from a hole edge. Closer than that, the hole distorts during forming and your screw clearance turns into an interference fit. If the hole must sit near the bend line, add a relief notch or move the hole into a separate tab.

Hole clearance matters more than people expect. An M3 screw in a Ø3.2 mm hole gives 0.2 mm of play. Over a 40 mm lever arm, that play becomes about 0.3° of rotation, which is often acceptable. If the sensor is sensitive, use a shouldered screw or a dowel pin in a reamed hole to remove the slop.

• 1
  1.0 mm 304 stainlessInside radius 1.0–1.5 mm; plan flange length after the radius.
• 2
  1.2 mm CRSInside radius 1.0–1.2 mm; easy to form, needs plating.
• 3
  Hole near bendKeep 2× thickness from the bend line, or add a relief.

Indoor doors in dry warehouses are the easy case. Cold-rolled steel 1018 or 1045 at 1.0 mm to 1.5 mm, zinc plated or powder coated, holds alignment and costs little. If the bracket is inside a plastic housing, corrosion is a non-issue and the steel choice is really about stiffness and cost.

Outdoor gates and dock doors see rain, salt, and temperature swings. Here 304 or 316 stainless at 1.0 mm to 1.5 mm is the standard answer. 316L is worth the premium within roughly 10 km of salt water or in food plants with chloride cleaners. Stainless springs back more during bending, so the press brake setup needs a slightly different over-bend to hit the target angle.

Aluminum 5052 or 6061 at 1.5 mm to 2.0 mm is a reasonable middle ground for doors where weight matters, such as high-speed roll-up doors. Aluminum does not rust, but it needs a conversion coating or anodize if the environment is wet. Bare aluminum in contact with stainless fasteners can set up galvanic corrosion in a wet joint.

Magnets themselves are usually nickel-plated neodymium. The bracket should not trap moisture against the plating. A small drain slot or a raised pocket that keeps the magnet off the flat surface helps. If the door is washed down daily, consider a stainless bracket with an over-molded or potted magnet instead of a press-fit one.

• 1
  Dry indoor1.0–1.5 mm CRS, zinc or powder coat.
• 2
  Outdoor or washdown1.0–1.5 mm 304/316L stainless; expect more springback.
• 3
  Weight-sensitive1.5–2.0 mm 5052 or 6061 aluminum plus coating.

Every bracket has a stack. Start at the mounting hole, go through a bend, across a flat, through the magnet pocket. Each feature has its own tolerance, and they combine. A ±0.2 mm hole position plus a ±1° bend angle plus a ±0.1 mm pocket depth can add up to more than ±0.5 mm at the magnet face. On a sensor with a 10 mm switching gap, that is 5% of the working range.

The bend angle is usually the biggest contributor. Over a 30 mm flange, 1° equals about 0.52 mm of tip movement. That is why the flat pattern and the bend deduction have to be right, and why the first article should be checked on the actual formed part, not on the flat blank.

Springback is the reason bend angle drifts. Steel springs back 1° to 3° depending on grade and radius-to-thickness ratio; stainless springs back 2° to 5°. A shop that runs the same program for 1018 and 304 will miss one of them. The fix is either over-bending in the program or a bottoming/coining setup that forces the material into the die.

Pocket depth for a press-fit magnet needs its own control. If the pocket is too shallow, the magnet sits proud and the gap closes. Too deep, and the magnet sits low, weakening the field. For a 3 mm thick magnet, hold pocket depth to ±0.05 mm and check it with a depth micrometer or an optical comparator.

• 1
  Bend angle dominates1° over 30 mm equals about 0.52 mm at the tip.
• 2
  Springback variesMild steel 1–3°, 304 stainless 2–5°.
• 3
  Pocket depthHold ±0.05 mm on press-fit magnet pockets.

Finishes add thickness. Zinc plating typically adds 5 μm to 15 μm per surface; powder coating can add 60 μm to 100 μm. On a bracket with a press-fit magnet pocket, that coating can close the pocket and make assembly impossible. Either mask the pocket or cut it oversize by the coating thickness.

For outdoor stainless brackets, passivation is usually enough. It removes free iron from the surface and restores the chromium oxide layer. Electropolishing goes further and smooths the surface, which helps in food and medical equipment, but it also removes material and should be accounted for on thin edges.

Aluminum brackets for wet doors are commonly anodized. Clear anodize adds roughly 5 μm to 25 μm depending on the type. Hardcoat anodize is thicker and more wear resistant, but it is brittle and can chip at sharp bend radii. If the bracket has tight bends, keep the anodize thin or use a chromate conversion coating instead.

Laser marking is a practical way to add a part number or a date code. Minimum character height is 1.5 mm for legible marking. Do not put the mark on a sealing surface or inside a magnet pocket where it can interfere with the fit.

• 1
  Zinc plating5–15 μm per surface; mask press-fit pockets.
• 2
  Powder coat60–100 μm; too thick for tight pockets.
• 3
  Anodize5–25 μm; hardcoat can chip at tight bends.

A paper certificate does not prove the bracket will work. What matters is the measurement plan. For a magnet bracket, the critical dimensions are usually the mounting hole position, the bend angle, the magnet pocket depth, and the flatness of the face that touches the door.

Bend angle is best checked with a digital protractor or a vision system on the formed part. Hole position is checked against the datum called out on the drawing, not against a convenient edge. Pocket depth needs a depth micrometer or an optical comparator, because calipers on a small pocket are unreliable.

Flatness matters when the bracket mounts to a painted or uneven door surface. A bracket that rocks on a high spot will not sit at the designed angle. For critical doors, specify a flatness callout on the mounting face, typically 0.2 mm over the face length.

GreatLight runs raw material checks, in-process monitoring, and a final inspection before shipment, with reports available on request. For a bracket that holds a safety-relevant sensor, we recommend a first article inspection and a small capability run before full production.

• 1
  Hole positionMeasure from the drawing datum, not an edge.
• 2
  Bend angleCheck on the formed part, not the flat blank.
• 3
  Pocket depthUse a depth micrometer or optical comparator.