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Countersunk Cup Magnets
Flush Screw-Mount Rare-Earth Assemblies
Countersunk Cup Magnets (Flat-Head Screw Mounting Bases)
CMS Magnetics manufactures commercial-grade countersunk neodymium cup magnets (round base screw-mount pot magnets) engineered for zero-clearance mechanical fastening and heavy-duty magnetic clamping. Each assembly seats a high-energy sintered NdFeB ring magnet inside a precision-machined, triple-plated mild steel cup with a concentric center hole beveled at standard 82° (imperial) or 90° (metric) angles. This geometry allows flat-head wood, sheet metal, or machine screws to drive completely flush with or slightly below the magnetic contact surface, eliminating the fastener interference and edge cracking common to raw countersunk discs.
Looking for threaded studs, rectangular channels, or raw discs? If your assembly screws into pre-tapped holes, explore our Male Threaded Stud Cups or Female Threaded Cups. For anti-twist door latches, view Neodymium Channel Magnets. If you need uncased magnetic rings without a steel cup, see our Neodymium Ring Magnets, or pair cups with Steel Magnet Strikers.
Sub-Flush Screw Seating
Precision-chamfered center bore ensures screw heads sit 0.2 mm to 0.5 mm below the contact rim.
Steel Shunt Flux Boost
Low-carbon steel cup concentrates magnetic field lines onto the open face, multiplying pull up to 4x.
Torque & Impact Armor
Rigid steel cup walls absorb fastener tightening torque and buffer against direct metal slamming.
Standard Hardware Compatibility
Engineered center bores fit standard #4, #6, #8, #10, and 1/4" flat-head countersunk screws.
Filter by Cup & Assembly Style: Countersunk Cup Magnets Male Threaded Cup Magnets Female Threaded Cup Magnets Neodymium Channel Magnets All Neodymium Cups
Engineering Guide: Fastener Fitment, Shunt Mechanics & Installation
Countersunk cup magnets are designed specifically for rigid mechanical mounting using flat-head fasteners. By encasing the brittle neodymium core inside a machined steel cup, these assemblies solve the two most common mechanical failure points of raw countersunk magnets: wedge-stress fracturing during screw torque-down and microscopic air gaps caused by unseated screw heads.
Standard Fastener Sizing & Pull Force Guide
| Cup Outer Diameter (OD) | Compatible Screw Size | Countersink Angle | Rated Pull Force Range | Typical Mounting Use |
|---|---|---|---|---|
| 0.63 in (16 mm) | #4 or M3 Flat Head | 82° / 90° | 10 – 18 lbs | Light cabinet catches, electronics enclosures |
| 0.79 in (20 mm) | #6 or M4 Flat Head | 82° / 90° | 20 – 35 lbs | Tool racks, architectural access doors |
| 1.26 in (32 mm) | #8 or M5 Flat Head | 82° / 90° | 65 – 90 lbs | Heavy machinery guards, commercial fixturing |
| 1.89 in – 2.36 in (48 – 60 mm) | 1/4" or M6 Flat Head | 82° / 90° | 140 – 300+ lbs | Industrial workholding, structural marine mounts |
Countersunk Cup vs. Raw Countersunk Disc: Critical Differences
Understanding the physical differences between an uncased countersunk disc and a steel-jacketed cup magnet prevents premature installation failures:
- Torque Tolerance: Tightening a flat-head screw directly into a raw sintered NdFeB disc creates outward hoop stress. Because sintered neodymium is a ceramic-like material with low tensile strength, slight over-torquing will snap the disc in half. In a countersunk cup magnet, the low-carbon steel cup absorbs fastener seating pressure, isolating the magnet core from cracking.
- Flush Surface Seating: If a screw head protrudes even 0.5 mm beyond the magnet surface, it introduces an immediate air gap that cuts holding force by 50% or more. CMS Magnetics countersunk cups are engineered with deeper lead-in chamfers to ensure the screw head sits completely below the outer steel lip.
- Focused Clamping Force: While a raw disc radiates half its magnetic field into the wall behind it, the steel cup acts as a magnetic shunt, redirecting rear flux lines forward into the contact plane to boost holding power by 300% to 400%.
Vertical Shear Slide vs. Direct Breakaway Pull
Published pull capacities represent perpendicular breakaway force (Case 1) tested on flat, ground, 1/2-inch structural low-carbon steel. When mounting countersunk cups vertically on a wall or machinery frame:
- Friction Coefficient Factor (μ): The coefficient of friction between smooth nickel-plated steel surfaces is typically μ ≈ 0.15 – 0.20. Under pure gravity shear, a vertical cup magnet will begin sliding downward at approximately 15% to 20% of its rated perpendicular pull force unless mechanically supported.
- Target Metal Thickness: Cup magnets require an adequately thick steel target (at least 1/4" thick) to fully absorb the concentrated magnetic circuit. Thin 20-gauge sheet metal saturates quickly, reducing real-world holding strength significantly.
- Non-Ferrous Mating: When securing to drywall, wood studs, or aluminum extrusions, mate the assembly with matching pre-drilled Steel Magnet Strikers.
Frequently Asked Questions
What screw type should I use with countersunk cup magnets?
Always use standard flat-head countersunk screws (such as 82° imperial wood/drywall screws or 90° metric machine screws). Do not use pan-head, round-head, or socket cap screws, as their heads will protrude above the magnetic rim and prevent flush contact with your steel target.
Should I use magnetic steel screws or non-magnetic brass/stainless screws?
Both work, but non-magnetic fasteners (such as 300-series stainless steel or brass) are preferred because they do not draw magnetic flux through the center hole, preserving the intended flux loop between the magnet face and the outer steel cup rim. However, standard zinc-plated carbon steel flat-head screws are acceptable for non-critical holding applications.
What is the temperature limit for countersunk neodymium cup magnets?
Standard countersunk neodymium cup magnets are rated for maximum continuous operating temperatures of 80°C (176°F). Exceeding this temperature results in permanent demagnetization. For high-heat installations, consider high-temp SmCo assemblies or naturally heat-resistant ceramic cup magnets rated up to 250°C (482°F).
Why does my countersunk cup magnet feel weaker when holding thin sheet metal?
A cup magnet concentrates high-density magnetic flux directly into the contact surface. To achieve 100% of the published pull rating, the steel substrate must be at least 1/4" to 1/2" thick. When placed against thin 18- or 20-gauge sheet metal, the steel saturates immediately, allowing magnetic field lines to escape through the back and cutting holding force by up to 50% or more.