Spice Grinder
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A precision-machined aluminum herb & spice grinder , designed from scratch and CNC-milled.
Design Objectives
Two interlocking hexagonal halves with interdigitating diamond-profile grinding teeth
Magnetic closure; neodymium magnets provide holding force without additional fasteners
Press-fit raceway interface (Ø2.460 in) for smooth rotational engagement between top and bottom
Teeth sized to grind tea leaves and dried spices; designed as cantilever beams with FOS ≥ 2
Manufacturable on a Tormach CNC mill from a single aluminum billet with minimal setups
Dimensions fully toleranced with GD&T for reproducibility
Design Considerations
By altering from our original design, we were able to reduce the CNC mill machining time by 60% to just under an hour for each half.
60% Reduction in Time
->
Final Design
Geometric Dimensioning & Tolerancing
The core functional interface of the grinder is the mating cylindrical raceway, the top piece outer Ø must slip into the bottom piece inner Ø with minimal play (so the halves stay concentric during grinding) but without binding. This is a precision clearance fit at Ø2.460 in across two separately-machined aluminum parts.
We applied a unilateral tolerance strategy: the top raceway was toleranced at +0.000 / −0.005 in (produced to or slightly under nominal) while the bottom was held at +0.000 / −0.005 in (nominal or slightly tighter). This ensured a clearance fit even in the worst-case stack-up. Circularity callouts of Ø0.005 in were added to both raceways on the GD&T drawing to control any out-of-round condition from the CNC circular interpolation. Measurements were taken with micrometers (rather than calipers) for the critical diameter features to achieve the required resolution.
Top raceway: Ø2.460 +0.000 / −0.005 in | ⊙Ø0.005(L) A
Bottom raceway: Ø2.460 +0.000 / −0.005 in | ⊙Ø0.005(M) A
Worst-case clearance: 0.000 in (slip fit) → No interference guaranteed
Manufacturing
While a majority of the operations were done using the CNC Mill, the following were done using a manual mill:
Drill Magnet hole (15/64”)
¼” reamer for magnet press fit
Removal of Pedestal
Challenges
Tooth Strength
Each grinding tooth is a 0.25 × 0.125 in diamond-profile protrusion standing 0.49 in tall, essentially a thin cantilever beam subject to lateral grinding forces. The primary concern was whether these features could withstand realistic loads without yielding, and whether they could even be cleanly machined at that aspect ratio.
We modeled each tooth as a cantilever beam and applied the bending stress equation σ = Mc/I = 24FL / (d₁ · d₂²), with Kₜ ≈ 2.5 to account for the stress concentration at the tooth root. Using the yield strength of 6061-T6 aluminum (~40,000 PSI) and a factor of safety of 2, we found the teeth can withstand approximately 2.7 lbs of lateral force before yielding, well above a realistic grinding load of 2 lbs. Deflection was estimated at 0.015–0.02 in, acceptable for this application. We also validated this with a SolidWorks FEA simulation, which confirmed the stress concentration at the root and showed the max von Mises stress well below yield strength.
Coolant Failure
Halfway through machining the bottom piece, the coolant system failed. Without coolant, chip evacuation stalled and chip buildup deflected the cutting path, resulting in an off-center bore. This shifted the central axis of the bottom piece and compromised the raceway alignment; the single most critical dimension for part function. We ended up having to re-do the bottom piece.
The key DFM lesson here is the value of monitoring critical in-process conditions (coolant flow, chip load) and designing in re-machinable features. The bore could not be re-centered without a new setup from a known reference datum. In production, this would be addressed by (a) implementing coolant flow monitoring with auto-stop, (b) machining the bore last so earlier features aren't wasted if it fails, and (c) using through-coolant tooling for deep bore operations.
Magnet Assembly
the original plan to press-fit the neodymium magnets into Ø0.250 in holes (tolerance +0.000/−0.002 in) proved unworkable, as the brittle magnets fractured under press force. We pivoted to cyanoacrylate adhesive bonding, which is actually a better DFM choice: adhesive bonding eliminates tight-tolerance bore requirements, works with brittle materials, and is repeatable without specialized equipment.
Skills
CAD Modeling: SolidWorks assemblies, section views, exploded drawings with mates
GD&T: Applying circularity, flatness, and positional tolerances to production drawings
CAM & Toolpathing: Fusion 360 toolpath generation; 60% reduction in CNC cycle time through path optimization
CNC Milling: Tormach setup, workholding, speeds & feeds for 6061 Al, multi-operation planning
Precision Measurement: Calipers and micrometers for bore, tooth, and fit verification to ±0.002 in
Structural Analysis: Cantilever beam stress analysis + FEA validation for tooth geometry under grinding loads
Fit & Tolerance: Clearance and interference fits for raceways and magnet holes; unilateral tolerancing strategy
Design Iteration: Pivoting from press-fit to adhesive when material behavior (magnet fragility) changed constraints
Finshed Product: Open
Finshed Product: Open
Finshed Product: Closed
Finshed Product: Closed
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