A pressboard dovetail transfers insertion force through sloping faces and a narrow root. Qualify the fit, restraint and load path together to establish an assembly range that preserves the part. [1]
Calculation basis. Project-specific values and gates in the illustrative screens below are assumptions; cited catalogue values retain their source test conditions.
Inspect the dovetail as a contact system. Nominal widths describe the opening and tail, but taper, angular mismatch, surface condition and end restraint determine where contact starts and how the part is loaded.
Locate the load path before inspecting the crack
- Sloping contact faces
- Look for concentrated marks at the mouth or deep end. Contact over a short region can produce a different local load from distributed contact along the face.
- Narrow root
- Examine the transition radius and nearby sheet direction. A force tending to spread the mating opening can create tension or splitting at this location.
- Bottom of the cut
- Check whether the tail bottoms before its sloping faces seat. An apparently complete insertion may then be supported by an unintended surface.
- Entrance and exit ends
- Inspect local chips and separation introduced as the tail enters or leaves the machined profile.
- Fixture support
- Identify which movements are prevented during insertion. Restraint can change the force path even when the dovetail dimensions are identical.
This map guides observations; it does not assign a stress concentration factor. Pressboard is a directional cellulose material, and its local response cannot be established by copying a metal notch formula or a tensile value from another grade.
Measure insertion together with damage
A qualification trial should record the insertion sequence and force-displacement behaviour under representative supports. Preserve the force trace alongside photographs of contact marks and any splitting. A sudden force drop may indicate a seating event or damage; the trace alone cannot decide which.
Inspect specimens before and after insertion. Examine the root on both accessible faces, then section sacrificial witnesses through the critical region. Document the sectioning method so separation introduced during examination is distinguishable from an existing crack.
Use the relevant material and conditioning state. A dry machined fit and a fit after the intended drying or impregnation sequence may differ. The starting board’s mechanical data support material selection but do not qualify the concentrated contact at the dovetail root.
Compare dimensional combinations near the proposed fit boundaries, not just the nominal pair. Width, taper and root transition can combine to produce local interference even when each individual dimension appears acceptable. Include the approved sheet orientations and the actual assembly direction.
A larger root radius may reduce a sharp transition while reducing engagement or changing the mating envelope. Evaluate both effects. Similarly, making the fit looser can lower insertion force while permitting movement under the subsequent duty.
Define the fit range from assembly response and retained function. Correct and retest geometry or loading combinations that produce root splitting.
Check dovetail interference at the root as well as the mouth
Keep conditioning, loading direction and aging state with pressboard comparisons [2].
Assume a spacer tongue measures 10.10 mm at the controlling section and its receiving slot is 10.00 mm, giving 0.10 mm interference. With an assumed ±0.04 mm tolerance on each, worst-case interference is 0.18 mm. The full assumed interference range is 0.02–0.18 mm. Assume a separate assembly trial on the same material and conditioning established an acceptable 0.03–0.12 mm fit window. Nominal fit passes, but the tolerance range crosses both boundaries. Reduce and verify the combined tolerances before release; a mouth measurement cannot qualify root loading.
References
- IEC, IEC 60641-2:2004, Pressboard and presspaper for electrical purposes — Part 2: Methods of tests.
- C. Oria, I. Carrascal, D. Ferreño, C. Méndez and A. Ortiz, Experimental dataset on the compressive mechanical properties of high-density pressboard used in power transformers spacers, Data in Brief 50, 109471, 2023. DOI: 10.1016/j.dib.2023.109471.