Mill Stiffness = Top Finish

Mill Stiffness = Top Finish is a fundamental equation for any milling machine big or small. Increasing stiffness reduces vibration.

Sieg SX2.7 stiffening bracket

I increased the column to base connection stiffness with this mild steel plate. The fundamental design principle was to add a thick plate and increase the bending stiffness of the plate using beams. So the 1/4″ thick mild steel plate has 3/4″ deep 1/4″ wide beams. This gives it an effective thickness of 1″ overall.

The plate also closes off the U-section column and connects the column to the mill base.

10 off M5 cap-head bolts secure the plate to the base. Then 10 off M5 cap-head bolts secure the plate to the edges of the column.

The design, build and fitting of this stiffening bracket took me a couple of days. One of my first jobs after fitting this bracket to the SX2.7 was to end mill the edge of some mild steel plates.

end mill finish with SX2.7 and bracket
2 off 1/4″ thick mild steel plates bolted together

I used a milling cutter store 3 flute carbide bit. At 1000rpm and with no cutting fluid this resulted in a great finish. I have to put a lot of that great finish down to the milling bit. However, the new bracket has significantly reduced the vibration that you can feel on the column when making this type of cut.

The simple measurement of the static stiffness of the mill showed that the bracket increased this by 1.5x. That is a significant step change. Therefore, I think I can happily say that Mill Stiffness = Top Finish.


Sieg SX2.7

Sieg SX2.7 Milling Machine

A small benchtop manual milling machine with brushless motor and belt drive.

  • Drilling Capacity: 20mm
  • End Milling Capacity: 16mm
  • Spindle Stroke: 70mm
  • Throat: 190mm
  • Spindle to Table Distance: 345mm (MT3)
  • Spindle Speed (Variable): 100-2000 rpm ± 10%
  • Cross Travel: 180mm
  • Longitudinal Travel: 395mm
  • Size: 520x100mm
  • Effective size: 595x140mm

Surface Roughness Ra

Surface Roughness

At a microscopic scale, no surface appears perfectly smooth. Surface roughness is determined by calculating Ra and Rz.

  • Ra = average surface roughness
  • Rz = information for any pore, hole or surface deformities. These can be detrimental to strength.

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