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The middle is a Felder F7 spindle cartridge. I arranged the spindles in what I believe are in order of strength. Left is a Knapp and being hollow, I believe it to be the weakest. Still a stout build but relative to others the shorter length is a clue that stacking is not an option for a hollow spindle. The newer Felder cartridges have four steps. On mine the smallest diameter is 40mm which would be the weak point- again only relative and maybe only when running a large 50mm spindle and heavy cutters which the F7 is not designed for. Right is a T130. Strength is related to spindle diameter as there is nothing unsupported below it as it fits into a taper. The old Martin system is very similar but as with all things Martin, just better. Takes a 1/4 turn of the collar to allow spindle to be unscrewed vs removing the whole collar on the SCMI. The real difference in the Martin vs the Felder or SCMI are in the bearings. Newer machines run light duty 60xx to allow for higher spindle speeds with standard grease shielded bearings. The good news is that the Felder bearings are much easier to replace than the SCMI. The old Martin runs precision 62xx bearings in a drip oil bath. Those bearings have a much higher radial load capability and still run at high speed. Expensive system to produce though.

I digress but spindle and quill designs are interesting and seldom considered when buying a shaper. Dave