Gunn Machine & Tool Co.

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Specialist in on-site spindle taper seat grinding for repair and restoration of precision to your machine spindle.

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Spindle Information


Spindle Interior With Damage

Symptoms & Causes Spindle Taper Seat Damage or Wear

Is your spindle taper seat damaged or worn? Learning the common symptoms and causes could save you a lot of money. Contact Gunn Machine & Tool Co. for more information or to book a complete inspection.


Common Symptoms:

  • Tool Run Out
  • Loss of Precision
  • Out of Round Bores
  • Erratic Bore Sizing
  • Tool Chatter
  • Overall Machine Performance & Repeatability Faltering
  • Stepping of Cutter Blends
  • Fretting
  • Wear on Face Keys
  • Tool Holders Sticking
Bell Mouth Diagram

Common Causes:

  • Wear — Tool pressure and heat generated from cutting contribute to the deterioration of the taper seat. Normal wear usually causes a "bell mouth" condition, which causes further and more rapid wear. Bell mouth condition is typical of tooling seat wear. Often, the wear is concentric to the spindle bearings, so when checking for spindle run out with an indicator, the condition cannot be detected. However, when the machine is working and pressure of cutting applied, the tool holder unseats, causing the problems associated with a bad spindle taper seat.
  • Wreck — Broken cutters, retention knobs, and drawbars could be the result of preexisting taper damage. As the power is transmitted through the taper seat, a taper that fits badly may let go. Wear on the keys in the face of the spindle may be evidence that the taper is not holding.
  • Dirt — Dirt and chips find their way between the tool and spindle. Automatic tool changers may be equipped with air shot cleaners, but when you add coolant to the mix, they become less than reliable.
  • Automatic Tool Changers — ATC misalignment may cause tool holders to drag on one side of the seat every time a tool is put in and taken out, causing wear on one side of the seat.
  • Manual Tool Changes — Heavy tools may cause the operator to bump the end of the tool against the taper seat, creating dents and raised areas around them. Look for dented or pitted surface of taper seat.

Bellmouth Wear

Bell-mouth condition is typical of tooling seat wear. The wear may be concentric to the spindle. So, when checking runout, with test bar, the condition cannot be detected. However, when the machine is working and the pressure of the cutting tool is applied, the tool holder unseats, causing vibration and other problems associated with a bad spindle taper seat.


Characteristics of Properly Ground Spindle Taper Seats

  • Greater Precision & Truer Boring — Grinding the spindle taper seat in place while the spindle is running in its own bearings will true up any error in concentricity. Even new spindles can achieve better concentricity by grinding after assembly, eliminating the stack up of tolerance error. Out-of-round and erratic sizing of bores are commonly caused by movement between tool and spindle.
  • Increased Productivity — Faster feed rates and spindle speeds can be achieved, as equal cutting pressure is distributed to all cutting edges of tool by true running of the spindle and tool. This increases productivity at your facility.
  • Reduced Tool Chatter & Better Blends — Renewing the spindle taper seat tolerances by regrinding in place ensures a ridged connection between tool and spindle. This eliminates chatter and rock caused by poor tool seating.
  • Increased Cutter & Tool Holder Life — Your cutting tools will last longer. The spindle-to-tool concentricity allows the workload to be equally distributed among all the cutting edges of the tool so that no one edge is broken.