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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Inspecting Your Spindle

Inspecting the Spindle Taper Seat

Inspection of the spindle taper seat requires the proper tools and gauges. Some of the inspection procedures, such as contact testing, are best left to someone who does them routinely. Gunn Machine & Tool Co. will thoroughly inspect the spindle on-site. However, you can get a good idea of the condition yourself by performing the following procedures and carefully observing the results.

Taper Seat & Tool Holders

First, visually inspect the taper seat and tool holders. Is there fretting or other damage present? Fretting is often mistaken for rust or corrosion. It occurs when the tool holder and the spindle microslip relative to each other. It is generally caused by an improper contact angle, damage, or contamination in the interface between spindle and tool holder.

Drive Keys

Look at the drive keys. Do they show signs of wear? This is caused by the tool holder trying to slip in the taper seat only to be caught by the keys. The taper seat is designed to transfer the torque through it and should not rely on the keys to drive the tool holder. Contact us for trailer spindle replacement.


Next, thoroughly clean the taper seat. Use a soft cloth or paper towel that will not leave lint. Use a solvent that will not leave a residue. Never use Scotchbrite or any kind of abrasive to clean the seat or tools, as they only mask the problems, as well as create further wear. Clean and dry the seat.

Checking Contact

If you have the proper precision taper plug gauge, you should use it to check the contact. If you do not, you may use a new, high-quality tool holder. This is not recommended, but you should be able to get a basic idea on the contact. Use a non-drying blue, such as Dykem HI-Spot Blue or Canode water-soluble blue. Do not use layout blue. A small amount goes a long way, so coat the gauge with a very light film.

Be careful seating the gauge so as not to rub the sides and get a false reading. Once seated, you should rub or twist the gauge in the seat so that the high spots will mark in the spindle. If using a tool holder, you may clamp the tool holder and let it set for 15 seconds before carefully removing and inspecting the contact.

Reading the bluing is something that takes some getting used to. Different people can get different readings depending on their method. It is best done by someone who does it routinely. But even if it is your first experience, you should be able to get an idea as to how the tools are seating.

When we regrind the seat we typically get 95-100% contact. A result of less than 70% should be considered in need of regrinding. Also note where the contact is. It should be distributed evenly around the seat. A little heavier contact on the large end is acceptable, but heavier contact on the small end is not good. This is typical of a bell mouth condition, caused by normal wear, and should be reground.

Inspection with an air gauge will tell you if the gauge is seating properly, but it will not tell you what the problem is, if it is not seating properly. For this reason, we prefer to use bluing and plug gauge.

Check the Runout

Next, you should check the runout with a precision test bar. Again, if you do not have a test bar, you can use a tool holder. This is not recommended, but it will give you a rough idea of the condition. The tool holder or test bar should be clamped in the spindle and indicated. Note the total indicator runout (TIR) and mark the bar and spindle face to reference the high spot. You must then remove the test bar and replace it in the spindle at a 180-degree rotation from the first check. Note if the runout is the same. Also, note if the high spot followed the spindle or test bar. If the high spot mark stays with the spindle, it should be considered as spindle runout. If the high spot stays with the test bar, you have a test bar that is damaged.

Check the Draw Force

The next test that should be done is the draw force check. This is to make sure the drawbar mechanism is working properly. You will need a draw force dynamometer such as our ForceCheck unit. Completing all of these checks should give you a good idea as to spindle taper condition.

Checking the Runout