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Specialist in on-site, spindle taper grinding for repair and restoration of precision


Gunn Machine & Tool Company

Inspection of the spindle taper seat requires the proper tools and gages. Some of the inspection procedures are best done by someone who routinely does them, such as contact testing.  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.


First, visually inspect the taper seat and toolholders. Is there fretting or other damage present? Fretting is often mistaken for rust or corrosion. It occurs when the toolholder 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 toolholder.

Look at the drive keys. Do they show signs of wear? This is caused by the toolholder 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 toolholder. 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.


If you have the proper precision taper plug gage, you should use it to check the contact. If you do not, you may use a new, quality brand, toolholder. 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 gage with a very light film.


Be careful seating the gage, so as, not to rub the sides and get a false reading. Once seated, you should rub or twist the gage in the seat, so the high spots will mark in the spindle. In the case of using a toolholder, you may clamp the toolholder and let 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. Less than 70% should be considered in need of regrinding. Also note where the contact is. It should be evenly around the seat. A little heavier contact on the large end is  acceptable, but heavier contact in the small end is not good. This is typical of a bellmouth condition, caused by normal wear, and should be reground.


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


Next, you should check the runout with a precision test bar. Again if you do not have a test bar you can use a toolholder. This is not recommended, but it will give you a rough idea on the condition. The toolholder or test bar should be clamped in the spindle and indicated. Note the total indicator runout (t.i.r.) 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.


The next test that should be done is the drawforce check. This is to make sure the drawbar mechanism is working properly. You will need a drawforce dynamometer such as the ForceCheck unit that is available through us.

These checks should give you a good idea as to spindle taper condition. Gunn Machine & Tool Co. would be happy to answer any questions you may have or further discuss. So, feel free to call or email us.

ForceCheck in use
blueing in spindle
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