Service Home How To Procedures Spindle Deflection Test - Mill - AD0599
Last updated: 10/02/2025

Spindle Deflection Test - Mill

Revision 4

AD0599

Revision A - 10/2025

Introduction

Important: Download and fill out the Spindle Inspection Report Checklist below before replacing any parts.

NOTE: Only perform this procedure AFTER all other troubleshooting steps outlined in the Mill Spindle Inspection Report, linked below, have been completed. 

Spindle Inspection Report Checklist

This procedure applies to T-0140 SPNDL DEFLECTION TOOL KIT 40T/50T/HSK63A.

Machine Compatibility:

  • 40T Spindle
  • 50T Spindle
  • HSK spindle
     IMPORTANT:  Except on machines with a Gimbal Spindle.

This test determines if surface finish issues are caused by spindle bearings not having the correct preload force. 

Deflection Test

1

Place the piston assembly on the machine's table and put the T-nuts into the T-slots [1]. Put the half inch bolts through the holes on the tool's base and thread them into the T-nuts. (DO NOT TIGHTEN DOWN) They should still be loose. The tool should be able to slide along the X axis, and there should be limited movement in the Y axis. 

2

Put the 5/8" or 16mm threaded stud [1] into an end mill tool holder [2].

WARNING: Ensure the tool is held tightly in the holder. This test will put 500 lbf on the 40T/HSK tool, while the 50T tool will receive 1000 lbf. 

Place the tool holder with the threaded stud into the spindle. 

3

Connect the regulator to shop air and set the regulator [1] to 40 psi (0.3 MPA) for HSK and 40T machines and 85 psi (0.6 MPA) for 50T machines.

Extend the piston [2] by sending air to it by flipping the switch up. Flip the switch to neutral once the piston has extended about one inch. The test will not work correctly if it is fully compressed or fully extended.

Make a mark on the side of the piston 0.25" (6.5 mm) [3] below the top and 0.75" (19 mm) [4] below the top. See the video at the top of the page for an example. 

Slowly jog the spindle down until the stud compresses the piston slightly. Continue by slowly compressing the piston to the 0.25" line [3], followed by turning the spindle by hand until the 0.75" line [4] is visible. Continue this step process until the stud is fully threaded in. Then compress the cylinder 0.1" further down so the piston is in the middle of its stroke. See the video at the top of the page for a demonstration.

4

Rotate the spindle using your hand to thread the tool into the piston. At the same time, use your other hand to pull up the rod with the female threads on the air cylinder to meet with the male threads on the tool holder. Once the threads catch, both hands can be used to spin the spindle to thread the tool further.

At this point is important not to move either the X or Y axis as this will cause damage to the tool or the machine. You may need to move the machine's Z axis up or down to fully thread the tool. If it is necessary, do it slowly and in small increments. 

Once the tool is fully threaded, tighten the two T slot bolts.

WARNING: Turn on E-stop on the machine. If the machine tries to move at this point, the tool and machine will be damaged. 

The piston should be somewhere in the middle of its stroke. The test will not work correctly if it is fully compressed or fully extended.

5

Place the base of a dial indicator [1] on the spindle housing [2] and the tip on the spindle face. In the image, the dial indicator is placed correctly. Do not place the tip of the indicator on the spindle housing. This will lead to incorrect results. 

WARNING: This test looks for movement with the spindle, so the spindle housing needs to be used as the reference position. Placing the base on other parts will lead to incorrect deflection. 

When using a dial indicator, ensure it is at the correct angle to the spindle nose.

6

Both images below show the correct placement of the dial indicator tip on the spindle face [1]. The area labeled [2] is part of the spindle housing, and the indicator's tip should not be placed there. The image on the right is of an HSK spindle, and the left image is of a 40T spindle. 50T spindles will look the same as the 40T spindle. 

 

 NOTE: In the example shown, the spindle deflection readings are:

  • Push value: 0.0005"
  • Pull value: 0.0005"
  • Total Lost Motion: 0.0002"
  • Total Deflection: 0.001"

7

Set the dial indicator to zero.

IMPORTANT:  Indicator readings are critical make sure your indicator repeats.  Deflect the dial indicator a few times and make sure it repeats back to zero.

Push Test:

  • Push the air switch lever so the piston pushes into the spindle face.  Record the Push Test indicator value:______________.

  • Return the switch lever to the middle position.  Record the Lost Motion indicator value:_______________.

Pull Test:

  • Push the air switch lever so the piston pulls the spindle face.  Record the Pull Test indicator value:______________.

  • Return the switch lever to the middle position.  Record the Lost Motion indicator value:_______________.

Total Lost Motion

  • Add the sum of the Push/Pull Test Lost Motion indicator value:______________.

Total Deflection

  • Sum the absolute value of Push and Pull test values: _________ (do not add/subtract the (total) lost motion to the total deflection).

Put the Push, Pull, and Total Lost Motion indicator values on the checklist and submit it to Haas service.

The acceptable tolerance for mill spindles is: 

  • 40T/HSK: 0.0025" NTE
  • 50T: 0.003" NTE

8

To disassemble the tool, spin the spindle to remove the tool from the piston. It may be necessary to raise the spindle if the piston bottoms out. Once the threaded stud is free of the piston, the spindle head can be raised up. Next, remove the bolts from the T nuts and remove all the tool parts from the machine.