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Lathe Operations - Grooving and Tread Relieving
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Grooving or thread relieving is often done at the end of a thread to allow for the full travel of the thread to a shoulder. Tools can be ground for special-shaped relief grooves, although a parting tool (Figure 1) is often used for external grooving or thread relieving.

 

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Figure 1 (Click for larger view.)

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Figure 2

The setup for grooving or thread relieving is critical. Grooving creates high cutting forces; therefore, maximum rigidity in the setup must be maintained. The parting tool holder must be kept as close to the tool post hold down screw as possible (Figure 2)and the parting blade must not stick out further from the holder than is absolutely necessary.
Parting tools must also be set on center and square to the work. Maintaining center height is crucial when grooving (Figure 3).

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Figure 3

When using a high-speed steel, pre-formed parting tool, the back rake angle should be neutral (Figure 4).

This will ensure that the tool does not try to pull itself into the work causing the tool to over cut and snap off. Apply cutting oil to the point of the parting tool. This will aid the cutting process as well as keep the tool from binding in the groove. When using a pre-formed parting blade, never grind on the sides of the blade.

The RPM setting for grooving varies with the width of the parting tool, but a good rule of thumb is 1/3 to 1/2 the speed you would use for turning.

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Figure 4

 Procedure for cutting a groove:

  1. Layout or measure the groove location.
  2. Set the lathe to the proper R.P.M. setting.
  3. Mount the workpiece in the machine.
  4. Set the parting tool on center and square (remember: maintain maximum rigidity).
  5. Locate the tool to the work.
  6. Touch off on the diameter of the part.
  7. Set the cross-feed graduated collar to zero.
  8. Lock the carriage down using the carriage lock.
  9. Calculate how much the cross feed dial must travel to cut the groove to the proper depth.
  10. Slowly feed the tool bit into the work using the cross feed handle.
  11. Apply cutting oil.
  12. Before cutting to depth, stop and check the depth of the groove.
  13. If a wider groove is needed, move the tool over and repeat steps 6-12.

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