Introduction

A good blueprint will specify a part's finished length, within a given tolerance. This may be called out as an overall dimension or as the sum of multiple feature sizes. Regardless of how it's shown, the machinist is responsible for producing a part to that final length, within the given tolerance.

Most other part features are located by a dimension referencing one of the end faces. However, there are times where the design intent requires some features to be located in reference to other features.

Beyond that, in the process of machining a complex part, it is useful to hold tighter tolerances than required, so that the finished can be used as references in the creation of other features.

Challenge

  • Most workpieces start with saw cut ends, which make useful measurements impossible.
  • Measuring the overall length of a workpiece while it is still in the chuck is only rarely possible.

Strategy

  1. Face both ends to create clean, flat surfaces. Leave the workpiece slightly longer than the final dimension.
  2. The faced ends now provide reliable reference surfaces for accurate measurement with a height-gauge.
  3. Finally, knowing the difference between the current and desired lengths, the extra material can then be removed to bring the part to a precise final length.

Learning Goals

Understand

Perform

  • Facing a part on the lathe.
  • Measuring part length with a height gauge.
  • Facing a part to a specified length, ±0.005″

Index

  1. Demonstration
  2. Procedure
    1. Face both ends
    2. Measure the length
    3. Finish to length
  3. Sequencing Operations

Demonstration

Procedure

Face both ends

Setup steps

covered elsewhere, complete before continuing

  1. Start the spindle and touch the tool off on the face.
  2. Zero the carriage micrometer collar.
  3. Set a depth of cut by moving the carriage over by .02″.
  4. Feed to center, either by hand or with power feed.

    Adjust the tool height and face again, if needed.

  5. Remove the workpiece from the chuck and flip it end for end.
  6. Repeat the above steps to face the other end.

    Be sure to leave the workpiece slightly longer than the final length, so that it can be finished later.

Measure the length

HOW TO: Use a Height Gauge

  1. To remove the nubbin at the center of the face from the measured height, place the workpiece on a 1-2-3 block or over a hole in the granite surface plate.

    It's not technically correct to ignore the nubbin... it should be included in the measurement, per the blueprint. However, removing the nub is a post processing step, similar to deburring. For the purpose of facing to length, we want to ignore the nubbin and measure the distance face-to-face.

  2. Zero the height gage at the bottom of the workpiece: on the 1-2-3 block, or the granite.
  3. Measure the height of the workpiece.
  4. Subtract the desired length (finished overall length) from the current length: this is the total amount that still needs to be removed.

Finish to length

  1. Return the workpiece to the lathe and touch off as lightly as possible and zero the carriage micrometer collar.

    The goal is to remove as little material as possible while in the touch off, to locate without affecting the current length.

    The method that works best for me is to touch off normally, then back off enough for the tool to come out of contact. Then, so that the second touch off doesn't land in the groove left by the first touch off, I move the carriage out in diameter some. Finally, I use the backlash in the handwheel to bump the carriage back toward the work until the tool just barely touches the face.

  2. Set the depth of cut for the remaining stock and face off to the finished length.

    If there is more material than should be taken in one pass, take multiple passes!

Sequencing Operations

In making any part, it is typical to complete as many operations on the first end as possible before flipping it over to work on the second end. This includes when facing to length: the first end is faced, and then any OD turning, hole making, grooving, or whatever else that can be done in the first hold, should be done before the part is flipped.

This approach minimizes the accumulation of error, as all operations are referenced from the same setup.