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Determining the Lead of a Large Gear with Precision

This article describes an alternative method to measure lead of a large helical gear. This article complements the
previous article whose objective was to accurately determine the differential constant of a gear hobber when the sample
gear was small enough to mount on a milling machine. This method also has the potential to make a more accurate/precise
measurement of gear lead when compared with the previous procedure since a larger and longer sample gear is used here.
This procedure uses a lathe with an accurate Digital Read Out (DRO) to measure carriage position. Also, the user will
find there are significant time saving advantages:
no computations are required to find lead; the lead is directly measured

no change-gears are required (very time consuming)

no interpolation (between values) is required

many ‘steps’ that were required before are unnecessary

large, even very large, gears can be accommodated
The basic concept is to measure the linear distance along the longitude axis of the gear for a precise
angle of rotation. The linear distance is measured with a DRO Scale on the longitude axis of the lathe. The
angular displacement for this example is 90-degrees and is measured with a frame level and precision angle-plate.
Simply the lead will be measured as 4 X the DRO Readout.
To implement the following procedure, you will need a setup like this.




1st Step:

Make a helical gear on your gear hobber. Face width should be about 1-inch wider than the portion of lead cut into
the gear. By way of an example, we used a 32 tooth, 30 degree helix angle, DP 8 gear. The face width of our gear
is 7 inches. To be exact, the lead of this gear is 25.132741 inches, so 90 degrees of rotation would move 6.283185 inches
along the center axis of the gear.
2nd Step:

Install a precision angle-plate on the chuck, as shown above and below.



3rd Step:

Load and align your gear, as shown above.
4th Step:

Mount a dial indicator with ball-tipped stylus on the cross slide, with contact approximately on
centerline of the lathe, as shown below.



Set cross slide dial to zero without backlash.

5th Step:

Slightly rotate the chuck until the frame level reads exactly vertical.


Align pivot pin of the stylus to be parallel to the gear tooth (measure deflection perpendicular to tooth face).
Contact tooth face about half-way between top and bottom of tooth space. This initial contact represents
"0" lead, so set DRO to zero (0.0000).


6th Step:

Now, very carefully move the stylus using lathe carriage and simultaneously rotate the chuck by hand.
You want to rotate the chuch approximately 90 degrees (or to an angle not exceeding the lead of your gear).
Keep the dial stylus from getting bumped or reaching its limit of motion.



7th Step:

Do this coordinated move until the angle-plate is exactly horizontal as determined by the frame level.
Without further rotation, move the carriage to fine tune the stylus to the same "0" dial deflection obtained
at the initial contact.


If tooth space is large and you can keep the stylus in the tooth space during this coordinated ‘move’, this
should be the quickest procedure. Also, if you stay in the groove you guarantee that you will measure lead over
the same tooth.
However, if you fear 'bumping' the indicator or if the space is small, then mark the same tooth end-to-end
with chalk (or other marker). Mark your cross slide dial as "0" and withdraw stylus using cross slide. Rotate
the chuck 90 deg (or to suitably accurate angle) using the frame square.
After chuck is rotated, move the carriage longitudinally to place the stylus over the same tooth's space.
Don't touch the chuck! Carefully settle stylus into the space by moving both carriage and cross slide. When the
cross slide is at "0", move only the carriage until the stylus contacts the same tooth face. Carefully move
carriage until the dial reads the same "0" as it initially did.
If for any reason you 'bump' the stylus, start over.


If the chuck rotation was 1/4 of rotation (i.e. 90 degrees), then LEAD = 4 x DRO reading.
In general, you will divide the DRO reading by the fraction of chuck rotation. In other words, if rotation
is 1/12 of a rotation (i.e. 30 degrees), LEAD = 12 x DRO reading.

If you need to calculate your hobbing machine's differential constant, refer to the Step 6 of the previous article
and use the gear lead you just measured.
Note:

For narrow face widths (less than the equivalent of 90-degrees of rotation), you can still use this
procedure. The chuck will need to rotate through a smaller angle to stay within the limited section of lead
on your gear. You still start with the angle plate in the verticle position zero and the dial on a tooth face
as previously described in step 5 above. To achieve a smaller angle of chuck rotation, use a precision machinist level and
a sine-plate to restore the horizontal level after rotating the chuck (see photos below).



Notes:

ACCURATE RESULTS DEPEND ON ACCURATE ALIGNMENT
It is beyond the scope of this article to discuss the many issues related to alignment; however, I will
address some of the more important alignment issues in a later article.
Here is another picture of a frame level:





Information and photos contained in this article have been generously provided by:

August Lehman,
Managing Director
Lehman Associates
Manilla, Phillipines

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