9x20 Lathe Single-point Threading
Last updated on Wednesday, April 05, 2017 02:17:26 PM Eastern US Time Zone


3-Wire Method, 60 deg Center Gage, Thread Cutting Reference, Retractable Tool Bit Holder,
Lathe Gears, Gear Ratios, Thread Indicator, Thread Micrometer, Thread Gages, CNC Threading

Threads: parts & basic thread shapes, reading thread designations, 3-wire method of measuring pitch diameters.
Also refer to the Hand-tapping Section.

Metric & Inch Equivalents Inch to Millimeter Conversion Table
Millimeter to Inch Conversion Table Decimal Equivalents of 8ths, 16ths, 32nds, 64ths
Decimal Equivalents of Letter Size Drills Decimal Equivalents of Number Size Drills
Double Depth of Screw Threads American National /Unified Coarse/Fine Thread Dimensions & Tap Drill Sizes
American Standard Acme Screw Thread Dimensions ISO External Threads Medium Fit

3-Wire Method

3-wire set (chart) & plastic color-code wire (micrometer anvil ) holders used to measure thread size.
The Machinist Calculator has built-in 3-wire measurement functions.

Click on linked thumbnails




Click on linked thumbnails

60 deg Center Gage

60 deg satin chrome-plated tempered steel Starrett No. C391 center (or fishtail) gage.
The different scales measure threads/inch. The double depth of American National threads are
shown for different screw sizes but threads are best cut using the compound at left 29 degrees.

This Starrett attachment is useful for holding the center gage firmly against the lathe arbor or
faceplate when setting either internal & external screw cutting tools. A slot containing a flat spring
holds the gage. The V-groove in the other side permits locating the attachment against a round part.

Using the
center gage to setup the 60 deg tool bit cutting angle.

When cutting U.S. Standard Threads, set the compound at exactly 29 deg, adjust the chasing tool
on center & to conform to position with the 60 deg center gage. Move the tool in to just touch
the work. Re-check setup. Feed the tool in by moving only the compound.
By using the compound, there is less stress on the tool bit & the cut is cleaner.


DEPTH @ 29 deg


DEPTH @ 29 deg

2 .3713 18 .0412
4 .1856 20 .0371
5 .1485 24 .0310
6 .1237 27 .0275
7 .1060 28 .0265
8 .0928 30 .0247
9 .0825 32 .0232
10 .0742 36 .0206
11 .0675 40 .0186
12 .0620 48 .0155
13 .0570 50 .0148
14 .0530 64 .0116
16 .0464 72 .0103

Using an angle jig to set & then (four-bolt) lock the lathe compound at 30 degrees.

Retractable Tool Bit Holder

Retractable tool bit holder.

Lathe Gears

Gear Ratios

Gear ratios for various feed rates. See Jet feed gears.

Other 9x20 lathes have additional gear sizes that are interchangeable with the Jet BD-920N.
Gear ratios differ among the machines (e.g., Jet vs. Enco).
I bought additional gear sizes (e.g., 120t), 127t) to have more ratios.

Thread Indicator

A manual lathe has a thread indicator that shows the operator when to close the half-nut. For CNC threading,
Mach3 makes use of the pulse from the spindle index sensor to know when to start cutting. By timing
the index pulses, Mach3 calculates the feed per revolution that corresponds to the pitch of the thread.
As an  example, for one revolution of the lathe spindle, a 1/4-20 bolt has a thread every 0.050" (1/20).

Thread Micrometer

Screw thread micrometer with five different anvil pairs to measure both unified &
metric external pitch diameter. 0" to 1" measurement range with a 0.0001" scale.
A much easier pitch measurement method than the 3-wire method.









64 - 48

44 - 28

24 - 14

13 - 9

8 - 5


0.4 - 0.5

0.6 - 0.9

1 - 1.75

2 - 3

3.5 - 5

Anvil pairs are clearly marked for pitch range.

Example measurement of a 1/4-20 screw. The micrometer reading is 0.2160". Table pitch value is 0.2164".
A fast & accurate thread measurement tool. The bolt can freely rotate around the micrometer's anvil axis.

Thread Gages

Standard Unified & metric thread gages.

CNC Threading

Two movies showing a Taig CNC lathe threading a 3/8-16 aluminum bolts.

3-Wire Method, 60 deg Center Gage, Thread Cutting Reference, Retractable Tool Bit Holder,
Lathe Gears, Gear Ratios, Thread Indicator, Thread Micrometer, Thread Gages, CNC Threading