A permanently mounted thread cutting gearbox for the lathe

A permanently mounted thread cutting gearbox for the lathe
Tip 38 — A Permanently Mounted Thread-Cutting Gearbox/Jim Knighton
The screw-threading gearbox requires parts from
the standard Sherline P/N 3100 threading
attachment. The gear selection is for the most part
as per the threading chart that accompanies it. The
design of the gearbox places all eleven of the 24
pitch gears from the Sherline kit on the main shaft.
This makes it impossible to use those gears
elsewhere in the gear train, so a few “extras” are
necessary. As built, this gearbox requires one each
extra 20, 22, and 40 tooth gears and two 35 tooth
gears. Their usage and placement is described in
the following discussion.
The gearbox was built in two versions, the first
being a prototype and the second as shown in
these photos. One of the significant lessons
learned from the prototype is that aluminum is not
a good choice for the large parts. There is too
much flex and twist, enough so as to potentially
prevent the gearbox from operating appropriately.
In the final version all of the large parts are
machined from mild steel, either 1018 CRS or
The prototype also made use of bronze bushings
instead of the ball bearings used in the final
version. Bronze bushings did not provide the solid
support needed, especially in the idler arm
assembly. I strongly urge caution if prospective
users are inclined to go this route. The
combination of bronze bushings and aluminum
construction proved to be fatal, rendering the
prototype unfit for serious use. It did, however,
demonstrate the validity of the design and the
feasibility of going forward with this project. The
results are shown hereafter.
Gearbox 01 – Final Installation
This photo illustrates the final gearbox installation
with all attendant modifications. It is clearly
evident that the main motor and controller are not
in their Sherline standard locations. This is the
result of previous modifications described on the
Sherline web site, Item #20 in the Sherline
Workshop page. I didn’t have a digital camera at
the time that document was put together and I
apologize in advance for the poor photography.
This photo shows a slightly different setup than
illustrated in that document in that the pillow
block style motor mount shown there and also
visible in some of these photos has been replaced
with the one shown here. This was done because
the pillow block motor mount blocked the
movement of the gearbox’s idler arm making
adjustment difficult in certain situations.
Please note that the relocation of the motor and
controller are not essential to the construction or
operation of the gearbox. However, these
modifications open up considerable space around
the headstock making it much easier to see what is
going on and to make idler arm adjustments.
The motor mount is constructed of 3/8" x 3.5" x
7.625" mild steel (1018 CRS). Unlike the earlier
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pillow block mount with its 3" diameter hole, the
largest hole in this steel plate is 3/8" to provide
clearance for the main motor’s spindle shaft. It can
be machined in its entirety on a Sherline mill. Not
clearly visible in this photo is the 4" wide section
of ¼" aluminum angle on the backside. This has a
couple of slots that permit the motor mount to be
shifted fore and aft for adjusting belt tension. The
belt is a Singer sewing machine part.
Also, as noted in the aforementioned document
describing earlier modifications, the auxiliary
power supply (24vdc) is mounted underneath the
prominent aluminum channel I use as a base and
the controls are located on the left end. In addition
to the document noted above, better and clearer
photos of the power supply and controls are
located in the Yahoo Sherline User’s forum in the
files section in a folder bearing my name.
The following sequence of photos presents the
final gearbox assembly along with construction
notes as appropriate.
Gearbox 02 - Main Shaft
My drive motor has a .312" spindle and the main
shaft is .375" diameter. The left end of the main
shaft was drilled and reamed to .312" and the
small hole shown in the photo is for a pin that
locks this shaft and the motor’s spindle together.
There is a notable idiosyncrasy regarding this
shaft and the operation of the feed engagement
lever that I discovered when testing the prototype
gearbox. The lever can be rotated too far to the left
(off position) pushing the shaft approx .050" .070" too far out of the support tube. The standard
Sherline part, being short and light, has a
propensity to move back into it’s proper position.
The longer and heavier replacement has to be
“persuaded” to return to that position. If it is not,
the engagement lever will jam the next time the
operator attempts to use it. The solution to this
problem is to make sure that the feed motor is
positioned so that this leftward movement is
blocked, thus preventing the over-rotation. This is
an important consideration and failure to do this
can have serious and unpleasant consequences, as
I discovered to my chagrin with the prototype
Gearbox 03 – Pulley Gear
The main shaft for the gearbox is shown in this
photo. It is a functional replacement for the
Sherline fixed shaft, PN 15430, included with the
Sherline threading kit (shown in the foreground).
It is machined on the right end to exactly duplicate
the flat and groove of the Sherline part. The photo
also shows the narrow groove for a retaining ring
(snap ring), a shallow (.050”) channel for a 1/8”
square key, and at the left end the details for
affixing the shaft to the drive motor.
As seen in the photo, the 100 tooth 56 pitch “A”
gear is permanently attached to the pulley. Not
visible is a 1/16" thick, wide nylon washer that
acts as a spacer. This is the only non-reversible
modification to the lathe itself and is in fact not an
essential feature. Rather than the permanent
mounting shown here, the gear can be driven by
the small screw Sherline supplies for that purpose
in the threading kit. If this approach is used, a
small retaining collar with a setscrew should be
added to lock the gear into position.
Gearbox 04 – Gear Mounting Plate
The large cutout at the upper left is an adaptation
that allows easy access to the drive belt. When the
mounting plate is rotated to the right, the cutout
allows me to remove and replace the drive belt
without difficulty. Prospective builders with the
drive motor in the “normal” position will have to
work out their own accommodations to resolve
this issue.
The hole located close to the right edge of the
plate is for the cross slide travel stop and not
essential to the operation of the gearbox. More on
this later.
Gearbox 05 – Keyway, etc.
The gear mounting plate is machined from mild
steel and then blued. The bluing provides a nice
color match with the rest of the lathe but it is in
fact to eliminate the problem of surface rust that
will form over time if the steel is left unfinished.
Not visible in the photo is a clamping block on the
backside. The clamping block is machined from
aluminum and uses a SHCS to close the fixture
and securely lock the plate into position on the
“leadscrew support shaft” visible in the previous
In this photo the gears are being stacked on the
main shaft. Visible are the gears themselves, the
nylon washers used as spacers (notched to fit
around the 1/8" square key mounted in it’s mating
slot). Not visible is the retaining ring (snap ring)
that limits rightward movement of the gear cone.
The gear mounting plate has three ¼ x 20 tpi
threaded holes – the two on the left provide
alternate mounting positions for the idler arm, and
the one close to the small “C” gear is for mounting
an extra idler gear for cutting left-handed threads.
More on this later. The camera angle distorts the
apparent location of this threaded hole.
The 100 tooth 56 pitch “B” gear isn’t visible in
this photo, but it is mounted on the backside of the
plate on the same shaft as is the 20 tooth “C” gear.
This shaft is supported by and rides in two 3/8" ID
ball bearings pressed into a appropriately sized
hole in the mounting plate itself.
Gearbox 06 – Main Shaft Gear Cone
This photo shows the fully assembled gear cone as
well as the retainer collar. The retaining collar is
drilled to match the pin holes in the shaft itself and
also in the feed motor spindle. Nylon washers (not
slotted) are used here as spacers.
Gearbox 07 – Feed Motor
The feed motor shown in the photo is a Japan
Servo gear motor rated at 35.8 RPM at 24vdc. It is
reversible and as installed is driven by a simple
power supply with three speed settings, 24vdc,
16vdc and 8vdc. The details of the power supply
are in the aforementioned document on the
Sherline Workshop page, item #20.
Prospective builders will have to build a mount
and power supply to match the motor they select.
Gearbox 08 – Idler Arm, Shaft, and Gears
In this photo the idler arm is shown mounted in
the upper threaded hole and positioned for cutting
40 tpi threads, one of the two extreme positions.
The arm is machined from ½" x 1" mild steel and
as built is 3.75" long. The shaft is a rotating part
supported by and riding in a row of three 3/8" ID
ball bearings. Nylon washers are used on both
sides as spacers and retaining rings (snap rings)
are used on both sides to prevent possible
unwanted movement.
The two gears on this shaft are both 35-tooth
Sherline accessories not included with the
threading kit. The gear on the rightmost end of the
shaft is fixed in position and meshes with the “C”
gear on the mounting plate. The gear on the left
side of the arm is mounted to a sliding collar that
it allows it to be aligned with any of the 11 gears
on the main shaft. The channel visible in the photo
is for a matching setscrew in the sliding collar to
lock the movable gear into position. The setscrew
is actually a 4-40 SHCS in a counterbored recess,
not the tiny, headless creature normally suggested
by this expression.
Conceptually, the idler arm, shaft, and gears are
the logical equivalent of the “E” gear shown in the
Sherline threading chart. This is an idler gear that
transmits power and direction of rotation. It’s size
is not important and has no effect on the ratio
between the driving and driven gears. Since the
gears on both ends are the same size, think of the
shaft assembly as a single, variable length gear –
that is exactly what it is and how it operates in the
gear train.
In use, the fixed gear has to mesh with the “C”
gear on the mounting plate. The moveable gear
similarly has to mesh with the chosen gear on the
main shaft. The slot in the idler arm allows this to
occur and the operator has to position the arm so
that these two conditions are met. It takes longer
to explain than to actually do it as this is not a
difficult task. The operative word here is “mesh” –
the idler gears should not “bottom” on either of
the mating gears. In this regard, operation is
exactly as prescribed in the Sherline instructions.
Similarly, the 56 pitch “A” and “B” gears need to
mesh, not collide, at the upper end of the gear
Gearbox 09 – Whole Mechanism
When configured in this manner the feed motor is
used as a leadscrew power feed in the same
manner and using the same controls as I’ve been
using it for several years. The long rod with the
brass bumper on the right end is a travel stop that
limits cross slide movement when performing
operations close to the headstock. I have another,
very effective, travel stop that can be located at
any point on the ways, but it’s size is such that
close to the headstock and underneath the chuck it
isn’t effective – it’s too big for these close
quarters. Details can be seen in my folder in the
Yahoo Sherline User forum, in the “files” section.
This photo shows the whole gear train as
configured to cut 40 tpi right handed threads and
all of the gears are visible fully engaged in their
respective locations.
Gearbox 10 – Travel Stop Mounted
This stop is a very simple adaptation of the
mounting plate taking advantage of it’s width to
equip the lathe with this secondary travel stop for
circumstances when the other one’s size precludes
it’s use. The stop rod is locked into position by a
brass tipped SHCS. As an aside, I frequently use
brass tipped screws in my projects to prevent
marring mating surfaces.
Gearbox 11 – Threads cut with gearbox
In my shop, and presumably others as well,
threading is important but not the most frequently
used operation performed on the lathe. Most of the
time, therefore, the setup shown in this photo is
the “normal” mode of operation. The gear train is
fully in place, but disengaged. An oversight when
taking this photo is that the drive belt is not
installed. Sorry ‘bout that!
The gear plate has been rotated towards the front
of the lathe disengaging the “A” and “B” gears.
Similarly, the idler arm has been moved into a
neutral position disengaging those gears as well.
The Gearbox works very well, as this photo
illustrates. These are threads cut using the gearbox
and the threading setup shown in the photo. These
are 3/8" x 24 tpi threads cut in mild steel, and I
believe the photo illustrates the quality of the
finished product.
The threading cutter is the Sherline/Valenite
carbide insert threading tool. The chuck and live
center are non-Sherline modifications/upgrades
beyond the scope of this discussion.
Gearbox 12 – All Gearbox Parts
extra idler necessary for cutting coarse left-handed
threads although admittedly I didn’t pursue this issue
too hard. Please note that the instructions for the
Sherline threading kit clearly state that the lathe is not
designed for cutting right-handed threads coarser than
about 16 tpi or left-handed threads coarser than 20 tpi.
In making my personal choices about the design of
this gearbox I decided to accept these limitations at
face value, although as built it will cut right handed
threads as coarse as 10 tpi. Indeed, these limitations
are of no operational consequence in my shop.
This photo illustrates the gearbox fully engaged and
set up to cut the 24 tpi threads shown in the above
photo. In the foreground are two additional parts.
The “C” gear mounted on the gear plate has 20 teeth
and is used when cutting “fine” threads (even
numbers, 20 to 40 tpi as per the Sherline threading
chart). To cut “coarse” threads (10 to 20 tpi also as
per the Sherline threading chart) this gear is replaced
with the 40 tooth gear shown leaning against the
mounting plate.
The “C” gear is held in position with a retaining ring
(snap ring) which is easily removed and replaced
making this an easy swap. Please note that when
using the 40 tooth “C” gear and also when cutting
left-handed threads the idler arm needs to be mounted
in the alternative lower threaded hole in the gear plate.
In all other respects operation is as described above.
The gearbox can be used to cut left-handed “fine”
threads. This is accomplished by leaving the small
“C” gear as shown in this photo and adding the small
gear/carrier also shown in the foreground. This gear is
an idler that reverses the spindle rotation relative to
that of the leadscrew so that they rotate appropriately
for left-handed screws. When cutting these threads,
the cross slide table is fed left to right when cutting
and right to left for the return. This is the reverse of
the normal operation and an operational adaptation
necessary to keep the spindle rotating in the
appropriate direction relative to the threading cutter.
As I built it, I did not allow for cutting left handed
“coarse” threads. I did not find an appropriate location
on the mounting plate that would allow me to use the
Other possible adaptations are indeed possible. The
conceptual design of this gearbox is suggested by the
Sherline instructions and threading charts. Close
examination of the “inch” threading chart
demonstrates that if one eliminates very coarse and
very fine threads from consideration, i.e., all those
that require the use of a 50-tooth gear in the “A” or
“B” positions, all remaining threads can be cut with
either the 20- or 40-tooth gear in the “C” position, and
each of the 24 pitch gears in succession in the “D”
position. Conceptually, this is exactly what this
gearbox does, only it mounts all of them at the same
time selectable using the movable gear on the idler
Examination of the “metric” threading chart
demonstrates a similar situation, the main difference
being that the major variable is the gear mounted in
the “C” position. There are only two significant
options on the leadscrew shaft, once again the 20- or
40-tooth gears used to select “fine” and “coarse”
ranges. Consequently, the principles demonstrated in
my gearbox can be adapted to allow construction of a
metric version. If this was done, the cone cluster
would have to be in the “C” location probably
necessitating a heavier mounting plate or other similar
adaptation to provide adequate support for this shaft.
The idler arm assembly, then, would select the
appropriate “C” gear and mesh it with one of the two
possible gears on the leadscrew shaft. Admittedly, I
haven’t worked out the practical engineering
necessary to build this configuration, but this is
virtually the same starting point I used in designing
the gearbox shown herein.
— Jim Knighton
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