Split-nut animation.
Lead Screw
with a Split-nut & DC Motor Drive for the Taig Micro Lathe
Last updated on
Wednesday, November 21, 2012 05:42:48 AM
Eastern US Time Zone
Split-nut Animation,
Exploded View, Nut
Subassembly, Carriage Modification,
Split-Nut Mounted on Carriage,
Bearings & Lead Screw,
Lathe
Z1-axis Motor & Clutch
This lead screw modification has been removed for the lathe CNC conversion but the information has been retained for reference.
Split-nut Animation
Split-nut animation.
Exploded View
The aluminum base plate is 1.55" long x
1.15" wide x 0.1375" thick, the brass pieces are
1.50" long x 0.3150" wide x 0.3115" thick &
the steel rods are 0.1915" diameter.
The original rack & pinion carriage feed is
nice & fast but often you need the finer, smoother feed of a lead screw.
It is
desirable to retain both features. Using a split-nut mechanism is one method.
An exploded
view of my
split-nut subassembly is shown.
It is modeled after a full-sized lathe where
the split-nut opened & closed
(by complex means)
onto the lead screw
simultaneously from both sides. The two brass split-nut jaws are
opened &
closed using a single, 10-32 brass screw.
The left half of the screw has a right-hand
thread while
the right half has a left-hand thread.
Thus, when the screw is turned 3.5
times CW, both jaws close & vice versa.
Before drilling & tapping the brass faces
were covered with a layer of masking tape. The brass jaws were
then
clamped, drilled &
threaded for the lead screw using a left-handed ¼ -20 tap (Small Parts).
When the tape is
removed, there is a slight
gap left when clamping onto the lead screw.
This allows good
clamping &
compensation for wear over time. Be careful when
drilling
brass, it is very soft.
Sharp, standard drills can grab
the piece & pull it upward.
Clamp the drill vise or use drills with re-ground (flatter) angles.
The split-nut halves slide on two steel rods
that are held by the end brackets.
The end brackets have clearance
holes for the split-nut
adjustment screw &
are held to the plate by 4, 4-40 screws, which also capture the rod
ends.
This arrangement allows the split-nut to float perpendicularly
relative to the force
of the lead screw (toward &
away from the lathe bed)
to eliminate any potential
binding due to minor misalignment or thread rod bends.
The split-nut jaws must be able to
slide freely against the plate.
The two left corners were milled to allow the
subassembly
to be located
very close to the lathe bed. The notch towards the back allows clearance for
the
rack chip shield. When the nut screw is tightened halfway (CW),
the weak spring puts
mild tension onto the
lead screw (just the nut's left jaw)
which allows the threads to
engage. Rotate the lead back &
forth while
tightening to avoidt cross-threading.
Then, further tightening fully engages the lead screw & tension
can be
adjusted to the
desired level. Do not over-tighten. The two countersink holes
on the plate
are for mounting
to
the carriage & the large,
9/32" clearance hole is for the lead screw.
Nut Subassembly
The split-nut subassembly opened.
The split-nut subassembly closed.
The split-nut subassembly pushed to the side.
Split-nut back mounting plate.
The sides of the carriage are slightly slanted
so during production, the piece can be popped out of the mold.
The left side of the
carriage required milling so the mounted split-nut would be perpendicular to the lead
screw.
There is a small ledge that the split-nut subassembly mounting plate's top edge
rests against.
All milling &
drilling was performed using the lathe to assure
alignment,
especially of the two, lead screw holes (make sure
to clear the eccentric for
the pinion gear). I borrowed a Taig lathe
carriage because, of course, you need two
to do the operation. To perform the milling operation, I used a solid carbide, two-flute (for
soft materials, use a
4-flute for hard materials)
5/16" finishing end mill. I have also used a
fly
cutter to mill a carriage. Deburr
the
dovetail edge where it was milled.
Check the entire carriage dovetail for burrs
leftover from manufacturing.
Kerosene allows a smooth cut on aluminum.
This
mod was performed
before I had either the Taig
or the RF-25 mills.
Removed all calibrated dials, glass bead blasted
them (sandblasting too aggressive/coarse),
then using the
lowest lathe rpm, smoothed
the scale with 220, 320 then finally 400 grit sandpaper.
Do not be too aggressive
or you
can remove the markings.
This method only works for dials that have deep marks to begin
with.
Indicia
will be finer & the knob looks nice when the rough machine marks are
removed.
Careful, the dials are factory
Loctite on & screw off (not pull).
I put two
small flat sides on the non-threaded area of the lead screws
for a very
small, open-ended
wrench to facilitate disassembly/assembly.
Grease (not white lithium) makes these knobs
work very smoothly.
The two ends of the movable dial scale zero (the ¾ circumference
spring clip in the dial
bearing block grove)
may be rough; remove &
grind the ends
flat.
Replaced all of the OEM dial brass spinner pegs
with a pivoting-type (Wm. Berg) as per
MIL-STD-1472.
This one change alone greatly enhances the smoothness of operation &
is
the least expensive modification
for the largest increase in machine performance. Remember, that when
tapping
the steel knob, the tap hole
is (larger) for 50% threads not 75% like for softer
materials.
Always drill the exact recommended tap hole for
maximum thread strength.
Grease
on the stainless steel pivot screw makes it smooth. A small nylon washer,
just the size of
the pivot screw body, 3/16", (not the thread itself)
removes the last of the in-out
play of the
aluminum spinner knob. I also filed two small flats on the eccentric for the
pinion gear,
just behind the knob,
to accept a miniature 5/16" open-end wrench.
This
greatly facilitates rack & pinion engagement adjustment.
To
eliminate lead-screw flexing, a bronze
oil-impregnated bearing was press fitted &
then reamed to 0.25".
One corner of the set-screw nut was milled
so it would not interfere with the lead-screw.
The rack & pinion
eccentric
bearing setscrew area was milled to allow the locking nut to seat
evenly.
The
set-screw end was faced smooth to
eliminate the locking ridge which gouged the eccentric
housing. Also, note the milled flat area to the right on
the X-axis dial readout mounting
block. This is the area where the
Z1-axis dial readout plunger makes contact.
Split-Nut Mounted on Carriage
Split-nut subassembly mounted onto carriage.
Nut open.
Nut closed.
Detail of subassembly attachment. Used 2, 4-40
flat-head screws.
The closed jaws slide from side to side allowing attachment
to the carriage.
All screws had
Loctite applied.
Bottom view.
Right bearing block subassembly. Lead screw was
cut from 24" of 303 stainless steel ¼ -20, left-hand threaded
stock (Small Parts;
Y-TRLX-1420).
Right-hand threads work but right-handed operators
(may) find that there is a
natural tendency to turn the handle
CW, making the carriage move to the right instead of the
preferred
left.
Motion
from a
left-handed thread seems natural & easiest to coordinate
when
your left hand is working the cross slide
(also a left-hand
thread).
The ends were turned down to 3/16" while being held by
a collet.
Need a close fit but
allow easy, non-press fit,
assembly/disassembly with the ball bearings.
Each end has a
milled flat for setscrews.
From Wm. Berg; the two thrust
bearings that have red nylon ball retainers (B5-2-SS), with
supplied matching
washers, bear the lateral forces. Two, 0.3125"
(nominal) OD ball bearings (B1-40-S-Q3) hold the
shaft. Setscrew
collars (CS-29) keep things together.
The four-screw type
holds best. The hand crank (CN12-4) came with the
pivoting-type
spinner (CN8-1). The right bracket is held down by two, 10-32
cap-head screws.
The entire right
bearing block with the lead
screw can be quickly removed to
allow the carriage to slide off
for removal. These
use the most common-sized
hex wrench (5/32")
on my machine. I changed the tool bit holding screws to this
size, too.
Don't over tighten them. The brackets are milled (all
surfaces) from ¼" aluminum plate.
The base is
1¼" wide & 1"
deep. The top piece is 1¾" long (2" total bracket height).
The
corners were cut to 45º to reduce
bump hazards. They were assembled on a surface plate
using a machinists' square (2, 6-32
screws & Loctite).
They were glass bead blasted for the satin finish.
If I were to do this again, I would consider using an Acme
Thread.
Square threads would engage the split nut with less
chance of cross threading. The 9x20 lathe uses an Acme
Thread.
Before reaming the bearing holes, the brackets
were set at right-angles to the
lathe bed & then bolted to the
working surface. The
working surface (⅛" aluminum plate on ¾" plywood) was tapped for the
hold-down bolts.
Then, with a transfer punch placed in the
9/32" lead screw holes,
the carriage was moved to the far right & the
bracket marked.
The punch was reversed
& the carriage was then moved to the far left & the bracket marked.
This assured
perfect alignment of the lead screw (bearings) relative to the carriage. A hole, slightly
smaller than
the 0.3115" reamer was first drilled.
The reamer is then lubricated with
cutting fluid & at a low speed,
slowly fed
into the hole until its cutters pass all
the way through the plate.
Then stop & remove the reamer, do not pull it
back through
the hole while it is rotating.
The deburred holes were then lubricated with mutton tallow
&
then the
bearings press-fit into the brackets using an
arbor
press. They are flush to the outsides but that should not
make any difference.
The
thrust bearings & the ball bearings were lubricated with molybdenum disulfide.
Since
the pinion remains engaged to the rack during lead screw operation,
the pinion must be able
to turn smoothly &
without binding.
The pinion must be properly engaged to the rack
& it must not be pushed
in too far so as to
rub the lathe bed bracket.
I lubricated
the pinion in the eccentric bearing with molybdenum disulfide.
The
carriage gib should be
snug. I replaced the two adjustment screws with
5/8" long 10-32 cap heads, retaining
the locking nuts. This allows delicate finger adjustments of the gib.
Left bearing block has only one ball bearing
so the lead screw subassembly
can be easily pulled out towards the right. This bracket remains in place.
The carriage stop lock screw was moved to the
top by drilling through from the bottom & tapping (10-32).
Top & oblique views.
Entire lead screw.
Split-nut engaged. To not cross threads I move the lead screw handle back & forth while
engaging the split-nut.
An Acme square-shaped thread might have helped this potential problem.
Detail of split-nut, engaged. The spilt-nut
assembly is low enough so as to not interfere with the carriage
stop bar function & also clear the spindle housing allowing full carriage
travel.If I had it to do over, I would
have investigated using a
left-hand Acme (square-shaped) thread. Full-sized lathes use such a lead screw.
Finding the proper left-hand tap &
threaded rod might be difficult. A removable cover of some sort might
be nice to keep the chips out since you
have to be able to clean the split-nut area.
Detail of split-nut, disengaged.
Split-nut under spindle housing.
Reamed ¼" ID bronze bushing to eliminate
lead-screw flexing.
Lathe
Z1-axis Motor & Clutch
A
geared DC motor with a sliding
coupler-based clutch was added before the CNC
conversion.
The black rubber outer sleeve was slid right to
disengage (shown) & left to engage the
splines.
Kit-built
variable-speed
DC motor controller.
Split-nut Animation,
Exploded View, Nut
Subassembly, Carriage Modification,
Split-Nut Mounted on Carriage,
Bearings & Lead Screw,
Lathe
Z1-axis Motor & Clutch
