Lead Screw with a Split-nut & DC Motor Drive for the Taig Micro Lathe
Last updated on Thursday, February 02, 2012 06:45:52 PM 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This is an archived page of earlier modifications that preceded the Taig Lathe CNC conversion.

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.

Carriage Modification

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.

Bearings & Lead Screw

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 machinist's 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 variable-speed motor with clutch was added for a period.
The same design used to couple & drive the lead screw (X-axis) of the Taig mill.

Clutch - exploded view.
This is a 12 Volt DC surplus motor (95 RPM max) with a geared transmission (on the motor's right side) which reduces RPM & increases torque but then can not be freely rotated.  A clutch is needed to allow quick engagement to & disengagement from the mill's lead screw. The depicted arrangement allows for both manual & automatic feeds. Most full-sized mills have this arrangement. The motor speed is varied by a simple DC motor controller that utilizes pulse-width modulation. The controller allows high torque at low RPMs. (Employing an in-series variable resistor to reduce speed causes too much torque loss resulting in stalls, especially when making relatively heavy cuts.) The direction of shaft rotation is changed by simply reversing the motor power ± polarity via a double-pole, double-throw switch. One of the spline from the rubber-coupled universal joint is attached (via its only setscrew) to the motor output shaft but is located only halfway onto the shaft leaving a hole in the spline. The left end of the brass drive shaft fits into this motor spline hole & is thus allowed to rotate freely while maintaining axial alignment. The other spline goes onto the brass shaft. Its (blue Loctite) setscrew rides against the shown milled flat but it is not completely tightened. This 'play' allows the spline to be manually slid left or right while remaining engaged to the shaft during rotation. If positioned very closely, the right spline can be completely locked to the brass shaft & only the rubber coupler slides to the middle to engage both spline. However, sliding then requires more force & the spline are not fully engaged (therefore a somewhat weaker connection). A small ring on the left spline can be installed to act as a coupler stop, so you can just slide it over until it touches. The right end of the brass shaft is attached to a small, zero-backlash universal joint which, in turn, attaches to the mill's x-axis lead screw. Setscrews engage milled flats to prevent slippage. Note these are scanned photographs.

Motor & clutch bracket. Motor mount (exploded view). Left plate holds motor; center plate is a bracket;
right plate attaches to the (left) end of the mill Y-translation table, opposite the hand crank.
 The channel milled in the right plate allows Allen wrench (bottom) access to the zero-backlash coupler setscrews.

Disengaged clutch. This is the assembled unit with the rubber coupler & spline slid over to the right.
The motor & lead screw are disengaged. They are allowed to move freely & independently
while the motor & brass shafts remain axially aligned. The mill's hand crank can be manually turned.
The two larger cap-head screws to the right hang the entire assembly on the mill's x-axis bed.
The bed's end had to be drilled & tapped (10-32) & the lead screw's end was turned down to
accept the zero-backlash coupler. A small coupler (from Berg), about the diameter of
the ½" lead screw, was required to clear the confines of the bed ways.

Engaged clutch

Side view showing bracket construction. Coupler & spline slide to the left to engage the motor spline for
transfer of power. The rubber coupler is the only means by which  power can be transferred across the two shafts.
To allow easier engagement/disengagement, the squared spline corners were first cut on a lathe to a 45º bevel.
The spline was then mounted onto a rotary table set at a 45º angle & then each rib end was milled on each side.
The resulting 'chiseled' spline ends allow faster, smoother engagements into the rubber coupler.
 The same design was used to couple & drive the x-axis of the micro-lathe. 

Universal joint - exploded view (left to right) milled spline, original spline & rubber coupler

DC motor controller kit documentation.

Note the white universal joint at shaft/controller union.
Brass speed control knob shaft turns in a ball bearing.

Dedicated controls; speed (left) & x-axis direction (right).
Center is OFF & left/right positions moves bed left/right.

Black plastic motor housing. Strain relieved, plastic clad power cable.

Rotary table used to cut chisel points on spline ends.

Fly cutting using motor feed.

Split-nut Animation, Exploded View, Nut Subassembly, Carriage Modification,
Split-Nut Mounted on Carriage, Bearings & Lead Screw, Lathe Z1-axis Motor & Clutch