Lead Screw with a Split-Nut for the Taig Micro Lathe
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 left-hand thread while the right half has a right-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.
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. De-burr the dovetail edge where it was milled. Check the entire carriage dovetail for burrs leftover from manufacturing. A dab of 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. Go to OEM vise for a close-up photo
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 set screw 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 set screws. 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. Set screw 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. The square threads would engage the split nut with less chance of cross threading. The 9x20 lathe lead screw is 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 de-burred 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.
I
have since removed the Taig lathe lead screw &
mill table motors as I now only use the
lathe for very small parts & I
converted the
mill to CNC.
Archived Taig Mill X-Axis DC Motor modification.