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Watch the video: Screw Cutting on a Lathe - An Intermediate Method by handmadeextreme
Cutting a clean, functional thread on a manual lathe is one of those rites of passage in the home workshop. In this project, the goal is a motorcycle axle thread—and the method demonstrated is a clear, intermediate approach that’s both fast and forgiving.
You’ll see how to set slow spindle speeds, use the threading gearbox and thread dial, make a scratch pass, and confidently engage/disengage the half nuts at the right moment. You’ll also see why this method shines when you need to correct an undercut without losing alignment.
What you’ll learn
- A practical setup for metric threads using a 60° tool and the lathe’s gearbox
- How the thread dial indicator and half-nut timing keep you on the same helix
- Why slow RPM (35–180) buys you reaction time near shoulders
- Scratch-pass checks, pitch verification with a gauge, and safe returns between passes
- Mid-flight fixes (like deepening the undercut) without losing registry
Understanding Lathe Screw Cutting Basics
What is Screw Cutting? Screw cutting on a manual lathe is simply synchronizing tool movement with the rotating spindle so the tool traces a helical path with a defined pitch. In this video, the part is a shaft for a motorcycle axle and the thread is a metric 60° profile. The stock appears to be steel, though the exact material isn’t specified in the video.
Beginner vs. Intermediate vs. Advanced Methods
- Beginner: keep the half nuts engaged, reverse or drive back and forth. It’s slow and can be less flexible.
- Intermediate (shown here): run slow RPM, engage and disengage the half nuts at specific thread-dial numbers, and wind back with the tool retracted. It’s faster than the beginner approach and more forgiving than advanced methods.
- Advanced: set the compound at an angle (often near half the thread angle) to reduce tool load. The host notes it’s great for bigger tools/threads to lighten the cut, though the video intentionally demonstrates the safer cross-slide approach for hobby-scale work.
Preparing Your Lathe and Workpiece The shaft is turned to final dimension and a small undercut (gutter) is machined where the tool will run out. That undercut is your safe zone for disengaging the half nut at the end of each pass. The threading tool is a 60° tip suitable for metric threads, and its orientation defines the thread’s flanks.
Pro tip If you’re already comfortable around other precision tools—say you’ve set up an embroidery frame or similar alignment-sensitive fixture—carry that same “datum first” mindset to your lathe threading. It pays off when you start chasing tenths on the cross slide.
Setting Up Your Lathe for Precision Threading
Spindle Speed and Gearbox Configuration The spindle is set slow—“nice and slowly,” between roughly 35–180 RPM on this machine—to give you time to disengage the feed at the undercut. The threading gearbox is then set per the pitch lookup table; for M14×1.5 (1.5 mm pitch), the selector is moved to a setting labeled “BS6W” in the video. This ensures the leadscrew’s movement matches your intended pitch.
Watch out Running too fast at a shoulder leaves you little time to pull the half nuts. Slow speed is a built-in safety margin when you’re honing your timing.
Quick check
- Spindle speed: in the slow range (35–180 RPM)
- Gearbox: matched to the pitch lookup (shown as BS6W for 1.5 mm)
- Thread direction: right-hand thread toward the chuck as demonstrated
The Role of the Thread Dial Indicator The thread dial indicator gets a 16-tooth gear for 1.5 mm pitch in this setup. The dial shows you when to engage the half nuts so the saddle picks up the same helix each pass. On the machine in the video, the operator can engage on any of the numbers 1–8 and still land on the same thread.
From the comments Viewers often ask if you must use the same dial number every pass. On the machine shown, any number from 1–8 is acceptable for 1.5 mm pitch. The creator confirms this in the discussion.
Mastering Half-Nut Engagement Practice the lever action before your first pass. With the half nuts disengaged, the saddle moves freely. Engage them, and the saddle locks to the leadscrew and is dragged along at the pitch you set. The key is to disengage in the undercut and retract the tool in X before you wind back.
Pro tip Keep your hand poised on the half-nut lever as you approach the undercut. Building that muscle memory keeps you out of trouble at shoulders.
The Step-by-Step Thread Cutting Process
Marking and Datum Setting For visibility, the host colors the thread area with an engineer’s marker. Then, the tool tip lightly touches the surface and is jogged in Z to scratch a fine, visible line in the inked area—this establishes the X-axis datum. With the tool exactly kissing the surface, the X dial is zeroed and locked.
That tiny scratch pass on ink makes the helix pop visually. It’s the same idea as scribing on layout dye before you cut.
Quick check
- Even, clear ink application
- Tool only touches enough to mark, not gouge
- Cross-slide dial zeroed at the surface and locked
The Crucial Scratch Pass Now it’s time for a zero-depth scratch pass. Start the lathe with half nuts off. Watch the thread dial. When your chosen number aligns, engage the half nuts and let the saddle carry the tool to the undercut. Disengage in the undercut. This one pass creates a light helical groove you can verify with a thread gauge.
Safety reminder Never use a thread gauge while the machine is running. Stop the spindle, then check.
Executing Multiple Passes for Depth Retract the tool in the X-axis to clear the work, keep the half nuts disengaged, and wind the carriage back to the start. The spindle can remain running while you reposition—just don’t measure or gauge while it’s spinning. Bring the cross slide back to zero, set a small depth of cut (the video shows 0.25 mm on diameter per pass), then repeat: wait for the number, engage, cut, disengage at the undercut, retract, wind back, and go again.
Watch out Do not simply reverse the spindle while the tool is engaged after a pass. The creator warns that backlash will cause the tool to track a different helix on the return and ruin the thread.
From the comments Some operators prefer reversing instead of disengaging half nuts. The video’s method avoids backlash issues and, for this machine and pitch, allows fast, consistent pickups using the dial. If your machine and thread system differ (e.g., mixing metric on imperial), follow your manufacturer’s guidance.
Troubleshooting and Corrections on the Fly
Verifying Thread Pitch with a Gauge After the scratch pass, confirm the pitch with a thread gauge (1.5 mm here). This early check prevents wasting time cutting to depth on the wrong pitch.
Addressing Undercut Issues Safely In the video, a test nut shows the thread isn’t yet deep enough, but the undercut is too shallow—the threading tool would start nibbling into the root of the gutter. Here’s where the intermediate method shines: disengage the half nuts, remove the threading tool, and install a standard cutting tool to deepen the undercut.
With the undercut fixed, put the threading tool back in. Before taking metal, do a zero-depth pass to confirm the tool picks up the existing thread perfectly. This check avoids cross-threading after a tool swap.
From the comments A number of viewers discuss alternative methods—e.g., threading away from the chuck, or keeping the half nuts engaged and reversing. Those can work on some machines and geometries, but the demonstrated approach keeps the spindle running, preserves alignment via the dial, and gives flexibility to correct the undercut as needed without drama.
Resuming Threading without Losing Alignment Once the zero-depth pass tracks the existing grooves, continue with light cuts until the nut fits properly. The host then takes a couple of finishing passes with minimal depth to smooth things up.
Pro tip If you’re coming to machining from other precision hobbies where fixtures hold work securely—think of how magnetic hoops help align fabric in embroidery—you’ll appreciate how a good undercut functions as a repeatable, safe “run-out zone” for your threading tool.
Finishing Your Perfect Thread
Light Finishing Passes and Deburring The video shows a couple of gentle finishing passes, then “knocking the tops off” and using emery paper to improve the surface. Finally, a quick brush cleans out residual swarf from the roots so the nut can seat cleanly.
Watch out Use abrasives with care around rotating work. Keep fingers, clothing, and tools under control.
Cleaning for Optimal Fit A small wire brush clears out any remaining chips in the thread grooves before the final test fit. The result: a smooth-running nut on a clean thread suitable for its job on the motorcycle axle.
From the comments Some viewers suggest using cutting oil or higher RPM for a finer finish; others prefer advanced compound-fed methods. The video intentionally demonstrates a conservative, intermediate approach at slow speed for timing practice and safety.
Why the Intermediate Method is Ideal for Hobbyists
Balancing Speed and Safety Compared to the beginner approach (constant reverse/forward), this method is faster. Compared to compound-fed advanced threading, it’s a bit less efficient in tool load but reduces the chance of dialing errors and gives you time to disengage at the shoulder.
Versatility in Error Correction A standout advantage is the ability to disengage, correct the undercut with a different tool, and then confidently pick the thread back up using the dial. That flexibility saved the day in the video when the gutter needed deepening mid-process.
Achieving Quality Results in Your Home Workshop The finished thread runs cleanly with a standard nut, and the process scales well for small, hobby parts. If you’re just getting comfortable with lathe threading, this intermediate method is a solid, repeatable foundation.
From the comments If you’ve ever learned a new alignment-dependent skill—say setting up magnetic embroidery frames or squaring up embroidery machine hoops—you’ll recognize the value of consistent datums and repeatable engagement points. The thread dial’s numbered engagements play a similar role here.
Safety Essentials Recap
- Run at slow RPM for threading (the video references 35–180 RPM)
- Never check pitch with a thread gauge while the spindle is running
- Don’t reverse with the tool engaged after a pass—backlash will put you on a different helix
- Disengage the half nuts in the undercut and retract in X before winding back
Tooling and Settings from the Video
- Thread form: metric, 60° tool
- Pitch: 1.5 mm (M14×1.5)
- Thread dial gear: 16 teeth; engage on any number 1–8 for this pitch on the demo machine
- Gearbox setting: labeled BS6W for 1.5 mm pitch
- Depth of cut: initial scratch at zero; subsequent passes around 0.25 mm on diameter
From the comments: FAQ quick hits
- “Do I have to use the same number on the dial?” On the showcased machine/pitch, any number 1–8 works. Try it on scrap to confirm your lathe’s charted behavior.
- “Can I reverse instead of disengaging?” The creator advises against reversing with the tool engaged due to backlash. The demonstrated method avoids that pitfall.
- “Why not angle the compound?” The host calls that the more advanced approach; it can reduce tool load, but this video focuses on a safer, intermediate cross-slide method for hobby-scale work.
Wrap-up You’ve seen the whole chain: cautious RPM, clear pitch setup, a visible scratch pass, precise half-nut timing, on-the-fly undercut correction, and tidy finishing for a solid thread. Practice on scrap, learn your dial timing, and the “Go!” moment will soon feel second nature.
From the comments If you’re new to the broader making space and stumbled in from a different craft: your eye for alignment and fixturing will transfer. The way some makers rely on magnetic embroidery hoop alignment or a rigid embroidery machine for beginners setup is the same mindset you’ll bring to steady half-nut timing and repeatable datums.
Resource note The exact lathe model isn’t specified in the video. Always follow your machine’s chart and manual for pitch tables, dial gears, and engagement rules.
Bonus parallels for makers
- If you’ve ever used a magnetic embroidery hoop or mighty hoop to hold fabric square, you already appreciate fixturing that resists drift. On the lathe, the undercut acts like a “safety zone” fixture at the end of your run.
- Managing pitch alignment on threads is conceptually similar to keeping multi-pass alignment accurate in fabric work—think of how magnetic embroidery hoops or magnetic hoops maintain repeatability across multiple stitches.
Final test fit and next steps The nut glides on smoothly in the final check, confirming depth and finish. The part is ready for its remaining operations (the video mentions parting off and a drilled hole on the opposite end, which are not shown). Practice this intermediate sequence on a scrap shaft to build muscle memory before you tackle your next real component.