March 6, 2022

How to cut threads on a lathe

How do you cut threads on a lathe?       

  • Start by the workpiece as you machine it to the diameter of the thread cut using the turning center.
  • Place the workpiece in a chuck for threading between centers. The chuck connects to the spindle, engaged with the lead screw. To have the proper speed ratio between the spindle and the lead screw, use a quick change gearbox.
  • Set up the quick change gearbox according to the required thread pitch.
  • Use a thread gauge to align the head with the workpiece at an angle.
  • Move the threaded head towards the workpiece using compound feed and crossfeed.
  • Make sure the micrometer sets to zero for both feeds.
  • Due to the rotation of the lead screw, the slide carriage moves to a predetermined distance with each revolution in operation. Position the half-nut of the slide frame to engage the lead screw.
  • The half-nut or split-nut must engage at an exact time to make a correct continuous cut using a thread chasing disk or dial. This dial is attached to the slide and driven by a worm gear attached to the lead screw. This step is known as thread tool tracking.
  • The dial surface is into an even number of full and half divisions.
  • If the split nut is not engaged and the lead screw is rotated, the threaded dial is also rotated.
  • If the number of threads to be cut is an even number. The split nut should engage when the lines on the thread dial coincide with the zero line. When the number of threads cut is odd, any of the numbered lines should coincide with the zero line.
  • Scrape the part without lubricant. Disengage the half-nut at the edge of the cut, stop the lathe, and exit the tool using the cross feed. Return the carriage to the starting point.
  • Check the thread pitch with a pitch gauge. If it is correct, proceed to the next step.
  • Feed the compound using cutting oil into the first pass at 0.20 to 0.50 mm. As you approach the last pass, reduce the depth of the cut to 0.025 to 0.075 mm.
  • Continue to make successive cuts until the threads are closer to the desired depth or within 0.025 mm of the final depth.
  • Make a final cut at a depth of cut of 0.025 to 0.075 mm.
  • Check the dimensions using a thread micrometer or using a three-wire system.
  • When done, chamfer the ends of the threads to avoid damaging them.

      What is the first step to cutting internal threads on an engine lathe?

To cut a thread, calculate the pitch, lead, diameter, and depth. To cut threads, the single-point cutting tool turns the tool with a carbide insert.

      What tool cuts internal threads?

Taps and dies are metal threading tools used to cut and restore internal and external threads. A tap forms the female component of the lathe assembly and a die on the male.

How to cut threads on a manual lathe

  • Use a cutting bit aligned perpendicular to the work or the threads.
  • Use a 60-degree center gauge for this.
  • Get carbide inserts for threading that are precise. The tip needs to be at the midpoint of the work.
  • The other way is to single point it most of the way and use a die to finish. The single point assures that the thread is straight.

Threading on lathe tips for beginners

  • Set the speed to about one-quarter of the lathe speed used for turning.
  • Set the quick change gearbox.
  • Set the compound rest at 29 degrees to the right for right-hand threads.
  • Install a 60-degree threading tool bit, and set the height to the lathe center point.
  • Set the tool bit and right angles, using a thread gauge.
  • Position the tool for machining threads.
  • Coat the area to be threaded.
  • Set the micrometer to zero on both dials.
  • Move cross feed to the back tool, move the carriage to the end of the part and reset the crossfeed to zero.
  • Feed-in .001 to .002 inch.
  • Turn on the lathe and engage the half nut.
  • Take a scratch cut on the part without cutting fluid. Disengage the half nut at the end of the cut, stop the lathe and back out the tool using the cross feed. Return the carriage to the starting position.
  • Check the thread pitch.
  • Feed the compound in .005 to .020 inches for the first pass using cutting oil. Reduce the depth of the cut to .001 to .002 inches.
  • Continue this process until the tool is within .010 inches of the finish depth.
  • Check the size using a screw thread micrometer.
  • Chamfer the end of the thread to protect it from damage.

How to read a lathe threading chart

  • There are numbers in the columns and rows.
  • In the top chart, anything below the numbers 1,2…8,9 across the top is the Threads per Inch.
  • Below the 1, is 8 then 16 32 are the threads per inch.

How do you set up a thread on a lathe machine?       

      Which way should the thread come off a spool?

The thread should leave the spool at a right angle, in line with the direction of the wound thread. That will stop the thread from catching on the grooves at the end of the spool.

      How do you thread a single point on a lathe?

The screw thread has the lead equal to the pitch. See also pitch, lead, and screw thread. Single-start threads are more useful for general fastening needs.

      Should the point of the tool be set above or at the center of the spindle axis when taking a facing cut?

The single-point tool is fed linearly and parallel to the axis of rotation.

      Why is my bobbin thread not catching?

Your thread could be caught on something between the needle and your spool of thread. If so, your thread will be too tight for the needle to pick up the bobbin thread.

Can you cut threads on a manual lathe?       

      How do you cut left-hand threads on a manual lathe?

Turn on the lathe and cut but, at the end of the cut, do not disengage the half nut; instead, kill the motor.

      How do you cut fine threads?

Use a cutting fluid, like WD-40, and keep it wet. Check the tread depth often and use a feed for each cut.

      What are the tools that cut internal and external threads?

Taps and dies are tools used to create screw threads called threading.

      How do you pick up internal threads?

Pick up a thread on an external thread by coasting to stop with half nut engaged, shifting to high gear, and hand rotating chuck to get close. Bring tool close with cross slide with handle up at 10 o’clock and set the dial to zero, then engage in thread groove with compound & hand rotate chuck.

David D. Hughes
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