Tailstock in a Lathe: What's the Function?

Machining operations on a lathe involve turning long, slender workpieces at high speed.

This can often result in unwanted bending and vibrations of the workpiece, which can be overcome by engaging the lathe's tailstock.

But what exactly is a tailstock, and how does it aid the machining on the lathe?

The tailstock is an integral part of lathe machines, that is generally placed at the right end of the lathe bed, opposite the headstock. It acts as a support to hold long/slender workpieces and can also be used to mount cutting tools like drill bits and boring bars.

This article provides a detailed guide on tailstock in lathes by discussing its parts, functions, and types.

In addition, this article also discusses the practical aspects of everyday machining, like the methods to align a tailstock to achieve high-quality outputs.

What is a Tailstock?

Long and slender workpieces encounter high chances of bending while machining and the tailstock provide support to such workpieces to prevent unwanted bending or vibrations.

Unlike the headstock, the tailstock is a movable part of the lathe that can be positioned along the Z-axis of the lathe by gliding it over the lathe bed rail. 

The tailstock can also be used for mounting cutting tools such as drill bits, boring bars, etc., that are used for axial machining operations like drilling and boring.

Locking the tailstock at desired locations on the lathe bed helps secure workpieces of different lengths.

The ability to support rotating parts with a Morse tapered dead center makes the tailstock a versatile support accessory.

However, in lathes such as the capstan lathe and turret lathe, the tailstock is replaced by a hexagonal turret that can hold multiple cutting tools.

Parts of a Tailstock

The tailstock of a lathe is a complex structure that consists of various parts that work in coordination to perform the desired function.

Body

The body of a tailstock forms a housing for other parts and has a hollow split at the top.

The barrel (also called quill or spindle) is inserted via a T- shaped barrel key that sits inside the hollow portion. This mechanism allows the sliding movement of the barrel without any rotation.

Barrel

The barrel is a hollow cylinder that houses tools like a drill chuck or dead center.

A minor segment of the barrel length is threaded on its internal surface, while the rest of its length is a morse tapered, smooth hollow surface.

Morse taper is a standard taper series used in the shank of cutting tools. This tapered design provides comparatively more friction and holds the cutting tool firmly inside the barrel.

The threaded portion of the barrel facilitates the conversion of the rotatory motion of the screw spindle to the barrel's translatory movement.

With the help of the barrel key, the barrel is housed inside the body of the tailstock through a key slot cut on its external surface.

In this way, the barrel is placed concentric to the body of the tailstock.

Screw Spindle

The screw spindle is a stud rod with threads cut externally on one end and a key slot on the other.

These external threads match the internal threads of the barrel. Thus, the rotary motion of the handwheel is efficiently converted to the translation of the spindle.

For example, the left-handed threads cause the extension of the barrel forward when the handwheel is turned clockwise.

The screw spindle is attached to the body of the tailstock through a spindle bearing, which prevents the screw spindle from falling apart while giving the feed.

Hand Wheel

The handle wheel is keyed to the slotted end of the screw spindle, allowing the extension and retraction of the barrel.

The keyway prevents the hand wheel from slipping against the surface of the screw spindle and facilitates the transfer of the rotatory motion of the wheel to the screw spindle.

A handle is screwed on the hand wheel to provide easy access for rotating the wheel manually.

Lock Center

The lock center is used to lock the barrel in a particular position.

It is a stud with threads on both ends that tightens the split portion of the body, enabling it to lock the barrel in a specific position.

The lock center is tightened using a locking lever.

Locking Clamp or Nut/Lever

The tailstock is locked onto the lathe bed via a T-Bolt and a plate.  

In modern lathes, the T-bolt is replaced with a locking lever, that tightens the tailstock at the desired position.

The function of Tailstock in a Lathe

The main function of the tailstock is to provide support when working with long workpieces that are prone to bending under their own weight during machining.

The decision of engaging a tailstock depends upon the length-to-diameter ratio of the workpiece.

Length to Diameter ratio (L:D)	Usage
L:D < 3:1	Tailstock support is not required.
3:1< L:D < 10:1	The usage of a tailstock can be considered.
L:D > 10: 1	Usage of Tailstock with steady rest is highly recommended.

The length mentioned in the table is the unsupported length, i.e., the length measured from the chuck surface.

So, while calculating the length-to-diameter ratio, deduct the holding length (length of the job held by the chuck).

Engaging the tailstock is not required if the length-to-diameter ratio of the workpiece is less than 3:1. But for a ratio greater than 3:1, it is recommended.

In extreme cases where the ratio is greater than 10:1, tailstock along with a steady rest is highly recommended.

Steady rest is a holding device that provides support to the workpiece circumferentially.

Similar to a tailstock, it is placed on the lathe bed to provide smooth rotation of the workpiece without any eccentricity.

The tailstock also provides firm support to heavy parts that undergo distortion or bending while machining. 

As a general rule, the usage of the tailstock is also encouraged when the holding length (length inside a headstock chuck) is short.

Apart from that, the tailstock is also used to hold lathe cutting tools like drill bits, reamer, rotary broach, threading tools, etc., when performing machining operations on the cross-sectional face of the workpiece.

These tools are mounted by clamping them in the barrel of the tailstock and using the handwheel to provide the feed during the machining operations.

When using the tailstock as a tool holder, it must be ensured that the tailstock is locked in its place to deliver strong cutting force without any deflection.

In wood lathes, tailstock plays an important role in providing counterforce when using a spindle spur to hold the workpiece.

Generally, it is advised to operate the lathe at a safe speed and always engage the tailstock to ensure firm clamping.

How to Align Tailstock with the Headstock?

It is necessary to align the tailstock with the headstock.

Misjudging the alignment leads to higher noise levels, more vibration, and tapered parts (one end bigger than the other) that leads to additional machining or even scrapping of the workpiece.

The alignment process ensures the quality production of precise parts, and an improved working environment, i.e., with less noise and vibration.

Manual alignment of tailstock can be tiresome and time-consuming. In addition, there are high chances of human error that can lead to taper on the workpiece.

Different methodologies are available to perform the alignment process. Some methods are sophisticated, while others are simple.

Aligning the Tailstock Using an Alignment Bar

Aligning using an alignment bar helps in achieving accurate and good-quality parts.

An alignment bar is a pre-manufactured standardized bar made of high-strength anodized aluminum.

Using an alignment bar eliminates human error and helps avoid taper on the parts supported by the tailstock.

Procedure

Caution:  Perform this operation while the machine is unplugged from the mains.

1. Place a dead center with Morse taper inside the barrel of the tailstock.
2. Next, place a magnetic dial indicator (an instrument that displays the deviation from the desired value) on the carriage of the lathe machine.
3. Place the indicator of the dial gauge as close as possible to the tip of the dead center. In this way, we can read the deviation of the center from the zero scale reading.
4. After measuring the center at the tailstock end, place a straight shank dead center inside the chuck present on the headstock.
5. Now, place the alignment bar between the headstock and tailstock centers.
6. Place the dial indicator on the headstock end of the alignment bar and reposition the gauge's needle at the zero scale reading.
7. Rotate the chuck by hand and observe the dial gauge readings.
8. Choose the location on the alignment bar where the dial gauge shows the maximum deviation.
9. At this particular extreme, reposition the needle of the dial gauge to zero.
10. Rotate the chuck 180° to position the dial indicator at the opposite point.
11. Observe the deviation and then reposition the needle to half the value of that deviation.
12. Keeping the needle in this position, move the dial indicator via carriage to the tailstock end to ensure uniform alignment throughout its length.
13. In the event of deviation, loosen the tailstock locking clamp slightly and rearrange so as the reading approaches close to zero.

This method is best suitable for industrial applications where perfect alignment is of utmost importance.

Aligning the Tailstock Using a Test Bar

A test bar is used to find the misalignment between the tailstock and the headstock.

A series of turning operations for different diameters is performed along the length of the bar.

The diameters are measured upon completion of the machining procedure to know their deviation from the actual values. This is then corrected by rearranging the position of the tailstock.

Though this method is simple, it is time-consuming, and the test bar needs to be scrapped after the procedure.

Aligning the Tailstock Using an Indicator Mounted on the Headstock

This method requires an additional fixture on the headstock to hold the dial gauge in position.

A parallel bar placed in the tailstock barrel is brought near the dial gauge, which is mounted on the lathe's headstock.

The dial gauge is used to take readings at three or four random points along the circumference of the parallel bar by repositioning the needle to the zero scale reading.

This alignment method eliminates the need to purchase an alignment bar or use a test bar that has to be scrapped after alignment.

Making it ideal for DIY projects where the budget is low and the machined part does not require a tight tolerance.

Types of Tailstocks

A wide variety of tailstock is available commercially.

Based on manufacturers and machines used, tailstocks are available in the following types

• Manual bodied tailstock
• Manual bodied with programmable barrel
• Programmable body with/without a barrel.

A manual-bodied tailstock with a retractable barrel is the most commonly used tailstock, whereas a manual body with a programmable barrel tailstock helps hold long and slender parts.

The programmable tailstock is usually used in CNC lathes, where precise and accurate components are of utmost importance.

This type helps identify the right amount of holding pressure based on whether the components are long/slender or heavy.

Final Thoughts

To summarize, a tailstock is a machinist’s friend that provides support to eliminate or minimize the bending and vibrations caused in the workpiece during machining.

Parts, such as an axle of an automobile, are long and require accurate machining. 

The misalignment due to an uneven circumference or undesired taper can cost a lot for the manufacturer. In addition, such a product will cause damage to other parts engaged with it.

Therefore, it is important to understand the process of aligning the tailstock to ensure quality outputs. 

Frequently Asked Questions (FAQ)

How often do we need to align the Headstock and the Tailstock?

It is always good to check for misalignment before the start of any machining operation. An unseen error can be avoided by following this procedure. In this way, you can ensure achieving desired dimensions.

Does a CNC machine require a tailstock?

Yes, CNC lathes require a tailstock when working with long and slender workpieces. However, unlike manual lathes, CNC lathes use a programmable tailstock that can automatically set the holding pressure for the workpiece.

Are the methods for aligning the tailstock the same for manual and CNC lathes?

Yes, the method of aligning the tailstock is the same for both manual and CNC lathes. However, the most commonly used method to align the tailstock to the headstock in a CNC lathe is aligning using a test bar. Though this method is comparatively time-consuming, it is worthwhile rather than a faulty batch production.