Methods of Taper Turning

A surface may be turned in a lathe if the tool point is not fed parallel but at, a certain angle. Tool travel at an angle to the work axis may be obtained by the following methods: -

Taper Turning by Off Setting the Tailstock

Taper can be machined on a job that can be turned between centres by setting over the tailstock. The method of setting over the tailstock. In this method the axis of the job rotates with an angle to machine bed axis and the tool moves parallel to the machine bed. Under this method the tailstock is off set on the basis the value of, which is in mm or inches instead of degrees. If the dead centre is set over in the opposite direction it will turn a taper  its  larger  diameter  on  the side of the tailstock.

The set over can be determined by the formula: -

Off set =  D-d      x             L

                   2l

Where D = Major dia.

d  = Minor dia.

l = Taper length L = Full length.

When the entire length of job is to be tapered the formula:

Off set = D-d

                   2

Following process may be followed for setting the parallel position:

(a) Advance the nose of the tool to the left end of the taper so as to slightly nip a piece of paper between the tool and taper surface.

(b) Note down the reading on cross feed dial and withdraw the tool.

(c) Move the tool to the right end of the taper and again nip the paper between the tool and the taper.

(d) Check up the reading of cross feed dial. If it coincides then the set over is correct.

Off Setting Tail Stock Method

The setting over method includes the following steps:

(a) Loosen the clamp nut of the tailstock.

(b) Loosen one adjusting screw and tighten the other till the required set over is obtained.

(c) Mount the tool in usual way as in parallel turning.

(d) Take a rough cut.

(e) Test the taper and readjust if needed.

(f) When the required taper is obtained, finish the size.

Non-uniform wear on the centre holes may be prevented by using ball centres instead of ordinary centres. The ball centres may be used on both the ends.

Advantages of Tail Stock Method

(a) Longer jobs can be tapered.

(b) Good surface can be obtained by automatic feed.

(c) External taper threads can be cut.

Disadvantages of Tail Stock Method

(a) Steep taper cannot be cut.

(b) Internal taper cannot be cut.

(c) Since the centres are out of alignment the centre points do not seat exactly in the centre holes on the work and therefore the danger of the work being thrown out is more under severe machining condition.

Taper Turning by Swivelling the Compound Slide

This method is used to produce short or steep tapers. Compound slide is fixed on the top of cross slide and it can be rotated on its central pins in both ways. Its base is graduated with 0 to 45° and degrees are marked on both the sides of zero. Taper turning can be done outside the job or in the bore.

The principle of this compound slide is that axis of work piece rotates parallel to the axis of bed and the cutting tool moves at the desired angles. The compound slide is swivelled to the desired angle by loosening the nuts. Tool is set on the tool post. It is thus fed along a path at an angle turning its screw by hand. The tool travels on the path of compound slide and the desired taper is obtained. For internal taper turning the tool may be set as shown in the figure below.

Taper turning by compound slide has the following disadvantages:

(a) Production efficiency is low and finishing is not good because of hand feed.

(b) Only short tapers can be turned because the length of the taper is limited by compound slide travel.

Some important hints may be kept in mind while taper turning by compound slide and these are the following:

(a) Avoid the over hang of the tool.

(b) The height of the tool point must be in the centre. It should neither be low nor high.

(c) The hand feed should be uniform as much as possible.

(d) Use coolant sparingly.

Taper Turning with Taper Turning Attachment

In this method the work rotates around the bed axis and the cutting tool moves in an angle. Modern lathes are equipped with taper turning attachments for machining external and internal surfaces having an angle but not exceeding 12° maximum. On some lathes the angle is limited to 18°.

Working of Taper Turning Attachment - A taper turning attachment is in the form of a guide, which is graduated and bolted in back of the lathe. It is adjusted to an angle with the axis of the lathe. Its main function is to make it possible to keep the lathe centre in line but freeing the cross slide and then guiding it on the lines of taper to be turned.

In figure above a bracket attached to the lathe bed. It carries a guide bar and the guide can be swivelled to the required angle. The guide bar is clamped in the set position by bolt. Guide block is connected to the lathe cross slide by link and lock. It is free to slide along the guide bar to allow the cross to travel freely on its way it must be disconnected from the saddle by loosening the cross feed screw nut, which will disengage the cross slide on the longitudinal feed of the saddle. Guide block will slide along guide bar. The guide block is linked to cross slide, so it will travel with the tool in the direction of the guide bar. After each cut the tool is fed in the depth of cut by rotating the handle of compound slide. The compound rest should be positioned at 90° to its normal position. The guide bar should be set parallel with the taper on, which the tool is applied. The guide bar is swivelled on the taper angle as read on the scale of the indicator.

In some taper turning attachments the marking is in millimetres instead of degrees. In such cases we can find out the number of mm divisions by the formula.

MM= D-d            x H

              L

Here MM = required number of mm divisions to be moved by guide bar. D = Major dia., d = Minor dia.

L = Length of taper in mm.

H = Distance from the axis of rotation of the guide bar to its end in mm The angles for swivelling the guide bar can be determined by the Formula: -

Tan θ =  D-d

                2L

Where D = major dia, d = minor dia and L = length of taper in mm.

Advantages of Using Taper Turning Attachment

The following advantages are given below for using taper turning attachments:

(a) The attachment can be set quickly and easily.

(b) With the use of this attachment, tapers are turned without disturbing the normal set up of the lathe.

(c) External and internal tapers can be given.

(d) Tapers are turned with the longitudinal power feed and thus the work can be machined quickly and with better finish.

(e) Long tapers can be given.

(f) Taper turning attachment can be used for turning taper threads.

Taper Turning with Forming Tool

Short external tapers can be turned with the form tool. In this method the work is parallel to the lathe bed axis and feed is also parallel but the shape of the cutting tool is in taper angle. In this method both longitudinal and cross feed can be used. However from tool, which is also named as broad nosed tool can be applied only if the length of the taper does not exceed 20 mm. If wider tool is used, it will chatter and will vibrate considerably.

Defects of Taper Tuning

In taper turning following defects may occur:

(a) Incorrect Taper - It is due to incorrect set over of the tail stock or incorrectly setting of compound slide or incorrectly setting of taper turning attachment. It can be prevented by proper setting.

(b) Incorrect Dimensions of the Taper - It can be also due to wrong setting.

(c) Incorrect Diameters - Sometimes it so happens that the diameter of one end is correct and angle of tapers is also correct but diameter on the other end is incorrect. It is due to incorrect length of the taper. This defect can be avoided by carefully checking the length of the taper before starting the work.