Turret Lathes: Setting and Guidelines | Machine Tools | Engineering

In this article we will discuss about the setting and guidelines of turret lathes in industries.

Setting of Turret Lathes:

Setting turret lathes for operation involves mounting a work-holder and cutting tools, positioning stops for bar stack and for the slides, setting the control levers or cams to obtain the specified spindle speed and tool feed, machining two or three trial pieces, and correcting the position of the tools and stops.

Turret lathes with longitudinal motion of the turret slide, with no cross slide and cross motion of the turret, have limited processing capabilities. They allow such operations as centre-drilling, drilling, reaming, turning, threading, and facing narrow ends with a wide edge tool.

Bull grooving, facing wide end surfaces, form turning, and cutting off can be done on lathes of this type only with the aid of special tool holders that permit the cross traverse of the tool or through the use of the turret’s circular motion providing for cross feed of the tool.

The machining of form surfaces and cut-off operations are performed on turret lathes fitted with a cross slide (Fig. 32.16).

Turret lathes with a vertical axis turret are easier to set up for processing simple work than those with a horizontal axis turret. But the latter are easier to set when it comes to using several tools simultaneously, because the horizontal axis turrets employ simpler tool holders.

The lathes with horizontal axis turrets are switched over to another job by replacing an interchangeable plate of the turret head (Fig. 32.17) together with preset tools. At the same time the work holding fixture is also changed. The stops are positioned using a master work-part placed into the chuck, then a trial piece is machined, measured and corrections are made if required.

Turning and boring cylindrical surfaces is performed by tools in special tool holders clamped in the turret head or in the cross slide square turret. In the lathes with vertical axis turrets, turning tools are advisable to set vertically on the turret as this reduces machining errors due to the effect of turret indexing and so ensures a more consistent dimensional accuracy.

Generally, worn cutting tools are replaced together with tool holders, and new holders with tools preset against templates are placed instead.

The length of turning is held with the aid of stops. When the cross slide or adjustable tool holders are used, the required diametral size is achieved by trial cuts.

When tools have to be replaced separately from tool holders, it is necessary to machine a work piece, retract the turret, replace the worn tool with the new one, clamp the latter but slightly, advance it to the work so that its point bears against the machined surface, and clamp it finally. This method helps to keep the size setting. Forming tools are replaced in the same manner.

All tools (twist drills, core drills, etc.) except reamers are rigidly fixed in turrets. Reamers are held in floating holders. Helical flute reamers are recommended to obtain more accurate and better finished holes, both plain and with longitudinal grooves.

End faces can be machined with various lathe tools placed on the cross slide or in the turret. In the first case facing is effected with the cross feed of the slide. Stepped end faces can be machined with wide-edge tools using the longitudinal feed of the cross-slide saddle.

In the lathes with horizontal-axis turrets (without a cross slide), facing is carried out with tools fixed on the turret, which has a circular feed motion across the work. Grooving is done in the same way. Work piece 1 (Fig. 32.18) is passed through the oval opening in the turret and is cut off with tool 2.

On turret lathes the machining time is saved by the parallel action of the cutting tools set in the turret and on the cross slide, by the use of combination type tools, and by work with holders that carry several tools acting simultaneously.

Figure 32.19 illustrates operation of a lathe with a horizontal-axis turret head set up to machine two pieces at a time from bar stock.

When machining is done with several tools simultaneously, it is better to avoid combining the operations that involve roughing and finishing cuts (for example, core- drilling and reaming, or rough and finish boring).

For rational use, the turret head can be furnished with a duplicate set of tools, which makes it possible to repeat the working cycle in a full revolution of the turret and to machine an additional work piece.

The choice of cutting speed and feed in turret lathes stems from the following considerations:

(i) The possibility of combining operation stages for simultaneous machining of work piece surfaces by tools placed in the turret and on the cross slid;

(ii) The need to use for several tools operating simultaneously at a single cutting speed or feed rate determined for the tool that works under heaviest conditions (usually the tool machining the largest- diameter surface);

(iii) The possibility to apply such a cutting speed that will require one and the same rotational speed of the spindle in all operation stages, which will save time on speed changes.

Tooling Principle:

The jobs on turret lathe are simply a series of basic machining operations like turning, facing, drilling, boring, reaming, threading, etc.

The set up for any job consists of arranging these machining operations in their proper order. The best tooling set up fully considers the tolerances, quantity, and machining costs.

Once turret lathe is properly tooled, an experienced machinist is not required to operate it. However, skill is required in the proper selection and mounting of tools. In small lot production it is important that the work be done in the shortest possible time, so that much of the total production time is saved.

The production time consists of the following heads:

(a) Set up time.

(b)Work handling time

(c) Machine handling time.

(d) Cutting time.

It is usual to allow 12% as fatigue allowance to various operating times.

(a) Set-Up Time:

It is the time consumed in setting of tools on the machine. It can be reduced by having all necessary tools in good and readily available condition. For short run jobs a permanent set up of usual tools on the turret is an excellent means of reducing time.

(b) Work-Handling Time:

It is the time consumed in mounting or removing the work and is largely dependent upon the type of work holding devices. For bar work, this may be reduced to a minimum by having quick means for advancing the stock and by proper selection of collets, chucks and fixtures.

(c) Machine-Handling Time:

It is the time consumed in bringing the respective tools into the cutting position. This can be reduced by having the tools in proper sequence and position.

(d) Cutting Time:

It is the time consumed in actual operation and can be controlled by the use of proper cutting tools, feeds, and speeds. This can be saved by having multiple or combined cuts. In order to have an economic tooling the total production time should be reduced to minimum.

Standard times for various non-machining operations on turret capstan lathes are:

Feed to bar stop—0.06 min; Index turret—0.08 min

Change speed—0.05 min; Change feed—0.05 min.

Basic Analysis of Jobs:

Turret lathe set ups to be economical need to be analysed for each job.

Some guidelines in this regard are given below:

(i) Analyse the Job for the Types of Tooling:

A study of part design and specifications would enable determine whether it can be machined from bar stock or from castings or forgings. Roller-rest type of turning tools that support the work against the cutters are used for machining bar stock, and extended tooling that overhangs the hexagonal turret is used for machining castings and forgings.

(ii) Analyse the Machining Operations:

The usual order of machining operations is: internal cuts, external turning, operations from hexagonal-turret or square-turret station, facing cuts, chamfering, necking, grooving and cut off cuts.

(iii) Condition and Set the Cutters:

Proper cutting angles be ground on tool and these be set with minimum overhang and gripped firmly in the holders.

(iv) Set the Machine Stops:

Stops should be set only as accurately as part tolerances require. Hexagonal turret stops are usually set first and cross-slide stops last.

(v) Selection of Correct Speeds and Feeds:

Each cut should be run at as high a speed and at as coarse a feed as cutter and job permit. Usually hard materials are cut at slower speeds and heavier feeds, and soft materials at higher speeds and lighter feeds.