In this article we will discuss about:- 1. Meaning of Turret Automatic Screw Machines 2. Stages of Turret Automatic Screw Machines.
Meaning of Turret Automatic Screw Machines:
Turret automatic screw machines is a type of automatic lathe is the logical development of the capstan lathe.
These are actually automatic bar type turret lathes used for machining external and internal surfaces on work pieces from bar stock. Although these machines were designed for producing screws but now-a-days these are used for producing multitude of other products by using a number of special attachments. The bar is advanced to a stop automatically at the beginning of each cycle and then clamped in that position.
Upto six tools can be mounted on the tail stock turret and others on front and real tool slides. All these tools are automatically fed at the correct times during the cycle by cams. The various clutches are operated by trip dogs fastened to discs on the front shaft. These clutches engage with continued rotating rear shaft and are engaged at correct times in the cycle for providing power for indexing turret and reversing spindle for withdrawal of taps.
The general layout of a turret automatic is shown in Fig. 34.11 and Fig. 34.12 shows the principle of operation and the main feature. Tool slides and turret slides are operated by specially manufactured plate cams. Each cam actuates one slide and is machined to the required profile.
The radial throw of each cam lobe is equivalent to the required length of travel of the tool for which it is designed. The feed of each tool is also controlled by the radial throw of the cam lobe. The turret indexes automatically one station at the end of each reverse stroke of the turret slide.
Cam shaft located in the front of machine is driven through cycle time change gears depending on the component to be produced. Rear shaft runs as constant speed and it carries dog clutches which are operated through drum cams which in turn are actuated through levers from cam drums carried on the front cam shaft.
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By setting trip dogs in the correct angular positions on the cam drums, the dog clutches on the rear shaft can be made to operate when required in the cycle of operations. Clutches are used to open the collet chuck and advance the bar, change the spindle speed, and index the turret. One revolution of cam shaft produces one component.
The production time per piece is determined by totalling the times of the actual cutting operations and idle movements made up of such items as feeding stock, turret indexing, tool clearance etc. An important consideration which affects the efficiency of layout is the feature of overlapping i.e. performing an idle operation at the same time as a cutting operation, or carrying out two or more cutting operations at the same time.
Stages of Turret Automatic Screw Machines:
The work involved in laying out cams for this machine should be dealt with in the following stages:
(i) Determine the machine to be employed which mainly depends upon the size and shape of the bar to be used.
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(ii) Determine the operational sequence.
(iii) Determine the tools to be employed and the material from which these are to be made which mainly depends upon the material of the work piece.
(iv) Establish the cutting speeds permissible for the material to be machined, based on the operations to be performed.
(v) Calculate the R.P.M. of spindle.
(vi) Calculate the distance to be travelled by each tool. Usually an approach of 0.4 mm in allowed to avoid damage to tools.
(vii) Establish the feed per revolution of spindle for each tool. It depends on type of material, operations and type of tool.
(viii) Calculate the revolutions of the spindle for each cutting operation and the idle movements.
(ix) Determine cycle time, i.e. establish the number of revolutions of spindle required to complete one component, and correct to suit the revolutions actually available using standard change gears (correct revolutions/operation).
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(x) Calculate the hundredths of cam surface needed for both cutting and idle operations by converting revolution/ operation into hundredths.
(xi) Establish the time per piece.
(xii) Draw the tool layout and the set of cams.
The example given below will illustrate the procedure further:
It is required to design a set of plate cams for machining the brass component shown in Fig. 34.13 on turret automatic lathe.
The various steps are:
(i) Machine is already specified as turret automatic.
(ii) Operational sequence.
The various sequences (seven numbers) are shown in Fig. 34.14.
(iii) Tools. These are shown in Fig. 34.15. Material of tools in H.S.S.
(iv) Cutting speed for brass is taken as 2.50 m/sec.
... Rev ./sec of work spindle = 80.
(vi) Throw or travel of each tool by adding approach of 0.4 mm to avoid damage to tool is shown in Fig. 34.14.
(vii) Feed/rev. of tool. Depending upon the operation, these are selected varying between 0.004 to 0.25 mm/rev. as shown in Fig. 34.14.
(viii) Work spindle revolutions = (Throw of tool/Feed of tool). For various operations, these are shown in Fig. 34.14.
For each idle operation, one revolution of rear shaft is required which rotates at 4 rev./sec. Therefore, 1/4 sec. is required for operations like indexing of turret, feed stock, changing of speed, etc. 1/4 sec. is equivalent to 80/4 = 20 revolutions of work spindle. Five extra revolutions are added, to facilitate setting of trip dogs.
For indexing the turret, it has to be withdrawn first so that it does not interfere or clashes with other tools on slides or work piece. This time, wherever, encountered must be added.
(ix) Cycle time to produce one component is the total sum of all the revolutions required for each operation, disregarding overlapped operations. Knowing spindle revolutions and the nearest available standard cycle time change gears, the nearest revs./piece can be worked out and thus cycle time worked out.
(x) Additional revolutions obtained on account of (ix) above, have to be distributed judiciously to those operations which need them.
(xi) Revolutions/operation can now be converted into hundreds as shown in Fig. 34.14.
(xii) Fig. 34.15 shows the tool layout for turret automatic. Knowing the throw and the cam hundredths positions, the cams can be drawn. This is usually drawn with cams, one on top of the other in order to show the working position to which they will be set on the camshaft.
