Turning is a machining process that used for machining, basically, round shapes (cylindrical, conical surfaces, and flat surfaces). These processes are usually performed by rotating the workpiece (turning) on a lathe. Turning. Objectives of machine tools (lathe). Hold the job
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Turning is a machining process that used for machining, basically, round shapes (cylindrical, conical surfaces, and flat surfaces).
These processes are usually performed by rotating the workpiece (turning) on a lathe.
Objectives of machine tools (lathe)
Types of lathes
Center lathes is used for single operation work or for miscellaneous jobbing.
Rotary motion of workpiece about fixed axis (cutting motion, speed) with feed motion toward the workpiece without depth of cut. Cases are such as g, I, and k.
In the cylindrical work, all diameters should be concentric and all faced surfaces should be square to the cylinder axis. The best way is to perform as many operations as possible at one setting.
The three-jaw, self-centring chuck: (a) construction; (b) external and internal jaws
The independent four jaws; p100, fig 4-4. It provides the facility of setting work off-center to produce eccentric workpiece. The concentric diameter(s) are produced first then adjust the chuck for eccentricity by a dial gauge. The dial gauge readings must vary by twice the amount of eccentricity required.
the chuck is adjusted until the DTI maintains a constant reading whilst the chuck is revolved;
The four-jaw chuck
The collet chuck, p100, fig 4-5. It provides for a certain diameter, high repeatability, too small griping length (it is possible to cut close to the collet), it is made of medium carbon steel and is hardened and spring tempered so that when unlocked it will spring open and release the work. To enable it to close on the work three or four slots are made.
(c) push out type collet
(a and b) draw in type collet
Holding work between centers. P104, fig 4-6. It used for solid work. It should have a center hole in each end. There are two centers, live center attaché to the headstock and dead center attach to the tail stock. Virtually exact repeatability of position. To drive the work catch plate with a driving pin is fitted in the spindle nose and a work carrier (driving dog) is attach to the work. The parallelism should be checked.
For hollow shafts a mandrel can be used, fig4-7
Traveling support (steady) fig4-9
Fixed support (steady) fig4-10, used when the tailstock can not be used (the work is hollow at the end near the tailstock or that end is to be bored to size). The pads or rests are act as bearing and are usually made of brass.
Where; L is the full length of the workpiece
D is the largest diameter of the workpiece
d is the smallest diameter of the workpiece
l is the length of the tapered surfaces.
Using the compound top slide (for any angle but the length of travel is limited and hand feed must be used), give limited accuracy.
Taper turning attachment, the lead screw of the cross slide is released, so the control of depth of cut is taken from the sliding block around the guide bar, which is inclined with the required angle.
The effect of incorrect tool setting on taper turning; with low tool setting;
With 2 mm low, the error in the radius is 0.02 mm (p112&113)
Pitch of work
Pitch of lead screw
Screw cutting in the lathe
The form of the thread is coped from the tool shape. The tool should be ground to the correct shape (angle and nose radius and zero top rake angle).
Use the screw cutting plate gauge to check the correct shape of the tool and to set it to the correct position with respect to the workpiece.
Generating the thread helix; p116
Select of gear trains in the Screw cutting
Use gears with no. of teeth between 20 and 120 and step 5 teeth.
1- Depth of cut d =(Do –Df)/2.
D.o.c is usually taken as 3-5 mm for rough turning and 0.8 to 1.6 mm for finish turning.
2- Cutting speed mm/min where V= 3.14 DN
3- feed per rev (f mm/rev) where feed rate fr= Nf
Feed is usually taken as 0.3-1.5 mm for rough turning and 0.1 to 0.8 mm for finish turning.
Time to cut =L/fr
Volumetric rate of metal removal = vfd
Each metal has an optimum speed v
It is required to turn a 90 mm. diameter and 250 mm. length bar to the shown dimension with speed = 90m/min, feed rate = 0.38 mm/rev. and depth of cut = 5mm. The approach and over travel = 3 mm.
All dimensions in mm.
Determine the number of teeth for screw cutting train to cut thread of pitch 0.7 mm given that the lathe leadscrew pitch is 4 mm and the available gears have 20 to 120 teeth with step of 5 teeth.