7. Turning and Lathes

• Cutting is performed in lathes by rotating the workpiece, and then holding a relatively stationary tool against it. Where the tool touches, the work is cut down in round patterns.

7.1 Introduction

• A lathe is a large machine that rotates the work, and cutting is done with a non-rotating cutting tool. The shapes cut are generally round, or helical. The tool is typically moved parallel to the axis of rotation during cutting.

• Manual lathes have the following major components,

 

• General classifications used when describing lathes are,

Swing: the largest diameter of work that can be rotated.

Distance Between Centers: the longest length of workpiece

Length of Bed: Related to the Distance Between Centers

Power: The range of speeds and feeds, and the horsepower available

• The critical parameters on the lathe are speed of rotation (speed in RPM) and how far the tool moves across the work for each rotation (feed in IPR).

7.2 Operations on a Lathe

• Operations on a lathe include,

 

 

7.2.1 Machine tools

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• There are two tool feed mechanism on most lathes. These cause the cutting tool to move when engaged.

The larger screw (the lead screw) will cause the lathe cutter to advance quickly. This is used for cutting screws, and for moving the tool quickly. Typical feed rates range from about 0.05” to 0.5” per revolution.

The small screw (the feed rod) will move the cutter slowly forward. This is largely used when doing rough cuts, or finishing operations. Typical feeds with this screw range from 0.001” to 0.010” per revolution.

• On a lathe the axial distance of the tool on the part is set by the carriage. A compound rest is used on a lathe that allows the radial tool position and orientation or the cutting edges.

 

• Work is held in the lathe with a number of methods.

3 jaw self centering chuck

4 jaw independently adjusted chuck

Between centers

Face Plates

Mandrels

Collets

Soft Jaws

7.2.1.1 - Production Machines

• In production there are a variety of cutting machines used to increase throughput by automatically feeding stock (through the headstock).

 

• Other types of turning centers provide multiple operations on a single machine,

Multispindle: Multiple spindles in a single machine allows parallel operations in a single lathe. Between each operation the spindles are advanced to the next operations.

Rotary Transfer: Large machines where parts are moved to different stations, typically over ten stations. These may have other tools such as drills mounted.

CNC machines: These computer controlled machines are typically flexible, but a bit slower. Flexibility is enhanced by a wider variety of operations and multiple tools in the same machine.

Cam: For high production rates, cams can be made to drive the cutting heads. Their geometry will move the tool in complex patterns.

7.3 Lathe Toolbits

• A lathe toolbit is shown in the figure below, with a few terms defined.

 

• In general, as the rake angle increases (positive), the cutting forces are reduced, the surface finish improves, and tool life increases.

• The side edge cutting angle has two effects outlined below,

 

• The End Relief Angle prevents friction on the flank of the tool. The holder for the bit is often angled, and the end relief angle must be larger than the tool holder angle to prevent rubbing.

 

• The side relief angle has a function similar to the end relief, This angle must exceed the feed helix angle.

 

• Increasing the nose radius improves the surface finish. But this reaches a limit.

7.3.1 Thread Cutting On A Lathe

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• Threads are cut using lathes by advancing the cutting tool quickly so that it cuts in a helical band. This helical band is actually a thread. The procedure calls for correct settings of the machine, and also that the helix be restarted at the same location each time.

• The basic procedure is,

1. The tool point must be ground so that it has the same angle as the thread to be cut. Typical angles are 60° for Vee threads, and 29° for ACME threads. A thread gauge can be used to measure thread angles. (also called Center Gauge or Fish Tail Gauge).

2. The correct gear ratio is required between the machine spindle to the lead screw. This can be determined with the equation,

 

3. The compound slide is set at half the thread angle. This is so that as multiple passes are made to cut the thread (most threads require a few passes to cut), the tool will be advanced in by the compound slide in such a way that only one face cuts. If both faces were used for cutting there would be a good chance of vibrations and chatter. For example, if a 60° thread is being cut, the compound rest is often set at 29°.

4. The cutting tool is set in the holder perpendicular to the work, and the fishtail gauge is used to check the angle of the point.

5. The In-feed is set to the surface of the part for the first pass (quite often the first pass just scratches the surface to allow visual checking of the settings). On each subsequent pass the infeed will be set closer.

6. The cross slide is set at the same location for each cutting pass. i.e., the dial setting is zero.

7. The In-feed is adjusted on the compound slide for each pass by moving it in a distance. A simple measure of this distance is,

 

************************** INCLUDE CHASING DIAL FIG 31-13

 

8. The chasing dial is used to restart the thread cutting in synchronization with what has been cut before. (If this step is not done properly, the notches in a thread might be cut over existing ridges: effectively cutting the entire thread flat to the bottom). The carriage of the lathe in driven across by a split nut. When the split nut is closed over the lead screw, it begins to move. It must be clamped over the lead screw when it is at the right angle. The method for doing this is with the chasing dial. The chasing dial has 16 different locations to engage at. In some cases you can engage the nut at any time, in other cases there are only a few positions to engage at. The basic rules are,

 

7.3.2 Cutting Tapers

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• A taper is a conical shape.

• Tapers can be cut with lathes quite easily.

• The typical measures for tapers are shown below,

 

• Standard tapers include,

Lathe-Spindle Nose: Used for alignment of hole/shaft pairs

type D-1 (tpf = 3”)

type L (tps = 3.5”)

Self Holding Tapers: Used for stability

Taper shank drills, reamers, sleeves, etc.
Use “Morse Tapers” numbered 1 to 7

7.3.3 Turning Tapers on Lathes

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• There are some common methods for turning tapers on a lathe,

Off-setting the tail stock

Using the compound slide

using a taper turning attachment

using a form tool

• Off-Set Tail Stock: In this method the normal rotating part of the lathe still drives the workpiece (mounted between centers), but the center at the tailstock is offset towards/away from the cutting tool. Then, as the cutting tool passes over, the part is cut in a conical shape. The method for determining the offset distance is described below.

 

 

 

• The Compound Slide Method: The compound slide is set to travel at half of the taper angle. The tool is then fed across the work by hand, cutting the taper as it goes.

• Taper Turning Attachment: Additional equipment is attached at the rear of the lathe. The cross slide is disconnected from the cross feed nut. The cross slide is then connected to the attachment. As the carriage is engaged, and travels along the bed, the attachment will cause the cutter to move in/out to cut the taper.

• Form Tool: This type of tool is specifically designed for one cut, at a certain taper angle. The tool is plunged at one location, and never moved along the lathe slides.

7.4 Feeds and Speeds

• If we consider the speed and feed of a lathe,

Spindle Speed is in revolutions per minute

Feed is in inches per revolution

• The Feed Chart is used to select the speeds and feeds of the lathe, and is often attached to the lathe near the setting levers.

• There are some simple (geometric) equations that can be listed,

 

• Typical cutting speeds for a high speed steel tool are, [Krar]

 

• Typical feeds when using a high speed steel tool are, [Krar]

 

7.4.1 The mrr for Turning

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• considering the parameters defined in the discussion of speeds and feeds, etc, the mrr is given below,

 

7.4.2 Process Planning for Turning

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• The general steps when process planning for turning external parts are,

1. Rough cuts all diameters to within 1/32” starting with the largest diameters first.

2. Rough cut all shoulders and steps to within 1/32”

3. Do special operations such as knurling and grooving

4. Cool the workpiece to get it close to the final dimension.

5. Finish turn the diameters, then the shoulders and steps

6. Deburr if necessary

• If the part is to be mounted between centers, plan should precede by,

1. cut stock that is 1/8” larger than required.

2. Put the work in the lathe, in a chuck, and face and center drill the end.

3. reverse the pice in the chuck and face the piece to size, and center drill.

4. Mount the work between centers

• For work to be mounted in a chuck, (implies internal features),

1. cut the stock 1/8” wider in diameter, and 1/2” longer.

2. Mount the work in the chuck with 5/16” to 3/8” inside.

3. Use a facing operation (lightly) to square the end.

4. Rough cut the external diameters, from the largest to the smallest.

5. Drill out the center of the work using a drill chuck mounted in the tailstocks spindle. Start with a center drill, and increase drill sizes to increase the hole.

6. Mount a boring tool to cut the internal diameter to close to the final diameter.

7. Cut any special feature now.

8. Do finish cuts on outside and inside.

9. Reverse the part in the chuck and face off the material to size. Protect the work by placing a piece of soft metal between it and the chuck.

7.5 Problems

Problem 7.1 Given that a tapered piece is to be made with the tailstock offset method, determine the taper per foot, and offset required if, you are starting with a bar of stock that is 8” long, and 1.125” in diameter, and the final taper is to be 6” long and 1” at the small end.

Answer 7.1 tpf = 0.25”, offset = 0.0833”

Problem 7.2 Given the 1/2-12 UNC thread that is to be cut on the lathe,

a) What should the gear ratio between the machine spindle and the lead screw be if the lead screw is 5 t.p.i.?

b) What should the in-feed be for each pass?

Problem 7.3 Which of the following statements about lathe toolbits is correct?

a) a small nose radius will result in a smoother surface.

b) small relief angles will always increase friction.

c) large rake angles will decrease cutting forces.

d) none of the above.

Answer 7.3 c

Problem 7.4 Which of the statements about lathes below is most correct?

a) jawed chucks hold only standard sizes of pieces.

b) mandrels hold work pieces from the outside.

c) the chasing dial is used for measuring fine cuts.

d) lead screws and feed rods are lathe parts

Answer 7.4 d

Problem 7.5 When turning between centers a dog is required; what is a dog in this context?

Answer 7.5 It holds the work piece so that it can be driven with a face plate mounted on the lathe spindle

Problem 7.6 A center gauge (fish tail gauge) is employed in thread cutting. Suggest two uses for the gauge.

Answer 7.6 Aligning a cutting tool for threads.

Problem 7.7 Given an external 9/16-12-UNC thread, determined which tools would be used.

Answer 7.7 A turning tool to turn the outside diameter of 9/16”+1/12”, UNC tool to turn thread.

Problem 7.8 If we are rough cutting a 5” diameter bar of bronze on a lathe with a HSS tool,

a) what speed and feed should be used?

b) if the cut is 12” long, and will be made in two passes, how long will the operation take?

c) if the setup time is 5 minutes, and the machine rate is $50/hr., what will the cost of the operation be?

Answer 7.8

Problem 7.9 List the basic steps for setting up a lathe to cut a thread on a bar of stock, assume the stock is mounted between centers already.

Answer 7.9 See thread cutting section.

Problem 7.10 If a taper of 1mm in 10mm is to be cut, what will the offset distance be for a 10cm part?

Problem 7.11 Calculate the machine tool spindle speeds for the following,

a) turning on a lathe with a high speed steel tool in mild steel work with a diameter of 2.75”. The cutting speeds is 100 ft./min.

Problem 7.12 We have been given a mild steel bar that is to be turned on a lathe. It has a diameter of 14” and a length of 28”. We have been asked to make two rough passes, and one finishing pass. The tool we have selected is Carbide. When doing rough cuts we use a feed of 0.007”/rev., and for finishing cuts we use a feed of 0.004”/rev. How long will this operation take?

Problem 7.13 Which of the following statements about lathe toolbits is correct?

a) a small nose radius will result in a smoother surface.

b) small relief angles will always increase friction.

c) large rake angles will decrease cutting forces.

d) none of the above.

Answer 7.13 C or D

Problem 7.14 Which of the statements about lathes below is most correct?

a) jawed chucks hold only standard sizes of pieces.

b) collets hold work pieces from the outside.

c) the chasing dial is used for measuring fine cuts.

d) lead screws and indexers are lathe parts

Answer 7.14 B

Problem 7.15 Given the non-standard 3/8-19 UNC thread that is to be cut on the lathe,

a) What should the gear ratio between the machine spindle and the lead screw be if the lead screw is 4 t.p.i.?

b) What should the in-feed be for each pass?

Answer 7.15 a) 4/19, b) 0.039

Problem 7.16 Develop a rough process plan for the part below by clearly listing operation steps in the correct sequence. Feeds, speeds, times and costs are not needed at this time.

 

Answer 7.16

Problem 7.17 The aluminum component below is to be turned on a lathe using a HSS tool. Develop a process plan, including offset for the taper, speeds, feeds, etc. Put the process plan in a list similar to the format shown. Assume a cost of $45.00/hr. for the lathe, and $25.00/hr. for all other pieces of equipment. State all assumption clearly, and justify numbers in the process plan with calculations or references.

Problem 7.18 On a lathe toolbit what are the functions of, a) the side relief angle, b) end relief angle, c) back rake, d) side rake angle, nose radius.

Problem 7.19 What applications are large positive rake angles for? negative rake angles?

Problem 7.20 What is the difference between end and face milling?

Problem 7.21 What RPM should be used to rough cut a cast iron piece with a 3” dia. with a high speed steel tool. What RPM should be used for a similar workpiece of plain carbon steel? What RPM should be used for the two materials if finishing cuts are being made?

Problem 7.22 Calculate the time required to machine a 2” dia. copper rod that is to be turned for a length of 10”.

Problem 7.23 What are rough and finish turning operations used for?

Problem 7.24 What are two methods for cutting stepped shoulders on a lathe?

Problem 7.25 Explain the difference between self holding and steep tapers using the coefficient of friction.

Problem 7.26 Find the tpf and tailstock offset for tapers on the following work. a) D=1.5”, d=1.25”, TL=4”, OL=8”

Problem 7.27 Convert a metric taper of 1:50 to a tpf. Convert a 1”tpf to metric.

Problem 7.28 Define the terms, fit, tolerance, allowance, limits, clearance, press fit, precision.

Problem 7.29 For a 1”-8-NC thread find the minimum and maximum diameters and minimum width of the toolbit point.

Problem 7.30 Describe the differences in speeds, feeds and depths of cuts for roughing and finishing cuts.

Problem 7.31 What types of chips are desirable when setting up automated cutting processes?

Problem 7.32 Compare the time to cut a work piece using a high speed steel tool and a carbide tool. The 4” dia. aluminum work is to be rough turned over a length of 14”.

Problem 7.33 What operations can be performed on a lathe?

Problem 7.34 How are the parameters different for a lathe when turning, as opposed to finishing?

Problem 7.35 A taper is to be cut on the aluminum part below. Indicate how far the tailstock should be offset and the speed and feed settings for the lathe.

Answer 7.35 offset=0.4”, feed 0.005-0.010”, speed 760RPM

 

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