machining and cnc technology section 11 lathes and turning operations

Machining and CNC Technology

Section 11 Lathes and Turning Operations


Safety is the 1st Word

Turning Creates Kinetic Energy

With lathe work, it’s not the cutter that spins, rather it is the workpiece!

That often means a heavy or long metal mass is doing hundreds or thousands of revolutions per minute!


Identify and Manage the Risk For all machining, we create setups and

chose tooling able to withstand the dynamic forces, heat and vibrations of the intended operation – solid, strong setups!

Additionally competent machinists are sure of their actions before taking them – they know their machine.

The Point: the way lathes work, the potential is there for danger, but it is offset with good safety training and especially with attitude

Trade Tip

A good part of a pro-management program, is to double check your own setups always! But for those first lathe setups in training, have them checked by the instructor or designated person. On the job, a journeyman or lead machinist should approve them until you build the confidence that comes with experience.


Chip Danger Too Adding to the unique potential of lathe

work, some turned chips can come off the workpiece like razor wire!

Trade Tip

Never reach for stringy chips with your hands or even with pliers, use a chip hook made for the purpose!


Ideas to Offset the Potential

Pay particular attention to the way your work is held in the lathe.

Never pull a lever or start an action until you know exactly what will happen.

Review how to stop the lathe before starting it, so you’ll be able to react instantly to a degrading situation.


Section 11 Goals Unit 11-1 Basic Lathe Operations.

> Identify the Operation to Machine A Given Feature> List and Describe the Fifteen Entry Level Lathe Tasks.

> Recognize Basic Setup Components for an Operation Unit 11-2 How the Lathe Works

> Know the parts and function of the manual lathe

> List the bolt-on accessories. Unit 11-3 Work Holding Methods.

> Select the right work holding devise for the job> List selection factors for work holding

Unit 11-4 Turning Tool Basics> Identify the right cutting tool for the job.

> Mount that tool in the right tool holding accessory


Goals Continued Unit 11-5 Lathe Safety

> Identify potential danger on lathes

> Plan, practice and refine emergency actions Unit 11-6 Setting and Operating A Lathe

> Setup and perform a typical job on a manual lathe> Identify the actions to take to turn a part correctly and safely.

Unit 11-7 Threading on a Lathe > Making a thread up> Learn operator actions during threading> Measuring the finished thread

Unit 11-8 Measuring Threads> Access and define Pitch Diameter> Use thread Wires and Thread Gages


Unit 11-1 Basic Lathe Operations When planning a job in a modern machine

shop, it’s important to know when the machining is best done on a lathe and when it’s not.

A good place to start is to recognize what lathes can do – our goal for Unit 1, knowing something about 14 operations.

At the end, you’ll be given drawings then challenged to identify the kinds of operations required to finish them.

For example: here’s three operations.

Taper Turning

Forming

Grooving Internal/External


CNC Verses Manual Operations

For some operations, there is little difference in how it is performed.

But others will be different when the computer takes over.

Where it is important to know the difference, it will be pointed out.


For example, cutting threads

On a manual lathe, we must start and stop the action by quick hand coordination – a real challenge.

That means spindle speeds are kept too slow for good chip formation. So the manual tool bit is ground with extra positive rake to achieve the best finish possible.

However, the CNC lathe cuts threads at or near correct cutting speeds for the material, so the bit geometry is normal.


Watch CNC threading and parting off

Starting and pulling out of the thread by CNC, the program was able to cut at the correct cutting speed for steel therefore better finish and tool life.


Unit 11-2 How the Lathe Works

You’ll be safer and more comfortable during lab demonstrations and assignments, if you know the basics of how a manually operated lathe works.

Again, there are some big differences in manually operated and CNC lathes, and they’ll be pointed out.

Shop Talk

Manual lathe training isn’t less important than CNC. Even today, jobs are started and finished on them as planned secondary and offload work. They are also used for tooling, repair and in some shops for low volume production.


Building a Lathe We’ll learn by assembling a lathe one major

component at a time.

Then we’ll look at the bolt-on accessories


Unit 11-3 Work HoldingKnowing how to hold the workpiece is a critical

factor for safety and for quality.There are several choices – each with

advantages and disadvantages.Chucks, 3 and 4 jaw

Collet ChucksBetween Centers

Face PlatesMandrels


5 Work Holding Selection Factors1. What Kind of Work?

Heavy metal removal or light?Is it a rough bar or finished part needing further lathe work? What tolerances?

2. GripRelated to #1, how much holding pressure is required to withstand the removal rates?

3. Turnaround TimeHow many parts are to be made on what schedule? Some lathe chucks are fast for putting in a new part, but not so good for runout. Others just the opposite.

When planning a lathe setup, consider five factors toward making the right work holding choice


4. Runout How much part wobble is acceptable in

the workpiece? Various chucks and holding methods have different amounts?

5. Special Holding AbilitiesFor example: Does the setup need to hold odd shaped work?Is the object to be held, weak enough that a chuck might crush it?


ChucksUniversal and Independent

All 3 jaws move at the same time – quick turnaround

Each jaw moves individually very slow turnaround

The 4 jaw chuck is the only method that allows adjusting unwanted runout out of the workpiece setup.


Custom Bored Soft JawsFor holding very odd shapesOr for canceling all runout in the setup


Spring Collet ChucksFor holding small workCollets feature very little runout but only

a medium grip

Special shapes can be held


Between CentersNot a very

strong setupCan be

removed and replaced with no loss of concentricity


Mandrels for turning small objects that have holes through them

Mandrels divide into two types Threaded and Expanding


Face Plate WorkA flat plate is used to bolt/clamp odd

shaped objects.Caution – Face plate setups are not beginner holding methods. Never turn one on until it is checked by a competent journeyman!

In the modern job plan, this out-of balance transmission case could be bored or circular milled at a CNC machining center – much safer than spinning it on a lathe!

Bore this hole


There’s a ton of technique tips about chucks in Unit 3, don’t miss them!


Mounting Spindle AccessoriesAnother skill to

be learned in Unit 3, is how to correctly mount the chuck, collet or other holding accessories.

The key task is to clean everything! One chip left behind can destroy the accessory, the spindle and your reputation!


Unit 11-4 Turning Tool BasicsAll lathe cutting tools have the four

features we learned back in Section 9: Rake Angle Clearance Lead Angle Nose Radius (corner radius)


Lead Angle

Rake

Clearance

Another decision is whether to use a high speed steel bit or carbide

Positive or Negative Rake?Positive or Negative Rake?

Nose radius or not? What lead angle would work best? Even the shape of the tool shank behind the cutting edge must be carefully considered!

Nose radius or not? What lead angle would work best? Even the shape of the tool shank behind the cutting edge must be carefully considered!

What size bit? Bigger is usually better but more costly.

What size bit? Bigger is usually better but more costly.

It’s a complex decision, and a lot depends on getting it right. Look for the tips and hints in the text.

It’s a complex decision, and a lot depends on getting it right. Look for the tips and hints in the text.

Nose Radius


The Tool Holding Method After choosing the cutting tool the next

decision is, how will it be held in the lathe? There are four on manually operated lathes.

Solid Tool Posts Quick Change Indexing Rockers

Once again, each has its advantages and disadvantages.

Smaller CNC lathes use tool holding methods similar to manual lathes, but industrial CNC lathes feature tool turrets with tool holding blocks bolted into the slots.


CNC Tool TurretQuick Change Tool Post for Manual Lathes


Unit 11-5 Lathe SafetyThe choices we’ve just discussed add

up to a large part of the setup safety – but not all of it.

Lathe safety also includes the following


Best Practice Safety Tips Know the Equipment and recognize 3 Classes of Danger! Then have a

practiced reaction to each – know what controls to use for stopping the lathe.Class 1 Something doesn’t seem right – stop at the end of the cut while monitoring the action carefully.Class 2 Something is wrong – stop now.Class 3 All out crash – stop the lathe if you can otherwise duck and cover! No kidding, it’s the big one! Your main goal in these events is to protect yourself!

Trade Tip

The first way we usually know that something is going wrong will be sound – that’s why most shops ban personal music machines.


Unit 11-6 Setups that Work RightA big part of Unit 6 is dedicated to

learning to plan your setup. The first item is to calculate the

workpiece RPMThe slide rule speed and feed

calculators and the formulae we learned back in Section 10, work here too.

Calculating RPM

All that’s required is to use the workpiece diameter instead of tool diameter. All the RPM methods work by starting with the rotating object’s diameter, the drill bit, mill cutter or lathe part.

Trade Tip

However, when calculating RPM for smaller work, the speed may come out in the thousands of revs, too fast for the holding method if it’s a chuck!

Your shop will have limits for certain work holding devises – for example, in my shop 3 & 4 jaw chucks cannot exceed 1,000 RPM.

RPM Limit Codes

This over-speed danger is so important for CNC work that we must add an RPM limiter code to many programs telling the control to not exceed a given ceiling value.

RPM Limit Codes

This over-speed danger is so important for CNC work that we must add an RPM limiter code to many programs telling the control to not exceed a given ceiling value.


Feed Rates for Turning

Like the drill press, feed rates are expressed in thousands of an inch per revolution for turning

Or parts of a millimeter per rev. To start we need to know the recommended chip load,

that is how much advancement per revolution is within good practice.

Too heavy and we break tools or have problems holding the work.

Too light is less critical but it means slow production and can even burn up tools in some problem metals.


Reference Feed Rates

While there are others references, the most widely accepted is Machinery’s Handbook ©

Look under Speeds and Feeds, Turning

Shop Talk

This book is so universal in our trade that the center drawer on most machinist’s tool boxes, is made to fit it!

The Handbook is also available in digital form too – very handy for quick reference.

The Handbook is also available in digital form too – very handy for quick reference.


Chip ControlOne objective of a good setup is to not

make long stringy chips.There’s several ways to do it.The best solution is a combination of

feed rates and tool geometry such that the chips break in to small C shaped pieces.


Other Chip Control Practices Always use a chuck guard to catch coolant and chips Never stand in line with the chuck, where chips are

flung out. Coolant helps but may not be possible if it flings out

badly. Use the chip breakers discussed back in Section 9, a

chip dam or a breaking groove in the rake face of the tool.

Do not use gloves around moving machinery – a reminder!


Work SupportA last setup component not to be

missed.Long work will require tailstock supportExtra long work may need center

supports as well.


Unit 11-7Single Point Threading

This is one of the most complex manual machine operations you’ll do in training!

The setup has more components to check than any other we do (except gear making on the manual mill)

Turning the threads requires more hand/eye coordination than any other too (plus a bit of luck sometimes)


8 Threading Setup Factors

1. The holding method

2. Prepping the part, relief and chamfers.

3. Bit Shape and Geometry

4. Setting the Tool Compound

5. Positioning the Bit

6. Coordinating Axis Dials

7. Gear Selections

8. Taking the First Pass (White knuckle time)


Unit 11-8Measuring Turned Threads

OK, you’ve mastered the threading setup.

Applying cutting oil or threading compound, chips are coming off nicely.

Now, how do you measure those threads to ensure they are the right diameter?


3 Ways To Measure Threads at the Lathe

The most universal, 3 wire methodThe fastest, thread gages The most accurate, thread micrometers

Gages

3 Wire Method


Last Word - SafetySection 11 is big, because there’s a lot to know

about turning. The setups can be complex too. Use a check list to be certain you’ve covered all aspects of a safe setup before asking for a setup check. Work holding tightened well. Work support – locked in place Tool holder/bit not hanging out and tight Speeds and feeds within reason


Conclusion

Keep in mind that safety on lathes is managed with training, and good common sense, but checked by a journey level person for those early setups!

Also, that the larger part of manual lathe practice applies directly to CNC work.

Competent machinists can machine precise parts on both the programmed and the lathes operated by hand!

machining and cnc technology section 11 lathes and turning operations

Documents

lathe setups

lathe safety

cnc technology unit

manual lathe

lathe works

basic lathe operations

unique potential of

operation unit