Machining Operations: Lathes Flashcards
How is the size of a lathe determined?
By the largest diameter of workpiece that can be supported over the ways (the swing) and the greatest length of workpiece that can be supported between centers.
What are the parts of the lathe shown in the image?
- The headstock (A)
- The tailstock (B)
- The bed (C)
- The carriage (D)
- The quick-change gearbox (E)
How is the tailstock clamped in place?
The tailstock consists of a heavy two-part casting that sits on the ways and is clamped in place with the tailstock clamp.
How is the tailstock spindle moved?
The tailstock spindle can be moved in and out of the casting like the quill on a drill press by turning the tailstock handwheel.
What is the most widely used material for the bed of the lathe and why is it used?
Iron is the most widely used material for beds because it dampens vibration, thereby reducing chatter.
What are the four parts of the carriage?
- Saddle
- Cross-slide
- Compound rest
- Apron
The saddle is an H-shaped casting that rides directly on the ways. What is its direction of movement?
Saddle movement is longitudinal, or parallel, to the axis of the machine spindle.
The cross-slide sits on top of the saddle. What is its direction of movement?
At right angles to the axis of the workpiece. The cross-slide is moved by turning a handwheel, which turns a screw.
Where is the compound rest located and how does it move relative to the machine spindle face?
The compound rest sits on top of the cross-slide.
It can be swiveled 360º to obtain any angle to the machine spindle face.
What are four of the main parts of the apron and what are their functions?
- Carriage handwheel - moves the carriage.
- Split-nut lever - closes the split nut on the lead screw when thread cutting.
- Automatic feed lever - engages the power feed for the carriage or cross-slide.
- Feed change lever - select whether the carriage or the cross-slide moves under power when the automatic feed lever is engaged.
What is the quick change gearbox responsible for?
It allows you to change gears quickly and easily by shifting levers.
The movement of the carriage is timed with the rotation of the spindle, causing the carriage to move a specific distance every time the spindle completes a revolution. The timing is accomplished with gears and the gears must be changed every time a new ratio is desired.
What are the back gears responsible for?
The back gears link the spindle to the input shaft of the quick-change gearbox. It is often necessary to remove one set of gears and replace it with another in order to achieve a particular feed rate or pitch listed on the quick-change gearbox.
What are the lead, feed rod and rack responsible for?
- Lead - an accurate threaded rod that runs the length of the bed and moves the carriage.
- Feed rod - transmits power from the quick-change gearbox to another set of gears within the carriage. The feed rod moves the carriage at a slower rate (more suitable for turning)
- Rack - A pinion (a small gear) engages the rack, which is found just under the front ways, to move the carriage under power.
What is the starting clutch used for?
It is used to start the rotation of the spindle.
Typically, lifting the lever starts the spindle turning forward, and pushing it down starts the spindle in reverse. Returning the lever to the central position stops the spindle.
What is the steady rest used for?
It supports the workpiece in the middle or at the end.
Long, thin workpieces tend to whip around and deflect during a cut, unless properly supported.
It is not always possible to support one end of the workpiece with a tailstock center.
What is a follower rest used for?
It keeps the workpiece from deflecting away from the cutting tool by supporting it in the immediate area of the cut.
What is a taper attachment used for?
It allows a lathe to cut a taper by causing the carriage and the cross-slide to move at the same time.
What is a toolpost grinder?
It consists of a grinding wheel and motor that can be attached to the compound rest in place of the normal toolpost.
Why must the ways be covered with plywood or cardboard when using a toolpost grinder?
The abrasive grit generated by a toolpost grinder damages the ways.
What does the thread chasing dial indicate?
It tells you when to close the split nut. On top of the unit is a numbered dial that rotates slowly past a pointer when the split nut is not engaged.
What is a Jacob’s taper used for?
It is used to secure a drill chuck to its shank. A series number identifies the Jacobs chuck taper, with larger numbers accepting larger chucks.
What is the taper on a Jacob’s chuck?
0.591 to 0.979 inch per foot.
What ability does a multi-position quick-change tool post have compared to a regular tool post?
It allows the toolholders can be set around the tool post in increments of 15-degrees.
What is a live center?
A live center fits in the headstock spindle. It is called a live center because it turns with the headstock spindle.
What is a solid dead center?
It is much like a live center in appearance, except it is held in the tailstock spindle. It is called a dead center because, unlike a live center, it does not turn.
What is a half center?
It is a dead center with part of the tip cut away to make room for a cutting tool.
What is a revolving dead center?
It is used in the tailstock. The workholding end rotates on a bearing to prevent friction and heat buildup. A revolving center is more convenient to use than a solid dead center but is not as accurate because clearance in the bearing is transmitted to the workpiece.
What are lathe dogs used for?
They are used to transmit drive power from the spindle to a workpiece when it is held between centers. Without a lathe dog, the workpiece stops turning as soon as the cutting tool is applied.
Why should lathe dogs be used with great care?
Any protruding part on the lathe dog or drive plate can catch loose clothing and severely injure the operator.
What is a drive plate?
It is a flat disc that attaches to the spindle nose. It has grooves and pins to engage and turn a lathe dog.
What is the advantage and disadvantage of using a three-jaw chuck?
They are simple and quick to use, but not as accurate as four-jaw chucks.
There is a scroll plate inside a three-jaw chuck. A spiral groove on the face of the scroll plate engages teeth on the back of each of the three jaws. The scroll plate causes the three jaws to move in and out simultaneously. Why should you not use a three-jaw chuck to grip hot rolled steel or irregular shaped objects?
Those types of pieces will damage the scroll.
When installing the jaws on a three-jaw chuck what must be done?
When installing these jaws, ensure that the jaws and slots match up, such as placing the #1 jaw in the #1 slot.
What are the advantages and disadvantage of using an independent four-jaw chuck?
The advantages are that the four-jaw chuck is much more accurate and it can hold square or oddly shaped workpieces.
The disadvantage is that it is more time consuming to set up than a three-jaw chuck.
What is a cathead used for?
A cathead is used to support a long workpiece that is not round. A cathead attachment can be used to provide a true surface for the steady rest.
What are spider attachments used for?
They are used when long, tubular workpieces require a center, but the inside diameter is too large to run on the center. The spider mounts to the inside of the tube and the adjusting screws are used to true the workpiece to the center.
What is a taper mandrel (arbor) used for?
They are when a number of work pieces are to be held by the bore. They are especially useful when the outside diameter must run true with the bore.
What is cutting speed defined as on a lathe?
The speed at which a point on the workpiece moves past the cutting tool.
How is cutting speed measured?
Meters per minute or feet per minute.
What is the metric formula for spindle speed on a lathe?
rpm = (CS x 320)/Dia
Where
* rpm is revolutions per minute.
* CS is cutting speed in meters per minute.
* Dia is diameter of the workpiece in millimeters.
What is the imperial formula for spindle speed on a lathe?
rpm = (CS x 4)/Dia
Where
* rpm is revolutions per minute.
* CS is cutting speed in surface feet per minute (SFPM).
* D is diameter of the workpiece in inches.
How must spindle speed be adjusted when workpieces are turned down to smaller sizes?
The actual cutting speed decreases as the workpiece gets smaller if the rpm remains constant. Therefore, the spindle speed must be set higher for smaller work pieces to maintain the correct cutting speed.
How is feed rate measured on a lathe?
It is s the speed at which the tool is fed into the workpiece. It is measured in millimeters or inches per revolution.
What is the correct chip size to produce on a lathe?
Long chips can be dangerous, so produce short, broken chips.
What are turning tools used for?
They are used for cutting diameters. A right-hand turning tool (the most common) cuts from the tailstock to the headstock, while a left-hand turning tool cuts from the headstock to the tailstock.
On the cross-feed, how is a direct reading dial read?
Each line on the dial represents 0.001 inch (0.01 mm) on the diameter of the workpiece. This means that, when the dial is turned through one increment (such as 0.001 inch), the diameter of the workpiece changes by 0.001 inch.
On the cross-feed, how is an indirect reading dial read?
Each line represents 0.001 inch on the radius, meaning that, when the dial is turned through one increment (such as 0.001 inch), the diameter of the workpiece is reduced by 0.002 inch (twice the radius).
What should the feet rate be on a roughing cut?
Usually between 0.015 and 0.030 inch per revolution.
When facing a material what is the general rule for distance from the chuck?
Stay within two to three diameters of the workpiece from the chuck. Use a steady rest if you need to face at a greater distance.
When parting a hollow workpiece, what must be done to ensure a good cut?
When parting a hollow workpiece (pipe) the inside diameter (ID) must be bored true or the tool will break at the end of the cut.
When boring on a lathe, what must be done with the hole to be bored before starting the operation?
A boring bar enlarges an existing hole but cannot start a new one. Drill the hole first using a drill slightly smaller than the bore.
How do you reduce chatter when boring?
- Make the set-up as rigid as possible.
- Mount the boring bar with the minimum overhang.
- If chatter occurs, reduce the spindle speed and increase the feed rate until the chatter ceases.
- Dampen the chatter by holding a piece of wood against the boring bar.
- Use a boring bar made of carbide. Carbide boring bars are more costly, but they chatter less.
What is the most important part of alignment when reaming a hole?
Ensure the tailstock is properly aligned. If it is not, the reamer acts like a boring bar and produces an oversized hole.
At what angle must the threading tool align with the axis of the workpiece?
Use a thread gauge to set the axis of the threading tool at a right angle to the axis of the workpiece.
If using straight depth to calculate the final depth of cut when cutting treads, what is the formula to calculate straight depth of cut?
Straight depth of cut = 0.6134 x pitch
What is the purpose of the thread chasing dial?
It tells you when to close the split nut.
In the metric system how is taper expressed?
As a taper ratio measuring distance changed per 1 mm.
1:N
where N equals distance travelled.
In the imperial system how is taper measured?
As a taper per foot (TPF) or the amount of change in diameter of the piece over one foot of travel.
What is included angle and centerline angle?
Included angle = the angle between the sides.
Centerline angle = the angle between one side and the centerline.
How is the tailstock used to create a taper?
Offset the tailstock a certain distance and then hold the work between centers. Use this method for cutting shallow tapers because of the limited offset of the tailstock.
How do you calculate the imperial amount of tailstock offset for a desired taper?
TSO = (TPF x OL)/24
Where
TSO = tailstock offset
TPF = taper per foot
OL = Overall length
How do you calculate the metric amount of tailstock offset for a desired taper?
TSO = [(Dia - dia)/(2 x TL)] x OL
Where
TSO = tailstock offset
Dia = Large diameter
dia = Small diameter
TL = Taper length
OL =Overall length
