Mechanicals

CNC Lathe - Mechanicals

Bed

The Bed is the base structure of the lathe that holds all its other key moving and stationary parts – the headstock at one end and the tailstock at the other, guideways for the cross slide, the cross slide, etc.

 

The bed must have these key characteristics:

  • High rigidity, to not deform under the action of cutting forces and hence ensure high part accuracy.

  • Good vibration damping properties to reduce the vibrations caused during cutting and ensure high part accuracy and surface finish.

 

The bed is typically made of Cast Iron or Steel. Cast Iron has traditionally been preferred because of its better vibration damping and thermal stability characteristics. The bed is occasionally even made of composites like Cement Concrete or Polymer Concrete (where a polymer is the binder instead of cement), which have much better damping.

 

Flat bed lathe

Slant bed lathe

 

The slant bed design is preferred over the flat bed because:

  • In a slant bed chips fall towards the bottom fall due to gravity, where they can be carried away by a chip conveyor before they can ruin machine accuracy by transferring heat to guideways and castings.

  • The turret on a slant bed is tipped toward the operator, making tool mounting, inspection and maintenance easier than on a flat bed lathe.

Guideways

 

The function of guideways is to ensure that the machine element carrying the workpiece or cutting tool moves along a straight line path. The key requirements of guideways are:

  • High accuracy and high surface finish.

  • Durability - ability to retain the surface finish and accuracy over time.

  • Low frictional forces to ensure low wear and reduced motor power.

  • High rigidity.

  • Good vibration damping properties.

 

The contact between a guideway and the element moving over it can be:

  • Sliding contact resulting in sliding friction - the carriage slides over the guideways on the bed.

  • Rolling contact resulting in rolling friction which is much lower – the sliding surfaces are replaced by LM (Linear Motion) guides.

 

Lathe with ground guideways

Source: Ace Designers

Lathe with LM guides

Source: Ace Designers

बॉंल स्क्रू

बॉंल स्क्रू मोटर्सची वर्तुळाकार गतिमानता (rotary motion) एकरेषीय गतिमानतेमध्ये (linear motion) बदलतो. हा लीडस्क्रू (leadscrew) सारखाच असतो परंतु यात स्क्रू आणि नट यांच्यामध्ये सरकण्याचा संपर्क (sliding contact) नसतो. स्क्रू आणि नटमधील आटे (threads) यांच्यावर अर्धवर्तुळाकार छेद (semi-circular cross section) असतात आणि त्यांच्यावरून बॉंल्स घसरत जातात. बॉंल स्क्रूमध्ये परिघर्षण (rolling friction) हे अधिघर्षणापेक्षा (sliding friction) खूपच कमी असल्याने बॉंल स्क्रूची मेकँनिकल क्षमता ही नेहमीच्या एक्मे (Acme) थ्रेडस् असलेल्या लीड स्क्रूच्या ५०% मेकँनिकल कार्यक्षमतेपेक्षा जास्त म्हणजे ९०% इतकी असते. यामुळे एक्सीस मोटरसाठी लागणारी शक्ती कमी लागते आणि लहान ताकदीची मोटर उपयोगात आणली जाऊ शकते.

बॉंल स्क्रूवरील थ्रेडस् त्यांची अचूकता वाढविण्यासाठी घासलेले (ground) असतात. स्कृमधील बॉंल्स सतत फिरत राहण्यासाठी (recirculating) नट्समध्ये खास व्यवस्था केलेली असते. बॉंल स्क्रू हे लीड स्क्रूपेक्षा खूप महाग असतात.

स्क्रू बेडला गाईडवेजमध्ये जोडलेला/बसविलेला असतो आणि बेअरिंग्जमध्ये दोन्ही बाजूना फिरतो. नट हा मार्गिकेला (carriage) जोडलेला असतो.

 

Ball screw - animation

बॉंल्स स्कृच्या आतील भागात सतत फिरत राहण्याची यंत्रणा

कार्यरत असलेले बॉंल स्क्रू

चल/गती असलेला झेड एक्सीस --  लेथच्या  मागील बाजूने दृश्य - झेड एक्सीस मोटरची नोंद घ्या

स्त्रोत: Ace Designers

निरनिराळ्या आकारांचे बॉंल स्क्रुज

 

 

 

 

कट सेक्शन

 

 

 

 

 

मशीनवरील बॉंल स्क्रू

Source: Ace Designers

LM Guides

LM ( Linear Motion) Guides eliminate sliding friction between the guideways and the moving elements. Sliding friction is replaced by rolling friction.

Because of the high mechanical efficiency, the motor torque required is lower and the size of the axis motor is reduced. Since there is no sliding motion between parts, wear of the parts is very low and operational life is increased.

The key benefits of LM guides are:

  • High rigidity
  • Long service life
  • High accuracy
  • Quieter and smoother operation

The rails are bolted onto the guideways, and the blocks are bolted onto the underside of the carriage.

 

LM Guide

 

 

 

 

 

Internals

 

Cut section

Pic.

 

 

LM guides on machine

Source: Ace Designers

Spindle

The spindle does the job of holding and rotating the workpiece at the programmed speed with minimal vibration.

The spindle rotates in highly accurate bearings to reduce runout. At one end it holds the workpiece , and at the other it is connected to the spindle motor. The motor is connected to the spindle through a timing belt and pulley. A timing belt is used to avoid slippage and loss of power.

The spindle, gearing, bearings and sometimes the motor, are housed in the headstock. The headstock is essentially a housing that protects the enclosed machinery from contamination and damage.

Cut section

Spindle on machine

Source: Ace Designers

 

Spindle in action

Source: Ace Designers

Spindle in motion – view from the rear of the lathe. Note the belt going down to the motor.

 

Spindle back view

Source: Ace Designers

 

Chuck

The chuck typically has 3 jaws and is self-centering – the jaws move in and out simultaneously. The jaw movement is controlled hydraulically. The radial movement of the jaws, the jaw stroke, is typically between 2 to 5 mm. The chuck can be opened and closed through a command in the NC program. It can also be manually opened and closed using a foot pedal in front of the lathe.

 

The jaw force is controlled by a knob on the console, and a gauge shows the force. The jaw force is adjusted based on the geometry of the part. E.g., a slender tube can be crushed if the holding force is too high, so you need to reduce the force for such a part.

 

Chuck on a machine

Source: Ace Designers

 

Chuck

Chuck opening and closing

Source: Ace Designers

 

Chuck holding a part

Jaw motion in a 3-jaw self-centering chuck

 

Chuck without jaws

Source: Ace Designers

 

Chuck with hard jaws

Source: Ace Designers

 

Chuck foot pedal

Source: Ace Designers

 

Chuck force setting

Video ClampingPressure.mpg

Setting the clamping force of the chuck

Source: Ace Designers

 

Tool changer - Turret type

Tool changer - Gang type

Steady rest

Tailstock

Quiz

Compared to sliding guideways, friction in LM guides is (single choice Question)

  • Less
  • More
  • Same

Which of these is a slant bed lathe ? - single choice image question

A ballscrew has that move inside between the nut and screw. Filling the blanks

 Balls

Drag and drop the text in the right column to match the text in the Left column.

  • Slant bed
    Better chip disposal
  • Flat bed
    More floor space
  • Sliding guideways
    More friction
  • Ball screws
    Less friction

Mark each statement as True or False

  • LM guides have steel balls in them.
  • Flat bed lathes occupy less floor space than slant bed lathes.
  • Ball screws convert rotary motion of motors to linear motion.

Click on the area that is shows the Nut.