Chapter 5: Machine Shop Estimation – Turning Operations

Abstract:
To estimate turning operations, you'll need to consider parameters like cutting speed, feed rate, depth of cut, and spindle speed, which are crucial for determining cycle time and material removal rate. 
Here's a breakdown of the key aspects:
1. Cutting Speed (V):
  • Definition: The velocity of the workpiece surface relative to the cutting tool. 
  • Units: Often measured in surface feet per minute (SFM) or meters per minute (m/min). 
  • Calculation: V = (π * D * N) / 1000, where D is the workpiece diameter in mm and N is the spindle speed in RPM. 
2. Feed Rate (f):
  • Definition: The distance the cutting tool moves relative to the workpiece during one revolution of the spindle. 
  • Units: Typically measured in inches per revolution (IPR) or millimeters per revolution (mm/rev). 
  • Feed rate is also calculated as: Cutting length per minute divided by the spindle speed (RPM). 
3. Depth of Cut (d):
  • Definition: The amount the cutting tool penetrates into the workpiece during one pass.
  • Types: Axial depth of cut (facing operations) and radial depth of cut (turning/boring operations). 
4. Spindle Speed (N):
  • Definition:
    The rotational speed of the spindle and workpiece, measured in revolutions per minute (RPM). 
  • Calculation:
    N = (V * 1000) / (π * D), where V is the cutting speed in m/min and D is the workpiece diameter in mm. 
5. Cycle Time (Tc):
  • Definition: The total time required to complete a turning operation. 
  • Calculation: Tc = (Length of Cut) / (Feed Rate * Spindle Speed). 
  • Formula: Tc = lm / l, where lm is the length of the cut and l is the cutting length per minute. 
  • l = f × n: where f is feed rate and n is spindle speed. 
6. Other Factors:
  • Material: Different materials require different cutting parameters. 
  • Tool Geometry: The shape and type of cutting tool influence the cutting process. 
  • Machine Capabilities: The lathe's power and speed capabilities limit the achievable cutting parameters. 
  • Surface Finish: Feed rate and cutting speed influence the surface roughness of the machined part. 
  • Cost: Material cost, tool cost, and machine time all contribute to the overall cost of a turning operation. 
Example:
If you have a 100mm workpiece that is machined at 1000 RPM with a feed of 0.2mm/rev, the cutting time is: 
  1. l = f × n = 0.2 × 1000 = 200(mm/min)
  2. Tc = lm ÷ l = 100 ÷ 200 = 0.5(min)
  3. 0.5 × 60 = 30(sec)
So now let's dive deeper into Chapter 5: Machine Shop Estimation – Turning Operations:

5.1 Introduction to Turning Operations

Turning is one of the most widely used machining operations in machine shops. It involves the removal of material from the outer diameter of a rotating workpiece using a cutting tool. The primary objective of turning is to achieve a desired shape, size, and surface finish.

Turning is typically carried out on a lathe machine and is essential in manufacturing cylindrical parts such as shafts, rods, bushings, bolts, and similar components.

Objectives of Turning Operations:

  • Reduce material from workpiece diameter.

  • Produce cylindrical surfaces.

  • Achieve dimensional accuracy and surface finish.

  • Prepare components for further operations (like threading or grooving).


5.2 Types of Turning Operations

  1. Straight Turning – Producing uniform diameter along the workpiece.

  2. Taper Turning – Producing a conical surface by gradually reducing or increasing the diameter.

  3. Step Turning – Machining multiple diameters in steps.

  4. Facing – Machining the end face of the workpiece.

  5. Parting – Cutting off a portion of the workpiece.

  6. Grooving – Creating grooves or recesses on the surface.

  7. Threading – Producing screw threads on the workpiece.

  8. Knurling – Producing textured surfaces for better grip.


5.3 Elements Involved in Estimating Turning Operations

To estimate turning operations accurately, consider the following parameters:

  • Material of the Workpiece: Affects cutting speed and tool wear.

  • Dimensions of the Workpiece: Length and diameter.

  • Depth of Cut: Amount of material removed in one pass.

  • Feed Rate: Movement of the tool per revolution (mm/rev).

  • Cutting Speed (V): Surface speed of the workpiece relative to the tool (m/min).

  • Machine Time (Tm): Total time taken to complete the operation.


5.4 Estimation of Time for Turning

Basic Formula:

Where:

  • = Length of cut in mm

  • = Feed rate in mm/rev

  • = Spindle speed in RPM

Spindle Speed (N):

Where:

  • = Cutting speed in m/min

  • = Diameter of the workpiece in mm


5.5 Example Problem

Problem: Estimate the machining time for turning a 150 mm long, 60 mm diameter mild steel bar to 50 mm in one pass. Use cutting speed , feed rate .

Solution:

  1. Spindle Speed:

  1. Machining Time:

Answer: Approximate machining time is 3.14 minutes.


5.6 Estimating Material Removal Rate (MRR)

Where:

  • = Original diameter (mm)

  • = Depth of cut (mm)

  • = Feed rate (mm/rev)

  • = Spindle speed (RPM)


5.7 Cost Estimation in Turning Operations

Cost estimation in turning includes:

  • Material cost (based on volume removed and scrap).

  • Machining cost (based on time, labor, and machine rate).

  • Tool cost (wear and replacement).

  • Overhead cost (utilities, supervision, maintenance).

Total Cost = (Machining Time × Machine Hour Rate) + Tooling Cost + Material Cost + Overhead


5.8 Factors Affecting Estimation Accuracy

  • Variability in material properties.

  • Tool wear and replacement frequency.

  • Machine efficiency and idle times.

  • Operator skill level.

  • Coolant and lubrication usage.


5.9 Tips for Optimized Estimation

  • Always refer to machining handbooks for cutting data.

  • Use conservative estimates for safety margins.

  • Update feed and speed values with latest tool technologies.

  • Consider additional time for setup, measurement, and inspection.

  • Use computer-aided tools and software for complex estimations.


5.10 Summary

Turning operations are vital in metalworking, and proper estimation of time and cost is critical to planning and cost control in machine shops. Understanding parameters like feed, speed, depth of cut, and material properties allows for precise machining and efficient production planning. Using formulas and applying practical knowledge ensures accurate estimation and optimized machine shop operations.

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