Tuesday, December 29, 2020

Determining Machining Operation Type and Depth of Cuts for Multiple Cuts - Process Planning and Process Industrial Engineering

Lesson 69 of Industrial Engineering ONLINE Course.

Production Process Planning - Sub-Module of Process Industrial Engineering



Recommended Text: Process and Operation Planning by Gideon Halevi

Chapter 8. How to Determine the Type of Operation


In this chapter method of determining depth of cut and feed are given. In the next chapter, determination of cutting speed is discussed.

1 Boundary Limit Strategy

Technical constraints are set as boundary limits, and then, bearing in mind economic considerations, working point within these limits are determined.

1.1 Technological constraints

Metal cutting theory indicates, minimum and maximum values for depth of cut, feed rate and cutting speed.

If the depth of cut is very less, the metal of the workpiece will only get compressed and will spring back when the took is passed and chip formation will not take place. Similarly, a low feed rate chip forming will not take place and only abrasive forming will take place. Similarly if feed is very high, tool wear process will become crater wear. The required tool wear is flank wear so that there is a definite tool life. Crater wear gives  sudden failure. Hence feed rate has to be within a limit for the tool to be within flank wear process.

Similarly in the case of cutting speed also, above a cutting speed, the resulting temperature creates diffusion wear in the tool. At low cutting temperature, the built up edge occurs and proper cutting will not take place.  Hence there are lower and higher limits to cutting speed.


Boundary value as per technological constraints

a(tmax)  = maximum depth of cut
a(tmin)  = minimum depth of cut

f(tmax)  = maximum feed
f(tmin) = minimum feed

v(tmax)  = maximum cutting speed
v(tmin) = minimum cutting speed

1.2 Part specification constraints

To meet specified surface roughness and tolerances, the feed rate and depth of cut have to be restricted to maximum values.

a(smax)  = maximum depth of cut  (surface finish specification provides the limits)
f(smax)  = maximum feed

1.3 Definition of material constraints

The work  material also has an effect on allowable maximum depth of cut and cutting speed.

a(hmax)  = maximum depth of cut  (h denotes hardness of the material, an important property)
v(hmax)  = cutting speed

1.4 Machine constraints

Vibrations are associated with chip formation and they affect surface finish. These vibrations reduce tool life also.

Experiments indicate that very low depth of cut and feed cause vibrations (chatter). 
Some authors state 0.06 mm as the lowest limit for depth of cut
Similarly, a feed rate value of 0.04 mm per revolution is specified as the minimum feed rate below which chatter occurs.

There is also a maximum depth of cut and cutting speed above which chatter occurs. The author recommends the following values

a(vmax)  = maximum depth of cut due to chatter (7 mm)
a(vmin)  = minimum depth of cut due to chatter (0.15mm)

f(vmin) = minimum feed rate due to chatter (0.04 mm/rev)

v(vmax) = maximum cutting speed due to chatter  - not specified

1.5 Definition of tool constraints

The tool material also gives boundary conditions to cutting parameters. At high feed rates, plastic deformation of the cutting edge takes place which is not desirable. 
Recommendation: Use a maximum value of feed rate between 0.5 mm per revolution to 0.8 mm per revolution for a cutting speed of 150 meters per minute

There are limits for depth of cut and cutting speed.

a(kmax)  = maximum depth of cut related to tool material
f(kmax)  = maximum feed related to tool material
v(kmax) = maximum cutting speed due to chatter related to tool material

1.6 User specified constraint

If user provides any limits based on his experience they have to be taken into account by process planners.

a(umax)  = maximum depth of cut indicated by user.
f(umax)  = maximum feed related indicated by user.
v(umax) = maximum cutting speed indicated by user.

1.7 Boundary limits summary

The above boundary value symbols are listed as per depth of cut, feed rate and cutting speed.

From the list of boundary values for a given situation, the minimum value is selected for maximum values, and the maximum value is selected for minimum values. These selected values are given the symbols.

a(amax)  a(amin)  f(amax)  f(amin) v(amax)  v(amin)     a indicates acceptable value for various cutting parameters.

2 Analysis of Cutting Conditions vs Part Specifications

The part specification gives the required tolerances and surface roughness. The part has to machined to conform to these specifications

2.1 Effect of cutting speed on surface roughness

A low cutting speeds BUE will occur and it scratches surface giving poor surface finish. As cutting speed increases above a limit, surface burns occur.

Halevi in 2003 book said, tool material and machine rigidity are being improved constantly, and hence recently declared values have to be ascertained and used. 

In the absence of specific values, Halevi suggested the values of 400 meters per minute as the upper limit and 60 meters per minute as the lower limit on steel parts with tool material being carbide.

2.2 Effect of feed rate on surface roughness

2.2.1 Turning processes

Halevi derived the formulas:

feed rate f mm/rev =  0.1Ra  for Ra <= 3.2    Ra is in micrometers
feed rate f mm/ rev = 0.18 Ra^0.5  for Ra>3.2

The above equations are used for finish cut and it is given the symbol f(smax) maximum feed rate based on surface finish criterion. For rough cuts f(amax) has to be used.  A table is provided for maximum feed rate and maximum depth as function of Ra and BHN. Refer Table 1.

2.2.2 Milling processes  (Not covered)

2.3 Effect of depth of cut on surface roughness

Maximum depth of cut limit based on surface roughness criterion

a(smax) = 32Ra/(BHN^0.8)

Example: Ra = 1.2 microns;  BHN  =  160

BHN^0.8 = 57.98

a(smax) =  32(1.2)/57.98 = .66 mm



3 Operational and Dependent Boundary Limits

As process planner makes decisions, these decision create limits for other variables.

3.1 Depth of cut as a function of feed rate

As the feed rate is determined, it puts a limit on the depth of cut. There will be a limit on the cutting force depending on the machine rigidity. The force is related to feed and depth as per the formula

Fx = (Cp)(a^u)(f^v)

Where Cp = specific cutting force (for medium steel  it is 220)
a = depth of cut and exponent u = 1.0
f = feed rate and exponent v = 0.75

3.2 Depth of cut as a function of a selected operation

In rough cut operation maximum depth of cut possible as per the boundary conditions is used and any remaining deviations from final specification have to be modified through finish cuts. The amount of material to be removed to correct deviations is given the symbol a(smin) and its value is added to a(amin) and the total has to be less than a(smax) which is the maximum depth of cut allowed for the finishing cut from the boundary limits.



4 The Algorithm for Selecting Cutting Operations

5 Example of Using the Algorithm



Current Tools

Iscar
10mm Max Depth, 4mm to 5mm Width, External Left Hand Indexable Grooving Toolholder
135mm OAL, 25mm x 25mm Shank, Uses GI.. Inserts, GHD Toolholder, Series Cut Grip
https://www.mscdirect.com/product/details/52713054

Update 29 Dec 2020
First 20 July 2020



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