No12 Feeds Speeds Depth Cut

7/28/2019 No12 Feeds Speeds Depth Cut

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FEEDS, SPEEDS & DEPTH OF CUT

OVERVIEW

The three key mechanical inputs in metal

removal operations are eed, speed, and

depth o cut. Manipulating the eed, speed

and depth o cut can maximize the benets

o a particular cutting fuid and can increaseproductivity. However, like most decisions, the

choice o eed, speed and depth o cut must

be based on the customers objectives. What

is their goal in this application? Do they want

to manuacture parts aster or maximize tool

lie? How important is the surace nish and

dimensional accuracy o the part? Answers to

these questions will drive their decisions on

eeds, speeds and depth o cut.

DEFINITIONS

Speed: Speed is the rate o rotation o the

spindle where the tool is held. It is measured in

revolutions per minute (RPMs).

Feed: Feed is the rate at which the tool is

moved into the part or the part into the tool.

Feed is measured in eet, inches or millimeters

per time period.

Depth of Cut (DOC):The measurement

(normally in inches or millimeters) o how wide

and deep the tool cuts into the workpiece.

Speed, eed and DOC work together to

determine the Metal Removal Rate (MRR).

MRR is calculated as ollows:

MRR = Feed Rate X Width o Cut X Depth oCut. Speeds, eeds and DOC infuence many

aspects o machining perormance:

Tool lie

Surace nish

Dimensional accuracy o the

manuactured part

Power required by the machine tool

The useul lie o a tool is a measurement o

how long the tool will unction and still provide

acceptable parts. Innite tool lie can be

achieved by not using the tool at all. O course

productivity would be zero and this would

achieve nothing!

TOOL LIFE

Tool lie, as measured by the amount o

material removed, may actually increase when

the DOC is increased. However, tool lie, as

measured by time, will likely decrease when

the DOC is increased. Sometimes customers

measure tool lie by how many quality partsthey produce beore a tool change. How does

your customer measure tool lie?

Generally, increasing the eed rate reduces tool

lie. Removing more material creates more

heat. Heat degrades the work piece and the

tooling. I you reduce your eed rate, the tool

lie improves because it is not working as hard.


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All o this assumes that the concentration

o the metal removal fuid has remained the

same. In order to cut aster or deeper (i.e.

DOC), increasing the concentration provides

more lubrication. There needs to be a balance.

Spending a little more in coolant can producetremendous savings in tooling.

Coolant is always cheaper than tooling. Ask

your customer i there is interest in reducing

tooling costs.

SURFACE FINISH

As we learned in our Surace Finish Skill

Builder, surace nish or smoothness o

the nal work piece can be measured with

various devices and recorded in various units

o measure. Surace nish is important to

many customers. Without sucient lubrication

(i.e. proper fuid and concentration), surace

nish will generally decrease as you increase

the spindle speed. It may be worth it to the

customer to increase the cost o the coolant

(with a slightly higher concentration or higher

lubricity fuid) to improve the surace nish

at higher speeds. O course, this means that

surace nish has to be worth something to

your customers customer.

Increasing the DOC can promote chatter

because o higher orces. The machine tool

must be rigid enough to withstand these

orces. Vibrations in the machine tool can

have a large eect on surace nish. Machine

tools designed and engineered or high speed

machining have improved stiness to minimize

the vibration that impacts surace nish.

Ask your customer i their machine tool has

sucient stiness to achieve the surace nish

theyre seeking.

DIMENSIONAL ACCURACY

The overall accuracy o the part is dened asmeeting all o the dimensions as required by

the engineering diagram. Dimensional stress

occurs when cutting orces and defections

are not controlled. Whereas surace nish

is usually measured on a small section o a

part, dimensional accuracy relates to a larger

section or the entire part.

The Metal Removal Rate (MRR) is a key

variable eecting part accuracy. As the MRR

increases, the chances o decreasing part

accuracy increases. In simpler terms, more

power is required to remove more metal.

Unless the stiness o the tool is compatible

with the higher horsepower, then defection is

likely to occur. Defection leads to inaccuracy.

Thermal issues are also a consideration. Some

machine tools are very susceptible to changes

in their environment. It has been proven that

many machine tools do not produce the same

quality parts throughout the day unless the

shop foor temperature is properly controlled.

Also, the alloy being machined could be

susceptible to temperature changes. Many

operations that are sensitive to temperature

changes will employ the use o chilled

coolant. The fuid is maintained at a constant

temperature with a heat exchanger. By holding

the temperature o the fuid to a constant

temperature, there is decreased deviation in

the alloy.


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POWER REQUIREMENTS

Horsepower is an important requirement

when assessing the correct machine tool or a

specic application. There are specic tables

in machining data handbooks that provide

guidelines as to how much horsepower isrequired to unction in a certain application.

The more work your customer is trying to

achieve, the more power is required. As the

metal removal rate increases, the horsepower

to support it increases as well. Ask your

customer i their machine tool has been

conrmed as having the horsepower necessary

to do the job. An important realization is that

with dierent combinations o eed and speed,

with the same MRR, the application with the

highest eed rate will require less power.

CONCLUSION

In your next discussion with your customer, a

review o speeds and eeds should be easier.

There are several published guides that give

recommended starting points or eed, speed

and DOC or specic operations and metals.Depending on the customers objectives,

you may want to suggest deviating rom the

standard settings. Find out what the customer

is trying to achieve to understand how altering

speeds and eeds will help or hurt their

application. Being part o this conversation

will make you a more important part o your

customers team to success and will help you

demonstrate the best perormance your cutting

fuid can deliver.

No12 Feeds Speeds Depth Cut

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