August 2007 Edition
MACHINING Rxs FROM DR. J
The Need for Speed
Don’t confuse speed with acceleration. Determining the right parameters of each can affect productivity.
There has been a dramatic change in the metal-cutting market for both producers and users of machine tools in the past 10 to 15 years. As Thomas Friedman notes in his best-selling novel,
The World Is Flat, the rapid developments in software, the Internet, and communications, coupled with the emergence of Brazil, Russia, India, and China, has changed the ways of doing business forever.
Simultaneously, there is a lack of skilled labor from technical schools and a reduction of well-educated manufacturing engineers in the U.S. for a variety of reasons. Combining these factors places a significant amount of pressure on the U.S. manufacturing community to improve productivity with fewer resources. The primary objective to succeed in this new environment is doing things right the first time with the right technology.
Technically speaking, how much analysis is required to do things right the first time? The truth is, it takes a substantial amount of work to ensure that things go right. However, the amount of work is completely dependent on the starting point for every project. How many projects have started with the wrong specification, and then been reworked to achieve success? The combination of gut feel, experience, keeping up to date with the latest technologies, and ability to perform back-of-the-envelope calculations are all tools employed by a successful manufacturing applications engineer. While experience and good gut feel cannot be taught, my intention is to present a common sense approach to technology that can benefit you.
Understanding machine specifications is the heart of application development. Knowing which machine to select with the appropriate specifications enables the prediction of machine tool performance prior to running the program on the machine. In this month’s column, I’ll discuss one of the primary factors that affect productivity – rapid rates and its relationship to acceleration.
Ask this simple question, “Is rapid rate more important than acceleration to an application?” This is akin to asking, “Should I buy a car based on top speed or how fast it goes from zero to 60?” The short answer? It depends.
On what does it depend? The application.
Consider the following machine types in the table below.
Machine One has a lower rapid rate with higher acceleration compared to Machine Two. Machine Three has the highest rapid rate and acceleration.
Which one to choose?
If the application consists of long, continuous cutting moves, it is restricted by the allowable feed rate for the material; think driving on a congested freeway. Any of these three machines are typically acceptable.
If the application calls for a lot of short rapid moves because there are a lot of tool changes, positioning moves, or changes in direction – like driving in a city – then acceleration is going to be important.
Note that depending on the typical length of the moves, the machine with the lower rapid rate could be more productive than the machine with a higher rapid rate.
Of course, if both high acceleration and rapid rate, with Machine 3 for example, were available, that results in the highest productivity. However, it may be cost-prohibitive.
Performing these sorts of analyses provides a better understanding of which situations require certain performance features. I hope I have given you a taste of what’s to come. If you have a technology topic that you’d like me to address, send me an e-mail in care of
pnofel@nelsonpub.com.
Have a machining question or dilemma?
The Doctor is in. If you have a topic you’d like the good doctor to address, send it to Dr. J at
pnofel@nelsonpub.com.
Jairam Manjunathaiah, Ph.D., – Dr. J – serves as vice president of engineering for MAG Fadal, a division of MAG-IAS. He leads the new product development, product maintenance, and engineering team.
The Basics
Here is a quick primer on speed and acceleration.
Rapid rate is speed, which is the distance covered in a unit of time. In the machine tool world, rapid rate is typically measured in inches per minute – ipm – or meters per minute. Acceleration is a measure of how quickly it can get to a speed. It has units of (distance/time)/time, or distance/time2.
Rapid rate is typically measured as a ratio of acceleration due to gravity – 9.8 m/s2 – and given the units of gravity, g. A 1g acceleration is the ability get up to 9.8m/s [23,150 ipm] speed in one second.
A car that gets to 60 mph [63,360 ipm] in 6.0 seconds, has an acceleration of 0.46g.
Here are a few formulae to calculate some key parameters for motion starting from a standstill:
Acceleration a required to get to final speed v in time t: a=v/t
Distance traveled during this time t: S=½at2= v2/2a
Note that one has to ensure consistency of units. If you work with inches and minutes, acceleration has to be expressed as ipm2
and not in g units to make the equations work.
What do you think?
Will the information in this article increase efficiency or save time, money, or effort? Let us know by e-mail from our website at
www.ModernApplicationsNews.com or e-mail the editor at
pnofel@nelsonpub.com.