Speaking of Precision Blog

Speaking of Precision

Speaking of Precision is a knowledge preservation and thought leadership blog covering the precision machining industry, its materials and services. With over 36 years of hands on experience in steelmaking, manufacturing, quality, and management, Miles Free (Milo) Director of Industry Research and Technology at PMPA helps answer "How?" "With what?" and occasionally "Really?"

Previous in Blog: Steel Velcro - New Metallic Hook And Loop Fastener   Next in Blog: What Does a Molecule Look Like?
Close
Close
Close
12 comments

5 Ideas To Reduce Stress in Precision Machined Parts

Posted July 27, 2010 10:47 AM by Milo

Stress and stress risers are words that we often hear in our shops. Usually when bad things have happened to our work. Here are 5 ideas to reduce stress in precision machined products and a brief tutorial on what it is when the engineers say "stress."

Looking at this, someone is going to say "stress riser".

Stress (when used by designers, engineers, and metallurgists) refers to the measurement of load on a part or test specimen related to the area under that load. Stress can be considered to be have three modes, axial (in line) , bending (you know what that means), or torsional (twisting or torque). The following graphic illustrates some stress states our parts may encounter.

Diagram of Simple Stress State

Residual stress can be considered to be a kind of "internal pressure" in the material which may act in the same direction as the stress applied to the part. Because of this, it can actually reduce the load carrying ability of the part. This is what usually results in failures. Characteristics of the part may also contribute to the concentration of these internal stresses, leading to premature failure of the parts once in service and subjected to load.

Here are 5 ideas to reduce stress in precision machined parts.

  1. Assure a smooth surface.
  2. Use a larger not smaller diameter for threading.
  3. Always maximize the fillet or radius between section or diameter changes.
  4. Provide both pads and relief areas on parts where applicable.
  5. Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction.

Some things never change . . .

Assure a smooth surface. The creation of a smooth surface prevents the concentration of internal stresses at sharp changes in surface. Parts with smooth surface finish are much less likely to fail than parts where deep grooves, tool marks or pits can allow stresses to build up.

Use a larger not smaller diameter for threading. This is both related to the strength of the additional material as well as to the geometry and radii between change of dimensions. The more generous radius possible with the larger diameter for threading can improve the endurance limit of the part substantially. In heat treated 4340, the increase in radius from 0.015″ to 0.090″ increase the endurance limit from 34,000 to 65,000 psi.

Always maximize the fillet or radius between section or diameter changes. Any design which allows stress to concentrate locally will promote fatigue failure. Generous radii and fillets are inexpensive insurance against premature failure.

Make sure that the designer has provided both pad and releif areas on parts joining perpendicularly. Instead of having a single point or locus for the change in forces to be distributed through the part, pads and relief areas diffuse the stresses that would otherwise be concentrated, improving the performance of the part.

Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction. Many of the materials that we prefer to machine are resulfurized, and in these steels, the manganese sulfides can in fact lower the steel's transverse mechanical properties. Also, cold drawing and or forging prior to machining can influence grain flow which can enhance the ability of the material to carry the load. The material the designer selected could be a large reason for the material's ability to handle stress, or not.

There you have it. Stress = Load. Don't give it places to concentrate on your precision machined parts.

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which was originally posted here.

Register to Reply

Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Associate
Hobbies - CNC - New Member

Join Date: Dec 2008
Location: Winnipeg, Manitoba, Canada
Posts: 39
Good Answers: 2
#1

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/28/2010 9:58 AM

Thank you for putting that out. In our maintenance shop we have seen hundreds of broken parts which were avoidable, had the failure not been designed in. We have learned over the years to pay close attention to avoiding "stress risers" in the replacement parts that we make. I just wish the original manufacturers would have done that.

One additional point- Even a large radius fillet, polished to a mirror finish will become a "stress riser" if it is allowed to corrode.

__________________
too soon old, too late smart
Register to Reply
Guru
Technical Fields - Technical Writing - New Member Engineering Fields - Marine Engineering - New Member

Join Date: Sep 2008
Location: Vancleave, Ms about 30 miles inland from Biloxi and the coast
Posts: 3198
Good Answers: 105
#8
In reply to #1

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/29/2010 11:32 AM

Quote:

"In our maintenance shop we have seen hundreds of broken parts which were avoidable, had the failure not been designed in. I just wish the original manufacturers would have done that."

Sounds like some Micky Mouse OEM's. You should be looking around for a better OEM. If the original part was of poor quality, buying or making a replacement would give you a new part with poor quality. You would have to redesign the part so it wouldn't fail.

__________________
Mr.Ron from South Ms.
Register to Reply
Associate
Hobbies - CNC - New Member

Join Date: Dec 2008
Location: Winnipeg, Manitoba, Canada
Posts: 39
Good Answers: 2
#9
In reply to #8

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/29/2010 11:45 AM

We service/repair/overhaul hundreds of different pieces of equipment from lots of manufacturers. Most is pretty good. As I said, we tend to see the stuff that isn't so good. We do redesign parts to prevent reoccuring failures.

__________________
too soon old, too late smart
Register to Reply Off Topic (Score 5)
Guru

Join Date: Jul 2006
Location: "Dancing over the abyss."
Posts: 4942
Good Answers: 243
#10
In reply to #8

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/29/2010 11:45 AM

That is one of the main points of this blog, ronseto- The importance of DESIGN to the success of the part.

Regards.

Milo

__________________
People say between two opposed opinions the truth lies in the middle. Not at all! Between them lies the problem, what is unseeable,eternally active life, contemplated in repose. Goethe
Register to Reply
Anonymous Poster
#2

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/28/2010 12:52 PM

Over a large flat surface of plate material where major machining has occurred stress will cause warping as well. Solve the problem by machining the part slightly over size and let the part set over night or longer. Re-clamp and make the final skim cut.

Register to Reply Score 1 for Good Answer
3
Power-User

Join Date: Dec 2009
Location: New Zealand
Posts: 122
Good Answers: 4
#3

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/28/2010 11:55 PM

You did not mention adding stress to reduce failure, using techniques such as fillet rolling, shot-peening etc.

Many failures are caused by tensile stresses resulting in cracks, if the surface has a compressive stress built into it, it is less likely to fail. This is what protects toughened window glass.

Register to Reply Good Answer (Score 3)
Guru

Join Date: Jul 2006
Location: "Dancing over the abyss."
Posts: 4942
Good Answers: 243
#5
In reply to #3

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/29/2010 8:39 AM

Thanks for the great addition!

Milo

__________________
People say between two opposed opinions the truth lies in the middle. Not at all! Between them lies the problem, what is unseeable,eternally active life, contemplated in repose. Goethe
Register to Reply
Participant

Join Date: Jun 2010
Posts: 1
#4

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/29/2010 8:00 AM

REPAIR PARTS - Although a portion of OEM parts possess an inferior design, often OEM parts are designed using specific materials or require processing that is not easily duplicated by a maintnenace repair department. This may include: use of through hardened alloy materials; case hardened materials; localized hardening for bearing races; localized shot peening to achieve compressive stresses or even polisihing to remove machining marks that could act as stress risers.

Typical examples may include: maintenanced repaired machined splines or gear teeth vs. OEM rolled splines & gears; OEM axle shafts that have very distinct heat treated metalurgical hardening profiles and wear coatings applied to rotating steel shafts.

Often the replacement cost of an OEM part (if avaialble) in critical applications is a wise investment.

Register to Reply Score 1 for Good Answer
Associate
Hobbies - CNC - New Member

Join Date: Dec 2008
Location: Winnipeg, Manitoba, Canada
Posts: 39
Good Answers: 2
#6
In reply to #4

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/29/2010 9:25 AM

Amother good point. Yes most of what we use is well designed and manufactured. However the maintenance department doesn't get to see the good stuff much! Because we only see the failures, perhaps our point of view is skewed.

I will point out that we do, or have done for us, all of the above except thread/spline rolling. And yes, if the OEM piece is good, and available,that is our preference.

__________________
too soon old, too late smart
Register to Reply
Anonymous Poster
#7

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/29/2010 9:50 AM

Hello everyone,

A number of years back Mclaren of Formula 1 fame had sent out to the engineers of the world a request to find a better machining process that would not inherently add the microscopic cracks to the material produced by cutting bits. Under a microscope these cracks would resemble an image of a file, not as profound but smoother, where the cracks would be found on the leading edge of the metal as the cutting bit would "tear off" metal as it went by. Even EDM, as soft and slow a process, was considered to produce hot spots leaving behind heat stress in the surface thereby forming a stress riser.

Sure grinding and polishing will produce a perfectly smooth surface but grinding itself produces these "cracks" and polishing simply smears metal over the surface giving the illusion of a mirror finish.

Never heard of what was finally discovered by Mclaren or if there even was a better way to eliminate stress risers during machining.

Greg

An idea is simply the rearrangement of our prejudice's.

Register to Reply
Guru

Join Date: Feb 2008
Location: Large hole formally occupied by furry woodland creature.
Posts: 3385
Good Answers: 97
#11

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/30/2010 11:00 PM

I had wondered how the rotor shafts for the pumps I deal with were able to retain the minor diameter of 20mm of the drive end for a horsepower range of from 2.5 to 30 Hp. (so that one seal is common to all). After reading this, I examined a shaft and saw that the guidelines given here were followed to the smallest detail.

Thank you Milo

__________________
CRTL-Z
Register to Reply
Guru

Join Date: Jul 2006
Location: "Dancing over the abyss."
Posts: 4942
Good Answers: 243
#12
In reply to #11

Re: 5 Ideas To Reduce Stress in Precision Machined Parts

07/31/2010 7:34 AM

Thanks for the report from the sunny side of the street. We're pleased that these tools helped you make sense of your application.

Really glad to hear your motor shafts followed these guidelines and that yiu found this info helpful.

That has been one of my reasons for posting these blog entries.

Thanks again for the feedback.

Milo

__________________
People say between two opposed opinions the truth lies in the middle. Not at all! Between them lies the problem, what is unseeable,eternally active life, contemplated in repose. Goethe
Register to Reply
Register to Reply 12 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (2); Milo (3); ohiohacker (1); RickZillman (3); ronseto (1); Smeaton (1); Unredundant (1)

Previous in Blog: Steel Velcro - New Metallic Hook And Loop Fastener   Next in Blog: What Does a Molecule Look Like?

Advertisement