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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?"

6 Reasons Steel is Vacuum Treated

Posted May 15, 2015 10:00 AM by Milo
Pathfinder Tags: harden oxygen steel Vacuum

Vacuum treated (vacuum degassed) steel is used for critical applications that require steel with an exceptionally high degree of structural uniformity, internal soundness, and other characteristics which may be impaired by the effects of uncontrolled amounts of dissolved gases. Vacuum degassing treatments, along with various deoxidation practices are specified to control the amounts of dissolved gases in the steel. This post describes 6 benefits of vacuum treatment of steel.

Liquid steel after treatment in a Siemens RH degassing plant achieves improved properties.

Vacuum treatment of molten steel

  • Reduces Hydrogen content. This reduces the tendency of steel to "flake" or become "embrittled."
  • Reduces Oxygen content. This makes it easier for the steel to conform to restrictive microcleanliness requirements.
  • Improves the recovery and uniformity of distribution of alloying elements and other additives.
  • Helps control the composition of the steel closer than without vacuum treatment.
  • Results in higher and more uniform transverse ductility, improved fatigue resistance, and improved high temperature performance.
  • Can be used to achieve exceptionally low carbon content that are otherwise unobtainable by conventional means.

What are some situations where vacuum treatment is employed?

  • Large forgings and large cross sections where hydrogen would otherwise remain and contribute to flaking and embrittlement.
  • Bearings where uniformity throughout the section is important for critical performance.
  • Inverted delta, human critical safety applications where steel toughness and performance place high demands on the steels properties in all directions.

The removal of Oxygen by degassing is a challenge for the steelmaker, because this element is extremely reactive- it can exist in the steel in many forms, such as free oxygen, dissolved in the melt as a soluble nonmetallic oxide, can combine with carbon to form gaseous oxides, and it can exist as complex oxides in the accompanying slags and refractories in the process.

In post will describe some steel deoxidation practices and the types of vacuum degassing that are used in the North American steel Industry.

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which you can also read here.

2 comments; last comment on 05/17/2015
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Ra Is Different Than Rz

Posted April 10, 2015 11:15 AM by Milo
Pathfinder Tags: measurement Ra Rz surface finish

The methodology of measurement and what is measured are quite different. this is critical to understand if you will not be paid for your parts because the Ra you measured is not in fact the Rz surface profile that customer specified.

"Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results.

"Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Rz averages only the five highest peaks and the five deepest valleys-therefore extremes have a much greater influence on the final value."- George Schuetz, Modern Machine Shop

"Ra is the arithmetical average value of all absolute distances of the roughness profile from the center line within the measuring length. Rz is the average maximum peak to valley of five consecutive sampling lengths within the measuring length. Ra averages all measurements and does not have any discriminating value in separating rejects from acceptable cylinders."- Swedev

And by the way the definition of Rz has also changed over the years. Which definition of Rz exactly is your customer using? How do you know?

You will find "Conversion Ratios" on the internet provided by well meaning people. But how useful can these be when the range said to be equivalent goes from 4:1, to 7:1 to 2-:1?

4:1 is equivalent to 20:1? Really? Not in my math class!

Smart shops will avoid using these "approximations in name only" and communicate with their customers to determine the customer's true need. Gambling on conversion factors that you found on the internet is not professional, it is an example of poor engineering practice, and it fails to serve and protect your customer.

Don't Do It!

Read this well written, not terribly mathematical treatment of the subject on MMSOnline

It's a classic.

Authoritative standards : Surface finish measurement procedures, general terminology, definitions of most parameters and filtering information can be found in American Standard ASME B46.1 - 2009, Surface Texture, and in International Standards, ISO 4287 and ISO 4288.

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which you can also read here.

Top 10 Facts About Leaded Steel Bars

Posted March 30, 2015 11:00 AM by Milo
Pathfinder Tags: leaded steel machining top 10

Leaded steel bars historically have been a mainstay raw material in the screw machining industry. As more applications and newer technology move away to non leaded steel applications, we thought that a brief refresher about Lead and its role in our shops might be timely.

The 0.15- 0.35 weight percent of lead contained in these bars helps them machine 25% faster with less power required.

  1. Leaded steel bars are standard steels and widely available. In the U.S. 12L14 is the predominant grade. 11SMnPb30, 11SMnPb28, 9SMnpb28, and 9SMnPb36are German designations nominally equivalent to 12L14.The Chinese version of 12L14 is Y15Pb; Japanese nominal equivalents include SUM22L, SUM23L, andSUM24L.
  2. Leaded steels are selected for use for the savings achieved in producing parts by machining.
  3. Leaded steels are not appropriate for all parts- and parts with low amounts of stock removal may not create any noticeable savings.
  4. Today's Leaded steels are more consistent, more uniform, than they were when produced by the ingot process.
  5. The decision to use Leaded Steels for a specific part must be based on the economics for that part- volume, stock removal, part complexity, tolerances required, surface finish needed are all factors that contribute to that economic calculation.
  6. There is no sacrifice in mechanical properties when adding lead to steel. neither longitudinal nor transvers mechanical properties are affected by the addition of lead to steel.
  7. Leaded steels are currently permitted under European Union Regulations covering End of Life Vehicles, RoHS.
  8. The reduction in energy required and time needed (about 25%!) to machine a part make leaded steels environmentally friendly by reducing Carbon Dioxide emissions to create parts compared to using unleaded materials.
  9. In order to be dangerous to humans, lead must be in a soluble form. The lead in steel bars is a separate solid phase. IARC lists lead under its Group 2B category - "possibly carcinogenic to humans".
  10. Lead, as well as Chromium, Copper, Manganese, Nickel, and Phosphorous is required to be reported under Sara 313 (40 CFR 372.65) when above thresholds.

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which you can also read here.

2 comments; last comment on 03/30/2015
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Unknown Controls To Protect You- EPA and Ozone Overreach

Posted March 27, 2015 11:15 AM by Milo

"PMPA urges EPA to maintain the current 75ppb standards for ozone. Allowing for the full and continued implementation of the current law will continue to drive significant reduction in ozone emissions. The proposed rule fails to demonstrate benefits, relies on "unknown controls," and fails to consider natural influences in ozone levels and attainment. "

The Precision Machined Products Association (PMPA) today filed official comments opposing the U.S. Environmental Protection Agency's proposal to lower the National Ambient Air Quality Standards (NAAQs) for Ozone to as little as 65ppb. This reduced standard would place virtually the entire U.S. in nonattainment status, ignoring natural influences, restricting economic activity and manufacturing production.

A recent study showed the EPA's latest proposal would lower U.S. GDP by $140 billion annually. At a 65ppb level, the entire state of Ohio falls into nonattainment status.

"The EPA needs to give the current standards a chance to work," said Miles Free, co-Interim Executive Director of PMPA. "The White House delayed the rules twice for other considerations, I think they should consider the impact on manufacturing and stay with the current levels. Current rules have resulted in an 18% drop in ozone emissions between 2000 and 2013, with an additional 36 percent reduction on deck."

Furthermore, over 60 percent of the controls and technologies needed to meet the rule's requirements are "unknown controls," according to EPA terminology. How are "unknown controls" a key step in attainment. How do "unknown controls" have credibility in Science based policy?

"Unknown controls" in science are like "Then a miracle happens' in Theology.

Due to the "unknown control"status, the new regulation will likely result in the closure of plants and the early retirement of equipment used for manufacturing, construction and agriculture. In the precision machining industry, well-maintained equipment can last decades and small businesses like our members can ill afford to invest millions of dollars in new machines because of an EPA regulation.

It is not clear how EPA plans on curtailing manufacturing during bouts of seasonal nonattainment, regardless of whether the basis is natural or manmade causes.

However, should the EPA's 65ppb standard take effect, virtually all PMPA members will find themselves in a nonattainment zone restricting their manufacturing activity.

Our shops can expect

  • Face EPA ordered restrictions on their production due to this rule
  • An EPA estimated 6-12% electricity price increase resulting from the existing power plant emissions regulation on their own operations;
  • 20% or more increase in cost of raw materials used in our shops that are produced by electrically intensive means such as electric arc furnaces (which are actually recycling steel scrap into new useful material)
  • Reduced hiring
  • Reduced creation of new plants
  • Reduced production and sales
  • Reduced U.S. GDP

EPA Ordered Restrictions?

"They could also mean reducing energy-intensive economic activity, which could have substantial impacts on regional and state economies. States or AQMDs that are unable to comply with the new standards on time would also face harsh economic sanctions, too. No new industrial activity could open in that state or AQMD unless the state or AQMD was first able to obtain even greater emission reductions elsewhere."- source Pillsbury Law Blog

The EPA estimates of cost impacts are also low, as our suppliers are energy intensive and we will also face much higher raw material prices making us non-competitive globally.

Our members support sound environmental policies based on proven science and health benefits balanced with realistic economic expectations.

  • We do not believe that "unknown controls" rise to the level of mature thought let alone science basis.
  • We do not believe that EPA has shown that this proposal will have significant public health benefits over the current standards.
  • We do believe, that lowering the levels to 70ppb or 65ppb would have a significant negative economic impact on the entire country, especially small and medium sized manufacturers.

For these reasons, PMPA urges EPA to maintain the current 75ppb standards for ozone. Allowing for the full and continued implementation of the current law will continue to drive significant reduction in ozone emissions. The proposed rule fails to demonstrate benefits, relies on "unknown controls," and fails to consider natural influences in ozone levels and attainment. As businessmen, we do not base our plans on "unknown controls." We cannot see how smart policy can put the manufacturing sector and the overall economy at risk, by relying on unknown and unproven controls either.

Then a miracle happens cartoon by Sidney Harris via TrulyFallacious

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which you can also read here.

4 comments; last comment on 04/01/2015
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How Chemistry Determines Machinability--Manganese Sulfides

Posted March 17, 2015 12:30 PM by Milo
Pathfinder Tags: chemistry machining materials

Flank wear is the "normally expected" failure mode for tools to fail when machining steels.

The volume fraction of Manganese Sulfides is a determinant of the tool's wear rate. "The wear rate of high speed steel tools decreases rapidly up to about one percent volume fraction of MnS and then levels off to a constant wear rate as the volume fraction is increased."-Roger Joseph and V.A.Tipnis, The Influence of Non-Metallic Inclusions on the Machinability of Free- Machining Steels.

Manganese and Sulfur have a powerful effect in reducing flank wear on HSS tools

As sulfur rises beyond 1% volume fraction, surface finish improves, chips formed are smaller with less radius of curvature, and the friction force between cutting tool and chip decreases due to lower contact area.

Manganese sulfides are a separate internal phase.

How does Manganese Sulfide improve the machinability?

  • The MnS inclusions act as "stress raisers" in the shear zone to initiate microcracks that subsequently lead to fracture of the chip;
  • MnS inclusions also deposit on the wear surfaces of the cutting tool as "Built Up Edge (BUE)."
  • BUE reduces friction between the tool and the material being machined. This contributes to lower cutting temperatures.
  • BUE mechanically separates or insulates the tool edge from contact with work material and resulting heat transfer.

This is why resulfurized steels in the 11XX and 12XX series can be cut at much higher surface footage than steels with lower Manganese and Sulfur contents.

More info about Manganese in steel HERE

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which you can also read here.

Why Tool Life Can Vary: Carbon and Alloy Steels

Posted March 02, 2015 8:00 AM by Milo
Pathfinder Tags: life metal quality tool

Tool life can vary when machining carbon and alloy steels despite the use of our best technology and our efforts to control our processes.

Many ways that tools can fail. This post discusses factors in the steel that can lead to this.

Here are 6 factors that can affect tool life in your shop.

  1. Variations between suppliers. Suppliers' melt processes, scrap practices, melt recipes, and reduction in cold drawing and straightening practice can significantly affect the way that the chip breaks and resulting built up edge on tool and resulting surface finish. Even though the grade is the "same."
  2. Variations in Chemistry. A potential subset of variations between suppliers, the fact is that a plain carbon grade with 0.005 wt. % Sulfur will not machine at all like the material with 0.025 wt% sulfur.
  3. Variations in grain size. While this factor is typically more relevant in stainless steels, when machining forgings, blocky structures resulting from excessive forging temperatures can result in inconsistent machining performance.
  4. Variations in microstructure. In this case, it is not so much about the grain size, as it is about the structure present. This is particularly problematic in the ~0.40 wt% carbon alloy grades like 4037.
  5. Decarburization or scaling on the work surface. Decarb can result is a carbon poor gummy surface, only to then transition into a fully carbon containing microstructure. Scale on the work surface can result in excessive tool wear, due to the very high hardnesses of the various iron oxides that may be present (Hematite, the red oxide of iron, Fe2O3, has a microhardness of approximately 1030 DPH.
  6. Deoxidation /High Inclusion Count. Free machining grades such as 12XX and 11XX steels are typically sold to a "Coarse Grain Practice" with no deliberate additions of grain refiners or deoxidizers. Sometimes, you may find deliberate additions of Silicon to 1144 in order to improve the internal soundness of the steel, the resulting silicates can abrade the edge of the tool when running at the surface feeds expected for a resulfurized steel. The addition of aluminum as a grain refiner can cause rapid edge wear as well. Rarely, very rarely, one might encounter exogenous inclusions entrapped in the steel from melt and casting. These can be real showstoppers.

Takeaway: Purchasing the same item from different suppliers hurls the full range of global variation at your machining operations. Standardizing on a single supplier for an item will allow you to get to steady state in your process.

Photo courtesy of via CTE Magazine Plus

Editor's Note: CR4 would like to thank Milo for sharing this blog entry, which you can also read here.

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