The Difference Between Accuracy And Precision Measurement In Your Machine Shop

March 27, 2012

Your QA manager can put you to sleep explaining the difference between these two terms- but you really need to know the difference.

Accuracy describes 'close to true value;' Precision describes 'repeatability.'

Accuracy in measurement describes how closely the measurement from your system matches the actual or true measurement of the thing being measured. It is the difference between the observed average of measurements and the true average.

Think of accuracy as the “trustworthiness” of a measurement system.

Precision in measurement describes how well a measurement system will  return the same measure; that is its Repeatability.

As the targets above show, it is important to be both Accurate and Precise if you are to get useable information from your measurement system.

But the repeatability has two components- that of the measurement system (gage) itself and that of the operator(s). Differences resulting from different operators using the same measurement device- this is called Reproducibility.

In our shops, we cannot tell if our measurement system has repeatability or reproducibility issues without doing a Long Form Gage R&R study.

Gage repeatability and reproducibility studies (GR&R) use statistical techniques  to identify and discern the sources of variation in our measurement system: is it the gage, or is it the operator?

Gage error determined by the GR&R is expressed as a percentage of the tolerance that you are trying to hold.

Typically, 10% or less Gage Error is considered acceptable. Over 30% is unacceptable; between 10 and 30% gage error may be acceptable depending on the application.

Regardless- any level of gage error is an opportunity for continuous improvement.

Target Graphic

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OSHA Haz Comm-Your Work Is Just Beginning

March 21, 2012

The new HAZCOMM 2012 ‘Right to Understand’ will impact 5 million businesses at an OSHA estimated cost of only $201 million dollars.

Thats just $40 per workplace to cover:

  • Cost of classifying Chemical hazards to meet the new GHS criteria;
  • Cost to revise Safety Data sheets and labels to meet the new format and content requirements;
  • Cost to train 43 million employeeson the new format and content of material symbols and data sheets;
  • Cost to management of those 5 million workplaces to become familiar with the new GHS requirements, assess, revise, develop and implement new compliance materials needed to adopt GHS;
  • Cost for printing new packaging and labels in color;

OSHA thinks that we can do it for $201 million a year- that’s just $40 per workplace!

OSHA says this is all it will cost your shop to adopt this new standard, become familiar with its requirements, reclassify all your chemicals, train your people, change all labels and data sheets. WHAT were they thinking?

Using Department of Labor enforced Federal Minimum Wages, $7.25 an hour, that means that they think that it will only take 5,5 hours to get a shop into compliance.

(Please, correct my math if I’m wrong!)

5-1/2 hours!

The PDF of the final rule is 858 pages!

Just to read that in 5.5 hours would mean reading 156 pages per hour.

At a nickel a page, just printing the final rule puts us over at $42.90.

Who the heck does these estimates? What were they thinking?

We really understand that regulations can provide a benefit to workers, companies and communities.

Especially where hazardous chemicals are involved.

But when the regulators underestimate the potential costs of adoption and compliance by such a large factor, it makes us wonder what other assumptions are they working under that are just as wrong?

P.S.  Do you think that OSHA or OIRA actually have  employee’s that can read 156 pages of federal technical regulation in an hour? At $7.25 an hour?

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CNC Machining Is The Foundation Of Manufacturing- Peter Zelinski

March 20, 2012

Zelinski: “Any product you pick up and touch, it’s not too many steps away from a machining process.”

Most of the parts in your car engine come from a CNC machine. Medical devices, your kitchen cabinets — CNC machine. Your computer case, your iPhone earbuds — well, no. But the mold that created them — CNC machine.

The growth of these machines represents the biggest change in manufacturing over the last 20 years. The people who run them are factory workers.

But they also have to be computer programmers. And they are in high demand.

Marketplace on  American Public Media /National Public Radio Closed with a story on the importance of CNC machining last night.

You can access the podcast and read the full transcript at NPR CNC STORY 

Bottom line : Skilled operators of CNC machine tools are in high demand.

High enough demand to make the national financial news on NPR.

Tip of the hat to Peter Zelinski at Modern Machine Shop magazine, for effectively describing and communicating the opportunity of CNC machine technology for our workforce.

Modern Machine Shop is the Flagship publication of Gardner Publications, who co-produce Production Machining Magazine with PMPA.

CNC Podcast

Photo credit Dustin Dwyer at MarketPlace

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How Plastic Deformation Makes Machining Possible

March 15, 2012

If steel did not have the property of plastic deformation, the only ways to make parts from it would be casting  or cutting into shape.

No deformation processes like cold heading, cold rolling, swaging etc. would be possible.

Slip planes in the metallic crystal explain Plastic Deformation and Plasticity in Steel. This makes cold working processes like cold drawing possible.

If one subjects a piece of steel to a heavy load, the material will measurably stretch. When the load is removed, if the steel goes back to its original dimension, the deformation that it underwent when the weight was applied is called “elastic deformation.” In this case, the steel did not take a permanent “set.”

If one subjects a piece of steel to a much greater load, if, when the load is removed, the steel does not ‘spring back’ or recover to its original dimension, the new shape or dimension is a permanent deformation. (It is often said to have ‘taken a set,’)  This is called “Plastic Deformation.”

Plastic Deformation is explained by the movement of planes of atoms from their normal positions.

Steel and most industrially useful metals are able to withstand a great deal of this Plastic Deformation before they break.

Brittle metals will just fracture under such loads;

Cold drawing of steel is a process that applies a load to the metal to make the atoms in the steel take new positions with respect to each other, resulting in lowered ductility, increased  tensile and yield strength and new dimensions or shape. These in turn, are often helpful in improving the machinability of the steel, allowing you to more economically produce the parts and components that are essential for our current technologies.

Most people think of steel’s hardness as its main advantage. The facts of the matter are that it is steel’s plasticity or ability to plastically deform that makes it such a useful and versatile material for humankind.

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How Value Engineering Saves Money on Machined Part Costs- Fairchild Auto-Mated Parts Inc.

March 7, 2012

You could just send your part CAM files to one of those online services to just make the parts and ship them to you. Sounds pretty high tech. Sexy. New school. No humans involved.

Or you could send them to a company that actively involves its human engineering talent to add value for you, the customer. Old school. And worth it!

Imagine the cost of both the material lost by turning and the machine time to remove it if this were made from barstock of the greatest diameter.

Two of the major contributors of a part’s cost are material and machining time.

Value engineering at Fairchild Auto-Mated  involves engineers evaluating each part to seek ways to reduce these cost factors.

Imagine, engineers getting involved in evaluating your part before production begins.

Decidedly Old School. And decidedly worth it.

The valve component shown above was presented to Fairchild made as one piece carved out of oversize barstock in an single piece.

Fairchild’s engineers studied the design, application, and function.

They determined that this part would be less expensive to produce as two separate items assembled and staked together to form this single part.

This design eliminated the costly stock removal of large diameter  (expensive) stainless steel, and reduced the amount of (expensive) stainless steel chips produced to generate the stem.

There was no need for the disk portion of this part to be stainless, and so less expensive and more machinable brass was selected for this part of the component.

What was the pay off for value engineering versus the “download the file over the internet and have it go straight into production” process path.

At $1.00 saved per part, the Customer saved one of these for every 100 parts they purchased thanks to Value Engineering.

The savings identified by Fairchild’s value engineers resulted in a total cost savings of over $1.00 per part.

End result for the customer: $48,000 in savings the first year…

If you just want to email your part file to someone and have them make it with no humans involved, well, that is certainly your perogative.

But if you would like to have the benefit of a value engineering teams design review that can find, say, $1.00 per part in cost savings- then you probably ought to make a different decision.

Old school shops like Fairchild have been able to survive through all of the ups and downs in the market- because they continue to add real value and identify real savings for their customers.

And in quantities of 50,000 or more per release, that value engineering can add up to real money.

How do you know your part is optimized for production?

How do you identify real cost savings besides just having jobs rebid?

Do you have a process to involve your suppliers in value engineering?

Or do you just go with lowest bidder for the part as drawn on the print?

Why?

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Dimensional Contraction of 17-4 PH Stainless Steel

February 28, 2012

The mechanical  properties of 17-4 PH  must be fully developed by age hardening from Condition A in order to reduce risk of failure and to take full advantage of the material’s capabilities.

Dodge Viper Throttles made by Bouchillon feature 17-4 PH shafts

17-4 PH  is a martensitic precipitation hardening (age hardening) stainless steel that can provide both high strength and excellent corrosion resistance.

In the annealed (solution treated condition- Condition A) the density of this material is 0.280 lb/in^3.

H 900 density is 0.282 lb/in^3.

H 1075 density is 0.283 lb/in^3.

H 1150 density is 0.284 lb/in^3.

These changes in density values show that this alloy undergoes a volume contraction when it is hardened. This volume contraction is predictable and must be taken into account if you are trying to hold close tolerances.

The contraction factor for the change from Condition A  to Condition H 900 ranges from 0.0004 to 0.0006 in/in or (mm/mm).

Hardening  from Condition A to Condition H 1150  contracts in the range of approximately 0.0009 to 0.0012 in/ in or (mm/mm).

Here are three reasons to NOT use 17-4 PH  in the Condition A  state:

  • The structure is untempered martensite. This means low fracture toughness.
  • The structure is untempered martensite. This means low ductility.
  • Without age hardening, this material is more susceptible to stress corrosion cracking.

17-4 PH  martensitic stainless steel can achieve high strength and superior corrosion resistance when precipitation hardened from Condition A to one of the Condition H tempers. It is used in many high performance applications made by our industry including valve parts for oilfield and chemical plant use; Fittings for aerospace and aircraft use; Jet engine componentry; Fasteners; Shafts for pumps; Dodge Viper carburetors! Many others.

In applications where high performance is mandatory, it is also mandatory to follow needed thermal treatment practices to assure the development of the full range of material properties that the material can provide.

For the savvy machinist, that also means understanding the pootential effect of that thermal treatment on final size due to dimensional contraction when hardened.

Thanks to Bouchillon for the throttle photo.

Material on Dimensional Contraction was taken from Schmolz + Bickenbach 17-4 Datasheet.

Density and European Equivalency data from  Rolled Alloys data sheet.

European designation note: Officially 17-4 PH is designated as UNS S17400. It is the US available nominal equivalent to DIN 1.4548, X5CrNiCuNb 17-4-4

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What Is The Most Expensive Training In The World?

February 23, 2012

No training at all!

The money saved by not training won’t begin to cover the direct and indirect costs of failing to train, let alone actual damages, consequential damages, potential liability, and possible loss of customers or even the business itself.

What is your training budget this year?

How does it compare to your cost of claims last year?

Your cost of expedited shipping?

Photo courtesy FlightGlobal

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Machinery’s Handbook-29th Edition of the Machinist’s Bible

February 21, 2012

Authoritative. Comprehensive. Invaluable. Practical. Updated.

Updated?

29th Edition of Machinery's Handbook now available.

I have relied on my 20th Edition copy since I entered the metalworking industry as a supervisor in the early 1980′s. It has served me well through the years, and while respectfully used, is showing evidence of ‘serious use’- missing thumb tabs, dust jacket in tatters, a host of bookmarks…

Here are 5 reasons why I’ll probably upgrade to the new 29th Edition:

  • New sections added on Micromachining, Statistics, and Calculating Thread Dimensions;
  • Expanded Metric content. The jobs we see in our shops today are increasingly metric as we serve a growing global market;
  • Easier to use- they have added tables of contents at the beginning of each section;
  • Extensive revisions to key sections including Mathematics, Gaging and  Dimensioning,  and Machining Operations
  • It has been re-typeset (including tables and equations) and many figures redrawn.

Now the problem for me is choice: Do I get the ‘regular edition’ to replace augment my current 20th edition handbook? Do I jump into the electronic age with the CD version? Or do I acknowledge I no longer have the eyes of a younger man and buy the “larger print” edition?

It’s time for me to buy. My investment in the 20th Edition sure paid off. How about you?

Which  would you  choose? What other books have you found critical to your practice in our precision metalworking field?

Machinery’s Handbook 29th Edition can be purchased direct from Industrial Press.

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5C Collets Are Cool- Somma Tool

February 15, 2012

In order to machine precision parts, you need to first hold the workpiece securely, accurately and precisely. 5C collets do just that.

5C collets are the result of 100 years of continuous improvement.

Work can be held using methods other than collets- 3 and 4 jaw chucks come to mind, as well as vises-  but for continuous high volume work with barstock, collets are ideal.

Here are 7 reasons Somma Tool says 5C collets are cool:

  • Collets are easier (and faster!) to set up than chucks.
  • Collets are more concentric. With chucks, tolerances stack up degrading concentricity.
  • Collets more affordably provide higher precision.
  • Collets more affordably provide higher accuracy.
  • Collets provide high holding force. As the collet is pulled axially into the bushing, the tapered sides compress radially generating static friction (holding force).
  • Collets are versatile- they can be made to hold over capacity stock; they can have steps built in; come in extra long sizes as well as have internal stops.
  • Emergency collets are available that can be custom bored to your exact need.

Hardinge invented the 5C collet back in 1901. It became a preferred choice for precision workholding in lathes, mills and grinders. Exacting standards, special alloy steel, heat treatment and spring tempering combine to assure accuracy and durability at low cost. The 5C collet became an industry standard. 5C collets range from 0.5 mm (thats 0.0196″ ) capacity to 1-1/16″  round; 5C collets hold up to 3/4″ square and and 29/32″ hex.

PMPA member Somma Tool sells 5C collets from Hardinge.

Thanks to Matt at Somma and Tom at Hardinge  for teaching this ‘steel guy’  7 reasons why 5C collets are cool.

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Can Your Robot Do This?

January 31, 2012

QinetiQ makes a line of “Throwable robots.”

Check out the video:

Dragonrunner

The Dragonrunner has multiple camera and payload options. While this robot is not used to maufacture  precision products like the big yellow ones we’re used to seeing in our shops,  this is an ideal embodiment of a robotic solution for reconnaissance and surveillance, and first-responder teams in hostile or life threatening conditions. Weighing in at around 10 pounds (depending on payload and equipment) the Dragonrunner is rugged enough to function in hostile environments and has the ability to climb stairs and handle “rugged dismounts-” Like throwing from the back of a speeding truck or upper story window.

We use robotic technology to reduce variation in our shop operations and to create highly efficient work cells combining different machine tools.

This one is not throwable...

But at the other end of the spectrum, Robots can be, well, Pretty AWESOME

QinetiQ Homepage

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