Burrs and foreign object damage are consideration that are increasingly critical as precision machined parts are engineered from more challenging materials and to demanding geometries and applications.-  Guest Post by John Halladay of Vectron, Inc.
Burrs are unwanted raised material remaining on a machined part as a result of prior manufacturing operations. Link here

Looks like unwanted raised material  to me!

FOD can be either Foreign Object Debris or Foreign Object Damage  as defined by the
National Aerospace FOD Prevention, Inc. (NAFPI),
Foreign Object Debris (FOD): A substance, debris or article alien to a vehicle or system which would potentially cause damage.

Foreign Object Damage (FOD): Any damage attributed to a foreign object that can be expressed in physical or economic terms which may or may not degrade the product’s required safety and/or performance characteristics.

“Cause damage, degrade product’s safety or performance characteristics, and economic damage” – These are serious issues to manufacturers and their customers making critical human safety reliant systems- like automotive, aerospace, fluid power, or medical devices or systems.

So how do we deal with Burrs, and FOD? John Halladay of Vectron explains:

“Based on my experience (a few more years than I care to admit) there are some things to consider with difficult to see FOD and burrs.  Number one, hand deburring is typically out of the question—even with magnification.  If hand deburring does happen to remove the burr, the dislodged burr magically transforms itself into yet another source of FOD.  Mass finishing techniques (vibro, tumbling, Spinner, bead blast/water jet) will fall into the same trap.   They may be able to dislodge the burr or foreign object, but then that dis lodged item creates damage to the surface finish or features you fought hard to create in the part.”
“Thermal deburring is a batch process involving very intense heat in very short durations. It’s like being inside an explosion. Because it utilizes combustible gases under pressure, it has been proven to be extremely effective at removing the hard to see burrs we often encounter on the less machinable materials and alloys we see in our shops today. One advantage of  Thermal Deburring is that it does not create FOD, and the process will seek out other sources of FOD that may be lurking in some of the tightest geometries in the part.  There is nothing quite like an  explosive gaseous mixture to see and vaporize and remove all  unwanted debris on or in our parts!
Electro-Chemical Deburring, is usually referred to as  ECD.  It applies an electrical current to the areas where the burrs are located. The current carried by the electrolyte actually dissolves the burr material. This process can actually create a controlled radius on the workpiece by its action.”
Electro-Chemical  deburring is therefore quite useful for removing burrs at internal intersections, especially when a radius is either desired or required.  The downside of ECD is that it may not completely take care of other sources of FOD.  This is easily resolved with the addition of a special wash process in conjunction with ECD to get to “Yes” with your customer.
Due to the expense and engineering associated with these processes, and the intermittent need for them, these processes are seldom performed in house in contract manufacturer’s operations.  They are readily available from a number of job shops across the continent.  You will find that most shops will provide a no-charge feasibility analysis including sample processing, so there’s really no down side to investigating these options while you continue to search for possible in-house solutions.

This is what the customer wants. Thermal deburred and supercleaned.
This is what the customer wants. Thermal deburred and supercleaned.

More info on Burrs and Deburring
Vectron, Inc.

The tolerance on cold drawn steel bars for machining is always specified as plus nothing minus some value…

So why are the dimensions on the bars held to the minus rather than plus side? Don’t we want to get more steel  per foot for our money?

May I have your answer please?

And the answer is ...
And the answer is …

The reason for the dimensions being held to the minus side is so that the bars can easily pass through a hole of nominal size.

If the bars were the same nominal size as the hole, they would be very difficult to assemble. If the bars were  even slighty larger, they would not pass through.

So bars are held to the minus side of each nominal dimension to assure that they can pass through the nominal size hole- whether it be a bushing, pulley, gear, collet,  support bearing or any similar application.

The bars must measure less than the nominal hole size to permit assembly.

How did this come to be?

Before the era of electric motors, power was transmitted by means of shafting.
Before the era of electric motors, power was transmitted to each machine by means of belts  and pulleys running on cold finished shafting.

Line shafting! The power transmission shafts  that ran across the ceilings of shops while being held in bearings were called line shafting. The power was taken from the shafts by belts and pulleys. The shafts were held by bearings afixed to the ceiling joists. The shafting had to fit into these bearings and pulleys.(These shafts were driven usually by a single large motor, steam engine, or water wheel…)

It has been some time since power transimission shafting has been used commercially to drive our lathes and drills commercially.

But we have the legacy of cold finished shafting to thank for the foundational concept of tolerances on bar products being held to the minus side.

Thanks to John Halladay at  PMPA technical Vectron in Elyria Ohio for the archival shop photo from the Perry Fay company.

And if you have a burr problem with some of your production, you can call on Vectron to help you with that too.

Do you have memories  of working with machinery driven by line shafting in your career? We’d love to hear your story…