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Deburring basics.

Deburring basics

In theory, a precision-machined or cast component is free of burr or blemish and can go directly to its intended use. In reality, however, dull cutting tools, poor maintenance, varying materials, and production demands make secondary operations a necessary part of most production processes. Also, the high cost of machinery, cutting tools, maintenance, and labor dictate maximizing machine running time before tooling is repaired or replaced. This emphasis on cost reduction often results in reduced part quality, and secondary deburring operations are required.

This article will explore some of the common automated and flexible deburring methods of mass finishing, particularly dry blasting. The best solution must consider part size and configuration, part material, burr location, desired finish, and downstream processing.

What's a burr?

Perhaps the most critical element in selecting a deburring process is defining a burr; i.e., what has to be removed. This can range from requiring only that no material come loose when the part is inverted to examination under 40X magnifications. In one case, a raised or rough edge may be judged part of the parent metal and of no concern, and in another, it's an imperfection that must be removed.

It's easy to overspecify a part's deburring requirements. Given enough magnification, it is impossible to ever completely eliminate burrs. Overspecifying limits your finishing alternatives, and adds unnecessary cost in both finishing and inspection. Thus, a realistic definition of a part's needs must be reached before searching for deburring automation.

Two general categories of burr are: 1. The obvious burr, protruding or hanging as a result of machining. Depending on its relative size and the material involved, it can usually be broken off. 2. Moved metal, securely attached to the parent component. This burr must be eroded away; it cannot be broken loose without affecting the parent material.

Deburring methods

Here are some common deburring alternatives:

Vibratory deburring systems use a mixture of abrasive media and a cleaning solution. The agitated container causes the solution to circulate and lubricate the media in a smooth, tumbling action. Burrs are slowly abraded away. Drawbacks of this approach include: media cannot be made small enough to reach narrow recesses in complex parts, the process is unselective--a slight radius is produced on all sharp edges, the process is slow--large tanks are required for large part quantities, and after deburring, parts must be separated from the media, and may require drying and cleaning.

Thermal deburring uses a combustible gas and oxygen to burn off loose or hanging burrs, and is commonly used in high-volume production of small complex parts. The larger the burr, the hotter the burn required, and if burr size varies widely, there is a danger of destroying the part. Combustion scale may reduce the part's finish and natural lubricity, or require further processing. Melted burrs may produce undesirable beaded edges or spatter. Energy and equipment costs are relatively high.

Hand deburring is still widely used on complex parts in low volume, or to handle unusual burrs. Although highly versatile, hand deburring is the most expensive and least consistent.

Blast finishing can be effective in high-production applications and yield low component-finishing costs. It is used where abrasive burr removal is required, particularly to break off the burr without affecting the rest of the part. Impact deburring is more descriptive of the finishing action--propelling a particle at the area to be deburred.

The blast media and propulsion method are chosen to suit the application. Nonabrasive media, such as a plastic, break burrs without affecting parent material. Abrasive media add abrasion to this impact effect. Media can be sized small enough to reach recessed and intricate part areas. Recovery systems handle media from 50 microns to 2 mm. Angular particles (aluminum oxide) or angular abrasive plastic are used to wear away material. Rounded or softer media material deburr by breaking away material. Steel shot additionally stress-relieve component surfaces. With dozens of material choices, plus dozens of size and shape choices within each material, picking the right media for an application requires expert advice. See Media choices and Media effects.

PHOTO : Media choices. Electron-microscope photos (80X magnification) show differences in blast media: Cubical nonabrasive plastic, A; abrasive plastic, B; abrasive aluminum oxide, C; nonabrasive glass bead, D; used glass bead, E; and metal shot, F.

PHOTO : Media effects. Series of 80X photos of the effects of deburring a hanging-edge burr on 6061 aluminum: Burr before treatment, A. Cubical plastic, B, breaks off large hanging burrs without abrading parent material and peens down remaining edge. Abrasive aluminum oxide, C, removes burr by erosion and the parent part surface is also affected relative to the size hardness, shape, and mass of particle used (here, a fine matted surface results because particles were the smallest of any in this group). Used glassbead media, D, because of fractured particles that are abrasive, produces an eroded surface similar to aluminum oxide. Metal shot, E, reduces the burr and peens the surface without abrasion. Angular plastic, F, has a semi-abrasive effect, compared to nonabrasive cubical plastic and abrasive aluminum oxide.
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Author:McHugh, Brian
Publication:Tooling & Production
Date:May 1, 1989
Words:844
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