Finish Marks

Many metal surfaces must be finished with machine tools for various reasons. The acceptable roughness of a surface depends upon how the part will be used. Sometimes only certain surfaces of a part need to be finished while others are not. A modified symbol (check mark) with a number or numbers above it is used to show these surfaces and to specify the degree of finish required. The proportions of the surface roughness symbol are shown in Figure 7.9. On small drawings the symbol is proportionately smaller.

Figure 7.7B Operations of the upright drilling machine.

COUNTERSINKING TAPPING

Figure 7.7B Operations of the upright drilling machine.

The number in the angle of the check mark, in this case "02," tells the machinist the degree of finish the surface should have. This number is the root-mean-square value of the surface roughness height in millionths of an inch. In other words, it is a measurement of the depth of the scratches made by the machining or abrading process.

Wherever possible, the surface roughness symbol is drawn touching the line representing the surface to which it refers. If space is limited, the symbol may be placed on an extension line on that surface or on the tail of a leader with an arrow touching that surface, as shown in Figure 7.9.

When a part is to be finished to the same roughness all over, a note on the drawing will include the direction "finish all over" along the finish mark and the proper number. When a part is to be finished all over but a few surfaces vary in roughness, the surface roughness symbol number or numbers are applied to the lines representing these surfaces, and a note on the drawing will include the surface roughness symbol for the rest of the surfaces.

Figure 7.8 A hand reamer and a machine reamer. Solid hand reamers are typically used when a greater accuracy in hole size is desired. Machine reamers can generally be expected to produce clean holes when used properly.

Figure 7.9A The proportions of the surface roughness symbol.

Figure 7.9B The method of placing surface roughness symbols.
Finish Codes

Figure 7.9C Examples of finish marks on a drawing.

SURFACES MARKEDvTO HAVEN

EXAMPLES OF TYPICAL SYMBOL USE IN MACHINE DRAWINGS

Figure 7.9C Examples of finish marks on a drawing.

Was this article helpful?

0 0
Greener Homes for You

Greener Homes for You

Get All The Support And Guidance You Need To Be A Success At Living Green. This Book Is One Of The Most Valuable Resources In The World When It Comes To Great Tips on Buying, Designing and Building an Eco-friendly Home.

Get My Free Ebook


Post a comment