Abrasion: The machine used by the U.S. Forest Service is a modified form of the Dorry abrasion machine. (See Fig. 42.) Upon the revolving horizontal disk is glued a commercial sandpaper, known as garnet paper, which is commonly employed in factories in finishing wood.

Figure 42

Abrasion machine for testing the wearing qualities of woods.

A small block of the wood to be tested is fixed in one clamp and a similar block of some wood chosen as a standard, as sugar maple, at 10 per cent moisture, in the opposite, and held against the same zone of sandpaper by a weight of 26 pounds each. The size of the section under abrasion for each specimen is 2" × 2". The conditions for wear are the same for both specimens. The speed of rotation is 68 revolutions a minute.

The test is continued until the standard specimen is worn a specified amount, which varies with the kind of wood under test. A comparison of the wear of the two blocks affords a fair idea of their relative resistance to abrasion.

Another method makes use of a sand blast to abrade the woods and is the one employed in New South Wales.60 The apparatus consists essentially of a nozzle through which sand can be propelled at a high velocity against the test specimen by means of a steam jet.

[Footnote 60: See Warren, W.H.: The strength, elasticity, and other properties of New South Wales hardwood timbers. Dept. For., N.S.W., Sydney, 1911, pp. 88-95.]

The wood to be tested is cut into blocks 3" × 3" × 1', and these are weighed to the nearest grain just before placing in the apparatus. Steam from the boiler at a pressure of about 43 pounds per square inch is ejected from a nozzle in such a way that particles of fine quartz sand are caught up and thrown violently against the block which is being rotated. Only superheated steam strikes the block, thus leaving the wood dry. The test is continued for two minutes, after which the specimen is removed and immediately weighed.

By comparison with the original weight the loss from abrasion is determined, and by comparison with a certain wood chosen as a standard, a coefficient of wear-resistance can be obtained. The amount of wear will vary more or less according to the surface exposed, and in these tests quarter-sawed material was used with the edge grain to the blast.

Indentation: The tool used for this test consists of a punch with a hemispherical end or steel ball having a diameter of 0.444 inch, giving a surface area of one-fourth square inch. It is fitted with a guard plate, which works loosely until the penetration has progressed to a depth of 0.222 inch, whereupon it tightens. (See Fig. 43.) The effect is that of sinking a ball half its diameter into the specimen. This apparatus is fitted into the movable head of the static testing machine.

Figure 43

Design of tool for testing the hardness of woods by indentation.

The wood to be tested is cut square with the grain into rectangular blocks measuring 2" × 2" × 6". A block is placed on the platform and the end of the punch forced into the wood at the rate of 0.25 inch per minute. The operator keeps moving the small handle of the guard plate back and forth until it tightens. At this instant the load is read and recorded.

Two penetrations each are made on the tangential and radial surfaces, and one on each end of every specimen tested.

In choosing the places on the block for the indentations, effort should be made to get a fair average of heartwood and sapwood, fine and coarse grain, early and late wood.

Another method of testing by indentation involves the use of a right-angled cone instead of a ball. For details of this test as used in New South Wales see loc. cit., pp. 86-87.