CON

194

CON

oblique angles, constituted by the surface of each piece onthe same side, supplements to each other; or whateveroblique angle or angles the one piece makes with a side, thecorresponding angle or angles formed by the joint or jointswith the same continued surface of the other, will, together,form two right angles, and thus the solid part of the one willbe equal and similar to the void of the other.

There are several methods of lengthening timber, eitherby joining whole pieces of the same transverse sections, andforming their ends, which are to come in contact in one orseveral planes, or by forming the connection by means of athird piece, or by building the piece to be lengthened inseveral thicknesses, making the joints abutting upon eachother on the solid of the piece with which they come in con-tact on the parallel joint. It is evident, that two bodiesunited, and intended to act as one in a state of tension, cannever be so strong as either piece taken separately.

Tabling is a mode of indenting the ends of the pieceswhich form the scarf, so as to resist a longitudinal strain ;the pieces, therefore, require to be held together, or otherwisethe notches and the tables which fit into them would requireto be dovetailed. In this construction, the tables betweenthe notches would be a very feeble support, as they are aptto split away. It must also be observed, that one singletable, or one abutting part of resistance, is stronger, andmuch more easy to execute, than two or four; and that theresisting part should have as little projection as possible,because such projection diminishes the cohesive force, by aquantity of the timber equal in section to the abutting parts.Two pieces of timber may be very firmly fixed, by makingthe ends of the tables, instead of abutting, to form a taperingmortise, so that when the two pieces are brought close at theconnecting surfaces, a wedge driven into the cavity willbring all the parts of the joint into contact.

Every two pieces of timber require to be held together bysome force compressing them equally on each side, particularlywhen the pieces are light; for which purpose iron bolts arevery convenient, they acting as a tie, and having the sameeffect as two equal and opposite forces would have in com-pressing the beam on each side of the scarf; and as iron is ofgreat strength, the bore made to receive the bolt will not beso large as to diminish the section, and consequently thefirmness of the timber at the scarf, in any considerabledegree; whereas, when wooden pins are used, they requirea large bore, which weakens the timber, and the two piecesthus connected are not so firmly compressed, or, indeed,compressed at all, but are held together almost solely byfriction.

No limited distance can be specified for the length ofthe scarf; though it may be observed, generally, that a longscarf has no effect in diminishing the cohesive strength of acompound piece of timber. On the contrary, a long scarfgives an opportunity of increasing the number of bolts,which are the only ties when no tablings are used, as is thecase where the abutting parts unite only by compression. Itmust here be understood, that all such abutting parts diminishthe cohesive force in the proportion of their abutting surfaceto that of the whole section at any one of the abutments ; sothat, should a scarf consist of a series of steps, formed byplanes parallel and perpendicular to two of the opposite sides,the transverse sides of these steps to those of the pieceshould be all equal; and the greater the number of steps,the less will the strength be impaired; but if they beunequal, the timber will be weakest at the greatest sectionor compressed abutment, and if few, the section will be large,and the piece consequently deprived of a proportional degreeof strength. We may also add, if the two pieces be strapped

longitudinally across their abutting parts, the cohesivewill be considerably assisted thereby. c0 r-

There is no part of carpentry which requires S reat . er < j en t9rectness in workmanship than scarfing; as all the inshould bear equally, otherwise the greater part of the st ^ ^gwill be lost. Hence we see how very unfit some ^complicated forms shown in the old works on carpentry (()for the purpose. It is certainly the height of absur ^ ^render the parts difficult to be fitted, when the whole o s ;strength depends on their fitting well. “But many> ^gglP \Professor Robison, “ seem to aim at making t * ie . ( j et at e Istronger than if it were of one piece; and this inconsi^j-j,g jproject has given rise to many whimsical modes otand scarfing.” und er i

Having already shown many varieties of scarfing 3 ;.^ oll t ,the general head of Carpentry, we shall here only P°^. a y of jthe most approved forms for practical purposes, by 'illustrating the preceding observations. J , „ a sing 16 j

Figure 1. Two pieces of timber connected by a j g Jos* ’ !step on each piece. Here more than half the power s joOneither is the scarf so capable of resisting the force ot ,

as a single piece of timber would be, were itthrough its thickness from the opposite side, at a i b/equal to the length of the scarf: however, if aSS1,straps, it may perhaps be capable of resisting a niuct t [j eforce, particularly if each opposite surface be boltedsides of the transverse joints through the straps. ^llo""Figure 2. An oblique scarf, bolted in three places- ^ tb eing the utmost cohesion of the part of the joint A B, 10 ^ p orsame as whole timber, and that the transverseB c, are one - fourth of the breadth, D E, the coj ^[idtimber will possess three-fourths of the strength opiece. _ . nle tab 16

Figure 3. A scarf with parallel joints and a s !!'°j n isl ieC "upon each piece. Here the cohesive strength is di ejectin' 1' in an additional degree to that of Figure 1, by tb % ^ r jviJ>g ?of the table; but this gives an opportunity ot ^gg,wedge through the joint, between the ends of the - a pa 0 ”thereby forcing the abutting parts to a joint. j )£lV erequires the scarf to be longer than those whic a j 3 o b®tables; and the transverse parts of the scarf Wstrapped and bolted. . aging a

Figure 4. Allows of the same opportunity ot ^g c otP ebefore: if we could suppose the parts ab and c P jj U tpressed by bolts as firmly together as if they ^e* a pd

nio/io fliotr iirnnld lio l\vr f tio ormtin itifir A f the fio ie pgfy

wit'ld b6

piece they would be, by the continuity of the p 8 * V,

the projection of the tables be equal to the transV are d " !ltof the joints at A and d ; the loss of strength, comp „„that of a solid piece, will be no more than what iat a and d. strupP’ 1 ’^

Let it be here observed, once for all, that tn ^ e ffeC tU jjacross the transverse part of the joint is the 1110 ^. or a e( ' c ^mode of preventing the pieces from being drawn s C& 1

other, by the sliding of the longitudinal parts 0 ,j

and thereby giving the bolts an oblique position- ^js, it ^Figure 5. A scarf formed by several steps. j on git°^iethe transverse parts of the steps be equal, and the w>' „parts as strongly compressed by bolts as the fibr^ ^ oll ld 0 -

be”

timber would adhere laterally, the loss of strength " re -be a fourth, compared to that of a solid piece; g [ a s 1four transverse parts, that is, the part which the enis of the whole. t j ire e

Figure 6. The end of each piece is formed by a tet ^ jand the abutting parts of the middle step being Sthat at either extremity, the loss of strength in ^ &ce s ® f6timber is the part which the middle abutting surthe whole section.