EXPERIMENT NO.5 Measurement of Screw Thread using AIM:To measure the Major, Minor and Effective diameter of external parallel screw threads using. APPARATUS: EDMM-100CL, Prism (A to D), Wire (1 to 4mm), Specimen, Master, Hooks etc. THEORY:In order to ensure the manufacture of screw threads to the specified limits laid down in the appropriate standard it is essential to provide some means of inspecting the final product. For measurement of internal threads thread plug gauge is used and to check these plug gauges is used for measuring Major, Minor and Effective diameter. FLOAING CARRIAGE MEASURING MACHINE: Fig: EDMM-100CL SPECIFICATIONS: 1. Weight of the machine : Approx. 25kg. 2. L x W x H : Approx. 350mm x 150mmx 140mm 3. List count of micrometer : 0.001 mm 4. Standard micrometer or electronic type. Electronic Micrometer has digital display and Std. 5. Dial type Fiducial Indicator with 0.01mm standard dial. 6. Admit between centre 200 mm 7. Max Diameter capacity 100mm Measuring machine shown in the figure has Base with two small and one big adjustable support knobs provided for leveling the assembled unit. Base hastwo parallel integral V grooves one short and other long. Long groove is for guide pegs located at the bottom of Intermediate Piece or Carriage (B) and smaller for a ball. One more V pair is in the Centre of the base, which is provided for accommodating Centers (E) to hold work -1-
piece. Carriage has two parallel V grooves, one to accommodate two balls and other to accommodate one ball. Underneath the floating top (C) there is one V groove on one side and flat portion on other side. Digital Micrometer is in one bracket (with less width) and dial type fiducial on the other side lever is provided to tighten it. Fig: Different views of -2-
DEFINITIONS RELATED TO PARALLEL SCREW THREADS Major Diameter: The diameter of an imaginary cylinder (termed the major cylinder) which just embraces the crests of the external thread or the roots of an internal thread. Minor Diameter: The diameter of an imaginary cylinder (termed the minorcylinder) which just embraces the roots of an external thread or the crests of an internal thread. Simple effective(or Pitch) Diameter: The diameter of an imaginary cylinder(termed the pitch cylinder ) which intersects the surface of the thread in such manner that the intercept on an generator of the cylinder between the points where it meets the opposite flanks of the thread groove is equal to one half the basic of the thread. The two methods for thread measurement as follows Two Wire Method: The effective diameter of a screw thread may be ascertained by placing two wires orrods of identical diameter between the flanks of the thread, andmeasuring the distance over the outside of these wires. The effective diameter is thencalculated as E=T+P Where T= Dimension under the wires =M 2d M=dimension over the wires, d= diameter of each wire -3-
Fig (a) Fig (b) The wires used are made of hardened steel to sustain the wear and tear in use. Theseare given a high degree of accuracy and finish by lapping to suit different pitches.dimension T can also be determined by placing wires over a standard cylinder of diametergreater than the diameter under the wires and noting the reading R1 and then taking readingwith over the gauge, say R2. Then T=S (R1 R2). P=It is a value which depends upon the diameter of wire and pitch of the thread. If P= pitch of the thread, then P= 0.9605p 1.1657d (for Whitworth thread). P= 0.866p d (for metric thread). to give the effective diameter. The expression for the value of P in terms of p (pitch), d(diameter of wire) and x (thread angle) can be derived as follows: In Fig. since BC lies on the effective diameter line BC= ½ pitch=½ p OP= (d cosec x/2) 2 PA=d (cosecx 2 1) 2 PQ=QC cot x 2=p 4 cot x 2 AQ=PQ AP=(p cot x 2) 4 d (cosec x 2 1) 2 AQ is half the value of P.. P value=2aq =p 2 cot x 2 d (cosecx 2 1) Two wire method can be carried out only on the diameter measuring machinedescribed for measuring the minor diameter, because alignment is not possible by two wires andcan be provided only by the floating carriage machine. In the case of three wire method, two wire, on one side help in aligning the micrometer square to the thread while the third placedon the other side permits taking of readings. -4-
Three Wire Method: This method of measuring the effective diameter is an accurate method. In this threewires or rods of known diameter are used; one on one side and two on the other side [Fig. (a) and (b)]. This method ensures the alignment of micrometer anvil faces parallel tothe thread axis. The wires may be either held in hand or hung from a stand so as to ensurefreedom to the wires to adjust themselves under micrometer pressure. M=distance over wires E=effective diameter r=radius of the wires d=diameter of wires h =height of the centre or the wire or rod from the effective x=angle of thread. Fig (a) From fig.(b), AD = AB cosec x 2 = r cosec x 2 H = DE cot x 2 = p 2 cot x 2 CD = ½H = p 4 cot x 2 H = AD CD r = cosec x 2 p 4 cot x 2 Distance over wires=m = E+2h+2r = E+2(r cosec x 2 p 4 cot x 2)+2r = E+2r (l+cosecx 2) p 2 cot x 2 or M = E+d (1+cosec x 2) p 2 cot x 2 (since 2r = 0 ) Fig (b) (i) In case of Whitworth thread: X = 55, depth of thread = 0.64 p, so that E= D 0.64 p and cosec x 2 = 2.1657 Cot x 2 = 1.921 M = E+d(1l+cosec x 2) p 2 cot x 2 = D 0.64p+d(1+2.1657) p 2(1.921) = D+3.1657d 1.6005p M = D+3.1657d 1.6p -5-
Where D=outside dia. (ii) In case of metric threads: Depth of thread=0.6495p So, E = D-0.6495p. x = 60, cosec x 2 = 2;cot x 2 = 1.732 M = D 0.6495 p+d(l+2) p 2 (1.732) = D+3d (0.6495+0.866)p = D+3d 1.5155p. PROCEDURE: 1. The diameter of setting master or a cylinder should be nearly same as the diameter of the thread gauge to reduce error. 2. First meet the anvil of micrometer to dial gauge and rotate micrometer in forward to rotate needle in dial by 10mm with setting its scale zero at needle positionand make digital micrometer to zero by pressing button. 3. Put master in between centers and make dial needle to zero by moving micrometer. Take the reading on micrometer as R S.. Similarly replace master with a threaded work piece again second reading is taken as R 4. Hang the required set of prisms (from Appendix-1 according to pitch of work piece) on the hooks provided on both sides of standard. Take the reading on micrometer and zero indicator in such a way that portion of prism touches master and flat portion of prism is on micrometer & zero indicator. 5. Take reading for master as R P and work piece as R q on micrometer as needle on dial indicating zero. 6. Now place two small wires or cylinders in place of prism. The wires should be choosen so that, when placed between the threads, they should contact about halfway down theflanks. Suitable sizes of these cylinders for various threads are given atappendix-1 7. Take the readings for master as R W and work piece as R OW on micrometer as needle on dial indicating zero. OBSERVATION: D = Diameter of Setting master = Dimension of work piece = 14M*2-6-
Observation Table: MAJOR DIAMET (M) MINOR DIAMET (C) R S M = D ± (Difference between R and R S ) = R R p C = D ± (Difference between R p and R q ) R q = EFFECTIVE DIAMETER (E) R W T = D ± (R W - R OW ) = R OW E = T + P = CALCULATIONS: Major diameter: R S = Micrometer reading over setting master. R = micrometer reading of threaded work piece. M = D ± (Difference between R and R S ) (Note: The + or is determined by relative size of the master and the work piece.use sign as master is of greater size than work piece and vice-versa for + sign.) Minor diameter: -7-
C = core or minor diameter of the work piece. R p = reading over master with prisms. R q = Reading over work piece with prisms. C = D ± (Difference between R p and R q ) Effective diameter: E = effective or pitch diameter R OW = Reading measured over work piece with wires. R W = Reading measured over setting master with wires. T = measured dimension under cylinders. T = D ± (R W - R OW ) E = T + P P = Constant depending on the pitch and angle of the thread and the mean diameter of wires used. (Reading P values for different threads are given along wires with wires to be used in Appendix -1) -8-
RESULT : Major Diameter = Minor Diameter = Effective Diameter = CONCLUSION: Using the diameter of imaginary cylinder i.e. Effective or pitch diameter can be evaluated. SAMPLE CALCULATIONS OBSERVATION: D = Diameter of Setting master = 16.004mm Dimension of work piece = 14M*2 Observation Table: MAJOR DIAMETERR S 16.05 Major Diameter = D ± (Difference between R an R 14.05 R S ) =14.004mm MINOR DIAMETERR p 26.37 C = D ± (Difference between R p and R q ) R q 21.491 =11.125mm EFFECTIVE DIAMETER R W 18.699 R OW 15.009 T = D ± (R W - R OW ) =12.314mm E = T + P =12.696mm Major Diameter = D ± (Difference between R and R S ) =16.004 (16.05-14.05) = 14.004mm Minor diameter C = D ± (Difference between R p and R q ) = 16.004- (26.37-21.491) =11.125mm Effective Diameter T = D ± (R W - R OW ) = 16.004-(18.699-15.009) =12.314mm E = T + P = 12.314 +0.382 = 12.696mm -9-