*Threads and Thread Cutting Session 11

*ThreadsThreadHelical ridge of uniform section formed on inside or outside of cylinder or coneUsed for several purposes:Fasten devices such as screws, bolts, studs, and nutsProvide accurate measurement, as in micrometerTransmit motionIncrease force

*Thread Terminology

*Thread TerminologyScrew threadHelical ridge of uniform section formed on inside or outside of cylinder or coneExternal threadCut on external surface or coneInternal threadProduced on inside of cylinder or cone

*Thread TerminologyMajor DiameterCommonly known as the outside diameter . On a screw thread, the major diameter is the largest diameter of the thread on the screw or nut.

*Thread TerminologyMinor DiameterCalled the root diameter, the minor diameter is the smallest diameter of the thread on the screw or nut.

*Pitch diameterDiameter of imaginary cylinder that passes through thread at point where groove and thread widths are equalEqual to major diameter minus single depth of threadTolerance and allowances given at pitch diameter lineThread Terminology

*Thread TerminologyNumber of ThreadsThe number of threads per inch.

*Thread TerminologyPitch The distance from a given point on one thread to a corresponding point on the very next thread

*Thread TerminologyLeadThe distance a screw thread advances in one revolution. The lead and the pitch of a single lead thread are the same. On double lead threads, the lead is twice the pitch. A double lead thread has two start points.

*Single LeadDouble LeadThread Terminology

*RootBottom surface joining sides of two adjacent threadsExternal thread on minor diameterInternal thread on major diameterThread Terminology

*CrestTop surface joining two sides of threadExternal thread on major diameterInternal thread on minor diameterThread Terminology

*FlankThread surface that connects crest with rootThread Terminology

*Depth of threadDistance between crest and root measured perpendicular to axisThread Terminology

*Angle of threadIncluded angle between sides of thread measured in axial plane

Thread Terminology

*Helix angleAngle that thread makes with plane perpendicular to thread axisThread Terminology

*Right-hand threadHelical ridge of uniform cross section onto which nut is threaded in clockwise directionWhen cut on lathe, toolbit advanced from right to leftThread Terminology

*Left-hand threadHelical ridge of uniform cross section onto which nut is threaded in counterclockwise directionWhen cut on lathe, toolbit advanced from left to rightThread Terminology

*Thread FormsISO MetricAmerican National Standard ThreadBritish Standard Whitworth (BSW) ThreadUnified ThreadAmerican National Acme ThreadSquare ThreadBrown & Sharpe Worm ThreadInternational Metric thread

*ISO Metric Thread60 included angleD (Internal) = 0.6143 x pitchCrest = .125 x pitchRoot = .250 x pitch

*ISO Metric Thread

*ISO metric pitch and diameter combinationsNominalThreadNominalThread DiameterPitchDiameterPitch (mm)(mm)(mm)(mm) 1.60.35202.5 20.4243 2.50.45303.5 30.5364 3.50.6424.5 40.7485 50.8565.5Portion of table taken from textbook

*American National Standard ThreadDivided into four main series, all having same shape and proportionsNational Coarse (NC)National Fine (NF)National Special (NS)National Pipe (NPT)

*Common Thread FormsCoarse Thread Series UNCThe one most commonly used in the mass production of bolts, screws, nuts and other general fastening applications. Less of a tendency for cross-threading than fine threads.It is also used on quick assembly or disassembly, or if corrosion or slight damage is possible.

*Common Thread FormsFine Thread Series UNFWhen used on external threads have greater tensile stress area than coarse threads of the same size. Resists stripping out better than coarse threads in areas where the external and mating internal threads are subjected to loads equal to or greater than the capacity of the screw or bolt. Used were the length of engagement is limited

*Common Thread Forms

*Common Thread FormsTaper Pipe Series NPTWhen used with a sealer (such as pipe dope or teflon tape) will produce a pressure tight joint.The taper of the external thread forms a joint with the taper of the internal thread.

*American National Standard Thread

*British Standard Whitworth(BSW) ThreadApplication is same as for American National form thread

*Unified ThreadDeveloped by U.S., Britain, and Canada for standardized thread systemCombination of British Standard Whitworth and American National Standard Thread

*Unified Thread

*Acme Screw ThreadReplacing square thread in many casesUsed for feed screws, jacks, and visesD = minimum .500P = maximum .500P + 0.010F = .3707PC = .3707P - .0052 (for maximum depth)

*Brown & Sharpe Worm ThreadUsed to mesh worm gears and transmit motion between two shafts at right angles to each other but not in same planeD = .6866PF = .335PC = .310P

*Square ThreadBeing replaced by Acme thread because of difficulty in cutting itOften found on vises and jack screwsD = .500PF = .500PC = .500P + .002

*International Metric threadStandardized thread used in EuropeF = 0.125PR = 0.0633P (maximum) = 0.054P (minimum)D = 0.7035P (maximum) = 0.6855P (minimum)

*Thread Fits and ClassificationsFitRelationship between two mating partsDetermined by amount of clearance or interference when they are assembledNominal sizeDesignation used to identify size of partActual sizeMeasured size of thread or partBasic size: size from which tolerances are set

*AllowancePermissible difference between largest external thread and smallest internal threadDifference produces tightest fit acceptable for any given classificationThe allowance for a 1- 8 UNC Class 2A and 2BMinimum pitch diameter of theinternal thread (2B) = .9188 in.Maximum pitch diameter of theexternal thread (2A) = .9168 in. Allowance = .002 in.

*ToleranceVariation permitted in part sizeTotal tolerance is sum of plus and minus tolerancesIn Unified and National systems, tolerance is plus on external threads and minus on internal threadsMaximum pitch diameter of theexternal thread (2A) = .9168 in.Minimum pitch diameter of theexternal thread (2A) = .9100 in. Tolerance = .0068 in.The tolerance for a 1 in.8 UNC Class 2A thread is:

*LimitsMaximum and minimum dimensions of partMaximum pitch diameter of theexternal thread (2A) = .9168 in..Minimum pitch diameter of theexternal thread (2A) = .9100 in.The limits for a 1 in.8 UNC Class 2A thread are:

*ISO Metric Tolerances and AllowancesISO metric screw thread tolerance system provides for allowances and tolerances defined by tolerance grades, tolerance positions, and tolerance classesTolerance grades specified numericallyMedium tolerance indicated by number 6, number below 6 indicates finer tolerance and number above 6 indicates greater tolerance

*Tolerance Example:An external metric thread may be designated as follows:PitchOutside NominalDiameterDiameter Metric SizePitchToleranceTolerance M 6 X0.75 -5g6gThe thread fit between mating parts is indicated by internal thread designation followed by the external thread tolerance:M 20 X 2 - 6H/5g 6g

*Allowance SymbolsFor external threads:e indicates a large allowanceg indicates a small allowanceh indicates no allowanceFor internal threads:G indicates a small allowanceH indicates no allowance

*Classes of Unified Thread FitsExternal threads classified as 1A, 2A, and 3A and internal threads as 1B, 2B, 3BClasses 1A and 1BThreads for work that must be assembledLoosest fitClasses 2A and 2BUsed for most commercial fastenersMedium or free fitClasses 3A and 3BUsed where more accurate fit and lead requiredNo allowance provided

*Classes Of Thread Fits-20 UNC 2AIn this particular case the class of fit would be a 2. The symbol A indicates an external thread. -28 UNF 3BIn this particular case the class of fit would be a 3. The symbol B indicates an internal thread.

*Thread CalculationsD = single depth of threadP = pitchCalculate pitch, depth, minor diameter, and width of flat for a -10 UNC thread.

*Thread CalculationsP = pitch = 1 mmD = 0.54127 x 1 = 0.54 mmWhat is the depth, minor diameter, crest and root for an M 6.3 X 1 thread?

*Thread-Chasing dialLathe spindle and lead screw must be in same relative position for each cutThread-chasing dial attached to carriage for this purposeDial has eight divisionsEven threads use any divisionOdd threads either numbered or unnumbered: not both

*Measuring ThreadsThree-wire methodThread ring gageThread plug gageThread snap gageScrew thread micrometerOptical comparator

*Three-Wire MeasuringThree wires of equal diameter placed in thread, two on one side and one on other sideStandard micrometer used to measure distance over wires (M)Different sizes and pitches of threads require different sizes of wires

*The Three-Wire Method of Measuring 60 Threads

*Measurement with Wireswhere M = measurement over the wires D = major diameter of the thread G = diameter of the wire size used N = number of tpiAny of the following formulas can be used to calculate G:

*Multiple ThreadsMay be double, triple or quadruple, depending on number of starts around periphery of workPitch: distance from point on one thread to corresponding point on next threadLead: distance nut advances lengthwise in one complete revolutionSingle-start: pitch and lead equalDouble-start: lead twice the pitchTriple-start: lead three times pitch

*SingleDoubleTriple

*Internal ThreadsCutting threads in a holeOperation performed on work held in chuck or collet or mounted on faceplateThreading tool similar to boring toolbit except shape ground to form of thread to be cut

*The compound rest is set at 29 to the left for cutting right-hand internal threads.Internal Threads

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