Gages Unit 14

Objectives Recognize and describe the uses of three types of plug and ring gages Check the accuracy of a part with a plug or ring gage Check the accuracy of a part using a snap gage

Gages: Basic Terms Basic dimension: exact size of part from which all limiting variations made Limits: maximum and minimum dimensions Tolerance: permissible variation of part unilateral: one direction only Bilateral: both plus and minus (two directions) Allowance: intentional difference in dimensions of mating parts

Fixed Gages Used for inspection purposes Provide quick means of checking specific dimension Easy to use and accurately finished to required tolerance Generally finished to 1/10 the tolerance designed to control

Cylindrical Plug Gages Used for checking inside diameter of straight hole (generally "go" and "no-go" variety) Consists of handle and plug on each end ground and/or lapped to specific size Small-diameter plug ("go" gage) checks lower limit of hole Larger-diameter plug checks upper limit Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Cylindrical Plug Gages Go Dimensions usually stamped on handle at each end. "go" end longer than "no-go" for easy identification Many made with carbide tips to increase gage life Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

To Use a Cylindrical Plug Gage Select plug gage of correct size and tolerance Clean both ends of gage and hole (dry cloth) Check gage and workpiece for nicks and burrs Wipe both gage ends with oily cloth to distribute thin layer of oil

To Use a Cylindrical Plug Gage Start the "go" end squarely into hole If hole within limits, gage will enter easily Plug should enter hole for full length with no excessive play between plug and part Hole tapered if gage enters only part way and excessive play indicates hole is elliptical After checking with "go" end, check with "no-go" end End should not begin to enter hole, if does then indicates oversize, bell-mouth, or tapered hole

Cylindrical Plug Gage SAFETY PRECAUTIONS Do not force or turn it. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Plain Ring Gages Used to check outside diameter of pieces Ground and lapped internally to desired size Size stamped on side of gage Outside diameter knurled and "no-go" end identified by annular groove on knurled surface Precautions and procedures similar to those outlined for a plug gage

Taper Plug Gages Used to check size of hole and taper accuracy Made with standard or special tapers Some have "go" and "no-go" rings scribed Gage fits into hole between two rings means within required tolerance Some have steps ground on large end to indicate hole-size limits Wobble between plug gage and hole evidence of incorrect taper

To Check Internal Taper Using a Taper Plug Gage Select proper taper gage for hole checked Wipe gage and hole with clean dry cloth Check both gage and hole for nicks and burrs Apply thin coating Prussian blue to surface of plug gage Insert plug gage into hole as far as it will go Maintain light end pressure on plug gage, rotate it counterclockwise for ¼ turn

To Check Internal Taper Using a Taper Plug Gage Check diameter of hole Proper size indicated when edge of workpiece lies between limit steps or lines on gage Check taper of hole by attempting to move gage radially in hole Movement or play at large end indicates excessive taper; movement at small end indicates insufficient taper Remove gage to see if bluing has rubbed off evenly along length of gage (proper fit)

Taper Ring Gages Used to check both accuracy and outside diameter of taper Often have scribed lines or step ground on small end to indicate "go" or "no-go" Procedures like taper plug gage When checking work not ground, three equally spaced chalk lines around circumference and extending for full length of taper used to indicate accuracy of taper Work ground, use three thin lines of Prussian Blue

Care of Plug and Ring Gages Gage life dependent on following factors Materials from which gage is made Material of part being checked Class of fit required Proper care of the gage

To Preserve Accuracy and Life of Gages Store gages in divided wooden trays Check them frequently for size and accuracy Correctly align gages with workpiece to prevent binding Do not force or twist plain plug or ring gage Clean gage and workpiece thoroughly

To Preserve Accuracy and Life of Gages Use light film of oil on gage to help prevent binding Make provision for air to escape when gaging blind holes with plug gage Have gages and work at room temperature Never use inspection gage as working gage

Thread Plug Gages Used for checking internal threads of the "go" and "no-go" variety Based on same principle as cylindrical plug gages "go" end (longer end) Should be turned in flush to bottom of hole "no-go" end Should just start into hole and become snug before third thread enters

Precautions When Working with Thread Plug Gages Have chip groove cut along thread to clear loose chips Prolong life of gage, remove burrs and loose chips by means of old tap Before using, apply a little oil to surface Never force the gage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Thread Ring Gages Adjustable thread ring gage (most popular) Used to check accuracy of external thread Has threaded hole in center, with three radial slots and a setscrew to permit small adjustments Outside diameter knurled "no-go" gage identified by annular groove cut into knurled surface Thread being checked should fully enter "go" gage but not enter "no-go" by more than 1 ½ turns

Snap Gages One of most common types of comparative measuring instruments Faster to use than micrometers Limited in their application Used to check diameters within certain limits by comparing part size to preset dimension of snap gage

Snap Gages Have C-shaped frame with adjustable gaging anvils or rolls set to"go" and "no-go" limits of the part Several styles Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Using Snap Gage to Check a Dimension Thoroughly clean anvils of gage Set "go" and "no-go" anvils to required limits using a standard Lock anvils in position and recheck accuracy Clean surface of work Hold gage in right hand, keeping square with work

Using Snap Gage to Check a Dimension Lower anvil in position on the workpiece with left hand Push gage over work surface with rolling motion Do NOT force gage Advance gage until "no-go" anvils or rolls contact work if gage stops at this point, work within limits