1. Gages
Unit 14

2. 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

3. 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

4. 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

5. 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
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6. Cylindrical Plug GagesGo
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
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7. 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

8. 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

9. Cylindrical Plug Gage SAFETY PRECAUTIONS Do not force or turn it.
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10. 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

11. 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

12. 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

13. 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)

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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
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20. 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

21. 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

22. Snap Gages
Have C-shaped frame with adjustable gaging anvils or rolls set to"go" and "no-go" limits of the part
Several styles
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23. 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

24. 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