This document discusses limits, fits, and tolerances in mechanical engineering. It defines key terms like basic size, zero line, shaft, hole, deviation, allowance, and tolerance grade. It describes different types of fits - clearance fit, transition fit, and interference fit. It also explains hole basis and shaft basis systems for defining fits and tolerances. Different tolerance grades from IT01 to IT16 are used depending on the required accuracy and application, such as precision parts, general engineering, or casting.

1. INSTITUTE OF ENGINEERING & TECHNOLOGY KHANDARI
CAMPUS AGRA
MECHANICAL DEPARTMENT
A Seminar on
LIMIT FITS AND TOLERANCES
SUMBITTED BY
RANJEET KUMAR JHA
(19ME27)
BE-ME 4TH YEAR

2. TERMINOLOGY
Basic Size:- It is the standard size of a part, with
reference to which all the limits of variations of
size are determined.
Zero Line:- the line corresponding to basic size is
called as zero line.
Shaft And Hole:- These terms are used to
designate all the external and internal features of any
shape and not necessarily cylindrical.
Hole Designation:- By upper case letters from
A, B,... Z, Za, Zb, Zc (excluding I, L, O, Q, W and
adding Js, Za, Zb, Zc) - 25 nos. Indian Stds.
Shaft Designation:- By lower case letters from
a, b, ... z, za, zb, zc (excluding i, 1, o, q, w and adding
js, za, zb, zc) - 25 nos.

3. DEVIATION
UPPER DEVIATION:- The algebraic difference
between the maximum limit of size (of either hole or
shaft) and the corresponding basic size.
LOWER DEVIATION:- The algebraic difference
between the minimum limit of size (of either hole or
shaft) and the corresponding basic size.
FUNDAMENTAL DEVIATION:- It is one of the two
deviations which is chosen to define the position of
the tolerance zone.

4. LIMITS
Limit:- The limit are two extreme permissible size of a part between
which the actual size of the part is contained. They are fixed with referance to
the basic size of that dimension.

5. HOLE & SHAFT
• Basic Shaft:- It is the shaft whose upper deviation is zero or whose
maximum limit of the size is equal to basic size.
• Basic Hole:- It is the hole, whose lower deviation is zero or whose
minimum limit of the size is equal to basic size.

6. ALLOWANCE
 Allowance is the prescribed difference between the hole dimension and
shaft dimension for any type of fit.
 It is the intentional difference between the lower limit of the hole and
higher limit of the shaft.

7. FITS
• FIT :- Fit is the clearance between two mating parts.
1. CLEARANCE FIT :- In this type of fit, the largest permitted shaft
diameter is smaller than the diameter of the smallest hole, so that, the
shaft can rotate or slide through, with different degrees of freedom
according to the purpose of the mating members.

8. TRANSITION FIT
 In a fit of this type, the diameter of the largest allowable hole is greater
than that of the smallest shaft, but the smallest hole is smaller than the
largest shaft, so that, small positive or negative allowance between the
shaft and hole members are employable.
 In this type of fit, the size limits of mating (shaft and hole) parts are so
selected that, either clearance or interference may occur depending upon
the actual size of the parts.

9. INTERFERENCE FIT
 In this type of fit, diameter of minimum allowable shaft is greater than
that of Maximum allowable hole.
 In this type of fit, the sizes of the mating parts are so selected that,
interference or negative allowance will always occur.

10. TOLERANCE
The algebraic difference between upper and lower deviation.
It is absolute value.

11. TYPES OF TOLERANCE
1. Unilateral Tolerance:- If the is allowed on one
side of the basic size the system of tolerance is said
to be unilateral.
2. Bilateral Tolerance:- If the tolerance is allowed
on both side of the basic size, the system of
tolerance said to be bilateral.

12. HOLE BASIS SYSTEM
 In this system, the design size of hole, whose lower deviation
(fundamental deviation) is zero, is assumed as basic size and different
class of fits are obtained by varying the limits of the shaft only.
 In other words, the limits of the hole are kept constant and those of the
shaft are varied so as to obtain the necessary fit.

13. SHAFT BASIS SYSTEM
 In this system, the design size of a shaft, whose upper deviation
(fundamental deviation) is zero, is assumed as basic size and different
class of fits obtained by varying the limits of the hole only.
 In other words, the limits of shaft are kept constant and limits of holes are
varied to obtain the necessary type of fit.

14. DESIGNATION OF FIT
 To describe completely a hole or a shaft, its basic size followed by
appropriate letter and the number of tolerance grade is given.
 Holes are designated by capita letter.
 Shafts are designated by small letter.
Example
20 mm hole 'H' with tolerance grade IT7 is designated as 20H7. 20 mm
'f' shaft with tolerance grade IT8 is designated as 20f8.
 For Shafts a to h – Clearance Fit.
 j to n - Transition Fit.
 p to z -Interference Fit.

15. GRADES OF TOLERANCE
Grade of Tolerance:- It is an indication of the level
of accuracy. There are 18 grades of tolerances –
 IT01, ITO, IT1 to IT16.
 IT01 to IT4 - For production of gauges, plug gauges, measuring
instruments.
 IT5 to IT 7 - For fits in precision engineering applications.
 IT8 to IT11 - For General Engineering.
 IT12 to IT14 -For Sheet metal working or press working 14.
 IT12 to IT14-For Sheet metal working or press working.
 IT15 to IT16 - For processes like casting, general cutting work.

16. APPLICATION
Manufacturing process and applications :-
 Super finishing process, such as lapping, diamond boring etc. Use: Gauges
Grinding.
 Precision turning, broaching, honing.
 Turning, boring and reaming, Boring.
 Milling, slotting, planing, rolling and extrusion.
 Drilling, rough turning.
 Metal forming processes.
 Die casting, stamping Sand casting.
Machine Required :-
 Super finishing machine, Grinding machine.
 Boring machine, honing machine, Lathes, capstan and automats Boring
machines.
 Milling machine, slotting machine, planing machine and extruders Drilling
machine, lathes Presses.
 Die casting machine, hammer machine.