7. Indication of fits on the straight reamer

In assembly and interchangeable manufacture, a system of standardization of fits is internationally accepted:

It classifies fits in definite ranges of accuracy. Such range is called range of tolerance and is limited by a maximum limit and a minimum limit.

ISA SYSTEM OF TOLERANCES AND FITS

Example of a bore true to size:

(This designation is indicated on the shank of the reamer.)

Ø 8K7

Ø 8 = nominal diameter 8 mm
K7 = range of tolerance

The tolerance limits can be 8.005 mm - maximum limit
taken from a relevant table: 7.990 mm - minimum limit

Conclusion:

By a reamer of Ø 8K7 a bore true to size can be made within the range of 7.990 mm and 8.005 mm.

For the operation itself knowledge of the maximum and minimum limits is not required if the fit is known.

Note:

The designation of the fit is identical on the tools and on the testing equipment - in connection with a certain tool the corresponding testing instrument roust be used.

Figure 13 - Plug limit gauge Ø 8K7

1 - Go end (minimum size),
2 - Handle with indication of fit,
3 - Red colour ring indicating the not-go end,
4 - Not-go end (maximum size)

What is the designation of the plug limit gauge by which a bore shall be checked that was reamed by a reamer marked Ø 8K7?
__________________________________________________________________________

By the plug limit gauge it can only be tested whether or not the bore is within the fixed range of tolerance - the exact diameter cannot be determined by the plug limit gauge.

Exercise:

The following tool and machine values have to be determined for making a cylindrical through hole in steel true to size and of a diameter of Ø 8K7:

Diameter of the drill (D):	______________________________
Rotational speed (n):	______________________________
Countersinking diameter (Ds):	______________________________
Rotational speed (n):	______________________________

Enter the individual steps of operation and the values calculated for this bors in the below table; fill in blank spaces:

No.	Operation	Tools, testing and auxiliary equipment	Tool and machine values
1.	Clamping		-
2.	Scribing/ prick-punching		-
3.	Drilling	Drill vernier caliper lubricating and cooling agents	D = n =
4.	Countersinking	Countersink vernier caliper	Ds = n =
5.	Reaming	Hand reamer tap wrench cutting oil	Ø 8K7
6.	Cleaning		-
7.	Testing	Plug limit gauge	Ø

This form of table can be used for preparing the practical exercises in reaming.

Exception

This sequence of operations does not apply to the use of structural reamers - the reaming of offset holes is no fitting operation.