6. Reaming of taper holes

Tapers can also be produced by the reaming technique by means of the reamer. Here, too, the holes are close-tolerance holes. The cutting is also done on the inner surface of the workpiece in its centre of rotation by means of a solid or floating reamer. Because of the intricacy of pre-machining the hole, the following rules are to be observed.

- The taper hole may be pre-machined as straight stepped hole (Fig. 15) but also by top slide adjustment (Fig. 16) or by means of the taper guide bar (Fig. 17).

Figure 15 Straight stepped hole

1 diameter 1, 2 diameter 2, 3 diameter 3

Figure 16 Taper turning by top slide adjustment

1 workpiece held in chuck, 2 top slide, 3 carriage, setting angle

Figure 17 Taper turning by means of taper guide bar

1 chuck and workpiece, 2 taper guide bar, 3 top slide, 4 carriage, a/2 setting angle

In any case it is to be ensured that the diameter after pre-machining does not fall below the minimum taper diameter (see Fig. 19) but is provided with a machining allowance for reaming.

- Roughing reamers can also be used for taper reaming of holes pre-machined as straight stepped holes (Fig. 18).

Figure 18 Pre-machining reamers (1) roughing reamer (2) semi-finishing reamer

- Top slide adjustment is the method mostly used for pre- machining of taper holes. The taper guide bar is normally used where the taper length (see Fig. 19) exceeds the working travel (crank length) of the top slide.

Figure 19 Pre-fitting check with taper plug gauge

1 = d minimum taper diameter, 2 = D maximum taper diameter, 3 = 1 taper length, 4 = a pre-fitting length, a/2 setting angle

- The hole is properly pre-machined when the taper plug gauge is supporting over its entire length. The entire length of the taper plug gauge is marked by means of try-out paste, chalk or similar material. When turning the taper plug gauge in the hole the chalk, for instance, must be equally distributed over the length and diameter.

The pre-fitting length (see Fig. 19) can be calculated by the formula

The actual taper reaming is performed as per sections 4 and 5 above.

In taper reaming the cutting is not done by the lead of the reamer but by the entire length of the taper reamer. The cutting forces, therefore, are considerably higher and, consequently, the cutting values are to be reduced to 50%.

The taper plug gauge (Fig. 12) is used for testing.

When is a taper hole considered prepared for reaming?
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The reaming technique shall be applied to produce a taper hole M.T. 4 (Morse taper size 4); a/2 = 1° (degree) 29’ (minutes) 15” (seconds); d = 25.71 mm; D = 31.5 mm.

Calculate the machining allowance and the pre-fitting length a!

Formula:	a =
Given:
Required:
Calculation:
		================== mm