7. Reaming and tapping

Reaming is made with reamers. As a rule, the reamers are firmly mounted in the drilling spindle by means of taper sleeves. They are fed into the hole by manual feeding. Cylindrical as well as taper holes can be made by the technique of reaming.

Machine reaming is applied, if for a great number of workpieces a high surface finish is to be reached and another fine-finishing operation (e.g. grinding) is not possible.

Table 1. Recommended values for machining allowances

Final size in mm	Machining allowance (undersize) in mm
< 5	0.1 - 0.2
5-20	0.2 - 0.3
21 - 32	0.3
33 - 50	0.5
51-70	0.8
71 - 120	1 - 1.2
> 121	1.3 - 1.5

For reaming the workpiece bore must be precisely premachined and have a sufficient machining allowance (see Table 1). The machining allowance depends on the diameter of the hole to be machined.

The dimensional variations of bores (fitting holes) can be taken from corresponding standard tables (see Table 2), if necessary.

Example:

Fitting hole

Ø 50 H7	upper limit 50.025
Ø 50 - nominal size	lower limit 50.000
h - position in the tolerance zone
7 - quality index

When is the technique of reaming applied?
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Tapping on the drilling machine is made by means of taps and the necessary tool holders (normally tapping chuck). The feed is mostly operated by hand. Depending on the thread lead, it can be effected, however, also mechanically (s = p, feed = thread lead). The tapped hole is always premachined. Kind of thread, minor diameter and thread lead must be known.

Figure 32. Thread sizes (simplified)

1 minor diameter, 2 major diameter, 3 lead

This information is mostly compiled in tabular form and must be available.

Tapped holes are made as through bores and blind bores. The tap is to be chosen accordingly. Taps with long chamfer are used for through bores and taps with short chamfer are used for blind bores. Tapping on the drilling machine is applied when non-permanent joints are to be manufactured with uniform quality. It is applied for tapped holes below 50 mm and for simple workpiece shapes (not bulky).

Table 2. Common kinds of fit (except from ISO system of fits/basic holes)

Tolerance zone		E 8	F 7	H 6	H 7	H 8	H 11
nominal	size
range	(mm)	nominal sizes in µm
over	1	+ 28	+ 16	+ 7	+ 9	+ 14	+ 60
up to	3	+ 14	+ 7	0	0	0	0
over	3	+ 38	+ 22	+ 8	+ 12	+ 18	+ 75
up to	6	+ 20	+ 10	0	0	0	0
over	6	+ 47	+ 28	+ 9	+ 15	+ 22	+ 90
up to	10	+ 25	+ 13	0	0	0	0
over	10	+ 59	+ 34	+ 11	+ 18	+ 27	+110
up to	18	+ 32	+ 16	0	0	0	0
over	18	+ 73	+ 41	+ 13	+ 21	+ 33	+130
up to	30	+ 40	+ 20	0	0	0	0
over	30	+ 89	+ 50	+ 16	+ 25	+ 39	+160
up to	50	+ 50	+ 25	0	0	0	0
over	50	+106	+ 60	+ 19	+ 30	+ 46	+190
up to	80	+ 60	+ 30	0	0	0	0
over	80	+126	+ 71	+ 22	+ 35	+ 54	+220
up to	120	+ 72	+ 36	0	0	0	0
over	120	+148	+ 83	+ 25	+ 40	+ 63	+250
up to	180	+ 85	+ 43	0	0	0	0
over	180	+172	+ 96	+ 29	+ 46	+ 72	+290
up to	250	+100	+ 50	0	0	0	0
over	250	+191	+108	+ 32	+ 52	+ 81	+320
up to	315	+110	+ 56	0	0	0	0
over	315	+214	+119	+ 36	+ 57	+ 89	+360
up to	400	+125	+ 62	0	0	0	0
over	400	+232	+131	+ 40	+ 63	+ 97	+400
up to	500	+135	+ 68	0	0	0	0

The connection between drilling spindle and square end of the tap is effected over the tool holder which can be designed as tap chuck (see Fig. 24) or as square hole with tool taper. In the tap chuck the tool is firmly clamped, in the square hole with tool taper it is held loosely.

In order to avoid tool breakage, tap chucks should be used, if possible. This possibility is limited, however, to taps smaller than 10 mm, as the force balance of the overload clutch would otherwise be nullified too quickly.

Therefore, tapping of larger bores requires considerably greater skills and attention.

Tapping is carried out as one technological process according to the following working steps:

1. centring
2. drilling of the minor diameter
3. chamfering of the minor diameter
4. tapping
5. checking

When machining tapped holes, always mark the depth of the thread on the cutting part of the tap (wound wire, colour marking) for checking purposes to prevent tool breakage.

When changing the running direction of the drilling spindle, note its lag time (time from switching off to the change of the running direction).

Tapped holes are made, as a rule, with the use of cutting oil. The following hints on labour safety must be followed:

- Carry out operations near the tool only when the drilling spindle is at rest.
- An emergency tripping of the machine must always be guaranteed.
- It must be possible to apply coolants or lubricants on the tap without any danger.

What is the minor diameter of an M 14 x 1.5 tapped hole?
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What kind of clamping is to be preferred for the tap?
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