|Centring, Drilling and Counterboring / Countersinking - Course: Techniques for machining of material. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 22 p.)|
Normally (except for eccentric turning) these working techniques are used to produce rotationally symmetrical parts by means of the drill or boring tool. The workpiece is machined on the end face, base face and internal surface. The tool is to be selected depending on the required type of hole and size specified.
- There are through holes and blind holes.
Figure 27 Drilled holes
(1) Blind hole,
- Twist drills have straight shanks (up to 10 mm dia., standard design) or taper shanks (taper 1-6 for drills exceeding 10 mm dia.).
For very hard material, carbide-tipped drills are used.
For drills the rotational speed, corresponding with the cutting speed, is determined depending on the diameter of the drill (not the diameter of the workpiece).
- To prevent off-centre running of the drill when penetrating into the workpiece, which would produce an offset hole, a centre hole is to be produced prior to drilling with the twist drill.
Centre first, then drill!
- The drill is to be checked for correct diameter and proper drill point grinding. Dull or improperly ground drills are subject to increased stress during drilling, because of heavier friction, and may break easily.
- The drill is to be safely clamped. Small-diameter drills are chucked in a drill chuck. Drills with bigger diameter have a taper shank which is to be inserted in the taper hole of the sleeve. If the tapers do not match, the difference is compensated by taper sleeves.
Figure 28 Taper sleeve
- Using a holder for the twist drill - the drill block -, the power feed of the lathe can be utilized. The drill block is clamped instead of the turning tool. The axis of rotation and the drill axis must be exactly in line (see Fig. 16).
- Bigger drills can be protected against distortion by attaching a driver (lathe dog) as per Fig. 29.
- When clamping the drill in the sleeve, the surfaces of the outside and inside tapers must always be clean; otherwise the drill will move in the sleeve and destroy the surface.
- Almost any drill will drill a hole which is a little bigger than its diameter. Therefore, it is recommended to drill and exactly measure a test hole.
Figure 29 Protection by driver
- Correct cutting speed, correct feed and correct cooling are essential conditions for the production of a hole meeting the quality requirements.
- Clogging of chips in the spiral flutes of the drill will cause heavy friction at the wall of the hole which may result in breakage of the drill. Therefore, the drill should be frequently retracted from the hole and the chips be removed.
- Generally the tool and not the workpiece is to be cooled during drilling. The drill is to be lifted several times so that the coolant can reach the principal cutting edge, too.
- Short-chip materials (grey cast iron, brass) are dryly drilled.
- Economical machining is ensured by combination tools (step drills, subland drills).
- The use of single-edged tools (boring tools, boring bars) permits requirements for closest tolerances and high surface finish to be met.
- Such tools are used for enlarging/boring of premachined holes.
- For clamping of turning tools with round cross section of the shank, a Vee support is required.
Figure 30 Boring bar for through holes
1 turning tool,
- Boring bars are tool holders to hold small cutter bits which are used for machining of through holes and blind holes.
Figure 31 Boring bar for blind holes
1 turning tool,
3 thrust pad,