Deep hole drilling machines

Deep hole drilling machines are specialized machine tools that are used to drill deep and precise holes that are many times larger than their diameter.

Deep hole drilling machines

For the purpose of machining using deep-hole drilling methods or methods associated with deep drilling, in most cases deep-drilling machines are employed in the form of standard (multi-purpose) or special machines. Frequently when holes with smaller drilling depths (up to approx. 40 × D) are required, gun drills on machining centres are used. Ejector drilling is used primarily in conventional machine tools. Since deep drilling has a high level of productivity, only comparatively high-performance machines tend to be used. Basically a cutting oil system is required which supplies the cutting oil at a higher-than-average flow rate (compared with other drilling methods) at higher pressures. A deep-drilling unit means the grouping consisting of the deep-drilling machine itself together with the cutting oil tank and other peripheral equipment for cutting oil preparation and chips handling.

Modes of operation

Figure 1 shows which combinations of cutting and feed movements are possible when making a deep cylindrical hole. Depending on the application case these movements are to be provided by the machine.

Characteristic features of drilling with a rotating tool are:

Use with workpieces which cannot be rotated on the machine ( for example, workpieces with eccentric holes, workpieces of great (unbalanced) weight, prism-shaped workpieces)
Typical application case for gun-drilling machining
On machining centres
With single-purpose deep-hole drilling machines which do not have a workpiece drive (Figure 2, bottom)

Characteristic features of drilling with a rotating workpiece and feed movement effected by the tool are:

Used in the case of workpieces with a hole drilled at the axis of rotation
Most common method used with STS deep-drilling machines
For producing less run-out than when drilling with a rotating tool
With comparatively long boring bars
With pull boring
Used in older machines to obtain high cutting speeds
With heavy or very long workpieces
Usually has the least run-out

Machine types

Standard and special deep-drilling machines

Figure 2 shows some schematic drawings of common types of deep-drilling machine. The two universal machines shown at the top provide all of the combinations of tool and workpiece movements described in the previous Section. The remaining machines are cut back to just those modules required for the particular application case. More information about the machines may be found in guidelines VDI 3208 and VDI 3209.

Characteristic features of standard deep-drilling machines

Infinitely variable feed and speed ranges
Cutting oil equipment with filtration and possibly cooling unit as well
CNC control system
Safety and monitoring equipment ( for example, for feed monitoring, oil pressure limiting, and so on)
Relatively short retooling times
Rotary movement for workpiece and / or tool

Characteristic features of special deep-drilling machines

Single or multispindled for drilling several holes (in one workpiece or in several at the same time)
Automatic work loading and unloading device
Automatic tool-changing
Automatic chip disposal, workpiece washing
Used in mass production, in transfer lines, and so on
In some cases, other types of machining may be possible, such as skiving and roller burnishing, turning of internal or external diameters, or front-face machining. In special cases, honing can also be done on deep-drilling machines.

Other machine tools

Ejector drilling was developed as a deep-drilling technique for use with conventional machine tools. The use of gun-drilling is in particular usual in machining centres on mass production lines (Figure 3). Apart from this, especially in the borderline area between deep-hole and conventional drilling, deep-drilling methods are used with machine tools which are not regarded as deep-drilling machines. However various requirements need to be satisfied:

Suitable speeds and feed rates (infinitely adjustable)
Adequate drive power to the work spindle
Suitable cutting oil system (quantity, pressure, filtering)
As a rule, special facilities for conducting the cutting oil
Adequate chip removal, anti-splash protection

**Figure 3:** Machine tools used for deep drilling^[3]

Additional devices and equipment

Accessories for gun-drilling

Boring bar clamping unit

Boring bars used in gun-drilling have an adapter (clamping sleeve) brazed or glued onto the unfluted end depending on the clamping system used.

Cooling lubricant inlet housing

In case of the gun drill, cooling lubricant is introduced at the clamping point via the machine spindle (through the rotary transmission leadthrough). If this is not possible, between the spindle and the drill tool so-called front-end feeds (lubricant inlet housing) are used which allow the cooling lubricant to be introduced laterally.

Starting bush - starting-bush holder

In single lip gun-drilling the starting bush is inserted into the starting-bush holder which also catches the mixture of chips and cooling lubricant coming laterally out of the flute, sealing it off from the environment, and routing the mixture away. The starting bush is used for guiding the tool during the pilot drilling phase. Its design, such as central positioning and oversize, has a great influence on the entire drilling process. In many cases – in gun-drilling on machining centres, for example – the job of the starting bush is taken over by a drilled pilot bore hole made by (or reamed out by) a carbide-tipped twist drill or other short-hole drills.

Anti-whip devices (steadies)

Gun drills operate at comparatively high speeds. Since the tool shanks also have an asymmetric cross-section, whipping or bowing must be prevented in the case of longer tools. For this purpose one or more anti-whip devices are used along the tool shank. They also improve the centric position of the tool and in addition can have an damping effect. In addition to improving safety, the devices also help to improve the drill-hole quality.

Accessories for deep-drilling with external cutting oil feed (For STS and similar methods)

Boring bar

Not only the drill tips but also the boring bar is counted as part of the tool. In practice certain standard sizes have become established for the bar diameter. The threaded connection may be either single-start or four-start special threads. Large tools are also flanged on.

Boring bar steadies

Particularly in the case of longer boring bars or tools (with high l/D ratios, from about 40), steady rests are used in addition. These guide the boring bar or tool along the drilling axis. With STS drilling and especially with rotating boring bars, mechanically or hydraulically operated vibration dampers are also used instead of the steady rest (support shell) which only provides guidance.

Boring bar clamping unit

Various designs of clamping chucks are in common use.

Cutting oil delivery system

This is used for delivering the cutting oil to the workpiece, for sealing off with respect to both the workpiece and the boring bar, and also as a holder for the starting bush. In most cases the cutting oil delivery system also supports the workpiece, by means of a conical or cylindrical holder, for example. The back-end seal to the boring bar, which usually takes the form of a stuffing box, also performs an additional task that of guiding the boring bar. In the case of push boring the drill bushing is installed in the cutting oil delivery system. It is used for guiding the tool during the pilot-drilling stage. The characteristics of the starting bush, such as central position and oversize, have a great influence on the entire drilling process.

Starting-bush holder with pull boring (lantern chuck)

In case of pull boring the starting bush is located at the end facing away from the cooling lubricant feed apparatus. Therefore it can be installed in the workpiece holder or the hollow spindle of the workpiece drive. In this case the tool must be threaded onto the back end of the hollow spindle. A closable cover will be required for this. However, in many cases the method of starting-bush installation and tool changing described here is not actually used. Instead, a workpiece counter bracket, the so-called “lantern chuck”, is used. The lantern chuck is a device which is fitted on one side of the workpiece collet chuck (if applicable, of the spindle nose). On the side facing the workpiece it carries the starting bush and the workpiece holder. On the side of the device there is a door which allows the operator to reach into the interior so as to thread the tool onto the boring bar. It is also possible to seal the lantern chuck off against the hollow spindle by inserting a “pot”. Chips and cooling lubricant may also exit via the lantern chuck in special cases. If so, a cowl will be needed to provide sealing off from the environment.

Vibration dampers

Particularly in the case of long boring bars a vibration damper should be used. This is used for reducing torsional and flexural vibrations which not only have a negative impact on bore quality but also reduce the service life of the tool. Both mechanical and hydraulic vibration dampers are used.

Other equipment

Torsional vibrations

Here the tool vibrates at a high frequency (from about 500 Hz up to 1 natural frequency) about the longitudinal axis of the bar or shank. The vibration occurs in the first or in a higher torsional eigenfrequency. It produces a high level of noise which is perceived as unpleasant. Tool wear is increased and frequently the surface quality of the bore is impaired. The use of a vibration damper is recommended as a way of preventing this kind of vibration. Another remedy is to adjust the cutting parameters, in this case the feed rate in particular.

Twisted drilling (flexural vibration)

Here the boring bar vibrates at low frequency (approx. 50 Hz) vertically to the longitudinal axis. The bar bends within its range of elasticity. Tool wear is increased by spiralling. This vibrational mode does however have a very marked negative effect on the hole shape on both the macro and micro levels. A typical result is the polygonal formation. Frequently the cause of lobed holes is the displacement of the drilling head axis away from its ideal position (centric and parallel to the desired drilling axis). This displacement can occur for various reasons: unsatisfactory pilot drilling

excessive feed rate
poor tool regrinding
warped boring bar or warped tool shank
inaccurate boring bar clamping, imprecise boring bar steadying
geometrical error in the machine
position of the damper

Literature

VDI 3208: Tiefbohren mit Einlippenbohrern
VDI 3209: Tiefbohren mit äußerer Zuführung des Kühlschmierstoffs (BTA- und ähnliche Verfahren)
VDI 3209: Blatt 2 Tiefbohren; Richtwerte für das Schälen und Glattwalzen von Bohrungen
VDI 3210: Blatt 1 Tiefbohrverfahren
VDI 3211: Tiefbohren auf Bearbeitungszentren
VDI 3212: Abnahmebedingungen für einspindelige und mehrspindelige Tiefbohrmaschinen

References

^ VDI 3210
^ VDI 3210
^ VDI 3210

[1] VDI 3210

[2] VDI 3210

[3] VDI 3210

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[2]

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