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Background ... F3
Boring operation types ... F4
Boring tools ... F5
Choice of boring tool type ... F6
Roughing ... F6
Finishing ... F7
Inserts for boring ... F8
Building and setting boring tools ... F9
Ways of countering vibrations in boring ... F9
Success factors for boring ... F10
Tool overhang and diameter ... F10
Cutting data and chip control ... F10
Insert nose and geometry ... F11
Cutting fluid ... F11
Damped tools ... F11
The tool coupling ... F11
Specially for reaming ... F11
How to select boring tools ... F12
Coromant Capto for boring ... F13
Boring tools for roughing ... F14
Boring tools for finishing ... F15
CoroBore 820 ... F16
CoroBore 820 range ... F17
Duobore ... F18
Duobore range ... F19
Heavy duty boring tools ... F23
CoroBore 825 ... F25
CoroBore 825 range ... F26
Single edge fine boring tools ... F27
Fine boring head ... F27
Boring bars ... F28
Start values for finish boring ... F29
Setting values for high speed boring ... F29
Contents
Boring
Mounting dimensions – fine boring units ... F37
Setting the boring unit ... F38
Cartridge replacement in fine boring unit ... F39
Cutting speed recommendations for boring ... F41
Coromant flexible boring tool ... F44
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Boring
Background
Boring operations involving rotating tools are applied to machine holes that have been made through methods such as pre-machining, casting, forging, extru-sion, flame-cutting, etc. Roughing opera-tions are performed to open up the exis-iting hole to within large tolerances and usually to prepare for finishing, which makes the hole to within tolerance and surface finish limits.
Typically, boring operations are per-formed in machining centres and vertical boring machines. The rotating tool is fed axially through the hole. Most holes are through-holes, often in prismatic or round components. External boring operations can be accomplished using specially adapted boring tools.
The most common size holes being bored are 30 to 100 mm diameter. The range of Sandvik Coromant boring tools covers 23 to 550 mm for roughing and 3 to 975 mm for finishing.
As regards hole depths, four times the hole diameter is generally the maximum recommended depth. Various tools have individual maximum hole depths. For rough and fine boring of deeper holes - of six times the diameter - damped boring tools with tuned tool bodies should be employed.
As regards hole quality, finish boring can typically achieve holes within tolerances of IT 9 with some achieving IT6. Surface
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Boring operation types
Boring operations are divided into: - single-edge cuts- multi-edge cuts - step-boring - reaming
Single-edge boring is usually applied for finishing operations and for roughing and finishing in materials where chip control is demanding. A single-edge boring tool may also be a solution when machine power is a limiting factor.
Multi-edge boring, involving two or three cutting edges, is em-ployed for roughing operations where metal removal rate is the first priority. High productivity levels can be maintained by allow-ing two or three inserts, set at the same axial height, each to machine at the recommended feed per tooth. This results in a high feed per revolution through the hole.
Step-boring is performed in roughing by a boring tool having the inserts set at different axial heights and diameters. This also improves chip control in demanding materials with the different depths of cut of up to 1.0 or 1.5 times the cutting edge length. Depths of cut of 0.5 times the cutting edge length can be di-vided into smaller cuts, providing smaller chips.
Reaming is a light finishing operation performed with a multi-edge tool giving high-precision holes. Very good surface finish and close dimensional tolerance are achieved at high penetra-tion rates. The pre-machined hole needs to be within close lim-its and the radial depth of cut is small.
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Boring tools
A boring tool is usually a modular tool, consisting of a basic holder, adaptor and a cutting unit in the form of a cartridge, slide or fine boring unit. A fine boring head holds a boring bar which is radially adjust-able and a reamer has a tool shank with a close pitch head with brazed blades. The tools are radially adjustable within a spe-cific diameter range.
The smallest diameter holes, from 3 mm upwards are bored with the fine boring heads while largest holes of up to 975 mm are finish-bored with a single-edge tool having an adjustable extension-slide mounted on an eccentric bar.
A dedicated boring tool is the best so-lution for operations on components in small to medium batches, with the key features being good productivity – pen-etration rate -and high versatility – one tool used for several hole diameters.
Boring tools for roughing with more than one cutting edge have relatively simple ad-justment possibilities and it is preferable
to use a tool pre-setting facility, however, a modern design such as CoroBore 820 can also be set using Vernier calipers.
Boring tools for finishing with one cut-ting edge must be settable to within mi-crons and, for this reason, the Sandvik Coromant range of boring tools are set-table to within 2 microns. Tools should be set in a tool pre-setter. With only one cutting edge, the finishing tool will experi-ence some degree of radial deflection during machining. This means that a pre-measuring cut is normally needed, fol-lowed by an adjustment of the tool.
Different insert carriers are employed on boring tools and these are dedicated to the type of tool (slides, cartridges, boring bars and fine-boring units). The fine-bor-ing unit is used in special tool solutions and can be mounted in blind holes with-out the need for locking after setting.
The mechanism of CoroBore 825 can be used for special tool solutions. It is more rigid than the fine boring unit but
does require access from the back of the shank and for the mechanism to be locked when set.
The reamer provides through-holes with high surface finish and dimensional accu-racy at high penetration rates.
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The choice of tool for single-edge, multi-edge or step-boring operations is very much down to capability and requirement. Roughing tools are primarily multi-edge and finishing are single-edge tools. Roughing tools include CoroBore 820, DuoBore (R391) and heavy duty boring tools (R391.B…-R), all tools being capa-ble of all the three operations.
Finishing tools include CoroBore 825, fine boring tools (R391.B..-F, fine boring head and the Reamer 830.
Roughing
CoroBore 820 is first choice providing high metal removal capability through boring with three inserts. Power require-ment is higher with several cutting edges engaged. All three inserts are set to the same diameter and height for high-pro-ducitivity boring while for step-boring, the inserts are set at different diameters and heights. This achieved by using different shims placed between the tool adaptor and slide, where the highest insert is set for the smallest diameter and the lowest for the largest.
When single-edge boring with CoroBore – particularily for finishing or when ma-terials with demanding chip control is involved - only one insert-slide is used, with the other two slides replaced by cov-ers to maintain the balance of the tool and to be protected.
Duobore has two inserts and needs less power. For high-productive rough step boring, the two inserts are set to the same diameter and height in the tool providing a penetration rate giving dou-ble the recommended feed per tooth. For step-boring the tool setting principle is the same as for CoroBore 820.
The maximum recommended depth of cut is half the cutting edge length. Each insert can be set accordingly making it possible to have depths of cut up to the full cutting edge length. The depth of cut of half the cutting edge length can be divided into two smaller cuts, giving smaller chips – advan-tageous in demanding materials.
When single-edge boring is employed, only one of the insert-slides is used, leaving the other seat to be protected by a cover that radially supports the slide.
The heavy duty boring tools, primarily for larger diameters, are also capable of all three operations types. The tool design re-sembles the boring tools for the smaller diameter ranges but with mounting dimen-sions for standard milling arbour. Approxi-mate settings are performed by moving sides across the adaptor or bar followed by fine setting on the cartridge.
With two cartridges, each mounted on an extension slide, which is radially adjustable on an adapter and using shims, the cutting edges can be set in accordance with the operation type to be employed. The tool for the larger diameter range uses a wide bar mounted on the adaptor and on which two extension slides can be set.
CoroBore 820 with tree cutting edges.
Heavy duty boring tools for large diameters.
CoroBore rough boring tools with two cutting edges.
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Finishing
CoroBore 825 is the first choice for gen-eral finish boring within the diameter range of 23 to 250 mm. This boring tool has a single cartridge, with high stability and easily adjustable radially to within 2 microns, to achieve IT6 tolerances and surface finishes within Ra 1 micron. The CoroBore 825 finsihing tool complements the CoroBore 820 roughing tool for the same hole diameter and depth range to provide a modern solution for boring.
The fine boring tools for larger diameters consists of two different designs : one for single or twin-edge boring of internal diameters 250 to 575 mm and external of 138 to 447 mm and one for internal boring of 250 to 975 mm.
Common for both are adapter, bar, slide and fine boring head. Rough setting is carried out by moving the slide along the bar and then adjusting the fine bor-ing head. To avoid large holes becombor-ing oval at high spindle speeds, recommen-dations for a counterweight can be sup-plied. Selecting the most suitable insert and entering angle and the ability to turn the unit provides a high-performance bor-ing tool for high tolerance and surface finish demands.
Fine boring heads hold dedicated boring bars fo diameters 12 to 25 mm and car-bide bars of 16 mm to provide the capa-bility to produce close tolerances (IT6) at high spindle speeds (7000 rpm). With a diameter range of 3 to 44 mm to depths of 13.5 to 128 respectively, these super-finishing boring tools machine at small depths of cut (0.05 to 0.5 mm) and small feeds (0.01 to 0.1 mm). Generally, the first choice for the boring bar is a posi-tive knife-edge insert in a cermet grade machining at high cutting speeds.
The Reamer 830 covers the diameter range 10 to 31.75 mm with varying hole depths, hole tolerances of IT 7 are achieved along with high concentricity. Easily exchangeable heads are secured by front clamping. The heads accom-modate 4 to 8 cermet brazed blades depending upon diameter. Effective chip evacuation is secured by cutting fluid directed to each cutting edge. There is high concentricity and repeatability with a precision coupling.
The combination of multiple cutting edges capable of high cutting speeds re-sults in very high penetration rates. The heads are equipped with left-hand chip flutes which push the chips ahead of the reamer, thus necessitating through-holes for machining, but providing an ad-vantage for machining with interrupted cuts as in crossing holes. This cutting action generates cutting forces directed axially into the spindle, increasing the ri-gidity and providing accurate control of the hole diameter accuracy.
CoroBore 825
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Damped boring tools (Silent Tools) should generally be considered for hole depths of more than four times the diameter. DuoBore damped tools are available for roughing operations in the diamter range 25 to 150 mm and with the ability to per-form single-edge, multi-edge and step-bor-ing. Damped fine-boring tools are available from 25 to 269.5 mm diameters.
The tool coupling is a critical part of the total set-up and interface to the machine spindle. If the tool coupling is unsatis-factory as regards stability, strength, ac-curacy and handling, the chain of factors that ensure performance will be broken. A good modular system – Coromant Capto – with a large range of tool types for roughing and finishing is an important
part of the machining equipment for bor-ing with rotatbor-ing tools. The system pro-vides the means for a tool and spindle interface with a self-centering coupling that provides the centre of rotation for minimizing the boring tool run-out and making it possible to adapt each tool as-sembly for the operation in question.
Inserts for boring
The selection of indexable insert for bor-ing has a decisive influence on the out-come of the boring operation. Productivi-ty, reliability and quality is directly related to the performance of the cutting edge. Chip formation, tool-life and cutting data are determined by the combination of in-sert geometry and grade.
As in all internal machining operations, chip control and evacuation are critical. Inserts for boring are essentially the same as for turning, where
chipbreak-ing areas are defined for each geome-try type. Short, thick chips can lead to excessive cutting forces with deflection and vibration as the result. Chips that are too long, often in the form of stringy swarf, can accumulate in the hole and cause surface finish deterioration and then chip stocking, leading to break-down.Chips in the form of commas or determined spirals are the ideal.
The depth of cut is determined by the recommended values for each insert ge-ometry. The sharper a geometry is, and the smaller the nose radius, the smaller the depth of cut possible. It may very well be that an increase in depth of cut may lead to harder chipbreaking, while a reduction may lead to insufficient cut-ting edge engagement, with rubbing as a result.If chips are generated sporadically during machining, the depth of cut may be insufficient or excessive.
Damped tooling for long tool reach.
The right indexable insert provides the vitally correct cutting action. Coromant Capto
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Building and setting boring tools
Building a tool for boring with modularholders, adaptors, extensions, reduc-tions and cutting units provide a broad range of possibilities to perform opera-tions and for optimization. Long reach tools with the best stability, the inclu-sion of damped tools and flexibility for use in other operations are some of the main advantages. This is especially im-portant when building finishing tools as the cutting edge should be moved away from the machined surface in the bore to avoid retraction marks. It is then also vital that the recommended torque val-ues are followed for assembly and that a suitable fixture is used and that a good tool pre-setter is used.
Each boring tool is delivered with setting instructions. For finishing cuts, it is often advisable that a measuring cut is made to see what adjustment is needed to compenstae for tool deflection. Special boring tools should be considered for opti-mization, especially when large batches or repetitive orders involving the same operation. CoroBore 820 can be combi-ned with CoroBore 825 to accomplish roughing and finishing one hole diameter. Tool combinations for machining differ-ent diameters, chamfers and counter-bores can be made.
The factors for successful boring with tool assemblies are similar to those as-sociated with internal turning.
With all parameters related to accuracy and surface finish generation under con-trol, it is possible to achieve very good tolerance levels and surface finishes in boring. The surface finish in boring opera-tions is influenced by various parameters component and fixturing stability, ma-chine tool condition, tool holding stability, cutting data and the choice of indexable insert. The choice of Wiper inserts will
influence not only surface finish but also the productivity of the operation.
Surface finish is directly related to the same parameters as for rough boring but with the depth of cut also playing a role. Generally depths of cut are quite shallow but there is a lower limit to depth which relates to insert geometry and feed rate. Most boring tools for finishing have only one insert and are as such prone to de-flection during cut. When the cutting depth is smaller than the lowest depth-limit, the insert will tend to ride on the pre- machi-ned surface, only scratching and rubbing it, leading to unsatisfactory cutting action and and hence the final result.
Ways of countering vibrations in boring:
- Uset the largest tool-diameter possible. - Use shortest tool over-hang possible.
- Use tapered shanks/reductions whenever possible. - Use dedicated tools for longer overhangs - damped tools. - Check machine spindle, run out, wear, clamping force etc…
- Check that all units in the tool assembly are assembled correctly with the correct torque.