Boring Cutter: Characteristics And Selection

Boring can generally be carried out on lathes, boring machines and machining centers.

The boring process is mainly used to process cylindrical holes (see fig. 5-39), threaded holes, end faces and grooves in holes on workpieces such as boxes, supports and bases.

When special accessories are used, inner and outer spherical surfaces and taper holes can also be machined.

Boring can obtain high accuracy and high surface quality.

The hole accuracy can reach IT6 ~ IT7, the hole spacing error is not more than 0.015mm, and the surface roughness Ra can reach 0.8 ~ 1.6 μ m.

Fig. 5-39 boring cylindrical holes

Boring cutter is a kind of boring cutter, which can be divided into single edge boring cutter, double edge boring cutter and three edge boring cutter according to the number of cutting edges;

According to its processing technology, it is divided into rough machining boring cutter and finish machining boring cutter;

It can be divided into through-hole boring cutter, blind hole boring cutter, step hole boring cutter and end face boring cutter according to the machining surface;

According to its structure, it can be divided into integral boring cutter, assembly boring cutter and adjustable boring cutter.

1. Single edge boring cutter

This kind of boring cutter can correct the deflection or position error of the original hole axis.

It is one of the simplest tools in structure.

It is easy to use and grind.

It is usually a clamping structure (see Fig. 5-40).

The boring cutters shown in Fig. 5-40a and Fig. 5-40b are respectively used on the lathe to process through holes and blind holes.

The boring cutters shown in Fig. 5-40c, Fig. 5-40d, Fig. 5-40e and Fig. 5-40f are respectively used on the boring machine to process through holes and blind holes.

These structures are applicable to rough and finish machining of holes.

The boring bar can be used for a long time, which can save the materials and man hours for manufacturing the boring bar.

The single edge boring cutter can also process different apertures by adjusting or changing the boring cutter head, but the adjustment is time-consuming, the accuracy is not easy to control, the workers have high technical requirements for operation, and there is only one cutting edge, so the cutting efficiency is low.

single edge boring cutter

Fig. 5-40 single edge boring cutter

a) ,b) respectively used for boring through holes and blind holes on the lathe

c) , d), e) and f) are respectively used for boring through holes and blind holes on the boring machine

2. Fine tune boring cutter

As shown in fig. 5-41, it is a fine-tuning boring cutter suitable for automatic line boring machines, coordinate boring machines and CNC machine tools.

It has the characteristics of simple structure, easy manufacturing, convenient adjustment and high precision.

The adjusting nut and boring head are pressed on the boring bar by the tightening screw and washer.

During adjustment, loosen the tightening screw slightly, turn the adjusting nut with scale, and then fine tune the boring tool, and then tighten the tightening screw.

fine tuning boring cutter

Fig. 5-41 fine tuning boring cutter

1-washer

2-tension screw

3-boring bar

4-adjusting nut

5-blade

6-boring head

7-guide chain

3. Floating boring cutter

(1) Floating boring cutter is shown in fig. 5-42, and its characteristics are as follows:

When the floating boring cutter is installed into the square hole of the boring bar, it can automatically balance and center its cutting position by virtue of the cutting force received by the cutting edges on both sides of itself.

Therefore, it does not need to be clamped, and it can automatically compensate the machining error caused by the boring cutter installation error, the radial circular runout of the boring bar or the deviation of the machine tool spindle.

Therefore, the machining error caused by the radial force on the boring bar during boring can be eliminated.

floating boring cutter

Fig. 5-42 floating boring cutter

(2) The machining accuracy can reach IT7 ~ IT6.

Processing cast iron hole Ra can reach 0.8 ~ 0.2 μ m.

Processing steel hole Ra can reach R1.6 ~ 0.4 μ m.

(3) Since the straight-line error and position error of the hole cannot be corrected, the prefabricated hole is required to have good straightness, and the surface roughness value Ra ≤ 0.4 μ m。

(4) The structure is simple and the grinding is convenient, but the operation is troublesome.

It can not process holes with too small aperture, and has high requirements for the manufacture of boring cutters and boring bars.

In particular, the cutting efficiency of machining square holes on boring bars is lower than that of reaming.

4. Clamp type deep hole boring cutter

This kind of boring cutter has the commonness of deep hole cutters in structure, including guide blocks, chip removal holes or cutting fluid inlet holes.

The machine clamp deep hole boring cutter is divided into machine clamp indexable deep hole boring cutter and mechanical clamp fixed deep hole boring cutter.

Their structures are similar, but the clamping of blade and tool pad is different.

The former is easy to use and the latter is rigid.

Fig. 5-43 shows the machine clamp indexable carbide deep hole boring cutter.

The guide block is welded by the guide pad and the carbide guide block, and is clamped on the boring tool body with screws.

The indexable blade on the tool pad is fastened on the boring tool body through the screw, and the adjusting screw is turned to drive the adjusting inclined iron to move axially, so the radial dimension of the boring tool can be adjusted.

machine clamp indexable carbide deep hole boring cutter

Fig. 5-43 machine clamp indexable carbide deep hole boring cutter

1-adjusting the inclined iron

2-knife pad

3-indexable blade

4-guide block

5-cutter body

6-adjusting screw

7- cutter pad pressing screw

8-guide block compression screw

5. Modular boring cutter

In order to meet the needs of various hole diameters and depths and reduce the variety and specification of boring cutters, a modular boring cutter is designed.

Modular boring cutter (see Fig. 5-44) means that the boring cutter is divided into several parts: Foundation handle, extender, reducer, boring bar, boring head, blade seat, blade, etc., and then freely combined according to the specific processing content.

This not only greatly reduces the number of tool handles and reduces the cost, but also can quickly respond to various processing requirements and extend the overall life of the tool.

Obviously, modular boring cutter has incomparable advantages over integrated boring cutter.

Of course, this also requires modular boring cutters to have high connection accuracy and high connection rigidity, as well as high repetition accuracy and high reliability.

In a word, modular boring tool system has great advantages, but it does not mean that as long as it is modular.

It must be measured from the connection rigidity, accuracy, operability, price and other aspects.

modular boring cutter

Fig. 5-44 modular boring cutter

Selection of boring cutter

According to the number of cutting edges, boring cutter can be divided into single edge boring cutter, double edge boring cutter and three edge boring cutter;

According to its processing technology, it is divided into rough machining boring cutter and finish machining boring cutter;

It can be divided into through-hole boring cutter, blind hole boring cutter, step hole boring cutter and end face boring cutter according to the machining surface;

According to its structure, it can be divided into integral boring cutter, assembly boring cutter and adjustable boring cutter.

The selection steps of boring tools are shown in table 5-12.

Table 5-12 selection steps of boring tools

StepStep nameStep content
1Determine operation typeDetermine the type of operation and note down the characteristics, constraints, materials, machine tools, etc. related to the hole to be machined.
2Select boring systemUse the tool product family overview to find the system that meets the process (rough or finish) requirements and conditions.
3Determine boring diameter and boring requirementsSelect the tools that cover the boring diameter range according to the process, surface quality and tolerance.
4Select main deflection angleBased on the selected main deflection angle, select the corresponding blade carrier and the most appropriate blade type.
5Select tool handleSelect the joint handle according to the interface size and boring depth.
6Select bladeDetermine cutting parameters according to blade type and size: select blade brand according to machining materials and working conditions.

(1) Selection of rough boring cutter

The rough boring cutter aims at high metal removal rate and its main function is to improve work efficiency.

Therefore, the rough boring cutter should have high strength.

For holes with low accuracy requirements, rough boring can be used as the final process.

For holes with high accuracy requirements, rough boring can cut most of the allowance of the blank hole, improve the dimensional accuracy of the hole, reduce the surface roughness value, and prepare for semi fine boring and fine boring.

As shown in Fig. 5-45, the commonly used tools in rough boring are shown.

rough boring tool

Fig. 5-45 rough boring tool

1. Multi edge boring cutter

In order to improve the boring efficiency, it is recommended to select a double-edged boring cutter in low to medium power machine tools, unstable processes or large-diameter machining. Its two cutting edges can be adjusted to the size of the machined hole in advance.

The machining accuracy of double-edged boring cutter can be controlled to IT9 level.

For medium and large power machine tools, it is recommended to use a three edge boring cutter, which has higher machining efficiency and boring accuracy up to IT9.

As shown in Fig. 5-46 and Fig. 5-47, they are double-edged and three edged boring cutters respectively.

double edge boring cutter

Fig. 5-46 double edge boring cutter

Fig. 5-47 three edge boring cutter

2. Heavy duty boring cutter

For large-scale and heavy-duty boring operations, the large-scale and heavy-duty boring cutter shown in fig. 5-48 can be selected.

In addition, the boring head with tool holder and adjustable extended sliding plate can be selected, and the rough boring head can be installed.

Fig. 5-48 heavy duty boring tool

3. Step boring cutter

For holes with large allowance or when it is necessary to strengthen chip control and machining of eccentric holes, the stepped boring cutter shown in fig. 5-49 can be selected.

A tool pad with different thickness is added between the tool base slider and the tool body, so that the three blades can process three layers of step surfaces with different diameters and depths.

Fig. 5-49 step boring cutter

4. Indexable deep hole boring cutter

When machining deep holes with a diameter of more than φ30mm, it is recommended to use an indexable deep hole boring cutter.

The geometric angle of the cutting part is selected as follows.

(1) The selection of front angleγo, rear angleαo and auxiliary rear angleαo′ is shown in table 5-13.

Table 5-13 selection of front angle, rear angle and auxiliary rear angle

 Cutting-toolAngle angle  materialGeneral materialHardened and high strength steelsDifficult to machine materials such as 38CrMoAl and stainless steel
Γo°)05~-105~10
αo°)8~108~108~10
αo°)10~1510~1510~15

(2) Main deflection angleκr, auxiliary deflection angleκr′ and blade inclination angleλs.

The main deflection angleκr is generally 45 ° ~ 65 °. When the back cutting amount is large, κr take the large value, otherwise κr take the small value.

The secondary deflection angleκr′ is generally 3 ° ~ 5 °. The deep hole boring cutter can not grind the polishing edge and blade belt, and the blade inclination angleλs is generally 0 °.

(2) Selection of fine boring cutter

1. Single edge boring cutter

When the power of the machine tool is low, the cutting force needs to be reduced and there are vibration problems, the single edge boring cutter is the appropriate choice.

It can achieve small tolerance, accurate roundness or ideal surface quality.

2. Fine tune boring cutter

In the fine boring process, in order to obtain high dimensional accuracy, the boring cutter is required to adjust the dimensions conveniently and accurately.

Fine tuning boring cutter can precisely adjust the boring size on the machine tool.

3. Floating boring cutter

For single piece and small batch production, it is appropriate to select floating boring cutter for machining holes with large diameter.

4. Vibration damping boring cutter

When the tool overhang is 5 times the tool diameter or more, the vibration is usually the limiting factor for the machine tool to achieve high production efficiency;

Thus, the cutting speed, feed rate and back feed rate can only be reduced.

At this time, it is recommended to use a vibration damping boring tool (see Fig. 5-50).

The tool has a vibration damping system, which can reduce the vibration to a minimum, so as to increase the cutting parameters, and achieve a more reliable vibration free machining process, small tolerance, ideal surface quality and significantly improved metal removal rate.

Fig. 5-50 vibration damping boring cutter for finish machining

(3) Possible problems and Countermeasures in rough boring

Table 5-14 shows the possible problems during rough boring, their causes and solutions.

Table 5-14 possible problems and Countermeasures during rough boring

Problem

Possible reason

Solution

Poor chip control

Feed rate too low

Increase feed rate

Excessive cutting depth

Using the step boring cutter

Chattering and vibration

Cutting speed too high

Reduce cutting speed

Excessive aspect ratio

Shorten the tool to increase stiffness

Increase the outside diameter of the clamping handle and extension rod

Use hard alloy or heavy metal extension rods

The arc radius of the tool tip is too large

Use a blade with a smaller tip fillet radius

Unstable conditions

Ensure rigid clamping in contact with the spindle surface

Check workpiece clamping

Check that all units in the tool assembly are assembled to the required torque

Check the machine tool spindle, clamping, wear, etc

Main deflection angle kt is 80 °

Select kt=90 °, and match with corresponding blades

The blade edge is slightly chipped or broken

Blade selection error

Use a tough blade material

Severe intermittent cutting

Reduce cutting speed and feed rate

Shoulder cutting blockage

Check the clearance of bore diameter of boring bar

Improve shoulder cutting control and feed rate

Low tool life

Blade selection error

Use blade materials with higher wear resistance

Cutting speed too high

Reduce cutting speed

Blade edge chipping

Check back feed and feed

Cutting fluid pressure too low

Increase cutting fluid pressure

Chip cannot be discharged

Excessive back cutting

Use step boring cutter

Insufficient space under hole

Place the workpiece higher on the workbench

Poor chip control

Use chip breaking groove blade

Insufficient machine power

Feed rate too high

Reduce the feed rate (no more than 25% of the fillet radius of the blade tip)

Excessive cutting depth

Using the step boring cutter

Low machine power

The speed is in the low torque area of the main shaft, increase the speed

Speed in shift area: adjust speed

Increase front angle

Reduce cutting parameters

Excessive burr at hole outlet

Feed rate too high

Decrease feed rate

Cutting force too high

Reduce back cutting amount

Reduce the tip fillet radius

(4) Possible problems and Countermeasures in fine boring

Table 5-15 shows the possible problems, causes and solutions in the process of fine boring.

Table 5-15 possible problems and Countermeasures during fine boring

Problem

Possible reason

Solution

Low tool life

Improper selection of blade material

Use more wear-resistant tool materials

Cutting speed too high

Reduce cutting speed

Excessive back cutting depth

Reduce back cutting depth

Chattering and vibration

Cutting speed too high

Reduce cutting speed

Excessive aspect ratio

Shorten the tool to increase stiffness

Use a stronger boring tool

Use hard alloy or heavy metal long rod

Blade selection error

Reduce the tip fillet of the blade

Use finely ground blades

Unsuitable machining allowance

Change the diameter of the pre machined hole

Poor aperture accuracy and repeatability

Unstable condition

Ensure rigid clamping in contact with the spindle surface

Check workpiece clamping

Check that all units in the tool assembly are correctly assembled with the correct torque

Check the machine tool spindle, clamping, wear, etc

Incorrect tool change

Replace worn and damaged tool bars

Clean the spindle and tool bar

Low spindle stability

Use sharper fine grinding blades

Roundness difference

Excessive unbalance of boring tool

Check spindle runout

Use a suitable boring head instead

Check balance ring setting

Reduce cutting speed

Excessive cutting force

Check machining allowance and feed rate

Insufficient clamping of workpiece

Check the consistency of workpiece clamping

Workpiece asymmetry

Reduce cutting force and use finely ground blades

Increase cutting speed and decrease feed rate

Poor position accuracy

Excessive back cutting

Reduce the back cutting depth and carry out two times of cutting

Low surface roughness

Improper blade tip fillet radius

Use a larger nose fillet radius

Excessive feed

Reduce the feed rate (no more than 30% of the tool tip fillet radius)

Poor chip control

Internal cooling should be used

Use a blade with a larger front corner

Check the back cutting depth

Taper hole error

premature failure

Change to a more durable blade material

Adjust the cutting speed

Increase cutting fluid flow

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