Rough boring

Rough boring is primarily focused on metal removal and to prepare the hole for finishing. Roughing machining is performed to open up an existing hole made by methods such as pre-machining, casting, forging. Rough boring tools can be configured for productive-, step- and single-edge boring.

Productive boring

Productive boring is the basic set-up for most boring applications and is the best choice for highest productivity.

It involves two or three cutting edges and is employed for roughing operations of holes, with tolerance IT9 or larger, where metal removal rate is the first priority. Feed rate is obtained by multiplying the feed for each insert by the number of inserts (f_n = f_z × z).

Step-boring

A step-boring tool has the inserts set at different axial heights and diameters. This method is used when a large radial depth of cut is required or to get improved chip control in long-chipping materials since the chips can be divided into smaller and easily handled chips. The number of tools and tool changes might be reduced when step boring.

The feed rate and produced surface finish is the same as if only one insert is used (f_n=f_z). Produced hole tolerance is IT9 or larger.

Single-edge boring

Single-edge boring is performed when using only one cutting edge. It can be beneficial in materials where chip control is demanding (e.g long-chipping materials), or when machine tool power is limited (f_n = f_z).

Choice of inserts for rough boring

Carefully select insert style, entering (lead) angle, geometry and grade to achieve good chip control and machining performance.

Positive or negative insert style

Negative inserts: Choose negative inserts in stable conditions for better insert economy and in tough applications that require strong inserts and improved process security.

Positive inserts: It is an advantage to use positive inserts in rough boring, as they give lower cutting forces compared to negative inserts. A small nose angle and small nose radius also contribute to keeping the cutting forces down.

Entering/lead angle for rough boring

The entering angle (lead angle) of boring tools affects the direction and magnitude of axial and radial forces. A large entering angle (small lead angle) produces a large axial force, while a small entering angle (large lead angle) results in a large radial cutting force.

90° entering angle (0° lead angle)

First choice for general operations, step boring and for shoulder operations.

84°/75° entering angle (6°/25° lead angle)

For interrupted cuts, sand inclusions, stack boring etc. Through holes only.

95° entering angle (-5° lead angle)

For high feeds or improved surface finish with Wiper inserts in stable conditions.

Positive inserts

Negative inserts

Boring insert geometry and grade

Component material, type of operation and machining conditions define what insert geometry and grade to use. For boring, turning inserts are used, find out more about how to choose correct turning insert.

Recommendations

• Choose a roughing geometry for large cutting depths
• Choose a medium geometry for smaller cutting depth or to achieve improved chip breaking
• A large insert nose radius (RE) will improve process security and enable larger feed but might generate vibrations. Recommended starting nose radius is 0.08 mm (0.031 inch)

Machine considerations for rough boring

When rough machining, make sure the machine has the required power and torque needed for the specific boring application. Feed, number of inserts, hole diameter and depth of cut are main affecting parameters.

Boring large diameter holes

Large diameter holes require larger torque than small diameter holes. Large diameter rough boring tools have larger inserts and can therefore take larger depths of cut than small diameter tools. Make sure the machine has the required power and torque.

Boring blind holes

When machining a blind hole it is very important to secure an efficient and correct chip evacuation.

• Correct cutting data is essential to achieve proper chip forming
• Ensure that the chips do not jam or wear the inserts
• Coolant pressure and flow should be sufficient to evacuate the chips
• Vertical machines are more critical than horizontal machines for efficient chip evacuation

Boring interrupted cuts

Machining of interrupted cuts, such as cross holes, set high demands on insert cutting conditions.

• Choose a tough grade
• Choose a strong, square negative basic-shaped insert for improved process security, valid for stable conditions
• Reduce cutting data if there are severe interruptions

Boring sand inclusions – cast component

Sand inclusions in cast components increase wear on the inserts.

• Choose a tough grade
• Reduce cutting data
• Choose a strong square negative basic-shaped insert for improved process security and minimized wear

Boring gas burned holes

Gas burned holes could have local hardened areas that will generate more wear on the inserts.

• Choose a tough grade
• Reduce cutting data
• Choose a strong, square negative basic-shaped insert for improved process security and minimized wear

Large depth of cut in boring

If a really large depth of cut is needed, step-boring could be considered as a good alternative. Make sure the machine has the required power and torque.

Boring off-centre holes

If the centre lines of the pre-machined hole and the boring tool are not concentric, the cutting depth could be really large on one side of the hole. This could typically occur on cast components.

A good solution for these applications is step boring in order to be able to take the large depth of cut. Unsymmetrical cutting forces might bend the tool and cause some small remaining off-centre or vibrations, especially when long overhang tool assemblies are used.

Application checklist for rough boring

• Choose productive boring (triple-edge or twin-edge), step boring or single-edge boring
• Choose the largest possible coupling size
• Select appropriate entering angle
• Ensure proper chip control. Short/hard chips might lead to vibrations and long chips might deteriorate surface finish causing insert breakdown
• Horizontal machining and cutting fluid improve chip evacuation in blind holes
• Select insert geometry and grade:
• Choose the shortest possible overhang, use damped boring bars for overhangs longer than four times the coupling size
• Select appropriate cutting data and consider overhang.
  Note: Do not use maximum recommended feed and cutting depth at the same time. Maximum recommended starting speed is 200 m/min (656 ft/min) to ensure proper chip evacuation
• A large insert nose radius (RE) will improve process security and enable larger feed but might generate vibrations. Recommended starting nose radius is 0.8 mm (0.031 inch)
• Use positive inserts as they provides lower cutting forces compared to negative inserts
• Choose negative inserts in stable conditions for better insert economy and in tough applications that require strong inserts and improved process security
• Insufficient cutting edge engagement can increase vibration through friction during cutting
• Rigid clamping with face contact to spindle improves stability
• Excessive cutting edge engagement (large depth of cut and/or feed) can increase vibrations
• Make sure the machine can provide the required torque and power for the specific boring application
• Wiper inserts can be used to improve surface finish or increase feed but are not recommended for unstable conditions and long overhangs
• Make sure to get proper clamping of boring tool and workpiece
• Use cutting fluid to improve chip evacuation, tool life and hole geometry
• For best performance of multi-edge boring tools it is recommended to machine in the higher area of the cutting depth and feed recommendations, especially for longer overhangs. Note: Do not use maximum cutting depth and feed at the same time
• Productive boring machining enables maximum penetration rate. If a really large depth of cut is needed, it could be more productive to go for step boring since the cycle time can be minimized and fewer tools might be needed

See also How to apply boring for general recommendations.

See the handling instruction below to get the recommended tightening torque.