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Are hollow bars the key to driving down machining costs?

03 August 2018

In an increasingly crowded marketplace, buyers of steels for machining often find themselves balancing the initial price of investment with longer term cost implications.

Hollow bar after 5 minutes & Solid bar after 5 minutes

Selecting less expensive materials initially reduces outlay but may be false economy if the component generates additional costs in maintenance, tool wear or reduced tool settings long-term.

When selecting steel bar, manufacturers can choose between solid or hollow. The question is: which provides better overall performance and cost savings? Can hollow bar deliver longer tool life and lower manufacturing costs over time?

Rinaldo Brivio, Field Sales & Machining Applications Specialist at Sandvik Materials Technology, shares the results of a detailed study undertaken by the company, pitting the Inconel 625 solid bar against the new Sanicro 60 hollow bar.

The contenders: established solid bar vs new hollow bar

The competing grade Inconel 625 (UNS designation N06625) is a nickel-based HRSA (heat resistant superalloy) which, since its initial development in the 1960s, has found applications in the marine, nuclear and other industries due to its high strength properties and resistance to elevated temperatures, with enhanced protection against corrosion and oxidation.

The new challenger is a hollow bar variation of Sanicro 60 (also called Alloy 625). Designed for better performance in some of the niches already occupied by Inconel 625, Sandvik’s new hollow bar is made of a high-strength nickel-chromium alloy that withstands extremely high temperatures, resists intergranular and stress corrosion in chloride-containing environments and has a pitting resistance equivalent (PRE) above 48.

The purpose of the investigation was to fully assess and compare the machinability of Sanicro 60 (diameter =72mm) versus Inconel 625 (diameter=77mm). The criteria assessed would be tool life, surface quality and chip breaking. Which would come out on top: the new hollow formulation or the traditional solid bar?

Methodology: how the tests were carried out

Conducted at Sandvik Coromant Center in Milan, Italy, the assessment programme was divided into three sections: turning, drilling and tapping.

An MCM horizontal machining centre (HMC) was used for the drilling and tapping tests. The turning operations would be implemented with a Mazak Integrex Mach 2, using a Capto tool holder with internal coolant.

Tool life was assessed by evaluating tool wear at a range of cutting speeds from 60 to 125m/min, using an insert grade of S05F suitable for medium to rough machining. To measure the productivity of each test, the volume of material removed was measured for each cutting speed by three main criteria: 

• Flank wear (the portion of the tool in contact with the finished part erodes) = 0.3mm 

• Edge fracture on the cutting tool’s main or secondary cutting edge

• Notch wear (small v-shaped cut is formed on the cutting edge) = 0.5mm

Chip formation was assessed and monitored as another indicator of machinability. Testers assessed the chip formation for various insert geometries at a cutting speed of 65m/min (the Mazak Integrex 2 was used with a PCLNL shank tool for turning, and a CNMG120412SM S05F insert). 

Surface quality was judged according to strict criteria: the workpieces should exhibit a surface roughness of no more than Ra =3.2 micron Rz =20 micron. They should also display no evidence of vibration marks, wear or built-up edge (BUE – material accumulating against the cutting tool).

Drilling tests were carried out by first cutting several discs from each of the same 60 mm bars which had been used for the turning experiments. The machined holes were drilled parallel to the bar axis for five minutes, and tool flank wear was recorded at regular intervals. 

Thread tapping tests assessed the suitability of hollow Sanicro 60 and solid Inconel 625 to carry out this indispensable process. All the holes produced in the previous drilling experiment were used, and tapping was performed with a Cutting Coromant Tap M6x1. Six were entered into the MCM horizontal machining center, both to experiment with different tapping variations and to ensure these remained rigid throughout the tapping cycle. After tapping, the diameters of the holes produced were measured with a gauge

Results: hollow bar performed better overall

The outcome of the tests was clear: Sanicro 60 hollow bar outperformed the solid Inconel 625 by delivering longer tool life and better surface quality. It also matched the solid bar on chip formation, drilling, threading and tapping, performing equally well in these tests.

The hollow bar product showed a significantly longer tool life than the solid bar at higher speeds, lasting more than three times longer than the solid bar at the cutting speed of 140m/min. At this upper speed the solid bar lasted only five minutes while the hollow bar had a tool life of 16 minutes.

As the speed of cutting increases, the tool life of Sanicro 60 remains more consistent, dropping by only 39 percent when the speed is doubled from 70 to 140m/min. This is compared to the Inconel 625, where the tool life drops by 86 percent for the same change in speed.

The Sanicro 60 hollow bar workpieces exhibited significantly smoother surfaces than the Inconel 625 solid bar workpieces. This was measured both objectively (surface roughness no more than Ra =3.2micron Rz=20 micron) and by visual examination for wear, built-up edge, vibration marks or damage on the surfaces due to chip formation.

The Sanicro 60 hollow bar matched the performance of the older solid Inconel 625 bar in the thread tapping test, as well as delivering similar results in tool flank wear after drilling and comparably low levels of chip formation.

Conclusion: hollow bar lasts longer, works harder

The findings strongly favour hollow bar as an enhanced alternative to the solid bar product. With a tool life up to three times longer than its competitor at high cutting speeds, Sanicro 60 delivers both longevity and increased efficiency, remaining reliable while working harder and faster. 

Emerging as the competitive global marketplace drives machine operators towards a longer term view of their investments in materials, the ability of Sanicro 60 to save wear on machining tools is significant for those looking to extend profit margins and price their products more competitively. 

Not only could machine tools last longer and call for fewer replacements, but using hollow bar sidesteps an entire machining process by eliminating the need for a central bore – potentially delivering significant time and money savings.

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