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Rough cutting strategies run on optimised toolpaths
12/10/2013 9:00:00 AM

Fagersta, December 2013 – Toolpath optimisation through the use of CAM systems has been commonplace for quite some time, especially in the die and mould industry sectors. However, only recently have shops begun to pair that capability with relatively new machining strategies and special designed solid rotary cutting tools to optimise rough-machining operations.

These CAM-based rough-machining, or dynamic-milling, strategies are ones that centre on a cutting tool’s arc of contact and its average chip load. By manipulating a tool’s arc of contact via its CAM-generated toolpath, shops can boost roughing speeds, effectively control process temperature, apply higher feeds per tooth and take increased depths of cut to significantly shorten overall part machining cycle times — all without placing any additional strain on machine tool spindles.

Arc of contact and thermal load in relation to cutting speeds
A cutting tool’s arc of contact is an independent variable that influences thermal load on the tool and is the key to optimised roughing operations.

Maximum arc of contact on any tool is 180 degrees, or basically its diameter. So at a full arc of contact, the radial cutting depth (or cutting width) is the same as the cutter diameter and represented by ae (radial depth of cut) = Dc (cutter diameter).

In manipulating the arc of contact, shops can reduce the amount of heat generated during roughing operations. As the radial depth of cut decreases, so does a cutter’s arc of contact. A smaller amount of contact results in less friction and, therefore, less heat between the tool’s cutting edges and the workpiece it is machining. What occurs is that the tool’s cutting edges gain more time to cool from the time they exit the cut, revolve around and re-enter the cut. These lower machining temperatures, in turn, allow for increased cutting speeds and shorter cycle times. 

Average chip thickness and physical load
A cutting tool’s average chip thickness (hm) is based on physical load and maintained through a combination of feed per tooth and arc of contact adjustments. Because chip thickness constantly changes during cutting, the industry uses the term average chip thickness (hm).

A full 180-degree arc of contact will generate the thickest chips at the centre of the cutter’s width. So, a smaller arc of contact – less than 90 degrees (ⱷe, engagement angle) – reduces the chip thickness, thus allowing for increased feed per tooth (fz) as compensation.

For example, consider a 10-mm diameter cutter side roughing at 10-mm ae (full arc of contact). At that ae, the cutter is generating its largest average chip thickness/heaviest physical load. Within the first 90 degrees, the cutter is up milling until a maximum chip thickness (fz) is reached, then once into the second 90 degrees, it is down milling where chip thickness decreases again to zero.  But if ae drops (ae < Dc) to 1 mm (10 percent), then average chip thickness will become smaller, allowing for faster roughing by applying increased feed per tooth (fz). While the cutter removes less material, it does so at a much faster pace and with less tool and machine spindle strain as compared with taking heavier radial cuts but at slower feedrates. In slot roughing applications, a lower ae also allows for a heavier ap (depth of cut) for even faster material removal.

Cutter designs for optimised roughing
While most cutting tool suppliers offer products for specific materials, others, such as Seco Tools, also develop tool geometries for advanced machining methods. In the case of CAM-based rough-machining strategies, this tooling addresses the key issue of chip control along with necessary flute and length requirements.

Seco, for instance, developed its Jabro®-HPM (high-performance machining) cutters specifically to rough machine at their full arc of contact and take heavy depths of cut for extremely high-volume material removal applications. These cutters feature special geometries for aggressive performance in specific materials.

To cover a wider range of workpiece materials, Seco recently modified the geometries of its Jabro®-Solid² 550 line of cutters specifically for optimised rough-machining strategies. The cutters feature double-core designs that provide extra stability and reduced tool deflection.

Within the JS550 Series are longer length tools that the company determined work best for deep pocket and 3D shape roughing/dynamic milling. Tool lengths are typically in between three and four-times diameter.

When a steady arc of contact is maintained, these tools experience consistent and evenly distributed wear along their flutes and provide a much more predictable tool life. However, long cutters produce equally as long chips that can be difficult to evacuate from cutting zones and from the machine tool.

To create chips that are smaller and more manageable, Seco modified its JS554 L (long version) cutter design by adding chip splitters – tiny grooves on the tool’s cutting edges and reliefs. The modified cutter, now known as the JS554 3C (C indicating chip splitters), features chip splitters spaced apart at a distance equal to 1 X D (cutter diameter). So a 40-mm-long, 10-mm diameter cutter would produce chips no longer than 10 mm that are quickly evacuated from the cut zone and eliminate the risk of jamming machine tool chip conveyors.

Shorter standard-length cutters are also well suited for optimised roughing strategies. Seco, using one of its standard JS554 cutters (2 x Dc + 2 mm cutting length), rough machined a pocket in common SMG-3 steel and achieved equally impressive results as it would have using a longer length cutter. Seco ran the shorter cutter at the typical 10 percent ae: Dc ratio that would have been applied with a long cutter, but simply modified feed per tooth for generating the same metal removal rates.

When applying a small arc of contact, the more flutes a cutter has, the faster it can feed and the higher the productivity. Feed speed = number of cutter flutes x feed per tooth x spindle speed. While roughing cutters typically have, at most, four flutes, Seco is currently researching the potential of five-fluted cutters.

Complex part shapes
In straight line machining paths (side milling), the arc of contact, once set, remains unchanged. However, with a more complex part shape, for instance one that includes inside and outside radii, inconsistencies arise concerning the set arc of contact.

When a cutter finishes a straight cut and engages an inside radius/corner, its arc of contact will increase – meaning cutting parameters no longer coincide with the current arc of contact. If toolpaths fail to adjust for these situations, the results will be chatter, vibration and even cutter breakage.

Today’s CAM packages offer toolpath strategies specifically for inside/outside radii shapes where changing arcs of contact occur along conventional toolpaths. These software packages automatically apply different feeds to control arc of contact and keep chip loads consistent. To maintain arc of contact, these CAM packages employ trochoidal machining and peel milling techniques when entering a radius. Next to the choosen toolpaths, those CAM packages reduce L movement significantly to even further reduce cycle times.

When using an optimised roughing toolpath and maintaining consistent arc of contact, the cutter’s radius can match that of the inside radius being cut without risk of cutter overload, grabbing or overcutting. This capability allows shops to remove more stock in the roughing pass, thus reducing the amount of stock the finish pass has to cut —all of which translates to faster machining cycle times.

Optimised roughing strategies also apply to specific workpiece materials. Seco has conducted extensive testing with steel, stainless steel, cast iron, titanium, aluminium and steels with hardnesses up to 48 HRc. The company recommends that shops first apply a 10 percent ae to the diameter ratio – 5 percent for the tough-to-machine materials such as titanium and superalloys. Seco has established optimised speed and feed data for these specific arcs of contact as well as many others. Shops can apply increased amounts of ae than is recommended, but then cutting speed, along with feed per tooth, must be reduced.

In regards to ap, Seco currently provides Jabro®-HPM cutters designed for ap up to 2 x D for full slotting in steel (JHP951 & JHP993). While this is considered extremely aggressive machining, Seco’s general-purpose cutter, the JS554 3C, easily handles 4 x D when optimised roughing is applied.

Also, shops with machines unable to handle heavy roughing cuts can simply reduce the arc of contact and use a trochoidal machining path. Doing so reduces cutting forces and lessens the need for high machine power, yet still generates high productivity results by applying large depths of cut.

When roughing strategies are applied to difficult-to-cut materials such as stainless steel and titanium, coolant should be applied to the complete length of the cutter – top, middle and end. Cooling the complete cutting edge is important. And when cutting steel and cast iron, shops should use compressed air at maximum pressure to blow chips away.

It must be noted that shops are unable to apply CAM-based roughing strategies when programming at a machine because programs must be generated externally through special toolpath optimisation packages. Although, when programming at a machine, shops can manually enter arc of contact data that Seco has established, but only for simple straight-line roughing operations or fixed trochoidal cycles.

Field tests
While optimised roughing strategies are ideal for longer length cutters, Seco conducted arc of contact tests with standard length tools. In one test, Seco ran a standard Jabro 554 cutter at 300 m/min cutting speed, 20-mm depth of cut, 1-mm ae and 0.2-mm feed per tooth for a machining cycle time of 4 minutes and 26 seconds. Technicians then changed the ae to 2 mm and reduced the feed per tooth to 0.1 mm. And although the metal removal rate remained unchanged, the machining cycle time dropped to 3 minutes and 11 seconds. The shorter cycle time occurred because the higher ae did not increase machining speed, but it did reduce the number of passes needed. Consequently, the part was roughed machined in a shorter amount of time.

 

JS554 ø10 standard length in SS2172 steel (SMG-3)

 

ap

ae

Vc

fz

Cycle time

1

20 mm

1 mm

300 m/min

0.2 mm

4.25 min

2

20 mm

2 mm

300 m/min

0.1 mm

3.11 min

3

20 mm

3 mm

200 m/min

0.1 mm

3.13 min

4

20 mm

4 mm

120 m/min

0.08 mm

5.11 min

 

For one of its aerospace customers, Seco demonstrated the benefits of roughing strategies using the customer’s BT40 spindle machine and one of its actual components. The customer typically roughs these parts using conventional toolpaths and standard machine parameters for a roughing time of one hour per piece.

Seco applied the biggest diameter cutter applicable – a 25-mm-diameter JS554 3C long length cutter with chip splitters. Together with optimised roughing strategies and toolpaths, the cutters reduced the old roughing cycle time down to an amazing 8 minutes. Plus, Seco estimates that further times savings could be achieved (most likely a 6-minute roughing time) if a more powerful machine was used.

Another Seco customer experienced the benefits of optimised roughing strategies and toolpaths for an automotive component.  Not only did the shop slash overall part cycle time from 8.5 minutes to only 1.1 minutes, it also boosted tool life from 80 parts to 250 parts per cutter.

For a Seco customer rough machining a motorcycle component mould, optimised roughing and tool paths reduced machining cycle time from 900 minutes to 400 minutes. The customer was using an indexable high-feed cutter for both first and second roughing operations, but then switched to a 25-mm diameter JS554 3C for the first and kept the high feed on the second one.

Conclusion
Arc of contact and average chip thickness are keys to optimised rough-machining operations. Through special CAM software packages specifically for toolpath optimisation and dynamic milling methods, today’s manufacturers can manipulate/control a cutting tool’s arc of contact and maintain consistent loads.  And in so doing, they effectively control process temperature, apply higher cutting speeds and feeds per tooth and take increased depths of cut to significantly shorten overall part machining cycle times.

However, manufacturers must keep in mind that optimised roughing requires the right CAM packages for external programming. And while most cutting tool suppliers offer products for specific materials, few develop tool geometries for the particular advanced machining cycles and tool paths required. With the right cutter and those dynamic cycles, manufacturers can increase metal removal rates by as much as 500 percent when compared with traditional machining methods.

By:

Teun van Asten MSc., Engineer Marketing Services Solid Milling, Seco Tools

 

Headquartered in Fagersta, Sweden and present in more than 50 countries, Seco Tools is a leading global provider of metal cutting solutions for milling, turning, holemaking and toolholding. For more than 80 years, the company has provided the technologies, processes and support that manufacturers depend on for maximum productivity and profitability. For more information on how Seco’s innovative products and expert services bring success to manufacturers across all industry segments, please visit www.secotools.com.