Mold Machining

A CNC control should have features to automatically smooth the axis movements or allow the operator to set tolerance limits manually.

1. Spatial plane function: Many mold cores or bases require five-sided machining without excessive coordinated movements. The CNC should have a spatial plane function that allows the machinist to set the plane that is machined on each side of the part. That way, the machinist can program each side of the part in an X-Y-Z plane without changing the CAM program and ultimately improve the tolerances between features on each side.
2. Contour deviation monitoring: The control directs axis movements to follow the 3D surfaces within predefined tolerance bands that consist of simple line segments. The control should automatically smooth the block transitions while the tool moves continuously on the workpiece surface. An internal function that monitors the contour deviations controls automatic smoothing, enabling the user to define the maximum-allowed contour deviation. The machine tool builder determines the default value in a machine parameter of the control (typically 0.01 to 0.02 millimeter). The tolerance also affects the traverse paths on programmed circular motions, which is important if the core or cavity has cylindrical details that a user machines with either an interpolation milling or a mill-turn function.
3. Linear encoder compatibility: The CNC control should ensure that all machine axes follow the exact path when moving from X-plus to X-minus and then moving from X-minus to X-plus after a stepover. There must be an exact reproduction of adjacent paths after cutting direction reversal.
4. Vibration mitigation: A machine axis moving very fast and changing direction on a point or using a higher-than-permitted feed rate for the cutting tool can generate vibration, impacting part quality. The CNC control should monitor any tool vibration caused by either high dynamic movements or higher-than-permitted feed rates, and then adjust feeds and speeds to avoid chatter marks on the final part.
5. Operator flexibility: The CNC interface should optimize the machine dynamics based on each part’s feature priorities. The control should allow the operator or programmer to verify and optimize the program on the fly. For example, several different components made on the same machine, each with different accuracy, surface finish and lead time requirements.
6. Automatic feed and speed adjustments: Milling through mold cores or cavities with variable workpiece thicknesses is another machining challenge. A solution is a CNC that can detect how much material is currently cut, so the feeds and speeds can be automatically adjusted without operator intervention. This is accomplished with sensors connected to the CNC that measure spindle load and vibration and then identify and adjust speeds and feeds within milliseconds. This technology ensures that the machine maximizes chip removal rate based on workpiece cutting tool engagement, as well as cutter and spindle life.

Source: Six CNC Control Features for Moldmakers

Justifying new technology is a multi-step process that requires a thorough assessment of a number of factors at work in your manufacturing operation.

1. 1. Understand your costs

• Use a transparent costing system
• Evaluate all cost factors
• Evaluate the influence of new technology on the cash flow
• Calculate production cost and evaluate the influence of investment of new technology on production cost and return on investment

2. Understand potential benefits of CNC technology; measurable benefits

• Productivity
• Reduced setup and tool change time
• Improved uptime
• Throughput
• Scrap rate
• Tooling cost
• Maintenance cost
• Job preparation cost

There are also intangible benefits that may not be calculable, but can have a major effect on your machining success—including quality, accuracy and surface finish, which should not depend on operator skill and should not require constant checking; and, preset quick change tooling with standardized inserts that save production time (up to 15 percent or more compared to setting the tools on the machine so the machine is productive for a greater part of the shift).

3. Understand the costs of older equipment

1. • Lower efficiencies
2. • Longer than planned setup times
3. • Requires vanishing skills to set up and operate

4. • Requires tweaking to successfully run good parts—only a few setup/operators in your company have this ability

   • Incapable of today’s quality requirements
   • Cannot statistically hold tolerances
   • Produces excessive scrap parts, hurting efficiency, adding to material cost
   • Frequent unplanned downtime for repairs
   • In-house maintenance personnel (overhead)

5. Source: How to Justify New Machine Tool Technology

Shops can reduce idle time using tool presetters. High-quality tool presetters enable shops to take fast, accurate and repeatable measurements for each tool offline. This capability makes it possible for machinists to set tools while the machine is still running, which reduces idle time and increases cost efficiency in the shop. In addition to time and accuracy, the nature of presetting inherently promotes greater emphasis on best practices on ideal ways to store and measure tools. Toolboxes and drawers with random tooling ultimately migrate to more centralized tool-management areas.

The important attributes of a presetter are:

1. Accuracy. In the era of Industry 4.0, presetters should be able to maintain repeatable measuring accuracy of 5 microns or better. This usually means that the system should be equipped with an optical measuring device, which is more accurate than physical contact (or indicator) devices.
2. Repeatability. It is not just a function of measuring accuracy, but also of thermal stability. Presetters that use multiple material types in their base construction are more prone to non-uniform thermal expansion in the shop environment, which can require daily, repeated calibrations to output repeatable measurements.
3. Capacity. Before making a purchase, shops should consider the size and weight of their largest tooling assembly. Here is where size matters, so shops want to be sure the machine can handle the maximum weight, height and diameter of the tools they need to measure.
4. Ease of use. Adoption rates in the shop are key. The machine with the greatest capability in the world will end up being one of the largest paperweights in the world if no one can figure out and remember how to use it. When looking at functionality, examine the usability of the software, how many options and screens one has to look at to measure basic dimensions, and the overall ergonomics of the machine.

Source: Why You Need a Tool Presetter

With careful consideration given to the specific application and the various alternate means of achieving five-axis capabilities, there are a large number of benefits that five-axis machining can bring to the moldmaker.

By using a flat bottom end mill and maintaining perpendicularity to the complex surface, machining time can be greatly reduced. This is accomplished by stepping over the full diameter of the cutter, which reduces the required number of passes across a surface. The same principle applies to side milling of angled surfaces. This same process will also eliminate ribbing caused by ball-nose end mills, resulting in a much better surface finish, and cut down on manual millwork and handwork required to clean up kellered surfaces.

Five-axis technology also eliminates multiple setups required to re-position the workpiece at complex angles. This not only saves time, but greatly reduces errors and costly tooling and fixture expenses required to hold the workpiece in place. It also provides the ability to machine complex parts that are not otherwise possible—including holes, pockets and tapping required to be normal to a complex surface.

Through the use of shorter tools, five-axis milling can complete an entire part without re-fixturing or using the long tools required in a similar three-axis application and still deliver the final product in less time and with a better surface finish. Because the cutter extends out from the pivot point, a spindle-based attachment can actually mill outside the host machine’s stated travels when working inside a mold cavity.

Source: Five-Axis Machining: What Are The Benefits and How Can You Achieve this Capability?