High Feed Insert Tooling

Another design consideration is the speed at which the tool is designed to operate. Different materials machine better at certain surface speeds. Surface speed is the distance covered by a point on the circumference of the tool.

For example, a 1in diameter tool being run at 1000 rpm has a surface speed of 265 Surface Feet per Minute (SFM)

Aluminum and plastic materials require the highest surface speed. In discussions with Dr. Eberhart and other readings, surface speed is determined by both material yield strength and toughness, but also a machining constant measured in watts of power to remove a specific amount of material.

Some exceptions do not seem to fit this rule. Steels and stainless steel with vastly different yield strengths require the same surface speed. (1018 – 350 SFM, 17-4 PH 300 SFM vs 6061 Al 1200 SFM). Titanium is another exception. I initially thought that hardness and density might have been what influenced surface speed. Titanium is machined at much lower surface speeds yet it has a high yield strength.

Source: Carbide tool manufacture - Harvey Tool [1]

The proper surface speed is also influenced by the material doing the cutting. Carbide tooling can machine at much higher surface speeds and therefore much higher material removal rates. I believe that carbide can do this because exerts a higher pressure by creating a more precise shear plane. Carbide tools are not sharp like a knife or a steel milling tool, instead, I believe they are designed to exert the cutting force across a smaller area and therefore create more shear force. Carbide tools more ‘slam’ into the material than they do cut it. Dr. Eberhart also brought up the idea of different specific heats influencing how fast a blade material can cut. I think that this is also a significant factor but in the workpiece rather than in the cutting blade. Carbide has half the specific heat of traditional steel tools. [2] Another factor might be that carbide simply holds a sharp edge longer than high-speed steel. Carbide measured around 75 HRC and steel tools are around 60 HRC. This will lead to longer edge life and will generate less heat. Carbide can also withstand more heat than a steel because its hardness is less dependent on heat. It will not lose an edge in a cut because it got hot.

With my research, I now think that the hardness of the cutting tool, the yield, and the modulus of elasticity has the largest impact on surface speed. Having both a high yield and a high modulus of elasticity will result in a slow surface speed. In general carbide tools cut at twice the SFM of high-speed steels.

TOOL DESIGN

I have decided to use high-feed geometry to direct as much of the cutting force into the spindle to reduce deflection. It will use carbide inserts because they allow for a higher surface speed and are easily replaceable. Furthermore, the tool will have 5 cutting flutes to allow for fast feed rates on low surface speed materials where deflection will be most apparent.

I started the design by looking at existing tools. I then started modeling an initial tool in CAD and running analysis. The first designs had too shallow of a blade angle that would not allow for the cutting insert to properly engage the work.

After redesigning, I had a tool that had an 11-degree tilt. This would direct nearly 20% (sin(11deg)) of the cutting force into the spindle. Anything past 11 degrees of tilt, the cutting insert would not function correctly. By doing FEA, I was able to determine that the material used for the tool body would need to have a yield strength of over 110 KSI to not fail. For FEA, I assumed full spindle torque being constrained by a single insert. Below are images of the CAD and the results of the FEA.

CAD Model

Analysis Result – Global FOS

-Assumes full drive and stall torque of spindle applied to one insert pocket with shank fixed

The next steps were to machine the blank and then heat treat. Initially, I expected to machine a rough blank, heat treat, then finish machine to remove any warping caused by the heat treat. I later decided to not finish machine the blank because the steel reached the same hardness as the tools that would be cutting it. This would make finishing to tool extremely difficult.

MACHINING

Below are pictures of the machining process. It was initially roughed on a lathe then the pockets were machined on a 4th axis mill.

Lathe Blank – Part is spun with stationary tool to cut a circular profile. Total time – 30 mins

4^th Axis Pocket Machining. The part will rotate and be robotically cut according to programs I write.

Heat Treat:

I selected 1566 steel because of how easy it is to heat treat and harden. According to a datasheet online [3], to achieve a yield strength of 110 KSI, the material should be heated to 850C then water quenched, and then annealed at 315C. Before heat treating, Dr. Bourne helped take surface hardness measurements to determine how well the tool hardened. Initially, the tool was at around 15 HRC. According to the chart, a yield strength of 110 KSI would correspond to around 30 KSI. I choose to not let the blank soak at the full temperature hoping that the core of the tool would not harden as much as the surface. This would result in a stronger tool that would be able to deform more without shattering.

Heat treating causes hardening because it allows for new structures to form. What new structures form and their distributions are specific to a material and can be found on phase diagrams.

[4] 1566 steel phase diagram

In 1566 steel, with .75% carbon, the initial heating allows for both cementite, graphite, and Austenite material to form. I only saw this chart after heat treating and after our lecture. Interestingly, the initial heat-treating temperature passes through a triple point in the chart. It is also interesting that 1566 steel has a carbon percentage of 0.75% which corresponds to this triple point. [3] The process also calls for a quenching step, Quenching stops the crystals from slowly forming and causes more grain boundaries. This is what causes the hardness. The material is later annealed to remove some grain boundaries to reduce brittleness. Annealing is like shaking the tray of BBs from lab. It allows molecules to reorganize into a more uniform crystal structure. This will increase the resilience of the material and cause a more ductile failure. After hardening, the tool was measured to be at 60 HRC. This is significantly harder than expected. I do not know what caused this and suspect that the material I was working with was mislabeled.

Post Heat treat

RESULTS

The tool was a great way to apply what I learned in the CHGN 125 class regarding phases, energy distributions, and material properties. Because of this project, I understand the materials better and see the real world uses for what we are covering in class, and see more of what is possible in material chemistry. Hardening and heat-treating have made me consider doing an MME minor because of how well material science compliments mechanical engineering.

I do not think the tool has any future in machining parts because of the extreme insert geometry. At an 11-degree sweep, the inserts cannot efficiently clear material and become bogged down. This leads to high spindle loads and tool deflection. Despite this, I think with different inserts designed for a higher sweep angle, the tool could be useful. I learned quite a bit during the process and have ideas for the future such as adding additional angles to further change the cutting geometry. The only way to make these might be to make my own carbide inserts which require sintering.

Finished tool with carbide inserts

Chips made by cutter

Surface finish left