As engineers, we apply our skills, knowledge and experience to produce the best looking and most accurate parts. We take great pride in the products we make, and we want others to see the pride in the finished product. But what do we do when we don’t get the results we want? Dimensionally, the part meets the blueprint specs, but the surface finish and overall appearance are less than ideal? When that happens, we need to go back to basics and make sure We use the best machining methods we know.
We need to look at things like a workholding fixture to make sure it’s sturdy and that it doesn’t promote harmonic issues or vibration during machining. We need to make sure we don’t use unnecessarily long tools that can easily turn or increase the chance of chattering. In high-speed processes, we need to ensure that we use a mass-balanced tool that is rated according to the programmed RPM used. But what if all the things mentioned above are fine?
Consider the following options:
- Control chip: Chip evacuation is a key factor in producing a good surface finish. The control chip is probably the first thing you should consider. If the chips produced are in contact with the workpiece during machining, or if you are re-cutting chips, it will most likely affect your surface finish in a negative way. Consider the possibility of changing the style of chip breaker you’re using to help break down the chips for better control. While using air and coolant are both good options for controlling chip evacuation, watch out for coolant. Coolant should be avoided when cutting intermittently. Thermal cracking of the cutting edge can occur…due to intermittent heating and rapid cooling of the cutting edge…and can cause premature insert failure, or at least start to affect your surface finish due to overstressed cutting edges and fail.
- Increased speed: This is especially true when using carbide tools. Increasing the speed will ensure that the material is in contact with the tip for less time…thereby reducing edge buildup on the tool, which can lead to poor surface finish. Increasing the rake angle of the cutting tool also helps reduce and control edge build-up.
- Use the correct nose radius: A larger nose radius will be able to accommodate faster speeds. The insert was able to feed at about half the TNR per revolution and still produce good results. If you exceed this TNR to IPR ratio, the tool will create more of a “line-like” surface finish rather than the glossy smooth finish you want. Therefore, the larger the TNR, the faster feed rates it can accommodate and still produce the desired results. However, using a very large TNR can create chatter – reducing cutting pressure – so be careful and consider the speed you need to cut the material – use a TNR tool that matches your needs. It’s also worth mentioning that using a larger nose radius means you have to leave more material for the finish pass. For the tool to function properly, you must have a TNR equal to or greater than the TNR in order to complete the tool removal. If you’re getting chatter around the corner, then you might want to try a smaller TNR. Always use a TNR smaller than the corner radius you are cutting – so you can “form” the desired radius – especially on finishing tools. This will help reduce cutting pressure and eliminate chatter. When milling, try using a bullnose or spherical end mill instead of a flat end mill. Something with a corner radius will give you a higher finish on sharp corners and will definitely help with tool life.
- Try wiper insertion: as much as possible. The wiper insert has a small flat area adjacent to the tip radius. This plane actually “wipes” the finish as the tool is fed along the workpiece, and helps eliminate the line-like finish that faster feed rates might encounter – which allows the use of a smaller TNR to help control chatter.
- Increase the lead angle of the tool. Higher lead angles and positively sloping inserts produce a better surface finish than tools with shallower cutting angles. For example: a face mill with a 45° cutting angle will produce a better surface finish than a face mill with a 90° cutting angle.
- Eliminate dwells and pauses: Every time the tool stops moving in contact with the part surface, it leaves a trace. Change the process if necessary, but do your best to ensure that the knife never stops or hesitates during the cut.