Jul 28 2016

Plastics routing

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Modern plastics vary greatly throughout their manufacturing processes. The differences between various types of plastics and the different uses machinists will have for their materials cause problems for some people. A good machinist, whether you are a hobbyist or a professional, should be aware of the problems you could run into and steps you can take to get around those problems.
Not all plastics are made equal, not even plastics that share the same name when you are purchasing your raw materials. Something as simple as a change in color can alter how one plastic cuts compared to another. You need to be able to recognize and classify what kinds of categories plastic falls under when you are working with it. To start, determine whether you are working with hard or soft plastic. This is easy to determine; you can check this by seeing how rigid or flexible your material is. Alternatively you can see what type of chip is produced when the material is cut. Soft plastic chips will curl while hard plastic chips retain their solid shape. A single type of plastics is not always universally hard or soft; some plastics can be hard or soft depending on how they were manufactured. Knowing the geometry of your cutting tool is the second step to success. Tools that are best suited for cutting plastic have high rake and low clearance. These days there are thousands of different tools made for plastics cutting that you can choose from. Soft plastics tools have “O” shapes and are usually straight or spiraled in shape. Hard plastics tools can have an “O”, spiral shape or a “V”, straight shape.

Routing plastics

Routing plastics

One of the most irritating problems you can run into is the plastics material welding. This problem can happen due to the direction of your cut, using a cutting tool that is too small or having a chipload (the thickness of a chip) that is too large. Chipload is the most common cause of this problem. Your chipload is determined by your spindle speed, feed rate and the number of edges on your cutting tool. In plastic routing you want your chips to be just the right size to distribute heat correctly. Another potential problem is your finished project having poor finish. Having good finish is extremely important on plastic items, especially ones that are made to be put on display somewhere. The quality of your finish is going to be heavily affected by your chipload. Problems aside from chipload tend to come from the condition of your CNC router. You want to make sure that your CNC router is up to speed on maintenance and performance at all times. Learning to work with plastic correctly is a matter of practice and with time you will become experienced enough that you will run into these problems less and less, until you stop running into them almost altogether.
Making sure you have the right bits for plastics is one of the most important parts of preparing for projects. Your best option for hard plastics is a solid carbide router tool. If you cannot get one of those a carbide tipped tool is a good second choice but they will not perform as well as a solid carbide tool. You can use the same kind of tools on soft plastics but high speed steel tools will also work well on these softer materials. Your CNC router should be capable of speeds of at least 15,000 RPM to work on plastic. A CNC router is the best type of CNC machine for working on plastic, being the machine that is best suited for working at the high speeds your projects will require. Ideally you want to be working at 18,000 RPM, if your machine can operate at that speed, at a feed rate of 200 in. per minute. This is a general figure for plastics in general and the exact speed and feed rate you should use will vary a bit between different types of plastic.


Mar 3 2016

Leather CNC and Rubber Cutting

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Leather CNC and Rubber Cutting

Many of the materials that you will find yourself working with on a CNC machine can be cut using standard techniques. When you boil it down to the basics cutting different materials like wood, metal and plastic all use techniques that are pretty similar during the cutting process. Some of the less common materials that you might work with have different properties from the more frequently used materials and therefore require you to do your work in less standardized ways. Two types of materials that you need to cut differently are leather and rubber.

CNC leather cutting

Leather cutting

Leather is difficult to work with because of how it cuts differently compared to more common materials; you cannot just lay the leather down and cut it with a bit or you will ruin the material and your project will not look presentable. Hobbyists who already have a CNC machine can equip it with a drag knife in order to cut leather but there are a few things to keep in mind while using a drag knife. When getting your machine ready turn off the power to your spindle or router. The drag knife should never spin while it is cutting. Because the drag knife does not spin at all on its own you will have to manually realign its cutting edge when you want it to move in a new direction. Keep in mind that it can only move in the direction that the cutting edge is facing when you are programming your tool paths. In addition to a more specialized tool you will also need a very strong way to hold the material down. Leather needs to be held down as tightly as you can manage when you cut it. When you are selecting a hold-down method you cannot choose a vacuum. If a vacuum is used the pressure from the suction will go through the leather, making it almost impossible to cut. While a drag knife does work it is more suited for working with thin leather but it is not your only option. For any leather (thick leather especially) a laser cutter works very well. Because the laser is not physically cutting through the material you do not have to worry about how the leather will cut. Additionally, there will be no frayed edges and the leather does not require as strong of a hold-down method since the laser does not put any physical pressure on the material. Overall a laser cutter is better for cutting leather but many hobbyists do not have access to a laser cutter.

leather cut cnc

Leather cut

Rubber is a bit more challenging than leather. While rubber can be cut there is not necessarily an “easy” way to do it. First off, do not even try to cut rubber if you are using a laser cutter or a plasma cutter. It will catch fire. When working with rubber you are going to have to mill it. The key to cutting rubber is to freeze it so that it become more solid and easy to work with. To what extent you want to freeze it can depend on the type of rubber you are working with. In most cases freezing it with dry ice will work (-80° C). You can also freeze it with liquid nitrogen (-320.8° C) but that might be overkill. Once the material is frozen you are going to have to machine it very quickly before it can heat back up (especially if you are using dry ice). You will also need a tool that is extremely sharp. This can be a challenging process so do not be surprised if you cannot do it perfectly the first time. Rubber can be cut just as easily as any other material with a water jet cutter but most people will not have access to one of those due to their price. You are more likely to go with the freezing-then-cutting method.