Sep 22 2016

Pocket Milling

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The combination of CNC machines and CAD/CAM software allows machinists to cut materials faster and more accurately than what is possible when cutting by hand. One of the most common types of cutting is pocket milling (or pocketing), which sees extensive use in the aerospace and shipyard industries along with the uses a hobbyist has for the technique. Pocketing milling can be used with 2D or 2.5D projects and roughly 80% of all mechanical parts can be created with this kind of toolpath. Using this type of toolpath can make a big difference in the quality of a finished project.

Pocket milling

Pocket milling

Pocket milling allows you to use an end mill and machine away large parts of your raw material in a roughing process prior to finishing your project. This process allows you to save time and money while maximizing the amount of material removed. There are several styles of pocket milling such as traditional lace or zig-zag, concentric or offset In/Out and high speed cutting paths. The type of toolpath you pick can be categorized as linear or non-linear and there are parameters that can be set in many CAD/CAM software programs to control the pocket milling process.
Linear tool paths are unidirectional and the two major types are zig-zag tool paths and zig tool paths. Zig-zag milling causes the tool on your machine to move back and forth while removing material. Cutting is done with and against the rotation of the spindle. This method makes machining faster but also increases tool wear and machine chatter. Zig milling causes your tool to move in only one direction which results in better surface quality. However, you will have to lift and retract the tool after each cut so it takes more time to finish machining.
Non-linear tool paths are multi-directional with contour-parallel tool paths being an good example of this type of tool path. Contour-parallel tool paths use the pocket boundary to derive the tool path. Using this method your cutter is always in contact with your material, allowing you to eliminate time that would be needed to position and retract your tool using other methods. This method is used frequently for large-scale material removal and there are two different approaches that can be classified as contour-parallel tool paths. Pair-wise intersection approaches bring the boundary of the pocket inwards in steps. Offset segments intersect at concave corners and intersections are trimmed off to obtain the required contour. Voronoi diagram approaches segment the pocket boundary and the voronoi diagram is constructed for the entire pocket boundary. Voronoi diagrams are used to generate tools paths and this method is considered to be efficient since it avoids the topological problems machinists can run into using traditional offsetting algorithms.
Toolpath parameters for pocket milling in your CAD/CAM software will include data about roughing and finishing tools, pattern selections, cutting direction options (climb vs. conventional), toolpath cutting angle and step over % input fields. There should also be data that allows you to control side and bottom allowances for finishing as well as depth controls for single or multiple roughing steps. Many CAD/CAM software programs allow you to define single, even cutting depths based on the overall depth and define or force depths. Finally, there should be lead-in and lead-out options to control how the tool enters and exits the cut. Some CAD/CAM software programs will offer all or at least some of these options and some will not; it will depend on what software you are using. There are CAD/CAM programs that offer explanations and tutorials on pocket milling and these programs are usually the ones that are well equipped for pocket milling operations. Examples include BobCAD-CAM, CIMCO CNC-Calc and SimCam (CNCSimulator Pro).


Sep 15 2016

Milling Materials & Cutting Speeds

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Milling machines are used for a large amount of differing tasks. Some of these tasks are what would come to mind when you think “milling” but the machines are also capable of drilling, boring, tapping, reaming and other jobs. There are rules you can following to determine what cutting speed and RPM should be used depending on what you are doing and what kind of material you are working with. For example, reaming is generally done at half the speed and double the feed of drilling.

Milling machine

Milling machine

First consider your cutting speed. On a milling machine “cutting speed” refers to the speed of the outside edge of the machine’s cutter while the cutter is active. Cutting speed is sometimes referred to as surface speed. When determining surface speed keep in mind that it is directly related to surface footage and surface area. Say you have a one inch cutter and a three inch cutter and you roll each of them by one full turn. The three inch cutter will have traveled further because it has a larger surface area. This principle is used when determining cutting speeds. If you have two cutters that are different sizes and move at them both at the same RPM the larger one will have greater surface speed. Surface speed is measured as surface feet per minute (SFPM). Cutting tools all work using this principle. Your cutting speed will mostly depend on the type of cutting tool you are using and the kind of material you are cutting. How hard the material is determines what cutting speed you should use. Harder materials require slower cutting speeds and vice versa. If you were cutting steel and switched to iron you would want to speed your machine up. If you were cutting aluminum and switched to iron you would want to slow your machine down. The cutting speed you should use will also be greatly influenced by the material your cutting tool is made out of. Cutting tools made of harder materials allow for faster cutting speeds. Feed rate and the depths of the cuts you are making will affect speed as well but not as much as the hardness of the cutting tool and the material. Cutting speed, feed rate and the depth of the cut are called cutting conditions when you group all three together. Your cutting conditions are determined by machinability rating (comparing materials on how well you can machine them).
A milling machine must be set up so that its cutter will operate at the right cutting speed. To find out what the right speed is you need to calculate the RPM (revolutions per minute). The equation for this is (cutting speed * 4) / (diameter of cutter). There are many charts available on the Internet and in machinability books that you can use to find the hardness of materials. These charts make good cheat sheets for machinists who are still learning or for people who do not have the time to look up the information on material hardness frequently. Say that you are using a quarter inch cutting tool on a material with a SFPM of 110. That would be 70 * 4 (which is 280) divided by 0.25. This equals out to 1120 RPM. There will be many times where you cannot set your speed settings to the exact number you can but try to get as close as you can. When you have to decide whether to go a bit faster or a bit slower than the number you calculated think about things like how deep the cut will be, if you are roughing or finishing your project and whether or not you are using coolant. What you want the end result of the project to be should influence your decision. There are also software programs available online that you can use to make the calculations you need. Just be sure to have access to all the numbers you need for your calculations, regardless of whether you are making the calculations yourself or are using software.


Sep 1 2016

Aluminum and CNC Machines

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Aluminum is one of the trickier metals used on projects made with CNC machines. Its properties are a bit different from other metals so there are usually considerations that have to be taken before you start cutting. This article will review how aluminum reacts when cut with a water jet cutter, a laser cutter and a plasma cutter so that you can be better prepared to work with aluminum regardless of what type of CNC machine you are using.
Water jets are the most common type of CNC machine used for cutting alu, being able to cut as thin as 0.001” or as thick as 12”. Because water jets are a cold cutting process you do not have to worry about heat while you are cutting like you do with laser or plasma cutters so there is no need to worry about deformation or the material hardening. The lack of heat creates a safer work environment since there are no dangerous fumes being created by the cutting process. Abrasive water jets also have the advantage of being the cheapest of the three machines discussed in this article. Almost every type of cut you could need for an aluminum project is possible with a water jet.

Aluminum and cnc machine

Aluminum and cnc machine

Laser cutting is a bit unusual for working with metals (generally mechanical tools are used for metal cutting) but it is still a viable option. Because alu reflects light and conducts heat efficiently there are considerations that need to be taken before using a laser cutter on aluminum. Due to aluminum’s properties a standard laser will not work. Instead you would need a laser with a high power setting that also uses compressed gas. Using gas during cutting is fairly common with multiple types of machines so it should not be difficult to make the modifications you need. Laser cutters supported with nitrogen can cut aluminum without any loss in the quality of your project. The downside is that the increased power usage (i.e. more electricity) and the gas that you will use are going to make cutting aluminum more costly to cut than other materials. Additionally, laser can only cut aluminum that is fairly thin; at the maximum a laser cutter can be used on aluminum that is about ⅛” thick.
Plasma cutters generate a lot of heat and are capable of cutting many materials, including aluminum, very quickly. It may take a little time for a plasma cutter to get to full power after you turn it on but once the machine is running it will do its job very well. Selecting the correct type of gas to use for the plasma is paramount in ensuring that the aluminum cuts correctly. Your choice will also determine how messy the cutting process will be (what type of aluminum you are using will also be a key factor in determining this). Plasma cutters can use less expensive gasses than laser cutters can when cutting aluminum so costs will be a bit better over a laser cutter (though they likely will not be cheaper than a water jet).
Each of these three methods has their own ups and downs when it comes to cutting aluminum. If you are choosing between all three of these methods a water jet is going to be your best bet in most cases. Not every machinist will have a water jet available so if it comes down to a plasma cutter or a laser cutter do a little research and see which one would work best on the type of aluminum you are using. Regardless of what type of machine you have access to there is usually a way to make any project work (within reason) on a CNC machine. You just may need to think outside the box a little to figure out what that way is.


Jul 7 2016

Climb and conventional milling secrets that you should know

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When a cutter moves through material it can mill up (conventional milling) or down (climb milling). Many machinists use climb milling for most, if not all, of their CNC projects. Climb milling is known for producing better surface finish than conventional milling and overall does have more advantages than conventional. However, climb is not always the better of the two options and any machinist should know that there are times when conventional is preferable, and when those times are.

Climb and Conventional Milling

Climb and Conventional Milling

Using climb milling, each tooth on your cutting tool makes contact with the material you are working with at a defined point and moves out, cutting thinner parts of the material until it is no longer touching the material. So, the width of the material being cut starts at the maximum length and decreases to zero as the cutter moves. This causes chips to be thrown behind the cutter, making chip removal an easier process while machining. Tool life is also extended because each tooth on the cutter is not rubbing against the material. One of the major downsides of climb milling is that it can potentially produce a lot of backlash. As a result this method should mainly be used on machines that can eliminate large amounts of backlash and it may not always be usable with older CNC machines.
Conventional cuts in the opposite direction of climb. Using conventional milling, the teeth of the cutter will start at zero thickness and work their way up to the maximum thickness that you are cutting. When first making contact with the material your cutter does not even cut the material; it slides across the material surface until enough pressure is built up for the tooth to dig in and begin cutting. This causes the work material to become hard and somewhat deformed and also causes cutters to dull faster than when using climb. The sliding and biting of this cutting process also tends to leave an inadequate surface finish on work materials. On the upside this process does not generate anywhere near as much backlash as climb and is a perfectly sound cutting method on almost any CNC machine. The two methods do not have to be used independently either; climb can be used for rough passes while conventional milling is used for finishing passes.
Climb does have a few distinct advantages over conventional when your machine can manage it. As mentioned before, your tool life will be longer, surface finish will be better and chip removal is much easier. Additionally, you do not need as advanced of a hold-down system. Climb exerts force downwards instead of upwards like conventional milling. You can also use higher rake angles while climb milling, saving you a little money on the amount of power needed. Just do not forget about the excessive backlash when looking at all of the positives of climb milling.
When using climb milling, deflection can causes some problems with surface finish. Climb cutting causes tools to deflect, deforming the surface finish of projects and leaving you with less than adequate results. If you run into this issue try switching to conventional cutting; that will likely make a big enough difference to correct any issues you are having with maintaining a good surface finish. While conventional cutting can help it will not always fix your problems. If you have tried climb cutting and conventional cutting and are still having issues you can decrease deflection further by reducing the depth of your cuts. Using a small amount of your cutters diameter will make it less likely that you will experience any deflection.


Jun 2 2016

Aluminium Cutting and tips for cnc routers

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Aluminium Cutting and tips for cnc routers

Some hobbyists may ask if a CNC router can cut aluminum(Aluminium Cutting) since you most often see other users cutting wood or plastic. You can use your CNC router to cut aluminum if you know how to do it right. Aluminum (and other metals) are different to cut compared to wood or plastic. They have a smaller “sweet spot” for optimal cutting. If you leave the sweet spot your cutter will start breaking and wearing out faster and the surface finish will not be very good. Another thing to keep in mind is the “stickiness” factor (this applies to some other metals too). Aluminum wants to stick to your tool. It will do this to the point where it welds itself to your tool(tools). When sticky bits of aluminum have clung to your cutting edges your tool is not going to have much more life in it. Even though it is challenging you can cut aluminum on almost any CNC router. This article will give you a few tips to make sure you stay on track doing that job safe and proper.
While a CNC router can cut aluminum it is not the best tool for big parts, like vehicle parts. To cut successfully you need to do the job slowly. Just let your machine do its job and do not be in a rush. Cutting aluminum is not easy and will probably push your CNC router to its limits(Aluminium Cutting). Use a Feeds and Speeds calculator to optimize your settings so that you are not dodging the tip of your cutter that broke off and is flying across the room. Ideally you want to use a calculator that takes minimum RPM, CNC router cutter types, deflection, rubbing warning and chipping thinning into consideration. Your first problem will be recommended RPMs being too low. A CNC router cannot always go as slow as you want to go for cutting aluminum but there are ways to bump you’re the recommended RPM and fix this problem.

CNC router cutting aluminum ((Aluminium Cutting)

(Aluminium Cutting CNC router

The first tactic is to user cutters that are good at high speeds. The measurement to look for is Surface Speed which is high is carbide cutters. Cutters like HSS and Cobalt will be too slow so look for carbide TiAIN coated cutters. They cost a bit more but the improved results of your project will be worth it. Say you were using an HSS Endmill and the recommended RPM is 5877. Your CNC router’s spindle may have a minimum speed of 20,000 RPM (this is a very common minimum speed for routers). A TiAIN Carbide Endmill may have a recommended RPM of 16,897, putting it much closer to your machine’s minimum RPM. For aluminum try to get as close to 20,000 RPM as you can and you should be fine.
Another way to increase RPM is using small diameter cutters. The largest cutter you want to use should be ¼” but go smaller than that if you can. You will most rigid cutters to stop tool deflection from being a problem (this is where having tool deflection on your calculator becomes important). The smaller your cutter is the closer you can get it to 20,000 RPM. Be extra careful about clearing away chips. Recutting chips is an easy way to break a cutter. Be paranoid about this and do not just assume that your dust collection system is good enough without testing it for this first. It will be harder to get chips out as you cut deeper. Make more passes to cut down as far as you need to and you will open up shallower depths for better access. Lubricate your tools that that the chips will not stick to your cutting edges. The best way to do this is with a mister if you can set one up.
Now you need to think about avoiding tool rub. Going too slow with your federate can cause this to happen. Using your calculator should let you dodge this but be careful because the high speeds of your CNC router can result in tool rub happening easily. It can even start to happen while you are unaware that it is going on. If you cannot feed fast enough use fewer flutes. You should use a maximum of three flutes at once regardless. Using few flutes is good and is the reason 1 flute cutters are sold. There is a lot to remember when cutting aluminum but once you know what you are doing you should be able to use your CNC router for aluminum projects safely and accurately.