CNC Trade Secrets. James HarveyЧитать онлайн книгу.
id="ulink_75897a1c-82ca-5b4b-8f62-d166e9e53d1e">Figure 1-2 Some parts can be planning nightmares.
Figure 1-3 This setup was chosen based on using the longest contact area possible with the vise jaws.
2.Use the longest contact area. (see Fig. 1-3)
One of the first and simplest things to look for is the length of the contact area between the part and the vise jaws. In general, choose the longest contact area possible. The setup on the left of Figure 1-3 is rigid before any material is cut away. Initially, aggressive cuts can be made with high feed rates to remove material. As the “window” cut shown on the right nears completion, the part becomes much weaker. Lighter cuts must be used at this stage. These types of machining scenarios are common and must be planned for in advance.
Figure 1-4 The deep windows in this part were cut first because that’s when the part had the most rigidity. The shallower windows being cut here are cut last because the part is significantly weaker.
3.Rigidity is an important factor. (see Fig. 1-4)
As you envision or simulate material being cut away, think about the rigidity of the part and setups. When possible, choose a sequence where you can use short, sturdy end mills.
Long end mills are more sensitive and unforgiving than short end mills to feeds and speeds. They usually need to be run slower. Long end mills have a tendency to chatter and push away. If you need to use long end mills to cut tall or deep features, then sequence the setups so that the long end mills are run when the part has the most rigidity. If you use long end mills on a part that has already been weakened by previous machining, you’ll likely compound difficulties.
4.Spend time visualizing. (see Fig. 1-5)
Planning begins by visualizing the different ways you may be able to hold a part for each setup. As you think about the different possible setups and machining operations, keep track of what material remains on a part that can be used to hold the part for future setups and also what material remains so you don’t run into any uncut material by mistake. Some parts may not have simple solutions. Look for a sequence that uses the least amount of setups. Multiple setups take time and are ripe for inducing dimensional errors. CAD systems help you visualize all the possible holding combinations because you can easily flip and rotate the model on the computer screen. In general, I prefer doing a job that uses the fewest setups possible, even at the expense of having to run long end mills less aggressively or cutting angles by 3D milling.
Figure 1-5 Try to plan jobs using the fewest setups possible.
5.Determine if you can machine a job holding the material in a standard milling machine vise. (see Fig. 1-6)
CAD systems help you visualize all the possible holding combinations because you can easily flip and rotate the model on the computer screen.
Plan to use a vise when possible. A vise eliminates a lot of variables such as the accuracy and rigidity of your work-holding device. A good quality vise is both accurate and rigid.
Figure 1-6 Plan to use a vise when possible. Holding parts in a vise eliminates the need for time- and material-consuming fixtures.
6.Face off the backside of a part to finish the overall thickness. (see Fig. 1-7)
Often when using a vise to hold a part, you will at some point have to face off the backside to bring the part to the correct thickness. This is a technique I use often. It eliminates the need for a fixture and allows you to do a lot of machining in one setup. Furthermore, the material thickness you start with is not critical within reason. Generally 1/8” of material is plenty to hold in a vise.
Figure 1-7 1/8” of material is generally all you need to hold parts securely in a vise.
Figure 1-8 Using multiple matched vises is an easy way to hold long parts for machining.
7.Use multiple matched vises to hold long parts. (see Fig. 1-8)
Indicating multiple vises in line may take some time. However, once the vises are in place, they can usually be left alone for a while. Multiple vises can be used to run multiples of the same part using different starting points such as G54, G55, G56, etc. Unused vises can also be used to run other jobs to avoid disturbing a setup.
8.Determine how you are going to square the material. (see Fig. 1-9)
Squaring the five sides of a part that are accessible in a setup means that you are going to have to side mill four sides. The top surface can be face milled. Side milled surfaces are more prone to inaccuracies than face milled surfaces. An end mill used for side milling that is dull, chipped, or tapered will likely give you a poor, inaccurate surface. This is especially true when using longer end mills. Therefore, if you are going to side mill a surface and expect an accurate surface, use a fresh end mill.
Figure 1-9 Side milled surfaces are more prone to inaccuracies than face milled surfaces, especially when using long end mills.
Figure 1-10 Small, stocky parts are less prone to warping than thin hogged-out parts.
9.Machine stocky rectangular parts in one setup. (see Fig. 1-10)
Small, stocky rectangular parts are less prone to warping and falling out of tolerance than hogged out parts. You can usually square them and do other machining operations in one setup. Using this method, you have to plan for holding excess stock in the vise. The excess material on the back side can be faced off later using a separate program.
10.Square larger parts independently. (see Fig. 1-11)
On larger, tight tolerance parts, I often do the squaring independently. In other words, I concentrate on getting good blanks first, then use other programs to add the necessary features.
Figure 1-11 Sometimes it is wise to focus on getting precision blanks before adding features. The mold cavity above was precisely squared to size before doing any other machining operations.
11.Plan for material warpage.
Material warpage is a fact of life in machining; it is often an issue that must be addressed, especially with long, slender, and hogged-out parts. It may not be much of an issue on stocky, block-type parts or parts that need just a few simple features added.
One of the most effective