Читать книгу CNC Trade Secrets - James Harvey - Страница 8
ОглавлениеThere are a lot of advantages to having a work piece held rigidly. When a part is held rigidly, feed rates can be increased and cutting time reduced. Cutters last longer and you’ll likely end up with better surface finishes and more accurate parts. At “tool shows” where vendors offer impressive demonstrations, you rarely see flimsy, difficult-to-hold parts being machined. .
1.Hold parts securely and accurately. (see Fig. 2-1)
In real life, it’s common to come across parts that are flimsy and difficult to hold. Holding parts for second operations such as drilling holes on edge can also be challenging.
Figure 2-1 This part is being held for edge drilling with the aid of 2-4-6 blocks.
2.Find simple ways to hold parts securely. (see Fig. 2-2)
When presented with a part that is difficult to hold, your ingenuity will be put to the test. However, don’t spend excessive time constructing complex fixtures when you don’t have to.
The first options to consider when a standard vise won’t do the job include using standard shop tooling — such as 1-2-3 blocks, 2-4-6 blocks, long parallels, grinding vices, angle plates, and V-blocks. You can often use these items effectively to provide the added support you need.
Figure 2-2 Some parts are difficult to hold. This part was ultimately machined using a standard milling machine vise in conjunction with a couple of spacer blocks.
3.Make a fixture, if needed. (see Fig. 2-3)
I’ve found that when you finally decide you need a fixture and take the time to make one, you’re almost always glad you did. Often a fixture is needed to hold parts for perimeter cutting if the material is already to the correct thickness. In these cases, you can’t use excess stock to hold the part for perimeter cutting.
Figure 2-3 A 2-4-6 block in combination with a gauge block stack is used to support the center section of a bar so it doesn’t “push away” during the cut..
4.At the end of a CNC program that drills holes in the part, add a tap drill to drill deeper into the mounting fixture. (see Fig. 2-4)
Here’s a simple but effective trick for making fixtures you can bolt parts to. Suppose you are making a part that has no excess material to hold for cutting the perimeter, but has through holes that can be used for clamping. When you are done with the drilling program, hand tap the fixture through the holes in the part. After that, you simply screw the part to the fixture. You are now ready to cut the perimeter. The beauty of this method is you never have to move the part.
Figure 2-4 A tap drill can be added to the part drilling program to drill into a fixture plate.
5.Save fixtures for future use. (see Fig. 2-5)
Most fixtures can be constructed using aluminum. We have a couple of drawers full of simple fixtures that machinists have constructed over the years for various jobs. One of our biggest problems with fixtures is that we don’t have enough room to store them. Therefore they get thrown here and there, scattered around, which makes them difficult to find.
Figure 2-5 Fixtures become worth their weight in gold when you need them.
Although sheet metal parts look simple enough, they are usually a pain to hold and machine. I dislike seeing guards, housings, panels, electrical boxes, etc. come through the door simply because they are flimsy and difficult to hold (see Fig. 2-6).
You’re often forced to make elaborate, hare-brained, time-consuming setups to hold these parts just so you can machine a few simple slots, holes, or windows in them. You may find tips 6 through 12 to be beneficial; they are all related to work holding.
6.Use a good quality angle lock milling machine vise.
We made the mistake some years ago buying a couple of cheap imported milling machine vises. What a blunder! Our thinking at the time was: “All the vise has to do is clamp the part. How difficult can that be?”
We suffered with these vises for about a year before we upgraded to high-quality vises. Table 2-1 shows what we found about using cheap milling machine vises.
A milling machine vise is something used nearly every day so the benefits of having high quality vises are compounded.
Cheap milling machine vises
• You are constantly struggling to keep parts held securely against parallels.
• There is a “spongy” feel when you start clamping on a part. You’re never quite sure how much to tighten the vice, which results in inconsistent clamping pressure and part location.
• They have a lot of sharp edges and burrs.
• They are constructed of cheap metal; tolerances are wide open; even the handles don’t fit well.
Figure 2-6 Sheet metal parts are often difficult to hold for further machining.
7.Plan setups so you can see the cut. (see Fig. 2-7)
This is a rule I always try to follow. There is no sense working blind if you don’t have to. Being able to see the cut has advantages: You’ll be able to keep an eye on cutters for anomalies such as chip packing, chatter, coolant coverage, surface finish, cutter flexing, gouging, jamming, and anything else that might become an issue. As an added benefit, features you cut will likely be easier to inspect.
Figure 2-7 The programming and setup for this cut were planned in advance so the cut would be facing the operator. Being able to see the cut makes it easier to watch for programming and cutting anomalies.
Figure 2-8 A planer gage is used to quickly add stability to this setup.
8.Aim for stability in your setups. (see Fig. 2-8)
Theoretically, you have stability when pressure applied anywhere perpendicular to an unclamped part doesn’t move the part. Having stability is more important when machining flimsy parts —the strength of the material may not provide enough rigidity to make a cut.
Figure 2-9 Dog leg parts such as this can often be given additional support by mounting a temporary vise to the table.
9.Use a grinding vise for additional support. (see Fig. 2-9)
This arrangement works great for supporting odd-shaped parts that can’t be held conveniently in the main vise.
10.Construct a set of long jaws that can be bolted to the vise. (see Fig. 2-10)
Cut a groove down the middle of two aluminum bars mounted side by side to create a set of long jaws. We have a few sets of these parallels that have come in surprisingly handy over the years. Long jaws that extend beyond the width of the vice will not provide the same clamping pressure at the ends of the jaws as in the middle. Use a couple of Kant-Twist clamps at the ends of the jaws to provide adequate clamping pressure to hold parts firmly.
Figure 2-10 Long jaws such as these come in handy for holding parts that extend beyond the vise jaws.
Figure 2-11 V-blocks provide a quick way to hold cylindrical parts for further machining.
11.Use V-blocks to hold round parts. (see Fig. 2-11 and Fig. 2-12)
There are a few issues to keep in mind when using V-blocks to hold parts for machining. First, V-blocks don’t apply clamping pressure over a large surface area. The clamping pressure is applied to tangent lines on the circumference of the part. If you squeeze down too hard on parts held with V-blocks, you may dent the parts and throw the center line of the part off location.
If you use two V-blocks to hold round parts, theoretically you get four lines of clamping contact. If I know cutting pressures will be light, I often use just one V-block and the vise jaw to apply clamping pressure. You can also use some soft material between the vise jaw and the part to avoid denting the part.
If the diameters of cylindrical parts are not precise and consistant, using a V-block to hold parts may cause them to tilt or may throw center lines off location. That’s one reason you shouldn’t always take advantage of wide-open tolerances. Tight tolerances, although not necessarily needed for the part to function, may be needed to locate and hold parts accurately for further machining.
Figure 2-12 This V-block has a relatively small “V” groove cut which is used to hold a small copper pin for additional machining. Plastic material is being used here to apply pressure to avoid denting the part.
Figure 2-13 Perishable vise jaws can be used effectively to hold parts that may be difficult to hold using other methods.
12.Use perishable vise jaws. (see Fig. 2-13)
Use perishable vise jaws as a last but effective method to hold parts. I use perishable vise jaws as a last resort for the following reasons. It’s often difficult to find a set that hasn’t been all cut up. You can always make new vise blanks, but if you leave them out, they’ll likely get used quickly. Furthermore, the appropriate nests have to be machined into them, which takes extra time.
Suggestions for Get a Grip
1.Hold parts securely and accurately.
2.Find simple ways to hold parts securely.
3.Make a fixture, if needed.
4.At the end of a CNC program that drills holes into the part, add a tap drill to drill deeper into the mounting fixture.
5.Save fixtures for future use.
6.Use a good quality angle lock milling machine vise.
7.Plan setups so you can see the cut.
8.Aim for stability in your setups.
9.Use a grinding vise for additional support.
10.Construct a set of long jaws that can be bolted to the vise.
11.Use V-blocks to hold round parts.
12.Use perishable vise jaws.
