CNC Trade Secrets. James Harvey
to cut through the skin of the stock material before doing other precision machining operations.
Occasionally, when a finished part has to be precisely square, parallel and on size, and has a lot of hogged-out features, you may have to rough in the large features first, then go “back” and complete the precision overall squaring and sizing of the part. Once the overall size is correct, you can add or finish close tolerance features such as dowel pin holes, knowing that the material will be stable. Using this technique is somewhat time consuming, but will likely produce parts that are in tolerance.
12.Make a few extra parts.
Running a few extra parts makes running a job faster and easier, I believe. When you have only one piece of material or the exact amount of material to complete an order, you necessarily have to be more careful — which usually translates into more time and stress.
13.Do a trial run on new programs
When making a first run test on a new program, note the changes you make at the machine for feeds and speeds. Also note any depth of cut and cutter path changes that would make the job run more efficiently. Until you watch a part being machined, you can’t be certain what changes should be made.
After making notes at the machine, go back to the computer and update the settings or the cutting parameters in the CAM software; then re-output the program. Sometimes I do this a number of times, depending on how complicated or tricky a part is to run. The higher the quantity of parts, the more energy I put into this editing process. Ultimately, I like to have settings saved in my CAM software match what I am running at the machine so that very little editing, if any, is needed when the part is run at a later date.
You may have libraries of cutter feeds and speeds in your software. These are somewhat useful, but they don’t account for the rigidity of the setup and the part you are machining. There’s a lot of “feel” and intuition that goes into machining, whether it is manual machining or CNC machining. “Feel” is an acquired skill. It is sort of like hand tapping. If you tap enough holes, you eventually develop a feel for how much torque you can apply to a tap before it breaks.
Suggestions for Proper Planning
1.Initiate a new project with proper planning.
2.Use the largest contact area.
3.Rigidity is an important factor.
4.Spend time visualizing.
5.Determine if you can machine a job holding the material in a standard milling machine vise.
6.Face off the backside of a part to finish the overall thickness.
7.Use multiple matched vises to hold long parts.
8.Determine how you are going to square the material.
9.Machine stocky rectangular parts in one setup.
10.Square larger parts independently.
11.Plan for material warpage.
12.Make a few extra parts.
13.Do a trial run on new programs.
There 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