CNC Machining Part 3

Literature Review

The important of CNC machine are

1. Accuracy

2. Repeatability

3. Programming capability

The biggest CNC machine called Conventional-CNC machining. It is a combination both Conventional and CNC function and put in single machine. This type of machine has large amount of cutting tools. This makes this machine complete to do all type of manufacturing process including combination Lathe and milling works.

This machine can do several types of process like Lathe, milling, turning, boring, cutting and finishing. The common cutting materials are coated carbide, ceramic and uncoated carbide. It depends on how strong the workpiece is. The good cutting material is stronger than the work piece to prevent from the tools break. There are 2 types of machine cutting tool which are:

• The workpiece moving while the cutting tool in static position
• The workpiece stay static and moving cutting tools

Mechanism and Theoretical Background

As the name is Computer Numerical Control, CNC has a control system that can control the machine by a specific program. By doing the programming on CNC machine, the machine then read the instruction and performs the series of motion.

CNC machines have a specific range of motion–typically between 2 to 5 linear and rotational axes. Each direction of motion of a CNC Machine is called an axis. It is simply a direction of motion under the influence of the CNC controller. It can either be linear or rotary. For machine movement two types of motor are used either stepper motor or servo motor.

For trainer machine use a small machine. It just for training purpose and no large quantity of product has been produced. There are possibility to have large amount of defect on product because no expertise in skill. It is for skill development purpose.

For production machine it cost a big cost and also big amount of quantity of product in a short period. The defect of product is less since the machine is conducted by professional. Usually the machine used in production is bigger than the machine used for trainer.

Typical usage and Application

The entire machine component must be rigid and very strong material. The spindle should be high strength, sustain a high temperature and supported by large bearing. The clamp system should be strong enough to hold work piece as the machine facing a moveable part during the manufacturing process. In addition clamping system should be efficiently moved fast in clamping or unclamp the work piece with fast movement during the process. The importance of choosing cutting tools is important in order to make sure it will not break when cut the work piece.

Attached to the side of the machine usually collections of cutting tools. This also called a tools magazine. The machine will take a fast time to change the suitable cutting tools following what the machine is programmed. The range of time to change the cutting tool is between three to ten seconds depending on the machine. The bigger machine may have up to 60 cutting tool putting inside the tool magazine. Each will loading automatically follow the instruction of the programming.

The place that holds the work piece while machining is called a bed or work table. It consists of hard steel named “ways” which is usually covered by flexible guards. The worktable can flexibly move with required angle depends on what kind of process. For example process like turning and finishing sometimes need work table to stay on an angle so that the cutter can cut with required shape of the design work piece. Cast iron or Meehanite used to be the material of choice for metal working machines. Today, most machines make liberal use of weldments of hot-rolled steel and wrought products such as stainless steel to reduce cost and allow fabrication of more intricate frame designs.

Adopting in CNC machine technique is easy for large amount of production. CNC is using automated machines that work as what the machine is program and controlled. The essential part that user must know before conducting CNC machine:

• Spindle
• Work table
• Cutting tools
• Controller
• Computer programming
• Safety management

CNC method of Machining and Process

Referring to the Sketch:

1. Horizontal Machining Center – Take away the chip box and the gundrill bushing and you have the horizontal machining setup.
2. Lathe, Screw Machine or CNC Turning Center – This will look very similar to the horizontal machining setup, except that normally the work piece is rotating and the gundrill is stationary in a tool holder.

3. Vertical Machining Center – Rotate the sketch 90 degrees counterclockwise.

(Caution: Once you determine the length of drill you need for your application, assure that you have adequate clearance in your machine setup)

4. Coolant System – You'll require high pressure to do a reasonable drilling job. Many later CNC’s have high pressure coolant systems installed at the factory. They will tend to be in the 1000 or 1500 psi range. In such cases, follow the filtration recommendations of the manufacturer.

If you attempt to install a high pressure unit on an older machine, proceed with caution. It’s conceivable that you could blow the seals on the machine; so consult the original equipment manufacturer.

Another consideration is whether your machining center has ‘coolant thru spindle’. If it does nott, you can use a special ‘live’ gundrill holder (known as a ‘coolant-induced’ holder) which allows the coolant to flow thru the holder into the hollow shank of the gundrill. These holders are a boon for limited production, but they are not always adaptable to a high production environment.

Also, what size holes do you need to drill? 1800 psi will allow you to drill a very small hole- probably much smaller than you need. 1500 psi will cover you for a .080" dia. hole, and 1000 psi will let you drill somewhat smaller than 1/4” dia. If you’re going to drill a 3/4” dia, you can get by with 400 psi. If you’re limited as to what pressure you can generate, you may still be able to drill small holes within reason by adjusting your parameter downwards.

5. Coolant – Your CNC would not necessarily be running a coolant that is suitable for gundrilling. The anti-weld characteristics required for gundrilling are much more critical than for most CNC machining. This is important! If you're considering trying a gundrill, check into your coolant capabilities. Your manufacturer/supplier probably has an alternate choice that is acceptable for gundrilling as well as conventional machining. For more on this important subject, see "The right coolant”.
6. Starting the Hole – A gundrill bushing is very seldom practical in a CNC operation. Accordingly, you have to make your own "bushing" by machining a pilot hole in the workpiece with a center drill or end mill (using circular interpolation). The hole ideally should be toleranced to nominal drill diameter +.0010–.0000", preferably tighter, but never undersize. if you get an acceptable hole and have minimal drill breakage, you're home free! The depth of the pilot hole must be sufficient to bury the outside corner of the gundrill without the tip touching the bottom of the hole.
7. Sequence – Following this sequence of operation should keep you out of trouble:

1) Index gundrill into pilot hole slowly with spindle turning at bare minimum speed. Stop index just short of contacting drill tip with bottom of pilot hole.

2) Initiate coolant flow.

3) Ramp up rpm and feed slowly until you have a bite into the material. Then ramp up to specified parameters. For the drilling sequence, always set your program to read inches/rev rather than inches/min.

4) At bottom of hole, stop spindle and then shut off coolant prior to retraction.

5) Pat yourself on the back, trying hard not to break your arm!

CNC machines are very safe to use as they are designed to be as safe as possible. One of the main advantages of CNC machines is that they are much safer than manually operated machines. The picture below shows a Boxford A3 HSRi² CNC Router with many of its safety features labeled.