DIFFERENCE BETWEEN NC AND CNC

In strict adherence to the terminology, there is a difference in the meaning of the abbreviations NC and CNC. The NC stands for the older and original Numerical Control Technology, whereby the abbreviation CNC stands for newer Computerized Numerical Control Technology, a modern version of its older relative. However, in practice, the CNC is the preferred abbreviation. To clarify the proper usage of each term we can tell the major difference between the NC and CNC systems.

Both systems perform the same tasks, namely manipulation of data for the purpose of machining a part. In both cases, the internal design of the control system contains the logical instructions that process the data. At this point the similarity ends.

NUMERICAL CONTROL

In The Manufacturing field, and particularly in the area of metal working, Numerical Control Technology has caused something of a revolution. Even in the days before computers became standard equipments in every company and in many homes, the machine tools equipped with Numerical Control System found their special place in the machine shops. The recent evolution in the microelectronics and the never ceasing computer development, including its impact on Numerical Control, has brought significant changes to the manufacturing sector in general and metalworking industry in particular.

Numerical control can be defined as an operation of machine tools by means of specifically coded instructions to the machine control system.

In instructions are combinations of the letters of alphabet, digits and selected symbols, for example, a decimal point, the percent sign or parenthesis symbols. All instructions are written in logical order and a predetermined form. The collection of all instructions necessary to machine a part is called an NC Program, CNC Program, or a Part Program. Such a program can be stored for a future use and used repeatedly to achieve identical machining results at any time.

Using a Vice

Vices use hardened and serrated steel jaws to grip the work-piece. These jaws are removable and can be replaced if worn. In normal use, make sure the jaws are kept tight.

The serrated jaws keep a tight grip on the work-piece but they also inevitably mark it. So in most situations, the jaws are covered with soft metal jaws. The easiest way of making these is to cut some short sections of aluminium angle a little longer than the width of the jaws.

The work-piece is positioned within the jaws of the vice and the vice just nipped-up.

The work is then rechecked for location, keeping in mind that you don’t want your hand tool (eg a file) to touch the hardened jaws of the vice. The vice is then firmly tightened by hand.

Normally the full width of the jaws is employed to hold the work-piece (or the work-piece is centred, as here) but sometimes the item must be positioned at one end of the jaws. Note, however, that doing this frequently over a longer period will distort the jaws – as a result, the clamping force will no longer be even across the jaws’ full width.

Mounting a Vice

Strange as it sounds, mounting the vice is perhaps the most important aspect of vice-use to get right. If the vice is mounted incorrectly, it will be hard to use and may even be dangerous.

Height

A vice must be mounted so that when you are standing next to it, the top surface of its jaws is level with your bent elbow. If the bench top is too low, the vice can be raised with a hardwood block.

If the vice is too high, you can stand on a rubber or timber board. Having the vice at the correct height will allow much more accurate filing and hacksaw cutting – your fore-arm will be able to move back and forth horizontally.

Rigidity

A vice must be very securely mounted. This has implications both for the way in which it is attached to the bench, and how secure the bench itself is.

Some books recommend the use of coach screws to attach a vice to a bench – but don’t! Instead, use heavy-duty bolts, nuts and washers to securely bolt the vice to the bench. Large vices use four bolts, while smaller vices may use only two.

It’s no use attaching the vice securely to a bench if the bench can then ‘walk’! The heavier the bench, the better. If you’re working with a bench that is not bolted down and is lighter than desirable, at least make sure that it has a lower shelf on which weighty objects can be placed. A bench that is not bolted to the floor needs to have a mass of at least 150kg if it isn’t to be easily moved around by objects being manipulated in a 150mm vice. If used to mount a vice, this bench should be bolted to the concrete floor. Note the 'feet' that can be used for this purpose.

Location

As we’ve indicated above, vices are normally mounted on heavy benches. But where on the bench? This is a deceptively tricky question. As shown in this diagram, the parting line of the vice is best positioned so that it very slightly overhangs the edge of the bench. In that way, long items can be vertically mounted in the jaws of the vice – and that’s even more important when mounting a vice with offset jaws.

Bench Vice / Work Vise

In order that work may be held rigidly for the performance of hand operations, the machinist uses what is termed a vise. They are made in a great variety of forms and sizes, but all consists essentially of a fixed jaw, a movable jaw, a screw, a nut fastened to the fixed jaw, and a handle by which the screw is turned in the nut and the movable jaw brought into position. The sectional view, Fig. 43, shows these parts clearly and also a device, present in some form in all vises, by which the movable jaw is separated from the fixed jaw when the screw is backed out of the nut. In the machinist's vise, both jaws are made of cast iron with removable faces of cast steel. These may be checkered to provide a firm grip for heavy work, or may be smooth to avoid marking the surface of the plate operated upon. When holding soft metal, even the smooth steel jaws would mar the surface; and in such cases it is customary to use false jaws of brass or Babbitt metal, or to fasten leather or paper directly to the steel jaws. The screw and handle are made from steel and the nut from malleable iron.

Standard Wire Gauge (SWG)

• lA gauge for measuring the diameter of wire, usually consisting of a disk having variously sized slots in its periphery or a long graduated plate with similar slots along its edge.
• lA standardized system of wire sizes.
• lWires are manufactured to standard sizes and labeled with their SWG.
• lAs the SWG increases the diameter decreases. 

 

How to use Micrometer?

• •Thimble

–The thimble rotates around a cylinder which is marked in millimeters.

–The left-hand side of the thimble has markings all around it.

–The line labeled 0 is the primary pointer.

• •Close the jaws

• •Calculate the least count
• •Place the object between the two jaws
• •Record the position of main scale where the two measuring rods met.
• •Note the position of the circular scale which coincides with main scale 

  

 

Reading of the Instrument

 

 

lReading of the instrument = MS div + (coinciding CS div x L.C)

l= 8+ (12 x 0.01)

l= 8 + 0.120mm

l= 8.120 mm = 8120 µm

 

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