# Machine Shop Estimating

For estimation purposes, machining cost is calculated after finding the material cost. Machining is done on castings, forgings and bar stocks etc., for getting the exact size and shape of the product.

We shall discuss in the chapter on 'Estimation in Foundry Shop' that certain amount of material is added as machining allowance. For this purpose, we take slightly bigger dimensions than that shown on finished drawings on the sides which are o be machined.

## Machine Shop Operations

Generally following operations are performed in the machine shop on different machines:

1. Turning. 2. Knurling. 3. Facing. 4. Drilling. 5. Boring. 6. Reaming 7. Threading. 8. Tapping. 9. Milling. 10. Grinding. 11. Shaping. 12. Planning.

In estimating, our aim is to find out the time, which an operator takes for performing the machining operation, for calculating their wages. In addition to this machining time (also known as operation time), following time considerations are taken:

(i) Setting up the job and tool or cutters.
(ii) Setting up the machine.
(iii) Inspection of job.
(iv) Fatigue allowance.
(v) Tool changing and sharpening time.
(vi) Machine cleaning and servicing time.
(vii) Personal allowance.

In the study of machining time following terms are generally used:

## Cutting Speed

It is the distance which tool travels along the material in one minute. Its unit is meters/min.

Let us consider an example, in which a job of D dis is revolving at a speed of N r.p.m. Then, distance travelled by the tool point in one min = Distance moved in one revolution x Revolutions performed in one min.

Therefore, cutting speed = 3.14DN/100 m or min

Cutting speed depends on the following factors:

(i) Hard material requires a lower cutting speed than that of soft and ductile materials.
(ii) High speed tools and tools of special cutting alloys can cut at higher cutting speeds than carbon steel tools.
(iii) If the depth of cut and feed is more, then less cutting speed is taken and vice versa.
(iv) By using good cutting fluids, cutting speeds may be increased.

An estimator should consider above factor while selecting a suitable cutting speed.

Table on next page gives the cutting speeds for different materials on different operations. The cutting speeds are with H.S.S. tools.

Note:

(i) Cutting speeds for grinding in Table show the speed of the work at which it travels against the grinding wheel, while grinding wheel has its speed for external grinding at 1800 m/min, for internal grinding as 1200 m/min and for surface grinding as 1500 m/min.
(ii) When the tungsten carbide tools are used, the cutting speeds are 2 to 3 times of these speeds.

## Feed

It is the distance, through which the tool advances into the workpiece during one revolution of the workpiece or the tool or cutter. Its unit is mm/rev.

As the feed depends on the depth of cut, cutting speed and power of the machine, hence no specific values for this can be mentioned.

## Depth of Cut

It is the amount by which a tool or cutter is inserted into the metal during one cut. In other words, it is the thickness of the metal removed in one cut. It is generally measured in mm.

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