CUTTING FLUIDS AND GRINDING ALLOWANCE

CUTTING FLUIDS AND GRINDING ALLOWANCE

Introduction
Application of a cutting fluid is very important in a grinding operation. The cutting fluid should be applied in adequate quantity and at very low pressure. The cutting fluid should be directed on the work just above the point where it makes contact with the wheel is. Contact zone.

Advantages of cutting fluid:
o It removes the heat generated.
o It improves the surface finish of the work piece.
o It maintains the hardness of the heat-treated works.
o It removes burn marks on the ground surface.
o It washes away the metal chips and discharged abrasive grains.
o It keeps the grinding wheel face clean.
o It acts as a lubricant for the work piece and the moving parts of the machine and prevents rust formation.
o It controls the grinding dust from flying around which may present a health hazard.
Recommended cutting fluids
Soluble mineral oil and pure water are mixed in the ratio of 1:40 or 50 (depending upon the grinding wheel) and is used for grinding the following materials.
➢ Cast iron or hardened steel
➢ Soft steel
➢ Connected carbide tools (using silicon wheel)
In India we use IOC Servocut oils as cutting fluid.
There is a special grade oil manufactured by IOC for grinding alone. It is called Servocut - clear.

Important points to be noted while using cutting fluids
➢ Always add pure water to soluble oils.
➢ Always the coolant should be kept clean. Periodically clean the coolant tank.
➢ Do not allow soluble oil to mix with other oils.
➢ Always mix water to the oil in correct proportion and not oil to the water.
➢ It should be kept cool under normal temperature.
➢ Use suitable splash guard so that the operator is protected from the splash of the fluid.

Grinding allowance
Machine parts are processed in different machines such as lathes, shaping machines, milling machines, etc. in such a way that their final dimensions have some stock left, which is finished during the grinding operation. The amount of this stock left is called the 'grinding' allowance. No definite value of the grinding allowance can be given as a general rule because this depends upon too many variable factors. In general it varies from-0.2 mm to 0.50 mm.The grinding allowance is given in microns. Closely spaced, fine abrasive grinding wheels generally give better finishes than coarse ground, widely-spaced abrasive wheels, and, hence, less grinding allowance may be sufficient. Keeping more grinding allowance than necessary ultimately increases the cost of the grinding operation.

SOME IMPORTANT FACTORS GOVERNING GRINDING ALLOWANCE

Hardness of material to be ground
For hard material less grinding allowance and vice versa.
Whether the part has to be heat-treated
During heat-treatment, deformation and scaling may occur; therefore, more grinding allowance is recommended.
Case depth of case-hardened workpiece
Less grinding allowance for shallow depth of case.
Grit and grade of grinding wheel
For fine grit and hard grade more grinding allowance and vice versa.
The surface roughness left by the previous machining process i.e. shaping, milling or turning etc.
Parts finished by a shaping machine require more grinding allowance than parts finished by milling and turning processes.
Whether grinding is done wet or dry
Less grinding allowance for dry grinding and vice versa

METHODS OF SELECTING THE HOLDING DEVICES
Selecting the holding device
The correct method of holding the work piece is determined by the type of grinding operation, the nature of the work piece and the machine to be used.
▪ The shape.
▪ Whether it is hollow or solid.
▪ Whether it includes centres.
▪ The position of the surface to be ground.
▪ The strength of the work piece.
▪ The accuracy required.
▪ The type of material.

The results of this check will determine whether the work should be held in
▪ A jaw-type chuck
▪ A magnetic chuck
▪ A vice
▪ A steady rest
▪ Centres
▪ A mandrel.
Selection of an unsuitable method may cause damage to the work or the machine. It may also cause injury.

Mounting in a chuck
There are two types of chucks.
❑ The three-jaw chuck
❑ The four-jaw chuck
Each type is designed to screw on to the headstock of the grinding machine.
In the four-jaw chuck, each jaw is moved independently with the chuck key. The jaws are reversible and stepped to engage different sizes of work. A work piece may be centered with superior accuracy in a four-jaw chuck but the operation is time consuming.

NOTE:
The jaws of a three-jaw chuck are moved in unison with. the chuck key. Centering may be carried out quickly, but. is not always as accurate as with the four-jaw chuck. The centering accuracy of the three-jaw chuck is dependent on the wear of its jaw surfaces, but should be within 0.05mm to 0.08mm
The three-jaw chuck is used to support.
Short cylindrical work pieces for internal or external cylindrical grinding operations.
One end of a long cylindrical work piece not provided with centres. (Fig 4)

 

Caution
❖ Use soft metal packing pieces to prevent scoring of the work piece surface. (Fig 6)
❖ Do not tighten the jaws excessively on the work piece as the jaws may mark or deform its surface.
The three-jaw chuck may also be used to hold hollow work pieces for external or internal grinding operations.
In this case, select the appropriate jaw set and adjust the jaws to bear against the interior surface of the
Work piece.
Use of the four-jaw independent chuck
A four jaw chuck will hold work more accurately than the Three Jaw chuck. As the jaws move independently, it may also be use for holding work pieces that are not perfectly round.

To use the chuck
- Open each jaw with the key
- Place the work between the open jaws and locate as centrally as possible against the chuck
- Use the guide rings to help centre the work piece
- Tighten the jaws against the work piece
- Rotate the chuck and mount a dial indicator to rest against the workpiece so that it touches only the high spots during rotation. (The dial indicator will show any eccentricity of the workpiece.)
- Position the workpiece so that the dial indicator is in contact with the high point on the workpiece circumference.

- Carefully retract the jaw or jaws of the chuck opposite the high point by an amount approximately equal to half the run-out of the high spot.
- Tighten the opposite jaw or jaws as necessary to move the workpiece into contact with the retracted jaw or jaws.
- Rotate the chuck again and note the dial indicator reading.
- If eccentricity is still present read just the jaws until true running of the workpiece is obtained.

Holding in a magnetic chuck
A magnetic chuck may be used to hold ferrous workpieces during cylindrical grinding operations. Some magnetic chucks are designed to screw on to the headstock spindle and some to be clamped to a faceplate.
Holding the workpiece in a magnetic chuck is also useful where the workpiece has an unusual shape not suited to other mounting methods.

Mounting in a collet
Collets are designed to hold work which is close to its required final diameter. A collet should only be used for holding wok which is within about 0.13 mm of size.
The type of collet selected will depend on the shape and size of the workpiece. The following example is one of a spring back collet to hold a solid cylindrical workpiece of small diameter for an external cylindrical grinding operation.

To mount the workpiece:
• Select a round collet with an internal diameter suitable for holding the workpiece. (Fig 10)

• Fit the collet to the headstock spindle and secure it in position with the collet hood. Usethe hood tool provided. to screw the hood to the spindle and lock it in position.
• Insert the collet by passing it into the hood and along the interior of the spindle towards the end of the pull rod.
• When the collet meets the pull rod, rotate the hand wheel carefully to engage the collet thread loosely with the pull rod thread. Do not tighten by turning the wheel further.
• Pass the end of the workpiece into the collet, and towards the pull rod. If necessary, the extremity of the workpiece may pass into the bore of the pull rod.

Mounting with a vice
When the shape of a workpiece is irregular.  It may be necessary to use a machine vice to hold it during grinding.

The steps to follow are as follows:
▪ Mount a face plate of suitable size on the machine headstock.
▪ Loosely bolt the vice to the face-plate.
▪ Secure the workpiece in the vice, taking any precautions necessary to protect the workpiece and ice jaws from damage.
▪ Centre the workpiece as accurately as possible by sight, moving the vice and workpiece as required.
▪ Firmly fix the vice to the face plate and mount the workpiece in the vice.
▪ Remove the handle from the vice.
▪ Disengage the headstock drive to allow the face plate to rotate freely.
▪ When the face-plate comes to rest, mark the uppermost part of its edge with chalk. This point is diametrically opposite the heavypoint.
▪ Balance the face-plate by bolting or clamping counter weights on or near an imaginary radius from the face plate centre to the chalk mark. Position the weights as close as possible to the vice. (Fig 16)
▪ Adjust the size and, if absolutely necessary, the position of the counterweights until the face-plate is statically balanced.
▪ Re-engage the headstock drive.

WARNING
Make sure the clamps are correctly fitted, and firmly secure the counterweights.
Rotate the face-plate at a SLOW speed and check the centering of the workpiece. Adjust the position  of the work and vice until the required centering accuracy is obtained.

Mounting on a mandrel
To grind the exterior surface of a cylindrical workpiece with an accurate centre bore, a mandrel to mount the workpiece between the centres of the grinding machine may have to be used.
Procedure
Select a mandrel with a diameter appropriate for the bore in the workpiece.
Clean the surface and centres of the mandrel and apply a small quantity of oil to the surface.(Fig 7)
Distribute the oil evenly.

Check that the mandrel runs true between the machine centres.
Remove the mandrel from the machine and slide the workpiece on to the narrow end of the mandrel.
Tap the THICK end of the mandrel gently with a soft faced hammer to firmly locate the workpiece on the mandrel. (Fig 8)

Fit a carrier to one end of the mandrel, and a face plate to the headstock of the grinding machine.(Fig 9)The headstock must be fitted with a dead centre.

Mount the mandrel between the machine centres.
Failure to clean and oil the mandrel before fitting the workpiece may result in the workpiece binding to the mandrel. This will make it difficult to remove the workpiece without damage to the mandrel and the workpiece.
To remove the workpiece from the mandrel, clean the mandrel surface and then tap the NARROW end of the mandrel with a soft faced hammer.When using the soft faced hammer to tap the workpiece on to or off the mandrel, support the workpiece to prevent it from falling to the floor.