Fixture Design Considerations For Machining Computer Science Essay

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Cylinder head is an integral part of combustion engine and provide many key functions to the engine. The cylinder head design can be very complex in 4 stroke engines and hence manufacturing process of machining the cylinder head has great demands for quality, accuracy, repeatability and efficiency. This paper provides technical overview of types of fixture for milling process and determines that modular and dedicated fixtures are preferred for multi-step machining process of the cylinder head. Fixture setting as well as machining process contributes to a dimensional variation of the parts and means for mitigation of the influence from fixture and machining is also discussed in the paper.

Keywords

Cylinder head, Fixtures, Dedicated, Modular, machining errors.

Table of Contents

Introduction 4

Objectives 4

Principles of workholding for machining 5

Classification of fixtures for machining 5

Components of machining fixtures 6

Locating fundamentals 7

Location and clamping systems for cylinder head 8

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Comparison of Modular and Dedicated Fixtures 10

Selection of fixtures for machining cylinder heads 11

Machining Errors 12

Conclusion 15

References 15

Introduction

Cylinder heads for automotive applications are almost exclusively produced with casting process that requires extensive machining to meet all functional requirements of the cylinder head. Sand cast aluminum (AL 356 T6 heat treated) heads for most common they contain internal cavities for intake, exhaust ducts and water jacket. Main functions of the cylinder head can be characterized as:

Facilitates sealing of the combustion chamber and withstands mechanical, chemical and thermal loads associated with combustions events.

Accommodates inlet and outlet duct and valve train systems for sequenced exchange os air fuel mixture and exhausted gas exchange from combustion chamber

Accommodate ignition mechanism (spark plug) and injections ports (injectors)

Allows for proper cooling (water jackets) and lubrication (oil gallery) to dissipate heat and control a friction in moving components

Fig. 2 Cylinder head shows Cylinder head in different view and shows the complexity of part. Many of the functional features that require rather tight tolerances (size, position, flatness, roughness, etc.) must be machined. This paper provide overview of concept selection for fixturing strategy that allows located raw casting to milling machine, to machine datum features used in 2nd setup when final machining of the cylinder head is done.

Objectives

To research the principles of work holding for machining

To determine locating elements for cylinder head.

To find out the differences between dedicated and modular fixtures

Methods to reduce machining errors.

Workholding for machining

A Fixture is a special workholding device that holds the workpiece during machining or assembly operations, at the correct location to keep manufacturing reliability and accuracy during machining. In addition, the fixture should be able to clamp the workpiece fast and reliably.

Workholding devices provide fundamental functions like locating and clamping.

Locating refers to orienting and positioning the part relative to the cutting tool.

Clamping refers to holding the part in its proper orientation with enough force to resist the force of cutting but not deform the part.

Advantages of fixtures for machining process when compared with direct locating:

Better reliability and accuracy.

Making the process period shorter.

Decreasing the manufacturing cost.

Decreasing the labor intensity.

Decreasing the worker skill requirement.

Safe working environment.

Expanding process capabilities of a single machine.

Classification of fixtures for machining

Universal fixtures - Have great versatility to hold different workpieces regardless of geometry. (Chucks, jaw vices, indexing disk)

Modular fixtures - consists of standardized components with different shapes, sizes and uses, are applicable to the testing of new products, products often replacing the single-piece, small-batch production and temporary assignments. Example of modular fixture is as shown in the Fig.1a [2].

Dedicated fixtures - specifically designed for particular workpiece or working procedure have the highest manufacturing efficiency, reliability and accuracy. However, they are single functional equipment that can only be used on large and relative steady volume of production.Fig.1b [2] Workholding pallets - special type of work holding fixture, mostly used in automated production lines where the workpieces along with fixture are mounted on the workholding pallet and are automatically transferred through multiple machining stations. Workholding pallet has hydraulic clamping system that allows for rapid set-up in the machining station.Fig.1c [1]

http://www.piersonworkholding.com/images/ez-pallet2.jpgDescription: http://www.xiangyang.com.cn/pictures/chproduct/pictures/2011729172159127.jpg

(c)

(b)

(a)

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Fig: (a) Modular fixture, (b) Dedicated fixture, (c) Workholding pallet

Components of machining fixtures

Fixture body is the major parts of machining fixture keeping all the elements together.

Locating elements are used to locate workpiece accurately, reliably and steadily;

Clamping elements and Fastening elements are used to keep the workpiece unmovable and rotatable during the manufacturing process and supply the clamping force;

Supporting elements are used to connect and support the workpiece on the fixture body or support the fixtures on machine body;

Other elements are the elements may not be used in every fixture (i.e. indexing elements, rotating elements and Tool guiding element).

Following shows the components of the fixture Table 1

Table Components of fixture

1 Fixture body

2 Clamping element

3 Fastening element

4 Supporting element

5 Locating element

6 Guiding element

7 workpiece

Different types of clamping methods

Table Different types of clamping methods

Type

Cost

Magnitude of force

Speed

Manual Clamping

Very low

Low, imprecise controllable

slow

Pneumatic Clamping

low

Low, accurate controllable

fast

Hydraulic Clamping

high

High, accurate controllable

fast

Magnetic Clamping

low

Low, predesigned, uncontrollable

fast

Locating fundamentals

The principal function of the fixture is to accurately locate, support and hold the work piece in a manner so that features on the work piece produced utilizing the fixture will be alike within specific limits. Tooling engineers need to factor several objectives into a fixture design concept such as tooling equipment cost, tool making time, manufacturing cost (set-up cost, per-piece cost), production capacity and required work piece accuracy. Additional considerations are taken when locating and support of the work piece is developed [1]:

Locating features should use datum features specified on the print wherever possible. If cannot use datum features, then locate from features used to dimension the part from.

Locate from machine features wherever possible

Don't locate from parting lines, gates and overflows

Attempt to use same locating surfaces for all machining operations in order to avoid accumulation of dimensional errors from changing the locating surfaces.

Locating features should be as far apart as practically possible to provide greater stability.

Follow the 3-2-1 principle (a.k.a. 6 point method) to determine number of locating features for primary, secondary and tertiary datums and if additional support points are needed, they should be manually adjustable or self-registering types.

All 12 degrees of freedom (6 transitional movements, 6 rotational movements) should be constrained on the work piece by properly constructed locators and clamps.

Points of Contact should be as small as possible, but robust enough to prevent damage at the contact point from clamping forces, tool forces and from loading/unloading the work piece.

If possible support should be places equally from center of gravity and be easily replaceable.

Avoid duplicate locators and when using 2 locating pins, one of pins should be relieved.

Locators should be placed above or away from chip collecting areas and path of coolant drainage.

Locators should allow quick and easy loading of work-pieces without interface due to rough parts variation.

If needed, develop retractable datum that allow sliding heavy work piece into a fixture rather than lifting it.

Locators should stay away from cutter path in case of overrunning of deep cut.

Fixture locators should be foolproof and allow for safe operation

Locating and clamping system for cylinder heads

Cylinder heads require significant amount of machining of multiple areas and complexity of the machined features require several locating set-ups. Locating and clamping systems is not only influenced by complexity of machined features but also greatly affected by availability of existing tooling. One of the main objectives in the design of the locating set-up is to minimize number of re-locations (to minimize machining downtime and dimensional variation due to re-locating). Proper selection of features machined in different locating set-ups also ensures that features are dimensionally insensitive to tolerance stack-up from features machined in previous locating set-up. Main functional areas and features on cylinder head to be machined (also shown in Fig.2):

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Valvetrain side (valve cover face, injector port, spark plug port, cam shaft bearings, etc.)

Combustion chamber side (engine block mounting face, valve seats and ports)

Intake side (intake manifold mounting face)

Exhaust side (exhaust manifold mounting face)

Front cover side (front cover mounting face)

Back cover side

Typical process sequence for locating and machining of the cylinder head includes following steps:

Cubing - 1st fixture setup to rough (casted) locators. (example of rough locators near combustion side shown on Fig. 3

Machining of 1st functional face and 2 index holes for 2nd set-up (shown in Fig. 3). Typically selected side with minimum functional features and low or no tolerance dependence to features machined in remaining setups (i.e. exhaust side).

Set-up change - Work piece located to machined datum features (exhaust flange as primary datum, index holes as secondary and tertiary)

Machining of features on remaining sides - intake side , combustion chamber side, front cover side, and valvetrain side (Note: Cam shaft bearings must by machined roughly only)

Install bearing caps

Final machining of bearing gallery

Fig: Cylinder Head

Fig: Locating set-ups for machining of cylinder head

Dimensional accuracy of the machined work piece and ability to meet dimensional specifications has significant effect on the scrap rate. Dimensional variation of the part can be influenced by fixturing system in following ways:

Work piece locating dimensional variance (fixturing positional accuracy)

Work piece re-locating dimensional variance for series of consecutive operations

Variation caused by clamping distortions (apparent once part is unclamped)

Warpage caused by release of internal strains after material is cut-away

Comparison between Modular and dedicated fixtures:

Design considerations: While designing a modular fixture it takes a lot of time in planning and analysis. Choice for different variety of parts for dedicated fixture is wide but for modular fixture it is limited to a few numbers of standard parts to get the fixture. So dedicated fixtures have better flexibility compared to modular fixtures. The accuracy, surface finish, and productivity are affected by rigidity. The cutting tools produce severe shock, pressure, and vibration on the work piece which must be alleviated with good Fixturing. Time taken to design a part for manufacturing using modular fixture is less because standard CAD templates are available.

Fabrication: For fabricating a dedicated work piece, it requires raw stock and fixture components. The raw stock should be machined to get the necessary part. They are further hardened or heat treated. The parts are fitted and assembled together. The next step must be inspection for proper fits and tolerances. Modular work holders require machined components. This reduces the cost.

Inspection: After fabrication, the fixtures are sent for inspection to see to the errors. Both modular and dedicated fixtures take same time for inspection process. So there is no major difference between them in this phase. Acceptance/rejection will depend on the accuracy of manufacturing processes.

Storage: It is the final stage of dedicated work holder. A few hours are spent every year in putting the tool in storage, maintaining it there and unloading it when required. Modular fixtures are disassembled and each part is free to be used for other work holders. This saves both time and cost.

Tool life: A machine tool collision will damage a dedicated fixture whereas in modular fixture it is usually confined to one or more components of the fixture. So modular fixture is better than dedicated when tool life is concerned. [2] [3]

Cost Considerations: Unlike Modular fixtures, dedicated fixtures are costly, difficult to design, occupies lot of space and not useful for variety of jobs. Modular fixture can be dissembled into components and can be assembled with other components of the system to hold work pieces of different designs features. A complete modular system could be initially more expensive than a dedicated fixture but it can be used for a variety of parts, so on the long run Modular Fixtures are less expensive than dedicated fixtures. Furthermore, engineering changes are difficult to be made in dedicated fixtures whereas in modular fixtures they are typically less difficult. Properly designed fixture has an effect on cost of the actual part as it enables to reduce set-up time, reduce machining errors and scrape rate. Cost of the part is typically determined as:

Selection of Fixture for Machining Cylinder Head

Assuming in this project has a long term and large volume of production, so that some advanced method or equipment can be used to get high product rate. And the unit cost can be reduced by large volume of production. E:\Documents\notes\Manufacturing process - Ghassan Kirdli\Assignment - 2\References\pm0520-07.jpg

In the initial stage the Modular Fixture would be used for rough locating and milling of datum surfaces of the cylinder head. Since the Modular Fixture has a better balance on cost and accuracy, and modular fixture is more cost effective than Dedicated Fixture While a dedicated fixture is designed to hold a specific part, modular can hold irregular shaped workpieces.

Fig: Hydraulic Dedicated Fixture for cylinder headDedicated Fixture would be used in subsequent locating set-up to finish the rest of major machining process in the milling machining center. The cylinder head has a high manufacturing quality requirement because it is an important and complex component in the engine. Using Dedicated Fixture should be one of the most efficient ways to reduce machining errors and improve the manufacturing quality. Due to a large production volume, the cost of Dedicated Fixture is justifiable especially if improves production throughput due to reduced set-up time.

The cylinder head is a large and complex component which requires high manufacturing quality. In the project, the Hydraulic Clamping would be used to get stable holding for the workpiece with steady and controllable clamping force. The total cost of Hydraulic Clamping may higher than others, but due to the long term and large volume of production project assumption; the initial unit cost can be justifiable.

Machining errors: These are the errors caused during the process of material removal from a work-piece to obtain the geometrical features and dimensions.

The common errors caused due to machining are;

Geometric errors of machine components and structures

Kinematic errors

Errors induced by thermal distortions

Errors caused by cutting forces including

by gravity loads

by accelerating axes, and

by the cutting action itself

Material instability errors

Machine assembly-induced errors

Instrumentation errors

Tool wear

Fixturing errors

Other sources of errors like servo errors of the machine (following errors and interpolation algorithmic errors)

Geometric and kinematic errors: These are one of the biggest causes of inaccuracy. They are caused due to the existing errors already present in the machine such as Spindle run out, Spindle plays like Radial and axial, Clearances in the machine tool slides, improper movement of work table slides, Geometrical errors of spindle with respect to work table, Flatness of work table, Squareness, Parallelism & perpendicularity of different  slides with respect to spindle and column, Lead screws Play and backlash, Ball screw/ Ball cage errors, rigidity of machine spindle and slides  etc. These errors are also affected by factors like surface roughness, surface straightness, bearing pre-loads etc. These errors in the machine and kinematics result in  Circularity of hole, Cylindricity, concentricity, Flatness, straightness, Squareness, Parallelism, perpendicularity, surface finish and dimension inaccuracy on the work piece (cylinder head). Kinematic errors are concerned with the relative motion errors of several moving machine components that need to move in accordance with precise functional requirements. These errors are particularly significant during the combined motion of different axes. We can reduce these errors by using compensation algorithms in CNC machining centers. [8] [9]

Real time error compensation technique: The following error compensation control was implemented on the turning center. A compensation control system had been developed by integrating a PC computer with the existing CNC controller. This compensation control system performs the following functions:

It reads the cutting force information from an embedded piezoelectric force sensor in real time.

It reads the current positions of the slides from the rotary encoders or linear scales.

It calculates all cutting force induced error components using mathematical models.

It calculates the errors in two directions (YZ) using the cutting force 2-dimensional error synthesis model.

Transfers the planar error signals to the CNC controller to implement the planar error compensation in real time [8], [9].

Fig: Tool WearTool Wear: It is one of the most complex and important aspect of machining process. The cutting forces, temperature generated during cutting operation, tool and work piece materials, tool geometry and properties of cutting fluids are some of the different variables which results in tool wear. The various regions of tool wear are flank wear, nose wear, crater wear, chipping of the cutting edge and built-up edge as shown in Fig.5. The tool life is generally based on flank wear which has been extensively studied by F.W.Taylor who proposed an equation which provided an estimation of tool lifeE:\Documents\notes\Manufacturing process - Ghassan Kirdli\Assignment - 2\References\images.jpg

Taylor tool life equation,

We have two general categories that are used to monitor the tool condition:

Direct method: it involves optical measurements of wear, by periodically observing the change in the tool profile. This method is commonly employed and reliable technique, which is done using a microscope. But this requires the cutting operation to be stopped.

Indirect method: it involves in correlating the tool condition with process variables such as forces, power, temperature rise, surface finish, and vibrations. Acoustic emission is one such technique which uses a piezoelectric transducer attached to the tool holder. The transducer picks up the acoustic emission signals resulting from the stress waves generated during cutting . by analyzing the signals, tool wear and chipping can be monitored [11]. Some of the other research methods used for monitoring tool wear are "Tool Wear Monitoring by Means of Artificial Neural Networks" [12] and "Tool wear monitoring in reconfigurable machining systems through Wavelet analysis" [13].

Fixturing errors: In the machining process the fixtures are used for the orientation and clamping. Once the workpiece is oriented and clamped with locating pins and clamping devices the workpiece can be machined in order to obtain the required geometrical conditions. The errors can be classified in two categories: errors due to orientation of the work-piece in the fixture and errors due to deformations of the work-piece-fixture system during clamping and machining.

The errors due to fixtures are major ones which influence the work-pieces machining accuracy; the errors can amount to 20-60% of the overall machining error. Therefore, to reduce these errors the work-piece-fixture system has to be properly designed and optimized for performance and ease of use. Proper clamping force has to be calculated so there is no or very less elastic deformation. The fixture must have proper locating pins which have to be very accurate because it affects the part which is placed and which in turn causes errors. The material also deforms due to contact deformation to compensate this error one of the way is by using analytical method. The full research can be found in [10]

Human error: The inherent errors in the instruments used also reflect as machining errors. The wear out of instrument's contact points, zero error, backlash, pitch errs, Play in moving parts are some of the common errors in the instruments. The parallax error, reading errors are the errors caused by the human elements which are basically dependent on the skill of the operator. The instruments errors are taken care of by periodical calibration of instruments.

Conclusion

After analyzing and understanding different types of fixtures we found that the dedicated hydraulic clamping fixture is good for finished machining and modular hydraulic fixture is good for roughing operations in high volume production.

Furthermore we expect to use the same dedicated fixture for rough and final machining of cam shaft bearing gallery.