Conceptof Design for Manufacturability (DFM)
DFMrefers to the proactive design of products in order to optimize allmanufacturing functions such as assembly, fabrication, shipping,procurement, tests, delivery and repair (Farmer and Harris 1984, p.267). Also, DFM plays a major role in assuring the best costs,regulatory compliance, reliability, time-to-market, customersatisfaction and safety. Early consideration of the issues relatedwith manufacturing plays a major role in shortening the productdevelopment time, ensures smooth transition from production to marketand minimizes the development costs (Farmer and Harris 1984, p. 269).
DFMis critical in reducing huge costs since the products can beassembled quickly from fewer parts. This ensures easier building andassembling in little time and with high level of quality. The designof parts is based on ease of commonality and fabrication with otherdesign. This encourages standardization of parts as well as maximumutilization of the purchase parts, standard design features andmodular design. As such, the designers tend to save money as they donot have to ‘re-invent the wheel’ resulting to broader productline responsive to the needs of the clients (Badeneset al. 1997, p. 404).
Accordingto research by Dr. Anderson, companies which have applied DFM haveleaped substantial profits (Badenes et al. 1997, p. 406). In thiscase, costs and time to market are reduced almost by half withconsiderable improvements in reliability, quality, serviceability,delivery, product line breadth and competitive posture. Any move toDFM should ensure that all team members involved in productdevelopment are involved. This ensures that they gain a generalunderstanding of the products manufacturing process throughexperience in multi-functional design teams and participation.Specifically, DFM should involve design of the processes to be usedin building the product. For instance, if the product is to be builtusing the standard processes, then the design team should understandthem well and design them. On the other hand, if new processes are tobe followed, design team should concurrently design new processesduring the product design (Markus and Keil 1994, p. 11).
Beforethe formulation of DFM, the product development used the motto ‘Idesigned it and you built it’. The design engineers used towork alone or in company of other design engineers within theengineering department. By then, designs were thrown over the wallleaving the manufacturing team in dilemma of either rejecting orlaunch the product that was never designed for manufacturability. Inmany cases, this delayed the launch of the product as well as thetime ramp up for attaining full production, which is the only measurethat is meaningful for time-to-market (Manivannan et al. 1989, p.153).
Developmentof products within a multifunctional team is a major avenue throughwhich the manufacturability can be assured. This involves active andearly participation of all departments ranging from manufacturing,marketing, purchasing and industrial designers amongst others. Theteam should work together to optimize on functionality, quality, easeof assembly, testability, reliability and customization (Manivannanet al. 1989, p. 155).
CostImplications of DFM
Bythe time the design of the product is complete, only 8-20% of thetotal budget is spent and by that time, the design would havedetermined 80% of the total committed cost of the product. This isbecause design determines manufacturability which in turn determinesa considerable proportion of introduction and production costs 80%of the product. Manufacturing may be unable to remove this cost onceit locks in. it is critical to note that architecture or conceptalone determines 60% of the total cost (Ngoi and Ong 1998, p. 910).
Costexpenditure of DFM
Paradoxically,the team has to make optimal use of the off-the-shelf parts torealize substantial benefits. These parts are cheap to designconsidering the related cost of design, documentation, testing,prototyping, overhead purchasing costs, testing andnon-core-competency manufacturing cost. Also, the off-the-shelf partshelp in saving time considering the design, documentation,administration, building, testing and fixing of the prototype parts(Ngoi and Ong 1998, p. 910).
Theprinciples of DFM play a major role in helping the designer minimizethe difficulties and cost related to a manufacturing item. Theseinclude:
Reduction of number of parts
Reducingthe number of parts in a product presents one of the bestopportunities for reducing the manufacturing cost since fewer partsimply less inventory, purchases, engineering and processing time. Anypart that does not meet the specifications should be eliminated(Arisset al. 2000, p. 14).
Development of modular design
Modulesin product design play a major role in simplifying the manufacturingactivities like inspection, assembly, testing, redesign, purchasingand maintenance amongst others. Modules play a major role inenhancing versatility of the product update during the redesignprocess as well as during testing before final assembly(Arisset al. 2000, p. 15).
Itis cheaper to purchase standard components than custom-made itemssince they are easily available hence reduce the product lead time.Also, they are well ascertained in terms of reliability factors andplays a major role in relieving the manufacturer concern in meetingthe production schedule (Arisset al. 2000, p. 16).
Thedesign parts should be multifunctional in order to reduce the totalnumber of parts in a design. For instance, an electric conductor canalso be used as a structural member or dissipating element. Someelements can be used besides their principle function to do othersupport functions such as alignment (Arisset al. 2000, p. 17).
Design parts for multiple usages
Differentproducts can be designed in such a way that they share some parts.The parts can have different or same function when used for differentproducts. This can be accomplished by identifying parts that aresuited for multi-use (Chase and Parkinson 1991, p. 23).
Ease of fabrication
Theteam should design in such a way that it is easier to fabricate. Thiscan be done by selecting the optimum combination between thefabrication process and material in order to minimize the totalmanufacturing cost. Some operations like polishing, painting andfinish machining should be avoided. Surface finish requirement andexcessive tolerance are some of the major problems that increase theproduction cost (Chase and Parkinson 1991, p. 24).
Avoiding separate fasteners
Fastenersplay a major role in increasing the manufacturing cost due to thefeeding and handling operations required. Besides the high equipmentcost involved, in most cases, the operations are not successful andcontribute to overall efficiency reduction. Therefore, fastenersshould be replaced or avoided by using snap or tab fits. If thefasteners have to be used, their selection should be based onspecific guidelines. The variation, number and size of the fastenersshould be minimized and standard components should be utilizedwhenever possible. Screws that are too short or too long, separatewashers, flat and round heads should be avoided. Chamfered andself-tapping screws should be preferably used since they play a majorrole in improving success of the placement. Screws having verticalside heads should be selected using vacuum pickup (Chase andParkinson 1991, p. 24).
Minimizing the assembly directions
Assemblingof the parts should be done from same direction. The recommended wayis to add the parts from vertical directions and parallel togravitational direction. This ensures that the effect of gravityreinforces the assembling process contrary to compensating for theeffects when other directions are used (Chase and Parkinson 1991, p.23).
Thevariations in dimensions and inaccuracies of positioning device cancause errors during insertion. Such a faulty behavior can damage theequipment or parts of the equipment. Examples of part build-infeatures include chamfers or tapers and moderate size radius tofacilitate insertion and the nonfunctional external elements thatenhance detection of hidden features. Selection of rigid-base partswith vision systems and tactile sensing capabilities should be usedin assembling process to enhance compliance. Use of high-qualityparts presents one of the simplest solutions (Chase and Parkinson1991, p. 23).
Handlingprocess entails positioning, orientation and fixing of components orpart. To enhance orientation, the symmetrical parts need to be usedwhenever possible. If impossible, the asymmetry should be exaggeratedto prevent failures (Anderson 2004, p. 56). The external guidingfeatures play a major role in orientation of the parts. Subsequently,the operations should be designed so as to maintain the orientationof the parts. Also, tube feeders, magazines and parts strips can beused to maintain the orientation between operations. Slave circuitboards may be used to prevent use of flexible parts. When cables areused, a dummy connector has to be included in order to plug therobotic assembly or the cable to enhance easier identification of itslocation. It is recommended that the designer to ensure that the flowof parts and material waste is maintained at minimum during themanufacturing operations. Also appropriate and safe packaging shouldbe selected for the product (Anderson 2004, p. 56).
Threetypes of Surface Finish Standards and their Uses
Asurface finish, surface texture or surface topography is the natureof a surface as described in terms of surface lay, roughness andwaviness. This encompasses the small local variations of a surfacerelative to a perfectly flat ideal plane. Surface finish controls theformation of transfer layer and friction during sliding. There arethree types of surface finish standards (Markus and Keil 1994, p.11). These include surface roughness,
Thethree types of surface finish standards include
Surfaceroughness is a constituent of the surface texture and is measured bythe variations in direction of normal vector for the actual surfacefrom the ideal form. Surface roughness is used in determining how theactual object relates with the environment. Surface roughness isdisadvantageous to the performance of machine parts due to thefriction involved. This compels manufacturing prints to set up ahigher limit of surface roughness instead of lower limit withexception of cylinder bores where oil is normally retained in thecylinder and little roughness is a requirement.
Wavinessentails the measurement of the surface texture components which aremost widely spaced. It’s a broader view of surface roughness sinceit is austerely defined as any irregularity with a spacing that iswider that the length of sampling length. Waviness cannot ordinarilybe compared with surface roughness. For instance waviness in bearingballs and bearing races results in vibrations and noise in ballbearings. Another application involves waviness in chatter on acircular shaft surface and flat milled sealing surface.
Layrefers to surface finish standard which describes the direction ofprincipal surface pattern that depends on the method of manufacturingused. Some examples of lay patterns include the isotropic, vertical,circular, horizontal, cross-hatched and radial.
Changes in the tolerance of shaft and hole and the implication on manufacturing of parts
Aclose running fit (RC4) is a clearance fit that is critical forrunning on machines that are accurate as well as locating accuratelyat moderate speed and journal pressure. This tolerance allowed foraccurate location of the shaft within the hole with minimum play whenused under moderate speeds and loads. This is a good choice forobtaining high level accuracy. The change in the dimensionaltolerance of the shaft and hole is a reflection of designed-inclearance. Normally, tolerance is designed to parts because ofmanufacturing purposes as limits for the acceptable build. There isno any machine that can hold dimensions precisely to a nominal valuehence, there must be acceptable levels of variation (Chase andParkinson 1991, p. 23). Manufacturing a part with dimensions that areout of tolerance may not render that part unusable based on designintent.
A sketch of figure 2 using CAD and adding the information using relevant ISO symbols.
Thefollowing have been represented in the above drawing
A geometrical tolerance box showing that the center of the slot must be contained within a 50um diameter cylindrical tolerance zone which is perpendicular to the bottom face. This is a perpendicularity geometrical tolerance.
A dimension showing that the slot width is nominally 4mm with an h8 tolerance. This tolerance is to be shown not by the designation ‘h8’ but in actual upper and lower tolerance values.
A surface finish tick mark showing that the root-mean squared roughness of the inside of the slot is to be a 5um as produced by the molding process with no additional machining.
Anderson,D.M., 2004. Design for manufacturability & concurrentengineering: how to design for low cost, design in high quality,design for lean manufacture, and design quickly for fast production.CIM press.
Ariss,S.S., Raghunathan, T.S. and Kunnathar, A., 2000. Factors affectingthe adoption of advanced manufacturing technology in small firms. SAMAdvanced Management Journal,65(2),p.14.
Badenes,G., Hendriks, M., Perelló, C. and Deferm, L., 1997, September. AHigh Performance 0.18 um CMOS Technology Designed forManufacturability. In Solid-StateDevice Research Conference, 1997. Proceeding of the 27th European(pp. 404-407). IEEE.
Chase,K.W. and Parkinson, A.R., 1991. A survey of research in theapplication of tolerance analysis to the design of mechanicalassemblies. Research in Engineering design, 3(1),pp.23-37.
Farmer,L.E. and Harris, A.G., 1984. Change of datum of the dimensions onengineering design drawings. International Journal of Machine ToolDesign and Research, 24(4), pp.267-275.
Markus,M.L. and Keil, M., 1994. If we build it, they will come: Designinginformation systems that people want to use. Sloan ManagementReview, 35, pp.11-11.
Manivannan,S., Lehtihet, A. and Egbelu, P.J., 1989. A knowledge based system forthe specification of manufacturing tolerances. Journal ofManufacturing Systems, 8(2), pp.153-160.
Ngoi,B.K.A. and Ong, C.T., 1998. Product and process dimensioning andtolerancing techniques. A state-of-the-art review. TheInternational Journal of Advanced Manufacturing Technology,14(12), pp.910-917.