Engineering Design and Material Selection Lecture Notes PDF

Engineering Design and Material Selection Lecture 8 - Technical drawing: Dimensioning Dr Tino Stankovic Prof. Dr. Kristina Shea Prof. Kristina Shea 1  Course Schedule Week/ Topic...

Engineering Design and Material Selection Lecture 8 - Technical drawing: Dimensioning Dr Tino Stankovic Prof. Dr. Kristina Shea Prof. Kristina Shea 1  Course Schedule Week/ Topic Case study Quiz Lecturer Dates 1 Introduction and Sketching 2 Introducing Engineering Design Health Prof. Dr. Kristina Shea 3 Technical Drawing: Projections and Cuts 4 CAD: Introduction and Modeling Operations 5 CAD: Features and Parametric Modeling Future Mobility 6 CAD: Freeform Modeling Dr. Tino Stankovic 7 CAD: Assemblies and Standard Mechanical Parts X (45 min) 8 Technical Drawing: Dimensioning Health 9 Sustainability in Engineering Design 10 Materials and their Properties 11 Manufacturing Processes with Focus on Additive Manufacturing Sustainable Materials Prof. Dr. Kristina Shea 12 Material Selection 13 Review and Q+A X (75 min) Prof. Kristina Shea Engineering Design + Computing Laboratory 2  Learning Objectives ▪ Understand the different types of dimensioning and when and where to use which type ▪ Be able to give and read dimensions in technical drawings according to the norms ▪ Learn how to give and read simplifications for dimensions ▪ Learn how to dimension special elements, e.g. threads Prof. Kristina Shea Engineering Design + Computing Laboratory 3  3D Model of a Mechanical Ventilator Prof. Kristina Shea Engineering Design + Computing Laboratory 5  Mechanical Ventilator Main Parts and Functions Prof. Kristina Shea Engineering Design + Computing Laboratory 6  How should we add dimensions to a drawing? Prof. Kristina Shea Engineering Design + Computing Laboratory 8  Technical drawing: process 1. Choose the principal (front) view 2. Choose other required views 3. Draw the views 4. Add dimensions 5. Verify Prof. Kristina Shea Engineering Design + Computing Laboratory 9  What are elements of dimensioning? Leader line Extension line Symbol Ø12 Dimensional value Reference line 350 450 150 Indicator Dimension line Terminator of origin Prof. Kristina Shea Engineering Design + Computing Laboratory 10  Extension and Dimension Lines Terminator Extension line Dimension line Prof. Kristina Shea Engineering Design + Computing Laboratory 11  Example for Simple Dimensioning not allowed to add unnecessary dimensions Prof. Kristina Shea Engineering Design + Computing Laboratory 12  Dimensioning for… function manufacturing inspection Prof. Kristina Shea Engineering Design + Computing Laboratory 13  Dimensioning Process 1. Define outside dimensions 2. Define functional dimensions 3. Add required manufacturing dimensions 4. Add any dimensions for inspection 5. Add auxiliary dimensions 6. Verify the dimensioning Prof. Kristina Shea Engineering Design + Computing Laboratory 14  Functional and Non-Functional Dimensions NF F F F NF Prof. Kristina Shea Engineering Design + Computing Laboratory 15  Rules for Dimensioning Prof. Kristina Shea Engineering Design + Computing Laboratory 16  How many dimensions have to be added for the dimensioning to be complete? 4 missing at In Prof. Kristina Shea Engineering Design + Computing Laboratory 17  Auxiliary Dimensions () () Prof. Kristina Shea Engineering Design + Computing Laboratory 19  First Example: Cover plate Prof. Kristina Shea Engineering Design + Computing Laboratory 20  Cover plate: functional dimensions Parts-bolt interaction Overlap with the bearing 𝜙 ℎ Bearing centering However, this is achieved with the housing!!!! Prof. Kristina Shea Engineering Design + Computing Laboratory 21  Cover plate: outside dimensions Prof. Kristina Shea Engineering Design + Computing Laboratory 22  Cover plate: functional dimensions Functional Prof. Kristina Shea Engineering Design + Computing Laboratory 23  Cover plate: manufacturing dimensions Manufacture dimension Prof. Kristina Shea Engineering Design + Computing Laboratory 24  Orientation of Dimensional Values 30° 30° 60° 60° 56 56 30 56 30 56 ° ° ° ° 60 56 60 56 56 56 56 56 56 60° 30° 30° 56 56 6 56 ° 56 ° 55 56 56 55 60 56 ° 60 ° 5° 5° Prof. Kristina Shea Engineering Design + Computing Laboratory 25  Second example: motor holding plate Holdingplate motor Housing Prof. Kristina Shea Engineering Design + Computing Laboratory 26  Motor holding plate: outside dimensions Prof. Kristina Shea Engineering Design + Computing Laboratory 27  Motor holding plate: functional dimensions Prof. Kristina Shea Engineering Design + Computing Laboratory 28  Motor holding plate: manufacturing dimensions Prof. Kristina Shea Engineering Design + Computing Laboratory 29  Dimensioning of Special Elements Bevels and countersinks 1 45° or 90° only Prof. Kristina Shea Engineering Design + Computing Laboratory 30  Arrangement of Dimensions: Chain and Parallel Dimensioning Chain Parallel Take EF to over dimensio Prof. Kristina Shea Engineering Design + Computing Laboratory 31  Arrangement of Dimensions: Combined Dimensioning Chain Running Parallel Prof. Kristina Shea Engineering Design + Computing Laboratory 32  Arrangement of Dimensions for Internal and External Dimensions Prof. Kristina Shea Engineering Design + Computing Laboratory 33  Representation of Internal Threads Major Minor Major Minor Prof. Kristina Shea Engineering Design + Computing Laboratory 34  Representation of External Threads Minor Major Prof. Kristina Shea Engineering Design + Computing Laboratory 35  Representation of Thread Run-Outs Root I Depth of thread Prof. Kristina Shea Engineering Design + Computing Laboratory 36  Connection of Threaded Parts - Hatching + = External Internal thread thread Combined Prof. Kristina Shea Engineering Design + Computing Laboratory 37  How many threaded holes are there on the shown views? A. 11 B. 10 C. 1 D. 9 Prof. Kristina Shea Engineering Design + Computing Laboratory 38  Checklist for verifying dimensioning: ▪ Here are some questions that guide you in verifying the dimensioning: ▪ Are there outside dimensions? ▪ Are there functional dimensions? ▪ Are all geometric elements defined? ▪ Does it match the desired manufacturing process? ▪ Are the relevant dimensions for inspection shown? ▪ Have you reduced chains and is there is no over-dimensioning? ▪ Do the dimensions follow the norms? ▪ Are all of the dimensions clear and readable? ▪ Tip: Try to resketch the part based only on the dimensions. Prof. Kristina Shea Engineering Design + Computing Laboratory 39  Checklist for verifying your technical drawings (Lecture 3) Here are some questions that can guide you in verifying the quality of technical drawings: ▪ Is the principal view the most informative view? ▪ Are there sufficient views (projections and cuts) to fully define the geometry? ▪ Are the smaller details clearly visible? ▪ Are the views positioned correctly? ▪ Are the proportions and the scale correct? ▪ Are you satisfied with it? Prof. Kristina Shea Engineering Design + Computing Laboratory 40  Drawing comparison 1 A B Better cuz readable Prof. Kristina Shea Engineering Design + Computing Laboratory 41  A Drawing comparison 2 Both suck B is slightly B better Prof. Kristina Shea Engineering Design + Computing Laboratory 42  A Which drawing is better? Both such bad for manufacturing B Ang 0 Prof. Kristina Shea Engineering Design + Computing Laboratory 43  Case Study Mechanical Ventilator – Wrap-Up The Need Phase 1: Phase 2: Phase 4: Phase 5: Concept System-Level Phase 3: Testing & Production Phase 0: Planning Detail Design Development Design Refinement Ramp-Up Prof. Kristina Shea Engineering Design + Computing Laboratory 44  Dimensioning – Wrap Up Dimensions in a drawing ensure the function, manufacturing and inspection of a part. The norms define how dimensions are placed on drawings. It is up to you to create a complete and unambiguous geometric definition in your drawing. Always verify your drawing and check that the dimensioning is complete and not over dimensioned. CAD tools can help you to create drawings but it is up to you to validate them! Prof. Kristina Shea Engineering Design + Computing Laboratory 45  Exercise 8: CAD Drafting Understanding a part’s function: Can you create a completely dimensioned, unambiguous drawing of this linear guide from a 3D model in NX? Lathe turning machine Prof. Kristina Shea Engineering Design + Computing Laboratory 47  Environmental Impacts Global Warming Resource Depletion Solid Waste Water Pollution Air Pollution Land Degradation www.buildbabybuild.com www.flickr.com Ben Rad www.wonkroom.thinkprogress.org www.co.rockingham.nc.us commons.wikimedia.org www.adb.org Prof. Kristina Shea Engineering Design + Computing Laboratory 4  Global Warming ▪ The radiation balance keeps the Earth’s temperature in equilibrium SUN ▪ Human activities are generating additional 2023:+1.31°C (1) greenhouse gases (like CO2) in the atmosphere. ▪ The additional radiation increases the temperature +GREENHOUSE ▪ Sea level, local climates and ecosystems ATMOSPHERE GASES are very sensitive to temperature increase Additional radiation ▪ Global warming should(2) be limited to ~15°C +1.5°C compared to pre-industrial times EARTH Human activities (1) https://climatechangetracker.org/igcc (2) Paris Agreement, 2015 Prof. Kristina Shea Engineering Design + Computing Laboratory 5  Global Warming 2 tons of CO2 equivalent ▪ Indicative carbon budget for 2050 2 tons CO2 eq. / year / person ▪ Average carbon emissions(1): ▪ 12,000 km flight(2) ▪ Switzerland: 12 t CO2 eq. / year / person Round-trip Zurich NYC, or ▪ World: 4 t CO2 eq. / year / person ▪ 12,000 km with IC engine car(3), or ▪ ETH Goal: Net Zero for 2030 ▪ Avoid, reduce and compensate ▪ 43,000 km with electric car, or ▪ 220 days of meat-based meals(4), or ▪ 430 days of vegetarian meals (1) https://ourworldindata.org/grapher/consumption-co2-per-capita?time=latest (2) https://co2.myclimate.org/en/flight_calculators (3) https://ourworldindata.org/travel-carbon-footprint (4) Nabipour Afrouzi, H., et al. “A Comprehensive Review on Carbon Footprint of Regular Diet and Ways to Improving Lowered Emissions.” Results in Engineering, vol. 18, June 2023, p. 101054. ScienceDirect, https://doi.org/10.1016/j.rineng.2023.101054. Prof. Kristina Shea Engineering Design + Computing Laboratory 6  “Doughnut Economics” New model of “ideal economics” Societal well-being is also important to sustainability The center of the doughnut is the shortfall for people. No individual should be in there! The outside is the environmental overshoot The goal is to be “in the green”. This means that we are caring for everyone while not hitting the ecological ceiling. Raworth, K. (2017), Why it's time for Doughnut Economics. IPPR Progressive Review, 24: 216- 222. https://doi.org/10.1111/newe.12058 Prof. Kristina Shea Engineering Design + Computing Laboratory 8  Definition: Sustainable Development ▪ “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” - UN Commission on Environment and Development (1983) ▪ Creating a sustainable society is the main challenge of our century! http://www.un.org/sustainabledevelopment/sustainable-development-goals/ Prof. Kristina Shea Engineering Design + Computing Laboratory 9  “Conditions” for Sustainability ▪ Consider the earth as a closed system with limited solar input and natural bio- cycles. ▪ Resources must be used in balance with the rate at which the Earth creates them, including fossil fuels. ▪ Toxic wastes, heavy metals, radiation, and other “molecular garbage” must be eliminated because they are not part of the bio cycle. Prof. Kristina Shea Engineering Design + Computing Laboratory 10  Product Life Cycle Materials Manufacturing End of Life Transport (Disposal) Use Prof. Kristina Shea Engineering Design + Computing Laboratory 11  Non- renewable Post-industrial Circular Economy resources Recycling Renewable Materials Manufacturing Resources Resources Remanufacturing Post-consumer Recycling Natural Industrial “Biological” “Product” Life Cycle Life Cycle Transport Toxics Recovery Natural Reuse Decay Organics Inorganics Deposit Use ▪ “… a systems solution framework that tackles global challenges like climate change, biodiversity loss, waste, and pollution.” – Ellen McArthur Foundation ▪ Aims to be regenerative in terms of material and energy consumption Prof. Kristina Shea Engineering Design + Computing Laboratory 12  Manufacturing and Circularity Primary Manufacturing Secondary Manufacturing Nature Product Sourcing Preparation Feedstock Forming Finishing Assembling Creation Circular Economy Disassembling Reusing Repairing Prof. Kristina Shea Engineering Design + Computing Laboratory 13  Circular Economy – 9R Framework Strategies Circular Make product redundant by abandoning its function or by offering R0 Refuse Smarter the same function with a radically different product economy Product Use and R1 Rethink Make product use more intensive (eg. by sharing product) Manufacturing Increase efficiency in product manufacture or use by consuming fewer R2 Reduce natural resources and materials Reuse by another consumer of discarded product which is still in good R3 Reuse condition and fulfils its original function Repair and maintenance of defective product so it can be used with its R4 Repair original function Extend Increasing Circularity Lifespan of R5 Refurbish Restore an old product and bring it up to date Product and Its Parts R6 Remanufacture Use parts of discarded product in a new product with the same function Use discarded product or its parts in a new product with a different R7 Repurpose function Linear Process materials to obtain the same (high grade) or lower (low grade) economy R8 Recycle quality Application of Materials R9 Recover Incineration of material with energy recovery Kirchherr, Julian, et al. “Conceptualizing the Circular Economy: An Analysis of 114 Definitions.” Resources, Conservation and Recycling, vol. 127, Dec. 2017, pp. 221–32. DOI.org (Crossref), https://doi.org/10.1016/j.resconrec.2017.09.005. Prof. Kristina Shea Engineering Design + Computing Laboratory 14  Stokke Tripp Trapp Chair R1 Rethink Peter Opsvik (for Stokke, 1972) designed the award-winning Tripp Trapp chair to grow with the child, increasing the effective lifetime of the chair. www.stokke.com Prof. Kristina Shea Engineering Design + Computing Laboratory 15  Sustainability Aspects of the Kyburz PLUS II ▪ Maximum part reuse between the different models (modular design) ▪ Use of 1st life parts for the manufacturing of 2nd life vehicles ▪ Multi-life concept for batteries (3rd life) and recycling-driven battery design (EMPA) R7 Repurpose R8 Recycle R5 Refurbish R6 Remanufacture Source: www.kyburz-switzerland.ch Prof. Kristina Shea Engineering Design + Computing Laboratory 16  Freitag Bags – since 1993 Freitag truck inner seat to create belts tarps tubes repurposes: bags and accessories. Freitag repurposes: ▪ truck tarps ▪ inner tubes R7 Repurpose ▪ seat belts to create bags and accessories. www.freitag.ch Prof. Kristina Shea Engineering Design + Computing Laboratory 17  Dunlop Recycled Wellington Boots ▪ Dunlop Wellington boots are made from polyurethane, PVC, and rubber. ▪ Dunlop developed a line of recycled boots. ▪ Dunlop takes back used Wellingtons from customers. Old boots are re-ground and re-manufactured into new boots. ▪ This helps to reduce production of new PVC and keeps it out of the waste stream. R6 Remanufacture R8 Recycle Source: www.biothinking.com Prof. Kristina Shea Engineering Design + Computing Laboratory 18  Life-Cycle Assessment (LCA) ▪ Quantifies environmental impact over product life cycle ▪ Steps in LCA: 1. Goal and Scope: Define the system and its boundaries 2. Inventory Analysis: Collect all the environmental inputs and outputs associated with the product 3. Impact Assessment: Quantify the environmental impacts of each material, energy, waste 4. Interpretation: Evaluate the results, limitations and possible implementation. Source: ISO 14040 / 14044 norm Figure adapted from L. Golsteijn, “Life cycle assessment (lca) explained,” 2020, https://pre-sustainability.com/articles/life-cycle-assessment-lca-basics/ Prof. Kristina Shea Engineering Design + Computing Laboratory 19  Software to assist LCA (Granta EduPack Eco-Audit) ▪ Measuring the environmental impact of a product is very difficult with much uncertainty ▪ Three measures that can be used: ▪ Mass (kg) ▪ Embodied energy (Joules/kg) ▪ CO2 (kg/kg) Reference: Ashby et al., “Granta EduPack Eco Audit Tool- A White Paper”, Ansys, 2021. Prof. Kristina Shea Engineering Design + Computing Laboratory 20  Sustainability properties (Granta EduPack Eco-Audit) Factors to Consider: Material: Stainless Steel Data taken from CES EduPack ▪ Recyclability ▪ Biodegradability ▪ CO2 footprint (processing and recycling) ▪ Water Usage ▪ Energy Consumption ▪ … Prof. Kristina Shea Engineering Design + Computing Laboratory 21  Assessments: CO2 and Energy (Granta EduPack) CO2 footprint (kg CO2e / kg) ▪ CO2-equivalent mass (kg CO2e) of greenhouse gases emitted for 1 kg of the material. Energy (Joules / kg) ▪ Energy required for 1kg of the material Distinction between the life-cycle phases: Material Manufacture Transport Use Disposal ▪ Example: Embodied energy ▪ “Energy required to make 1 kg of the material from its source: Granta EduPack Eco Audit Tool ores or feedstocks.” Prof. Kristina Shea Engineering Design + Computing Laboratory 22  Example: PET bottle Scenario: ▪ 1 liter PET bottles with polypropylene (PP) caps. ▪ Each bottle weighs 40 g and each cap 1 g. ▪ Bottles and caps are molded. ▪ Bottles are filled, transported 550km from the French Alps to London, England, by a 14 tonne truck ▪ Bottles are refrigerated for 2 days requiring 1 m3 of refrigerated space at 4°C and then sold. ▪ Bottles are recycled. Reference: Ashby et al., “Granta EduPack Eco Audit Tool- A White Paper”, Ansys, 2021. Prof. Kristina Shea Engineering Design + Computing Laboratory 24  Approximate Values for the Energy Use in Each Phase (without Disposal) ▪ Approximate energy breakdown (total 100%) ▪ Some products have a much higher environmental impact in the use phase, e.g. aircraft, auto, appliance. Heating Water ▪ Others have a much higher environmental impact from the material, e.g. car park, house, carpet. Reference: Ashby et al., “Granta EduPack Eco Audit Tool- A White Paper”, Ansys, 2021. Prof. Kristina Shea Engineering Design + Computing Laboratory 25  Which “Eco Audit” (CO2) corresponds to the electric car (CO2)? CO2 CO2 ▪ Internal combustion ▪ Electric car A. engine car CO2 CO2 B. Electricity mix: = Switzerland = Germany CO 2 CO2 CO2 C. CO2 CO2 D. Image: Fredriksson et al., “Level 3 Industrial Case Study Electric Cars: Sustainability and Eco Design”, Ansys, 2021. Prof. Kristina Shea Engineering Design + Computing Laboratory 26  Design for Environment (DFE) Design for Environment (DFE) is a method to minimize or eliminate environmental impacts of a product over its life cycle. Effective DFE practice maintains or improves product quality and cost while reducing environmental impacts. DFE integrates social-ecological aspects with conventional factors of form, function, and costs, and at times inspired by nature‘s design solutions. DFE expands beyond the traditional product and manufacturer’s focus on production and distribution of its products to a closed-loop life cycle. Prof. Kristina Shea Engineering Design + Computing Laboratory 27  Four Simple DFE Rules 1. Design products and processes with industrial materials that can be recycled continually with no loss in performance, thereby creating new industrial materials. 2. Design products and processes with natural materials that can be fully returned to the earth’s natural cycles, thereby creating new natural materials. 3. Design products and processes that do not produce unnatural, toxic materials that cannot be safely processed by either natural or industrial cycles. 4. Design products and processes with clean, renewable sources of energy, rather than fossil fuels. Prof. Kristina Shea Engineering Design + Computing Laboratory 28  DFE in the Engineering Design Process Basic Working Detailed General Layout Verification Principles Structure and Function Phase 1: Phase 2: Phase 4: Phase 5: Phase 3: Phase 0: Planning Concept System- Testing & Production Detail Design Development Level Design Refinement Ramp-Up 1. DFE Goals 2. DFE and Material 3. Assess Impacts 4. Improve DFE and Team Guidelines and Refine Designs process Prof. Kristina Shea Engineering Design + Computing Laboratory 29  DFE Product 1. Set DFE Agenda Process Planning 2. Identify Potential Environmental Impacts Concept 3. Select Material and DFE Development Guidelines System-Level 4. Apply DFE Guidelines to Design Initial Designs 5. Assess Environmental Impact (e.g. LCA) Detail 6. Refine Design Design Compare to DFE Goals N Y Process 7. Reflect on DFE Process Improvement and Results Prof. Kristina Shea Engineering Design + Computing Laboratory 30  DFE and Material Guidelines Example DFE Guidelines Example Material Guidelines ▪ Do not combine materials incompatible in ▪ Use recycled and recyclable industrial materials recycling ▪ Use natural materials which can be returned to ▪ Label all component materials for recycling biological decay cycles ▪ Enable easy disassembly into separate material ▪ Use processes which do not release toxic recycling streams materials ▪ Use no surface treatments ▪ Capture and reuse all hazardous materials ▪ Eliminate packaging ▪ Reduce weight and size for shipping A more comprehensive list can be found in Chapter 12, Ulrich et al 2020. Prof. Kristina Shea Engineering Design + Computing Laboratory 31  Reduce Weight - Substitution of Materials in an Automobile Bumper ▪ Case study: A family car that is driven 250,000 km in its lifetime ▪ The steel bumper weighs 14 kg; the aluminium bumper substitute weighs 10kg; reduction in weight of 28% ▪ The embodied energy of aluminium is greater than steel. Is there a net savings for the material and use phase? What about replacing steel with CFRP (Carbon Fiber Reinforced Polymers)? Reference: Ashby et al., “Granta EduPack Eco Audit Tool- A White Paper”, Ansys, 2021. Prof. Kristina Shea Engineering Design + Computing Laboratory 33  Practice-Oriented Course Addressing MAVT Grand Challenges Health Future mobility Sustainable materials breathe Kyburz Ski workshop Low-cost ventilators Electric vehicles Sustainable skis Concept generation exercise Balloon power car project Material selection exercises Prof. Kristina Shea Engineering Design + Computing Laboratory 37  Case Study: Ski Design https://edac.ethz.ch/education/current-lectures.html Prof. Kristina Shea Engineering Design + Computing Laboratory 38  Case Study: Ski Design Process Phase 1: Phase 2: Phase 4: Phase 5: Phase 3: Phase 0: Planning Concept System- Testing & Production Detail Design Development Level Design Refinement Ramp-Up Design Requirements Design in CAD Fabrication Testing Concept Generation Parametric Modeling 1st Prototype Evaluation Prof. Kristina Shea Engineering Design + Computing Laboratory 39  Skis – An Engineered Sandwich Top Sheet 14 mm Fiber Composites Base and Edges Wood Core Final Lamination Prof. Kristina Shea Engineering Design + Computing Laboratory 40  Impact of Material Selection on CO2 3500 3000 CO2 Emissions [g/Pair of Skis] 2500 2000 1500 1000 500 0 Prof. Kristina Shea Engineering Design + Computing Laboratory 41  LCA of the Skis Made at ETH Not Compatible Considered Trial Adopted Smarter Product Use R0 Refuse and Manufacturing ▪ Skis are inherently hard to integrate into a R1 Rethink circular economy. R2 Reduce ▪ Why is this the case? R3 Reuse ▪ Highly integrated design (multiple non-recyclable Increasing Circularity Extend Lifespan of Product R4 Repair materials) and Its Parts ▪ Hard to comply with DFE guidelines R5 Refurbish ▪ Very specific use case and relative low volume R6 Remanufacture ▪ Manufacturing process hinders larger repair and R7 Repurpose refurbishment of Materials Application R8 Recycle ▪ What can we do to improve our skis R9 Recover environmental footprint? N. Czopek-Rowinska, “Life Cycle Assessment and Sutainability of Skis Built at ETH Zürich” [Unpublished Bachelor Thesis]. ETH Zürich, 2023. Atomic, “Atomic Impact Report,” Atomic Ski, Tech. Rep., 2023. Prof. Kristina Shea Engineering Design + Computing Laboratory 42  LCA of the Skis Made at ETH Total CO2 Emissions per Pair of Skis incl. Waste ▪ “Cradle to Gate” analysis. 4466g 7799g End 22.7% 39.6% ▪ The CO2 produced during production due to laminating materials and the tools is 415g 2.1% shown to be a significant contributor to 87g the overall CO2 output of the ski.