Engineering design needs practise, so here I share the points I run through

I wanted to do something entirely different this month and focus on a core topic of manufacturing ‘Engineering design’ and where I have had the most experience which is the designing of Jigs & Fixtures.

Again, I won’t go over the basics of what each does (Google can help you there), but what I wanted to do is use my knowledge/experience with further research to produce what I would call ‘Lean Jig & Fixture engineering design considerations’.

So what is engineering design? We take these 3 ideas

• Traditional design
• Lean Principles
• SMED Principles

And formulate a set of guidelines which you can follow (a checklist if you prefer) when you do your own designs. Note – You don’t need to satisfy all, just consider them and decide if they are required.

My aim here is to create a starting point, as always, I encourage others to get in touch if there’s anything I’ve missed.

Starting considerations for Engineering design

After or before designing we need to consider what can we affect (control) with our engineering design?

• Initial setup onto machine / bench
• Load / unload of workpiece
• 6 Big loses
  • Setup scrap
  • Reduce speed
  • Production scrap
  • Breakdowns
  • Adjustments
  • Slow cycles
• Cycle time is affected by
  • Choice of process
  • Tooling used (cutting, moulds, forge etc..)

These are affected by variation and complexity of the product family which needs to be minimised or eliminated first.

Cost considerations are required & this ties into demand and capacity – low demand and capacity equals no need for jig / fixture. However, work out your ROI based on your engineering design to see if it’s viable.

Also look at the current process method, would a jig/fixture make life better for operator/company?

Design several concepts & score them against set of generic and specific criteria for the product you are designing for (Rank these in score order and test).

For manufacturing to be flexible, work holding devices should be able to accommodate all the parts within the family of parts.

The guidelines to follow can be apply to any type of Fixture or Jig for Manufacturing and Assembly processes, your experience will help you to decide how to incorporate the points below that you need.

Location

1. Locating points (Datum faces)
2. Orientation of workpiece
3. If your fixture / jig holds a part that becomes hot as part of the process, minimise surface contact area so the part can be removed
4. When positioning parts, remember to restrict movement in the unwanted axis
5. Design using dowels to fit parts of the fixture together if alignment of parts is critical
6. If the workpiece has a pre-existing hole, use that for internal location.
7. Use Vee blocks to locate cylindrical workpieces
8. 3 – 2 – 1 method.
   1. The 3-2-1 method is a work-holding principle where three pins are located on the 1st principle plane, i.e., either XY, YZ, ZX. And two pins are located on the 2nd plane perpendicular to the 1st plane, and at last, one pin on the plane is mutually perpendicular to the 1st and 2nd planes.

Clamping

1. Clamping to resist machining forces
   1. Understand the direction and magnitudes of forces being applied to the workpiece and clamp with enough force to counter.
   2. https://article.sciencepublishinggroup.com/html/10.11648.j.sr.20150304.19.html

1. Locate clamps in the best location to resist the force from the cutting tool
2. Soft surfaces depending on where the jig is used (i.e. in final assembly)
3. Clamping devices must be simple and easy to operate
4. Adequate clearance for variation is workpiece supplied (i.e. MMC)
   1. Understand tolerances and stability of manufacture
   2. Pre-machined tolerances of the workpiece (i.e. from supplier or last process)
5. Locate clamps opposite some bearing points to avoid springing action
6. Calculate clamping forces and/or stress distributions in the fixturing elements
7. Determine loads that will deform the fixture or workpiece elastically or plastically
8. Spread clamping forces over a large area to minimise distortion to workpiece
9. Freedom from part distortion

Materials

1. Consider materials used to manufacture Jig/Fixture
   1. Tool Steel – Milling
   2. Mild Steels – Milling/Welding
   3. Stainless – Wire erroder
   4. Acetal/plastics – Assembly
2. Harden surfaces for wear
   1. Harden & Temper
   2. Durability – What process is being used and what considerations to how it will wear the Jig / Fixture?
3. Workpiece material
   1. Max deformation
   2. Max shear of contact faces

Configuration

1. Repeatability – Can your design repeat the process on every part? – Is it impossible to incorrectly load the workpiece?
2. Interchangeability – Can your design be changed to different parts or is it dedicated to one?
3. Rigid design
   1. Use ribs and fillets to increase rigidity
4. Ease and speed of loading and unloading workpieces
5. Easy disposal of swarf/waste from process
   1. Consider swarf removal, allow it to escape and not build up on the fixture or get trapped
6. Machine’s extent of automation
7. Machine’s current clamping system (i.e. what it uses to hold jigs/fixtures)
8. Any tool paths you will need to account for within your design
   1. Low profile – Clear of toolpath
9. Ease of assembly for your jig/fixture
   1. Design and construction ease
10. Ease of access to parts within your assembly considered ‘consumerable’
    1. Ruggedness – Elements subject to wear – easy to replace
    2. Ease of cleaning and maintenance
11. Design for family of parts that have similar features (Group technology)
12. With Assembly fixtures, design so the part can be built from the bottom up
13. For prismatic parts that require lathe work a mandrel (adaptor) may be required to mount
14. Reliability – Easy to maintain and lubricate

Safety

1. Safety considerations
   1. No sharp edges
   1. Safety critical dimensions
   1. Bolts / Nuts should not protrude on the body but be placed inside
2. Ease of handling
   1. Portability
   1. Design handles to help handling easier
3. Consider the weight of the jig and how it will be used and transported
   1. Two-man lift is any weight above 25Kgs
   1. Reduce weight where possible is the design
      1. Change materials that aren’t load bearing
      1. Cut holes/slots to remove material where you can’t change it
   1. Ergonomics & safety – max 30-40lb to open/close clamps

Output

1. CTQ (Critical to quality) dimensions
2. Accuracy – Can your design produce the feature within tolerance? What is required from the drawing?
3. Optimal output – Cycle time/Takt required vs No. of parts produced
4. Inspection requirements
   1. Does the workpiece need to be checked during machining process?
5. Machine size & capacity / capabilities
6. Machine’s accuracy

Modular

1. Flexibility

Twenty principles of jig and fixture design – From DeGarmo p.821

• Determine the critical surfaces or points for the part
• Decide on locating points and clamping arrangements
• For mating parts, use corresponding locating points or surfaces to ensure proper alignment when assembled
• Try to use 3-2-1 location, with 3 assigned to largest surface. Additional points should be adjustable
• Locating points should be visible so that the operator can see if they are clean. Can they be replaced if worn?
• Provide clamps that are as quick acting and easy to use as is economically justifiable
• Clamps should not require undue force by the operator to close or to open, nor should they harm hands or fingers during use
• Clamps should be integral parts of device. Avoid loose parts that can get lost
• Avoid complicated clamping arrangements or combinations that can wear out or malfunction
• Locate clamps opposite locators (if possible) to avoid deflection/distortion during machining and spring back afterwards
• Take the thrust of the cutting forces on the locators (if possible) and not on the clamps
• Arrange the work holder so that the workpiece can easily be loaded and unloaded from the device and so that it can be loaded in the correct manner (mistake proof) and in such a way that the location can be found quickly
• Consistent with strength and rigidity, make the work holder as light as possible
• Provide ample room for chip clearance and removal
• Provide accessibility for cleaning
• Provide for entrance and exit of cutting fluid (which may carry off chips) if one is to be used
• Provide four feet on all movable work holders
• Provide hold down lugs on all fixed work holders
• Provide keys to align fixtures on machine tables
• Do not sacrifice safety for production

Cost to Implement

From DeGarmo’s book (Page.841), I present their engineering design equation when considering the costs involved when developing Fixtures and Jigs (bear in mind to adjust for inflation).

(Labour cost per piece without tooling + Machine and overhead cost per piece without tooling) – (Labour cost per piece with tooling + Machine and overhead cost per piece with tooling) >= (Cost of tooling + interest on tooling cost)

This gives two values and if the left hand side is equal to or greater than the right hand value then the tooling is justified.

SMED Considerations in Engineering design

1. Use springs to ease ejection
2. Design the jig or fixture so human assistance isn’t required to hold the work piece down while clamping
3. When possible, design your fixture or jig so that it can be operated by one hand
4. Design using common sizes of materials and fasteners – don’t go bespoke unless you need to
5. Use what you have around you (if your company stocks bolts/nuts/screws for products design using them, save cost/time and spares are ready)
6. Use boltless clamping
   1. Example – Spring or Magnetic clamping
7. Use One-touch clamping where possible
   1. Consider changeover process, can you reduce this via design?
8. Use cassette type designs (modular)
   1. If the machine uses specific fixturing, can you design an insert that can be removed and swapped for another that does another job?
   1. Save weight
9. Note – Screws / bolts only hold and release on the last and first turn
10. Design fixtures and jigs with quick release features
11. Design self centering jigs
12. Use alignment plates that permanently stay on the machine but has features to enable quick alignment like square blocks
13. Always eliminate the need for adjustments, use setting pieces
14. If a machine uses lots of fixtures, minimise setup by standardising the overall H/W/D dimensions – Look at functional dimensions to standardise.
15. Design systems where intermediate jigs can be setup with the next job while the machine is running and swap jigs over when needed
16. Use C-washers in designs, one turn to loosen the bolt then slide washer out to give enough space to remove jig
17. Instead of using overhead cranes and lifts, use roller trollies to slide jigs on and off machines
18. Least common multiple method
    1. Make settings, not adjustments.
    1. Change only functions; leave mechanisms as they are.
19. Use pear shaped holes to quickly remove jigs without removing bolts
20. To make engagements easier, use rads on holes and chamfers on pins
    1. Frequency of use and handling distance
21. Single motion clamping – P.60 Shingo
22. U-slot method – P.59 Shingo
23. The Clamp method – P.59 Shingo
24. The split thread method – P.58 Shingo
25. The Pear hole method – P.57 Shingo
26. U washers – P.57 Shingo
27. Reduce number of parts in assembly
    1. After design is finished, question is every part needed – can it be simplified?
28. Identify internal and external factors your design will introduce to the process; can you move internal (when machine is idle) to external setup?
29. Ease of setup
    1. Setup time reduction by design

Lean Manufacturing Considerations in Engineering design

1. Standardise parts used in assembly
2. Eliminate / reduce waste in design & in process the design is used
   1. Reduce variation
   1. Reduce complexity
   1. Reduce mistakes
      1. Poke yoke
3. Takt time to use the assembly – is this increased or decreased from the current?
4. Move design to single piece flow
5. Organise workplace
   1. Storage of assembly
   1. Transportation of assembly
   1. Identification method
6. Consider if your design will cause the process to become a constraint (i.e., Cycle time does not meet takt time)
7. Reduction of lead time from current – will this engineering design effect this?
8. Demand of part – is Takt time too short for manual operations, giving the reason to produce a Jig/Fixture?
9. Minimise the number of steps/operations required in your design, and the process it’s designed for.
10. Consider a design that highlights misalignment of the work piece (poke yoke)
11. Design jigs that incorporate the idea of flow
    1. Example – if the part has 3 operations (machining different side) have those positions on the same jig and program to machine all three, once completed you just move the part to next stage.

Remember that these engineering design considerations will be used during the life of the Jig/Fixture, so with each iteration, come back to these guidelines to help consider the modification you need to make. Compromise is inevitable in the designing of jigs and fixtures.

References

• DeGarmo et al “Materials and Processes in Manufacturing” – 1988
• Shingo ”A revolution in manufacturing: The SMED system”
• “An application of SMED technology” – Berna Ulutas 2017
• “Rethinking modular Jigs’ design regarding the optimization of machining times” – S.Kumar et al 2019
• “Design and Analysis of Jigs and Fixtures for manufacturing process” – H.Radhwan et al 2019
• “The design and need for Jigs and Fixtures in Manufacturing” – Charles Chikwendu Okpala et al 2015
• “A literature survey of fixture-design automation” – J.C Trappey and C.R Liu 1990

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