Fabrication :- Alignment, tolerances and tolerance propagation, and quality assurance

1. Fabrication :- Alignment, tolerances and tolerance propagation, and quality assurance Simon’s thoughts/musings and HSX experiences Feb 05

2. 1 Computer Design : CAD model • The coils and winding surfaces should be generated from the ORIGINAL computer model used to generate the magnetics • Feeds and busswork should be included in the model • A script file should be used to go from the computer to CAD or CAD to computer • Once generated, this is a non-changeable script ! • Generating appropriate input from CAD for a computer magnetics model is not trivial • The same coordinates used for NC machining should probably be used for checking and verification, and used in the CAD model • How many coordinate system does one use and keep track of ?

3. 2 Scripts and coordinates • Create Scripts or Lisp files or ?? for the CAD program • A script to move the coil or object and ALL reference points to correct coordinate system for the current operation • A script to reverse this to get everything, including measurements, back to the original coordinate system • Only these tested scripts are used on a copy of the original CAD design drawing for either direction • Keep a propagation-series of this drawing at each stage annotated to show what and where and why this differs from the previous version • Scripts provide reliable and reproducible and checkable design manipulation

4. 2 Scripts and coordinates - #2 • Without an electronic design, measurement and tolerancing from CMM become very hard and difficult to trace • Comparisons of complicated surfaces and shapes are difficult to make • Measurements are often offset by a ball diameter when using the CMM probe • A line or set of points are measured, and then compared to a CAD-generated surface. • These measurements are then later used to generate a set of surfaces or filaments for computer modeling • Immediate comparison to CAD models needs to be made to verify winding etc. • CMM measurement output conversion to computer input needs to be possible

5. Coils from scripts Coil 4 as generated from computer model – 7 turns and 2 layers In HSX space as it will be assembled

6. Coils from scripts #2 Coil 4 in position for winding, generated from script – plus winding surfaces Coordinate system is built into winding base, plus reference marks inserted for winding form and for fabrication

7. Coils from scripts #3 Final winding mold design with reference planes and winding mold components All from original coil drawing

8. 3 Reference marks • One needs reference marks for each stage and each type of fabrication • Tooling balls for CNC mill/lathe and CMM for checking this stage • Tooling balls or detents for CMM checks during coil winding, and jigging fixtures for quick tolerance checks (David A’s go/no-go blocks) • Tooling balls or detents for CMM checks at assembly • Tooling balls or detents for finished assembly to check other components like port locations and orientations, diagnostic locations, etc.

9. 3 Reference marks #2 • References need to be easily/sensibly accessible during each stage • A new set of measurements might be measured relative to a second or third order set of references, so errors will propagate unless original absolute reference marks are always accessible through all fabrication and assembly stages. • One really does not align a model in general, but aligns a set of reference points! Then measurements are performed relative to this alignment. So the reference points are crucial. It is difficult to have too many initial accurate reference points.

10. 4 Tolerances • Fabrication and assembly has to have an allowable size/location for each measurable component or stage in fabrication, and an overall tolerance/allowance for a completed part • Each stage of fabrication • The winding mold • Each turn of a winding • Each layer of a winding • Cross-overs and feeds • The full coil • The full assembly

11. 4 Tolerances - #2 • Decisions need to be made at all stages on allowable tolerances, and overall tolerances, based on computer models. One needs to set these tolerances/allowances before construction, and reassess during fabrication and assembly • The style of component and method of coil fabrication dictate the final magnet-current distribution – insulated turns, spiraled copper, keystone effects, conforming to an internal bend, cooling channels, … • Model errors and tolerances • Model As-built coils to get B Spectrum and magnetics

12. 5 CMM Alignments • The style of alignment for a CMM to a set of reference points dictates accuracies and a possible bias in alignment • 1,2,3 alignment locates the first measured point EXACTLY on the appropriate CAD reference point, puts point #2 along the line joining the EXACT CAD 1-to-2 line, and the moves everything to get point number 3 into the 1-2-3 plane. So, my point is that #1 reference is fully accurate, but the others are not and progressively so! • Best-fit alignment does an average fit over all chosen points • how many point should one chose? • Always use the same points or number of points? • The style of alignment needs to have an error analysis done and an error propagation considered.

13. 6 Consistency • One must consider the effect on low and high order m and n numbers of consistency in fabrication and assembly and alignment. • The same winding crew will probably use a consistent method to make a coil which might lead to a higher order error – half-period periodicity • The same winding crew will probably get better at a coil, so it might get tighter in tolerance in some dimension (flatness or radial build or tightness on a corner). If these coils are assembled in the order they are built, then most probably a low-order error is introduced • Different crews making the same coil will do it differently - with tolerances and allowances in different locations

14. 6 Consistency #2 • A consistent alignment methodology probably influences some magnetics components more than others – modeling is necessary • Assembly by different crews will possibly lead to consistent differences, and thus non-random errors • Overall, is random better, is low-order better, is the B spectrum crucial, are surfaces crucial, or is iota crucial (coil aspect ratio affects this) ? • One needs to be able to go reliably from computer model to CAD and from CAD to computer modeling – Scripts ! • Try to model the as-built components on as low a level as possible – each coil turn? • Can the CAD model or CMM measurement generate and ascii output/excel file which can then be read into a computer file as input to generate a current model of current paths?

15. 7 Changes • If a design changes, these have to be signed-off by both the physicists and the engineers • ATF had a huge field error from an on-the-fly engineering change splitting the main coil-feed busswork to go around a support, which was not discussed with the physics crew. • One needs to build as the CAD model dictates, generated by the computer model if appropriate (coils and feeds) • Current paths dictate I and thus B • Supports affect alignment and assembly, and therefore B • Structures and breaks affect image current and eddy current and thus B • Final as-built, or as intended to be built, components need to be reverse engineered (from scripts) into the computer model • Finite-mu modeling is difficult. What is acceptable?