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Printing and variances

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9 comments

  • Kevin Spencer

    The print file was set to the high quality setting

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  • Jacant

    Change the 'Resolution' for the exported STL file.

    File - DSM Options - File Options - STL - Resolution - Custom

    You may still have to set a 'Tolerance' in your design file. A 1" peg will not go into a 1" hole, it needs a clearance.

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  • Tim Heeney

    Talk to the supplier...

    But perhaps a repeat print, printing parameters exactly as before except make the hole 25.4mm would actually make the hole very close to 25mm in theory.

    All manufacturing methods have tolerances or inaccuracies - designers/ manufacturing engineers take these into consideration when choosing the making process. Clearances allow assembly and proper part 'working' operations.

    Provided it's only the 25mm hole that's important , meaning all other faces can be a slightly different size / location, and that's ok, then i think the above method will work fine.

    Golden rule: Talk to the supplier about you expectations and usually what Jacant says as well ;)

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  • Kevin Spencer

    Thanks Jacant and Tim

    I read an article answering a question about making a smooth print which suggested to do the same thing Jacant but I didn’t know if this would reduce the variance.

    Tim The OD of the frame of the splint is 29 mm to match the EVA rod grip. The EVA butt cap can only be reduced to 25 mm maximum OD which is to fit into the item's ID of 25 mm.  This would give me a 2 mm wall to hold the butt cap in place. It will need this thickness to cope with wear and tear.  I originally wanted to make this out of carbon fibre but couldn't find anyone to do it so I went with nylon and added a carbon fibre strip internally in a different section that takes most of the load.

    Its hard to describe the design without seeing it.  The basic design has been in my head for years but DesignSparks Mechanical has enabled me to put it to paper and share my mental thought so people can see and understand what i'm talking about.  It has enabled me to change the design to overcome unforeseen issues as the design took place. 

    I will do another print but at $150.00 each plus the carbon fibre strip it will get very expensive quickly.  I’ll make a few changes to the first design before printing and hopefully it will be the last.

    Thanks again to both of you

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  • Jacant

    Maybe this will explain more.

    An STL file is an approximation of the outside surface of your model, split up into flat faces. Again split up into at least two triangles. The more triangles the smoother the model.

    Even in Cad a 'Circle' is drawn with many 'Facets'. This only shows when you save as an STL file. In fact if you change the 'Rendering Quality' in DSM Options you can see it here.

    The first image shows the 'Variance' in the measurements across three circles.

    The second image shows them as STL 

    The 50mm dimension is from opposite points. The other measurements are taken from opposite 'flat' faces. 

    You will never get a perfect round surface from an STL file.

    The model was drawn in Sketchup with the amount of 'sides' changed to 24, 48 and 96

     

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  • Kevin Spencer

    Thanks I did observe the triangular nature of the STL file and I understand the flat facets and circles idea (i facet gems as a hobby).  So the smaller the surface area of the triangles, changes the angle they meet at, reducing the peaks. 

    In your first image of changing the STL file from fine to custom:

    ASCII reduces the size of the file,

    File is set to design so the whole design has these printing specifications. Can a surface have different printing specs? See below image.  The outer surface of the of the spine and gimbal housing I would like to have some texture since these will be covered with leather which will be stitched and glued in place.  The internal design of the gimbal housing needs to be smooth.

    Resolution deviation and angle, are these your recommended settings or just an example?

    Deviation is set at 0.25.  What is meant exactly by the deviation and what happens as this is changed?

    Angle is the degrees the triangle meet at.  I suppose how small this could could be depends on the variance of the printer. To obtain the minimal settings i will need to talk with the printer. 

    Sorry for the dumb questions but i would rather ask it and learn than make many errors.  The image below took me about 2.5 weeks to complete.  Its not to bad for a beginner.

    Thanks 

     

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  • Jacant

    ASCII reduces the size of the file

    A 'Binary' file is more compact

    Resolution deviation and angle, are these your recommended settings or just an example?

    These are my own settings through trial and error on an 'Ultimaker' printer.

    What is meant exactly by the deviation and what happens as this is changed?

    Deviation controls how far facet edges are away from model edges. The images below show the effect of Max distance on the mesh of a cylinder. These examples keep the Aspect ratio fixed at 3 and the Facet maximum edge length set to 4mm.

    Copied from Help Files (Fn key + F1) Search for STL - File import and Export options.

     

    There may be another way of doing this.

    The 'Facets' are decided by the 'Resolution'. What if you used a Circumscribed Polygon?

    You would then know exactly the size, flat face to flat face. There is a maximum number of 64 facets in DSM

    The image shows 'Polygons' Inscribed and Circumscribed. (64 faces)

     

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  • Tim Heeney

    Kevin,

    Jacants 'work around' nicely side steps the issue of 'resolution' - but it might be more work to pull / change it's size or add other features in or intersect with it. Nonetheless it would produce a very accurate STL file where a 25mm circle would fit perfectly.

    I also had a thought...if a hole size came back at 24.6 and you wanted 25, why not sand paper / abrade out to 25? wouldn't that take only 5 minutes to do. 'Material off' situations are much preferred to the other way as it might not be possible at all !

    Also, once you have decided a hole size and it's construction method ( maybe using a circumscribed polygon etc), to keep costs low, split the model up and only print the part you're really concerned with i.e gimbal housing bit. Talk to supplier, you want it to be 100% representative of the whole part and a quote.

     

     

    Everything below is the makers resposibility - it's included in their general making tolerances.

    There are sadly other factors here though in the making...it comes down to when they extrude the plastic through the nozzle.

    How accurately they can place the nozzle in a perfect position.

    The speed of deposition and 'wander or slump' as it solidifies.

    The attitude of the part in the printing machine - if you want it made in a certain orientation, it must be specified or there's a chance it might be put with other parts and orientated anyhow to fit inside the printing cubic volume.

    I say the above because there's certainty in my mind that a nominal perfect hole 25 mm will be produced at 25 + /- 0.1 by the very best machines and +/- 0.2 as a normal makers tolerance.

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  • Kevin Spencer

    I'm not an engineer, i have a degree in social work so i've had to look up some of the terms used by both of you.  Such as circumscribed polygon and still scratched my head a little LOL.  Before going on I'll mention i have Quadriplegia, more movement in my arms than most people think when mentioning quadriplegia and grouse movement in my hands, due to tendon transfer operation.   

    Tim yes I  tried making some tools to sand the ID but they just compressed.  I brought a Ozito the same as a Dremel hand held tool with a flexible drive.  The drive had a wobbled/vibration of 1-2 mm so I didn't use it.  I tried to find out if this was normal for this type of tool but couldn't get any replies.  

    Now remember i'm a novice. I have to change the design a little.  When i place the curve end on the splint, to spread the load over a wider area on the fore arm near the wrist, it change the angle of the rod.  This would pace more pressure on the hand.   

    When i knew it was to be printed in nylon I thickened the spine  from 2 mm to 6 mm.  This is so could insert a sleeve to accommodate a 2 x 19 mm carbon fibre strip to increase its strength and rigidity.  Changing this one element has given me opportunity to remove the bent end of the spine and have the angled curved instep just on the flat spine.  I'll will lengthen the flat spine and that should change the angle of the rod back to the original angle.

    Having it commercially made: I had thought of having the two ends moulded to a carbon fibre strip.  Finding a commercially available strip about 3 x 25 mm is hard.  I've been looking. I could having it made made of carbon fibre but having the mould made would be expensive.  This model is just  a prototype to test the idea but also be useful for me.

    Sorry I talk to much sometimes especially since it takes me so long to type. 

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