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Thread: My CNC Router Build Adventure

  1. #81
    Member StrawberryBoi's Avatar
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    Default Re: My CNC Router Build Adventure

    EDIT: Deleted because it didn't actually matter to the OP

    Summary: Real products have variation in manufacture and supply, function, and aesthetics. This results in real design reasons for not expecting complete depth thread engagement. In structurally critical applications it may be a loss to have unused hole/thread feature that has removed material from a stressed location.

    Last edited by StrawberryBoi; 09-21-2021 at 01:35 PM. Reason: Eliminate needless fighting


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    Default Re: My CNC Router Build Adventure

    Quote Originally Posted by JayneV View Post
    I appreciate all comments. I am here to learn and share my journey with other like minded people. Please keep any type of bickering out of this thread. There are always multiple solutions to any one problem and hearing different opinions is very interesting and informative. Healthy debate is good and I welcome it, all I ask is that you are all respectful.

    StrawberryBoi, thanks for the tip about double checking bolt lengths before installation. It would be difficult to track down a poor connection due to a bottomed out bolt in a blind hole. I can see how it would be easy to pickup an incorrect length bolt out of a box of hundreds and not notice it’s the wrong size.

    Mactec, I may have not mentioned it before, the current design with all those brackets is aluminium. I could build the same design out of steel with welded connections, eliminating most of the brackets. There are valid reasons for choosing either material. I actually have two copies of the model in CAD, one for an aluminium base and one for a steel base. For the steel base, I am planning to take Peter’s suggestion and use tabs and slots to locate and weld each part. I need to update the steel version of the model to include the tabs and slots then I’ll generate 2D drawings of the parts to send to laser cutters for quotes.

    I don’t have a problem with using bolted connections. I work on large aircraft and the entire wing is held on by a handful of bolts and these bolts remain untouched after installation for the life of the aircraft unless there is an unusual circumstance that warrants their removal. Why do you say that threaded holes deeper than what is need for max strength if the joint is a fail? Surely the hole would need to be a little deeper than the max depth of the bolt to eliminate any possibility of the bottoming out. Could you clarify what you meant?


    Jayne
    With you being involved in aircraft you should already know the importance of hole depth and thread depth, aircraft parts have very strict spec's for this, it is unimportant for what you are doing, with a machine build like this, the Machinery's Handbook is a good place to find anything to do with engineering standards for anything you want to build. even this snip, I have attached will give you enough information for your build regarding fasteners

    Tabs are good, and have been used throughout manufacturing for years, the tab weld area has to be prepared correctly for welding

    Attached Files Attached Files
    Mactec54


  3. #83
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    Default Re: My CNC Router Build Adventure

    Quote Originally Posted by StrawberryBoi View Post
    EDIT: Deleted because it didn't actually matter to the OP

    Summary: Real products have variation in manufacture and supply, function, and aesthetics. This results in real design reasons for not expecting complete depth thread engagement. In structurally critical applications it may be a loss to have unused hole/thread feature that has removed material from a stressed location.
    Oh yes, of course. It makes sense to not remove more material than necessary. I can see how that would have varying impact depending on the application. Thank you for the explanation.



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    Default Re: My CNC Router Build Adventure

    Quote Originally Posted by mactec54 View Post
    With you being involved in aircraft you should already know the importance of hole depth and thread depth, aircraft parts have very strict spec's for this, it is unimportant for what you are doing, with a machine build like this, the Machinery's Handbook is a good place to find anything to do with engineering standards for anything you want to build. even this snip, I have attached will give you enough information for your build regarding fasteners

    Tabs are good, and have been used throughout manufacturing for years, the tab weld area has to be prepared correctly for welding
    Is this the book you are referring to?
    https://www.angusrobertson.com.au/bo.../9780831136314
    I wasn’t aware of its existence. It looks like a very useful reference to have on hand. I will add it to my shopping list of useful things to buy. And thank you for the attached PDF it also has lots of good information.

    I’m off to do some research on welding tabs to see what I can learn about how to use them effectively.



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    Default Re: My CNC Router Build Adventure

    Quote Originally Posted by JayneV View Post
    Is this the book you are referring to?
    https://www.angusrobertson.com.au/bo.../9780831136314
    I wasn’t aware of its existence. It looks like a very useful reference to have on hand. I will add it to my shopping list of useful things to buy. And thank you for the attached PDF it also has lots of good information.

    I’m off to do some research on welding tabs to see what I can learn about how to use them effectively.
    Yes that is the Machinery-handbook, sometimes you can find them at a lower cost than that one, for what you need on your bolt fab-machine, the PDF should have enough information for most of what you need including thread depths

    Here is some good information on the use of Tabs the video is short about FEA it's what I say as well.
    https://mentoredengineer.com/warning...xturing-parts/

    Mactec54


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    Default Re: My CNC Router Build Adventure

    Quote Originally Posted by mactec54 View Post
    Yes that is the Machinery-handbook, sometimes you can find them at a lower cost than that one, for what you need on your bolt fab-machine, the PDF should have enough information for most of what you need including thread depths

    Here is some good information on the use of Tabs the video is short about FEA it's what I say as well.
    https://mentoredengineer.com/warning...xturing-parts/
    Thank you. The link for the book was just the first result I found from a local book seller. I've already found cheaper links since that post. I also asked my brother and he has the book which I can borrow in the meantime. I also found and watched that video about tabs. It's good information and it highlighted a few things I had not considered.

    Jayne



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    Default Re: My CNC Router Build Adventure

    Hi All - I have briefly looked at the video on FEA and the concept of stress flow is incorrect. Stress does not start somewhere and flow like a river to somewhere else in a structure. It has been conceptualised like this for many decades as the iso stress contour visualisations look like streamlines from fluid flow. Stress is a static local value and is a simplification of strain. In regard to tabs and fatigue this is valid in cyclically loaded structures but in machine tools like we are discussing the stress is well below the values that lead to fatigue. Most stresses are well below 10Mpa and this is insignificant for fatigue. I do recommend that you speak to the laser cutter and they will give you the clearance allowance for a tab/slot that they find works. In regard to corner stresses in conventional FEA its poorly explained in the video. Since stress in FEA is calculated buy averaging adjacent elemental stresses, at edges you only have one element to average across, plus an edge is a mesh discontinuity which means it will have infinite stress at it generally. The code adjusts for this in the element formulation but stress at edges or small features amongst big features are difficult to interpret sometimes. There are several mesh tools and strategies to correctly estimate edge stresses. Big subject for somewhere else... Peter

    edit - the unbrako doc pretty much lives on my desk - its a good one...



  8. #88
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    Default Re: My CNC Router Build Adventure

    The Unbrako dock now lives on my desktop as well, primarily because it's a more to the point reference than the Machinery's Handbook, which is more like the catch-all reference.

    Jayne, the Machinery's Handbook is absolutely invaluable for machine designers and engineers. While the information is not always representative of optimal design, it gives you a known functional basis utilizing many many decades of theory and practice. It also includes a great many mechanical standards as well that you would otherwise have to purchase or pirate to utilize correctly.

    I have an e-book from the publisher of version 31 and a print of 28. I use it at least weekly.

    That said, it's a starting point and not an end all. A great many standards are established for a known good and not an optimal result. Not for your project now, but for any precision or extreme performance projects in the future, searching through university research is often a way to improve upon these standards. That and testing, lots of testing.

    I do note than neither Unbrako or Machinery's Handbook states an opinion on maximum thread depth vs bolt length, only on the statistical average thread engagement for a given thread length. This is an example of where designers must justify choices on an application basis and not simply be instructed on what to do.

    After all, you can't just look at the Machinery's Handbook section on building CNC routers and find a step by step instruction with governing standards telling you exactly how to design or build each element or whole

    This is the e-book version: https://ebooks.industrialpress.com/p...l-edition-31st



  9. #89
    Member JayneV's Avatar
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    Default Re: My CNC Router Build Adventure

    Oh wow! I have to admit that some days it feels like I am way out of my depth with this project. Today is one of those days. I am currently on a very steep learning curve and it's going to take a while to digest all this information. I'm very grateful for all of your help and shared knowledge, please keep it coming. At some point, soon, I am going to have to commit and start building this machine. I learn better with practical experience than from theory.

    Thank you for the ebook link to the Machinery's Handbook. I will probably end up ordering a hardcopy of the book. I find it easier to navigate huge reference book like this when they are a hard copy. Still haven't figured out a way to not get lost in large PDF documents that are not meant to be read sequentially from start to finish.

    Wouldn't it be nice to simply open up some book to the section about whatever your current project is to find a step by step guide. But then again, where is the fun in that. LOL

    Time for this girl to head to bed and rest my overworked brain cells. Good night!

    Jayne



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