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  1. #11
    Quote Originally Posted by D.C. View Post
    As far as the ballscrews go, would a 20mm 5mm lead not offer better accuracy, I'm not intending to do large volume runs of stuff so speed isn't that important (as long as it doesn't case any material problems) but precision for engraving is.
    Yes, but overall you're still better off getting 10mm pitch ballscrews since you can use pulleys and have a 1:2 (motor:ballscrew) ratio to get the same resolution as a 5mm pitch screw on 1:1. If you got 5mm pitch then the ballscrew has to spin twice as fast, so unless the critical speed is twice as great you will be limited. The critical speed is just proportional to the diameter, so using 20mm will only gain 25% speed hence there's no point. If the 20mm diameter screw was 10mm pitch I probably still wouldn't use it since the required torque to accelerate it is much greater.

    Also the resolution you get from 5mm pitch is hardly ever required. I cut this with (roughly) 10mm effective pitch:
    Click image for larger version. 

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    The area shown in that image is roughly 50mm wide...

    Quote Originally Posted by D.C. View Post
    online resource for explaining the basic maths and mechanics for working this stuff out properly so I can stop vaguely throwing combinations of parts around in the hope that someone will work it out for me? :)
    ... preferably with emc2 examples?
    Er, me? But seriously, the information on this forum is all you need. If you've not already then read as many of the build logs as you can as the same things come up regularly, so it shouldn't take long to find out what the best way to each part is. The fact you're intending to use LinuxCNC doesn't make a lot of difference to the process - in most cases it'll be just selecting a different option when you export the G-code from the CAM program.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

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  3. #12
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    This is vagulely seeping in now, thanks! I really need to brush up on my basic mechanics I've forgotten it all.

    I was reading through the build logs and found a link to a section of this site I never new existed, the FAQ... ooops.

  4. #13
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    I'm going to order the cnc cookbook from amazon, does mycncuk have an affiliate account I can click on?

    I love the finish on your ply chair, it almost looks like spalted beech.
    Last edited by D.C.; 12-10-2012 at 02:10 AM.

  5. Well I hope you find it useful - definitely a bit basic for some of the guys on here but for relative newbies like me it's good.

    Glad you like the chair - that was what got me into this lark! I made it for my son and cut it all out by hand - never again! Next time I'm Cncing all the way...

  6. By all means buy the CNC Cookbook, but you'll get a lot more useful & detailed info from here. Its a bit basic and out of date (2009) in many areas and a lot of things are glossed over, like pros and cons of microstepping for example. In fact that subject gets 1 line in the book and not even a index entry!

  7. #16
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Quote Originally Posted by irving2008 View Post
    By all means buy the CNC Cookbook, but you'll get a lot more useful & detailed info from here. Its a bit basic and out of date (2009) in many areas and a lot of things are glossed over, like pros and cons of microstepping for example. In fact that subject gets 1 line in the book and not even a index entry!
    It wasn't a slur on the quality of the information on this site but I have zero experience with this sort of stuff and I'll be happier when I have a basic grasp of the fundamentals. In that situation I normally find an academic textbook/guide is useful as it covers all the fundamental topics in a logical order (or they should!) and gives you something to build on. At the moment I'm kinda grasping at threads of information and trying to knit them together randomly.

    For example in one of the build logs a tip is given about the positioning of the screw in relation to the rail, I would like to actually know this stuff and the mechanics behind it to work it out instead of stumbling across nuggets of information and thinking 'oh yeah, of course'.

    I'll see if the library has a copy if it's not worth buying and in the mean time dust off my old Sadler and Thorning 'Understanding Mechanics' that has been sitting on the shelf untouched for years.

  8. Ah... then you wont find that level of detail in the Cookbook... or at least only superficially... its a cookbook, much like if you beat some prescribed quantities of eggs, flour, bicarb of soda and water together and put it in the oven you get a cake-like thing... it says nothing about the chemistry of cakemaking or how the different ingredients might affect the final result in any detail. So if you're looking for the maths on acceleration/mass calculations, bending moments/rigidity, stepper motor torque v leadscrew pitch, etc etc. you'll find it light on detail...

    (for a test, go to the Amazon page for the book and try some search terms in the "Search Inside This Book" field)

  9. #18
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Are there any good recomendations for a basic introduction to cnc design/use including explanations of the maths and principles then? First year undergrad level is probably about right for me.

  10. Now there's an interesting question... none of the books I've looked at appear to have much of the maths in them. The bulk of the books are either "Build a CNC like I did" and then are essentially a set of plans (or not in one case) but with little explanation why... or are more focussed on the programming and usage but relatively little on the design aspects... something we have great debates on the forum about, between those that prefer the suck it and see based on experience of doing it several times before (and there's nothing like experience I agree) and prefer empirical evidence of what works or doesn't work and others (and I include myself in this category) who want to understand what constitutes a good design from good engineering principles and want evidence to demonstrate that, and indeed want to understand how to make it an optimal design.

    Of course, those that build big commercial CNC machines must, we assume, have done the full blown structural design, but most of the low-end 'Chinese' machines are almost certainly 'empirical' in nature (and it shows in many of them). There is very little on the web about the calculations, though plenty of generic 'bending of beams' etc stuff which is 1st year undergrad (or when when I did it 30y ago). As always its in knowing how to apply the knowledge that counts. There are nuggets out there, like this thread on CNCZone and this one here, but again they don't explain too much of the 'why'.

    That is one of the reasons we say that no question is stupid... and reading through build threads past and present will give a good cross-section of both the 'calculated' and 'empirical' approaches. If you have specific questions about calculations abd/or principles then ask away...

  11. #20
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Hmmmm, perhaps you guys should put one together, then you can answer all daft questions with a BTFM and link to amazon. :)

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