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  1. #11
    Quote Originally Posted by Leadhead View Post
    The bevel gear add-in for F360 produces what I want but it is a trade off between graphic presentation and real gear form.
    That is stunningly poor!
    I'm Shocked!
    You think that's too expensive? You're not a Model Engineer are you? :D

  2. #12
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 4 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,014. Received thanks 221 times, giving thanks to others 5 times.
    Quote Originally Posted by Neale View Post
    Trouble is, tooth needs to scale with distance along - tooth width and gap aren't constant. There are some standard ways of getting close using a dividing head and taking three cuts per tooth space but I'm not sure how you would fully automate that.
    Yeah, you need to take the standard form, then scale it to/from the disappearing point where the centre of the two shafts meets.
    As Leadhead isn't hobbing, the variable tooth size doesn't really add to the complexity of the machining, just the designing.

    Gear machining is something that fascinates me, especially when you see some of the setups and tools from pre-computer days.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #13
    Currently adjusting tooth parameters to be able to rough with a 3mm and finish with a 2mm bullnose. That is, of course, the small end of the small pinion.
    Not able to answer you until that is resolved. Will post a screenshot then.

  4. #14
    MartinS's Avatar
    Lives in Leeds, United Kingdom. Last Activity: 18 Hours Ago Has been a member for 0-1 years. Has a total post count of 17. Received thanks 1 times, giving thanks to others 5 times.
    The bevel gear add-in for F360 produces what I want but it is a trade off between graphic presentation and real gear form.
    I also have often looked at the "pretty pictures" that these cad systems have produced and wondered just what the shape of the tooth is based on. Is it a true involute created from first principles, or is it part of an arc of a circle which just "looks right".

    Also, I cannot figure out how you are going to machine a bevel gear with just three axes. (I will muse on it.........).

    A previous post mentioned taking the cross section of a tooth and extending it forwards and back. Beware of this approach as the tooth surface shape changes along it's length.

    Click image for larger version. 

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    At the small end of the gear the involute tooth surface is generated from a small base circle diameter, whilst at the large end a larger BCD is used. You can see from the pic above that from the root outwards, the radius of curvature of the tooth surface is constantly changing up to the tooth tip. But this point radius of curvature changes from small end to large end also.

    With a spur gear the involute shape can be machined onto a cutter (e.g. disk milling cutter to end mill) and the gear form cut with indexing from tooth to tooth. This approach won't work with a bevel.


    Thought!
    How good is CAD and/or CAM at "topological milling"?

    If the gear is mounted with the PCD on the XY plane so it can be indexed per tooth, can the tooth space be roughed out and then finished with a ball ended cutter? It could be somewhat rough if at all possible.


    Another way is

    Click image for larger version. 

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    Mount the gear as above but via a 4th axis on an inclined plane. Then the tooth can be generated with a straight sided cutter in successive passes whilst rotating the gear. The yellow in the pic is a rack relative to the two rotating gears. Therefore each tooth flank is conjugate to a straight sided cutter (e.g. 20 deg included angle end mill). Therefore this principal can be used to cut the tooth surface!

    Hope this helps


    PS

    You are wanting to make a straight bevel not a spiral?

    Martin

  5. #15
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 4 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,014. Received thanks 221 times, giving thanks to others 5 times.
    Martin, thanks for clarifying the BCD/scaling issue.

    From what I gather, Leadhead is planning on machining the gear, by mounting the blank flat on the machine I.e. so the gear axis and spindle are in the same plane.

    If you've not seen it before, you might want to have a play with Gearotic (www.gearotic.com). It's written by Art (the original man behind Mach), and allows you to design/simulate all sorts of weird mechanisms. It originally started as just a gear designer, but has grown. The only limitation without a license, is you can't export different file formats, but if you do license it, it can produce various 3D file formats, or even generate the required G-code using either 3 or 4 axis.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  6. #16
    Yes I am doing a simple straight conical form. I just took a form from an internet software. Modified the base form to make it a little deeper so that my 2mm cutter form corner did not foul. Then lofted it as a 2D major circumference 32tpi to the scaled down minor circumference 32tpi for the small gear. This looks quite good. I am then doing the same with the gear form onto the 64tpi. Maintaining the same tooth pitch on the mating major and minor circumferences. This looks fairly promising.

  7. #17
    MartinS's Avatar
    Lives in Leeds, United Kingdom. Last Activity: 18 Hours Ago Has been a member for 0-1 years. Has a total post count of 17. Received thanks 1 times, giving thanks to others 5 times.
    m_c:

    I'll have a look at the Gearotic software. What I can do with it might be limited as I do not yet have a CNC machine. I have planned to build one since retiring and have decided that I AM going to start one in the new year (I guess I'll be posting many questions so please be prepared to be patient with me!). In my professional career, besides bespoke s/w, I used one from Kisssoft AG http://www.kisssoft.ch/english/home/index.php. It is totally trustworthy. I was using it for calculating gear tooth stress analysis, bending, deflections, optimising shape for low noise/wear etc. As we had many machines designed for hobbing/shaping and tooth grinding, we did not need CAD/CAM for producing tooth surfaces. Have just looked at Kisssoft and it is now owned by Gleason and looks to have gone up-market somewhat. There is a test/demo that will allow generation of reliable data.

    Leadhead:

    You have challenged me to a project for when I have built a CNC machine!. I will be following this thread with interest. I guess that by 32 tpi you mean "32 teeth per inch of PCD"? That would be 32 DP which is quite small (tooth depth of 1.78mm). Heck that is small. If you need any help with gear geometry, just shout.

    Good luck

    Martin

  8. #18
    Martin - Yes an error of scale on the TP.
    Thank you for your offer of help. I certainly am in the needy category.

    Just selling a medium sized 700x 500 footprint, table top CNC router. Fully complete with Mach 3. Including prepped laptop and smoothstepper.
    A much bigger one flew under my radar before I got chance to use it. So as new, plug and play. Not actively on sale till I get round to advertising it. Let me know if it might be of interest. Can do a teach in and demo.

  9. #19
    MartinS's Avatar
    Lives in Leeds, United Kingdom. Last Activity: 18 Hours Ago Has been a member for 0-1 years. Has a total post count of 17. Received thanks 1 times, giving thanks to others 5 times.
    Thanks for the early offer but I am hoping to get a lot of pleasure out of the design and manufacture of "my own" machine. So will my wife, she will have the Sky remote all to herself!

  10. #20
    Martin - Your wife will still be disturbed by you continually flopping in the armchair expecting consolation for frustrations she could not possibly understand or appreciate. This has to factored into your relationship. I am qualified to make this observation!!!

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