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Thread: Cutting a gear

  1. #1
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    Default Cutting a gear

    This will be my first gear. A worm wheel. Replacing this:
    IMG_3184.jpg
    moulded Bakelite/Tufnol gear in a chicken rotisserie. The chef is tired of buying new gears, as they don't last long before being chewed up. So I will try to make one out of Brass or Bronze.


    First challenge was what to cut it with. The cheap way would be to grind some HSS, but that (plus finding a holder) sounded like too much work. So I went to McJing, and managed to buy a single involute cutter from one of their sets.

    Of course, it is a modern cutter. Metric 27mm bore. I have two "suitable" arbors that fit my mill:
    IMG_3183.jpg
    On the right is a slitting saw arbor. A series of steps, something like 15/18/21/24mm.
    On the left, an FMB style 1".

    Hmmm. Slitting arbor isn't really suitable. Not enough gripping depth.
    Either I turn up a thin sleeve/spacer for the FMB, or a solid parallel arbor to adapt it into an ER32 collet.


    Unless anyone in Sydney happens to have an MT3->27mm arbor?

  2. #2
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    Hi Nigel,

    ARC euro do MT3 30 mm blank end arbours, or they used to, I've not looked lately.

    But like me you would find a suitable piece of bar and turn it to fit into your largest ER32 collet, then with the arbour in the collet turn it to fit the cutter.
    Best Regards:
    Baron J.

  3. #3
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    Not the most straight forward thing to cut as a first gear, as there is the helix angle to take into account, as well as the tooth form.

    I'd agree with John - make up a straight arbor and hold it in an ER collet chuck.

    I'd also be wondering why the gears are always stripping - is the spit being overloaded, or the bearings that it runs on need a clean? One reason that gear may be bakelite is to limit heat running back into the motor from the spit so replacing with metal may help gear life but at the expense of motor life.

    My 2 cents.

    Michael

  4. #4
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    Quote Originally Posted by Michael G View Post
    I'd also be wondering why the gears are always stripping
    I suspect the motor and shaft are mis-aligned.

    I also wondered about the Bakelite. If it was something softer I would say minimising vibration, but this horrible brown stuff seems like it would not absorb anything.
    Anyway, the immediate need is a spare gear, and the customer wanted something tougher.


    I thought about the ER32 arbor, but decided on the smallest possible machining task with the smallest raw material even if it did require a little more precision a sleeve/bush for the 1" FMB...
    IMG_3185.jpgIMG_3186.jpgIMG_3187.jpgIMG_3188.jpgIMG_3189.jpgIMG_3190.jpg


    Now, almost ready to work out the helix angle:
    IMG_3191.jpg
    That table is currently set to 10 (er, 80 from horizontal), which is to steep. Probably around 5 or 6.

    Was thinking of mounting the existing (trashed) gear to this rotary, to check alignment against the cutter (feeler gauge? paper?). But I just realised I can calculate it from the worm gear! I can measure the diameter and pitch, and plug them into a calculator or web page
    Attached Images Attached Images

  5. #5
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    Quote Originally Posted by nigelpearson View Post
    Now, almost ready to work out the helix angle:

    ... I just realised I can calculate it from the worm gear!
    Just about to suggest doing it that way. It won't be much - I'd guess 3 to 5 degrees.

    Michael

  6. #6
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    Quote Originally Posted by nigelpearson View Post
    But I just realised I can calculate it from the worm gear!
    This is the only accurate way to do it. KHK has an excellent online calculator that's free to use, you just need to create an account https://khkgears.net/new/gear_calculator.html

    The camera angle might be playing tricks on me, but that worm looks much steeper than 20 pressure angle, so....yeah...might want to confirm that before you get too carried away.

    Also bear in mind that plunging with one of those cutters can never replicate a properly generated tooth profile in a worm wheel. It might look close (and it might be good enough), but it will not be the right shape.
    Gear cutting specialists and general engineers www.hardmanbros.com.au
    Fine pitch gear cutting from 0.1 Module www.rigear.com.au

  7. #7
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    Before I jump into Brass/Bronze, I'm going to try making one in wood.

    Start with an old MT3 arbor, making a rough spur drive:
    IMG_3202.jpg

    and then some timber from my a BBQ firewood bin (Gordonia prunings, badly dried) get Hercus'd to a 50mm dowel:
    IMG_3203.jpgIMG_3204.jpgIMG_3206.jpg


    I'll bore that tomorrow, mount on the dividing table, and start experimenting.


    Quote Originally Posted by elanjacobs View Post
    ...KHK has an excellent online calculator that's free to use...

    Thanks. The only other good page I found was SDP/SI:
    https://www.sdp-si.com/resources/ele...logy/page5.php
    and their calculations are based on knowing the lead and helix.
    (my brain is getting too old to reverse the trigonometry)

    The camera angle might be playing tricks on me, but that worm looks much steeper than 20 pressure angle, so....yeah...might want to confirm that before you get too carried away.

    No, your eyes are good.
    I selected the cutter based on the worn worm wheel, and only got the worm off the motor a few weeks later. The worm profile looks like a 30/60 or 35/55 to me.


    I could maybe re-cut the worm, or maybe plunge off-centre on the worm wheel to change the side cut angle.
    Or, I could just try with what I've got, and see how it meshes and laps in?

  8. #8
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    A crude way to check pressure angle might be to take a photo square to the axis that you can then print out and run a protractor over. Get as far back as your zoom lense will allow (less distortion).

    Michael

  9. #9
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    I'd guess 25-30. Not only is your first gear one of the trickiest options, it's not even a standard tricky option

    In order to calculate anything, you must have the axial pitch and the pitch diameter of the worm.

    You could try the off centre plunge to fiddle with the pressure angle. It'll wear itself in eventually, but with such a big discrepancy it will be point contact until it gets there and it will need to wear away a lot of material.
    Gear cutting specialists and general engineers www.hardmanbros.com.au
    Fine pitch gear cutting from 0.1 Module www.rigear.com.au

  10. #10
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    Default Wood worm wheel whittled!!

    Machined a hollow locating sleeve:
    IMG_3208.jpg
    12mm thru for threaded rod, 19mm to locate in the dividing plate, 16 in the gear blank.

    Set the plate tilt at about 3, and fine tuned it against the worm with a square (V block):
    IMG_3210.jpgIMG_3211.jpg

    Instead of edge finding and/or indicating, used a square against the plate to draw the centreline:
    IMG_3214.jpg

    which I will line up with the middle of the cutter for plunging:
    IMG_3249.jpg


    Then, rotate 20, plunge, rotate 20, et c.:
    IMG_3250.jpg


    I went around again, and made the cut a little deeper, so that the involute side just started to relieve the middle of the gear cylinder.
    End result:
    IMG_3251.jpgIMG_3252.jpg

    Not the best mesh. About half way down. Looking at the worm, it is lathe cut (judder marks), and non-symmetrical (different angles on the two sides), and has some burrs on the top land.

    Slightly wider spacing between the teeth seems like it would mesh better. I could do a slightly larger diameter (e.g. 53mm instead of 50mm)? Or I could try to cut the worm wheel deeper, which might get closer to the 30? Or I might also try to improve the shape of the worm.


    But, for now, I'm happy-ish with the worm wheel profile. So, trip to Edcon, $81 chunk of Brass from the offcuts bin, and into the lathe to make a real gear blank:
    IMG_3253.jpg

  11. #11
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    Default Shiny gears!

    The wooden gear was mostly meshing on one tooth of the worm, so I will try at 53mm, to see if that gives me better engagement.


    Lathe the big slug-o-Brass down to 53mm, bore it out,
    machine some grooves to guide the later cutting, and mount on dividing head:
    IMG_3254.jpgIMG_3255.jpg

    Had to adjust X/Z, because the larger "flange" made it a different size/length to my wooden mock up.
    Check I'm close to the middle, and plunge 20 times:
    IMG_3256.jpgIMG_3257.jpg
    I went fairly deep. Also upped the cutting speed. Maybe from 120 to 300. Mostly without problems, except when the gear started vibrating (started to work itself loose). Glad I wasn't trying this with something harder like Bronze.
    IMG_3258.jpg
    Note the middle now down to a thin point/web, and some cutter judder in one of the tooth valleys.

    Checked the worm in there, but engagement wasn't any better than the wooden gear version. Pitch looked a little too big. I guess +6% (50mm -> 53mm) was too much.


    Into a bandsaw to remove my 70mm diam. (to save it for another project).
    Had to use some V blocks. And I don't want them to rust. Almost forgot to turn the coolant off:
    IMG_3260.jpgIMG_3259.jpgIMG_3261.jpg
    (one size wasn't cutting true, despite the groove I had machined in there with the lathe!)



    So, carefully into the lathe, and turn other side down to about 51mm:
    IMG_3262.jpgIMG_3263.jpg


    Then back on the dividing head. Had to reset the X/Z. Cut another gear:
    IMG_3264.jpg
    Meshing is closer, but still not perfect. Will look closer in better light (out in the sun?)

    Back into the bandsaw to seperate these siamese (co-joined?) brother gears:
    IMG_3265.jpgIMG_3266.jpg
    If you look closely, you can see a step on the top (53mm diam.) gear.
    I was out by 1 on one tooth, and had started plunging before I realised, but...

    Nice and shiny!

    Feel like I have achieved something.
    My first 2 gears (4 if you count the wooden ones).
    Just gotta drill and tap for a grub screw.






    Next day spent a while looking closely in the sun. Worm wheel was bottoming out, on the web, between the worm wheel teeth. Worm was also a rough shape. Here is a 55 thread fish template:
    IMG_3267.jpg


    Here I am in the lathe, hoping to follow the thread with a few passes to clean it up, trying to get closer to that magical 20.

    Measure the pitch. 8mm. Look at my thread cutting gear settings. Max 7mm. Hmmm.

    Now, in a perfect world, I would use my neighbour's larger lathe. But they have gone home for the day, and when I look up the specs, it also maxes out at 7mm. Plus, I haven't put an ELS on my Hercus yet.
    So, I have to do it the hacky way:
    IMG_3268.jpg
    (hand turning the chuck with my left hand, filing the tooth profile with my right)
    That helped a little. As will a little bedding in time on the brass gears.

  12. #12
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    Curious to know how long it lasts
    Gear cutting specialists and general engineers www.hardmanbros.com.au
    Fine pitch gear cutting from 0.1 Module www.rigear.com.au

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