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  1. #46
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    As Bryan says, you would cut the radius on the face first. It's for clearance so the exact form would not matter. However, I can see several issues with using a fly cutter for the 'tooth'
    • A worm is a helix. For a small helix angle a circle (that is, the tip of a fly cutter) would approximate a segment of helix but the smaller diameter the helix is, the greater error
    • Simon hasn't passed on any measurements yet, but I'm guessing that the worm is 25mm or less in diameter. That's not much room to swing a fly cutter. Remember the worm wheel is an involute tooth form, so the cutter will need to be full form. A hob has a straight sided tooth and generates the appropriate worm wheel profile as part of it's action
    • Lastly (and perhaps most importantly) trying to hob a worm gear (stepping bravely into the unknown) sounds like more fun to me...


    I was thinking about this while getting ready for bed last night and one thought that did occur was whether the worm tooth angle was 29 degrees (imperial) or 30 degrees (metric). Being a German machine it could well be based on some metric form for an extra degree of difficulty.

    Every reference I've looked at so far has pointed out the importance of keeping worm gear sets lubricated. I suspect that as soon as the sight glass was broken and the oil escaped, metal on metal contact between the worm and gear started and without lubrication just accelerated. Replacement value for that saw is probably several thousand, so the failure to replace a part worth (probably) less than $50 can really hurt (Oh, and having fools who don't maintain machines properly)

    Michael

  2. #47
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    Hi Bryan, Shed, Michael & Ueee,

    Thanks Ueee, I hope it still turns out to be a bargain!

    Thanks guys for taking the time to enter into the discussion. I too have since re-read my book on gear cutting and it is now apparent to me that worm and gear cutting is not so trivial!

    I have been formulating a plan in my mind and I think it's similar to what you guys have come up with. Turn the blank to correct diameter on the lathe and pre-cut the radius profile. Then take it to the mill, set the mill head to take into account the helix angle and feed with the Y axis, use a gear cutter to cut the teeth and then finish it off with a hob on the mill.

    Once I work out the modulus of the worm, I'm hoping to just purchase the gear cutter, but the hob I'm hoping to be able to make. Would a mild steel hob work on brass or bronze or would I need to make it out of some silver steel and then heat treat it?

    If I'm too cheap to use bronze, will brass do, or even aluminium (at a stretch)?
    I see if I can take some measurements today.

    Cheers,

    Simon

  3. #48
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    You really want something that will hold an edge, so MS is probably not the best. The best material to make the worm gear out of would be phosphor bronze. I find that with PhBr you really need to keep the tool sharp. It depends on what grade of Al you get, but one of the softer ones may literally stretch when loaded. Brass will probably work.

    Remember too that the hob pitch will be the circular pitch of the worm wheel - it's unlikely that you will be able to dial that up on your lathe so you may need to make/ get a change wheel or 2.

    Michael

  4. #49
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    Michael, is your CVA worm-capable? I would have thought so but can see no mention of it on the lathes.co.uk CVA page, nor the 10EE one. My Graz is equipped for cutting worms so consider that option available.

  5. #50
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    Thanks Bryan!

    Oh boy my learning curve with all this has gone into overdrive! I thought all lathes could du worms, thought it was just the tool geometry and the correct pitch to be chosen? Oh boy

    So I have some basic measurements. They are very approximate due to a number of factors, being damaged gear and limitations on my measurement skills on the worm.

    WORM:

    Inner diameter (root) 24.0mm
    Outer diameter 36.5mm
    Pitch 10mm

    GEAR:

    No. teeth 40 (definitely got that bit correct!)
    Outer diameter 122mm

    Working back I worked out that the gear module must be between 3 - 3.5 being closer to 3.5. What are your collective thoughts on this?

    Cheers,

    Simon

  6. #51
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    Quote Originally Posted by Michael G View Post
    So first I'm to blame and now can I help? Wow Simon - you really know how to make friends!

    Yes I wouldn't mind having a go at cutting one (once I work out how). I think I'd have to make a hob up as Bryan suggests. To start with as well as the number of teeth on the gear I think you'd also need to know major and minor diameters of the worm; the worm screw pitch and the axis to axis distance between the worm axis and the worm gear axis.

    Michael
    Hi everyone don;t quote me but I recall a long time ago a worm wheel being hobbed after teeth gashed with a coarse tap tap between centres and it rotated the wheel to be cut until depth of cut completed one would assume [[ that word] that the no of teeth on worm wheel matched or was divisible in relation to the tap.

  7. #52
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    Quote Originally Posted by Michael G View Post
    So first I'm to blame and now can I help? Wow Simon - you really know how to make friends!

    Yes I wouldn't mind having a go at cutting one (once I work out how). I think I'd have to make a hob up as Bryan suggests. To start with as well as the number of teeth on the gear I think you'd also need to know major and minor diameters of the worm; the worm screw pitch and the axis to axis distance between the worm axis and the worm gear axis.

    Michael
    Hi everyone don;t quote me but I recall a long time ago a worm wheel being hobbed after teeth gashed with a coarse tap tap between centres and it rotated the wheel to be cut until depth of cut completed one would assume [[ that word] that the no of teeth on worm wheel matched or was divisible in relation to the tap. oh I did forget use a straight mineral oil 140 sae used for worm and wheel rear axles ep 90 eats the phosphur bronze. you need hi viscosity oil for the hi pressure between teeth Cheers John my 2 bobs worth.
    Last edited by j.ashburn; 18th May 2013 at 11:28 AM. Reason: more to add

  8. #53
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    Hi simon

    have you actually tried to source a replacement gear form the local bearing joint it may be a standard size and there for and easy fix


    cheers
    harty

  9. #54
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    Hi Ashburn,

    yes that's what I'm thinking of. I'm just trying to work out a few thing before that process starts:

    OD of the blank, Module of the gears so I can purchase the correct gear cutter and the best way to go about making the hob (If I can)

    Also, news flash:

    My pitch measurement of 10mm is incorrect. I re-measured over it's entire length and it's about 47mm for 5 threads. That makes it about 9.4mm. I don't know what to make of that. Now i know even less than I did before. Not sure if it's an imperial (would have thought not likely) or some weird special metric pitch. Why would a bandsaw need a very special pitch? surely not. Surely the maker would just use standard tooling to cut the gears and any special ratio requirements could be done with the pulley and belts, where the motor is connected to the gearbox?

    DUNNO!

    I'm just starting to get my teeth (pun intended again!) into this and now I gotta get all prettied up for (yet another) wedding!

    My family are used to seeing me with hands with ground in dirt!

    Cheers,

    Simon

  10. #55
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    Quote Originally Posted by harty69 View Post
    Hi simon

    have you actually tried to source a replacement gear form the local bearing joint it may be a standard size and there for and easy fix


    cheers
    harty
    Hi Harty,

    Not yet. There is a service plate on it with a number. I was hoping to give it a ring but it's a very old number. I will still give them a try. One half of me says yes, just buy another gear, the other part of me says NO! making one will be an really educational exercise. It really depends though, if it's technically outside the scope of my expertise then I'm probably dreaming and all it will end with is me being frustrated and the cost of a blank (probably anything up to $50) being ruined. It's a fine line between pushing and challenging myself and being realistic!

    I always tend to have a "can do" attitude and feel I can do most things if I take the time but sometimes I think I get out of my depth. It takes just as much wisdom to know when it's out of my league as it does to do the job!

    Cheers

    Simon

  11. #56
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    It's not a special pitch as such - it just has to match an integer number of teeth on the wheel. As that number Pi is involved with the circumfrence, it comes out in circular pitch calculations.

    Bryan, both the CVA and Monarch can cut worms but it is not built in. A bit of gear swapping will be required.

    Michael

  12. #57
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    OK. According to page 88 of Ivan Laws' book on Gears and Gear cutting, a linear pitch of 9.42 is mentioned and it corresponds to a Module of 3 which is in agreement with previous measurements. I may be getting somewhere here.... or on a wild goose chase!

    This also corresponds to a tool depth of cut of 6.47mm which is also in agreement with my worm measurements of 24.0mm inner and 36.5mm outer, give or take a little allowing for my inaccuracies in measurement.

    I'm on my way to working out the OD of the required blank. I'm yet to see what the closest pitch to 9.42 I can create with my existing gears....

    I also rang the local steel place for a price on 100mm length of 150 diameter Phosphor Bronze bar. It's a special order and they will have a price for me on Monday. Can't wait to see that one! I may end up making it out of cheese Bless'd all the Cheese makers!

    Simon

  13. #58
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    Simon, just did some maths.
    A gear with a circular pitch of 9.42mm is M3 (it's that pesky Pi thing again). Cross checking with the major and minor diameters of the worm it looks about right.

    Michael

    Damn. Took too long.
    What thickness of blank is needed?

  14. #59
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    Ueee is offline Blacksmith, Cabinetmaker, Machinist, Messmaker
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    If the price of the bronze bar is too much, you should be able to get bronze bearing stock, although this will be softer. You could make the gear with a steel center and a bronze ring around the outside. The other thought is what about cast iron? Some 4e wouldn't be that pricey, and I would think it would wear ok. Not sure on the groups wisdom on that one.....(ducking for cover)
    At least a trial run from CI might be a good idea if you do go bronze.

    Cheers,
    Ew
    1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.

  15. #60
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    What ratio is your gear box Simon ? if it is 20:1 then I'll pull this to bits and measure the worm wheel, the output shaft measures 24.93mm

    Tiz a punt, but who knows
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