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  1. #76
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    Hey BT, standby for one of my out there ballpark no measurement ideas. Would it be possible to mount your blank in the Hoz spindle then cut the gear with the Y axis? then you could bore it to size in the same setting

    Stuart

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  3. #77
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    The horizontal spindle drives the vertical head Stu.

    033 (Large).JPG

  4. #78
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    Bob, FWIW I cut all my gears on mandrels between centres. Ideally you make a hardened ground mandrel with a very slight taper and push the gear blank on it firmly enough so it won't move.

    Naturally you cut so the cutting force pushes the blank *up* the taper not down it.

    This gives the best concentricity you're going to get in the average shop. Over time you end up with a nice collection of mandrels too.

    Most of mine are shop made and not hardened/ground and for one off jobs, perfectly serviceable. A lot have shoulders and threaded collars because I didn't feel like turning long shallow tapers but all were turned between centres.

    Except for the PITA factor of cutting prime number teeth, that gear is a really simple job with the correct gear cutter.

    PDW

  5. #79
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    Quote Originally Posted by Anorak Bob View Post
    The horizontal spindle drives the vertical head Stu.
    another great idea bites the dust lol

  6. #80
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    Quote Originally Posted by PDW View Post
    I cut all my gears on mandrels between centres. This gives the best concentricity you're going to get in the average shop.
    I must admit I always forget turning between centres. One day I'll make up the bush I need to do it but it's something like an extended B&S 9 (or is it 10) on the outside with a MT2 on the inside, so I'm unlikely to find one in a shop somewhere.

    Like the gear, except for the PITA factor of cutting accurate internal tapers, that adaptor is a really simple job with the correct compound setting...

    Michael

  7. #81
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    Bob, cutting the Schaublin thread isn't difficult and I can send you the gear ratios for the 260 if you want to do that (I'm away at the moment). The trick is not to think about it being 1.6667 pitch, and instead 1 2/3 and it all falls into place. I have an external tool ground I'll happy to send you too if you wish. I've never had need to cut an internal.

    I cut gears on a mandrel and have never had an issue with concentricity, but they have all been lathe gears so not super high precision. Nevertheless it wouldn't be something I'd be getting too hung up on.

  8. #82
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    Quote Originally Posted by Michael G View Post
    I must admit I always forget turning between centres. One day I'll make up the bush I need to do it but it's something like an extended B&S 9 (or is it 10) on the outside with a MT2 on the inside, so I'm unlikely to find one in a shop somewhere.

    Like the gear, except for the PITA factor of cutting accurate internal tapers, that adaptor is a really simple job with the correct compound setting...

    Michael
    Bit of a digression but still...

    If it's the plug for the DH spindle you don't need to have an internal taper, just the external taper to fit the DH then a male centre out far enough for the job. Easy turning job and you have a small cylindrical grinder to get the taper perfect anyway. I'm assuming your grinder table swivels.

    Same for the lathe actually - I often use a soft centre in a chuck then skim cut its point before using it. I know it's dead nuts that way and no risk of stacking tolerances.

    I wouldn't say I do a lot of work between centres but for anything requiring a few removals & replacements, it's a lot better than re-zeroing a 4 jaw and even good collets don't repeat quite as well. Besides, you're size limited with collets.

    PDW

  9. #83
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    If I use sacrificial blanks either side of the intended gear blank I'd probably opt for a shoulder and a nut on the mandrel. As for the between centres setup, I have a W20 centre, thanks to Phillip "Metalman", along with dog driver ( not a catch plate) for the DH.

    I popped apart the scope's focusing handwheels and snapped a few photos to better illustrate the problem.

    The outer gear is two piece. The rear section is connected to the left hand coarse focus handwheel. The right hand handwheel's gear is engaged only by the nylon fine focus gear. The later is mounted on an eccentric which rolls that gear around 119 tooth gear providing the fine feed.

    DSC_4897 (Large).JPG DSC_4875 (Large).JPG DSC_4874 (Large).JPG DSC_4873 (Large).JPG DSC_4885 (Large).JPG DSC_4886 (Large).JPG DSC_4890 (Large).JPG DSC_4892 (Large).JPG DSC_4871 (Large).JPG

    BT

  10. #84
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    Quote Originally Posted by Pete F View Post
    Bob, cutting the Schaublin thread isn't difficult and I can send you the gear ratios for the 260 if you want to do that (I'm away at the moment). The trick is not to think about it being 1.6667 pitch, and instead 1 2/3 and it all falls into place. I have an external tool ground I'll happy to send you too if you wish. I've never had need to cut an internal.

    I cut gears on a mandrel and have never had an issue with concentricity, but they have all been lathe gears so not super high precision. Nevertheless it wouldn't be something I'd be getting too hung up on.
    Pete,

    I'm not sure if the 260 gearing translates to a 9", probably does if they share the same leadscrew, but mine is imperial and while I have a set of metric transposing gears, whether those gears will provide a combination to suit the W20 thread, I imagine only you will know.

    Bob

  11. #85
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    Bob,
    After seeing the photos and while you are progressing to cutting a new gear, a suggestion to repair the existing one would be to stitch across the break with some fine stainless steel wire, maybe 2 or 3 loops. This would require several small holes in the undercut area, while dimensions are difficult to judge, it appears enough, maybe with some delicate use of the Dremel. Some form of adhesive would also be required across the break. The stitching gives the strength across the break as adhesive only would likely not be enough.
    The downside of course of a repair, is the thing breaks somewhere else around the edge as is typical of some plastics as they age. Alan.

  12. #86
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    Quote Originally Posted by Anorak Bob View Post
    Pete,

    I'm not sure if the 260 gearing translates to a 9", probably does if they share the same leadscrew, but mine is imperial and while I have a set of metric transposing gears, whether those gears will provide a combination to suit the W20 thread, I imagine only you will know.

    Bob
    Bob the gear combination will be purely a function of the leadscrew and the lathe won't matter. However I'd forgotten you had an imperial lathe. If you take 25.4 and divide it by the pitch in mm, you will get the tpi. In this case 15.24, so 15 tpi is extremely close, and would likely do it for a short thread like this. To select threads that aren't available in your gearbox, take existing combinations and change the drive or driven gear into the gearbox to change the ratio. So at the most basic example, to obtain 15 tpi on your machine, set up the gearbox for 5 tpi, then exchange the drive gear for one with a third the number of teeth the handbook instructs, and the driven gear on the leadscrew for one 3 times larger than the handbook instructs. The idler gears in between make no difference to the ratio, and can be exchanged at will to allow the other gears to fit.

    By doing this you can go through many different gearbox opportunities, and the gears you have available, to find combinations that will allow threads to be cut that aren't normally available. In my case I select 0.5 mm on the gearbox, and the gear combinations to make 1.6667 mm.

    Sorry if this is telling you how to suck eggs, but many people here may not be aware of being able to do this or how to go about it. Combined with the basic 25.4/pitch formula, it's possible to approximate, sometimes relatively accurately, a metric thread pitch on an imperial machine, and visa versa, without needing to go through a metric/imperial transposing gear.

    I regard the cutter, I presume you have one ordered now? You're no doubt already aware that it's possible to generate the form off a V cutter by manually hobbing the form? I've not done that myself, and wouldn't fancy doing so for a 107 tooth gear, nevertheless it is an option to keep in mind, and turning what is essentially just a circular rack isn't at all complicated. The gear produced will be true involute.

  13. #87
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    Quote Originally Posted by C-47 View Post
    Bob,
    After seeing the photos and while you are progressing to cutting a new gear, a suggestion to repair the existing one would be to stitch across the break with some fine stainless steel wire, maybe 2 or 3 loops. This would require several small holes in the undercut area, while dimensions are difficult to judge, it appears enough, maybe with some delicate use of the Dremel. Some form of adhesive would also be required across the break. The stitching gives the strength across the break as adhesive only would likely not be enough.
    The downside of course of a repair, is the thing breaks somewhere else around the edge as is typical of some plastics as they age. Alan.

    Alan,

    If a catch up tomorrow afternoon is OK, I will bring the scope along. The gear is an H in section. I had thought that a couple of opposing steel rings might pull the gear back into alignment? A photo of someone else's snapped gear -

    5+Nikon+S+Nylon+gear+cross-section.jpg

    Quote Originally Posted by Pete F View Post
    Bob the gear combination will be purely a function of the leadscrew and the lathe won't matter. However I'd forgotten you had an imperial lathe. If you take 25.4 and divide it by the pitch in mm, you will get the tpi. In this case 15.24, so 15 tpi is extremely close, and would likely do it for a short thread like this. To select threads that aren't available in your gearbox, take existing combinations and change the drive or driven gear into the gearbox to change the ratio. So at the most basic example, to obtain 15 tpi on your machine, set up the gearbox for 5 tpi, then exchange the drive gear for one with a third the number of teeth the handbook instructs, and the driven gear on the leadscrew for one 3 times larger than the handbook instructs. The idler gears in between make no difference to the ratio, and can be exchanged at will to allow the other gears to fit.

    By doing this you can go through many different gearbox opportunities, and the gears you have available, to find combinations that will allow threads to be cut that aren't normally available. In my case I select 0.5 mm on the gearbox, and the gear combinations to make 1.6667 mm.

    Sorry if this is telling you how to suck eggs, but many people here may not be aware of being able to do this or how to go about it. Combined with the basic 25.4/pitch formula, it's possible to approximate, sometimes relatively accurately, a metric thread pitch on an imperial machine, and visa versa, without needing to go through a metric/imperial transposing gear.

    I regard the cutter, I presume you have one ordered now? You're no doubt already aware that it's possible to generate the form off a V cutter by manually hobbing the form? I've not done that myself, and wouldn't fancy doing so for a 107 tooth gear, nevertheless it is an option to keep in mind, and turning what is essentially just a circular rack isn't at all complicated. The gear produced will be true involute.
    Thank you Pete.

    It would be a great thing to be able to replicate or probably more correctly, approximate the buttress thread of the W20. An ER collet chuck to suit the DH would be a hugely useful addition to the 13's tooling.

    I have not purchased a cutter or a set of cutters for the job. I had been thinking of grinding a single point tool using the Dumore TPG set up somehow on the tool and cutter grinder. The Dumore has a collet holder that I could use to hold a small mounted stone, Dremel size, in an attempt to grind something close to the correct involute form. ( and this could be whimsical dreaming )

    Any chance of expanding on the manual hobbing bit?

    BT
    Last edited by Anorak Bob; 29th August 2015 at 11:44 AM. Reason: Additional question asked of Pete.

  14. #88
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    Quote Originally Posted by Anorak Bob View Post

    Thank you Pete.

    It would be a great thing to be able to replicate or probably more correctly, approximate the buttress thread of the W20. An ER collet chuck to suit the DH would be a hugely useful addition to the 13's tooling.

    BT
    For something like this Bob I think an approximation would be more than adequate with just a tad more clearance provided. The difference between 15 tpi and 15.24 tpi is only something like 1.5% error, so I reckon you would be sweet. You may even be able to get closer with some creative gear combinations, and one tooth could make all the difference. There may be a smarter way to do it with a spreadsheet etc, indeed I'm sure there is, but I just sat down with the gears I had and the gearbox combinations, and tried find combinations I could do.

    If you get stuck and work out a perfect combination but don't have a gear for it, let me know and I'll cut you one when I get a chance.

  15. #89
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    Quote Originally Posted by Anorak Bob View Post
    If I use sacrificial blanks either side of the intended gear blank I'd probably opt for a shoulder and a nut on the mandrel. As for the between centres setup, I have a W20 centre, thanks to Phillip "Metalman", along with dog driver ( not a catch plate) for the DH.

    I popped apart the scope's focusing handwheels and snapped a few photos to better illustrate the problem.

    The outer gear is two piece. The rear section is connected to the left hand coarse focus handwheel. The right hand handwheel's gear is engaged only by the nylon fine focus gear. The later is mounted on an eccentric which rolls that gear around 119 tooth gear providing the fine feed.

    DSC_4897 (Large).JPG DSC_4875 (Large).JPG DSC_4874 (Large).JPG DSC_4873 (Large).JPG DSC_4885 (Large).JPG DSC_4886 (Large).JPG DSC_4890 (Large).JPG DSC_4892 (Large).JPG DSC_4871 (Large).JPG

    BT
    Great pictures, as always BT, I'm pretty sure I read that nylon expands and contracts with moisture, you could google it and find out what conditions would cause the nylon to expand enough to close the gap, then at that point you could insert two thin machined steel rings, into the H section grooves, one either side, to hold the nylon in place while it slowly dries out.

    Ray

  16. #90
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    Quote Originally Posted by RayG View Post
    I'm pretty sure I read that nylon expands and contracts with moisture
    Yes that's definitely the case Ray. The amount will depend on what grade of nylon it is. I knew nothing about any of this until getting into squirting the stuff around the place. We keep nylon filament in plastic bags with descant for this reason. The good news is that it's reversible, and you can dry nylon in a LOW oven to remove the water. I can't recall the dimensional change in percent, but I don't think it's much unless it's actually immersed in water.

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