Thread cutting, making your own sizes?

  1. earthman Member

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    It is the only way, don't rush etc,.....it's really not that daunting.:)
     
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  2. 123hotchef Member

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    Er it is very daunting mate I can control.a tap or.die easily.
     
  3. Hopefuldave Intergalactic pot-mender

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    If you leave the halfnut engaged you can cut any pitch you can gear for without using the indicator - the table tells you which gear you need on the bottom of the indicator shaft (you should have 14, 15 and 16 tooth for it) and where you can engage halfnut for each thread pitch; my own lathe's destruction book recommends cutting everything with the halfnut engaged unless it's a *very* long thread, including imperial, metric, DP and module, plus the BA oddballs which most folks would use taps and dies for :) It does have niceties like a quick-retract on the cross-slide and a proper mechanical brake/clutch 2-speed box to let it reverse at 4x forward speed though...

    Dave H. (the other one)
     
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  4. earthman Member

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    What lathe do you have? I'm only going by mine obviously but I kept my hand on the on/off knob, turning the motor off or disengaging the lever, that's all you have to do really,......the whole operation is done at low speed so just keep your eye on the ball, after the first couple of passes, it's no so daunting honest.:)

    Next time I'll have more practice on scrap steel, make a through hole for example rather than a blind hole, that sort of thing. Make things a bit easier for yourself.
     
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  5. earthman Member

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    Thanks, I understand I think,......now if my lathe came with a 14 and 15 wheel indicator, I could just fit either one of those depending on the pitch of thread I were about to cut, one of those would tally up marking wise. I'm not that bothered, I'll be sticking with keeping the carriage engaged and turning the motor on/off I think, just don't feel comfortable with that lever disengaging at the mo.
     
  6. Hopefuldave Intergalactic pot-mender

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    Exactly, but if you need to cut imperial (tpi) threads you can't use the indicator as inch and metric threads have a different basis - tpi is in turns per length, metric in length per turn (hope that makes sense!), so you'd have to keep the halfnuts engaged like you're doing now. There's a technique when you need to stop at the end of the thread () shown for cutting metric in an imperial lathe that also works exactly the same for cutting imperial on a metric lathe, worth a quick (16 minute) watch, that can make things a bit quicker / less frantic when cutting up to a shoulder etc.

    Dave H. (the other one)
     
  7. earthman Member

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    Thanks for the link, I have seen some other videos from that guy but not that one.:)

    I'm getting way ahead of myself but I didn't think it were possible to cut imperial threads on my metric lathe period, surely the plastic cogs (sorry gears:laughing:) supplied are only suitable for the metric sizes mentioned in that chart??
     
  8. Pete.

    Pete. Member

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    Sure you can cut imperial threads on a metric lathe. Whether you can do it with the supplied gears depends on what you have. For perfect conversion (also called translation) you'd use a 127 tooth gear along with several others but you don't NEED one strictly for metric-inch translation. There are many combinations that will do the conversion with an error so small that it's less than the accuracy of the leadscrew.
     
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  9. Dr.Al

    Dr.Al Forum Supporter

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    I've got a calculator on my website for the combinations you can do with standard mini-lathe change gears (and another one for the M250). If the gears don't correspond to what you've got and your lead screw, it's really easy for me to add other combinations. Let me know if that's helpful.

    There's also a calculator for thread dial indicator use: that should work for any lathe if you set it up right.
     
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  10. Pete.

    Pete. Member

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    Those two calculators are VERY handy. Nice work.
     
  11. 123hotchef Member

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    I have a Boxford
     
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  12. earthman Member

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    Very good.:)

    You know a lot about these little machines, do you always refit the standard larger gears after you are done with threading or just leave them in place? Are the standard ones designed for the perfect speed for power feed cutting? I've always turned the carriage by hand when turning stock down to be honest,......am I right in saying that which ever cogs are fitted, they only get used when the lever is engaged so it shouldn't matter which ones I leave on the machine??
     
  13. Pete.

    Pete. Member

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    Threading is almost all more coarse than turning. If you're going to use the leadscrew for power feed you'll want the gears for fine feed installed. If you're doing all your turning by hand the best thing to do is disengage the gears and save on wear.
     
  14. eLuSiVeMiTe

    eLuSiVeMiTe Member

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    Very fortunate to have a reduction box between the drivechain and leadscrew for a quick switch between screwcutting and surfacing.

    Will be picking you and others brains when I finally start on adding a gearbox.
    Don't have the experience to draw on.
     
  15. earthman Member

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    I'll have to try turning via power feed now.:)

    Those gears should provide the perfect finish then?....depending on how fast the motor is turning?? I still have a lot to learn.:)
     
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  16. Hood

    Hood If it walks like a duck....

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    With a lathe you want to ideally keep the chip load constant no matter the spindle speed. In other words your feed is per rev so engaging the power feed will always result in the same feed per rev no matter the spindle speed, until you change the feed gearing of course :)
     
  17. Pete.

    Pete. Member

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    No the fine feed won't give a 'perfect' finish but it'll give a better one or one that's at least visually perfect. The only difference between cutting a thread and turning a finish on your machine is the shape of the tool, and the timing of where it starts cutting. If you put a round tipped tool in the holder and feed it slow it'll look nice and shiny and to all intents it WILL be a nice finish (if you're lucky) but it will still be a spiral groove like a thread, just a very very flat one.

    You can also go too slow and ruin the finish. If you're using a carbide tool the feed needs to be at least the amount of the edge radius (or sharpness of the cutting edge). Same for HSS but you sharpen that yourself.

    https://www.cadem.com/post/cnc-turning-surface-finish-nose-radius

    https://www.bigkaiser.com/en/applic...ols/demystifying-insert-nose-radius-selection
     
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  18. Dr.Al

    Dr.Al Forum Supporter

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    I'm not sure I'd say "the perfect speed": it depends on how deep your cut is. Having said that, when I use the mini-lathe for turning (as opposed to threading), I leave the 20/80/20/80 combination in place and do most turning under power.

    I found the ratio on the hand wheel was too coarse to get a good finish. That might juot be down to my cackhandedness though!

    If you are turning by hand though, it'll save the wear on the leadscrew and in that case you can leave any combination of change gears in place.
     
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  19. Dr.Al

    Dr.Al Forum Supporter

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    672
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    Agree with all of this.

    For the best finish I've ever achieved on the mini-lathe, grind a round nosed HSS tool like the one shown in Harold Hall's excellent book "Lathework A Complete Course". Then take a fine cut on slow power feed.

    On my big Harrison lathe I get a better finish with a CCGT insert (or CCMT on stainless or tough steel) taking a deeper cut, but the lathe's more powerful than the mini-lathe and you generally have to work carbide tools a lot harder than HSS to get a good finish.

    The best thing to do is play around as much as possible with different diameters and materials of scrap and see what works for you.
     
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  20. Dr.Al

    Dr.Al Forum Supporter

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    Glad you like them!

    If you think of things that could be improved, do let me know.
     
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