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  1. #1
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    Default Help needed on lathe threading gears

    Hello Metalworkers,

    I would like to cut some threads on my lathe and while I can follow the charts provided to cut standard metric thread pitches I would like to cut ones that are not listed, specifically a 3.1416mm pitch (i.e. Mod 1), unfortunately I have no clue at all on how to determine what I need to set gears to. I would like to learn how to fill in the gaps in the charts and learn how to determine what numbers of teeth etc I need so I can cut gears myself (which I suspect I will need to for a Mod 1 pitch).

    Mostly I do not understand what my A,B,C and I, II, III knobs do, my readings on gear ratio's etc have not help me with this.

    I have posted pics of my lathe chart and gears knobs if that will help. The 30/100 gears are also removable from the banjo so I don't know why these seem to be shown as fixed in the diagrams

    Any help/explanations would be appreciated

    Eric

    PS are there and clubs or gatherings in the north east suburbs of Melbourne to attend? say around Heidelberg/Bundoora?
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  3. #2
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    Default

    What pitch is the leadscrew?
    Light red, the colour of choice for the discerning man.

  4. #3
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    Quote Originally Posted by ericg View Post
    Mostly I do not understand what my A,B,C and I, II, III knobs do, my readings on gear ratio's etc have not help me with this.
    If you look at your first photo, the top chart has a column with AI, AII, AIII etc in it
    Help needed on lathe threading gears-img_20150602_192307-jpg
    So if you select A and I for example, combined with a 63/63 gear combination you have a feed rate of 0.22(traverse) or 0.05 (facing) mm per revolution.
    The second chart uses the same knobs as part of the thread pitch selection.

    Michael
    Last edited by Michael G; 9th Jun 2015 at 08:03 AM. Reason: added traverse and facing

  5. #4
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    Default

    Ah yes, I forgot to mention leadscrew pitch, it is 2mm.

    What I'm after is the maths involved on how to determine thread pitches not listed, or even how the ones listed are derived at from the gears and the control knob settings.

    Perhaps I'm missing the obvious however?

    Eric

  6. #5
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    I think you need a 1.57:1 ratio...

    Or you had a 157 tooth gear driving a 100 tooth gear and select the QCGB to 2mm pitch thread you would cut a 3.14mm pitch thread?

    Someone else can work out the compounding gear train...

    Of course I the question is could your lathe cut a thread where the pitch is greater then the leadscrew pitch...
    Light red, the colour of choice for the discerning man.

  7. #6
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    Default

    Quote Originally Posted by .RC. View Post
    I think you need a 1.57:1 ratio...

    Or you had a 157 tooth gear driving a 100 tooth gear and select the QCGB to 2mm pitch thread you would cut a 3.14mm pitch thread?

    Someone else can work out the compounding gear train...

    Of course I the question is could your lathe cut a thread where the pitch is greater then the leadscrew pitch...
    What are the factors one has to consider in order to answer that question please ?

    I have seen hints on a few web pages that you can put too heavy a load on a lathe with an inappropriate gear train. However I've never seen an explanation regarding how to assess beforehand what might be a demanding load.

    I have the impression that turning the mandrel by hand is a possible workaround is this true ?

    I'd hate to hurt my lathe through avoidable ignorance.

    Bill
    Last edited by steamingbill; 9th Jun 2015 at 12:54 AM. Reason: added manual turning option

  8. #7
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    Eric,
    Below attached is a chart I threw together that should be correct for your lathe based on the following assumptions:
    1. Leadscrew is 2mm pitch
    2. the 30 and 100 tooth gears in positions B and C are not changeable.
    3. The QCGB ratios based on the chart you supplied are correct.
    4. The change gears supplied are only what is shown on your supplied chart.

    Weiss Pitch Chart.pdf

    Of course, if the 30 and 100 tooth gears are changeable, then there are many more options available for screw cutting.

    Andrew

  9. #8
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    Default

    Quote Originally Posted by ericg View Post
    Hello Metalworkers,

    .......... The 30/100 gears are also removable from the banjo so I don't know why these seem to be shown as fixed in the diagrams
    I imagine its merely to keep things simple for that chart.

  10. #9
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    Quote Originally Posted by steamingbill View Post
    What are the factors one has to consider in order to answer that question please ?

    I have seen hints on a few web pages that you can put too heavy a load on a lathe with an inappropriate gear train. However I've never seen an explanation regarding how to assess beforehand what might be a demanding load.

    I have the impression that turning the mandrel by hand is a possible workaround is this true ?
    The issue is just the mechanical advantage that the screw has. The normal 'rule' is that you should not cut a thread coarser than the leadscrew (that is, if the leasdcrew is 4tpi, don't cut 3tpi or less) unless the threading chart or instructions say otherwise. The force acting on the leadscrew comes from the tool, so if you are taking light cuts instead of the DOC that you would usually take then that's less strain on the leadscrew. If turning by hand you can't physically put more than around 3/4 hp, so you would need light cuts anyway.
    Thinking through the transmission chain, the weak link is probably the half nuts, especially if being engaged/ released continually.

    Michael

  11. #10
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    Default

    Thanks for the replies folks,

    Andrew, that chart is what I would like to learn to do for myself, do you have the calculations you used?
    Also (me being a totally ignorant noob) I do not understand the QCGB ratios, where does that come from? is that the AI, BII etc knobs? how is that ratio found?

    Sorry to sound so ignorant but this is the first lathe I have ever owned or used, so I know less than a first year apprentice, but it is fun to learn (gotta keep the 60+ years grey matter exercised)

    Eric

  12. #11
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    When I aquired my lathe a few years back, I was in a similar situation. Bloody hell I didn't even know what the tailstock was! The owners manual came with a parts break down including a schematic of the drive chain including all gears in the QCGB. From here it was a matter of working out ratios and making up an excell spread sheet. All I really ended up doing was re-inventing the threading chart on the lathe, but at least I knew how it all came together.

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  13. #12
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    Quote Originally Posted by steamingbill View Post
    What are the factors one has to consider in order to answer that question please ?
    pitch to cut ................ driver gear
    ------------ =.............. ----------------
    pitch of leadscrew ....... driven gear


    That is your gearing ratio.. Ignore the full stops as I had to put them in as the forum software is retarded and will not allow you to have big spaces..
    Light red, the colour of choice for the discerning man.

  14. #13
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    Eric,

    Spreadsheet attached. The calculations are in the spreadsheet cells. The QCGB ratios were calculated back from the picture of the label you supplied.
    Weiss Pitch Chart.xls

    If you can change the 30 and 100 tooth gears (B and C), you can set up a chart for each new ratio in that place in the gear train.
    Can you list all the change gears (including duplicates)you have with the lathe?

    Andrew.

  15. #14
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    Default

    Thanks for the spreadsheet mahgnia,

    I can move the 30/100 gears as they are designed to be easily removed from the banjo.

    I have all the gears as listed in the chart

    I'm still confused about the QCGB ratio's, how are they derived and from where on the chart?, I can see that under the 70/56 ratio column all but B1 is listed, but why would that ratio be used? or am I missing something basic and obvious here?

    Sorry to be so thick headed,

    Eric

  16. #15
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    Quote Originally Posted by ericg View Post
    Thanks for the spreadsheet mahgnia,

    I can move the 30/100 gears as they are designed to be easily removed from the banjo.

    I have all the gears as listed in the chart

    I'm still confused about the QCGB ratio's, how are they derived and from where on the chart?, I can see that under the 70/56 ratio column all but B1 is listed, but why would that ratio be used? or am I missing something basic and obvious here?

    Sorry to be so thick headed,

    Eric
    Eric,

    I can update the spreadsheet to show pitches that result from other combinations of gears in B and C if you need.

    Working back from the chart (using the screwcutting values, as the feed values are rounded), the ratios are as follows:

    I/II = 1.75
    II/III = 0.4
    I/III = 0.7
    A/B = 0.4
    A/C = 0.8
    C/B = 0.5

    Given that you have a 2mm pitch leadscrew, and the shown pitch with the QCGB in position BIII is 2mm, then the QCGB ratio in BIII is 1:1, and all other QCGB ratios can be worked back from that to give (calculate in this order):

    BIII = 1
    AIII = 0.4 (BIII ratio x the A/B ratio)
    BI = 0.7 (BIII ratio x the I/III ratio)
    BII = 0.4 (BIII ratio x the II/III ratio)
    CIII = 0.5 (BIII ratio x the C/B ratio)
    AI = 0.28 (AIII ratio x the I/III ratio)
    AII = 0.16 (AIII ratio x the II/III ratio)
    CI = 0.35 (CIII ratio x the I/III ratio)
    CII = 0.2 (CIII ratio x the II/III ratio)

    each of which multiplied by the actual leadscrew pitch of 2mm will give an effective leadscrew pitch for that QCGB ratio.

    Hope this makes sense.

    The missing combinations on the original chart are for non-standard or duplicate metric pitches, so they aren't shown.

    Andrew

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