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Thread: DSG worm gear I.D and manufacture

  1. #1
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    Default DSG worm gear I.D and manufacture

    Ok my first proper challenge 2017 to get my new DSG lathe up and running.

    The worm in the apron driving the power feed is finished (by finished I mean f-ck-d). For the moment I am going to call the worm wheel serviceable.

    Here they are:

    [IMG][/IMG]

    So i want to make a new worm and also a hob for cutting a new wheel in the future. (worry about the hob details later but obviously turn the basics in one piece)

    I have been previously advised that this is probably a diametric thread. On the worm a twin, two start thread.

    How do I measure the original to determine the diametric thread pitch needed to cut this on the lathe?

    I've tried to get my head around diametric thread stuff but please explain it like i'm 5 years old. Birthday in June so will be six then.

    Just to be clear, I think I'me ok with the the practical thread cutting procedure but it's clarifying the diametric pitch combined with the twin threads where i'm stuck.

    Thanks

    Al
    Last edited by blckbx; 01-02-2017 at 04:18 PM.

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    k, i'll go first.

    Diametrical pitch in its simplest way is just saying how big the gear tooth is. if you have 16 DP gears for example, its describing the size of the tooth. The actual profile/shape of the tooth will change as the number of teeth on gear changes (obviously with the same size teeth, a gear with a lot of teeth is much bigger in diameter than one with few), but they will all mesh perfectly and be the same size. So, to talk about gears, we use the unit diametrical pitch. Thinking that way lets us define all gear by their tooth size so our stuff meshes.....and it avoids us trying to cutt a gear of say 32.5 teeth.

    Gear sizes are determined by the number of teeth on them of a given diametrical pitch

    Of course DP = the number of teeth you can get on gear of 1" pitch diameter. The list bit, pitch diameter makes it a bit awkward, but its just the imaginary circular running through the middle of the teeth of the gear - ie if the gears were wheels instead of gears it would be the diameters where they both meet.

    So, 16DP, means on a gear 1" diameter of the imaginary line through the middle of the teeth, there will be 16 teeth on it. on a 2" pitch circle, there'll be 32, etc.

    You could express that gear as having 16 teeth around the imaginary line of 3.14" (pitch dia of 1 x PI). Or, each tooth have a lead/pitch of 0.196" (total length of 3.14" / 16 teeth).

    So here's the part where we really blow your mind......a rack or a worm is like a gear of infinite diameter. If you measure the pitch between two teeth, and its .196, its a 16DP worm! DP of the worm = PI/measured pitch, in this case 3.14/.196 = 16. Therefore to cut a 16DP worm (or a hob) you a lathe that'll cut a .196 pitch (or in your case .392, double the .196 for the two start). Fortunatly you have a DSG which will easily do that as well as the double start.

    The double start matters a great deal toward machining it, but not to determining the DP (afaik). Its like a thread profile - it doesn't change as the number of starts changes, only the lead changes.

    Since you have to make a cutter, I'd make a new worm as well. It's the identical set up

    clear as mud?
    Last edited by Mcgyver; 01-02-2017 at 03:31 PM.
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  3. #3
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    Quote Originally Posted by Mcgyver View Post
    k, i'll go first.

    Diametrical pitch in its simplest way is just saying how big the gear tooth is. if you have 16 DP gears for example, its describing the size of the tooth. The actual profile/shape of the tooth will change as the number of teeth on gear changes (obviously with the same size teeth, a gear with a lot of teeth is much bigger in diameter than one with few), but they will all mesh perfectly and be the same size. So, to talk about gears, we use the unit diametrical pitch. Thinking that way lets us define all gear by their tooth size so our stuff meshes.....and it avoids us trying to cutt a gear of say 32.5 teeth.

    Gear sizes are determined by the number of teeth on them of a given diametrical pitch

    Of course DP = the number of teeth you can get on gear of 1" pitch diameter. The list bit, pitch diameter makes it a bit awkward, but its just the imaginary circular running through the middle of the teeth of the gear - ie if the gears were wheels instead of gears it would be the diameters where they both meet.

    So, 16DP, means on a gear 1" diameter of the imaginary line through the middle of the teeth, has 16 teeth on it.

    You could express that gear as having a 16 teeth around the imaginary line of 3.14" (pitch dia of 1 x PI). Or, each tooth have a lead of 0.196" (total length of 3.14" / 16 teeth).

    So here's the part where we really blow your mind......a rack or a worm is like a gear of infinite diameter. If you measure the lead between two helix, and its .196, its a 16DP worm! DP of the worm = PI/measured lead, in this case 3.14/.196 = 16. Therefore to cut a 16DP worm (or a hob) you a lathe that'll cut a .196 lead (or in your case .392, double the .196 for the two start). Fortunatly you have a DSG which will easily do that as well as the double start.

    The double start matters a great deal toward machining it, but not to determining the DP (afaik). Its like a thread profile - it doesn't change as the number of starts changes, only the lead changes.

    Since you have to make a cutter, I'd make a new worm as well. It's the identical set up

    I would
    Sorry i think the post title was confusing. I am making the worm not the wheel. I just thought as i was making the worm I might as well make a hob to cut a wheel in the future

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    Quote Originally Posted by blckbx View Post
    Sorry i think the post title was confusing. I am making the worm not the wheel. I just thought as i was making the worm I might as well make a hob to cut a wheel in the future
    you captured all my typos lol

    its all the same - you asked for basics so I was trying to explain how it ties together. Measure the distance from one worm crest to the same spot on the next - that's the pitch. divide into 3.14 for the DP. You could measure across two teeth to get the lead directly, but it looks like you only have a bit on the end that is in good enough shape to measure

    Grind your tool for that profile, and to cut a two start thread and set the lead to be double the pitch (ie tool will advance the .392 per revolution. There are formulas out there that will let you figure out the thread wire measurements for it so you can get it dead on (based on the pitch diameter of the worm)
    Last edited by Mcgyver; 01-02-2017 at 03:34 PM.
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    So the pitch diameter of the worm wheel is approximately 2.8 inches and it has 34 teeth so the diametrical pitch is ?

    Please show your math (or maths in the UK)

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    Quote Originally Posted by Mcgyver View Post
    you captured all my typos lol

    its all the same - you asked for basics so I was trying to explain how it ties together. Measure the distance from one worm crest to the same spot on the next - that's the lead (if a single start). divide into 3.14 for the DP. You could measure across two teeth to get the lead directly, but it looks like you only have a bit on the end that is in good enough shape to measure

    Grind your tool for that profile, and cut a two start thread and set the lead to be double the pitch.
    Out of step here but the pitch on the worm measures almost exactly 0.25'' (so the lead for manufacture on the two start is 0.5 then). What has confused me is that an older post discussing this suggested that "it was unlikely that the pitch was exactly 4TPI as it was probably a diametrical pitch"

  7. #7
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    Think I'm getting it!

    So if the lead is 0.25'' the DP is 12.56 which is the same as 4TPI ?

    Edit DP needs to be a whole number? guessing thats down to my measurements.)

    DSG thread cutting table gives 12 or 13 in DP?

    I'm still confused: Can a thread defined as 4TPI also be described in DP?

    I feel a 'Eureka' moment is close but I haven't got there yet.
    Last edited by blckbx; 01-02-2017 at 04:28 PM.

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    Stage 1/ The DP for gears is just (teeth +2)/diameter. The +2 is a fiddle factor to account for the fact that you measured over the outside fo the teeth not the actual pitch circle. so (34+2)/2.8.
    Stage 2/ But for a worm it is the distance along the pitch circle per tooth that matters. So calculate the circumference as 2.8*pi (except you really want the real pitch circle dia not the outside of the teeth) so about 9inches for 34 (+2 fiddle) teeth or about 4tpi. (if it wasn't 2 start).

    But I approximated at the 2.8+pi =9 which is why it isn't exactly 4tpi.

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    Quote Originally Posted by Baz View Post
    Stage 1/ The DP for gears is just (teeth +2)/diameter. The +2 is a fiddle factor to account for the fact that you measured over the outside fo the teeth not the actual pitch circle. so (34+2)/2.8.
    Stage 2/ But for a worm it is the distance along the pitch circle per tooth that matters. So calculate the circumference as 2.8*pi (except you really want the real pitch circle dia not the outside of the teeth) so about 9inches for 34 (+2 fiddle) teeth or about 4tpi. (if it wasn't 2 start).

    But I approximated at the 2.8+pi =9 which is why it isn't exactly 4tpi.

    OK but the teeth on the worm wheel are concave by design to mesh with the worm better? The maximum diameter measurement on the worm wheel teeth externally are 3'' on the outside edge and 2.9'' on the centre line. The width of the gear is 0.86''.

    Measuring the TPI on the worm its so close to 4TPI that I find it hard to believe it isnt. This doesnt mean that someone could have made a new worm incorrectly making the same assumptions as I have.

    Accurate PD measurement on concave gear teeth?

    This is why I'me trying to define the new worm off of the worm wheel dimensions.
    Last edited by blckbx; 01-02-2017 at 04:59 PM.

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    Quote Originally Posted by blckbx View Post
    Think I'm getting it!

    So if the lead is 0.25'' the DP is 12.56 which is the same as 4TPI ?

    .
    yes, if the lead really was .2500", but its dangerous to think of it as tpi, because most of the time it won't be a whole number. if it looks like .25", 3.14/.25 = 12.56 DP.....(assuming it is DP) its not that. But we get a hint.......try 12DP... 3.14/12 - .261 pitch. if you just using the mark I eyeyball, that .250 could very well be .261". If you think the gear is standard, you do an forward and back calc like that and it suggests pretty darn strongly its 12 DP

    if that's it, and it sounds like it, you need to cut the hob at 3.821 teeth per inch (that's why the DSG is nice - lets you do it). Actually need to double the pitch because of the two start. (which the DSG also has built in stuff for)

    Can a thread defined as 4TPI also be described in DP?

    yes, but it doesn't do much for you (if you are trying for standard gears) 4 TPI would be 12.56 DP, so no, not a normal gear. You could make though, it just wouldn't mesh with anything standard. It might become apparent pondering this: 4 tpi is an even number for a linear inch. But gears, because the teeth are around a circumference, involves PI. For a 1" (pitch circle) diameter 12 DP gear.....there are 12 teeth in 3.14 inche long circumfrence (3.14 x 1" pitch diameter). So its not going to work to an even number of tpi, which why for lathes its pita to cut.
    Last edited by Mcgyver; 01-02-2017 at 05:32 PM.
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