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  1. #1
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    Default Setting a cutting angle question

    I cut my first thread last night on my 9" Hercus AR . First a bronze test run 5/8 11 TPI . It worked so I then did the same thread on 4140 3/4 x 3/4 bar for a part of a machine I'm restoring .
    I was so happy I sat it on the bedside table for the night so I could keep having a look .
    The cutting tool was just moving right to left which I suppose is the basic way of cutting thread like that. I managed to get the threading dial worked out as well . Just small steps with that so far.
    The other way I see threads cut is to bring the tool in each step along an angle so that half of the thread angle face ( right Name ?) is cut each pass . Rather than both . Giving better finish. So I think for a 55 degree thread angle the compound moves at an angle of 27.5 degrees each pass.

    Ive had a little look on YouTube but most demos there they have degrees marked on the cross slide . My Hercus just has a couple of 0 marks for 90 or 180 ? Im not in front of My lathe atm but there is no full 360 increments . I dont know if that's how I would set 27.5 If I did have it . I assume that is the way .

    How do I accurately set the compound to move at 27.5 degrees if I want to try cutting the next thread that way ?

    I imagine I could have a protractor on the work being held or a temporary bar held in the chuck but that still doesn't solve what I would line it up to on the compound slide . There is only the tiny side of the cutting tool ?? One of the cutting edges of the 1/4" HSS.

    Rob

  2. #2
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    I've never bothered to use the compound slide method, but in my understanding it's not necessary to be super accurate, as long as you err slightly towards not enough rather than too much. As in, assuming 0 is with the compound parallel to the spindle, shoot for 27 to 27.5. I think the protractor against the compound would be fine (retract the compound and use the exposed dovetail if there are no machined flat surfaces on the side of the compound, chuck a dowel pin in against the dovetail if needed, maybe a 123 block to get you up to centre height).

    If you do want to do it 'properly', and don't have a sine bar, do the trig and set it up with a dial indicator off a test bar, a machined bar in the chuck or the tailstock quill. Work out how much movement you need to see on the dial indicator for say 75mm (or whatever works) of compound travel, and adjust the angle until you get it.

    I assume you realise this and it's what you need, but 55 degrees is Whitworth, 'standard' imperial and metric are both 60 degrees...

  3. #3
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    Thanks J&H

    Yeah its Old English woodworking machinery restoration so Whitworth is what Im doing .

    I have seen some of the YouTube videos of very accurate dial indicator methods since first posting asking here . Id have to replay them 8 times to understand I think . I would if I had to or if it was the only way .

    If super accuracy is not necessary as long as its under, and simple is what I want then maybe I scribe the angle on a suitable brass sheet with a straight back edge and holding that to the work or Bar then adjusting the compound until the tool follows that line . That's the same as the protractor but easier to hold up or set in position maybe ?

  4. #4
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    Quote Originally Posted by auscab View Post
    I have seen some of the YouTube videos of very accurate dial indicator methods since first posting asking here . Id have to replay them 8 times to understand I think . I would if I had to or if it was the only way .
    Pretty straightforward really, the hard part is doing the actual trig work. But you can use online calculators like this: https://www.calculator.net/right-tri...erv=&x=89&y=20

    Should come up already filled in with 'c' (the compound travel) as 20mm, and angle 'β' as 27.5 degrees. You can see that it's calculated side 'a' to be 17.74mm. So when you set the compound to 27.5degrees, 20mm of travel on the compound must give an increase in reading on the dial indicator of 17.74mm if you're measuring off a test bar/machined workpiece/tailstock. Obviously this will require a long travel indicator, however it also gives you side 'b', so you could use an indicator on the face of the chuck, and in that case 20mm of compound travel must give a increase on the dial indicator of 9.235mm.

    For 'rough' work and a permanent easy quick reference, you could always take the compound off, use a protractor or degree wheel as a reference and scribe a bunch of lines on the top of the cross slide as suits your needs (maybe every 5 degrees, more if you have space and can be bothered?) Could be well worth the time spent in terms of later convenience. I'd at least cover the 90 degrees between perpendicular to spindle (handwheel to front of machine) and parallel to spindle (handwheel facing tailstock), and probably also the 90 degrees from parallel to spindle (handwheel to tailstock) to perpendicular to spindle with handwheel facing the back of machine. For most purposes no practical use in marking the other 180 degrees out, exception being if you're likely to run a long bar and need to cut a taper on the tailstock side of the carriage.

    Keep in mind if using the angle marks on the compound, the angle you actually want for this purpose depends on where the '0' marking is on your compound. If it faces the front or rear of the machine, you're actually going to want to rotate around to 62.5 degrees (90-27.5). If the '0' mark faces the spindle or tailstock, then yes you'll want 27.5 degrees. If you look at the link I gave, you should see a gray triangle below the results. Swap the 27.5 angle from 'β' to 'α', and hit calculate, and you'll see the triangle change shape - side 'c' represents the path of the compound.

    So if your '0' faces the front, now put 62.5 in as angle 'α' and you'll see the triangle will change back to the shape it first was, which is what you'll want, and that is the angle you'll actually set on the machine when using the marks on the compound. The idea of setting the compound over for threading is that it basically traces the tailstock side of the valley, and only cuts on the headstock side.

  5. #5
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    Quote Originally Posted by auscab View Post
    How do I accurately set the compound to move at 27.5 degrees if I want to try cutting the next thread that way ?
    Annoying that there are no marks on the compound. I've attached an extract from the Hercus Textbook Of Turning. They advise having the compound parallel to the bed and advancing the compound slightly to the left with each pass. So instead of advancing the compound at an angle (27.5 or 30 degrees) you advance the compound left and the cross-slide in. Which I guess achieves the same thing. The maximum 2/5ths ratio it refers to equates to 22 degrees (or less), so the shape of the cutting tool determines the thread form rather than the movement of the compound.

    I've also attached the ToT notes about the thread chasing dial.
    Attached Images Attached Images
    Chris

  6. #6
    BobL is online now Member: Blue and white apron brigade
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    Unlike vertical angles where a gravity activated digital angle finder can be used, horizontal angle setting can be tricky.
    A few years ago my son bought me this as a Xmas present and it's turned out to be one of the more useful Xmas presents I have ever received from him.
    Screen Shot 2021-04-23 at 6.01.22 am.png
    It doesn't suit every situation and I have not used it very often but when I have had to used it, its been excellent.
    Mine is a bit long (300mm) for lathe work but I have seen them as short as 125mm.

  7. #7
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    You would still need to use the cross slide on the final pass just to lick off the the surface on both sides of the "V" groove.

  8. #8
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    Quote Originally Posted by jack620 View Post
    Annoying that there are no marks on the compound. I've attached an extract from the Hercus Textbook Of Turning. They advise having the compound parallel to the bed and advancing the compound slightly to the left with each pass. So instead of advancing the compound at an angle (27.5 or 30 degrees) you advance the compound left and the cross-slide in. Which I guess achieves the same thing. The maximum 2/5ths ratio it refers to equates to 22 degrees (or less), so the shape of the cutting tool determines the thread form rather than the movement of the compound.

    I've also attached the ToT notes about the thread chasing dial.
    Thanks Chris . I think the word BINGO suits here? Its as simple as that and right in front of me . Ive got the book but didn't see it .

    I was doing a 5/8 11tpi on a stud sticking out of the chuck . Not centered and supported on its right side . Ill go to the effort next time of supporting the right side and parting it off I think . I could see that stud flexing whe the cut was too heavy . The last light passes fixed the roughness.

    With the section of the book "TBOT" on the Chasing dial ( inch thank goodness as well ) Which mark on the dial suits an 11tpi according to that ?




    I didn't approach it by trying to understand all of that . I go blank at the first point that something doesn't make sense and from then on its all a mess .

    What I did was I turned the part down to its outside diameter . Painted it with my Methyl blue spirit mix to Blue it . I then engaged the lead screw and cut a fine line at 11tpi along the part . And checked that spacing with my pitch gauge .
    I saw all this on YouTube .
    I then disengaged and withdrew the cutter and moved it back to the starting point and just re engaged the lead screw at different points to see which mark followed the first path by using the point of the cutting tool off a little as a pointer to the first scribe line .
    It worked out that if I engaged the half nut at the first opportunity just past ( 3mm ) the smaller non numbered marks , .5 marks ? ( 4 of them on the dial ) I was on track .

    Rob

  9. #9
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    Quote Originally Posted by auscab View Post
    With the section of the book "TBOT" on the Chasing dial ( inch thank goodness as well ) Which mark on the dial suits an 11tpi according to that ?
    I think you would use the method I've boxed below. However, note the last sentence which advises keeping the half nuts closed for certain thread pitches.

    My lathe is a metric 260 which has a placard that tells me on which chasing dial numbers to engage the half nuts for any given pitch. Perhaps a Hercus 9 AR owner can confirm the method to be used?
    Attached Images Attached Images
    Chris

  10. #10
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    Hi Guys,

    I just plunge straight in ! Ever since I stopped trying to grind my own threading toolbits and went onto threading inserts all the problems went away ! Well except for some Chinese rubbish inserts that I got. A decent Sanvic insert showed me that it wasn't my technique that was at fault.
    Best Regards:
    Baron J.

  11. #11
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    1. Like Byron, I just plunge in.

    2. If you have a set of thread pitch gauges, they often come with a stamped angle plate that looks like a fishtail:
    2.jpg

    If you are lucky, your set came with both a 55 and a 60 version?

    You mount that on a chuck jaw, and align a flat side on your compound to it
    (or wind your compound back and forth, and align so the cutter side touches at both ends)

  12. #12
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    Quote Originally Posted by BaronJ View Post
    Hi Guys,

    I just plunge straight in ! Ever since I stopped trying to grind my own threading toolbits and went onto threading inserts all the problems went away ! Well except for some Chinese rubbish inserts that I got. A decent Sanvic insert showed me that it wasn't my technique that was at fault.
    Where did you buy the Sandvic insert from ? Ive just bought a quick change toolpost, it arrived today, I still have to buy tools and inserts .

  13. #13
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    Threading inserts are great, but you shouldn't have any trouble threading most metals with a sharp HSS tool bit.
    Chris

  14. #14
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    Quote Originally Posted by auscab View Post
    Where did you buy the Sandvic insert from ? Ive just bought a quick change toolpost, it arrived today, I still have to buy tools and inserts .
    Hi Rob,

    Recall that I'm in the UK, unless you have a "Michel Fox" store down under.
    I only bought one to replace one that was given to me to help me out, bloody expensive around 11 posted.
    Best Regards:
    Baron J.

  15. #15
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    Quote Originally Posted by jack620 View Post
    Threading inserts are great, but you shouldn't have any trouble threading most metals with a sharp HSS tool bit.
    Hi Chris, Guys,

    I agree, but I was really struggling to get the angles right grinding my own HSS toolbits ! Pre formed carbide inserts relived me of that pain . So that is all I use for threading now.
    Best Regards:
    Baron J.

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