Turning cap screws?

Discussion in 'Metals' started by Naiveambition, Feb 26, 2016.

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  1. Feb 26, 2016 #1

    Naiveambition

    Naiveambition

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    My boring head needs a leadscrew and the plans call for 6/40. Well I don't have this size or dies for this thread. But the bigger issue is turning an existing socket head screw.the ones I have are hardened . I have to be able to turn one according to plans so I can't simply change the bolt, or make one since I need the socket for adjustment. Can they be annealed, with it being a leadscrew in a brass threading I would think I can get away with it not being hardened afterwards. Any help , tips?.??

    And how do I make the screw come out to .001 adjustment. If plans call for 6/40 ,can I use one of bolts I have say a 1/4-20 and still be say .0005 or would it stay the same. Can I use any of the bolts pictured?

    image.jpg
     
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  2. Feb 26, 2016 #2

    gbritnell

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    Standard socket head screws aren't hard. They are made from good quality steel but it they were hard they would snap off when tightened. When I need to make screws and bolts from good steel I regularly turn down socket head screws.
    gbritnell
     
  3. Feb 26, 2016 #3

    Naiveambition

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    Every one I've tried so far has ended in failure. Try again I guess.

    What other common threads could I use for distance for .001
     
  4. Feb 26, 2016 #4

    Blogwitch

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    Hi N,
    As you most probably know, the reason for the 40TPI is because that gives you a lead of 0.025" per revolution of the screw, so 40 revolutions will give you the 1" you desire, and this is what is used in most imperial micrometers.
    The nearest thing you have in your picture is 20TPI, which gives you a lead of 0.050", and that could be used, but you will have to use a much larger scale as that will need to have 50 markings per circumferance to give you the 0.001" accuracy rather than the 25 for the 40 TPI thread.

    It is fairly easy for us in the UK as we use Model Engineering threading systems, which contains a range of 40TPI taps and dies.

    To get around the hardness problem you could heat the bolt up to cherry red, hold it there for a couple of minutes, then quickly move it and bury in a tin of sand to allow it to cool down slowly. It might work or it might not, but this will be the first step to try. You will be trying to anneal a piece of metal of unknown contents and heat treatment.

    The easy way is to ask if some friendly person on here is willing to make or thread the parts you need to the specs on the drawing. They can only give you one of two answers and I am sure you will soon get the positive reply if you are willing to pay for postage and any material costs. It is a shame to spoil the whole project for a few pennies spent on postage.

    Hope this helps

    John
     
  5. Feb 26, 2016 #5

    Naiveambition

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    Ok turning problem is solved. I swear every cap screw I've ever tried has ended badly then this one is normal. Go figure.

    And no I did not know how they were measured using standard 40 threads per inch. This is exactly the info I was looking for so thank a bunch. Looks like I'm buyin the tap and die. The dial is small anyways so the less marks the better.
     
  6. Feb 26, 2016 #6

    bazmak

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    Normal bolts are 4.6 tensile strength
    High tensile bolts are grade 8.8
    Hex socket cap screws are 10.9,very tough but easily turned
    I always use a std drill to c,bore for cap heads and chamfer the underside of the head at 120o,better than buying c,bore
    40 tpi is 25 thou per rev as is a micrometer for imperial
    You can use a fine metric cap screw if you know the pitch you can mark the dial to suit as long as you know the divisions its not too critical
    I have in the past used 3/8W which is 16tpi or 1/16 per rev
     
  7. Feb 26, 2016 #7

    goldstar31

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    I agree and may I also disagree?

    There is, well, in my opinion, far too a slavish acceptance of what has been written. My boring head was built to the design of George Thomas and the thread was screwcut to 40TPI. This is sort of where most of you 'come in'. There is nothing wrong with 40TPI but there is nothing wrong with 8TPI or 10TPI which are fairly standard on most 'English speaking' lathes and machine tools. Again, 10TPI is OK to because it can get a fairly easily 'dividing head' out of a change wheel. 8 is a bit more tricky but the degree of error comes with 62'5 divisions( I think).
    However, I built a tool and cutter grinder( well, 2) and for the Stent which had 3/8th BSF studding- as standard. My old Westbury mill drill had 1/2" BSW studding as lead and feed screws- because it was all that I could afford and more importantly achieve on my ( then) crude gear.

    However, my Quorn was line bored with boring bar with a 9/32" x 40 - angled at 40 degrees whilst my fine infeeds on the top slide can be in tenths of a thous if angled correctly.

    To complicate matters, my unimat clone is metric.

    Where most fail is because our tutors in maths really haven't much of a clue about the real demands of workers. It is OK to sit down and explain how Mr Euclid or Pythagoras got 'the son of the Squaw on the Hippopotamus is equal to the Sons of the Squaws on the opposite two hides' but there is a practical application other than the school boy joke.

    We fail to be able to crunch numbers because we have these damned calculators and I- things and whatever is next on the list of 'goodies' I was brought up on a good old fashioned slide rule where I had to guess the 'third number' in the series. My protractor came from a child's set from 'Everything is a Pound or Dollar'. I often work with a packet of cigarette papers and a bit of spittle.

    So let's have a bit more of this good old Irish lad remark about how he made his pies and it was obviously 'Twer that size'.

    I'm sorry if a few sacred cows were accidentally killed in the writing of this.

    Norman
     
  8. Feb 26, 2016 #8

    gbritnell

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    Norman,
    You are correct in stating that almost any size thread can be used but when trying to take out that last .001-.002 from a bore the finer the lead screw thread the easier it is to adjust the positioning of the bar.
    gbritnell
     
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  9. Feb 26, 2016 #9

    petertha

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    I would think the boring head pretty much necessitates fine pitch thread, at least for relatively fine work. Somebody check my math:

    40 tpi thread, means 1/40=.025" advancement for 1 full turn. So to make an incremental 0.001" bore diameter change, the screw must advance the cutter head 0.0005" (0.001 / 2). That works out to 7.2 degrees of screw rotation (360deg * 0.0005 "/ 0.025"). If the readout wheel on the boring head is say 1/2" diameter, then 7.2 deg works equals 0.031" rotation increment along the periphery, like a clock hand sweep. Hopefully the lines are engraved nice & thin :)

    Do the same calc for a 20 tpi thread, that works out to half: 3.6 deg of rotation or only 0.016" clock sweep.

    If the dial was a large diameter like on a lathe, the same 0.0005" infeed works out to 0.094" of rotation sweep. Much easier to see, even on my Taiwan lathe.

    A boring head is constrained as to how large the increment dial can be made, but I've always wondered on regular machines - why not make the dials large & lines as thin as practical, all other things equal?

    **oops** I think I somewhat repeated what John said a few posts up, just in a different way
     
    Last edited: Feb 26, 2016
  10. Feb 26, 2016 #10

    goldstar31

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    Moderators???

    I have made TWO posts here following G Britnell. They have not appeared.

    A 40TPI gives a dial of 25 divisions

    Norman
     
  11. Feb 26, 2016 #11

    Blogwitch

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    N,

    If you are going to get yourself a 40tpi tap & die set, then cut your internal thread first then gently come down in size for your adjusting screw, that way, you should get an almost perfect fit, and that shows itself by having very little backlash when adjusting.

    John
     
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  12. Feb 27, 2016 #12

    Naiveambition

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    Should of used a different title for this thread, my bad:p

    But while we're here. Can I just jump up one size then finish tap to correct size and still achieve low backlash? I planned on just buying the single size vs a whole set.

    I like the 20 divisions. If I'm reading the plans correctly the dial is .500. All it give me is a . 249 radius.
     
  13. Feb 27, 2016 #13

    Blogwitch

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    The thing is N is that you have no control when using a tap to cut an internal thread and finishing with a plug tap, what you cut with it is what you have to go with, this is where the die comes into it's own, it is adjustable to a certain degree so that you can gently bring it down to fit exactly the internal thread you cut with the tap, and so reducing backlash to an absolute minimum.

    I gather you are not up to a stage where you can cut your own threads, if you were, you wouldn't have any problems at all. But forget that for now, you will pick that up the further you get into model engineering

    When going with your larger sized thread of 20 TPI, to retain your accuracy of setting, which normally with a 40 TPI, you have 25 divisions on the head, so for every division moved, it will move the tip of the tool 0.001". If you use the 20 TPI with 25 divisions, each segment will move the tip 0.002", so halving your accuracy, and that was why I suggested going with 50 divisions, which will give you the same movement per segment as a 40 TPI thread, and as you have mentioned, the only down side is getting the head of the adjusting screw large enough to fit the 50 divisions. If you can squeeze them onto a 1/2" head, then you are cooking with gas.

    It is such a shame you don't live in the UK, I could most probably done the job for you in the time it has taken this post to get to this stage.

    I hope this helps

    John
     
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  14. Feb 27, 2016 #14

    goldstar31

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    Unfortunately, the gremlins or whoever got in and my posts disappeared. Of course, John is quite right but it is important to mention about avoiding or minimising backlash. You should aim to get perhaps an 80% engagement when drilling prior to tapping.

    Again, it is difficult to achieve some of the demands from perhaps limited tooling and improving skills. I wrote to a newcomer about the use of a metallic filler to take up some of less accurate items. Little did I think that my 'new' little Myford would have had a broken lever arm to control reversing and forward driving my leadscrew. It was cracked in some alloy and the thread was 3/8th BSF( with 20TPI)
    It is an awkward casting and now unobtainable but I went out and bought a tin . Clearly, I will have to make a new one. Whether steel or otherwise, I have yet to decide but that is what is 'working' at the moment.

    So there is no harm in making 'adjustments' until things improve.

    Regards

    Norman
     

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