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Other Working out the size of a threaded bar

Discussion in 'General' started by CrapBag, 5 Mar 2018.

  1. CrapBag

    CrapBag Well-Known Member

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    We collected an old glass tv cabinet that we want to use for photographing guinea pigs on due to it being glass and providing better all round lighting but its too big.

    It's made of three glass shelves held together with cylinders that have lone thread bars going through them.

    The bars are only threaded about an inch and a half on each end and I want to do away with two of the shelves thus I need a shorter length of bar.

    I have a digital Vernier and have measured the bar at 7mm, the thread diameter is 7.66mmand the internal measurement of the threads is about .74mm.

    From what I have googled this looks to be between M6 and M8 so is it more likely to be imperial?
     
  2. Wakka

    Wakka Yo, eat this, ya?

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    Quickest and easiest way - Find/buy an M8 nut and see if it threads on. I very much doubt by those measurements that it's M6 though.
     
  3. Xlog

    Xlog Active Member

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    Threads are characterized by their major diameter (tread diameter) and pitch. So you most likely have imperial bolts.
    Is just cutting them to length out of the question?
     
  4. CrapBag

    CrapBag Well-Known Member

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    Yeh thought the nut idea would be best, just I don't have one guess I'll just take the old bar in with me.

    I believe I have an M8 nut and that screws a little way on then gets stuck so I think this is an imperial threaded bar.
     
  5. CrapBag

    CrapBag Well-Known Member

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    The bar is only thread and inch and half on each end and the bar is 2 foot longish so not I cant do that.
     
  6. edzieba

    edzieba Virtual Realist

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    Go to a nearby hardware store (Wickes, B&Q, etc) with the current rods and one of the 'feet' (or whatever part the rod screws into). They have many lengths of threaded rod you can have cut to the desired size, and if you have the original parts on hand you can check directly that they thread in properly.
     
  7. CrapBag

    CrapBag Well-Known Member

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    B&Q only hold metric, my local privately owned hardware store ahs a vast range of stuff so should be able to help.
     
  8. cobalt6700

    cobalt6700 Active Member

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    My Zeus chart is at home - I will look up the size later for you - is 0.74mm the distance between the peaks of the threads? Can you count how many threads there are in an inch?

    Can you replace with M6 threaded bar?
     
  9. blackerthanblack

    blackerthanblack Member

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    Not sure if I'm getting how you've described the sizes right, but if the thread pitch is around 0.74m as you stated, then you're in the realms of a 5/16 UNEF thread. The thread may well be beside the point though for what you are wanting?
    It sounds like you would be best off just buying some threaded bar and nuts/washers to completely replace what you have. I'd check the holes that the bar goes into to see if that will take M8 bar (i.e. holes are bigger that 8mm) and use some nuts and washers to fix it up - nice ghetto mod, done.
     
  10. CrapBag

    CrapBag Well-Known Member

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    Ok looks like 18 threads per inch, the 0.74 was measured in between the threads, peak to peak looks like about 1mm but had to tell.

    No M6 bar wont screw into the previous fittings, I've just tried test fitting a small 1 inch bolt in and it starts to screw in but then stops after about 5mm, the original bar goes all the way in.

    I cant use nuts as the table comes with its own fittings, four of which are bonded to the glass.
     
  11. legoman

    legoman breaker of things

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    Sounds like you may need an M8 X 0.75 nut for that bar, so an M8 threaded rod should work.

    That or if you feel competent you can saw down the bars and chase a new thread into them as most threaded bar is threaded along its whole length.
     
  12. cobalt6700

    cobalt6700 Active Member

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    Mr CrapBag if possible would you be able to post a pic of the threads next to the verniers at 1" and at peak to peak? 18TPI works out as ~1.4mm pitch, 1mm pitch is 25.4TPI and 0.74mm is around 34TPI.

    If your first quoted measurement of 0.74mm is pretty spot bollock, and with 0.79mm being 32TPI, I would agree with blackerthanblack on 5/16 UNEF.

    There's a few die's around online - just make sure they are UNEF as a lot of the results just come up with UNF. Let me know if you need a die wrench to borrow for postage :thumb:

    Edit - Your rod size is 7mm OD? If so then that's about the same size as the inner diameter of a 5/16 UNEF. If you were to get hold of some 8mm plain steel bar then that would be the right diameter to take the die :)
     
    Last edited: 5 Mar 2018
  13. johnnyboy700

    johnnyboy700 Active Member

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    You can get M8 threads with different thread pitches, the typical coarse M8 thread has 1.25 mm pitch, I recently bought some cheap taps from Amazon at 1.0mm pitch and I already have 0.75mm pitch.

    To make a rough guess as to whether or not you have metric or imperial see if there is anything to identify country of origin. If it's made in the US it will be an imperial pitch, just about anywhere else will probably mean a metric thread - check for a CE mark which shows European origin. But beware that a lot of the stuff from China has a copy of the CE mark, one has a bigger space between the letters than the other but I never remember which way round it is.

    Don't be fooled by a thread being smaller than its identified size, by this I mean that 7.66mm could easily be an M8 thread because most of them are mass produced from poor quality materials and would be a loose fit on regular M8 nuts. It could be imperial as its works out at 0.3015 of an inch in diameter and is close to 5/16 which is 0.3125 but like I said unless its made in the US or is a very old UK unit it should be M8 but it might not be a standard thread pitch. Any M8 nut you might try in a hardware store will be the 1.25mm pitch so it will perhaps screw on a little and them jam.

    To be really sure you need to measure the thread pitch with one of these:-

    https://www.amazon.co.uk/Deals】OriG...&qid=1520416333&sr=8-1&keywords=thread+gauges

    Any old school hardware store will have one so you could take the bolt with you and they could measure it for you. Alternatively you could buy some fine pitch M8 nuts but I suspect that your looking for something like the hardware you get when building flat pack furniture, again your local hardware sore might be able to help. But post some pics so we have a better idea of what we're talking about here, there will always be a way to do it, just need to figure it out.
     
  14. CrapBag

    CrapBag Well-Known Member

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    Last edited: 7 Mar 2018
  15. johnnyboy700

    johnnyboy700 Active Member

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    Okay, just made a comparison with an M8 bolt I have here and I reckon its the same as the photo so it should be a regular M8 x 1.25mm thread, I'd still get an M8 nut and run it on the thread just to be sure. I measured the OD on my bolt and I got 7.47mm using calipers and trying to measure thread pitch with calipers is about as successful as herding cats. You should only ever use digi calipers as a rough approximation as any OD measured will vary depending on how much pressure to use to make the measurement.

    To be honest, if all you're looking to do is have a shorter bolt it might be easier if you get an M8 x 1.25 mm die plus holder (assuming the regular nut does fit) and run it up the thread past the shoulder (where the thread stops) as far as you need it to go. It looks like the bolt you have there is the same material that all flat pack furniture fittings are made from so it should be easy to do, make sure you you put some oil on the bolt before you use the die as a little lubrication does help. Then you chop off the excess thread and file the cut end flat, make sure you have a nut below the part where you make the cut. Once you file it flat when you unscrew the nut it should fix any part of the thread you crush whilst cutting the thread. If feels rough or is too tight to remove when unscrewing the nut, run it back down and file it a bit more.

    I'm sure there will be YouTube vids by the dozen on how to do this if you've never done it before.
     
  16. CrapBag

    CrapBag Well-Known Member

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    Yeh I have an M8 nut that wont fully go on.

    The M8 bolt I have in all honesty threads in far enough (7mm) to do the job maybe a little further with force.
     
  17. cobalt6700

    cobalt6700 Active Member

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    Hmm.

    Looks to be a rolled thread which would explain the undersize body of the rod. With a thread pitch of around that measurement (I agree hard to see with the rounded peaks) I would still go 5/16 but UNC as that has a 1.4mm pitch. I’ve never seen M8x1.5 available on the market (Which doesn’t mean it doesn’t exist)

    As Johnnyboy suggested - sticking it up to a thread gauge is the best way to confirm it - finding out wether its a 55 or 60 deg thread tells you a lot.

    I think you will struggle to get a decent thread, wether it be M8 or 5/16 onto a 7mm dia bar though.

    From what you’ve said about the M8 bolt and nut that further supports my second suggestion of 5/16UNC.
     

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