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  1. #1
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    Default Test Bar Suggestions

    I have an issue with the alignment of the tailstock on my lathe that in its previous life was used for some or maybe a lot woodturning. I have created a previous thread or two on the shimming of the tailstock to compensate for the wear probably caused by the use of sandpaper while turning but I'm thinking now that the tailstock dips towards the front left hand side.

    So far, my attempts to measure the situation have been a bit Mickey Mouse. For a test bar I've used a 200mm length of 5/8" precision ground 4140 in a Rego Fix collet mounted in a Schaublin 2 Morse chuck. Probably reasonably accurate but the 0.004" droop along 170mm of its length when mounted in the TS could be gravity, collet and chuck runout or a worn tailstock.

    The 4 thou has been measured by simply traversing a dial indicator mounted on the carriage. The 4 thou is pretty consistent along the lathe's long bed.

    The real drama is when I run an headstock mounted indicator around the outboard end of the bar. Using a Centricator ( http://www.woodworkforums.com/showth...34#post1539534 ) for measurement, there is a staggering 0.017" reading.

    I'm not sure what's going on. I've ordered a replacement used tailstock base from Mal Colomy at Australian Metalworking Hobbyist based on the thinking that whilst it will be worn, it probably will be worn more evenly that mine and I hopefully will be able the scrape and shim things back into some semblance of accuracy. Hopefully.

    To facilitate a more precise survey of the situation I need to improve on the test bar. Chronos in the UK offer this bar - http://www.ebay.com/itm/2MT-LATHE-TE...item4d2a6e8ef6 Shipping is about 20 bucks.

    Something cheaper would be nice but Chronos suggest their bar is accurate to two tenths.

    Any other recommendations?

    BT

  2. #2
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    G'day Bob,
    Perhaps I'm missing something or over simplifying things but I would have thought if you put a simple dead centre in the tailstock then just run an indicator (or a centricator) mounted in the headstock taper around it with the quill fully withdrawn and again with the quill fully extended then compared the two wouldn't this give a good indication of any problems with the alignment of the tailstock axis?
    Cheers,
    Greg.

  3. #3
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    Quote Originally Posted by kwijibo99 View Post
    G'day Bob,
    Perhaps I'm missing something or over simplifying things but I would have thought if you put a simple dead centre in the tailstock then just run an indicator (or a centricator) mounted in the headstock taper around it with the quill fully withdrawn and again with the quill fully extended then compared the two wouldn't this give a good indication of any problems with the alignment of the tailstock axis?
    Cheers,
    Greg.
    Thanks Greg,

    I will have a go with a dead centre tonight.

    Bob.

  4. #4
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    Default

    Bob, you do not need to buy a test bar just to check the tailstock alignment. Of course, if you had a test bar you would use it. But the job can be done just as accurately without. It may need a little more time and care and thought, that is all.

    The way I do it, I put a shaft to the rear dovetail clamp of a DTI, and clamp it to the spindle (any chuck or collet chuck will do, runout does not matter). Then I let the tip of the DTI indicate the inside of the tailstock taper. Whilst rotating the spindle with the DTI, wath the dial. You need to do this twice, once close to where the taper begins, and another time as far inside the taper as possible. The first time you check if the center line of the TS barrel and the spindle line up. The second time you additionally check if the center line of the TS barrel is parallel to the center line of the spindle. Finally, you need to repeat the above first measurement once with the TS fully retracted, and once with the TS fully extended, this verifies that the bore in the TS casting for the barrel is also parallel to the spindle center line. So, in total its 3 things that you need to check, to be sure the TS is properly aligned.

    Now last time I explained this procedure here, not too long ago, I got flamed about neglecting the influence of gravity and the curvature of space/time. Rest assured it does not matter, unless the lathe in question did cost new more than some $30k. In doubt hang some garlic to the wall and put a voodoo puppet into the lathe cabinet before taking the measurements. I remember I also posted pics of my measurement setup on the Hercus 260.... just can't find the thread anymore...

    By my practical experience, tailstocks tend to be some 0.05mm high when new. Depending on lathe size of course - I only have hands on experience with small hobby lathes of good build quality. If you can get the TS hight adjustment between 0 and +0.2mm (high) you will be fine for most tasks, unless you sometime need to drill very small diameter holes like for jets, or you need to drill very deep, or you sometimes run a reamer without a floating chuck....

  5. #5
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    Default

    Thank you Chris.

    I think my problem with measurement so far has been that I've only run the indicator over a short section of the TS barrel. What might have appeared acceptable isn't when the combination of drill chuck and bit extension amplifies a small error. A 1/2" bit in a chuck probably projects 170mm from the barrel nose. I have always observed that when using long large bits that the bit rides up into the previously drilled smaller hole. It rises up from about an 8 o'clock position. For work that matters I've always bored the hole. The more critical issue is that turning using a tailstock mounted centre results in some taper. Not a lot because the centre's tip is not far from the position the tailstock was accurately shimmed to.

    I never ran an indicator over the tailstock base when I had it apart. It is something I will do to see if the base does really nose dive. Previously I assumed the base was evenly worn and too low.

    http://www.woodworkforums.com/showth...80#post1523980

    Bob.

  6. #6
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    Jul 2014
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    eindhoven the netherlands
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    Default

    i got my testbar out of an old copier machine
    the bar that presses the paper onto the transfer roll
    it is 57 cm long, 5 cm wide, ground, chromed and has centers on both ends
    the visible wear is not measurable

  7. #7
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    Dec 2014
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    melbourne
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    A good quick and simple way of checking the alignment of a tail stock is the turn a piece of bar to the diam of the quill.
    Move the quill close to it and using a DTI mounted on the saddle measure the difference between them in both planes.
    The bar doesn't have to be dead accurate so long as you know the difference between quill and what you have machined.

  8. #8
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    Quote Originally Posted by shedeng View Post
    A good quick and simple way of checking the alignment of a tail stock is the turn a piece of bar to the diam of the quill.
    Move the quill close to it and using a DTI mounted on the saddle measure the difference between them in both planes.
    The bar doesn't have to be dead accurate so long as you know the difference between quill and what you have machined.
    This method makes two assumptions:
    - that the outer diameter of the tail stock barrel (quill) is concentric with the Morse taper bore inside it
    - that the bore for the barrel inside the tail stock upper casting is parallel to the bedways

    It is only fair to make these assumptions on an unworn good quality lathe. Low cost lathes are rarely machined that accurately. Wear in the barrel bore or the tailstock base remains unaccounted for, as are score marks or dents in the barrel taper.

  9. #9
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    eindhoven the netherlands
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    Quote Originally Posted by cba_melbourne View Post
    This method makes two assumptions:
    - that the outer diameter of the tail stock barrel (quill) is concentric with the Morse taper bore inside it
    - that the bore for the barrel inside the tail stock upper casting is parallel to the bedways

    It is only fair to make these assumptions on an unworn good quality lathe. Low cost lathes are rarely machined that accurately. Wear in the barrel bore or the tailstock base remains unaccounted for, as are score marks or dents in the barrel taper.
    If you repeat the test with the quill fully extended, you will know whether the quill is parralell to the ways the tailstock is running on.

    The best would be to take two morse chuck tapers, one for the headstock and one for the tailstock, and turn them both to the same diameter. This would take the concentriity of the inside bore of the quill into account.

  10. #10
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    First, an apology. I just reread my initial post and realised that I had incorrectly spelled Mal's surname. It should be Conomy. Sorry Mal.

    Alan "C-47" called by on Friday with a 300mm long 2 Morse test bar. We conducted a few tests together and it appears that my Jacobs Super Ball Bearing chuck might be less than super and a major contibutor to my alignment problems.

    I inserted a 3M dead centre in the headstock spindle and though it was errors cancelling errors, there was less than half a tenth TIR measured off the 60 degree taper. Hercus fitted Class 3 precision bearings in my vintage lathe and these were good for .0003" TIR when correctly aligned and when new. Hence the error comment. I don't want to tamper with the 3M centre again. The test bar showed a total runout of .0005" at both the headstock and tailstock ends and an error of only .001" when the DTI was traversed only its length on the X centreline when mounted between centres.

    Horizontal or Y error was about the same as X between centres. All well and good but the tailstock could still be skewed. With the test bar only installed in the tailstock there is droop. Measuring error on the Y axis is difficult because of the fall in the centreline. I'm thinking of jacking the outer end of the bar up to eliminate droop ( but not to prevent skew ) and traversing the indicator on Y to determine whether the problem lies in the tailstock. Any better ideas?

    Also, a parallel, centre drilled, taperless test bar would be of use. Not a monster like Jan's but something 150 to 200mm long by 16 - 20mm in diameter would be useful. A search on eBay has proven fruitless. A shaft from a gearbox? Can anyone think of a readily available bar? The precision ground 4140 I've been using dosn't live up to its name.

    The Jacobs chuck has been relegated to drill press duties.

    BT

  11. #11
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    Default

    Quote Originally Posted by Anorak Bob View Post
    The Jacobs chuck has been relegated to drill press duties.
    An opportunity to get an Albrecht!

    As for a precision ground bar, what sort of specs are you chasing?

    Michael

  12. #12
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    Gday Anorak Bob,
    This may be what you are after, centre in the 4 jaw and centre drill
    http://www.ebay.com.au/itm/SF16-200m...item43d20c13f4

    Cheers Max

  13. #13
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    Quote Originally Posted by Michael G View Post
    An opportunity to get an Albrecht!

    As for a precision ground bar, what sort of specs are you chasing?

    Michael
    another Albrecht ! I have a 0 -3 which is in good nick and a 0 -10 which looks good but isn't. Probably give the Jacobs a run in the wobble stakes. I think I'll wean myself off buying second hand chucks. Have you checked out the price of a new Albrecht!!

    Spec wise, something better than PG 4140.



    Quote Originally Posted by Maxi77 View Post
    Gday Anorak Bob,

    This may be what you are after, centre in the 4 jaw and centre drill
    http://www.ebay.com.au/itm/SF16-200m...item43d20c13f4

    Cheers Max
    Thanks Max but I probably would come unstuck with the centre drilling. The seller's spiel....


    • As the shafts are hardened, they are very difficult to cut (i.e. you will not be able to use a hacksaw, etc to cut). If you do need to cut, use either a lathe with a tungsten carbide cutter, a specialized saw (i.e. brobo), or a grinder (not recommended as the heat produced from the grinder affects the shaft hardening). So basically it is highly recommended to buy the length you need as it is a b**ch to cut.

  14. #14
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    G'day Bob.

    Not that, that's the instrument you need, centre drilling it is easy. Its only case hardened, the core is soft enough to drill / tap.

    Still wouldn't help as its not a ground MT2 to parallel shaft test mandrel.

    Phil

    (on Edit) Not sure the sellers spiel is correct. That would be some sort of Brobo. Maybe he meant a friction cut off saw.

  15. #15
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    My 3 morse taper test bar is Rimet brand, the agent is here http://www.robinsoninternational.com...ls_testing.php
    Gold, the colour of choice for the discerning person.

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