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  1. #1
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    Talking Surface Grinder Adapter Thread Differences

    My Kent SG came with one adapter fitted with one wheel installed on the spindle.

    As a first step in building up a set of wheels and adapters I purchased a cup wheel from Blackstones and a new adapter from Kent (Taiwan).
    The washer on the new adapter was too big for the cup wheel so I made a new nut and washer.

    Tried the cup wheel out on the new adapter, no problems.

    Got some old wheels from a friend and purchased a new wheel through another friend from an industrial supplier south of Sydney, so started playing around with various combinations - problem.

    I now have 2 adapters - old and new, and 3 nuts - old, new and made.
    The fits are: adapter old, new
    nut old: Ok, Ok
    nut new: NBG, Ok
    nut made: NBG, Ok

    Measured the nominal M30 x 1.5 thread with micrometer over OD and 0.040" thread wires:
    OD, Thread Dia
    Old: 29.79, 30.76
    New: 29.73, 30.51

    Measured the nuts with internal micrometer:
    Old 28.70
    New 28.63
    Made 28.44

    The new adapter thread is significantly smaller than the old adapter thread.
    My made nut used the new adapter as the gauge - wrong call.

    The problems keep us on our toes, and a couple of photos of the measurement are attached.
    The pads holding the thread wires are blue tack.
    Thread Wires 180311 compr.JPGAdapter Thread Pitch Dia 180311 compr.JPG
    John.

  2. #2
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    Default

    Why is the new one smaller? Generally the thread diameter is about that of the bore size of the wheel for those grinders. As that is 1.25" one would think the nominal dia of the thread would be 1.25" x 16tpi. Worth another look.

  3. #3
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    A 16 tpi thread gauge does not fit at all, the thread OD matches M30x1.5 requirements.
    When I made the nut, the lathe was set for 1.5 mm.
    The wheels are 1 1/4" bore, marked as 31.75 mm.
    The registers on the adapters are both 31.77 mm using a (cheap Chinese) vernier caliper.

    I will take the bits to lunch tomorrow for perusal by all.
    I think Kent have simply applied a bit of 'design improvement'.
    My plan is to make some new adapters/nuts/washers using the new one from Kent as the model.

    John

  4. #4
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    Quote Originally Posted by electrosteam View Post
    A 16 tpi thread gauge does not fit at all, the thread OD matches M30x1.5 requirements.
    When I made the nut, the lathe was set for 1.5 mm.
    The wheels are 1 1/4" bore, marked as 31.75 mm.
    The registers on the adapters are both 31.77 mm using a (cheap Chinese) vernier caliper.

    I will take the bits to lunch tomorrow for perusal by all.
    I think Kent have simply applied a bit of 'design improvement'.
    My plan is to make some new adapters/nuts/washers using the new one from Kent as the model.

    John
    Ok on that as you didn't say initially there was a step down from the wheel register diameter to the thread diameter.

  5. #5
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    John the first thing that strikes me is why you're using a cup wheel on a surface grinder?

    As far as the thread fits, you didn't accidentally use a 55 degree tool when cutting by chance?

  6. #6
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    Pete,
    The cup wheel was experimental, I was interested to see what could be done.
    Any future activities requires the addition of a micrometer feed on the cross-slide.

    The thread is correct shape with 60 degrees, note that the new Kent nut does not fit the original Kent (assumed) adapter.
    There is a possibility the adapter on the SG is a non-Kent alternative supplier.

    John

  7. #7
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    I'd be very careful of using that cup wheel John. The cross feed on a SG tends to be relatively coarse and I can see tears in that arrangement. I've never even heard of anyone doing that and I'd suggest there's probably a good reason. Should the cup wheel be driven down by mistake I'd say there's a good chance it would explode.

    You can side wheel with regular SG wheels and they're designed to be able to do that (if done correctly). I must say however I don't like going it personally. I think the key is to still cut on the front of the wheel by taking full depth cuts. Robin Renzetti (Robrenz on Youtube) is the maestro of doing this type of thing and his grinder work is worth watching.

    I'm not sure what to suggest with the threads. I didn't get what you meant by this,
    The new adapter thread is significantly smaller than the old adapter thread.
    Old: 29.79, 30.76
    New: 29.73, 30.51

    I'd be leaning toward the thread being deliberately different in some way. In my experience the products being made these days all come off CNC machines and it's unlikely to be just a little bit wrong, especially if there's a matching component that fits correctly. If the replacement came from the factory and you bought it yourself, that's the one I'd be assuming is "correct". 16 tpi is commonly used for these and is VERY close to 1.5 mm pitch (1.5 mm pitch = 16.93 tpi and 16 tpi = 1.59 mm pitch. ) My money is the old adaptor is 16 tpi, either from or designed for the US market. The new one is 1.5 mm, original from Kent who "should be" (but aren't necessarily) using metric exclusively. It could easily be the other way around. Why they would use nominal 30 mm instead of 1 1/4 is anyone's guess, but it's not the first time I've seen that

  8. #8
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    Pete,
    The need for care if using a cup wheel is accepted, nothing will be done without the introduction of a controlled stop micrometer cross-feed.

    The thread is clearly 1.5 mm pitch, the adapters compare perfectly against each other over a well-lit white background, and each compares perfectly against a 1.5 mm thread gauge, and very badly against a 16 tpi thread gauge.

    My SG dates back to 1981, things change.
    The old adapter may not be a genuine Kent.

    A 1.5 mm pitch has a thread depth of about 0.93 mm.
    The pitch diameter difference old to new is 0.25 mm, or 26 %, a very significant difference when cutting threads.

    We are left with an anomaly which I will manage in the future,
    Future made adapters will follow the new Kent one as the model.

    Of interest is that all the threaded holes in the body provided for accessories are UNC.
    John.

  9. #9
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    Sorry John but I'm not sure I understand what you're suggesting the manufacturer(s) have done here.

    Unless I'm mistaken, by my calculations the "new" thread is outside tolerance (too small) for a 6g thread based on the wire size and 3-wire measurement you provided. If the tolerances are opened up to be unusually generous, something I wouldn't expect even for a non critical application like this, then it may just squeak in with a bit of squinting, but it's not something I would be proud of shipping!

    I find it difficult to believe that Kent would produce a thread that's kinda, sorta, not really M30 x 1.5 mm, together with accompanying nut, that just happens to be slightly outside tolerance, but enough that a properly produced part won't fit. It just doesn't make logical sense to me.

  10. #10
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    Further to the above, I just ran the figures for a 30 mm x 16 tpi thread, and that "new" adaptor is absolutely smack on for a 16 tpi thread at 6g. The minimum 3 wire measurement would be 30.46 mm and the maximum would be 30.61 mm. It actually measured 30.51 mm, pretty much bang in the middle, just as one would expect for a machine/manufacturer like this.

    I appreciate this thread has been checked with a thread gauge, but it seems an awful coincidence that these adaptors are produced with LH 16 tpi threads, the numbers precisely match a 16 tpi thread, and the difference between a 16 tpi and 1.5 mm thread is quite small. 1.5 mm = 16.93 tpi. Alternatively 16 tpi = 1.588 mm.It would be enough to mean the nut wouldn't fit but would otherwise look very similar. 30 mm x 16 tpi is of course a bastard thread, but I've seen that done before that's for sure.


    Of interest is that all the threaded holes in the body provided for accessories are UNC.
    I believe there's your giveaway. I would strongly suggest that is a 30 mm x 16 tpi thread

  11. #11
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    Pete,
    I hear clearly what you are saying, that is why I posted the thread.

    The thread gauges show a 3/8" W bolt as exactly 16 tpi (as they should) and not 1.5 mm, so the gauges appear Ok.

    An observation is that the nuts are very loose on the threads.
    This is in keeping with my understanding of the fit required to mount a wheel on an adapter, a very close fit on the register and only a firm tightening of the nut pulling the parallel side of the wheel against the ground shoulder on the adapter.
    We are not dealing with a high performance thread/nut requirement.
    On a SG adapter, perhaps looseness is an advantage.

    I have a good mate who is a professional CAD/CNC designer and CNC operator with responsibilities for designing special threads in very critical applications.
    We should be able to discuss these adapters this weekend.

    As I expect to be making more adapters/nuts/washers, clearing the anomaly up would be helpful.
    John

  12. #12
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    G'day John. The bolt you checked is Whitworth (presumably checked with a Whitworth thread gauge), the thread I'm suggesting is UNC, the thread gauges are not the same.

    The pitch between a Whitworth and a UN thread pitch gauge is obviously the same, however the gauge won't lay in the (wrong) thread like it should. That could lead to an impression that it is not the correct thread pitch. That's why there are two versions of "imperial" thread gauges sold, one is either marked Whitworth (or marked 55 degrees) the other should be marked UNC/UNF (or marked 60 degrees, "American", etc etc) but don't be surprised if the markings are a bit cryptic on it. If it's made for the US market many people there wouldn't have even heard of a Whitworth thread, never mind feel a need for a gauge, so threads will either be "standard" or metric, both 60 degrees. Here's just a random supplier with a few options, no endorsement just first Google hit https://www.lprtoolmakers.com.au/thread-gauges/

    In this instance the two threads are very nearly the same pitch, and it will take some close inspection to see the difference between a 1.5 mm 16 tpi thread based purely on pitch. You will need to get a UNC thread gauge to check that properly (luckily they're cheap).

    Sorry I don't buy the story about loose threads deliberately made outside tolerances on surface grinders. I don't recall the nuts on mine being anything other than pretty standard in terms of fit (other than being left handed ). I haven't made any adaptors for mine, so change wheels in situ. I last changed a wheel a number of weeks ago. I don't take much notice of the nut, and don't want to disturb the wheel that's on there at the moment as it's a diamond wheel and an absolute bugger to true. I should have it off again in a few days and will check then. However a loose fit doesn't seem logical to me. You're absolutely right, as the nut is running in it will wobble all over the shop and could conform to a non-parallel surface. However when it hits the wheel it needs to tighten against something (the adapter's threads) and if the threads are a poor fit I would think it would tend to put uneven pressure on the wheel. If anything I would expect this nut to be a good fit, with the face reasonably accurate to the threads. Don't quote me on that as I'm just thinking out in writing and trying to remember what my nuts look like (that comes with age).

  13. #13
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    Quote Originally Posted by Pete F View Post
    In this instance the two threads are very nearly the same pitch, and it will take some close inspection to see the difference between a 1.5 mm 16 tpi thread based purely on pitch. You will need to get a UNC thread gauge to check that properly (luckily they're cheap).
    I just checked various combinations of Whitworth, metric and UNC in 1.5 mm and 16 tpi and take back part of what I said here. The difference between 1.5 mm and 16 tpi should be enough that it should be enough to see it on a thread longer than about 10 mm or so.

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