Modern Direct Drive Toolpost Grinder
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    Default Modern Direct Drive Toolpost Grinder

    Hi
    The traditional tool post grinder has a separate motor and a belt driven spindle.

    The modern spindle motors like this one Spindle Motor look like a simple path to a decent tool post grinder with minimal work. This is the link to the Jianken website

    The concern I have is whether the RPM will be too high. The consequences would be that reducing the rpm would reduce power output. So operating a 2kW motor at half speed would expect to give half power.

    What is the optimal rpm range for an external and internal grinding wheel?

    Dazz

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    Interesting idea but I'd always understood that a major reason for using belt drive was safety as, if set correctly, power transmission is limited so if you get a heavy contact between wheel and work the drive will slip and the wheel stall rather than explode. I imagine those fixed spindle motors deliver all the power available at whatever rpm its running at so may well grenade the wheel rather just stall out if things go wrong.

    On a tool post grinder the set-up is inevitably improvised and work specific to some degree so you need to be sure the wheel won't come apart if it all goes badly wrong.

    Seems to me that the spindle motor could be good in a proper, permanent machine, set-up but maybe not as a bolt on - bolt off accessory.

    Hafta say I won't use a tool post grinder as the potential to make big mess up scares me. I've never used anything other than ordinary bench type and T&C grinders. I know I've not got proper feel for grinders so I don't do it.

    Clive
    Last edited by Clive603; 07-10-2019 at 07:29 AM.

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    Quote Originally Posted by dazz View Post
    ...What is the optimal rpm range for an external and internal grinding wheel?

    Dazz
    That question is a clue to why general purpose tool post grinders like Dumore and Themac come with a set of pulleys. The RPM needs to be in proportion to the wheel diameter, so little internal wheels run much faster than large external wheels.

    Larry

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    For instance - a 1/2" ID wheel making 6000 feet a minute is turning not quite 46,000 RPM

    A 3" OD wheel making the same 6000 feet a minute is naturally 1/6 of that - less than 8000 RPM

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    I think it could be done, but it's just to easy to over-speed. There's also the torque thing. The larger the motor diameter, the longer the lever arm to the magnetic gap where the force is. Thus larger motors are good at torque. Smaller motors can produce high HP via high RPM, but aren't very good at making torque at low RPM unless they use a gear head. A tool post grinder spindle should be small to clear the work and tailstock, so the typical small spindle combined with a medium size motor, a la Dumore, makes sense.

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    The advantage of a belt drive is,the ability to use a 1 1/2 to 3 inch wheel to grind the o.d. of a LONG shaft right up to the chuck area and still kiss a square shoulder.You can set the grinder spindle parallel to the lathe spindle or a few dedrees reverse angle. The direct motors are usually larger than the wheel diameter. The motor would stike the o.d. of a long shaft and more suited for shallow depth i.d. work.Either way they are both clumsy to use and are usually on a shelf in a back room in most machine shops. Edwin Dirnbeck

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    My understanding of one of the reasons to use a belt was the reduction in vibration from the motor.
    I would think a direct drive would transfer the motors "quirks" into the finish

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    Agree with all of the above.

    Just to expand a little on mjk's point. I imagine that it's cheaper to produce a really high quality spindle as a separate element from the motor that drives it.

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    As far as RPM - perhaps the spindles you link to, or others like them, are capable of variable RPM?

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    Size/capacity wise the units appear to be in the range of this Dotco 60000rpm grinder made for attachment to machine tools.
    At the very center of the photo the Dotco grinders long neck is clamped to the wheelhead of a cutter grinder for light work difficult to do with regular attachments.
    I dont know the cost of the electric grinders, the Dotco air grinder is not cheap. The Dotco has a locking off/on button, speed is controlled by air pressure.


    This is my home made tool post grinder, but has some of the same constraints as the factory made models, for example as can be seen in the photo the considerable size difference in the pulleys will only produce 16000 rpms, but just enough speed to nicely grind the jaws on small chucks with a 3/4" stone.

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    Regarding power loss by reducing speed, most 'tool post grinders' are WAY under 2Kw to start with. I doubt power loss would be a problem. Lack of vibration isolation may be a factor as mentioned above, depends on the balance of the spindle/motor used.

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    The Dotco illustrated is but one of many air-powered uber-speed spindles. Look for them. NSK is another.

    But the puzzle to me is why direct-drive electric is considered "NEW?"

    My go-to include the classical belted McGonegal (Themac) J35...

    and one of two ancient "Precise" - a jig grinder with built-in fine traversal, and a plain block-mount that relies 100% on the machine-tool for location, fine or otherwise.

    Both are variable-speed by use of series-wound motors and ignorant Variacs.

    They also:

    - are HEAVY and bulky.

    - REQUIRE nearly 20 minutes of ramped-up warm-up to stabilize!

    - use spindles and bearings that the present owner, Fischer Spindle, charges over a thousand dollars to re-rebuild and balance if/as/when they run their bearings. "Clean room" required.

    Electric ones are still made, but are also still a costly option that isn't always easy to justify:

    https://shop.fischerspindle.com/Grinding-spindle

    Might want to look at the air spindles again? Dotco is not exactly "cheap".. but it is still well below the prices of several other really good ones.

    CAVEAT: The El Cheapo's intended for de-burr & c. need NOT apply. My ones are "OK" for a pantograph engraver, but their bearings aren't up for fine work, such as cleaning-up a precision spindle taper.

    Even so, as a tribe, air-powered are smaller and lighter than electric drive, direct OR belted, also by their nature, self-cooled.

    NB: ALL of these are for a person with lots of patience. None of them exactly "hog".


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    Hi
    Some good points raised.
    I think the risk of a disk exploding is always there which can be mitigated with a suitable guard.

    All of these spindle motors are driven by VFD at whatever speed is required. Speed is power so if the speed is reduced, so is the power.

    A good VFD will allow fine control of overload. The VFD will react far faster than any human could.

    I can see that a fat spindle motor would be limiting for external grinding but no problem for internal work.

    The accuracy of any grind is going to be defined by the lowest quality bearings, most likely the ones in the lathe head stock.

    I think the biggest issue would be rigging the water and air supplies. OK if you have them, but not if you don't.

    Dazz

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    Quote Originally Posted by dazz View Post
    Hi
    Some good points raised.
    I think the risk of a disk exploding is always there which can be mitigated with a suitable guard.

    All of these spindle motors are driven by VFD at whatever speed is required. Speed is power so if the speed is reduced, so is the power.

    A good VFD will allow fine control of overload. The VFD will react far faster than any human could.

    I can see that a fat spindle motor would be limiting for external grinding but no problem for internal work.

    The accuracy of any grind is going to be defined by the lowest quality bearings, most likely the ones in the lathe head stock.

    I think the biggest issue would be rigging the water and air supplies. OK if you have them, but not if you don't.

    Dazz
    I can see.... that if you are in NZ... you have fewer options. Used goods especially.

    Even so, I can't see re-inventing what the "usual suspects" have been shipping for close-on a century by now. Better to hunt-up a Dumore, better-yet Themac, rebuild it yourself, then add a decent air spindle.

    A "Precise" or Fischer is likely to break the bank, new or used. Even their collets are "special"

    As to the Chinese ones? If you really MUST, go water-cooled, not air, and skip the VFD they kit with them. By a DECENT VFD separately instead.

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    Quote Originally Posted by thermite View Post
    I can see.... that if you are in NZ... you have fewer options. Used goods especially.

    Even so, I can't see re-inventing what the "usual suspects" have been shipping for close-on a century by now. Better to hunt-up a Dumore, better-yet Themac, rebuild it yourself, then add a decent air spindle.

    A "Precise" or Fischer is likely to break the bank, new or used. Even their collets are "special"

    As to the Chinese ones? If you really MUST, go water-cooled, not air, and skip the VFD they kit with them. By a DECENT VFD separately instead.
    I have never seen any of the brands you mention here, used or new. If they were available new, they would cost more than gold. I don't actually recall seeing any used toolpost grinders for sale. The market here for that sort of stuff is tiny and if they ever came up for sale, the price would be high because of rarity.

    Getting anything sent across the USA border is stupidly expensive. The price for shipping something as heavy as a tool post grinder would be eye watering. Getting stuff sent from China is slow but cheap. I am currently waiting for 30 orders from China. In all cases it is cheaper for me to order an item from China than buy the same item locally.

    I would never buy a cheap Chinese branded VFD. I have Yaskawa (Japan) and ABB (Germany) VFD installed at present. Both are probably made in China.

    The Chinese spindles come fitted with ER collets so that makes them easy to use. Given the near zero options I have, the Chinese spindles can only be attractive. I have found that if you pay a good price, you get a good product from China. Cheap crap is just that, cheap and crap.

    Dazz

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    Perhaps you could find a used Dumore (most common brand of TP grinder here) spindle only ( minus motor, but with the sheaves and other bits) in the USA market. Shipping would be reasonable, and you can re-motor locally?
    Yes, buying a used high speed spindle untested is risky. Just a thought though. Perhaps contact a rebuilder here such as Sopko, and ask if they might be able to help?
    Being the most common, parts and accessories (belts sheaves and collets etc) are common and readily available so they are versatile.

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    dazz-The accuracy of any grind is going to be defined by the lowest quality bearings, most likely the ones in the lathe head stock.

    I believe that to be true in my experience, the cutter grinder above, must have very good bearings in the workhead for a good surface finish when it is powered.

    These electric spindles are fairly new I think, smaller versions are used in dentist drills currently, I would expect there is an electronic controller for them.
    The spindles taking a common colket is a plus, the Dotco spindles and grinders often use the common Erickson 300 DA collet.

    When grinding on a lathe, keep in mind the lathes controls are coarse for grinding, so its easy to crash a wheel into the work with a small movement. So, with gaurds or not, stay out of the line of the blast. The most tricky wheel to use is a cup wheel, try to use a dish wheel instead.
    The homemade post grinder above, runnig a 4" cup wheel at 4500rpm will sound like a gun if the wheel blows, and will punch holes in the walls or you if in the path.

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    Quote Originally Posted by dazz View Post
    The Chinese spindles come fitted with ER collets so that makes them easy to use.
    Lets' zoom-in on that.

    ER typically spec @ .0006 "average" TIR. Meaning it can change over their collapse range of a full mm. Better ones can be found spec'ed at .0002" TIR. Using "on size" limited-collapse range ER collets, as is common for taps and milling cutters, reduces your exposure to runout a bit more.

    Our man in Walla Walla pointed out - more than once - that your lathe spindle may have greater runout than your grinder spindle. And his spindles (10EE, more than one) left the factory with a max TIR of but 50 millionths, 35 millionths as a standard "option". Even if a few of us lucked into those, we might not know it. It gets hard to measure.

    Bottom line is that no matter what you buy or build, you'll have to expect some risks.

    My own grinders sit idle for YEARS between any use at all. I have a few - very few - "projects" left, after which I really should just sell them.

    So I have to ask.. how often, and on WHAT are you ever going to USE a TP grinder? And might it make more sense to send an item out to a full-time grind shop who aren't under-equipped, nor out of practice - they grind stuff every day, we do not - if YOUR need is only once in every few years?

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    Hi
    I have a Nardini lathe with Russian taper bearings. I have no doubt they would be the weakest link.

    I can see that the biggest problem with the direct drive spindles is the rated speed. Some will do 60,000rpm. For a 25mm wheel, that would be 4700metres per minute. Any slight imbalance would be a major issue. I doubt any normal 100mm grinding wheel is rated to that rpm.

    I think a toolpost grinder is something I have always done without, but would use if I had one. It's like angle grinders. I never had one for years before I brought one, then I found it so useful, I brought another one.

    With an ER collet and a simple mounting arrangement, I can see that a direct drive spindle could be used for other applications. I could make a mount for my Deckel clone to use it for high speed milling for example.

    Dazz

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    Quote Originally Posted by dazz View Post
    Hi
    I have a Nardini lathe with Russian taper bearings. I have no doubt they would be the weakest link.

    I can see that the biggest problem with the direct drive spindles is the rated speed. Some will do 60,000rpm. For a 25mm wheel, that would be 4700metres per minute. Any slight imbalance would be a major issue. I doubt any normal 100mm grinding wheel is rated to that rpm.

    I think a toolpost grinder is something I have always done without, but would use if I had one. It's like angle grinders. I never had one for years before I brought one, then I found it so useful, I brought another one.

    With an ER collet and a simple mounting arrangement, I can see that a direct drive spindle could be used for other applications. I could make a mount for my Deckel clone to use it for high speed milling for example.

    Dazz
    100 mm? Think 5 mm to 10 mm for the stones, max, even less for dental burrs, or micro-milling cutters.

    Also a whole lot less than 60,000 RPM. The Precise Jig grinders top out at 17 or 18,000 and aren't necessarily run at full speed anyway.

    As to "high speed milling", in a way, that's what my "pencil" air-powered grinders are for.

    But the "mill" they were adapted to is an H.B. Preise Panto-Engraver. I've added an electric engraver head salvaged off their Hermes competitor.

    You might find a "laminate trimmer", junior cousin to a carpenter/cabinetmakers router (I have both..) easily as useful, and right handy to have in its own right.


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