Pilot Tip Grinding on HSS 3/32 Drill Bits Advice (KO Lee B360 grinder)
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  1. #1
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    Default Pilot Tip Grinding on HSS 3/32 Drill Bits Advice (KO Lee B360 grinder)

    Not being a grinding expert, I would appreciate some advice from the grinding experts regarding proper wheels and techniques for grinding pilots on 3//32” HSS standard drills. We are grinding “pilot” points, 0.055” dia. x 0.22” length on standard .094 HSS drills. This pilot is non-cutting, and the shoulder is square, so no flutes or angles required.

    We have done hundreds using the toolpost die-grinder as shown in the picture. The wheels used are 1”dia x .25 wide “smooth blue” aluminum oxide bits from McMaster.
    This works reasonably well, but is sort of slow and prone to overheating the bit. These bits glaze somewhat, and are dressed every dozen bits.

    In quest of doing this sort of thing more efficiently (and some other related prototyping tasks), I purchased a KO Lee B360 grinder for $300. It looked in fairly good shape from the photos, and all the pieces present. The shipper did not secure it properly, so it banged around on trucks, and arrived off the pallet, broken handles, chipped castings, crushed oiler rollers, shipped rack gear, etc. But wear on the ways was minimal, leadscrews and nuts good, and the heavy-duty spindle and motor sound very good. The maintenance folks had also packed the gearbox full of grease (likely due to a leaking shaft seal), so there were worn bushings and bearings. So after fixing the aforementioned, and installing new bearings and seals, repairing gear-shafts, etc. VFD, it’s up and running. (if anyone needs any $.02 on refurb of a similar grinder, I'm more familiar with the innards and repairs than I cared to be...)

    I also motorized a spin-indexer as shown in order to have a compact way to grip those 3/32 bits, and other small parts.

    To the question, what would be the best approach and wheels for grinding pilots on the 3/32 bits with the ‘new’ grinder. In a quick test, the Norton 32AA 80 K 7” diameter wheel shown seems to not work well with rotating the part and feeding the wheel directly vertical onto the bit. There appears to be chatter and gouging of the wheel versus a lot of grinding. I had the same issue trying to use white aluminum-oxide wheels on a Dumore toolpost grinder. The spin fixture currently runs at 20 rpm max (I can replace a motor/belts if necessary to vary the speed). I assume the hardness of those bits used on the die-grinder are significantly harder than the K wheel, which is why they work adequately with the die-grinder.

    Thanks very much for any advice, pardon the long-winded post!

    tp-diegrinder.jpgkolee1.jpgspinner1.jpg

  2. #2
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    Good morning car2:
    There are a number of things with your setup that are creating the problems you describe:

    1) The Die grinder and mounted grinding point photo...that little grinding wheel is way too hard for the task.
    That means the abrasive grains are so tightly bonded that they will not be stripped off the wheel when they get dull, but will glaze and burn the work.
    So that's why you had problems with that approach.

    2) Your cylindrical grinding setup is sub-optimal for the following reasons:

    a) you need to spin the workhead 90 degrees and the wheelhead 90 degrees too so the axis of the workhead and the axis of the wheelhead are both pointing down the long axis of the table.
    This is because the table longitudinal travel is driven by a rack whereas the cross travel is driven by a screw, so you can control the diameter of the work better by using the screw for infeed, and because the rack does not restrain the workhead so it can bounce back and forth as the wheel is advanced into the drill.

    b) it is greatly to your advantage to lower the wheelhead so it's axis is on the same horizontal plane as the workhead.
    When you do that, you can use the cross slide (saddle actually) dial to accurately advance the drill into the wheel by reading directly off the dial.

    c) I presume your drill is held in a simple collet or drill chuck and is sticking out from that collet a fair ways.
    This is the kiss of death for grinding...the drill will vibrate and chew up the grinding wheel instantly.
    There are a few ways around this; the common one is to use a 6 jaw scroll chuck to hold the drill and to chuck onto the flutes so as little as possible of the drill is sticking out of the chuck.
    A collet will work OK too, just be sure you're chucking on the drill flutes, not the drill shank, and with only 0.050" more than your pilot length sticking out of the collet.

    3) Last you need to learn about wheel selection and wheel dressing.
    This is a whole 'nother subject and gets pretty complex when you get into the weeds of it, but for general tool grinding on small stuff like what you're describing, an aluminum oxide 60 grit "K" or "L" hardness wheel is a decent first choice.
    It will be friable enough to release dull abrasive grains, but hard enough to retain decent form for a decent amount of time and will run cool enough to keep from burning the steel if you don't push it or even better if you run coolant on it.

    Dressing consists of running a single point diamond dresser back and forth past the wheel using the machine motions and taking off around 0.001" per pass until the entire periphery of the wheel has been trued up.
    Typically this will take between 5 and 10 passes if you start with a freshly mounted un-trued wheel.
    When it's completely dressed, the sound of a dressing pass changes noticeably, and the wheel will grind smooth true surfaces until the surface of the wheel is degraded by wheel wear, at which point, you need to dress the wheel again.

    So that's basically it in a nutshell...lots to absorb but straightforward after all.

    Above all, be careful...grinders are pitiless bitches and will bite you if you do something wrong.
    A very good idea is to either read up what you can, Youtube video learn, or find a mentor who can show you the ropes.
    It's not dangerous if you know what you're doing but it's frickin' scary if you don't.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Thanks very much for the help Marcus. I'm fairly familiar with with toolpost, bench, t/c grinders, basic wheel grades, dressing, and have a healthy respect a chunk of ceramic spinning at 3000 rpm. However this is my first experience with such a tool/cutter grinder. I will do some more experimenting with your suggestions. I reoriented things with the spindle parallel to the table (needed to flip it and the motor also, as I wasn't paying attention when put back together).

    The spin fixture is a 3/32 5-C collet, and the drill sticks out only enough to accomplish the grinding (.25") (I may have to put a different motor on it since it only spins at 20rmp).

    Thanks again, will report back after some testing. Cheers

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    No expert with T/C grinders either but have done a fair amount of similar grinding in surface grinders with a Harig-type and spin fixtures. You've not stated tolerance so I'll presume that your current spin-dex fixture is adequate. You've also not stated batch numbers but if you've already done hundreds I'll guess that this indicates a need to make a few dozen at a time. In this regard you will have some amount of wheel wear, wheel type and grade will affect the tolerance and/or taper for the pilot. If this is so then you may want to consider working in stages of rough/finish grind with a wheel dress or two in between. You may have already planned this. I might also suggest using the thinnest width wheel that will still get the job done to avoid wasting the unworn portion of the wheel in dressing. Cost per wheel will determine your choice. Just my opinion but I prefer to plunge down and walk back off the work to minimize the unavoidable radius on the leading edge of the wheel that develops when walking INTO the work. I'll presume you also intend to grind wet to keep heat down. Rotation should be against wheel rotation (in this application) or you're crushing the wheel grains. If this is a repeat job, sounds like it, then any dedicated tooling stops and diamond locator will improve the speed and repeatability of the operation. If vibration seems present a small spring steel finger on the workpiece can help dampen the vibes. All info from the post by Implmex is solid advice particularly the quote below. Lock ALL table movements any time your hands have to be near the wheel. Good luck.

    "It's not dangerous if you know what you're doing but it's frickin' scary if you don't."


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