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Thread: Mini-Mill Improvements, A Companion WIP

  1. #1
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    Default Mini-Mill Improvements, A Companion WIP

    As some people are doubtless aware, ive made a long-term project out of making improvements to my Grizzly G8688 mini-lathe:
    http://bbs.homeshopmachinist.net/thr...s-A-WIP-Thread

    As a companion to that tool, ive also got a Grizzly G8689 mini-mill, and i figured "hey, if im spending this time working on the lathe, why not put some effort to the mill as well?". Much like the lathe, the mill is a tool that worked plenty well straight from the tent sale i bought it at. It spun bits, it drilled holes, it made parts, it worked about as youd expect; passably, and thats all you can expect for something you grabbed off a pallet for $100. Not great, but workable.

    Being honest, the only reason im touching either of these machines is A) im a firm believer that your work will never be more accurate than the tools you use to produce said work, so it pays to make sure the tools are as accurate as you need them to be, and B) im bored. I have also found some problems with the machine that are affecting the desired accuracy of my work, and those are what ill eventually be addressing.

    As it is now, the tool has already had a few mods made to it. The head has been fitted with a gas spring and a belt drive conversion kit, and it has been fitted with a DRO courtesy of a few igauging absolute digital scales and a TouchDRO controller. Planned improvements to the mill are as follows:

    -Replace the head bearings with a set of angular contact or tapered roller bearings. As with the mini lathe, i feel that this will improve the machine where the rubber meets the road, at the spindle
    - Replace the base and column with a fixed-angle version. My mill is one of the tilting column types, and theres an unfortunate lack of rigidity to it. The solid column conversion kit that LMS offers is a very desirable upgrade to me
    -Check and overhaul general alignment. One of the biggest problems ive noticed with my mill is that the table does not seem to be parallel to the direction of movement, which makes for wedge-shaped parts and problems tramming the mill
    -Improve fitment of sliding surfaces. Id like to find a way to make the bearing surfaces on the ways just a touch better, be it through scraping or surface grinding.

    Thats all that springs to mind at the moment, so off to the races!

  2. #2
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    The first thing ill be addressing is the table of my mill. Ive noticed for a while that parts i face off in the jaws always seem to have a wedge shape to them, no matter how firmly theyre seated on the parallels. Ive also noticed that no matter how in-tram my indicators say the head is, the cuts always heavier on the left side. Before anybody asks, yes, i did test the vise on a surface plate, and parallelism was spot on, so starting this i was 80% that the problem was in the table itself. First step, as it always is, was measuring to see exactly how bad the problem was. An indicator was set up in the spindle, this time my new Mitutoyo digital dial indicator, and zeroed at the back left corner:


    Shifting over to the front left corner, still on 0:


    So, the top of the table is parallel to the Y axis of movement. Now to check the X axis. Sweeping to the front right corner:


    Well that aint good. Halfway across the table and already a .004" drop, which is the same at the rear of the table:


    So, this got me 100% certain that the problem was with the machine itself, not the vise. The next step was to disassemble the mill and try to isolate exactly where the problem lies, which i did by starting with the table off at the surface plate.

  3. #3
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    Over at my surface plate, i started by setting the table down on its dovetails and doing the tap test, seeing if any corner was obviously off by listening for a knock, and right here i screwed up my methodology. I didnt realize until after the job done, but i was doing all these measurements by referencing off the TOP of the dovetail, and not the bearing surface like i shouldve been. Now, theoretically the top of the dovetail and the bearing surface are coplaner anyways so it doesnt matter anyways, but i did not verify that, referenced off the wrong surface, and got stupidly lucky. Im presenting this at this point as an example of what not to do. Again, my reference surface shouldve been the bearing surface, not where i measured from.

    Now that that disclaimers out of the way, over at the surface plate and verifying that the (wrong) reference surface is flat enough to take some measurements, i swept the surface with a DTI on a surface gauge, just to test and see if one side of the table was actually higher, zeroing at the front left corner and moving from there. The results:


    As you can see, the table is a little wonky, with the right side being roughly .006" lower on the left side. At this point, i got a little stuck. I knew i needed to take some material off the top, but at 15" the table is too large to blue up on my surface plates reliably for scraping, and my surface grinder is only a 6x18" so i cant- wait, i know exactly how im going to do this:


    There it is, all set up on the grinder ready to true up the top of the table, and take out some dings as an added bonus. Here again, warning, i did not do this correctly. Again, im referencing off the top of the dovetail, not the bearing surface that the table references off of on the machine. What i shouldve done was found a way to fixture the table so that it way sitting on those bearing surfaces, to make sure that the table top and bearing surfaces were coplaner. Anyways, before grinding the top:


    And after:


    Finish isnt perfect by any means, but its a lot better than it was, and its flat and parallel to the (wrong reference surface!) bottom of the table. Just for giggles, while i was at the grinder i flipped the table over and just barely dusted off the bottom. Serves no real purpose, but was nice to do. Now, another trip to the surface plate!

  4. #4
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    Time to measure the top of the table again, and here its more of the same as the last batch of measuring: reference off the wrong surface like an idiot, check the top to see how consitent the thickness is in a dimension that doesnt matter at all:





    DTI reads 0 at all 4 corners, and stays at 0 no matter where you sweep it, so the table top is as flat as i care to measure, and of consistent thickness in a way that has no real use! In case you havent noticed yet, im really cant believe i made such a boneheaded mistake in my process here, because it was sheer stupid luck that prevented me from making things worse. Anyways, now that the measuring is done on the plate, time to reassemble everything and redo the check on the machine, off to the next post

  5. #5
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    Omitting the assembly steps again, time to repeat the same measurement process from the start of all this. Indicator is zereod at the back left:


    And then traversed to the back right:


    Flat 0 across the full length of the travel. Against all odds, my idiot error actually ended up making things better, since the right side used to be >.004" lower than the left, which was what caused my thickness issues. Somehow i fixed the problem i set out to fix. With the indicator still zeroed, the front corners also need to be checked:



    As you can see, the front left of the table is high by ~.0025", and the right is high by ~.0035". This is where my referencing off the wrong face came back to bite me in the ass, once i realized my mistake in reference surface i went back and measure the thickness from the top of the table to the bearing surface of the dovetail. Sure enough, the table is thicker at the front, which confirmed 2 things; the top of the dovetail and the bearing surface were no coplaner, and i was an idiot for referencing everything off the wrong surface. On to the conclusion section

  6. #6
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    There she is, all reassembled and shiney, complete with a few marks already in that nice ground finish. Errors in referencing aside, the end result turned out okay, ive not got a flat table thats parallel to within a few thou of the direction of movement, and even the error is within the acceptable level of tolerance for my work. Once everything was put back together i checked for tram, which i didnt photograph because everybody photographs tramming a mill. Left-to-right rotation was easily dialed in to under a thou off on a 9 inch circle, but theres still about a .004" nod to the head. Adjusting that would require shimming the column, and being that i plan on replacing the column i didnt see it worth fussing with at this juncture.

    So, all the effort, was it worth it? Well, an indicator run across the base of the vise shows 0 misalignment, so it looks likely. Proofs in the pudding though, so time for a test:


    Bar of 1"x2" aluminium, tossed in the chuck tight against the parallels, then skimmed on both sides with a face mill. First thing i noticed was the double-cut pattern to the surface, where the cutter actually cut on both sides. First time ive seen that, and a nice confirmation that my mill is now actually in tram. The nod in the column didnt present as much issue as youd think either. Beyond subjective measurements, a micrometer read on on both ends of the bar showed a difference in thickness of .0012" across the 9 inch span of the part. Given that im used to seeing about 4 times as much change on a part 1/4 that length, im going to say yeah, it was worth the effort!

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