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Building the Trent Pinion Mill

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  • #61
    Originally posted by QSIMDO View Post
    Just want to say, enjoying your work and videos very much!
    Thank you!

    Sent from my BLN-L24 using Tapatalk
    Max
    http://joyofprecision.com/

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    • #62
      Nice.

      I kept wondering why you used the end mill and not a fly cutter.... then you brought in the fly cutter to finish the surfaces....! You really need to add scraping to the repertoire, assuming you have not already done so. Does a good job when well done, and you would do it well.

      That stuff you mentioned about lapping surfaces together.... Agree on it being better to leave them milled. That "lapping" is best left to the folks who do it so they can turn their pens to a tenth of a thou tolerance on their cheap import machine......
      CNC machines only go through the motions

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      • #63
        Originally posted by J Tiers View Post
        Nice.

        I kept wondering why you used the end mill and not a fly cutter.... then you brought in the fly cutter to finish the surfaces....! You really need to add scraping to the repertoire, assuming you have not already done so. Does a good job when well done, and you would do it well.

        That stuff you mentioned about lapping surfaces together.... Agree on it being better to leave them milled. That "lapping" is best left to the folks who do it so they can turn their pens to a tenth of a thou tolerance on their cheap import machine......
        Haha yeah I found that milling the rough cast skin away before fly cutting saves a lot of sharpening the fly cutter. Regarding scraping, I've just picked up what I can over the years, most recently from Stefan and his second-hand information from Mr. King. I can get a surface blued up real nice, but probably takes 20 times as long as it should. I'm going to buy his scraping DVD, it sounds like the next best thing to taking his class.

        Scraping already started on the knee and the column, I think somewhere earlier in this thread there was discussion about that. The rear and top surfaces of the knee are scraped, just because I want them to be good references for scraping the dovetails. The fronts of the column dovetails are scraped too, because they were easy to do any time. Now the trickier parts, the rear surfaces of the column dovetails will need a little fixture made, and the dovetails in the knee need small angled straight edges. I'm about 1/3 of the way through scraping the larger of those 2 angled straight edges, it's one that I made large enough (and with a shallow enough angle) to scrape in the cross and compound slides of the Rivett and Hjorths if I want to. I'm not going to scrape the CRS bed of the Trent, after a test cut with Randy Richard's insert dovetail cutter, I'm confident that the machined surface against the scraped knee will be excellent. If I scrape the undersides of the indexing head and tailstock castings, it will be very rough just to get decent static bearing against the bed. They don't slide along it in use, they just need to be solid when locked in place during setup.
        Max
        http://joyofprecision.com/

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        • #64
          For sliding surfaces, you have one amount of points and bearing. For a static "location" surface (even of you move it around) you need a lot less points. I see for ordinary slideways, 10-to 15 points per sq inch, and 50% bearing. For static support surfaces, more like 5 points per sq in, possibly 30 to 50% bearing.

          And then there is "scraping for alignment". Shading the scraping off to one side or one end, to bring surfaces into proper relation. That is as important as just for "bearing", plus you need to do both. But it is a LOT more "sensitive" than machining, you can achieve basically anything you can measure as far as alignment and so forth, so scraping is very versatile.

          If I had to boil down just the process of getting a good surface, I would say the following:

          1) Scrape off all the blue, and dig in, don't just scrape at the surface, until you have blue distributed all over the surface. Once you know you have a good bit to take off, you can make the chips fly at this stage

          2) Then start splitting the blue spots, without removing them entirely, until you get reasonably close to the points per inch and amount of bearing surface. You can be a good deal lighter handed with this.

          3) look around at the surface, and where you see bulls eye spots, with a light color surrounded by blue, take off the light center areas. You can add in a bit of what you did in stage 2 if it looks needed. keep doing that until you have good distribution, points per inch and area. Then you are done.

          Most of the time should be in #1. #2 and 3 should go surprisingly fast once you get to them. Each time you blue up, scrape at right angles to what you did before. If you are not seeing the surface change in step 2 and 3, just moving points around, then you are probably not splitting points, you might be removing too much of them at a time.

          For tilting a surface, "step scrape" at stage 1. Start at the end that needs to go down. divide the area up into crosswise strips maybe an inch or so wide. Scrape the last one at the end that needs to go down. Then scrape that again, adding the next one in. Then scrape those plus one more,, until you have scraped the whole area that needs to tilt. Alternate directions as before. Repeat as needed. One heavy scraping pass is usually around 2 tenths of an inch change of the surface

          Do that until you get pretty much "there", when you can start lightening up as stage 2 and 3. Stage 2 may still have some step scraping. Stage 3 should not.

          If you take a class from Rich King you will get better and more detailed instruction for sure.
          Last edited by J Tiers; 07-25-2017, 08:17 PM.
          CNC machines only go through the motions

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          • #65
            Great build Max! Looking forward to the next part.
            www.thecogwheel.net

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