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  1. #1
    rpmachining Guest

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    Does anyone have any good tips or information/experience relating to methods of resurfacing a Bridgeport table?

    Last week, I purchased a Bridgeport for use in my home shop. It's an early 1970's machine, but it was just rebuilt. New bearings and bushings throughout, new lead screws, all ways rescraped, dovetails and gibs realigned/adjusted, and new paint. NICE! The only thing that wasn't done was a regrind of the table (probably to save $'s). Not that it really needs it. The table shows only the normal wear of a 30 yr old machine, a few minor pits and dings, and a few slots made by careless operators. It's more of an aesthetic issue.

    I talked to a local grind shop that specializes in Bridgeports and he can grind the top and sides of the table for $115. Sounds reasonable, but he warned that this sequence is backwards - once you break the "skin" of an old casting there is a (good) chance that the table will stress-relive itself and warp. This would require the bottom dovetails to be reground, which would then naturally lead into a new round of rescraping an aligning the saddle ways etc. He suggested that if it is just a cosmetic issue, I simply go to work on the table with a good stone and plenty of kerosene.

    Has anyone tried this? Were the results worth it? Would rescraping be a better option? Again, itís not that the table is mechanically defective. I just want the appearance of the table to reflect the quality of the other work that's already gone into the machine as it embarks on its new life.

    Thanks

    RP

    [This message has been edited by rpmachining (edited 02-12-2003).]

  2. #2
    Fred T is offline Cast Iron
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    rpm
    If what the grinder says is true, about breaking the case, then how do you know that the table isn't warped from the grinding that was done on the underside of the table? Not trying to be a smartass but am questioning the local grinder. Personally I would use and orbital sander first then a stone with some kerosene on it and live with it. If it has slots and holes in the top that you can't stand then but some steel filled Devcon and fill them. What I did was to take a piece of cold rolled and surface grind one side of it. Then apply the devcon and clamp the cold rolled over the spot and let it cure. Son't forget to use some kind of release agent if it is a big spot. To remove the plate just bump with a hammer.
    Good luck
    Fred T

  3. #3
    Forrest Addy is offline Diamond
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    No orbital sander! Showers abrasive and depresses the crisp corners. Control the process. Stone only, file if you absolutely have to.

    Large machine tools have seemingly acres of table. In most shops using a power sander to dress the dings is a firing offense.

    I suggest you squelch your feelings for your marred table. Transfer your interpretation! Call them - battle scars honorably earned.

  4. #4
    Cass Guest

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    I agree with Forest. Stone off the high places. If you have some master surface you might scrap the table if it has minor problems. You want the table to be flat, not just pretty. A sander will knock the surface out of flat in a big way in my experience. It will take a lot a longer to screw it up with a hand stone, although you could do it with a sanding block. Unless you know what you are doing with scraping and have a lot of time on your hands just get the bumps off with a stone and be proud of the battle scars as mentioned above.

  5. #5
    Tumbleweed Tim is offline Stainless
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    Question about stones,
    I have a Blanchard burr buster that used to be a very nice tool for getting the high spots and raised edges off of tables and vises. It is shaped like a figure 8 and relieved in the middle. I suppose you would use this tool with some kerosene or something to carry away the metal partials, but it has been abused and is loaded up. It will not cut anymore. How do you clean or dress such a stone? Or any other stone? Or am I looking at a piece of scrap? I do not know if Blanchard is around any more but I did find the same thing available from Pacific abrasives and would buy again because it is a very effective tool, however if I knew better how to care for such an item it would not be necessary.

  6. #6
    rpmachining Guest

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    Thanks to everyone for the info. I have since spoken with two other local machinery rebuild shops and a precision scraping shop. They all reflect opinions similar to those expressed here: ďleave it alone, messing with it now will just open up a can of worms!Ē

    All of you are right. This is more of a personal problem rather than a mechanical one. I must get over my need for ABSOLUTE perfection and move on with my life (my wife tells me this too)!

    Seriously though, thanks again for all of the info.

    RP





    [This message has been edited by rpmachining (edited 02-12-2003).]

  7. #7
    Forrest Addy is offline Diamond
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    Slip stones and bench stonse glaze, go out of flat, load, and sometime just quit working.

    The solution is to dress the stone but in the past that involved laboring over a piece of steel plate covered with a slurry of silicon carbide grit. It was a dirty job and in the absense of a pressure washer it took longer to clean up the refreshed stone as it did to true it.

    If you had the equipment you could grind the stone with a diamond cup wheel and I have blocked them immovable on the magnetic chuck of a surface grinder and ground them with a coarsely dressed wheel.

    In all cases the reconditioned stone came out too smooth. Besides consider the cost of replacement Vs reconditioning. Doesn't take a ten minutes of company time fooling around run up the cost of a stone. Besides you have to lightly sandblast the working face to rough them up.

    Stone dressing is made easy these days when relatively low cost diamond bench stones are available. I have a sharp coarse (80 grit, I think) diamond bench stone I use just for tuning up my slip stones. The job is done in minutes under running water. No sandblast, no abrasive muck to clean up. Dry the stones in the oven for a few hours and you're ready to go.

  8. #8
    Spin Doctor's Avatar
    Spin Doctor is offline Hot Rolled
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    One thing that I think I would check is mount an indicator in the spindle whether in a collet or indicol doesn't matter. Tram the table through the whole working range of the machine. Post back what the results are then we'll kick it around some more.

  9. #9
    Fred T is offline Cast Iron
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    Sheueeeeeeeee.........
    In my own defense, most of the bridgeport tables I have seen have a chamfer on all the square edges, so I never worry about knocking them off. As far as an orbital sander I am talking about a woodworking sander with about 180 grit paper on it and a hard felt backer, and like anything else some common sense while using it goes along way. At any rate I hope things work out like you want them to .
    Fred T

  10. #10
    Tumbleweed Tim is offline Stainless
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    I quess I need to buy some new stones. These guys really work good on tables.
    http://www.pacificgrindingwheel.com/.../burrbstrs.htm
    Thanks for the info Forrest

  11. #11
    rpmachining Guest

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    I finally got the machine setup in my garage. After some initial cleaning, I indicated the table over its entire travel. The results - absolutely DNO (Dead Nuts On)! I think I might have seen a tenth in one spot, but other than that, the needle was motionless throughout the whole range. It's great to see that someone really took some time to do a quality rebuild on this one.

    RP

  12. #12
    Greg B's Avatar
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    RP, what was the name of the local grind shop that gave you the quote? I need to have the column of my Atlas shaper reground. I would do it myself but, it is just a little to tall to fit under the wheel of the Okamoto at work. This shop sounds really reasonable. My shop sends their large workpieces to a shop over by San Pedro and I don't want to drive all the way over there. Thanks, Greg B.

  13. #13
    rpmachining Guest

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    Greg -
    Schaffer Grinding Co. Email me if you need the contact info. They came highly recommended by Orange Coast Rebuilding, one of the largest rebuilders of Bridgeports in SoCal. However, Schaffer is up in Montebello, so it may not be any shorter of a drive than San Pedro (traffic is probably just as bad too)!

    RP

  14. #14
    Greg B's Avatar
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    RP, actually Schaeffer is where we send our stuff. But thanks anyway.
    Greg B.

  15. #15
    AlfaGTA is offline Diamond
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    Have used Schaffer myself.....very good work and very reasonable. I am in Nor Cal so it is a bit of an effort to send them machines, but it is worth the effert! Ross

  16. #16
    rpmachining Guest

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    Apparently, Schaffer has several shops scattered throughout SoCal. I contacted one other and was told that they do all of the Bridgeport stuff out of Montebello. Didn't know they had one in San Pedro too.

    RP

  17. #17
    AlfaGTA is offline Diamond
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    RP: i saw the post about indicating the table. Unfornately you can't tell if the table is flat by doing that. The table can have a bow from sag over time and usually it won't shoe by running the table back and fourth and reading with an indicator. The trouble is that the table will always show the thickness of the table at the point of measurement. If the table top and bottom are parallel then no indicator change, even though it could be bowed! Think of it as a round ring . If the ring is paralell (inside and out) and you support it on a roller (on the ID) ,that represents the table bearing of the Knee, and place an indicator above the ring (on the OD) as you rotate the ring (move your table from side to side) the indicator will read zero, no change, but the part is not flat!
    The only to test for flat is with some kind of standard...Surface plate known to be flat, paralell thaat is known to be flat (check agnist the calibrated surface plate etc...) You could also use a laser or autocolimator for this check. A precision level placed on one end of the table and leveled to zero and the table moved through its full travel right and left will give a clue about how tram and loose everything is. Cheers Ross

  18. #18
    Greg B's Avatar
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    RP,
    You are right, they are in Montebello, I just always had the impression that Montebello was near San Pedro. I don't get much farther north than Fullerton very often. When I do head that way, my wife and I are usually on our way to Monterey, so we just make a freeway run through L.A. Sorry for any confusion I may have caused.
    Greg B.

  19. #19
    rpmachining Guest

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    Ross-
    Thanks for the info. I actually had this same conversation with the head machinist at Orange Coast Rebuilders when I started researching this whole table-regrinding thing. According to him, there is no such thing as a "flat" Bridgeport table (or any vertical knee mill for that matter). Take any Bridgeport table, new or old, turn it upside down on a master surface plate and it will freely spin like a top. This is just a natural effect of gravity, once a table has been installed on a saddle. I asked him if there was some max allowable spec. He said no, the trick is just kowing that it exists, and controlling and accounting for it during the reginding/rescraping process. Like you, he also suggested that indicating the table, either from the spindle or directly from the saddle, is a good way to check the accuracy of the dovetail grinds on the table, not it's flatness.

    I think it would be an interesting experiment to try to measure this bowing on my mill. I'll try to rig up something to quantify this, maybe trying my machinsts level like you suggested. I would be curious to find out what other mills are like too. If you or anyone else are able to check your tables, it would be interesting to compare results.

    RP


    [This message has been edited by rpmachining (edited 02-21-2003).]

  20. #20
    jim king is offline Aluminum
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    I have never tried it but swing the head to the far right or left and then run the table to its extremes.Bet you will see the bow and sag as it moves off the saddle.

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