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Thread: Maintaining hole true position

  1. #1
    Registered Member chevynut's Avatar
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    Maintaining hole true position

    I'm looking for some tips on how to keep oversized holes centered where they're supposed to be when building welded assemblies.

    I have run across several projects where I wanted holes to be slightly oversize to account for tolderances, so bolts go in freely. What I typically do is drill the hole the nominal size, use a jig to locate the parts, weld them together, then ream the holes oversize.

    Do you guys have any tips for making this easier? A bolt with an oversized shank would be great, but I don't know where to get them. I could build a sleeve for the bolts, but I'd probably lose them.

    Any other ideas?
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  2. #2
    Registered Member NickP's Avatar
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    What tolerance value are you trying to maintain? Is the mating part that accurate? How many sub-assemblies form the completed assy.?

  3. #3
    Registered Member chevynut's Avatar
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    Nick, the problem is that if I use an oversize hole in the part, and a standard size bolt in the jig, the part can easily be skewed so the bolt touches the side of the hole. I want to try to keep the hole centered where it belongs. That way I can account for tolerances on the mating part.

    A tapered bolt or nut would do what I want to do, I'm just looking for some ideas how to do this. I run into this problem all the time. For example, let's say I want to weld a flange nut onto a part with a hole in it. If the hole is oversized, the flange nut may not be centered. So now I drill the hole nominal size, hold the nut in place with a bolt, weld the nut, then drill the hole oversized. If you don't do that, the bolt may bind as the welding pulls the nut off center even with the bolt in it.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  4. #4
    Registered Member NickP's Avatar
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    OK, what size hole? Drill bushings can be ordered in any size you need. Look at Carr Lane (http://www.carrlane.com/). A plethera of what you seek!

  5. #5
    Registered Member chevynut's Avatar
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    Nick, I've used drill bushings a lot when I worked as an aircraft mechanic. We used them to locate the center of an existing hole that was over a new panel with no hole. How does a drill bushing help me do what I want to do? Both of the holes I'm trying to line up are existing holes.

    As for sizes, I usually need typical 1/4, 5/16, and 3/8" holes.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  6. #6
    Registered Member NickP's Avatar
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    Well, I am at a bit of a disadvantage not being able to seeor touch what it is you want to do. CarrLane has an enormous selection of alignment pins, plates, clamps and otherassorted items. I was so in hopes ofsomething being there that might spark the idea/solution. Can you post pictures of the items and showwhat it is you want to do? If proprietary,email it to me with a non-disclosure and I'll work through it with you.

  7. #7
    Registered Member chevynut's Avatar
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    Nick, there's nothing proprietary about this. Let me explain better what the problem is.

    I am assembling my seat mounts that have two plates attached to a tube. The two plates have holes in them that match the holes in the tri5 floor, which are nominally 3 1/8" apart. Each plate has two holes, and they get welded to the tube. I want the holes to line up on any car, which has some tolerances.

    The holes on each plate are accurately located. However, I also need to locate the plates accurately relative to each other. I built a jig that has holes in it located precisely where I want each hole to be. I made the holes in the plates 5/16" in diameter, and the holes in the jig at 5/16" diameter. So if I bolt the plates to the jig, weld them to the tubes, everything is located where it's supposed to be. Once the welding is done, I ream the holes out larger, and they're concentric with the jig holes, and supposedly with the holes in the floor of the car.

    I was trying to think of a way to eliminate the reaming operation, and still have the plates located correctly. If I oversize them first, then use a 5/16" bolt to attach the plates to the jig, they can be off-center. As I mentioned, an oversize bolt with 5/16" threads would be one way to do it. I could make a bushing to fit in the hole to locate the parts.

    As I mentioned, I have a similar issue when welding nuts to the backside of a hole. If I oversize the hole, I can't get the nut centered. So I typically make the hole nominal size, hold the nut in place with a bolt, weld the nut, remove the bolt, and ream the hole oversize. If you've ever tried to do this, you know that welding can pull the nut off-center slightly, enough to bind the bolt.

    Just looking for some ideas, or off-the-shelf solutions.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  8. #8
    Registered Member NickP's Avatar
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    3/8" Shoulder Screws - http://www.carrlane.com/catalog/inde...2D252A5E415B59 Change the jig holes to 5/16 - 18 - determing the shaft length needed - assemble and weld.

  9. #9
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    That's a pretty drastic oversize on the hole - but it's a thought in the right direction.

    There are other things you can do.

    1. Weld up the brackets but don't drill the holes. Or maybe do one hole, or two holes on one end only. Use a threaded transfer punch screwed into the threaded holes to locate the clearance holes. Then center drill and drill to size.
    2. If possible, design your brackets so that there's no welds perpendicular to the length (minimizes shrink). Drill your holes to close fit for the welding. Then open them up to 1/32" oversize or something close, which is a pretty standard clearance hole.

    When I had race cars, the best made ones had close fit clearance holes, the builders were mostly using one of the two above procedures.

  10. #10
    Registered Member Run-em's Avatar
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    Hey, Laszlo, you want me to ask Ted? (See shade tree mechanics tips).

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