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What is acceptable tolerance for a frame??

Discussion in 'The Hokey Ass Message Board' started by Shaggy, Jan 15, 2010.

  1. In my opinion, there is nothing more important for a 32 Ford, than an accurate frame.
    The gas tank to body, body to frame, fenders to frame, hood to frame, grill to frame, all depend on an accurate, square frame.
    We've all seen examples of close, what some think is close enough, is good enough.
    HemiDeuce.
     
  2. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,394

    fiftyv8
    Member
    from CO & WA

    I believe we should all be aiming to get it perfect, but tolerance is different for various circumstances.
    How much error would you be prepared to tolerate is the question.
    The frame/chassis is the foundation so if it is out then, everything that follows will also be affected.

    I would say you guys as a vote would mostly agree that you would be prepared to accept 1/16" as your tolerance.

    Even putting weld anywhere can pull the frame etc slightly out of position that is why a good accurate jig to start with is good.
    Good tools good results.

    I think the more thought and times you do this stuff the more accurate you can become without all the stress and struggle that some folks seem to create for themselves.
     
  3. BISHOP
    Joined: Jul 16, 2006
    Posts: 2,571

    BISHOP
    Member

    And for those of you that don't have a jig, you can make a pretty accurate frame table from 2x4s and 3/4 in. MDF.

    Its not that hard to do, and clamping the frame down is easy. All you really need is for the frame to stay put and not move when welding. You can do it inexpensively with wood instead of a fancy jig. The only real down fall is the MDF gets smokey sometimes.
     
    Last edited: Jan 16, 2010
  4. in auto body school I was taught 1/8 in
     
  5. langy
    Joined: Apr 27, 2006
    Posts: 5,730

    langy
    Member Emeritus

    Always use the same tape as not all tapes are born equal :(

    I have a tape here that is up to 1/4" short in places :eek:
     
  6. torchmann
    Joined: Feb 26, 2009
    Posts: 787

    torchmann
    BANNED
    from Omaha, Ne

    long as it looks straight?

    this ladder frame would be thought of as perfect but not for a car:[​IMG]
     
  7. "Perfection" in manufactured parts is impossible. All measurements have errors, and they have a finite accuracy and precision. With something like a tape measure they are only as precise as their divisions, which if it is marked off in 16ths, would have a precision of 1/32" because you can estimate between divisions. So if you measure something that is supposed to be 2-19/32" long and it measures as 2-19/32" long, it might look perfect but really that just means that the error is smaller than our ability to measure it, as much as 1/64" with the tape measure. The same thing applies to measurements made with a caliper, something might be spec'ed out at 2.250" measures 2.250", but it could be 2.25018436572...", and since those number go on forever, you can't measure them, so you couldn't ever say it was perfect.

    Even if you could measure something with infinite accuracy and precision, it wouldn't do much good. The temperature of the frame, how its supported, where the loads are on it all deform the frame so whenever you change something, you have changed all the measurements. Interesting things happen when this isn't accounted for, or accounted for incorrectly, one example is the Hubble telescope's first mirror.

    Everything has tolerances, and if it is supposed to be "perfect", you could just assume that it is just accurate beyond our ability to measure it. A good tolerance for a frame is, as everyone else has said an 1/8" (if you can do better than that after burning the whole thing together you are a rockstar). But a good thing to remember is tolerance stack-up. Stack-up 16 things with a 1/16" tolerance and the new tolerance for the stack is +/-1".
     
  8. An alternative to a tape measure would be a tram gauge. On modesn cars measuring with a lazer measuring system + or - 4 mm is within tolerance.
     
  9. I forgot to say that 3mm is equal to about 1/8 th. My geuss is on a full frame car you could get away with twice that easily as long as you have enough suspension adjustment to keep your axles square. Like stated above some body styles may require the frame to be more precice for body alignment.
    Does anyone remember Chevies in the seventies with stacks of shims everywher in the front sheetmetal to make it line up ?
     
  10. tiredford
    Joined: Apr 6, 2009
    Posts: 560

    tiredford
    Member
    from Mo.

    If it drives straight and the tires wear OK, then it's good enough for me. Really, what more do you want. The first time you drive it, everything is going settle in where it wants to be anyway. JMO.
     
  11. rottenleonard
    Joined: Nov 7, 2008
    Posts: 1,993

    rottenleonard
    Member

    When I build a chassis one thing that seems to help is to try to use the same point of reference for as many measurements as possible, like as stated previously with a tape measure there is always some varience, lets say 1/16" so if you measure say from a rear frame corner to a say body mount and then from there to say the mounting point for the front crossmember you have the potential for two variences of up to 1/16" or a total of an 1/8", but if you had measured directly from the rear corner of the frame to the front crossmember you should end up within you 1/16" tollerance.
    I hope this makes sense.
     
  12. Shifty Shifterton
    Joined: Oct 1, 2006
    Posts: 4,964

    Shifty Shifterton
    Member

    Not hard to find 1/4" variation on 70s GM frames that have never been hit, and those cars generally have fabulous high speed tracking.

    To the person concerned about their front and rear axles tracking straight, that's why suspension designs have a degree of adjustablility built in.

    Also remember how much frames flex going down the road. Realistically, perfection ends the moment it's put in gear.

    Know what's more important than building to 1/16th versus a larger number??? Planning the assembly sequence so that the next piece can remove deviation instead of stacking it higher.
     
  13. johnny bondo
    Joined: Aug 20, 2005
    Posts: 1,547

    johnny bondo
    Member
    from illinois

    lets be honest here..... buggy spring suspension.... doesnt need to be a space shuttle. yall are overkilling it.
     
  14. shawnspeed
    Joined: Sep 10, 2009
    Posts: 165

    shawnspeed
    Member
    from Attica Mi

    Having worked in automotive prototype work in the late'90s , on full size and small P/U chassis some factory frame are so poor that they have to punch holes AFTER everything on the frame is welded to ensure body/chassis alignment....frame rails were out of parallel by a 1/2 " or 13mm over there length& width...they drove just fine...also without a fixtue table or surface plate, you will be hard pressed to get within an 1/8 " and most concrete floors have holes and dipsy doodles of more than 1/4" . measuring across the chassis from suspension mounting points to ensure the chassis is square within 1/8 would be more than acceptable ,as for twist, most early frames rely on the body for part of their stiffness and will flex corner to corner with wheel /suspension input alot more than you would suspect(1/2" +++just watch some of the P/U truck commercails and watch the box to cab movement)in the end , it's what YOU are comfortable with...myself on a concrete floor in a garage I would be happy if the cross measurment was 1/8" or under, and both rails looked level in a side veiw with each other ( no propeller)....Shawn
     
  15. falconsprint63
    Joined: May 17, 2007
    Posts: 2,358

    falconsprint63
    Member
    from Mayberry

    YOur grandfather rocks.

     
  16. SniffnPaint
    Joined: May 22, 2008
    Posts: 434

    SniffnPaint
    Member


    x 2
     
  17. fordcragar
    Joined: Dec 28, 2005
    Posts: 3,198

    fordcragar
    Member
    from Yakima WA.

    I have an old 1970 Chassis Service Manual for Chevrolet and in section 2 it says "When X checking any section of the frame, the measurements should agree within 3/16".
     
  18. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,263

    gimpyshotrods
    ALLIANCE MEMBER

    Sure did, had a '63 Falcon too!
     
  19. HotRod33
    Joined: Oct 5, 2008
    Posts: 2,570

    HotRod33
    Member

    I can read a tape measure too... but on buggy spring suspension you can be off alot...3/16 -1/4 inch and it is not really going to matter..... even on later models cars with independent suspension they have adjustments built into the suspension to get everything lined up..........
     
  20. Pete1
    Joined: Aug 23, 2004
    Posts: 2,253

    Pete1
    Member
    from Wa.

    We don't do very many frames any more but we subscribe to the old saying,
    "Do it right if it is worth doing".

    Or, "DEAD ON IS CLOSE ENOUGH"
     
  21. redo32
    Joined: Jul 16, 2008
    Posts: 2,137

    redo32
    Member

    Jeeeze!!! You guys with surface plates, lazers & 60' calipers KNOCK YOURSELVES OUT!!
    I thought you were all "TRADITIONALISTS" A lumpy concrete floor, apiece of chalk & a string, along with a good eye and the desire to do it the best you can will make it close enough!!
     
  22. redo32
    Joined: Jul 16, 2008
    Posts: 2,137

    redo32
    Member

    OK you know I meant 60"
     
  23. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,263

    gimpyshotrods
    ALLIANCE MEMBER

    Damn, I thought you might have the hook-up on some big calipers.;) Can never have enough tools.
     
  24. 29nash
    Joined: Nov 6, 2008
    Posts: 4,544

    29nash
    BANNED
    from colorado

    When I have removed every bolt and component and re-attached them. Frame straight? When I stand back and look at it and like it.
     
  25. Pete1
    Joined: Aug 23, 2004
    Posts: 2,253

    Pete1
    Member
    from Wa.

    Hee heee......Been there done it many times but the pleasure in driving a rod or setting up a race car that is "right" is far better. We still sit around the beer barrel and talk about the
    "crap" we ran in the olden dayz though.
     
  26. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    The 2 side rails can look like 2 different roller coasters, but as long as the suspension pick-up points are square enough that the axles and spindles etc. are in plane at rest it doesn't make any difference.
    "Dead on" sounds nice, and it's very important if the car has no suspension, no bushings, and steel tires.
     
  27. Pete1
    Joined: Aug 23, 2004
    Posts: 2,253

    Pete1
    Member
    from Wa.

    "Dead on" sounds nice, and it's very important if the car has no suspension, no bushings, and steel tires.

    "Dead on" is quite achievable with average home tools.
    Depending on the type of chassis, dynamic forces will move it under load no matter
    how good it is statically. The better is is to start, the better it will perform.
     
  28. fordcragar
    Joined: Dec 28, 2005
    Posts: 3,198

    fordcragar
    Member
    from Yakima WA.

    Dead on? I thought that we were talking about cars, when did we switch to airplanes or hogged out chunks of billet?
     
  29. KrisKustomPaint
    Joined: Apr 20, 2007
    Posts: 1,107

    KrisKustomPaint
    Member

    IMHO Trying to get a frame square, level or in any other dimension to a variance of less than a 1/16 is fuckin' crazy. There is just no point. Most frames flex quite a bit more than you think they would, unless they're built like a brick shithouse. Next time you got a frame striped put 3 jack stands on corners and stack a bunch of weight on the corner opposite of the one that doesn't have a jack stand under it(or even better tie it down). Then stand on the corner with out a jack stand and see how much it deflects. I guarantee you if you weigh more than 140 lbs. it'll move at least a 1/16th and if its a stock model A you'll find out why its crazy not to box the frame.
     
  30. sensor
    Joined: Feb 17, 2009
    Posts: 82

    sensor
    Member

    *ive only scratch built one frame and no i dont claim to be perfect*

    well.....when doing pulls at work we have a frame rack(chief ez liner since you asked) with scanner and targets so those will tell you everything...
    at home i use a tram(tape measures are fine but youll never get an accurate measurement over a big distance by yourself)...x measure as many times/places as possible ....when they all read the same thats when you start tacking and remeasuring...if it doesnt move and measures out start welding
     

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