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Thread: Explain runout please

  1. #1
    Join Date
    Mar 2006

    Default Explain runout please

    I have heard the term used in several applications and I wanted to get a better handle on it. I am about to purchase an older cable PR96, so I am sure my accuracies are fairly limited - but I do want to get an idea on what to expect.

  2. #2
    Join Date
    Sep 2005
    Mountain View Wood Works, Troy VA


    If you take a drill press and chuck a bit in it and turn it on, you expect the bit to spin, not wobble. The amount of wobble is "run out". The wobble could be due to a bent bit, bad bearings in the drill press etc...

    Now think about your router or spindle and the router bit under it. Same thing as the drill press.

    I hope this helps

  3. #3
    Join Date
    Jul 2004
    Valcourt, Québec, Canada



    depends what specifically you're referring to. Ed is on the right track but the example about the drill bit wobbling can be from more than one parameter.

    Say you wonder about the router/spindle to tool bit (shank) "assembly" runout, it refer to the amount of play there is horizontally (and vertically) AT the tool bit (shank) keeping the router/spindle steady while turning/wiggling from the tool bit (caution to sharp edge). You can check it with a dial indicator.

    Here I'm checking on the vertical runout (freeplay)...


  4. #4
    Join Date
    Feb 2004


    Honestly, Bret, I'd stay away from the cable machines. They are notoriously inaccurate and require constant maintenance, or so I've been told. I'd keep my eyes peeled for a used rack and pinion machine,...spend a little more, headaches. There is a REASON no one makes the cable machines anymore.

  5. #5
    Join Date
    Oct 2001


    Runout has become a catch-all word for the sloppiness, flexibility, non-repeatability or just plain "poor quality" of a machine. Runout could mean many things to many people and the word is often thrown around as a bogeyman.

    In the strict sense of the word, runout means the amount of deviation from the theoretical position.

    The term can be applied to rotary or linear motion, static or dynamic. Because most folk who actually measure runout use dial gauges (as in the pics above), the measurements are normally static, or at hand-turned speeds.

    It also stands to reason that if you push/pull, ie. apply force, to the shaft/carriage while measuring, that you will get a bigger reading of runout. (Somebody who wants to prove bad runout will push/pull rather hard - a machine salesman who wants to prove low runout will not push/pull at all)

    Why the different readings depending on force applied?
    - Because ALL machines are flexible and they WILL bend/deflect. (Cables are much more flexible than gear-rack teeth)
    - Because there is often freeplay/backlash in a connection between parts of a machine and applying force causes the parts to move within this free range.

    The other big cause of runout is a manufacturing defect, or damage. A bent shaft will not run "true" even if it was very stiff and had no freeplay. If a machined rail is not produced/setup straight then the rolling carriage will deviate from the theoretical straight line - ie. runout. If a 4-fluted router bit is put in a collet with a bit of dirt causing it to be held slightly skew, a dial indicator will get different readings from the flute edges - runout.

    Can one achieve zero runout? No. Any machine that is doing the work it is designed for will bend and deflect. Normally these claims/demonstrations are made without applying loads and with measuring instruments of low resolution.

    Is runout always bad? No. In the example with the 4-fluted cutter sitting slightly skew, it means that one of the flutes will take a bigger cut than the others as the cutter "wobbles". And the kerf will be wider than expected. For a single-fluted cutter, the only down side is the wider kerf. But, that wide kerf will be absolutely consistent in its width, and will be as straight as the carriage rails.

    Is runout consistent? No. Aside from the fact that it is load dependent (as described above), it is also temperature dependant. A bearing that has a lot of clearance (backlash/freeplay) when cold, will have hardly any clearance when at the right running temperature - if it has been designed and installed correctly.

  6. #6
    Join Date
    Jan 2006
    Toomey Studios, Orlando FL


    Man Gerald, that has got to be the best explanation of runout I've ever seen. I definitely agree with your observation about thermal effects on runout. I recently did a joinery project using Corian and had some extremely tight tolerances. The first batch fit perfectly but as time went on the fits got different. Stopping the machine for awhile to see what had happened, I couldn't find anything and started again. Amazing, everything fit again, a few minutes of cutting later, out of whack again. I narrowed it down to everything heating up and something moving just a little bit but enough to make a visible difference. In the end I think it was a combination of a collet that needed replacing, the spindle bearing and maybe even the bit itself. A new collet seemed to level everything out enough to fit hot or cold.

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