Anyone know what the perfect drill size bit is for an M8 threaded rod? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 13 Old 11-21-2012, 05:08 PM Thread Starter
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Anyone know what the perfect drill size bit is for an M8 threaded rod?

I'm making some spool clamps using an M8 threaded rod. I'm trying to get the hole to match the rod as close as possible but at the moment I only have an 8mm and 8.5mm drill bit and neither of these give a perfect fit.

I was wondering if anyone had experience of what I need to use.

Thanks
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post #2 of 13 Old 11-21-2012, 05:24 PM
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I expect rod diameter can vary slightly based on whether this was made as a metric version of an imperial dye or DIN.

Since you have the rod, why not measure the diameter with a pair of calipers.

You do not state if the 8 or 8.5mm drill bits are making a hole which is too large or too small.
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post #3 of 13 Old 11-21-2012, 05:36 PM
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It would be helpful to know what the thread pitch is for example 1.0 or 1.25 etc. I will check when I get back into the machine shop and let you know. Do you have a tap? If I remember correctly M8 usually 1.25. You can get a set of metric taps at Lowes. HF has them, but they are crap. They are OK as long as you are doing something soft like aluminum, brass or hardwood.
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post #4 of 13 Old 11-21-2012, 05:38 PM
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According to my table for both thread pitches it is 9mm or a letter T bit, but this will depend on how your die cuts, especially if it is an adjustable die.

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post #5 of 13 Old 11-21-2012, 05:41 PM
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Lots of places sell calipers and mics for cheap, harbor freight is one. Even the cheap ones will get you a fairly close measurement, unless you need accurasy in the 1/1000s you will be fine with a 10 dollar caliper. Use it to measure the threaded rod and find the right drill bit.

Thanks for your help
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post #6 of 13 Old 11-21-2012, 05:43 PM
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OK, I just checked, the correct size drill for a M8 x 1.25 is 17/64 (.2656). One size smaller is 6.7 mm, one size larger is 6.75 mm.
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post #7 of 13 Old 11-21-2012, 06:07 PM
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If all else fails drill a slightly smaller diameter hole in the spool and use a round file to fit it to the rod. I made some up many years ago and made brass bushings for the spools, the threads will dig into the walls of the hole and loosen them up eventually.

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post #8 of 13 Old 11-21-2012, 06:26 PM Thread Starter
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Thanks. I have a depth measurer which just uses a ruler which that accurate but its saying 8mm or maybe 7.5.

The 8mm drill was too large.

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Originally Posted by Fastback View Post
OK, I just checked, the correct size drill for a M8 x 1.25 is 17/64 (.2656). One size smaller is 6.7 mm, one size larger is 6.75 mm.
I'm being stupid here but are you saying the right dize drill bit should be 6.75mm?
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post #9 of 13 Old 11-21-2012, 06:27 PM Thread Starter
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Quote:
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If all else fails drill a slightly smaller diameter hole in the spool and use a round file to fit it to the rod. I made some up many years ago and made brass bushings for the spools, the threads will dig into the walls of the hole and loosen them up eventually.
I might try this.
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post #10 of 13 Old 11-21-2012, 06:33 PM
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Metric or inch, I just take one of those plastic drill index measureing things and start sticking it in the holes until I find one I like

John

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post #11 of 13 Old 11-21-2012, 06:45 PM Thread Starter
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Ok I tried using a 7.5 then expanding it slightly till i could get the rod through. It seems it flexes or theres as much movement as with the 8mm, I thought I could eliminate it more but I guess its not possible to remove it entirely.
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post #12 of 13 Old 11-21-2012, 08:31 PM
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a threaded rod will be slightly less

It will be slightly less than it's designated dimension...not much, a few thousands.
This conversion show a true 8.00 mm to equal 0.31 inches
http://www.sciencemadesimple.com/length_conversion.php
Result of your conversion:
  • 8.00 millimeters is equal to 0.31 inches
0.31 converts to 5/16"
http://www.seoconsultants.com/charts...al/#conversion

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 11-21-2012 at 08:34 PM.
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post #13 of 13 Old 11-21-2012, 09:15 PM
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I may have misunderstood your question. I thought you wanted to tap something. Oh, 5/16 = .312
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