# Rule of Thumb for Material Needed Either Side of Threaded Hole?



## Bolster

Is there a rule of thumb regarding how much metal should be left adjacent to a threaded hole? Let's say I have a piece of steel that's .250 in one dimension, for example. Is it OK to tap it for a #12 screw, major diameter being .216? That leaves just .017 of metal on either side, so I would not do it, just on intuition. But is there some rule of thumb?


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## New_Guy

is it a cylinder? you should be able to do it, i dont think you would break the tap if its all lined up right and you shouldnt cut out the wall but the thread wont be as strong maybe try using a smaller thread if you can


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## Bolster

Regardless the shape of the metal around the hole, how close to the edge of a piece of metal (steel) can the hole be, before thread integrity is compromised? 

What I am looking for is a rule of thumb, such as "the metal surrounding a hole should be ___ percent larger than the major diameter of the screw," or something similar. 

Preferably I'd like to know how much metal to leave around a hole, without weakening the threads appreciably. The dimensions above are just examples for clarification.


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## Ken I

I've been in engineering over 40 years and I don't recall any such rule of thumb.

Normally you need to pitch away from an edge sufficient to leave space for a cap screw, hex head or washer diameter.

I have placed holes at half the diameter from an edge (it doesn't break through because the major diameter is always a little less than theoretical) - not a desireable state of affairs - but then I can't see that it created any problems.

I recently placed a row of M5 holes along a 6mm copper plate to secure a steel "T" piece with no problems.

The only rule I would state is that the thread should not break through.

When tapping holes in the ends of shafts the two cross sections should be (at least) approximately equal

Example an M6 has a core of approx 5 diameter then the minimum OD would be (5^2 + 6^2)^0.5 or 7.8mm diameter - at this point the bolt and shaft have the same tensile strength - if made from the same materials.
For a cap screw having approximately twice the tensile strength of mild steel this would become ({2x[5^2]} + 6^2)^0.5 or 9.3mm diameter (calculations are ignoring all constants such a Pi - the results stay the same).

Repeating this for an M8 (same material) you get a minimum OD of 10mm.

Perhaps this clearance on a shaft could serve as your "rule of thumb" for distance to an edge.

Hope this helps.

P.S. in the medievil period the rule of thumb was that you weren't allowed to beat your wife with a stick thicker than your thumb.

Ken


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## Bolster

Yes! Very helpful! I see I have more leeway to get closer to the edge than I thought. Applause for the answer. 

And regards wife beating, I have been using a rod about the diameter of my little finger, so I'm sure we'll all be thrilled that I can move up in diameter to my thumb, thanks for the info!


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## Mainer

My general guideline is to leave enough space so the head of the bolt bears fully all around. Assuming we're doing this to scale and not taking into account "heavy pattern" bolts and such, a bolt head should be 1.5 times the diameter of the bolt (e.g. a full-size 1" dia. bolt has a head 1.5" across the flats) so we could get within 3/4 the diameter of the bolt to the edge before worrying about the bolt head not bearing fully (I'm neglecting the points of the hex).

Does this "rule" mean anything? Probably not.


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## bp

I agree with most of the above, but to throw a spanner in the works, when I was a draftsman if I left less than 1.5 x D between the centreline of the hole and the edge, I would have been sacked.
However making sure that whatever washer you will be using will be fully supported should do the trick.
In the main rules of thumb are based on precedent as to what has worked in the past. When in doubt ask like you have!!
Merry Christmas
and a safe new year
cheers
Bill Pudney


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## rcmadness

One thing I will throw into the mix is the fact that if you are tapping Alum or Brass, if you get too close to the edge it will bulge out the metal on the side of the part when you tap it. Ask me how I know!!!!!!!!

I would wager a guess that the edge of the hole to the edge of the work should be at least the width of the bolt. Remember this is just a guess.


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