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I am building a table and I have something like the following:

picture of joint

I want to know if I have a 3/8 inch bolt, how large should the hole on the diagonal piece be? I already secured it on the table leg.

  • Just larger to provide clearance. – Solar Mike Dec 1 at 6:38
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    @SolarMike Yet not too much or it's boing to be a very wobbly table. – Mast Dec 1 at 14:19
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    @Mast: The squared and smoothed (deburred) ends of the side and end frame rails are what hold the leg square and prevent wobbling. The bolt is only to keep the leg pressed firmly against the rails. – A. I. Breveleri Dec 1 at 23:18
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    @A.I.Breveleri true, but the better the fit (minimum clearance), the less off-axis wobble allowed to the bolt, plus the more area the washer can spread its pressure over. – Carl Witthoft Dec 2 at 15:17
  • @Carl Witthoft: Don't say "true" if you're gonna follow it with exactly the opposite of what I said. The shaft of the bolt should not touch the sides of the hole. I've posted an answer below. – A. I. Breveleri Dec 2 at 16:56
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It's important to remember the difference between a bolt and a screw (this is handy if you don't know the size of the bolt or screw)

  • Screws have the threads doing the work of holding. Your hole should be the size of the screw shaft. In other words, hold a drill bit above the screw. You should still see the threads, but not the shaft.
  • Bolts have a nut doing the work of holding. As such, the hole should be larger than the size of the outer threads. In other words, your bit should cover the shaft and threads.
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    For soft wood, the hole should be slightly smaller than the screw shaft. – MooseBoys Dec 2 at 0:50
  • If the bolt has thread all the way along the shaft, it would be structurally beneficial to treat the bolt as a screw and use the nut to prevent loosening as well as holding everything together. – Chris Rogers Dec 2 at 8:40
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    PLease make it clear that you've defined the screw tap hole diameter for the leg, not the cross-brace, which of course needs a clearance hole for screw or bolt – Carl Witthoft Dec 2 at 15:18
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    @ChrisRogers The problem with that is it can prevent the bolt from pulling the pieces together. If you are worried about it loosening, a second nut or a lock washer should suffice. – JimmyJames Dec 3 at 15:31
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    The distinction between the two ways of using rotary fasteners is useful, but machine screws are often used to join things with which the threads don't interact with, while lag bolts are used to join things the threads do interact with. – supercat Dec 3 at 19:27
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When you buy a hex bolt, the size on the label is the diameter of the shank below the head. The outside diameter (aka major diameter) of the threads will be no larger than this diameter.

So for a 3/8" bolt, it's simple - you drill a 3/8" hole.

thread diameters

13

This is a rare case where you want the hole to be somewhat larger than the bolt diameter, to allow some shifting and settling. For a 3/8" bolt the hole should be 1/2" or 5/8" diameter. You must use a fender washer under the nut.

The squared and smoothed (deburred) ends of the side and end frame rails are what hold the leg square and prevent wobbling. The bolt is only to keep the leg pressed firmly against the rails. Do not rely on the side loading of the bolt for anything.

Wood is notorious for not holding its dimensions while in use. Changes in temperature and humidity, or just drying and aging, will cause joints to shift and loosen over time. (In fact when a leg gets a little shaky I recommend first loosening then tightening the fastening nut, to allow the leg to resettle into the corner.) If the bolt is tight in the hole, it may prevent the leg from fitting snugly against the rails. Then the washer will cut into the angle brace and the leg will wobble under any serious stress.

Another cause of poor fit is the difference among the legs. It's very difficult to make all four corner braces, or all four legs, exactly the same. Some day someone is going to remove the legs without marking their positions, and if the bolt holes are a tight fit he will have only 1 chance in 24 of ever having a solid strong table again.

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    A common technique with drawers that were hand fitted was to clearly number them on the rear of the drawer's box, and on the framing straight below where the drawer slides in. So OP could simply number the legs 1...4 and put a corresponding number out of site on the table. – Criggie Dec 3 at 9:17
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It is called 10mm here, but the rule still applies - you drill for the advertised size of the bolt. Neither the hole, nor the bolt, nor the drill bit are exactly 10mm, but everything will fit.

The only notable exception is when you drill in stone/concrete/masonry with a hammer drill. The hole gets +10% - +100% larger and you never know how much.

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    Another exception is if you're doing any precision work in metal, in which case it's quite awkward getting say an M6 machine screw through a 6.0mm hole as the slightest error will cause it to bind, so there are standard tolerances for how much larger the hole should be for the screw (e.g. 6.3mm for M6 close fit littlemachineshop.com/images/gallery/PDF/TapDrillSizes.pdf ) – Pete Kirkham Dec 2 at 23:04
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    No doubt, but the precision work in metal is less related to "Home improvement". Eeer, depending what you call "home" - ISS crew will probably get interested. – fraxinus Dec 3 at 12:14
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You drill holes to the advertised size of the bolt. So a 3/8" bolt gets a 3/8" hole.

In your case, alignment may prove tricky. You may need to oversize a bit for that reason.

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I like the hole to be snug on the threads, with these screws you don’t have a lot of shoulder so if the screw is a wood screw with the shank close to the screw head it is better to have the screw tight in the hole so the head won’t pull thru the hole of over sized. I was talking about the 4 wood screws that attach the brace not the single wood/machine screw/bolt a washer can easily be fitted to the single screw/bolt but not the smaller wood screws.

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    I find fitting a washer more effective... – Solar Mike Dec 1 at 13:39
  • Not a carriage bolt , a carriage bolt has a square shoulder under the head to prevent it from turning and I have only seen them with machine threads. – Ed Beal Dec 2 at 15:05
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I drilled holes for a living.

For very small holes (numbered drills), a clearance hole should be 1 or 2 drill sizes up. For holes under 1/2 inch, drill a clearance hole that is 1/64 larger. For holes larger then 1/2, drill clearance hole 1/32 larger.

You should never be drilling the same dia clearance hole as the screw size. No to 3/8 hole for 3/8 screw. Screws and holes are not always straight and you dont want the two mating pieces to bind on the screw.

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  • In my experience, it's quite hard to drill a non-straight hole with a drill. Am I misunderstanding you? – Mast Dec 3 at 14:59
  • Hello, and welcome to Home Improvement. Thanks for the answer, but I hear @Mast's concern; would you clarify? And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Dec 3 at 15:39
  • My guess is it's less that the hole isn't straight as it isn't properly aligned. Screws and bolts of course can be bent, but I wouldn't use them in that case. – Darrel Hoffman Dec 3 at 18:57
  • I’ve drilled plenty of curved holes in wood, but the drill bit was 4 feet long. – canadianer Dec 3 at 19:01
  • My experiences are in machining, cutting steel. Drills are not the most accurate tool. If the leading edges on the tip are dull or uneven length then the hole may be slightly under-size or oversize. Drills also wander, it may be entering the material at an angle, and once it starts at an angle it follows that angle. Hole isnt really curved, just at an angle. Technically, purpose of the bolt is secure two pieces, not to ensure alignment. For alignment use a dowel not a bolt hole. Bolt holes should be clearance sized. – user109733 Dec 3 at 20:43
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You can use a drill & tap chart to see what size your holes should be. This will show you not only what size to drill for a tap, but also what size for the clearance. The one I found happens to include information for different materials, including specific numbered or lettered drills. The companion one is for metric.

Randomly found online charts:
https://www.physics.wisc.edu/ishop/tapdrillchart.html

https://www.physics.wisc.edu/ishop/metrictapdrillchart.html

In my experience, these are just guides. They are often really good guides, but due to the quality (or lack thereof) of the tools I'm using, I've needed to go up or down a size to make things work right. Sometimes it's a personal preference, such as liking a snug fit or a looser fit. Sometimes it depends on the materials or how much precision the project needs.

There are a wide variety of these types of charts. I'd recommend trying one out and see if it does what you need. If not, try another one. Or if it's reliably off, stick with it and make sure you remember (or write down) what the adjustment is. Since the one I linked above makes allowances for soft vs hard materials, it seems like it could be a pretty good chart. I haven't used it, so that remains to be seen.

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