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Screw/lag bolt selection. Dining Table

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Forum topic by sbjmg posted 08-24-2012 01:31 AM 2452 views 1 time favorited 18 replies Add to Favorites Watch
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sbjmg

6 posts in 848 days


08-24-2012 01:31 AM

I am building a dining table made of solid 8/4 Walnut. 73”x40”.

For the legs I am building 2 squares out of 1/4”x4” flat steel. 26”wide x 28.5” high.

The question is when screwing/bolting the legs to the table what type of screw should I use, what size, and what length.

I was thinking 6 lag bolts spaced about 6 inches apart for each leg, each bolt would be staggered half inch from bottom to half inch from top since I have 4 inches to play with. For size I was debating between 1/4, 5/16 and 3/8ths. For length I am contemplating between 1”, 1.25” and 1.5”. Keep in mind the steel is .25 thick so with a 1” bolt I will only get .75” of wood. Assuming the table top is finished at around 1.75” since I am starting with 8/4.

Thanks

Below is a picture of something similar


18 replies so far

View Doss's profile

Doss

779 posts in 1011 days


#1 posted 08-24-2012 04:08 AM

Depending on the grain orientation of the wood and environment, you’re probably going to have to allow for some seasonal expansion.

You don’t need to fasten the top like it’s a wing to an airplane. It just need to stay attached.

If you’re depending on the fasteners to keep the legs from tilting, you’re probably going to have to get a little creative and have it solidly mounted in the center room for expansion on the edges.

-- "Well, at least we can still use it as firewood... maybe." - Doss

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sbjmg

6 posts in 848 days


#2 posted 08-27-2012 04:04 PM

OK to clarify. The table will be made from 5 boards each 1.75” thick 7to11” wide and 73” long. I will go ahead and use 1/4” x 1-1/4” screws unless you feel otherwise.

I take it that it is a bad idea to screw the boards together via the top of the metal square as this will not allow for expansion? How would only screwing the two end pieces to the metal allow for expansion unless the top of my metal square is not present and is only a U shape with 2-3” bends on top to screw into just one plank. My main concern is wobble, I figure 4” wide steel flats would have taken care of that but if not what should I do?

In regards to the table top it will be made of solid walnut that I will oil with Danish oil and then finish with a wipe on Poly. It will be indoors in southern California temperatures will range from 60-90 at the extremes.

Thanks again.

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Doss

779 posts in 1011 days


#3 posted 08-27-2012 07:22 PM

I’m having a little trouble visualizing what you’re talking about (as it seems you’re talking about several options).

If you screw in directly to the metal (table top to top metal cross bar), then I’d probably slot the metal screw holes to allow for movement.

If you remove the top piece of metal (ending up with a U shape), that would allow for some movement I would think, but I don’t know how much resistance the wood would encounter (which may cause the wood to buckle at the seams).

That’ just if I’m understanding your design correctly.

You’re fine to bolt to the metal, you just have to allow for some expansion along the width of the table (not the length… if the grain is running parallel to the length which I’m assuming is the case).

The temperature is not so much of a concern on its own, it the humidity as well.

-- "Well, at least we can still use it as firewood... maybe." - Doss

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bondogaposis

2744 posts in 1097 days


#4 posted 08-27-2012 07:52 PM

If the grain direction of the table runs the length of the table Then I would attach using 3 lag bolts on each leg. One in the center and one each on the sides. Slot the metal for the lag bolt holes on the outside of the legs so that the table can move in width w/ changes in humidity. 1/4” by 1/14” lags should be fine, I’d use flat washers too.

-- Bondo Gaposis

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sbjmg

6 posts in 848 days


#5 posted 08-27-2012 08:14 PM

I am sorry. I came here to get help from you professionals but instead forced my ideas in. Let me start over. The above picture is my goal. What would you guys do to get there.

Again I apologize for assuming methods when I have little to no idea what I am doing hence the reason I cam here.

In regards to slotting the metal, how does that work, since when I bolt/screw it down. I assume I am torquing it quite a bit, when the wood expands does it just force the bolt sideways or am I not supposed to crank down on the bolt, as this brings up the “wobble” question if I am not torquing it down.

Sorry for all the questions, I truly am a novice. I spent a lot of my money on this beautiful wood I just do not want to mess it up.

Thanks

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Doss

779 posts in 1011 days


#6 posted 08-27-2012 09:50 PM

Okay, I’m going to try and help explain it a little more.

I have drawn these out so maybe it’ll help a little more. Here’s the explanation:

You’re going to need to make a slot cut (approximately 2 1/4” holes side by side) in the metal to allow for wood movement. You’re not going to torque down the table top so tightly that it can’t move. You want to allow some movement so it just gets “snugged down” – (tight enough that you can’t just move the top by hand but loose enough that you’re not crushing the wood fibers). If you tighten it down too much, the screws will almost definitely become small drill instead and you’ll end up removing wood that was being used by the screw to fasten.

So, here’s the picture:

EDIT: REDO

The part with the orange around it is a blow up of what I’m trying to explain.

Let me know if this helped or not.

-- "Well, at least we can still use it as firewood... maybe." - Doss

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sbjmg

6 posts in 848 days


#7 posted 08-27-2012 10:03 PM

Doss, you are the man. I now know exactly what you are suggesting. What size screws/lag bolts would you use?

What about wobble? Since I am not torquing down what can I do to be “creative” and ensure no wobble or do you think the legs being 4” wide I should be ok?

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Doss

779 posts in 1011 days


#8 posted 08-28-2012 02:54 AM

Well, I don’t know too much about how stable that design will be. My suggestion would be to run the screws zig-zagged or on both sides (as much as possible) of the top of the boxes.

In the picture (the right part where I have one oval hole illustrated), I’d instead use two (one near “This allows for…” and the other by those faint words at the bottom (“Top of metal box”) if you can understand that.

Like Bondo said, I think those 1/4” will be fine. If you run enough of them (maybe 4 per board per box in a three board top), that could work. Most people would probably put a stretcher across from one section of boxes to another, but that would possibly interfere with the design. You could weld on extension straps of 1/4-1/2” thickness extending from the tops of the legs down the length of the tabletop (maybe extending 1’ from the boxes) and secure those with a few extra screws. This would probably go a long ways to keeping the table from swaying (even though it’s a little hard with so many parallel and perpendicular supports).

-- "Well, at least we can still use it as firewood... maybe." - Doss

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sbjmg

6 posts in 848 days


#9 posted 09-14-2012 10:02 PM

Ok Table top is almost done. I am about to have the legs done up as suggested by Doss above with slots cut out. I am thinking now for stability purposes to have the bottom and sides 1/4” x 4 but the top pieces to be 6” or 8” wide to give me a more ridgid table/remove possible sway.

My last question is in regards to tightening the bolts. How can I get it tight enough to get rid of wobble but lose enough to allow for expansion? Should I use washers? What type?

Thanks again.

View muleskinner's profile

muleskinner

740 posts in 1183 days


#10 posted 09-14-2012 10:16 PM

Just my 2 cents worth, but I’d bolt those legs on with a rectangular bolt pattern – 4 or 6. That’s a better stabilizing pattern than a single line of bolts. I’d also think about using inserts and machine screws instead of lag bolts.

-- Visualize whirled peas

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sbjmg

6 posts in 848 days


#11 posted 09-14-2012 10:30 PM

I will be doing them in two rows as Doss suggested, in regards to your insert idea how does that help/allow for expansion?

Remember I am a complete novice so please talk to me as if I were a 10 year old.

Thanks again

View muleskinner's profile

muleskinner

740 posts in 1183 days


#12 posted 09-14-2012 10:59 PM

Because I’m the dad and I said so.

Actually, the machine screws and inserts are just my preference. I always think of lag bolts for picnic table and work benches, not dining room tables.

-- Visualize whirled peas

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Cosmicsniper

2199 posts in 1905 days


#13 posted 09-14-2012 11:18 PM

My thought on this is that screws are generally fine…the fewer mounting points, the better…which is why table tops are usually screwed to a table as opposed to glued. Glue is what keeps the table from expanding and contracting.

But the problem is that the fewer points of contact you have with the top, the more likely the boards will cup. I would be very concerned about being able to keep the table flat. Of course, this is why breadboard ends are common. I’d explore that option…or screw some splines underneath.

As far as slotted holes in the metal, that’s probably a good idea, but locking them down is okay…the idea is that you can just relieve the pressure every now and then, though I think by the time that’s a problem some of the top will be coming off the table.

Just my $.02.

-- jay, www.allaboutastro.com

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Cosmicsniper

2199 posts in 1905 days


#14 posted 09-14-2012 11:22 PM

Btw, the thickness of the walnut is definitely in your favor…it might not cup as much as I think.

Welcome to the most difficult and mysterious part of woodworking!

-- jay, www.allaboutastro.com

View Doss's profile

Doss

779 posts in 1011 days


#15 posted 09-15-2012 03:56 AM

Yeah, you could probably snug it up pretty good, but not so much you’re at risk of pull the wood out of the top (the screw will act like a drill bit if you apply too much torque).

I’d use washers but that’s mainly because I use them in a lot of situations.

Depending on what part of the log the boards are from (and what type of stress that part was in), you may get cupping or movement like Jay said. Then again, you may not.

Post up a pic some time.

-- "Well, at least we can still use it as firewood... maybe." - Doss

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