Recommended screw length and diameter for maple table tops?

WobblyHand

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Quick question. Have a new maple top for a workbench. It is 1.75" thick, I was thinking of using 3/8" x 1.25" long lag bolts to mount the top to the steel table legs. The mounting holes in the table legs are 1/2" diameter. This should give some float for the table top to grow and shrink. Is this reasonable? Or should I go with 5/16"? I need to buy the lag bolts in any case, might as well buy the right ones.

The manufacturer specifically warns against using full size bolts (1/2") for the top. They give no guidance for length. What's your advice?
 
I have never been a fan of lag bolts in wood, especially short lag bolts. T^hey work OK for shear forces but not so well under tension. They have a minimal thread engagement and tend to pull out. My preference for a blind hole would be a threaded insert bedded with epoxy. https://www.lowes.com/pd/Hillman-3-8-in-Brass-Standard-SAE-Wood-Insert-Lock-Nut/3012578

If a through hole is not objectionable, a counterbore could be cut with a Forstner bit and a washer and bolt installed, leaving a stud protruding from the bottom of the table. The bolt could be epoxied in place and a suitable plug installed and finished flush..

Tee nuts could also be used. https://www.lowes.com/pd/Hillman-3-8-in-Zinc-Plated-Standard-SAE-3-Prong-Tee-Nut/3226496 Any of these will be more secure than a lag bolt.
 
@RJSakowski I like the threaded insert idea. Found some made out of stainless at McMaster. I'll try the stainless ones, as it says it's designed for hardwoods. Thanks for the suggestion. Much better than lag bolts!

As for epoxy, is JBWeld ok?
 
I would do the through hole with counterbore, I did that on a work table in my wood shop that has a laminated maple top on an angle iron frame with square tube legs all welded together; a patternmaker told me that is how their work benches were made at Mare Island Navy Yard, so that the top could be removed when it got worn and it could be taken off and run through thee planer to true it up; I think the tables were used as sort of a surface plate for layout work.
 
It’s up to you and all suggestions are great.
5/16 x 1-1/2” lags will work. Predrill to the root size of the thread to avoid splitting . You can always go up to 3/8” down the road if you need too.
Cheers
Martin
 
I used E-Z Lok 400-4 brass threaded inserts (1/4"-20 Internal Threads, 0.500" Length) screwed into the bottom of the top spaced about 2' apart for my southern yellow pine 1 ¾“ x 27 ½” x 10' top. I drilled over sized the holes through the 2 x 4 base to allow for wood movement.

I created a simple 2 x4 template for installing my inserts as I wanted to have them aligned up with the bolt hole thru the 2 x 4 base. The 2 x 4 template has two counter sunk holes, one for the socket head and the other one for the thread insert. I used a bolt with two nuts locked together as my tool for installing the inserts and used a carpenters square to align the 2 x 4 template up with the bottom of the 2 x 4 base as I was drilling and installing from the bottom of the work bench top.

template.jpgtemplate 1.jpg
 
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I would just caution against using ¼" lag bolts. I've had way too many of them snap off right around the root of the top thread. That durn cheesecake-grade Chinesium "steel" is just too weak, and the threads weaken it even more. 5/16" minimum if you're going to use lags. And +1 on all the other suggestions above.
 
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