Bench build, elongating holes for movement

[Tyler Keniston]

Thanks for all the reply's.

This is the first time I have heard construction lags (i.e. spax, timeberlox, etc.) down talked over traditional lags. I have always understood it that construction lags/screws are a harder steel, therefor thinner gauge, but with sharper and larger threads. Old lags just have a sloppy look compared to construction lags to me. Thick, dull, and with flaky galvanized coatings. But I am no metalurgist so I don't really have a clue.

I do know that the Benchcrafted plans calls for tapped threads and bolts in the vise installation, but for the top to upper rail joining calls for spax. Not sure the specific reasoning to use tapped bolts in one application and spax in another, but that is what's called for.

"There's no space needed for clearance in the mortises.
Those are glue joints - expansion forces are directed elsewhere."

I am not sure I know what you mean Jim. The tenon joining the stretchers to the legs, and the tenons joining legs into the top have grain oriented 90 degrees to their accompanying mortises . So movement in the cheeks of the tenons are at 90 degrees to movement in the mortise walls and could over power a glue joint if sufficient.


The point about tradition roubo's with their 'bridle dovetail-esque' leg to top joint and how the leg vise would push on the leg itself is interesting. I never thought of that. With the Benchcrafted design the top does attach with large mortise tenons though, which should be more than sufficient to stay put (so long as those mortise tenons stay relatively tight, hence the question about tenon movement).

[Pat Barry]

The tenon joining the stretchers to the legs, and the tenons joining legs into the top have grain oriented 90 degrees to their accompanying mortises . So movement in the cheeks of the tenons are at 90 degrees to movement in the mortise walls and could over power a glue joint if sufficient.

This is an interesting point with regard to modern glues. Those glues are not cement, they actually willl creep to some extent. I don't know that I have ever seen it documented just how big a mortice and tenon would have to be for that particulate cross-grain situation to be an issue though. It will depend on wood type, wood fiber orientation - radial/tangential vs longitudinal, moisture content, the degree of temperature and humidity change with season, etc. Not an easy thing to figure out. The beauty is that the tenon is captured in the mortice and cannot really grow relatively speaking because it is restricted. It can however shrink a bit but my feeling is that with as large a glue surface as you would have it should not ever completely fail. I suppose you could always add a couple of cross dowels to lock it all up if you have concerns

[David Weaver]

This is the first time I have heard construction lags (i.e. spax, timeberlox, etc.) down talked over traditional lags. I have always understood it that construction lags/screws are a harder steel, therefor thinner gauge, but with sharper and larger threads. Old lags just have a sloppy look compared to construction lags to me. Thick, dull, and with flaky galvanized coatings. But I am no metalurgist so I don't really have a clue.

I think that has more to do with some of us having used lags, but not any of the new whiz bang thinner hardened stuff. Sometimes additional hardness comes at the cost of toughness, especially in commodity goods, and that's what would concern me. I wouldn't be concerned in something like attaching a top, though, just in a situation where something had racking forces.

[Daniel Rode]

Harder is not necessarily better. A thinner, harder fastener might be ideal for one application but a poor fit for another. For example, nails bend pretty easily. This can be a good thing when paired with wood movement.

[Prashun Patel]

My criticism of Spax was based on a sample set of 1 data point. So, take that for what it's worth. Also, I was speaking of the construction screws, not the lag screws, which are considerably thicker.

[David Weaver]

I get the sense reading about the spax that they are popular because on a paying job, you can drive them without pilot holes.

I'm sure a well made bench out of any of them would be fine.