Making square holes for carriage bolt heads

[wonderboy]

I'm putting tie-down anchors on my trailer. I bought a bunch and they are drilled with 1/4" holes. I want to use carriage bolts and so need to make the holes square. I used a triangular file to square off the edges of the two holes on one of the 8 anchors I want to install, but holy cow is it a pain. Is there any easy way to do this?

[El Camexican]

An iron worker with a square punch is the ticket. You're a wild man if you plan to square a round hole with a file.

What about welding bolts to the anchors to keep them from turning? Another option might be to drill a round hole just big enough that you could hammer the carriage bolts into them and they wouldn't turn.

[350xtrike.rider]

Wow that's gonna take some serious elbow grease! I'd use a die grinder if you want to just do it yourself.

[DohcBikes]

Porting tool with the right bit would get ya close.

[Flyingw]

I have seen videos of bits that drill square holes. https://www.youtube.com/watch?v=ALiqAXiTQBg

[Flyingw]

Broach it. https://www.youtube.com/watch?v=7n1r5XfVkyk

[cobradude]

Depending on the size of the hole, a jigsaw with a blade for cutting metal. Drill your round hole & finish out with the jigsaw

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[Jd110]

I believe you have the way, wonderboy. A file and patience!

[RUNMEDOWN]

I have a file that has a sawz-all connector on it, Home depot had it. You may want to look into that, makes it much easier. Snap it in the sawz-all and have at it.

[wonderboy]

I have a file that has a sawz-all connector on it, Home depot had it. You may want to look into that, makes it much easier. Snap it in the sawz-all and have at it.

I'm going to have to look for that!! I was imagining something like this would make quick work of it. I may search for a die grinder bit too, but it would have to be pretty small diameter...

Thanks for the advice!

[redsox]

square drill bit

[John Tice]

The suggestion of using a larger hole just big enough to catch the 4 corners. Drill a few sample holes to determine what size drill that you need. Smack the bolt home with a heavy hammer. If the material is thick enough, a square broach would be the best choice.
JT

[El Camexican]

I have seen videos of bits that drill square holes. https://www.youtube.com/watch?v=ALiqAXiTQBg

That's wild!

[250rAL]

Carriage bolts are made for wood. I think I'd use a little better grade of bolt for such an important item as a tie down point.

[El Camexican]

Carriage bolts are made for wood. I think I'd use a little better grade of bolt for such an important item as a tie down point.

They may have been used first in wood, but you can get high grade carriage bolts if you want them. Grade A449, which is equivalent to A325 (structural bolts)

[wonderboy]

1/4" stainless carriage bolts, 304 passivated stainless, minimum yield strength of 45,000 PSI.

Each bolt has a minimum yield strength of 2,209 pounds. Even at a fraction of that strength, I've got six bolts holding down my load (three hold downs per machine, two bolts per hold down)... I think I'll be fine.

I'll use either a pair of large washers (fender washers) or a square metal plate on the bottom side to spread the load out across the plywood floor, which I suspect will be the limiting factor in the amount of load each hold down will support.