How To... Attach Aluminum Waterjet Cut Letters To Backer Board

[auto.pilot]

I cut these large letters using a waterjet and plan to attach them to a backer board, most likely 3/4 inch thick maple plywood. They are 1/2 inch thick and approximately 14 inches tall. Yesterday, I made the spacers from 9/16 aluminum rod. They will hold each letter off the background by 0.75 inches. I'd like to use threaded rod, because I don't know how to weld aluminum and don't want to learn now! My plan was to use 1/4 - 20 threaded rod (or bolts).

Questions:
-Thinking about corrosion...can I use stainless, with permatex, or should I stick with aluminum? The sign will be inside.

-The letters will be drilled using a drill press. I suppose the threads only need to extend 1/4 inch into each hole. I've never tapped a partial hole like this. Is there anything to consider? I'm concerned that the tap will bottom out before making suitable threads.

Thanks

Jim

 

[RJSakowski]

With a 1/2" thickness, you should be fine. Use care drilling the holes so you don't break through. You will want to use a bottoming tap. I had tapped many hundreds of 4-40 holes in 1/4" aluminum in past years. One trick is to purchase an extra tap and grind the tip flat. I ground it so the the point where it just touched the thread, leaving no lead-in. I also slightly rounded the end to prevent digging in. If your "normal" bottoming tap has a pointed tip, grind that flat or slightly domed as well.

I first used the normal bottoming tap to start the tapped hole properly. Then I follow through with the modified tap to clean up the last thread or two.

If I had a choice, I would use 1/4-28 threads instead of 1/4-20. I would also use a tap guide to ensure the tap is running true.

 

[RJSakowski]

Aluminum will hold up well inside. Stainless will be much harder to tap.

 

[FOMOGO]

I think I would stick with Aluminum, and for attachment do a tight press fit to avoid having to tap them. Warm up the letters and put the standoffs in the freezer before insertion. Mike

 

[RJSakowski]

If you go the threaded option, you should use stainless fasteners. Moisture, aluminum, and steel don't play well together.

 

[auto.pilot]

Thanks all... I decided to tap the aluminum letters and use stainless threaded posts. I like the freeze/thaw idea, but I'm somewhat concerned about pull-out and durability, since this sign may be moved later.

Threaded aluminum rod is difficult to find. Long aluminum machine screws are impossible!

Thanks


Jim

 

[Wreck™Wreck]

I cut these large letters using a waterjet and plan to attach them to a backer board, most likely 3/4 inch thick maple plywood. They are 1/2 inch thick and approximately 14 inches tall. Yesterday, I made the spacers from 9/16 aluminum rod. They will hold each letter off the background by 0.75 inches. I'd like to use threaded rod, because I don't know how to weld aluminum and don't want to learn now! My plan was to use 1/4 - 20 threaded rod (or bolts).

Questions:
-Thinking about corrosion...can I use stainless, with permatex, or should I stick with aluminum? The sign will be inside.

-The letters will be drilled using a drill press. I suppose the threads only need to extend 1/4 inch into each hole. I've never tapped a partial hole like this. Is there anything to consider? I'm concerned that the tap will bottom out before making suitable threads.

Thanks

Jim


View attachment 139668

A ratio of 1 diameter of thread engagement is all that is needed anything beyond that does not increase strength. You may not have noticed that commercially available nuts are less then 1 thread major diameter long, heavy hex nuts are often 1 thread diameter in length, for an indoor sign application less then 1/4" thread engagement is more than sufficient.

An 3' length of 1/4-20 aluminum threaded rod may be purchased from McMaster-Carr for $4.46

https://www.mcmaster.com/#standard-threaded-rods/=154eatp