DIY small countersink for aluminum

[homebrewed]

I am working on an improvement for my cheap Craftsman table saw that uses a length of U-shaped aluminum extrusion that is attached to the left edge of the table. It is to add an additional guide for a sled. To attach it I want to use flat head screws that are counter-sunk so they are flush with the side of the extrusion. My plan is to drill all the way through the sides of the U so I can feed the countersink (and eventually a screwdriver) through the outer hole (which will be enlarged to accommodate the counter sink). The problem is that all the countersinks I've got are shorter than the width of the channel; and they also are pretty large in diameter (although the length is the most significan issue). So I'm looking at making my own countersink out of some 1/2" O1 drill rod.

My plan is to turn the drill rod down to clear the outer hole and then cut a cone on the end with the right angle. The business end of the CS will be a little larger in diameter than the head of the screw. Then put the countersink-to-be on my horizontally-mounted rotary table and mill a flat down to the point of the cone. This will only give me a neutral rake so I will rotate the workpiece 10-15 degrees and again mill down, but just down to the cutting edge. I believe this will give me some decent positive rake while leaving the cutting edge in line with the center of the bit. I will harden the tool and hone it afterwords -- I only have a air-propane torch but this part is small enough I figure I can heat the business end up enough to harden it.

I haven't done this before so I'm wondering if I missed anything, or if my idea on how to get some positive rake is faulty.

 

[mikey]

Could you use a hex shanked zero flute countersink on an extension to do this? Might be easier and more foolproof.

 

[pontiac428]

@homebrewed, you describe the correct procedure for forming your cutting edge.
I made this countersink using 15 degrees as you are planning. It's a D-style cut, but it would be the same idea for a quarter cut flute. The quarter cut is better for use with a hand drill that may wander, the D cut is better for rigidly held work in the mill vise. You can also use a 4-facet drill bit grind, but I like the way this one works.

 

[RJSakowski]

I am working on an improvement for my cheap Craftsman table saw that uses a length of U-shaped aluminum extrusion that is attached to the left edge of the table. It is to add an additional guide for a sled. To attach it I want to use flat head screws that are counter-sunk so they are flush with the side of the extrusion. My plan is to drill all the way through the sides of the U so I can feed the countersink (and eventually a screwdriver) through the outer hole (which will be enlarged to accommodate the counter sink). The problem is that all the countersinks I've got are shorter than the width of the channel; and they also are pretty large in diameter (although the length is the most significan issue). So I'm looking at making my own countersink out of some 1/2" O1 drill rod.

My plan is to turn the drill rod down to clear the outer hole and then cut a cone on the end with the right angle. The business end of the CS will be a little larger in diameter than the head of the screw. Then put the countersink-to-be on my horizontally-mounted rotary table and mill a flat down to the point of the cone. This will only give me a neutral rake so I will rotate the workpiece 10-15 degrees and again mill down, but just down to the cutting edge. I believe this will give me some decent positive rake while leaving the cutting edge in line with the center of the bit. I will harden the tool and hone it afterwords -- I only have a air-propane torch but this part is small enough I figure I can heat the business end up enough to harden it.

I haven't done this before so I'm wondering if I missed anything, or if my idea on how to get some positive rake is faulty.

My solution to an extended reach countersink. 3" of 3/8" round, drilled for 1/4" c/s shank and fixed with cyanoacrylate glue.

 

[derf]

I ran into a similar situation and had to make a countersink for 100 degree aircraft screws. I works lots better when you include a pilot on the counter sink, otherwise it tends to jump around and make holes kinda triangular.

 

[RJSakowski]

I ran into a similar situation and had to make a countersink for 100 degree aircraft screws. I works lots better when you include a pilot on the counter sink, otherwise it tends to jump around and make holes kinda triangular.

Performing the countersink operation first then drilling the hole will help to eliminate the three lobed pattern. If you need to drill a hole first, make it as small as practical and finish with the proper sized hole after the countersink. There are alternative countersink patterns which also help. The zero flute countersink is one and another has the flutes unevenly spaced. Finally, countersinking with as low an rpm as possible will help. I have sometimes resorted to turning the countersink by hand.

 

[Flyinfool]

Or just get a long c'sink

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

 

[ezduzit]

Or just get a long c'sink

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

This. I have extra-long countersinks from M-C.

 

[homebrewed]

Thanks for all the comments! I had thought of the extender idea and will use that as a fall-back if need be.

And the idea of including a pilot is good, too -- I was concerned that the unbalanced cutting load on the end of the rod would cause problems. A pilot would take care of that.

The cost of the extended-length CS is in the ouch! territory for me. I've got drill rod, a lathe, a mill, an RT and a torch already so the countersink I need is just hiding inside excess metal.

 

[homebrewed]

I made a test piece out of aluminum rod to vet the idea. Turning the 82 degree cone was trivial, and so was the step to machine the "vee".

Looking at Pontiac428's photo, I saw a compound-angle cut -- there is a rotation around the long axis of the CS to get the relief angle, but the direction of the cut is parallel to the "vee". That is necessary so I get the desired angle in my countersunk hole. I was thinking I needed to mount the RT so the work is horizontal and also (somehow) rotate the RT body 41 degrees to get the second angle but hit on a different approach. I'd been thinking in terms of using a standard end mill but realized that I could use a single-flute countersink to replicate the profile I needed. The CS has a spiral flute so it cut quite nicely -- at least in aluminum. Of course, I used my 82 degree countersink for this.

Including a pilot would just require leaving a spigot on the end of the vee, rather than turning the piece down to a point....I hope...

Here's a photo of my test piece with the countersink being used as a mill....