Tapping Holes - 1/4 Thick Aluminum Tubing

[RJS100]

I am almost done making parts for an Automatic Tool Changer and ready to start working on a micro adjustable arm that will hold the ATC in place. This design consists of a 2.5 x 2.5-inch aluminum tube which slides inside a 3x3-inch aluminum tube. Both tubes have .25" thick walls. The design consists of M5 tapped holes in the ends of the tubes. Since the wall thickness is .25 inches, this leaves approximately .027" between the major diameter of the screw and the walls of the tubing. On the inner aluminum tube, the tapped holes are comfortably away from the corner of the tube. On the larger tube, this is not possible (see images).

My question: Is there any special consideration I need to know with respect to tapping these holes? I plan on rigid tapping them with a chip breaking cycle to be extra cautious to minimize the chance of breaking the tap. Most of the aluminum I purchase if fairly inexpensive. These are coming from an alternate source at about $80.00 so I want to be extra careful.

Any thought on this?

Best... Richard

[suntravel]

Tubes are usually made from very soft aluminium, perfect for forming threads.

IMHO the design is overcomplicated and you will have lot of slack in the tube in tube design...

Uwe

[RJS100]

Uwe, I always appreciate your thoughts! The tubes are stamped 6061 so hopefully they will form threads nicely and with any luck the .027" clearance between the threads and outer wall will be ok.

With respect to the design, my plan is for the tubes to ride on the slots machined into the sides of the tubes and be guided by the shoulder bolts. Hopefully I can machine these with tight tolerances and with any luck, there will be minimal slack. There is a jack screw that will move the tubes with respect to one another, and the knobs will lock everything in place. Yes... I much prefer simpler designs... but I could not come up with anything that can be micro adjusted in the x and y axis and also offer a bit of rotation. Wish me luck... I will either end up with an adjustable bracket or a bunch of expensive aluminum, hardware and screws

I have included an image of a very simple bracket design that came with the ATC plans that I purchased. Since TTS tool holders require the tool holder to be PERFECTLY concentric with the spindle, and have very tight tolerances, I think the original design is a recipe for lots of fiddling to get this thing aligned properly.

[suntravel]

If you are lucky the tube will have play to fit.

If they have, tighten them only on two 90deg sides, otherwise they will deform.

You want two faces pressed together not a tube floating with a deformed bulge from tightening opposite sides.

Uwe

[RJS100]

Thanks Uwe. I was just examining the tubes. It looks like the tubes do not fit inside one another so I plan on facing the sides of the inner tube to fit snugly inside the outer tube. As a result, the inner tube will have a wall thickness of something less than .25 inches. This is not what the tube supplier told me.

With a wall thickness of less than .25 inches, I will have to use screws that are smaller than M5 I had intended. M4's looks a bit small to me, so I may use 8-32 which fall right between M4 and M5. I hope these are strong enough. I think there is a section in the Machinery handbook that calculates pull out strength.

[suntravel]

What about using solid 2.5x2.5 AW 2007 instead of the inner tube?

Nearly double the tensile strength compared to AW 6060

Uwe

[RJS100]

Thanks Uwe. I already own this material, so I am tempted to try to use it. This is an oversight on my part as I didn't expect to have to reduce the wall thickness of the inner tube. I just researched some M4.5 screws. These are bigger than the 8-32. If I use these, I will have around .030" of clearance between the threads and outer wall. I think I am going to test it with a small sample and see how it works. I am a bit concerned because the tool changer probably weighs 20 lbs. which would be supported by eight M4.5 screws.

I need to remove .010" to .012" from all sides of the tube. I can do this by either touching off the face mill to the tube and removing material or probing the top of the parallels and setting the top height with respect to the parallels for an exact height. The latter is my preference, but I am still waiting for my probe to be returned from service and not sure how to touch off the parrels without the probe.

[suntravel]

I would use M10x1 or similar UNF / UNEF for tightening.

A larger diameter fine thread will not pull the threads as easy as smaller M5.

For a simple facing you will not need a probe, touch off with the endmill and a piece of paper.

Uwe

[RJS100]

I don't understand. How can I use an M10x1 if the wall thickness of the tube is approximately .238 inches thick?

[suntravel]

I don't understand. How can I use an M10x1 if the wall thickness of the tube is approximately .238 inches thick?

Why not? more diameter same pitch = more area for the thread -> more pull out force

DIN439 M10x1 nuts have also only 5mm thickness

1.5 x diameter is only a rule of thumb to make sure the threads will not pull out before the screw is breaking

Uwe