Help me design - Custom Size threads for anodizing

macardoso

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Working on a fun project which is a light up coaster using RGB leds. The assembly is a stack up of plastic discs, 3.750" in diameter. Around the outside of this stack will be an aluminum tube with a lip on the front (for the discs to stack up against) and a fine thread in the rear for a threaded plug to be screwed in, thus sandwiching and sealing the assembly together.

I'd like to anodize the aluminum shell and plug with Type II dyed coating. I need to specify the thread dimensions such that the shell and thread on end cap will properly assemble after the anodizing buildup. I'd also like to keep the rear end cap as thin as possible - no thicker than 3/16".

Part of the complication is that I haven't found a "standard" thread specification for a 3.75" (ish) diameter thread with a fairly fine pitch. This would be similar to a thread used in a camera lens or some other delicate device.

Stock Dimensions: 4" OD, 3.5" ID, 1.25" L, 6061-T6 aluminum tube.
Thread Minor Diameter: >3.750"
Thread Pitch: >16 TPI (3 threads engaged minimum). 24 or 32 would probably be preferred as this limits thread depth.

Here is what I've come up with:

It is a 3.800"-24 thread with a 2B tolerance. This gives me a minor diameter of 3.755". This lets me bore the entire shell to a 5 thou clearance for the discs which need to go inside it and finish the thread minor at the same time. The major diameter will be no greater than 3.82" which gives a 0.090" wall thickness between the thread and the OD.


1658156617425.png

The anodizer quoted a 0.0004"-0.001" thickness. My understanding is that Type II anodizing has a 67/33 etch/buildup, so all surfaces will be built up by 0.0001"-0.0003". The issue is that this is equal on both sides of the thread flanks, so the pitch diameter is built up by 4x the growth of the surface ((growth * 2)/sin(included thread angle/2)) or (growth*2)/sin(30). This means my pitch diameter would be grown by 0.0004"-0.0012" on each part. I must have a minimum of 0.0024" of clearance on the pitch diameters between the parts before anodizing or the threads will bind.

Does this check out?

If so, do you think this spec would work for the end cap? This will have a external thread that mates to the one above.

1658156690415.png

This gives a 0.003" pitch diameter clearance in the worst case tight side of the tolerance, and a 0.014" pitch diameter clearance on the worst case loose side of the tolerance. I'll likely shoot the middle and get a 0.008" clearance or so.

The smallest major diameter on the endcap is 3.7898" and largest minor diameter on the shell is 3.7646" so the end cap cannot fall through the shell in the Least Material Condition.

Does this all make sense?

Thanks - Mike
 
Here is some CAD of the design.

The blue and black discs are polycarbonate and cut from the table top of the robotic bartender my buddies and I built years ago in college. There are windows through which WS2812B RGB LEDs shine (filled with resin to make them waterproof). The switch detects the presence of a glass to start the LEDs.


1658157212793.png

From the rear (end cap removed) you can the the custom PCB, a small low cost microcontroller to sequence the LEDs, and the switch. The yellow ring is a spacer to allow room for the wiring. The hole through the shell is for a USB cable (power and programming).

1658157253591.png

This section view shows the small lip on the edge of the shell that all the discs stack against. The highlighted blue part is the aluminum shell. The reduced diameter at the bottom is where the thread will be.

1658157364715.png

And the aluminum shell by itself. Not going to be an easy part to turn. Very thin walled. Need to make 8.

1658157412857.png
 
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Mike,

You've done a very good job trying to define your thread requirement. However I think you've misinterpreted one of your parameters.
I think the "67/33 etch/buildup" means that etch will remove X amount of aluminum and buildup will add X/2. Therefore, the anodized thread will be more loose than than pre anodize.

It's been quite a few years since I've had to be concerned about the etch/buildup issue but I think my memory serves.
 
Mike,

You've done a very good job trying to define your thread requirement. However I think you've misinterpreted one of your parameters.
I think the "67/33 etch/buildup" means that etch will remove X amount of aluminum and buildup will add X/2. Therefore, the anodized thread will be more loose than than pre anodize.

It's been quite a few years since I've had to be concerned about the etch/buildup issue but I think my memory serves.
I could be wrong, but I understood that the etch/buildup meant that the anodize replaces some of the subsurface material and then grows the surface by some amount as well. I thought that all anodizing grew all dimensions (shrinking holes, enlarging bosses).

 
Mike,

Looking at the linked info, I see how you came to your conclusion. However, they don't address the aluminum removed by etching except to say "Heavy etching before anodizing can also reduce buildup, by removing up to a few tenths of aluminum before the anodic layer is formed.". I'll stand by my previous statement.

Regarding the design, maybe the issue can be avoided altogether. When I look at the section view, I'm wondering what your plan is to protect a fine surface from being damaged by the aluminum base of the coaster. It occurs to me that making a base plate/retainer out of ABS (or other plastic material) solves your anodized thread problem. The plastic will be easier on soft surfaces and you just cut the male thread to fit. YMMV.
 
Mike,

Looking at the linked info, I see how you came to your conclusion. However, they don't address the aluminum removed by etching except to say "Heavy etching before anodizing can also reduce buildup, by removing up to a few tenths of aluminum before the anodic layer is formed.". I'll stand by my previous statement.

Regarding the design, maybe the issue can be avoided altogether. When I look at the section view, I'm wondering what your plan is to protect a fine surface from being damaged by the aluminum base of the coaster. It occurs to me that making a base plate/retainer out of ABS (or other plastic material) solves your anodized thread problem. The plastic will be easier on soft surfaces and you just cut the male thread to fit. YMMV.
Not a bad idea. I had originally planned to attach an adhesive backed felt ring to the bottom, but I could consider a plastic material. I wonder if the stuff I have on hand is too brittle...
 
Mike,

Here's a little more design perturbation suggestion. Design the thread diameter so you can use standard ABS or PVC plumbing as raw stock.
Turn the thread, continuously (no shoulder), on a length of tube. On the end of the tube, bond a disc of like sheet material (1/8" thick?). After bond cure, turn the disc OD to a few thousandths (or more?) larger than the OD of the aluminum ring, chamfer the thread and part off to length. You'll need a push plate to hold the sheet in place and your lathe will be tied up during bond cure cycles, but isn't that why it's called a hobby?
 
We make a lot of parts that have a 1.5 - 18 thread. Since we are making both the ''nut'' and the ''bolt'', we just cut the ''nut'' to a more or less standard size then cut the ''bolt'' just a bit loose, so the parts very easily screw together in the raw state. We have never had a problem assembling after anodizing.

Since you are controlling both pieces, do the internal threads first, then hand fit the external threads because they are easier to control. In this case, loose is better because there is not really any mechanical stress on the assembly, unless you have very heavy beer mugs. ;) :beer:
 
We make a lot of parts that have a 1.5 - 18 thread. Since we are making both the ''nut'' and the ''bolt'', we just cut the ''nut'' to a more or less standard size then cut the ''bolt'' just a bit loose, so the parts very easily screw together in the raw state. We have never had a problem assembling after anodizing
That was my thought. I tend to be a numbers guy, so I was curious how exactly to specify "just a bit loose".

I tend to work to exact measurements in my home shop and I'd like to make all the parts interchangeable if for no other reason than good practice.
 
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