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- Mar 26, 2018
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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.
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.
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
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.
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.
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