Help me design - Custom Size threads for anodizing

[macardoso]

Finished the threads on all 8 end caps. I still need to chamfer the corners and clean them up but I'm close.

The work started by cutting 4" discs from 1/4" aluminum sheet on the CNC. Doing it this way saved me from having to part off through 4" barstock and it let me complete a secondary operation (spanner wrench holes) in the same operation. The parts were fixtured with the "Super Glue" trick I first saw on NYCCNC years ago.







Then both sides of each part were cleaned up and coated in a single layer of masking tape which was firmly pressed down to the surface with a dowel pin. All 8 discs were then superglued to each other and held in the proper position by a constraint setup on my surface plate.





The entire stack was glued to a mandrel and turned as a single workpiece. Then whole piece was turned to the major diameter of the thread and then single pointed @ 24 TPI.



Measuring a 3.800"-24 thread over wires completely sucked. I lost the wires several times (one even snuck into the gearbox cover). Hit the pitch diameter tolerance perfectly, a bit on the looser side of the tolerance.



And all done!



Still need to chamfer in place and then break this apart into the 8 different discs.

Then I'm making (2) thread gages and then the aluminum shells.

 

[mattthemuppet2]

that's a very neat way of holding and threading all those caps at once. How will you chamfer them - on the CNC mill?

 

[macardoso]

that's a very neat way of holding and threading all those caps at once. How will you chamfer them - on the CNC mill?

Thanks! I thought it was creative.

They were chamfered with a 45 degree SCGT indexable tool right on the lathe as you see it. I plunged in at the interface between each of the two parts deep enough to cut the chamfer.

There were a few razor edges to the leading edge of the threads which I kissed with a fine cut fine. Then deburred the threads with a swipe of red scotchbrite.

This was a new one for me, but a medium sanding mesh on an orbital sander left a very appealing finish on the flat faces and cleaned up the extrusion marks. I held the sander in one hand and pressed the part into it with the other.

 

[mattthemuppet2]

Very neat, that makes sense

 

[macardoso]

8 bottom caps ready for anodize.



Detail of the perimeter 3.7980-24 thread.





Surface finish from the orbital sander

 

[macardoso]

Continuing onward on this project.

Made 2 thread gages just like the caps above. They were supposed to be a GO and a NOGO gage, but I made a few oops and got an undersized GO and a slightly oversized GO. I still don't know what went wrong, but on my skim pass for the NOGO, the tool took a huge cut and brought the pitch diameter undersized.

I could make a new one, but for now I'm going to ignore this issue and push forward. The gages are now more like a "you're getting very close" and "perfect". I want to make sure the biggest gage goes in by hand, and the actual caps have a tiny bit of rattle to them. Having them too loose is OK, but I don't want to end up in a situation where I can't assemble after anodize.

The "Shell" component is quite interesting from a machining perspective. The stock is 12" long, 4" OD, 3.5" ID round tubing. I could tell ahead of time that this would ring like a bell during machining so I had a smart idea...

When I interned at a machine shop, I saw them using some low melting point alloys as filler material for extremely thin walled aluminum castings prior to machining. These were transmission cases for indycar, weighed 6 lbs when done being machined. This stuff looked like a pain to deal with, but allowed machining without chatter. Rather than using the alloy, I purchased 10lbs of paraffin wax on Amazon for about $30. It can be melted at ~140*F using a double boiler on the stove like you might cook chocolate. The tube was filled and took overnight to solidify (wax has an extremely high specific heat capacity). This worked great with the only exception being that the wax shrinks very significantly when it cools. This required reheating the exterior of the tube with a hairdryer to make sure the wax was in good contact with the tube walls.



I didn't want to work on this piece with this much extension, so I opted to part it off into two 6" pieces. Each would yield (4) finished shells with plenty of remnants.



The cold paraffin wax machines like a breeze and did exactly as I needed. Cutting the tube walls sounds completely dead, maybe even more so than cutting from solid barstock. When cutting the bulk of the wax to make room to machine, I can collect the chips and melt them back down again to be reused. Maybe half ends up in the chip pan or on the floor and crumbles.



I started with machining the ID, inner lip, bottom face, OD, and thread minor diameter.



ID threads were cut with the "spindle backwards, tool on back side of tube, thread away from chuck method". This let me keep a very minimal thread relief and ensure I wasn't going to crash. The threading is a bit higher force and chatters just a tiny bit now that I removed so much wax and thinned the walls significantly. Maybe on the next part I'll do the thread first.



ID was polished with scotch brite and is dirty. Threads are still undersized and need to be opened up a couple thou. The caps and thread gages can start, but get stuck currently.



Once the threads are done, the front chamfer will be cut in-situ and the parted off. If I am lucky, I will have finished all sides in one operation. I can clean the parted surface with emery paper or make a quick lathe fixture to kiss the faces of the parts and clean them up.

 

[mattthemuppet2]

Great tip about the paraffin wax, I have some thinnish real tubing to turn soon where that will come in very handy. I'm the past I've simply held the wooden end of my chip brush against the back of the past, but that gets old pretty quickly!

 

[macardoso]

Great tip about the paraffin wax, I have some thinnish real tubing to turn soon where that will come in very handy. I'm the past I've simply held the wooden end of my chip brush against the back of the past, but that gets old pretty quickly!

This is what I bought...

https://www.amazon.com/dp/B07YXBHJFF?psc=1&ref=ppx_yo2ov_dt_b_product_details

 

[mattthemuppet2]

Thanks! I just asked my wife to add a couple of pounds to the Walmart order, I think it's around $4/lb