4-12 Acme thread issues

[kritter]

Ive got a 4-12 acme threaded steel can. A collar and a lock nut are machined out of aluminum and screw on to the steel can. The steel is electroless nickel plated .0008" thickness. The aluminum is andoized and is brought back to net.

We discussed with the machine shop to decrease the thread accordingly to allow for plating...they decreased the OD of the can.

Now I have the parts back, anodized and plated and pieces dont fit together.

I went and got an unplated piece and screwed on an anodized piece and it fit with radial play but no axial play so the problem is the threads were only decreased on the OD and the minor diameter or pitch diameter were unchanged. When I asked why they would only change the OD and not open up all the diameters to allow for plating he told me it cant be done.

I think it can be done...can the thread be loosened up to allow for plating?

So now I have 800 pieces of each part that dont fit...any ideas how to open up the aluminum since the steel cans cant be touched?

 

[metman]

Here is a thought: Make a steel threaded part just like your can except machine a tapered bore in it at 20 degrees included angle then saw cut numerous places axially into the new "tubular" can somewhat deeper than the max length of mating collars or nuts. Make a matching tapered plug. screw the collar onto the new can and insert the tapered plug. Place this assembly in a press and push the plug into the new can so that the Aluminum is stretched beyond its yield strength. Use a dial indicator to measure precise penetration of plug and creep up on the required penetration. Remove plug and unscrew the collar and repeat for the nut. But 800 times (x2 = 1600 times)? OK -- once you determine required penetration, make a cylindrical stop to lie on the base that the new can sets on so that the plug always bottoms the same so no more measuring of penetration just press to stop.

 

[BillPSU]

There are several Acme threads and before I give any advice I need to know what is the requirements. First is this a general purpose Acme or a centralizing Acme? What is the print designation for fit on the parts? Is it a 2G, 3G, or 4G or a 2C, 3C, or 4C?
How are you gaging the thread size? Measurement over pins or a go-nogo gage.

 

[ornerynorsk]

You certainly can machine the internal thread out further. You need good carbide tooling and the cut must penetrate below the anodizing if you want any kind of tool life. However, I don't believe that the newly exposed metal can be re-anodized with existing anodized surface already present, although I may be wrong on that point.

 

[metman]

ornerynorsk: This was one of my earlier thoughts but it would be difficult at best to re-register the cutting tool into the original thread form. I am assuming this was a single point threading operation.