# Erector set part number 'AC' Two-hole coupling



## BGHansen (Aug 30, 2022)

This string goes over the fixturing for making Erector set part# “AC”, two-hole coupling. The part is made from 5/16” diameter brass and is 5/8” long. There is a central hole ~11/64” in diameter (#21 drill), two cross-holes ~11/64” diameter and four tapped 6-32 holes clocked 90 deg. from the two cross-holes.


Sketch of the part# "AC"




Quick history lesson as this was a really esoteric part. Two of them were included in the largest Erector set sold in 1924 and 1925. I’ve looked through the manual multiple times and can’t find any use for the part. Did I waste my time making them up? Probably, but what the heck, it was “shop time”!

I use hardened steel drill fixtures for a lot of these parts. This saves the time of individually laying out each part. In this case, the bushing was made from 1” diameter drill rod. Started on the lathe to drill in a 5/16” center hole for slipping over the brass stock. Then on to the mill in a square collet block. The holes in the bushing were spotted, then drilled through; #21 for the cross holes and #36 for the 6-32 tapped holes.


Face, spot drill and drill a 5/16" hole in the 1" O-1



On to the Bridgeport for the cross-holes and tap dill holes using a square 5-C collet block




Next, the bushing(s) (made two), were cut off on the band saw. Then they were faced on the lathe to an overall length of 5/8”.





The cross-holes should be centered on the 5/8" long bushing.  The first one was drilled there on the BP, leaving the task of just facing the opposite side to the overall length of 5/8".



The second back-up bushing was too long on both ends.  Finished one face by slipping a #21 drill bit through the bushing and jammed it up against the chuck jaws.  Faced, and measured the depth from the face to the drill shank.  The math works out to 0.0455" from the edge of the hole to the face of the part (0.125" - (0.159/2)).  Faced to that dimension double-checking with a depth mic and drill bit.  Once there, flipped the part and faced the opposite face to an overall length of 5/8".






I also made a drill bushing out of ½” drill rod for drilling the center #21 hole in the 5/16” brass stock. We’d usually do this by first center drilling a hole, then come back with the #21 drill bit. That involves a tooling change, so I do the center holes with a drill bushing. The drill bushing is hardened steel with a 5/16” hole for slipping over the brass, and a through #21 hole for the center drill bit. The hardened bushing holds the #21 drill bit on center saving me the tooling change from a center drill to a #21 drill.


Faced, spotted, drilled a 5/16" hole for slipping over the brass stock, then finished with a #21 drill.  Didn't show it, but parted, flipped and faced the opposite side.




Heat treating was done in a Thermolyne muffle furnace. I left them full hard as they won’t be seeing any impact.





Finished drill bushings.




Then on to making some parts! Chuck up the brass, slip the center drill bushing over the brass and drill a #21 center hole. Then slip the 1” diameter drill bushing in place and flush the face of the bushing with the end of the brass. Then stage the parting tool to the opposite end of the bushing and lock the carriage. The bushing was machined to the finish length of the “AC” coupling; flushing the face with the end of the brass and staging the parting tool at the opposite side ensures the length will be 5/8”.


Slip the center-drill bushing over the brass and drill a #21 center hole.  The bushing locates on the OD of the brass and forced the #21 bit on center.  This saves spotting, changing tooling to a #21 drill bit and drilling that hole.



Set the cross-hole bushing in place, flush the face with the end of the stock, position the parting tool to the opposite face, and lock the carriage.  The cross-hole bushing is the overall length of the part so it serves a dual-purpose of a cross-hole fixture and length of the part fixture.




Hand drill the first cross hole, then pin the drill bushing to the part with the shank of a #21 drill bit. Then drill the second cross hole. Rotate the lathe chuck and drill a #36 for the 6-32 tapped hole. Stick a second #21 in the other cross hole, pull the first cross hole drill bit and drill its corresponding #36 tap hole. Pull the locating #21 drill bit, remove the drill bushing and power tap the two 6-32 holes. Lastly, part off.












I dress the parted face of the part with a Scotchbrite wheel. There isn’t much to hold on to, plus the brass gets pretty hot. So, the parts are held with a knock-out tool I use for multiple parts. It’s a piece of CRS with a 5/16”-plus center hole and a knock-out pin. That lets me stick the part in the steel holder and hit the end on the Scotchbrite wheel, then knock the part out with the center pin.


The knock-out holder is pretty handy as there's not a lot to hold onto.  Plus, the brass gets a bit warm after a dusting with the Scotchbrite wheel!



One hour of work (minus running in all of the set screws) or 17 parts in an hour.  




I made up around twenty of them which will probably last more than a lifetime. I did sell 8 of them at a recent show for $10 each, so I’m probably only about $100 in the red based on time involved! Hey, it was “shop time”!

Thanks for looking, Bruce


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## markba633csi (Aug 30, 2022)

That must be a pre-60's part cause I sure don't remember anything like that in my set- can't imagine what they would be used for
-Mark


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## BGHansen (Aug 30, 2022)

markba633csi said:


> That must be a pre-60's part cause I sure don't remember anything like that in my set- can't imagine what they would be used for
> -Mark


You are correct.  Only used in 1924 and 1925 in just one large set.  I've looked through the manual a number of times and have not found a use.  Not to belabor it (too late), but everything from the 1924/25 manual carried over into the 1926 manual.  The 1926 largest set no longer included the part making me think it never was used in an illustrated model.  Hence, 20 pieces will likely be more than a lifetime supply!

Bruce


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## C-Bag (Aug 30, 2022)

BGHansen said:


> . I did sell 8 of them at a recent show for $10 each,


WOW, cool! Being the terminally curious type upon closing the deal I would have asked them what the heck they were for.


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## markba633csi (Aug 30, 2022)

Yeah, next time you sell one ask what they are LOL
I'm thinking AC must stand for "axle coupler" or "axial coupler"


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