Best Of Notes on body-mount couplers (work-in-progress)

[DKS]

I really like how this is going. Just does anyone know how well the FTs hold up under long trains? Say 100+ cars?

FTs are remarkably strong. One reason is that the have a solid (not split) shank. I think they were tested and could withstand something like over 5 pounds of pull, but don't quote me on that.

[GaryHinshaw]

I don't how these would hold up to 100+ cars in N, but I think they do fine in 100-car coal trains in Z (in talgo-style trucks).  As DKS notes, the couplers and draft gear have plenty of strength; the key will be to maintain vertical alignment, which requires careful body-mounting.

Truth be told, we're just messing around here.  These are still not available as a standalone item, so you'd be hard-pressed to afford 100 pair.   We're just buying time until the NZT Proto-Mates come along.  That said, I do really like these FT's still.

[ednadolski]

According to this page on Atlas, 20 average N scale cars take about 0.8 ounces of pull on level+straight track.  I've tested a single FT to statically lift a 10-ounce weight, and I'm convinced it could do more than that.   So the strength doesn't seem to be too much of a concern.

We're just buying time until the NZT Proto-Mates come along.

Also there is the learning curve of building & using a box like this, which may come in handy by the time we have the NZTs.


In the meantime, I was wondering if any folks were interested in more FTs?   My understanding is that Will is all sold out, but he might be open to doing more if we can get him some kind of number.

Thanks,
Ed

[Ike the BN Freak]

I do like the box, being it seems like a good option for cars with open ends that make mounting MTs difficult.

I'd like to do be able to do these, however most of my cars will probably just be MTs...

[peteski]

- Mike, you're definitely correct that two clearance holes are best for cases where the box mounts directly to an under-frame.  My thinking was that I could start with a tap hole and ream it to a clearance hole for those situations (which are probably the vast majority).  A tap hole potentially gave me options for other applications like extended gear or maybe tank cars where the box is under a flimsy platform.  But that may not be true in practice, we'll see.

Brass is *VERY* easily soldered. The coupler box could soldered together (if needed) and any possible extensions or custom mounts (made of brass) could also be easily soldered to the box.

[ednadolski]

I've put together this sketch for a revised, standard-length pocket.   As always I'm very interested to hear critiques/comments/questions.



Here are the key changes in this version:

 - Added an etched air hose & pipe, in place of the fold-out tab for mounting a plastic part.  With this one, you simply fold it up along the two half-etches on the side of the cover, and bend the hose proper to whatever shape you want. This is closer to scale than the plastic part, and I also found that with the previous design the plastic air hose could interfere with the coupler swing.

 - I'm experimenting with a mounting system that uses two 00-90 screws instead of one.   The trick with the single screw is that I had to make it pretty snug in order to securely mount the pocket and keep it from moving; however that could easily cause the coupler to bind.  With two screws, one screw serves as the pivot for the coupler, while the other has the job of securing the pocket to the carbody.  Two screws also eliminates any rotation, and makes for a sturdier mounting overall. To install, simply drill and tap two 00-90 holes (and I'm considering a drilling template for the fret).   Note that the holes for the pivot screw are  sized for a 00-90 tap, while the other holes in the pocket are sized for a 00-90 clearance.  I think this will also be useful with an extended pocket design.

- I shortened the overall length of the pocket, so that it can be installed without trimming.  Likewise, I shortened the cover length, to avoid clearance problems with the second mount screw.  There is also now one cover instead of two.  The shorter length and one less cover means that more usable parts can be etched from a sheet of brass.

- The fold-down tab for a cut lever is now more like a 'hook'.  I think this combines the advantages of the two different tabs in the previous design.  It avoids a potential interference point for the coupler shank, and still allows a cut lever to be installed without having to make an additional bend in a fairly precise location.  Of course, if you don't want a cut lever, the entire tab is easily trimmed off.

- An extended pocket would be longer with more of the clearance holes. Also the pivot screw tap hole in the pocket body becomes a half-etched hole on the inside of the pocket.  The pivot screw is therefore shorter and goes not penetrate through the top side of the pocket; it is held in place by the threads through the cover.  The cover in this case would I think need to be CA'd or soldered into place. The air hose too would change, becoming like one of those 'looped' style ones that are on cushioned draft gear on the prototype.

Thanks for looking!

Ed

[Philip H]

Ed,
I like this design a lot - the hook for the cut lever is a nice touch. I'm curious however why not just do the pointed dimple that can fold into the car body in a whole? That said if you go to production I'd be happy to beta test some.

[ednadolski]

I'm curious however why not just do the pointed dimple that can fold into the car body in a whole?

Excellent point Phillip.  My main thought is that having one screw sort of 'overloads' the one screw with too many jobs -- it has to serve as a pivot point, and hold the cover on, and hold everything securely in place.  The would be OK except that those jobs tend to conflict, in that it is hard to secure everything yet still function as a pivot point without the binding.  So going with two screws is like a division-of-labor that removes that conflict, and it has the added benefit of also acting like the dimple without affecting the primary function of each screw.   (I think it's simpler than it sounds -- sometimes I just use too many words that only add to the confusion!)

Not that altho there are two clearance holes, the idea is to use only one of them.  The only reason for including both is that I thought it might be useful in situations where one hole or the other might line up better with a factory-drilled hole like on some cars.

Thanks,
Ed

[peteski]

I also think that having to drill and tap 2 screw holes for each coupler is overkill.  When I body-mount MT couplers I mount them using a single screw then I wick some CA glue around the pocket.  When the glue hardens, it creates a tiny lip around where the coupler pocket meets the cars floor. The pocket is not able to twist at all.  Using CA glue is much faster and less pain than installing another screw.

[GaryHinshaw]

Looks good Ed!  Thanks for pushing this along, since I let it languish...  I have a few questions/comments, some based on rereading my notes in this earlier post:

* Did you shrink the holes a bit relative to the spec for 00-90 tap and clearance holes?  In my notes above, I found the holes came out about 15-20% bigger than spec'ed.  That said, I'm not sure I see the advantage of etching the two clearance holes all the way through, vs. having maybe 3 half-etch dimples, zero or one of which could be drilled through for a 2nd screw.

* It seems like it would be really simple to add a small fold up tab on the back of the body part for stabilization, as has been suggested.  It could always be removed if desired.

* It looks to me like the back lip of the cover should bit slightly narrower still, to avoid interfering with the box sides.  Not by much - just a few thou.

* I like the hose.  I can't tell from the drawing, but is it sized to be roughly square in cross section?  If not, it could tend to buckle when bent.  (The hose detail looks great!)

* Nice design on the cut lever hook - that should work really well.  I would love to have a corresponding part to glue to the car body for those cases where I would bend my own levers from wire.  Something simple with a straight rod extension that would fit snugly in a #80 hole in the car.

* Finally - just to confirm - I am interpreting the drawing as if I were looking at both parts from above. Then the red lines fold up out of the page, and vice versa for the blue.  That makes the uneven tabs line up in the correct sense.

Thanks again!!
-gfh

[peteski]

* Did you shrink the holes a bit relative to the spec for 00-90 tap and clearance holes?  In my notes above, I found the holes came out about 15-20% bigger than spec'ed.  That said, I'm not sure I see the advantage of etching the two clearance holes all the way through, vs. having maybe 3 half-etch dimples, zero or one of which could be drilled through for a 2nd screw.

Gary,
I suspect that you already know this but I figured that I would mention this, just in case.  The etching process undercuts the design by about half of the thickness of the material being etched.  That is when the material is etched from both sides. So if you are etching from 0.010" brass, holes will be 0.010" larger in diameter than the artwork.

[GaryHinshaw]

You're right Peteski, it's an additive correction, not a multiplicative one.  Got that Ed? 

[ednadolski]

Thanks dudes for the comments!   Lemme see if I've got the main points:

I also think that having to drill and tap 2 screw holes for each coupler is overkill.  When I body-mount MT couplers I mount them using a single screw then I wick some CA glue around the pocket.  When the glue hardens, it creates a tiny lip around where the coupler pocket meets the cars floor. The pocket is not able to twist at all.  Using CA glue is much faster and less pain than installing another screw.

No question, a single screw would be easier if possible.  One thing tho about the MT pockets is that they have a center post which can support the cover and reduce binding when the screw is tightened.  However the pivot hole in the FT coupler is too small to allow for a post (it just about clears the 00-90 threads without much to spare).  I'm thinking that's not necessarily a bad thing, since a post would make both the coupler and the pocket significantly larger, even tho it does make it harder to get away with a single screw.  (BTW the original plastic FT pocket has a post, but it's solid, and no screw.)

Since this design doesn't rely on the pivot screw to hold into the body of the car, it isn't really necessary to tap threads for it (assuming I properly allow for undercut on the tap holes).  All that's needed in a car body hole is enough diameter to clear the 0.0470" thread diameter.  Also FWIW I do tend to prefer a glue-free installation (probably 'cuz I too often end up with CA where I don't want it...  )

* Did you shrink the holes a bit relative to the spec for 00-90 tap and clearance holes?  In my notes above, I found the holes came out about 15-20% bigger than spec'ed.  That said, I'm not sure I see the advantage of etching the two clearance holes all the way through, vs. having maybe 3 half-etch dimples, zero or one of which could be drilled through for a 2nd screw.

This draft doesn't contain any allowances.  My MT data sheet for a 00-90 shows a 0.0470" diameter, a tap of 0.0380" (#62 drill), and a clearance of 0.0465" (#56 drill, actually sounds a tad snug to me).  So for 0.010" brass it sounds like the tap wants to be drawn at 0.0280" and the clearance at 0.0370".   Does that sound about right?

Re: dimples, I ended up not using any of them on the original version.  With a two-screw approach it seemed to me that etched holes would save a bit of drilling.  What do you think of this idea: in place of several holes/dimples, how about an oval slot (etched to a clearance width)?  That saves on drilling, and might be a bit easier to use with pre-drilled holes in the car body.

* It seems like it would be really simple to add a small fold up tab on the back of the body part for stabilization, as has been suggested.  It could always be removed if desired.

Easy enough to add, tho I'm not sure if it means any less drilling - seems the choice is either drill for the second screw, or drill for the tab.  (Maybe I've overlooked something here?)

Also - if located on the back, does it (potentially?) encroach on the bolster?

* It looks to me like the back lip of the cover should bit slightly narrower still, to avoid interfering with the box sides.  Not by much - just a few thou.

Sounds like a good idea, but do we get about 0.005" back on each edge anyways, because of the undercut?

* I like the hose.  I can't tell from the drawing, but is it sized to be roughly square in cross section?  If not, it could tend to buckle when bent.  (The hose detail looks great!)

It's about 0.012" or 2 scale inches across.  I actually put a half-etch lengthwise on each side of the hose, on the back (blue) side so it is hard to see in this picture.  It's only a few thou wide -- not a true half-etch, but just knocking a bit off the edges to help make an impression of something round.   Considering undercut, I'm not sure if it is needed, but I might leave it in to see how it works out.

* Nice design on the cut lever hook - that should work really well.  I would love to have a corresponding part to glue to the car body for those cases where I would bend my own levers from wire.  Something simple with a straight rod extension that would fit snugly in a #80 hole in the car.

Let me see what I can come up with.  There are a lot of variations on the proto, so it will probably be generic-looking.

* Finally - just to confirm - I am interpreting the drawing as if I were looking at both parts from above. Then the red lines fold up out of the page, and vice versa for the blue.  That makes the uneven tabs line up in the correct sense.

Correct.  The one exception is the red half-etch lines on either end of the tab slots in the cover. Those don't fold, they are just recessed to receive the corresponding edge of the pocket side walls.


Thanks for the great feedback guys!

Ed

[peteski]

Ok Ed, I guess that a double hole is a reasonable solution. Even if it is there, it doesn't prevent someone like me from using a single screw/pivot, then preventing the coupler pocket from twisting with some carefully placed CA glue. 

I wanted to double-check what I stated earlier and PPD states that the undercut is only 20% (not 50%) of the material thickness.  So you'll need to recalculate the hole size. Better yet, you might want to double-check by contacting PPD directly. They should have no problem assisting with critical things like hole diameter. For more info see the "Tolerances" section in http://www.ppdltd.com/web_site_3/DG_Line_width.html

I'm not sure where I remember the half-thickness (or 50%) undercutting of the etched material. It was probably for single-sided etching.

[ednadolski]

Thanks Peteski... the PPD page shows the formula, B = 20% of T, where B is the undercut amount, and T is the material thickness.



But the way the pic shows here, a hole should actually be a smaller diameter... about 2 x 0.002" for a 0.010" metal.

This is triggering a dim recollection from somewhere in my murky brain.   Gary, at one point did you tell me that PPD was using a more "aggressive" etch process with these pockets?  Would that account for the 15%-20% larger holes that you saw?

Edit:  I think it's probably OK to undersize the holes on the drawing.  It's not too much of a deal to enlarge a hole with a drill, but making a hole smaller is a lot more difficult...


Thanks,
Ed