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

[Zox]

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?

Please forgive me if I messed this up--I've not done any etching--but I believe what we're discussing for a pocket with a tab would look something like this:



It would actually be less likely to interfere with the bolster, since you don't have the width of the screw-head to worry about.

As pointed out earlier, the hole for the tab wouldn't have to be nearly as deep as a hole that has to "grab" a threaded fastener, so you might not need to drill all the way through the floor. You're also saving the cost of a second screw, and possibly the time needed to tap the second hole.

[peteski]

Ed, you better contact PPD and ask them. From my past experience with etching I would be inclined to say that the peak as shown in the drawing is where the mask in the original original artwork would end (there is definitely undercutting going on during etching).  That drawing is a bit ambiguous.  As stated at the bottom of that page: If you have any specific questions not covered, or have any questions about these guidelines, please feel free contact us and we will be happy to discuss your artwork queries.

If you get this right then there will not be any drilling required. 

[Chris333]

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...

This is what I go by for myself.

Another way is to make a few tests next time you send something out, but that takes time and a few bucks.

[peteski]

Another way is to make a few tests next time you send something out, but that takes time and a few bucks.

Or as I said, ask the techs at PPD. That way there will be no doubts about what needs to be done to end up with the properly sized holes. 

[GaryHinshaw]

I dug up some old info about holes sizes from the first run of these boxes.  Sadly, I can't fully reproduce the numbers I had before, but my bottom line recommendation would be to spec the tap hole at .036 and the clear hole(s)* at .050.  Here is the exchange I had with James at PPD on the subject last January:

Hi James,

Just wanted to drop you a line to let you know that my etchings showed up last week safe & sounds, and they are awesome! There are a few things I'll want to tweak in the next run, but first I have a quick question for you about hole sizes.  As drawn, I had two holes specified: one at 40 thou dia. and one at 55 thou dia.  Since these were sized to be tap & clearance holes, respectively, for a 00-90 screw their dimensions are somewhat critical.  When I measure them, they come out as ~50 thou and ~62 thou, respectively, so about 5 thou larger in radius.  Does this sound about right to you?  Do you recommend that I draw the new holes about 5 thou smaller in radius?

Thanks,
Gary

Gary

It's a little difficult to say, since you drew bits that were under tolerance.  It is more likely that the guys pushed the etch to get the parts that should not have worked to work, leaving the holes over etched.  Does this make sense to you?  On material thicker than 0.4, we would suggest adding 5-10% if the dimension is super critical, but on thinner material we
would not bother, I suspect this is more likely the case that it was over etched.

Thanks
James Debnam

A few notes:
- Today, I remeasured some holes on the boxes I have left over and I can not reproduce the numbers I quoted above.  I am getting much closer to spec values.  That said, the tap hole is just a bit too large to be reliably tapped.  I agree that slightly undersizing is easier to deal with since the holes can easily by reamed to spec. afterwards.

- The comment James made about "pushing the etch" has the following context.  When he reviewed the original artwork, he  warned me that some items like the old air hose and cut lever tabs would be dicey.  He said it was worth a try though, and he mentioned beforehand that they would "push the etch" to see how it goes.  That aspect turned out really well.

- I'm not sure what James meant by the comment: "since you drew bits that were under tolerance."  That could either mean they were speced too small or they were subject to a spec.  I never followed up, but I'm getting a bit Talmudic now. 

-gfh

*P.S. I'll post a few more general comments in a separate post.

[Scottl]

Why is it critical that the screw tap the etch?  My thought was the screw needs to pass the brass and secure to the body of the car that has been tapped?

Great work, regardless and I love how the FT coupler looks.  I must admit I'm in coupler hold mode.  Waiting for an announced new product from a company that has three letters in it's name!

[GaryHinshaw]

Warning - this post contains no useful information, it's just an historical re-creation.    I still stand by the recommendation of .036 and .050.

Earlier today I measured the holes with my steel ruler and was getting numbers close to what were spec'ed in the drawing, with an uncertainty of a few thou.  This puzzled me because in January I had earlier measured them to be larger.  I now recall that I used my calipers back then, so I remeasured them today with those and I do get the larger values I got earlier.   Hmm.  Next I measured my ruler graticules with the calipers and I got numbers that are a few thou too large...  I re-checked that the calipers were correctly zeroed, and I calibrated them by measuring some code 55 rail, which came out bang on .055.  I think the problem is a bias in the way I measured the holes with the calipers.  I'm not able to get the caliper prongs into the holes, so I just place them in front of the hole and estimate the diameter (the same way I measured the ruler graticules).  This must introduce a small measurement bias.

Bottom line is that the etched holes came out a few thou too large, based on tests with actual holes and actual screws.

-gfh

[GaryHinshaw]

A few more general comments:

- I see that you changed the cover hole to a tap hole.  That's probably fine, but having both the top and bottom holes threaded could be tricky since that forces a quantized spacing (integer number of screw threads) between the two surfaces.  It would be easy enough to ream one or the other to a clearance hole though.  (BTW Scott, the original motivation for a tap hole was to allow the box to be "standalone".  In the situation you mention, both holes could be clearance.)

- I can't tell from the drawing, but is the shorter lid still long enough to clear the coupler shank?  Re the back flap, I think taking off .002 from each side is about right.  There is no harm having this flap be slightly narrow, but it's a pain for it to be too wide.

- How long is the overall part now (neglecting the fold up lip)?  I'm still keen to try these out on tank cars where they would mount directly to the tank.  As an example, the length I would want for an Atlas corn syrup tanker would be about .34", so I'm hoping your drawing is at least that long. 

- The slot idea sounds good.  Hopefully it's strong enough.  Your point about the pitfalls of an alignment tab are well taken.  Maybe there should be a short version, like Zox' drawing, with a tab, and a long version, like your drawing, with holes/slots?

- I honestly don't mind a generic cut lever bracket.  And roger on the cover slots - I knew those didn't fold.

Thanks again!
Gary

[Zox]

Why is it critical that the screw tap the etch?  My thought was the screw needs to pass the brass and secure to the body of the car that has been tapped?

The "screw tapped into the etch" was in the description of a possible long-draft/cushion coupler, where the shank of the coupler is beyond the body of the car, and therefore can't be supported by the same screw that attaches the coupler box to the car.

For the "normal" coupler as pictured, the screw does indeed pass through the box and shank and secures to the floor.

[Scottl]

OK, thanks.

[ednadolski]

Great feedback guys, and many thanks once again! This is one of the really awesome things about TRW!

Rob, thanks for the image, that does help me to 'get the picture' --- literally!     What you have there is shortened the pocket back into basically the single screw design, where the tab does the same job vis a vis rotation as the second screw.  I'm still unsure about the idea of one screw being as strong, so let me give you an example.  In this picture here, note how close the mounting screw had to be drilled to the edge of the car floor in order to put the pocket where it needed to go.  So I am a little concerned that in some cases, there might not be enough material in the right place for a secure screw attachment. And even with adding a tab (or using CA), there is still the concern about binding (since there is no center post).

Peteski & Gary, I'll definitely remember to check with PPD.  It sounds like I should start with the dimensions that Gary recommends for tap and clearance.  On James' comment about 'under tolerance' that sounds to me like he might have meant that the cut lever and air hose holes were drawn a bit too small to etch reliably for the thickness of the metal.  So to make sure the holes would etch thru, they decided to "push the etch" which I guess means leave the brass exposed longer in the etchant so more of the metal gets removed.  Which of course also means that the tap holes, dimples, and everything else had a little more material taken off too --- good thing you made the tags as large as you did (BTW what dimension did you use for those?)

Why is it critical that the screw tap the etch?  My thought was the screw needs to pass the brass and secure to the body of the car that has been tapped?

Well, we're using it in a couple of different ways.  If the screw is just to hold the pocket on, and serve as a coupler pivot, then the only hole that needs to be tapped is the one in the pocket body, the hole in the lid can be a clearance.   OTOH, with an extended coupler pocket, we don't want the pivot screw poking out the top of the pocket, so we use a shorter screw in that case and the tap-size hole in the cover allows the screw to hold om there.  BTW I was thinking in that case, the screw would not hold the cover on, so the cover would have to be glued (or even soldered) onto the body of the coupler.  Alternately, I could make the tabs longer so that they could fold up after the cover is put on, and thus hold the cover in place.

- I see that you changed the cover hole to a tap hole.  That's probably fine, but having both the top and bottom holes threaded could be tricky since that forces a quantized spacing (integer number of screw threads) between the two surfaces.  It would be easy enough to ream one or the other to a clearance hole though. 

Agreed.  I think this situation came from me conflating two separate use cases.

- I can't tell from the drawing, but is the shorter lid still long enough to clear the coupler shank?  Re the back flap, I think taking off .002 from each side is about right.  There is no harm having this flap be slightly narrow, but it's a pain for it to be too wide.

That part of it is unchanged.  I'll be sure to take the 0.002" off each edge.

- How long is the overall part now (neglecting the fold up lip)?  I'm still keen to try these out on tank cars where they would mount directly to the tank.  As an example, the length I would want for an Atlas corn syrup tanker would be about .34", so I'm hoping your drawing is at least that long. 

I think it's currently now more like 0.300".  If that's not enough then maybe an extended version could be used?   Do you have any dimensions that you want to see for that?

- The slot idea sounds good.  Hopefully it's strong enough.  Your point about the pitfalls of an alignment tab are well taken.  Maybe there should be a short version, like Zox' drawing, with a tab, and a long version, like your drawing, with holes/slots?

Perhaps.  I need to get started on the extended version, and see what I can tell from looking at that.


Cheers!

Ed

[GaryHinshaw]

Hi Ed, good point about the screw location being close to the edge of the floor.  I had the same issue with the grain hopper installation, in the 2nd photo of this post you can see how the screw was actually impinging the gap between the sill and end cage.  These couplers and boxes are small!

You're correct about the 'under tolerance' comment - James was especially referring to the cut lever tab in the old 2nd cover design: he wasn't sure the gaps were large enough to etch through.  By the way, I'm not sure what you mean by 'tabs' in that comment, the attachment 'sprues'?  I think those were .02" wide?

RE tank car dimensions, the only dim. I have is the 0.34" length.  Is there something else you wanted? I never had a problem trimming the longer version down wih etching scissors, so longer is better from my perspective. Same with lots of dimpled holes...   My one concern about a slot is that the pocket could slide forward under stress, if that is the only significant attachment point.  It's probably fine, it's just a minor concern.

Cheers,
Gary

[ednadolski]

Gary, it sounds that like for the tank car it might work to trim down the elongated version to the size that actually needed for the specific car.  Soldering too has been mentioned; for some open-end type installations it maybe would work to solder some brass strip onto the pocket, and then epoxy the strip to the frame members.   I'll dig out a tank or two and give them a look-over.

I'm not worried about the slot having any sliding.  The pivot screw should prevent that the same way that it prevents torquing.  On an extended pocket, the pivot screw couldn't work that way, but I'm thinking in that kind of installation it might be desirable to have a second mounting screw anyway.

BTW I found a really cool etching guide that talks about the etching profiles: http://www.hollywoodfoundry.com/Docs/Metal%20Etching%20Principles%20and%20Rules.pdf  (So now I have some reading to do!)

Ed

[ednadolski]

OK gents, here it is... Coupler Pocket 2b:  (apologies, that's not a Shakespearean reference!  )





Changes in this revision:

 - Re-sized tap holes to 0.036" and clearance holes to 0.050" diameters
 - Shaved 0.002" off the edges of the fold-up tab on the cover
 - Converted the two single clearance holes to a slot, to allow use of pre-drilled holes
 - Widened the long slots on the cover to allow for 'cusping'
 - Cleaned up some of the 'under tolerance' areas around the air hose

I still have to come up with the elongated version.  The main differences will be of course the length, plus the S-shaped hose, and the tap hole in the pocket goes from a full etch to a back-side (blue) half-etch.

Looking forward to your feedback!

Thanks,
Ed

[peteski]

Isn't the elongated hole a bit wide?  Or was that the goal?  Also, isn't the color of part which is to remain supposed to be black with white where everything will be etched through?  Or will the artwork colors be reversed later?

Also, what are the rectangular holes in the side walls for?  It was probably mentioned earlier and I just missed that (I'm a late arrival to this thread and I didn't read all of the previous posts).