How do you cut a "reverse draft angle" into an MTL coupler knuckle?

[Maletrain]

I know I saw a drawing of how to shape Knuckles to "reverse draft angle", but I can't find it anywhere (searching TRW and Google).  Does somebody here have that picture or a link to it?

[bbussey]

Look at the mold separation line of the coupler knuckle and see if the edge of the “fingertips” draft to the separation line, like a shallow triangle point. Use an X-Acto to cut inward to create a slight “valley” with the point at the separation line. This moves the small point of contact from the separation line to the outer edges which lessens the chance of the couplers riding up over each other.

You also can file the tip straight with a jewelers file.

If there is no positive draft angle on the coupler tip, then the reverse draft angle already exists. Micro-Trains has been utilizing RDA for at least two decades, so this would apply only to vintage MTL and Kadee couplers.

[Point353]

[peteski]

IMO, this is best done with a brand new #11 blade. Filing the slippery plastic produces fuzz which sticks to the coupler's surface, when then has to be trimmed with . . . a brand new #11 blade. At least in my experience. Delrin is fairly soft and it is really easy to trim a thin sliver off the material.

[bbussey]

1996.  A quarter-century ago. 

[railnerd]

1996.  A quarter-century ago. 

I remember buying my first set of these couplers when they came out from Rich Buike @ Trackside Trains.  He's been gone a long time, too.

-Dave

[Maletrain]

Thanks for the picture and instructions.

But, after looking through an assortment of MTL cars of various vintages, I am not very impressed with the "negative" angles on most of what I am seeing that I know is new, such as the singe window coach that I recently bought from MBK .  It looks like the parting line ridge and some offset in the mold halves is more dominant to the shape than the draft angle.  Most of the older cars I have were bought used, so it is not clear to me if those originally had Rapido or positive draft MagneMatics that were replaced with the negative draft couplers before I bought them.  But, "straight with a big bump at the parting line" seems to be a better description than "negative draft".  I did find one car with an clearly positive draft angle, and stopped looking once I had a sample.

So, considering Otto's problem with unwanted uncoupling in his thread about running long trains of MTL heavyweights, I am wondering if the negative draft angle is actually functional on some of the couplers.  The actual coupler faces are not as pretty as that picture, above.

[peteski]

As I see it there is no such thing as "negative draft angle". It is a marketing term. If it was really negative then the part would not come out of the mold.  The molds were redesigned to minimize the draft angle on the "fingers" area of the coupler, but this affected the shape/angle of the other surfaces of the coupler knuckle to allow for the part removal.  As you noted, there is also slight partying line flash where the mold halves meet.

[mmyers]

Long before MT announced the reverse draft angle couplers Bob Mohr showed me how to improve MT couplers so they would not deflect upwards and separate. He would use a small diameter round jewelers file. Turn it in the coupler only one or two times to score the smooth surface. That simple modification allowed for 100+ car trains before the RDA couplers.

[MetroRedLine]

WUUUUUUUUUTTTTTTTTTT!!!!

All this time I asked people what "Reverse Draft Angle" means exactly and either got responses like, "You can't really see it, but it's there" or a bunch of advanced calculus equations. I have been in N scale for exactly 14 years and had no idea this illustration existed until today.

[peteski]

WUUUUUUUUUTTTTTTTTTT!!!!

All this time I asked people what "Reverse Draft Angle" means exactly and either got responses like, "You can't really see it, but it's there" or a bunch of advanced calculus equations. I have been in N scale for exactly 14 years and had no idea this illustration existed until today.

If you look at an older and RDA coupler head under magnification, and from various angles, you can see the difference.

[MetroRedLine]

If you look at an older and RDA coupler head under magnification, and from various angles, you can see the difference.

I've only been in N scale since 2006. I don't think I've seen an older MTL coupler head.

[peteski]

I've only been in N scale since 2006. I don't think I've seen an older MTL coupler head.

Any older MTL (or Kadee) car will have the older style coupler.

[Maletrain]

I've only been in N scale since 2006. I don't think I've seen an older MTL coupler head.

If you buy stuff on eBay or at train shows, you will probably have some old style ones.

But, as I posted before, looking at a lot of the newer ones, I did not see anything that looked as clean as that drawing from MTL.  There seems to be much more ridge at the parting line and some offset between the top and bottom of the hook on the knuckle. 

So, to get back to the genesis of my OP, the trouble that Cajonpassfan is having with the MTL heavyweight cars uncoupling, as described in his other thread, may or may not have anything to do with how well that "reverse angle" actually came out on the latest renditions of the modern couplers.  That is why I was trying to remember how to use a blade to make it "right" in case it was cast wrong on some of my own cars.

[randgust]

The weird part about the RDA change is that it also depends on the INSIDE of the knuckle being molded in such a way that there's a slight peak in the center (inside).   If that's there, it really, really locks in to the opposing RDA 'finger' on the other side and if you pull hard enough, it will break the head off before it will separate.

Certain couplers never had that center mold line/point; I think original low-shank 1104's, likely some others.   If there's no center mold line, the RDA fingers probably don't have anything to grip on the other coupler.   

If you mix up consists you'll rarely see this as if one coupler is molded with the centerline ridge and the other one isn't, it still locks.   If BOTH of them have the mold line at the bottom, instead of the center of the head, no amount of RDA trimming is going to fix it.

I haven't checked my newer MT heavyweights but I haven't had issues either, and they are in a mixed consist where all the Rivarossi cars are converted over to 1015's.   1015's, depending on their age, may or may not be RDA.  And its more pronounced on some that others.  It always looked to me like the MT heavyweights simply had a normal 1015 shank, maybe not.

I've been preaching the RDA fix for years, and making sure the guy that pioneered it, the late Jim Fitzgerald, gets due credit for discovering it.   To me, it solved a nightmare of false uncoupling on my layout going back years, as well as the other major benefit - it eliminates trip-pin derailments resulting from one coupler pushing 'low' when the other one goes 'high'.   So to me, it not only solved the false uncoupling, it's allowed me to stay with magnetic uncoupling with no trip pin issues at all.

I do the sharp #11 blade treatment completely out of habit now.