Dart Reverser nut

Hi all,

Looking at the drawings for Martin Evans Dart Reverser nut it says 'Turn from solid gunmetal'

Should I follow the drawings or can I get a block of gunmetal.and add the pins?

If you insert pins … what material would you use ?

Presumably the integral gunmetal ‘pins’ provide some sort of bearing.

MichaelG.

I'd use gunmetal and they are guide pins that run in holes on the reverser arm, the top pin runs in a slot in the reverser stand.

Should be fine then

MichaelG.

Aren't they a bit more than guide pins? They presumably transfer the load from the nut to the reversing arm hence to the valve gear and so need to be reasonably sturdy. Agree, the top pin is basically a position indicator and so is not heavily loaded.

Edited By Clive Brown 1 on 03/07/2022 09:26:25

'Dart' builders and owners over on Model Engineering Clearing House can almost certainly give you suggestions and advice based on experience.

Phil

Turning the studs from solid has advantages:

• No need to drill holes that weaken the block, or make pins to fit them, and then ensure the pins stay fixed in place. Threading the 3/8 Whitworth LH is a forceful operation that might displace the pins unless rock solid.
• Easier to align the two studs by turning them in a 4-jaw from solid than risk a wandering drill.
• Pins of the same material are always slightly weaker than the solid equivalent; it may not matter, but...

I often use pins because they save time and material. Nonetheless, I'd follow the drawing in this example. Unusually, it looks like delivering the better result for less trouble.

Dave

.

I agree completely, Dave

… I just assumed that Steve was asking because [perhaps] he already had the smaller material to hand.

MichaelG.

It would be a right royal PITA to set up and machine the three spigots in the correct places. I would machine the basic block to size and then drill through for the 3/16" spigots, plus a blind hole for the 1/8" spigot. Then silver solder in a length of rod for the 3/16" spigots and a shorter one for the 1/8" spigot.

If it was felt necessary to stick with a Whitworth thread form I would screwcut it rather than try and find a LH tap. Alternatively I would make the thread M10 for which LH taps are readily available, albeit at a price.

Andrew

Agree that machining from solid would be tedious. Although the "Dart" drawing calls for that, Martin Evans used the built up method in eg. "Simplex". Mine's made this way, and it's withstood a lot of winding on up and down tracks. (Apologies for being sideways).

Don’t see what’s so difficult machining it from the solid, it’s not as though you’re on bonus and got 100 per day to make, nice little job for a small four jaw chuck and a happy hour or so machining.

I'm still undecided. It can be machined from solid, I'd start with a piece of 5/8 x 1" x 1 1/16" long bronze or gunmetal and with some careful setting up it can be machined as a solid. Getting a piece of 3/4" square is easier and drilling then silver soldering the pins in is easy enough.

I have looked at FleaBay and 3/8" LH Whitworth taps and dies are readily available and not too bad a cost, but as it would likely be a one off use I would seriously consider machining the threads and I already have the correct HSS tool bit. Hardest part would be holding it after soldering the pins as there isn't a lot to grip. That would be a very good reason to machine from solid and turn the pins after cutting the thread,

I have a while before the decision is required so I can ponder it for a while.

Thanks for all the replies which have given me food for thought.

Put a couple of washers over the pins and clamp on them and you have all the grip you need.

I have to bow to Andrew's much superior experience, but the part doesn't look hard to make from solid to me!

Is there a well-hidden booby trap? (I assumed starting from square unless that's cheating!)

I'll have a go later if I can find some time amidst all the other distractions. Due to lack of discipline and family intrusions I already have about 15 unfinished projects on the go.

Dave

Pah! Amateur!! Your unfinished project list ought to be in at least 3 figures if lack of discpline and family were really causing issues!!

Since the block is not too big, the cost saving from adding pins would not be much. I agree that the one piece block would be strongest and using a four jaw independent chuck in the lathe would be easy to make 90% of it.

Starting with some round stock about 28mm dia in the 3-jaw would make it easy to turn the two spigots on opposite sides so they are concentric then square up three other sides and turn the final top spigot.

Or a longer bit of rectangular stock would allow one spigot to be turned with conventional tools and the other with a parting tool all at the same setting which would again keep the two spigots concentric

I didn't say it was hard, or difficult, but a PITA. Making it from solid means that 3 three points have to be marked out and then picked up on in the 4-jaw chuck. Presumably the 3/16" spigots need to be concentric, so need to be picked up to a high degree of accuracy. Drilling a hole and silver soldering in a pin is simpler and guarantees concentricity. A sliver soldered pin will be as strong as solid.

Andrew

I would have a look at the GWR drawings, and start from there.

There are significant issues with the Martin Evans 'Dart' design.

Is the valve gear on 'Dart' capable of being 'notched up ' anyway?

Not if done as I suggested by starting with round stock as the 2 main spigots will be central.