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Dart Reverser nut

Dart Reverser nut.

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Steve Bright 203/07/2022 08:02:12
22 forum posts
22 photos

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?

reverse nutr.jpg

Michael Gilligan03/07/2022 08:09:41
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20289 forum posts
1064 photos

If you insert pins … what material would you use ?

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

MichaelG.

Steve Bright 203/07/2022 08:23:13
22 forum posts
22 photos

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.

Michael Gilligan03/07/2022 08:46:35
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20289 forum posts
1064 photos

Should be fine then

MichaelG.

Clive Brown 103/07/2022 09:23:44
869 forum posts
47 photos
Posted by Steve Bright 2 on 03/07/2022 08:23:13:

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.

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

Weary03/07/2022 09:33:07
388 forum posts
1 photos

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

Phil

SillyOldDuffer03/07/2022 09:59:17
Moderator
8903 forum posts
1999 photos

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

Michael Gilligan03/07/2022 10:42:28
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20289 forum posts
1064 photos
Posted by SillyOldDuffer on 03/07/2022 09:59:17:

Turning the studs from solid has advantages:

[…]

I'd follow the drawing in this example. Unusually, it looks like delivering the better result for less trouble.

.

I agree completely, Dave yes

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

MichaelG.

Andrew Johnston03/07/2022 11:25:11
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6678 forum posts
701 photos

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

Clive Brown 103/07/2022 13:18:39
869 forum posts
47 photos

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).

capture.jpg

Baz03/07/2022 13:52:28
757 forum posts
2 photos

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.

Steve Bright 203/07/2022 14:46:47
22 forum posts
22 photos

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.

Baz03/07/2022 15:48:41
757 forum posts
2 photos

Posted by Steve Bright 2 on 03/07/2022 14:46:47:

Hardest part would be holding it after soldering the pins as there isn't a lot to grip.

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

SillyOldDuffer03/07/2022 16:49:15
Moderator
8903 forum posts
1999 photos
Posted by Andrew Johnston on 03/07/2022 11:25:11:

It would be a right royal PITA to set up and machine the three spigots in the correct places.

Andrew

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

Nick Clarke 303/07/2022 17:24:34
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1475 forum posts
64 photos
Posted by SillyOldDuffer on 03/07/2022 16:49:15:

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!! cheeky

old mart03/07/2022 20:45:08
3912 forum posts
268 photos

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.

JasonB03/07/2022 20:56:05
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23070 forum posts
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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

Andrew Johnston03/07/2022 21:23:45
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6678 forum posts
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Posted by SillyOldDuffer on 03/07/2022 16:49:15:

...the part doesn't look hard to make from solid to me!

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

julian atkins03/07/2022 22:07:03
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1258 forum posts
353 photos

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?

JasonB04/07/2022 06:52:30
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Posted by Andrew Johnston on 03/07/2022 21:23:45:
Posted by SillyOldDuffer on 03/07/2022 16:49:15:

...the part doesn't look hard to make from solid to me!

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

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

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