Printed buffers for wagons

kelham
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Re: Printed buffers for wagons

Postby kelham » Thu Aug 10, 2017 9:22 pm

How easy would it be to scale up the files to 7mm scale? Yes, I know this is a 4mm scale Forum...I'm just asking! I'm thinking in particular of the LNWR buffer housings.


Richard

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Guy Rixon
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Re: Printed buffers for wagons

Postby Guy Rixon » Fri Aug 11, 2017 10:27 am

Sort answer: yes, I can scale up the buffers and will do so if you can help out with information and testing.

Longer answer: ease of scaling up varies with subject. The basic part is trivial - just change two functions in the source code that set the scale. Re-optimizing the print for the larger scale takes a little longer. In the case of the buffers I need to make three changes.

1. Re-engineer the bores for the springing conventions of 7mm-scale buffers, whatever they are.

2. Check the optimisation of the walls of the guide. Some of the buffers have been made slightly wider than 4mm scale to get the walls above minimum, printable thickness. I would remove this kludge for the larger scale.

3. Change the nuts from square to hex. When I coded the buffers I used square nuts because it was easier and the difference doesn't show in 4mm scale. Now I've written a library for fasteners and we might as well have them right.

To scale up the buffers, I'd need (a) advice on how 7mm-scale buffers are to be sprung (also, how they are fixed to the headstocK? how big should the spigot be made?) and (b) somebody to buy a set on spec to check the mechanical action. It's very likely that the springing will work on the first try, but I don't have 7mm parts around to test it.

For re-scaled versions of my other products, just ask.

kelham
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Re: Printed buffers for wagons

Postby kelham » Sat Sep 02, 2017 10:09 pm

I'm not entirely sure what the springing conventions for 7mm scale are either. I'll have to do some digging... Otherwise I'd be interested in being the guinea pig. I'll have a dig around.


Richard

Alan Turner
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Re: Printed buffers for wagons

Postby Alan Turner » Sun Sep 03, 2017 8:29 am

kelham wrote:I'm not entirely sure what the springing conventions for 7mm scale are either. I'll have to do some digging... Otherwise I'd be interested in being the guinea pig. I'll have a dig around.


Richard


Normally the buffer has two diameters of bore. The front bore accepts the diameter of the buffer shank and the rear accepts the diameter of the reduced threaded end of the buffer shank. Hence the spring which is fitted is held in the body.

the larger diameter is 1.66mm and the smaller is 1.25mm - these are for Slater's buffers.

regards

Alan

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Guy Rixon
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Re: Printed buffers for wagons

Postby Guy Rixon » Sun Sep 03, 2017 9:42 am

Buffer-dims-7mm.png
Buffer-dims-7mm.png (33.93 KiB) Viewed 4941 times


Thanks for the information on the bores. I now just need the three dimensions shown in the attached image. That's a section through the 7mm model, BTW.

The distance from the bottom of the wide bore to the buffing face I set to 6.25mm in 4mm scale. IMO, this gives the softest springing consistent with the buffer returning reliably. In the 7mm model, I've set it to 7/4 times the 4mm-scale figure, but it may need adjusting for the springs of choice.

Alan Turner
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Re: Printed buffers for wagons

Postby Alan Turner » Sun Sep 03, 2017 9:57 am

Guy Rixon wrote:Buffer-dims-7mm.png

Thanks for the information on the bores. I now just need the three dimensions shown in the attached image. That's a section through the 7mm model, BTW.

The distance from the bottom of the wide bore to the buffing face I set to 6.25mm in 4mm scale. IMO, this gives the softest springing consistent with the buffer returning reliably. In the 7mm model, I've set it to 7/4 times the 4mm-scale figure, but it may need adjusting for the springs of choice.


Based on a Slater's buffer:

Distance to buffing face = 6mm

Spigot depth = 4mm

regards

Alan

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Guy Rixon
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Re: Printed buffers for wagons

Postby Guy Rixon » Sun Sep 03, 2017 10:18 am

Alan Turner wrote:Based on a Slater's buffer: Distance to buffing face = 6mm


Really? Seems a bit short. Perhaps I wasn't clear in my picture; apologies. The distance in question is from the front of the front of the buffer head to the base of the wide bore. If 6mm is the depth of the wide bore in the buffer guide, then I can work out the dimension I need if you tell me the length of the guide and confirm that the sample buffer is 18" from headstock to front of buffer head.

Also, is 3mm OK for the spigot diameter?

Cheers,
Guy

Alan Turner
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Re: Printed buffers for wagons

Postby Alan Turner » Tue Sep 05, 2017 4:07 pm

Guy Rixon wrote:
Alan Turner wrote:Based on a Slater's buffer: Distance to buffing face = 6mm


Really? Seems a bit short. Perhaps I wasn't clear in my picture; apologies. The distance in question is from the front of the front of the buffer head to the base of the wide bore. If 6mm is the depth of the wide bore in the buffer guide, then I can work out the dimension I need if you tell me the length of the guide and confirm that the sample buffer is 18" from headstock to front of buffer head.

Also, is 3mm OK for the spigot diameter?

Cheers,
Guy


My quoted dimension is from the face of the casting to the end of the bore. I don't understand why the buffer face to end of bore is relevant to you.

3mm spigot dimension seems OK.

regards

Alan

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Will L
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Re: Printed buffers for wagons

Postby Will L » Tue Sep 05, 2017 7:35 pm

When I have converted solid buffers to sprung buffers by drilling concentric holes down buffer bodies, I have always drilled out the wide hole so it it just deep enough to ensure that when the spring is fully compressed the buffer head just meets the buffer housing. This is to minimise the risk that a hard shunt will push the spring deeper into/through the body of the buffer. It has always seemed to me that the depth of the wide section of the hole is slightly greater than, and entirely dependant on, the length of the wide section of the buffer shank plus the length of the compressed spring. These don't seem to be getting a mention. Am I missing something here?

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Guy Rixon
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Re: Printed buffers for wagons

Postby Guy Rixon » Wed Sep 06, 2017 10:31 am

Alan Turner wrote:
Guy Rixon wrote:
Alan Turner wrote:Based on a Slater's buffer: Distance to buffing face = 6mm


Really? Seems a bit short. Perhaps I wasn't clear in my picture; apologies. The distance in question is from the front of the front of the buffer head to the base of the wide bore. If 6mm is the depth of the wide bore in the buffer guide, then I can work out the dimension I need if you tell me the length of the guide and confirm that the sample buffer is 18" from headstock to front of buffer head.

Also, is 3mm OK for the spigot diameter?

Cheers,
Guy


My quoted dimension is from the face of the casting to the end of the bore. I don't understand why the buffer face to end of bore is relevant to you.

3mm spigot dimension seems OK.

regards

Alan


Assuming a standard length of ram and spring, the distance from the buffing face to the bottom of the bore determines how much the spring is compressed when the buffer head is pushed against the guide and how much pre-load is on the spring when the ram is fully extended. The length of the guide varies for a given, overall length of buffer. E.g., the LNWR, Emmett-period buffers for wagons with 3-link couplings come with either 10" or 12" guides for an overall length of 18". If I put the same bores in each, relative to the outer end of the guide, then one of the types will be 18" overall and the other will be wrong by 2".

So you've told me the distance from end of guide to bottom of bore, for which many thanks. If you could please now tell me how long your guide is then I can work out what I need.

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Guy Rixon
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Re: Printed buffers for wagons

Postby Guy Rixon » Wed Sep 06, 2017 10:46 am

Will L wrote:When I have converted solid buffers to sprung buffers by drilling concentric holes down buffer bodies, I have always drilled out the wide hole so it it just deep enough to ensure that when the spring is fully compressed the buffer head just meets the buffer housing. This is to minimise the risk that a hard shunt will push the spring deeper into/through the body of the buffer. It has always seemed to me that the depth of the wide section of the hole is slightly greater than, and entirely dependant on, the length of the wide section of the buffer shank plus the length of the compressed spring. These don't seem to be getting a mention. Am I missing something here?


I'm trying to set the buffers up with the deepest bore consistent with the rams returning reliably to the right position when uncompressed. I do this so as to have the lightest feasible springing. If I make the springing too light, then the springs don't have enough force to return the rams to a consistent position - we need a bit of preload to overcome the friction that varies between the buffers.

Conversely, the heaviest springing is going to have the shortest bore that allows the ram to go fully home, and that seems to match what you describe, Will. Softening the springing shouldn't make the system vulnerable to damage in rough shunts.

Alan Turner
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Re: Printed buffers for wagons

Postby Alan Turner » Thu Sep 07, 2017 9:09 am

Guy Rixon wrote:
Alan Turner wrote:
Guy Rixon wrote:
Alan Turner wrote:
So you've told me the distance from end of guide to bottom of bore, for which many thanks. If you could please now tell me how long your guide is then I can work out what I need.


It measures 6mm same as the guide depth. These are for Slater's buffers.

As regards springs wouldn't it be better to base everything on Slater's springs as they sold for this purpose and easy to buy?

regards

Alan

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Guy Rixon
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Re: Printed buffers for wagons

Postby Guy Rixon » Tue Sep 19, 2017 7:30 pm

The LNWR Emmett buffers in both 4mm and 7mm scales are now available in the shop (https://www.shapeways.com/shops/guyrixon). All four lengths are available. The 7mm-scale buffers are set up for Slaters springs and rams, as per the dimensions kindly supplied by Alan. The 4mm-scale ones are unchanged in this update.


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