Errors on drawings

[cplmickey]

Jul 20, 2021 7:36:04 GMT andyhigham said:

How do you machine the "rad to suit smokebox" without CNC?

Not easy at all. The way I have done this in the past is to make a radiussed washer to help seal to the smokebox - flat one side and radiussed the other. I do this for my outside steam pipes (both sides of the smokebox) and any fixing bolts too. Ian

[mbrown]

You can do the undercut ball seat with a suitably shaped D bit.

The radius can be achieved with careful hand filing, especially if you don't take the thread all the way to the shoulder so it won't be damaged by the file. However, I agree a shaped washer is the better solution.

Malcolm

[92220]

Jul 20, 2021 7:36:04 GMT andyhigham said:

Two other things strike me on that drawing.

How do you machine the "rad to suit smokebox" without CNC?

Hi Andy.

Can you not make up a large flycutter? In the early days, when I was making up my smokebox saddle, although it was fabricated from sheet steel, I still wanted the radii machined to a good fit. I made up a large flycutter to machine the radius in the steel plates, after I had roughly cut the radius by pin drilling. I did have to make up a very simple rig to hold the flycutter and set the cutter to the exact radius required. Unfortunately I didn't take photos of the build in those days, so can't show what I did, but I'm sure you get the idea.

Bob.

[andyhigham]

If you look at the part it is just the shoulder that is radiused to suit the smokebox.
The only way I can see without CNC is using needle files

[jcsteam]

These are not exactly errors, but probably best described as a better way of doing it unless someone here tells me otherwise. I am starting a 3.1/2" LBSC Molly, the holes for the buffers show a 3/8" hole the buffers themselves are tapped 3/8ME it would probably be better to have a plain section on the buffer, to press fit into the 3/8" hole and not thread all the way to the shoulder. Leaving 3/32" unthreaded would probably be satisfactory.
The boiler width is shown as 2.7/8", exactly the same as the inside of the frames leaving no room for expansion or rubbing on the 1/8" thick frames. I'll be making min former 1/16" less overall width where the boiler mounts into the frames, also the use of 3mm steel for the frames, combined with above will give me an overall clearance of 0.0675". Not a lot but enough to make sure the firebox isn't rubbing on the frames.

Another issue I've come across, been told about, is that the rear wheel brake hangers will foul the firebox if built to design. They are shown with a 5BA nut on the rear, given the small allowance already for the outer firebox, then I plan to drill the hole at the rear for the brake hangers to be a tapping size for 5BA.

If anyone knows of any other errors in Molly I'd like to hear from you.
Jon

[don9f]

Hi Jon, I don't know the "Molly" design, but have built a Martin Evans "Jinty" in 5" gauge. Does the 2 7/8" frame spacing you mention also have to accomodate any firebox stays protruding from the wrapper below the top of the frames? These would obviously make the overall firebox width a bit wider than just the wrapper itself.

Cheers. Don

[jcsteam]

Nov 6, 2021 18:17:37 GMT don9f said:

Hi Jon, I don't know the "Molly" design, but have built a Martin Evans "Jinty" in 5" gauge. Does the 2 7/8" frame spacing you mention also have to accomodate any firebox stays protruding from the wrapper below the top of the frames? These would obviously make the overall firebox width a bit wider than just the wrapper itself.

Cheers. Don

Hi Don,

Yes the width of the firebox is 2.7/8" Bare as it is described. So includes stays. On speaking with my boiler inspector, and another friend who has built many boilers the suggestion seems to be to countersink the holes slightly for the solder to form a nice fillet around the stay, then file them back flush. But for my own piece of mind, I'd prefer to reduce the width slightly.

[92220]

Jul 20, 2021 9:31:02 GMT cplmickey said:

Jul 20, 2021 7:36:04 GMT andyhigham said:

How do you machine the "rad to suit smokebox" without CNC?

Not easy at all. The way I have done this in the past is to make a radiussed washer to help seal to the smokebox - flat one side and radiused the other. I do this for my outside steam pipes (both sides of the smokebox) and any fixing bolts too. Ian

Quite right. You cannot produce that radius WITHOUT CNC if you have the thread, or anything in line with the valve body, except the end of the bar you are going to produce the curved face on. The curve cannot be produced on the lathe because the part is being rotated about a centreline at right angles to the centreline of the curved face. To produce that shape, machining movement can only be in one plane, i.e. at right angles to the centreline of the curve being machined. He has drawn an impossibility for most model engineers, except using the one sided, curved face, washer idea.

Bob.

[92220]

Just had another look at the drawing. There IS a way of getting what is drawn. Machine the body of the valve without the threaded end. When finished, the curved end can have a recess machined through to the inside, and a threaded piece machined up to fit in place. This can then be silversoldered in place. The drawing, obviously, doesn't show this, so a bit of extra drawing needs to be done to sort out how it will all fit together for a 'dry run' before silversoldering. Another problem arises though....you have made what is drawn, but how do you screw it into position? Actually impossible to screw in tight for a seal due to the curved face. Just had another thought...There is a way....If the thread is long enough and there is space, a thin gasket can be made up to seal the curved face to the mounting surface, and a nut used underneath to tighten the fitting to the surface.

Bob.

[firebird]

Hi
I have just got to the stage of making the cab for my current loco build. I won't mention what model it is but suffice it to say the plans were purchased from a reputable and longstanding supplier. Having had the experience of the numerous and costly errors on the plans of my first loco build I now scan and print out exact size copies and spray glue them to card so I can make a mock up. You would not believe the amount of errors in the measurements. For example, 6 3/8'' + 7 5/8''= 14'' or at least it did when I went to school (many moons ago) and not 15 1/4''!!!!!. There are so many inaccuracies that I have had to redraw the plans myself. Had I simply followed the plans as published 2 cab sides from sheet brass would now be residing in the scrap bin. My point is that with the ever-increasing cost of materials errors on plans should be corrected or at the very least the supplier should include a list of the known errors to the purchaser.

Cheers

Rich

[mightyradical]

kennions Butch 0-6-0 drawing errors.

I am currently building a kennions Butch 0-6-0 and one hole on the front section of both left and right frames simply states 1/4" on the Kennions plans.

Having finished the frames and looking into the brake gear in advanced planning now I can see that this hole on both frames should possibly have stated tap 1/4 x 26 as per a matching hole on the rear section of the frames. These are for the front and rear brake hanger pins which screw into the frame. Having already drilled mine out to 1/4" before noting this my fix is to now probably tap 9/32 x 26 hoping there is enough material left to cut thread at a later date and make the pin slightly bigger to screw into this hoping that the extra size does not interfere with any other components. Failing that I will make a step down insert.

I will update this with more info when I get to the problem I have caused, but If anyone who has built one can confirm this by pm'ing me that would be great.

[mbrown]

I am guessing you mean 9/32" - but it looks like a perfectly good fix to me!

Malcolm

[firebird]

Hi
I built a Chub which is the 0-4-0 version of the 0-6-0 Butch. I encountered and had to remedy many errors during the build.

One that springs to mind is the axle pump which has the wrong measurements on the plans. An expensive casting ended up in the bin on that occasion.

Cheers

Rich