Two Silver Soldering Questions

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Builder01
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Re: Two Silver Soldering Questions

Post by Builder01 » Fri May 18, 2018 8:44 am

Matt,

Silver brazing/soldering small parts can be done quite easily with mapp gas and it's a good way to gain experience. My boiler is for a 1" scale loco. The copper boiler is 5" in diameter and 19" in length. Fire insulating bricks are a must, not stove bricks for the large heat ups.

David

Bastelmike
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Re: Two Silver Soldering Questions

Post by Bastelmike » Sun May 20, 2018 7:05 am

Bill Shields wrote:
Mon May 14, 2018 5:04 pm

for many, +/- 0.01" is considered within tolerances..remember you are building a loco not a watch.
Hi,

that may be correct, but I think it depends on the scale of the build.
I'm currently building some 10.25" rolling stock and in one of my drawings I have specified a tolerance of ±0.15 mm (0.006"). I would use a piece with 0.01" error, no doubt. But with an engine for 3.5" gauge, I would consider that pretty poor Quality. :o

I have a study of mechanical engineering at a technical university and one teacher gave us a Guideline about permissible angle errors for things like axle bearings etc.

For angle tolerances we were told:

1:1000 very rough machines, e.g. farm machinery
1:5000 most machines
1:15000 and better precision machine tools e.g. jig Borers

1:1000 means for 1000mm shaft length you may have a Position error for the bearings of 1mm.
Using these guidelines 0.25mm (0.01") with 10.25" gauge is 1:1000, still useable,but low Quality
0.25mm with 3.5" gauge is 1:360 and I would call it really poor quality.

Just my 2 Cents
Mike

BClemens
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Re: Two Silver Soldering Questions

Post by BClemens » Sun May 20, 2018 2:25 pm

Bastelmike wrote:
Sun May 20, 2018 7:05 am
Bill Shields wrote:
Mon May 14, 2018 5:04 pm

for many, +/- 0.01" is considered within tolerances..remember you are building a loco not a watch.
Hi,

that may be correct, but I think it depends on the scale of the build.
I'm currently building some 10.25" rolling stock and in one of my drawings I have specified a tolerance of ±0.15 mm (0.006"). I would use a piece with 0.01" error, no doubt. But with an engine for 3.5" gauge, I would consider that pretty poor Quality. :o

I have a study of mechanical engineering at a technical university and one teacher gave us a Guideline about permissible angle errors for things like axle bearings etc.

For angle tolerances we were told:

1:1000 very rough machines, e.g. farm machinery
1:5000 most machines
1:15000 and better precision machine tools e.g. jig Borers

1:1000 means for 1000mm shaft length you may have a Position error for the bearings of 1mm.
Using these guidelines 0.25mm (0.01") with 10.25" gauge is 1:1000, still useable,but low Quality
0.25mm with 3.5" gauge is 1:360 and I would call it really poor quality.

Just my 2 Cents
Mike
The scale size of a mechanism has nothing to do with tolerances of the workings of the assembly nor the way the rules of mechanical fit for machine parts is basically applied. A 'slip fit' will hold the same standard tolerances in super small scale assemblies as a 'slip fit' in full sized mechanical mechanisms and assemblies because the relationships between machined parts remains the same for a 'slip fit' no matter who is doing it or what the mechanism is intended to perform. You can build and assemble junk in any scale but to provide a set of 'guidelines' for 'precision' assemblies depending on the 'scale' of a mechanism does not fit for a successfully assembled and working device. Your 'teacher' is working from 19th century texts that no longer apply in current machine shops whether they are making 'farm machinery' or 'jig bores'. (A 'jig borer' built in 1/12th scale will hold the same tolerances that a full or double sized sized 'jig bore' will and can hold due to standard physical properties.)

Bastelmike
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Re: Two Silver Soldering Questions

Post by Bastelmike » Mon May 21, 2018 4:34 am

That sounds like nonsense to me and is the opposite what mechanical Standards say!

For example, in ISO tolerances for shafts class h6 the tolerances are:

Ø3mm +0 -6 so 2.994 to 3.000 mm is okay
Ø20mm +0 -13 so 19.987 to 20.000 mm is okay
Ø100mm +0 -22 so 99.978 to 100.000 is okay

It's obvious that tolerances for the same fit are increasing with size of the part. And thats essential for manufacture !

Believe me or not, the Standard allowable tolerance for a micrometer for 250mm is significantly larger than for 25mm. So how will You measure those small tolerances specified for small shafts reliably on a very big shaft?

And there is thermal expansion. Thermal expansion for one degree more of temperature is very small, for a 20" shaft it is significantly larger. Consequence would be You would have to control the temperature of the large part very exact to measure it with the same error than the small shaft.

Mike

BClemens
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Re: Two Silver Soldering Questions

Post by BClemens » Mon May 21, 2018 6:39 am

Let me rephrase it so you can understand a bit better:

ISO tolerances and classes of fit can not be scaled. IT grades are as set out.... and you cannot scale an angle - that should be common sense.

BC

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gwrdriver
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Re: Two Silver Soldering Questions

Post by gwrdriver » Mon May 21, 2018 7:48 am

Bastelmike wrote:
Sun May 20, 2018 7:05 am
that may be correct, but I think it depends on the scale of the build.
This is, or should be, obvious, and I explain it to others in this way . . . as you go down (smaller) in scale, and therefore size of components, 0.001" (or 0.05mm) becomes a significantly larger percentage of a fit. As one goes up (larger) in scale 0.001"/0.05mm becomes a smaller percentage of a fit.

If you use a "fit" which is appropriate for a large scale model on a small scale model, you'll have "slop." If you use a "fit" which is appropriate for a small scale model on a large scale model you'll probably have binding. I learned this from a mentor very early on.

He had risen from apprentice machinist to QC manager for one of the largest T&D works in the South and he built his first locomotive (7.5"ga 0-6-0) to the standards he was accustomed to at the plant. It would not move - he had to dismantle the locomotive and relieve ALL the running fits to make it work, then it ran like . . a watch.
GWRdriver
Nashville TN

John Hasler
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Re: Two Silver Soldering Questions

Post by John Hasler » Mon May 21, 2018 10:29 am

I think we also need to be clear about the difference between tolerances and clearances.

Harold_V
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Re: Two Silver Soldering Questions

Post by Harold_V » Mon May 21, 2018 3:57 pm

Correct. It's not a tolerance issue, it's an engineering issue. Tight tolerance, in and of itself, won't be cause for an assembly to not function. In fact, it is instrumental in achieving consistency. If a fit is improperly designed, widening the tolerance exacerbates the problem, while tightening the tolerance improves the chance the fit will be correct.

It has been my experience that many lesser experienced individuals don't understand the true meaning of tolerance, often placing blame for failed efforts improperly. Look at it this way---an engine can be built with adequate clearance for proper operation, and still have stringent tolerances on all features. Loosen the tolerances and you risk building an assembly that no longer functions.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

John Hasler
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Re: Two Silver Soldering Questions

Post by John Hasler » Mon May 21, 2018 6:23 pm

Harold writes:
Tight tolerance, in and of itself, won't be cause for an assembly to not function.

But it may be cause for it to not be manufactured.

Harold_V
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Re: Two Silver Soldering Questions

Post by Harold_V » Tue May 22, 2018 1:14 am

John Hasler wrote:
Mon May 21, 2018 6:23 pm
Harold writes:
Tight tolerance, in and of itself, won't be cause for an assembly to not function.


But it may be cause for it to not be manufactured.
And that, too, is an engineering issue. Over engineering may not create a better widget.
The point one should take away from this discussion is that close tolerance work isn't the reason why an engine may not run properly. It's absurd to suggest that a machinist does work too well for it to function. Don't blame tolerance---blame clearance (engineering), which has nothing to do with tolerance. Properly dimensioned, a shaft on top tolerance will still fit in a bore at bottom tolerance, and perform properly. Tolerance, in that case, may be tight, of necessity.

Now, for ever changing dimensions (axles that move vertically), there must be clearance provided so they are able. That has nothing to do with tolerance. It has everything to do with dimensioning (engineering). If one opens up a bore, tolerance was not changed--dimensioning is. Once an acceptable dimension is determined, then an acceptable tolerance must be applied. If it is too wide in scope, you risk binding, or rattling fits. Tight tolerance isn't the enemy---it rarely is.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

matthew-s
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Re: Two Silver Soldering Questions

Post by matthew-s » Sun Aug 05, 2018 9:24 am

matthew-s wrote:
Mon May 14, 2018 10:39 am
2) I made a milling mistake on one of my lower bolster. I offset the slot that locates the columns by ~0.010". Is it reasonable to use silver solder to build the brass back up on the short-side and re-mill the slot, or should the solder be used for affixing parts to one-another? It would be nice to save this part and the machining that has gone into it.
Out of courtesy, and maybe for someone else to learn something, I'm going to close this story out:

I tried to braze the material back on per a suggestion above. That was a total failure. For kicks I thought I'd see if the silver solder would do the job. That did not work either. Maybe there was not enough heat from the MAPP torch, or a problem with the flux, or something more basic.

No matter, I decided to cut my losses and made a new lower bolster. All's well that ends well. I am going to post a different question about the upper bolster.

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Greg_Lewis
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Re: Two Silver Soldering Questions

Post by Greg_Lewis » Sun Aug 05, 2018 9:40 am

matthew-s wrote:
Sun Aug 05, 2018 9:24 am

Out of courtesy, and maybe for someone else to learn something, I'm going to close this story out:

I tried to braze the material back on per a suggestion above. That was a total failure. For kicks I thought I'd see if the silver solder would do the job. That did not work either. Maybe there was not enough heat from the MAPP torch, or a problem with the flux, or something more basic.

No matter, I decided to cut my losses and made a new lower bolster. All's well that ends well. I am going to post a different question about the upper bolster.
Ah well. All of us have made similar mistakes (note my sig line). But I've been on a success streak lately — it's been three days since I screwed something up. (Haven't been in the shop, either.)
Greg Lewis, Prop.
Eyeball Engineering — Home of non-interchangeable parts.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

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