Precision?

[Gordon]

I think that the main take away on this discussion is that there is a place for precision and a place where about that size is just fine. This is particularly true with those of us who are doing manual machining. We may be able to hold close tolerances but it is not required in many instances. Folks using CNC etc. are able to hold close tolerances without adding any time so it becomes less important.

 

[Lloyd-ss]

I think that the main take away on this discussion is that there is a place for precision and a place where about that size is just fine. This is particularly true with those of us who are doing manual machining. We may be able to hold close tolerances but it is not required in many instances. Folks using CNC etc. are able to hold close tolerances without adding any time so it becomes less important.

Gordon, yes, agreed.
Making one set of mating parts is one thing. But then if you want to job these out to a cnc house and get several sets with all interchangeable parts, then the chore is going thru all the drawings and adding a tolerance..... a well thought out tolerance...... to every dimension so that they can be inspected. Properly adding all those tolerances can be a major time-consuming PITA. I know, ha ha.

 

[Zeb]

Precision should always be considered as how much is necessary, and why. Functionality is the ultimate judge! I have watched mold making in this country migrate offshore since the 80's due to cost benefit. Medical mold making remains relatively strong due to liability issues. As one becomes familiar with the significance of the effects of geometrical precision ... squareness, concentricity, parallelism, perpendicularity, etc., combined with good tolerance application based on loads, forces, deflection, thermal, etc. we become more successful in our machining outcome. We don't need a degree in some type of engineering, but a well-rounded understanding of how the device behaves and reacts in its working environment. That's part of the fun of model engineering, isn't it? Sorting out what works sometimes comes from empirical testing, which is fun too! As we learn, we become more proficient in what's going on, firsthand, on a focused area, model engineering. We become the engineer. That's why many of us likely enjoy our hobby.

You nailed it there. I love starting with a dimensionless casting and making it come together a step at a time.

Any ASME14.5 nerds here? I got a copy for Christmas from my inlaws years back. They were a bit confused by my choice. I work through the first 120 pages every few years or so. Then I do remedial therapy to undo the temptation to use it.

No one would dare send an ASME 14.5 compliant drawing to a machine shop, but it sure looks nice to reference the standard on a title block.

 

[minh-thanh]

Precision
With real machines, it depends on the intended use....bla bla....... and spare parts....
As some information in the table below

For me, when I make the parts for the home made engine, of course I will follow the design dimensions.
I make cylinder , design is 20mm but if cylinder is good enough at 19.7 or 19.8mm it doesn't matter....
I think , all dimensions , tolerances ,,,..on that design is the perfect destination . but if I get to that point that's fine, but if I can only get close to that point, that's okay too

 

[Gordon]

There is a reason why the early automobile engines used selective assembly parts. Pistons were marked as oversize and undersize and the crank journal could vary on every throw.

 

[Lloyd-ss]

edit ........

Any ASME14.5 nerds here? I got a copy for Christmas from my inlaws years back. They were a bit confused by my choice .......

Is that the same as ANSI Y14.5? There are some real sickos around here. Did they tell their daughter to "run"? LOL.

 

[Asm109]

Once upon a time the standard was sponsored by ANSI. Since the 1996 revision it has been ASME Y14.5-XXXX.
I took a class from one of the guys that was on the working committee. I asked why the change. He laughed and said that ASME paid for the donuts at all the working meetings.

 

[Zeb]

Is that the same as ANSI Y14.5? There are some real sickos around here. Did they tell their daughter to "run"? LOL.

My inlaws approved when discovering my shop was quite the mess and there was some hopeful balance in life.

AZMEEfourteenfive is a common phrase in the vernacular of some circles. Leaving out the Y'all certainly is part of my remedial redneck therapy. The older ANSI does have a nice ring of anxiety to it.

Grandpa always taught me to leave a tuft of grass uncut. There's always a line that could be left unstraightened, a single character in the wrong font...anything to torment balance the OCD around those we hold dear.

 

[toadboy65]

My mentor was a precision machinist before he retired, a career that included making objects flown in the Apollo program. I am not sure he is at all comfortable with anything less than .001" or so. If he is asked for a backing plate of approximately 2"x3", it is probably going to end up well within a thousandth on every dimension.
He taught me that working on things where precision does not matter should be practice for when it does.
When you are trying to make a bunch of parts in a limited time at as low a cost as possible, acceptable tolerances matter quite a bit.
I would think that hobbyists would be more likely to take advantage of as much precision as their tools and measuring devices would allow.

 

[Zeb]

In terms of leveraging tolerances for the important and the not, a lot of us are going to find enjoyment in different ways. I'm jealous of the retired here, but I could see the instant loss of high work intensity a challenge to grapple with. Engine building for me is a break from the perfection required during the day.

An interesting tolerancing exercise is the statistical method where you have a stack of multiple parts. The formula biases wider tolerances for the more difficult parts to fit and machine. All parts machined at their extremes should also meet the fit and function goals of the final assembly. This could then be applied to a multi-cylinder engine valve train project like a RR Merlin, where you as a model builder would have to setup an assembly line with multiple copies of parts. If you were creating drawings, it might allow the builder at least the greatest chance of success.

 

[TonyM]

This has been a bit of a sore point over the last half of my career.
I started as a machinist, then after 20 years in manufacturing progressed to design and will be retiring next year as a chief engineer.
Before I retire, I am on a mission to try to build communications between the engineers and the manufacturing and assembly divisions of the company.
I am forever seeing designs which will cost a fortune and take forever to make and could have been replaced with a much simpler part or parts.
We have one or two engineers who have home workshops and are beginning to gain some appreciation, but what we really need is a reversal of the "us and them" culture which obstructs constructive interaction between designers and machinists and assembly technicians.

Pete.

I also had a similar job progression as you did and I can empathise. When I was on the tools back in the day. The company I worked for had a slow laborious process of hand wrapping a thin film of uncured rubber around a short brass tube. The film was cut from a roll and separated from a backing layer before being applied. I mocked up a machine from odds and sods that mimicked the hand process. It showed it could be done automatically, many times faster than by hand with no risk of contamination of the rubber film. The mock up was handed over to a young design engineer straight from uni, who did not want to discuss my original machine because he was 'going in a different direction'. He came up with the most complex piece of kit you could imagine which cost many thousand GBP. It never got through commissioning into production. The mock up continued to be used for some time until it was simply copied with more robust parts.

 

[Steamchick]

As a Design Engineer "on the board" (drawing board) and calculator for 6 years of my career, tolerancing to suit the available manufacturing processes was integral with the dimensioning of shapes..... like "Love and marriage", you "can't have one without the other"!
The real trick was designings something that didn't need really tight tolerances and still achieve the performance, cost, durability , servicability and assembly ability, etc. Etc. To the point where sometimes the Production En gingers a queried the "wide" tolerances I was stipulating. Usually on something that could be machined on one of the worn manual lathes and not needed in the new CNC machine centres that were being introduced. (With Closed-loop machining and measuring).
K2

 

[Ken I]

While studying "Cost Of Quality" for a thesis, I came across a US Army study - conducted under Demming's statistical process control and "Cost Of Quality" techniques during WWII.
It studied the manufacture of holes by drilling, reaming, boring, grinding & honing and came to the conclusion that there was no such thing as an "acceptable scrap rate", scrap only raised costs.
However the caveat was that the tolerancing had to be appropriate to the method employed - or more correctly vice versa.
The tolerance determined the method - the lesson being don't tolerance tighter than required and if it was required you had better use an appropriate method.
No use trying to hit reamed tolerance with a drill and hope for the best or worse live with a high scrap rate. Alternately adjust the tolerance to within the ±3+ sigma process capability for the process you wanted to use (assuming you could).

Apropos appropriate tolerances IIRC the Russians simply machined all their WWII diesel engines - no grinding or honing - they didn't see the point given the short life expectancy - apparently their tanks - if they made it to first service had something like 1½ kilograms of metal in the filters.

Regards, Ken I

 

[Steamchick]

Hi Ken I. I once had a discussion with the Marconi company - makers of torpedos - about engine manufacture. The Engineer explained that although the engines on had a 2 minute running time before the warhead was expected to go "bang", the engines had run over 100 hours of proof testing before and after instalation in the torpedo, to be sure they would start and run properly. A bit like a parachute. If only used once it must work!
K2

 

[SmithDoor]

This in not really a complaint or a criticism of others but I wonder about the "precision" implied on many of the plans posted here and other places. To me when you list a dimension as 27/64 it implies a level of precision required. I have frequently looked at dimensions like that and realized that the value could have been 3/8 or 1/2 without affecting the design. In the past I think that the original builder built the item without drawings and just built with nothing but sketches and gut instinct. Then after the fact they had someone else make finished drawings based on what was built and the person making the drawings was just blindly measuring and recording items without much attention to function or the amount of precision required. I think that today a lot of this happens because of the way cad drawings are made. Things are drawn out in space some place and then positioned after the fact. The cad operator just accepts the value without trying to make a value in round numbers. In my designs I have always tried to make things in round number size. If something will works at 1 1/2 long I see no reason to make the item 1.532. To each their own.

Typically you see that a 1/16" or 1/32" over bolt size.
Some new engineers today will today do that 27/64" or converting in a cad drawing.
It should be fix by the engineer/draftsman.

Dave

 

[Charles Lamont]

27/64 or 13/32 or 7/16 or 1/2? The counter argument is that they are all just numbers and you have no business inferring anything.

With non-toleranced drawings, I decide what tolerance to (try to) work to mainly from the function, not the size of the fraction's denominator.

But, then again, I slightly agree.

 

[GreenTwin]

I also had a similar job progression as you did and I can empathise. When I was on the tools back in the day. The company I worked for had a slow laborious process of hand wrapping a thin film of uncured rubber around a short brass tube. The film was cut from a roll and separated from a backing layer before being applied. I mocked up a machine from odds and sods that mimicked the hand process. It showed it could be done automatically, many times faster than by hand with no risk of contamination of the rubber film. The mock up was handed over to a young design engineer straight from uni, who did not want to discuss my original machine because he was 'going in a different direction'. He came up with the most complex piece of kit you could imagine which cost many thousand GBP. It never got through commissioning into production. The mock up continued to be used for some time until it was simply copied with more robust parts.

I worked with the most brilliant engineer I had ever met many years ago, and he was a card-carrying MENSA guy (no joke).

I was never considered very bright in school, and never made more than a passing grade.

The MENSA guy's design philosphy was basically make everything as complex as possible, in order to showcase his genius (and he was a genius).

The problem was that the maintenance folks who had to work on his designs basically just scrapped out the electrical enclosures, tossed all of his design in the dumpster, and installed something else that used 1/4 of the parts, worked better, and never needed maintenance.

So I generally think about what sort of design I would do, and then I think about the folks who would have to maintain it, and then I make a much more simple, robust, and maintainable design.

Its a good lesson to learn.
Overdesign can be as bad or much worse than not enough design.

The keys for me for design are flexibility, modularity, expandability, generic and readily available replacement parts from several manufacturers, simple, cost effective, reliable over the long term, and easy to access and maintain.

There are many important facets to a design, and your design is not done until you have considered all of them.

The saying is "Less is More".

.

 

[Charles Lamont]

Hmm, a genius - OK, but a brilliant engineer - evidently not.

I have probably misquoted Antoine de Saint-Exupery here before, but basically he said that you can consider a design is finished when there is nothing else you can remove.

 

[Brian Rupnow]

I have been designing machinery since 1965. First 32 years of that was done on a manual drafting board. We were taught from day #1 that precision costs money. 95% of things being built will go together quite easily with a plus or minus tolerance of 1/32".---Two digits to the right of the decimal point, thus 0.03". Machined parts are more critical and were generally toleranced to plus or minus 0.010", thus three digits to the right of the decimal point. And this was easy for us, because we had to print the dimension and the tolerance with a drafting pencil. Fast foreword to the computer age, and the dimensions are created by the computer, and the number of digits to the right of the decimal point is a "setting" on the computer. If you want a different number of digits to the right side of the decimal point, then you have to change a "setting" on the computer to make that happen. Time is money, so you end up just leaving the default setting of the computer to three digits to the right side of the decimal and assume that the person making the part is clever enough to establish what dimensions can be plus or minus a greater tolerance and which are critical enough to hold that three digit figure.---Brian

 

[GreenTwin]

Hmm, a genius - OK, but a brilliant engineer - evidently not.

I have found that there can be wide extremes between "highly educated" people, and people with common sense.

You need a little of both.

I knew one individual who was a literal walking encyclopedia.
He could tell you the types of moss growing in some obscure country 2,000 years ago, but he was totally unable to put that information into any sort of perspective, or comparision.

Its important to understand math.
It is important to be able to apply math.

But to me, the most important thing as a designer is to know how much math to use, and how to apply that math to create a practical product in an ecconomical way.

Its like I tell people; "don't use locomotives to crack walnuts".

One person asked me "how do I know when to use a locomotive, and when not to use a locomotive", and I said "that is why engineering is an art more than a hard science".

.