Precision?

[Gordon]

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.

 

[Zeb]

Most engineering graduates today lean heavier on theory and have less time in the workshop. The best are out there in their garage laying up carbon or bucking rivets.

Old hand drawings I've worked with have a lot of stacking errors or are not fully dimensioned. CAD tends to uncover stacking and mathematical errors.
CAD really hurts when everything is filleted with ball mills, thin walled, and made on a five axis machine when the same part could be made on a sheetmetal brake.

There are some very elegant modern CAD drawings out there, but we're not typically going to see proprietary data floating around. With CAD being so widely and freely available, we will likely only see the opposite.

I like how this image contrasts precision and accuracy.

 

[Lloyd-ss]

Times are changing and there is often a disconnect between the various disciplines. I haven't seen fractional dimensions used on new designs in over 30 years. And with decimal dimensions, often .xx =+/-.01 or .02 and .xxx=+/-.005. A simple approach, although not the best. Manufacturing tolerances are generally only understood by more experienced cad designers, or checkers, who have gotten lots of feedback. I worked as a manufacturing engineer at a full service facility that did everything from design to manufacturing and field service. Part of my job was working with the engineering and designers to ultimately get them to understand what a manufacturable part was. But with CNC programs taken right from the cad models, sometimes it makes no diff if a contoured surface is 1.50 or 1.517. Things like bushings and bores and fits matter, and that must be learned. It is nice when drawing tolerances reflect what is really acceptable, and not just an arbitrary tolerance taken from a table.
Bottom line, somewhere, the conversion of a cadd design to an actual manufacturable part has to take place. This process can be painful depending on how manufacturable the original cad model was.

 

[Peter Twissell]

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.

 

[Ken I]

Having spent most of my professional life designing one-off machines I have seldom bothered much with tolerancing except where necessary.

This has carried over into my model engine designing and building. The following caveat appears in my drawings and build notes :-

All dimensions are un-toleranced, make allowance for limits & fits, part numbers signify the manufacturing sequence that most facilitates subsequent adjustments.
Other than coordinate dimensions, No Decimal= Open, 1 Decimal = check fit / size with mating part, 2 Decimals = mating bearing parts – Generally ream holes and make shafts to suit.

Since that's how one off's and models are generally built I see little point in tolerancing anything that - if made over or under size - will not be scrapped when the option is to "adjust" the mating part to suit.

In my professional career this has of course bitten me in the butt when a customer orders a spare part - so now I add copious "as built" notes to the drawing.

Regards, Ken I

 

[Gordon]

In my working life I never used tolerances. The things that I built for the most part did not require precision. I designed and built custom material handling equipment. The overall size could be off by 1/8" and not be a problem. Obviously things like the bore of a sprocket had to be reasonably close. Therefore when I see some dimension in 1/64 or some odd decimal it immediately implies something which requires some extra care. Most of the folks in my line measured stuff in 1/4 or 1/8 or tiny marks or teeney marks on their ruler. Because I was the designer as well as the builder I was always thinking about how I was going to build the thing I was designing. Once I semi retired and started building small engines etc I was forced to pay attention to precision.

 

[Zeb]

The tightest I had to tolerance was a +/-.0002 taper, but it was absolutely necessary for device calibration. Lowest was a fractional drill bit size, because it's fun to torture QC and the application didn't require any more precision.

Something I really enjoyed about building my steam engines was tolerancing for break-in. Feeling the results after each assembly was an eye-opening experience. I'll often change thread pitches to match my lathe gearing. I also undersize my cylinder bores slightly, so I have enough meat on my piston raw stock if I mess something up. All the little things which might bring me to tears.

Setting up a smooth break-in, optimizing machining setups, and brainlessly making chips at low feed rates are a few of my favorite things. I wish I had these experiences and the fine folks in this forum when first starting out. There are many unknowns for a brand new mechanic waiting for an engine's break-in period to go through. You have all the pieces at the right size, and you follow procedures, but nothing beats having the old engine whisperers nearby for the fine important details.

 

[Lloyd-ss]

Edited for brevity .............
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.

Pete, that is a noble (and daunting) task that must come from the top down (from you) and I wish you luck. as a manufacturing engineer for 30 years I was intimately involved in the team approach. We manufactured precision marine navigation equipment for the US Navy and our allies around the world. We built everything in-house: design, software, machining, electronics, wire harnesses, mechanical and electrical assemblies. Some of the parts were indeed awful to make, but by the time a drawing was signed off and released, there was already a manufacturing plan in place.
A team was set up at the beginning of the project that included at the very least, the lead design engineer (the brain trust), the designer (cadd person), and the lead manufacturing engineer. The manufacturing engineer needed to have the backing of project management (and all management) so that tweaks to the design could be made early in the process before too much time had been invested and push-back became more difficult. But with the right team members, sometimes it worked like a dream and those projects were often used as an example in, "remember how well it worked on the XYZ project?"
Many of the parts were difficult to make, but if the manufacturing engineer knew why they had to be a certain way, a few tweaks might make all the difference: "We need some datum pads here and some fixturing holes here, and can you find a better spot for this feature. We can't get to it to machine it."

No designs were"thrown over the wall."
All dimensions on every drawing had a tolerance, even if it was difficult.
All the difficult tolerances and features had a good "reason."
Because we had the luxury of in-house machining, designs could be made that were compatible with our machines.
Engineers are often intimidated by the machine shop because they have no idea what they are looking at. This often manifests itself as what comes across as arrogance on both sides. The engineers can't be afraid to get out on the factory floor and learn how things are made. I remember there was one ornery production planner in the machine shop, and once he retired, the young engineers were no longer afraid to come down and ask questions.

We had one CEO at this 800 person facility who was a retired admiral. A super nice genius who exuded benevolent authority. Often, when he needed a break from his stressful CEO duties, he would take a walk around the factory. He knew the projects and would ask the folks on the floor how it was going and how they were doing. He knew many people by name. I realize this is a dream scenario, but it made such a difference in getting folks to talk to each other. It also made it clear to some of the supervisors that they needed to "back off" and allow interdepartmental "visits," and not be so anxious to run people off. (We have also had the opposite type where my wife told me not to talk about work at home anymore.)

Lloyd

 

[peterl95124]

when I see an odd dimension like 27/64" I immediately think this was a conversion from metric, and not an indication of need for precision.

if you are manufacturing lots of parts that need to be interchangeable then precision becomes critical,
but us engine modelers don't do that,

there really aren't any critical dimensions in a steam engine, and in an IC engine the critical parts
are the valves and their seats and the piston rings and their cylinders,

there are various rules of thumb for designing an IC engine, like
valve stem should be about 1/4 of the valve head, and valve lift should be about 1/4 of the valve head,
piston-top to cylinder clearance should be about .006" per inch of bore, and skirt about .002" per inch,
etc, etc, etc,
one item that's missing from Liston's "Aircraft Engine Design" is journal to bearing clearance,
I try to always have .001 to .002" on my model engine crankshaft and camshaft bearings.

but the actual diameter of a cylinder isn't important, the fit of the piston, and crucially
the fit of the rings, are what matter, and for a model engine these are made to fit,
regardless of what the plans say or what the size ended up being,
ditto with bearings and journals its the clearance that matters not the actual size.


remember, plans are merely suggestions !!!

 

[Zeb]

Regarding hole sizes in 64ths, I often drilled 1/64 under to get an unreamed size for "close enough". If someone is without a drill index with 1/64th increments and uses a standard, hand-sharpened bit, the hole is likely to be 1/64 over. I could see the designer shooting for numbers that accommodate a simpler set of tools. That's a bit of a stretch though.

Still dreaming of those fiscally unobtainable over-and-under reamer sets.

 

[peterl95124]

Regarding hole sizes in 64ths, I often drilled 1/64 under to get an unreamed size for "close enough". If someone is without a drill index with 1/64th increments and uses a standard, hand-sharpened bit, the hole is likely to be 1/64 over. I could see the designer shooting for numbers that accommodate a simpler set of tools. That's a bit of a stretch though.

Still dreaming of those fiscally unobtainable over-and-under reamer sets.

Zeb, I'm dreaming too !!!

in the mean time, whenever you have to use a drill and reamer make sure you make that part first,
then you can adjust the size of the part that fits in it while its on the lathe until it fits,

similarly with taps and dies, taps aren't adjustable, but dies are, so tap first, then make the part
requiring the die second and use the tapped part as your go-nogo check until the die is correctly
adjusted.

HTH, YMMV, some assembly required - batteries not included,

 

[Ozwes007]

Precision is relative. Simple but true statement. I’ve worked in many fields, from level 1 Machinist, CNC machinist, project management, maintenance management, leading hand, etc in machining , welding, fabrication, sheet metal. The one leading cause of failures, bar nothing, has always been engineers and draughtpersons with no knowledge of how parts are made, and/or what material specs. to use. While working as a project and/or Maintenance manager I always had my engineers and draftees spend at least 1 month a year on the floor working with the tradies. the need to know when relative and absolute measurements on drawings was required is paramount to cost effective projects. Precision of parts is relative to the job at hand. High precision takes time and so it costs more. You need high precision on parts that will be replaced due to wear and tear, or accidents. If you fit a bearing, you need high precision for it to do it’s job. Tolerance of parts is part of being a n engineer and a tradesman.

 

[Klrskies]

I'm 64 years old and coming to the end of my career. I started as a machinist out of trade school in 1977, went into cnc EDM, sinker and wire, then extrusion die design, then mold making and mold design. I would say the bigger the company gets, the bigger the disconnect between engineering and manufacuring toolmaking. The small mold making shops of 40 years ago produced some amazing tooling with few people ... but they had much depth in the fit and function of the tool as a whole, so it worked without so much critical engineering input.
As speed requirements forced specialization of tooling skills into distinct segments, much of the understanding of fit and function was indeed shifted to engineering and along came tight tolerancing and geometric tolerancing in an attempt to provide guidance in manufacturing the pieces required to rapidly assemble tooling.
Something was lost along the way. The understanding and involvement in the construction of tooling shifted from craftsman to technician. Designers and toolmakers have always been somewhat separated by egos. Don't know if that will ever change for the better as each seeks and enjoys control in a field that rewards precision and broad perspective. I spend these days doing final fit and assembly on medical molds. I work in sub .0001 tolerance in a poorly climate controlled grinding room yet understand the steel and machines behavior well enough to hold tolerance. The owner often reminds engineering that my job requires more skill that assigning a sub .0001 tolerance. But my generation is coming to an end and now more complexity will be required. But it will be expensive, require sophisticated machines ... and a new skill set. but things change, and always will.

 

[Gordon]

when I see an odd dimension like 27/64" I immediately think this was a conversion from metric, and not an indication of need for precision.

if you are manufacturing lots of parts that need to be interchangeable then precision becomes critical,
but us engine modelers don't do that,

there really aren't any critical dimensions in a steam engine, and in an IC engine the critical parts
are the valves and their seats and the piston rings and their cylinders,

there are various rules of thumb for designing an IC engine, like
valve stem should be about 1/4 of the valve head, and valve lift should be about 1/4 of the valve head,
piston-top to cylinder clearance should be about .006" per inch of bore, and skirt about .002" per inch,
etc, etc, etc,
one item that's missing from Liston's "Aircraft Engine Design" is journal to bearing clearance,
I try to always have .001 to .002" on my model engine crankshaft and camshaft bearings.

but the actual diameter of a cylinder isn't important, the fit of the piston, and crucially
the fit of the rings, are what matter, and for a model engine these are made to fit,
regardless of what the plans say or what the size ended up being,
ditto with bearings and journals its the clearance that matters not the actual size.


remember, plans are merely suggestions !!!

Where have you found the "rules of Thumb"? I have never heard of some of those.

 

[clockworkcheval]

Working on my master of science project in the early 1970's I visited Schaublin, the Swiss manufacturer of precision machine tools. Frédérique Schaublin told me that any fresh design engineer or manufacturing engineer had to spend at least two years in the service department to get a full understanding of what can go wrong in design and what apparently good design still leads to user's abuse. Since that visit I very much respect the design and manufacturing practices of traditional high class machine factories, which indeed should be based on team communication. And not only on next quarter results

 

[vederstein]

I wonder about the "precision" implied on many of the plans posted here and other places.

Simple response to your comment: If you have issues with existing plans, don't use them and design your own.

I put up my designs here on HMEM for free. I dimension them .xxx. That's not because the part really needs it, but it gives me the dimensions I should shoot for. Any critical fit dimensions usually are either tagged with "REAM" or say something about its fit to its mating part.

If this isn't good enough then don't build it.

...Ved.

 

[Zeb]

I'm 64 years old and coming to the end of my career. I started as a machinist out of trade school in 1977, went into cnc EDM, sinker and wire, then extrusion die design, then mold making and mold design. I would say the bigger the company gets, the bigger the disconnect between engineering and manufacuring toolmaking. The small mold making shops of 40 years ago produced some amazing tooling with few people ... but they had much depth in the fit and function of the tool as a whole, so it worked without so much critical engineering input.....

I was a moldmaker's apprentice in medical for a few years. A few years is still toddler level. That was amazing training and at a significant cost to my boss. Hats off to you for keeping things alive locally.
I've had the privilege to work under extremely talented engineers. Talent which is at an intimidating level, but with a foundation of humility. It rubs off on the new guys, and all of this is fostered under very competent management. The temptation for me is often "I could have designed this better" until the ball is in my court.

 

[Lloyd-ss]

Simple response to your comment: If you have issues with existing plans, don't use them and design your own.

I put up my designs here on HMEM for free. I dimension them .xxx. That's not because the part really needs it, but it gives me the dimensions I should shoot for. Any critical fit dimensions usually are either tagged with "REAM" or say something about its fit to its mating part.

If this isn't good enough then don't build it.

...Ved.

Ha ha. I like that. Similarly, patents only need to be written in a language and with just enough information so that someone "trained in the art" of such and such would be able to (eventually) duplicate the result. "Trained in the art" is also sometimes a per-requisite in this hobby.

 

[Klrskies]

I was a moldmaker's apprentice in medical for a few years. A few years is still toddler level. That was amazing training and at a significant cost to my boss. Hats off to you for keeping things alive locally.
I've had the privilege to work under extremely talented engineers. Talent which is at an intimidating level, but with a foundation of humility. It rubs off on the new guys, and all of this is fostered under very competent management. The temptation for me is often "I could have designed this better" until the ball is in my court.

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.

 

[Gordon]

Simple response to your comment: If you have issues with existing plans, don't use them and design your own.

I put up my designs here on HMEM for free. I dimension them .xxx. That's not because the part really needs it, but it gives me the dimensions I should shoot for. Any critical fit dimensions usually are either tagged with "REAM" or say something about its fit to its mating part.

If this isn't good enough then don't build it.

...Ved.

See the first sentence of my original post. I do not have an issue. I was expressing a thought.

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.