# machining accuracy



## Loose nut (Sep 4, 2008)

What do you consider to be an acceptable level of accuracy for model making? I have always strived for .001" as a standard where it counts, some times I make and ......

In the golden olden days of Model Engineering when the main measuring tool was a ruler marked out to 128 Th's and parts where made to fit each other, 5 or 6 thou was considered good but we now have much better tools today. 

The pros. keep talking about finishing parts to a couple of tenths of a thou. which is beyond my capability so what is a good and reasonable level of accuracy to try and achieve.


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## rake60 (Sep 4, 2008)

Loose nut

Invite any one of the pros to your shop to show you how they consistently hit that thenth
of a thou. size. I doubt you'll find many who have the time to come and show you. 

If it's straight and on size within .001 for me, it's Dead Nuts on!

If it actually needs to be closer than that I'll intentionally cut it +.001 and carefully hand
polish it to the perfect size.

Rick


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## Stan (Sep 4, 2008)

Loose Nut: Along with machine capabilities, there are a lot of other factors.
How fussy you are and how much pride you take in your work enter into it.
Do you have to have parts made to .001" if they work just as well made to .010"?
Will your friends be impressed if you tell them every part in this engine is made to .001" tolerance?
My friends just look at a running engine and say "That's nice, what does it do?"
My practice is that when fitting a bearing race, .001" is no where near close enough but when making a 3" diameter flywheel .250" is close enough if that is the material I have.
Everyone makes his own choice. Mine is the journey, not the destination.


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## chuck foster (Sep 4, 2008)

i have very good friend that started building a small up right steam engine about 15 years ago and he is about half done.
i asked him why it is taking so long and he said that every thing is machined to .0001" tolerance 

i said why would you work to that tight of tolerance and he said that that is the tolerance he works to at work.

i then proceeded to explain to him that you have to look at the parts you are making and determine what the part does and does it have to be that precise. 

like stan said in the previous post that if the flywheel is plus or minus .250" who cares if it will do the job then it must be ok.

anyway 2 weeks latter my friend finished the engine and now he is working on another steam engine model of some sort and he also told me that he is now having fun making his models. i guess it is because now he is not treating it like his job.

chuck


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## Bogstandard (Sep 4, 2008)

Loose Nut,

On general blueprints, if no tolerances were shown you would make the part +/- 0.002". 

In little engines, you would aim for getting it as close as you can, and if the engine runs, stuff the tolerances. Lives are not dependant on little engines, trying too hard can spoil all the fun.

If the bore size on the drawing says 0.500", and you made it 0.0510", by the time you made the piston to fit the hole, it is almost guaranteed to run.

Unless the drawings call for super fine fits, just do your best on fits and finishes. 

John


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## Bernd (Sep 4, 2008)

Bogstandard  said:
			
		

> If the bore size on the drawing says 0.500", and you made it 0.0510", by the time you made the piston to fit the hole, it is almost guaranteed to run.
> 
> 
> John



I'd say he would have to go abother .449" to get there. ;D Or make a darn small piston of .050"

Sorry John, I just couldn't resist. :-*

Bernd


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## Bogstandard (Sep 4, 2008)

Sorry about that, it is the middle of the early morning here, half asleep. 
I will leave the error in, just so your post means something.

What a gentleman I am.

John


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## Mcgyver (Sep 4, 2008)

> In the golden olden days of Model Engineering when the main measuring tool was a ruler marked out to 128 Th's and parts where made to fit each other, 5 or 6 thou was considered good but we now have much better tools today.



they were making them perhaps without aid of a micrometer but the does not mean their standards of fit were any less than today. There are many comparative technique to create extremely accurate fits.  On the contrary, i think you could make an argument that the more gadgets the newbie is equipped with today the lower the standard of work....why? they never learned the basics, filing, scraping, general bench work, layout etc whereas is less equipped learned many of these skills because there was no other way to do things. I haven't done it, but don't think it would be too difficult to fit a piston to a bore with less than a thou clearance without a micrometer.

Note that in making a one-of, its fit that matters not hitting a nominal dimension. if the the goal is 1 thou clearance between piston and cylinder that's what counts not whether the final bore is 1.000" or 1.005". In fact even with all of our fancy equipment today, its still good practice to turn the piston to the bore. 



> Invite any one of the pros to your shop to show you how they consistently hit that tenth
> of a thou. size.



and



> If it's straight and on size within .001 for me, it's Dead Nuts on!



there's, imo, several stops along the accuracy track between these two statements. Within a thou isn't good enough in many situations, yet you better be running a hardinge or monarch in temp controlled environment and have some very nice measuring equipment to consistently claim to hit a tenth. My best digital mic measures in tenths - if tenths is its best resolution, how can someone claim to hit a tenth without a mic that measures to even a smaller increment ...and does so accurately. show me a guy in his garage or even the average job shop running around with mic that accurately measure to a fraction of tenth :

However when it really really matters, like getting the proper press fit on something or housing or shaft for a roller element bearing, you need to machine to better than a thou.... a couple of tenths is a reasonable target imo....when you have to (as per chucks point).... also we tend to work with smaller stuff so clearances and tolerances are smaller.

My German tool and die maker turned high school machine shop teacher would unleash a tirade for filing in the lathe and emery was for finish not fit. Granted he was trying to train us, but the point is valid, once you start what that stuff you've no guarantee the part is round or straight. I do use fine emery to take off the minute burrs and ridges if its a moving part, but I've never lost what that very skilled and disciplined man taught us.....and those abrasive techniques are useless for bores unless you want a broken finger

Chuck point is right on - far more germane to us that how accurate you _can _ machine to is understands when and what accuracy is required. There are reasons model engineer drawings don't usually have tolerances or even clearance specified. Its up to us, acting as engineers rather than by rote following a tolerance schedule to understand what is required and machine to that - i like the engineers expression: Good enough is good enough! (though good enough is often in the eye of the beholder when we're talking model engines)


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## Bogstandard (Sep 4, 2008)

> Note that in making a one-of, its fit that matters not hitting a nominal dimension. if the the goal is 1 thou clearance between piston and cylinder that's what counts not whether the final bore is 1.000" or 1.005". In fact even with all of our fancy equipment today, its still good practice to turn the piston to the bore.





> Its up to us, acting as engineers rather than by rote following a tolerance schedule to understand what is required and machine to that - i like the engineers expression: Good enough is good enough! (though good enough is often in the eye of the beholder when we're talking model engines)



YES!!!

When in industry, if it was required I had to have a tolerance finer than 0.001", it was then ground to dimension, much more controllable.

I have actually seen, when I visited The Dowty Rotol Company, flat faces being reworked by disabled people (they seemed to have a knack of patience and tolerance for this sort of work) for use in the aircraft industry, using precision glass blocks laid on the surface with a light shining thru it from the end. The light was distorted slightly by the uneveness of the surface. The offending area was given a very light rub on a piece of paper to minutely wear away the surface until the light showed parallel lines running thru the glass block. The glass block was the rough stuff, they ended up under DTI's that had a face diamenter of about 24". Their tolerances were 0.00001" or better. Slip gauges were just chunks of 'near enough' metal compared to what they were doing.

John


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## Loose nut (Sep 5, 2008)

Thanks for the input, I have been making things on a fit to fit basis but have wondered if I should have been more conscientious of tolerances, a matter of prospective I guess.

I know that model loco. builders make everything to a rattling good fit they, have to be a bit loose to run properly.


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## rake60 (Sep 5, 2008)

At work this morning I finished up an order for 20 pieces of a part.
The OD of the parts was 9" something with a .0005" tolerance and a 
finish requirement of 16RMS.

These are OEM parts and ANYTHING out of tolerance is immediately scrapped.
Scrap enough of them and YOU may be scrapped!
Add into the mix you need to meet the quoted time for the job.
Often those times are estimated by someone who has never...........
Oh I'll leave that dog sleep... LOL

They were all good this time around. 
At least two of them were brought to size and or finish with emery cloth.
I don't know how many of the pieces my buddy made needed to be finished
in that manner.

I don't bring that kind of stress home with me.
There's no need to.
When your building an engine or anything else for that matter, you know what has
to fit and in what manner.

If you miss a size the mating piece size can be altered to accept it.
If you miss that one as well, pick the easier of the two to make and try it again. 

Rick


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## Kludge (Sep 6, 2008)

My Taig and Unimat are capable of .0005" accuracy. (That word seems to have different meanings to different manufacturers.) My watchmaker's lathes are specified to at least .00005" and one's half that. Do I work to that level? Heck no! I may be crazy & I may be insane (and I have the paperwork to prove both!) but I am not masochistic. How close do I go? Whatever it takes and no more. I may push a bit to get as fine as possible but that's just to push myself, not for any practical reason. 

Bogs, that light refraction technique sounds cool. .00001" ain't too shabby either. 

Not anywhere near in the same class, I often use glass plate salvaged from an assortment of sources for the final polish on some things, starting with "scary sharp" for things that need to be sharp to a bright finish using diamantine and other fine abrasives. 

What's this have to do with machining accuracy? Aside from a high polish, this approach can be used to "adjust" that last teeny tiny bit for whatever purpose. For me, it's part of the self-challenge, hence it's play. 

And Bogs said ... 



> In little engines, you would aim for getting it as close as you can, and if the engine runs, stuff the tolerances. Lives are not dependant on little engines, trying too hard can spoil all the fun.



That's pretty much it ... keeping track of the dismal point - er, decimal point - in the process. ;D

Best regards,

Kludge


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## Bogstandard (Sep 6, 2008)

I would just like to amend the quote that Kludge picked up on.

Most of the time, the above rule can be applied, but there are times that tolerances have to be kept very tight. In those situations, it usually takes a bit of experience in fits and finishes, coupled with a bit of tedious elbow grease. Good examples are flame lickers and Stirlings.

Other than that make it so that it runs.

John


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## Loose nut (Sep 6, 2008)

What annoys me the most is having a part that is 2 or 3 thou. over size and taking another cut without changing the settings on the cross side, just a cleanup cut to take out the spring of the tool and bring it to size and it usually ends up 2 or 3 thou. under size.

I usually true up the part in question while it is still 15 or 20 thou over, mike it and start the finishing to size but that last little bit.............!!!!!!!!!!


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## Stan (Sep 6, 2008)

Kluge:



> My watchmaker's lathes are specified to at least .00005" and one's half that.



I have never worked with watchmaker lathes so I am curious as to how the accuracy is determined. Do the handwheels have less backlash than .00005" or will it turn a shaft sticking out of the collet a couple of inches to that tolerance? 

Many years ago, I bought a Boxford (British Southbend) that came with a final inspection sheet that listed the deviation from zero in all possible movements of the tools. Does the same type of sheet come with a watchmaker lathe?


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## BobWarfield (Sep 6, 2008)

Loose nut  said:
			
		

> What annoys me the most is having a part that is 2 or 3 thou. over size and taking another cut without changing the settings on the cross side, just a cleanup cut to take out the spring of the tool and bring it to size and it usually ends up 2 or 3 thou. under size.
> 
> I usually true up the part in question while it is still 15 or 20 thou over, mike it and start the finishing to size but that last little bit.............!!!!!!!!!!



Lose nut, sounds like you're close, but taking a little too much depth of cut too late in the process. Back up so you have a couple shots taking very light cuts to get there. You'll want to experiment with your lathe to see what DOC it can be rigid enough to be accurate at might be. For my lathe, I shoot to take heavy passes until I'm within 0.010", and then 2 very light passes after that. It's nearly always dead on at that point. You need one spring pass that you know isn't going to take you over so that you've got an accurate read on where you are.

Cheers,

BW


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## Circlip (Sep 6, 2008)

Assuming that there's no free play in your slides Loose nut, do you give the tool a quick lick with the stone before the final cut just to get the edge back ?
 Regards Ian.


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## pelallito (Sep 6, 2008)

Circlip,
Please expand on that. What are your procedures in sharpening and maintenence of the cutting tool?
Thanks.
Regards,
Fred


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## Kludge (Sep 6, 2008)

Stan  said:
			
		

> Do the handwheels have less backlash than .00005" or will it turn a shaft sticking out of the collet a couple of inches to that tolerance?



I've never seen a cross slide with that little backlash (but then, all the ones I've owned have been very used and slightly abused) but the spindle sideplay is limited to that and is fairly easy to adjust since the watchmakers used to do so fairly regularly to keep them in true. In theory, that should include the collets which, in theory, should be bored true. I've never seen anything to support that they are or that they aren't. I do know that they are capable of a lot more than I can imagine using. Take a look at a mechanical movement for a lady's watch sometime and remember that they used to be hand made and repaired using these machines.

Of course, with any lathe, the effects of the sideplay are multiplied the further you get from the headstock so, as a rule, everything that could be done at the headstock was done at the headstock, and with a graver rather than a cross slide-mounted tool as often as not. (Reflective thought: probably more often in the original intended use.) Note that the cross slide on a watchmaker's lathe has no lead screw to move it the length of the bed since they were never intended to be used like the cross slide on a larger lathe. 



> Many years ago, I bought a Boxford (British Southbend) that came with a final inspection sheet that listed the deviation from zero in all possible movements of the tools. Does the same type of sheet come with a watchmaker lathe?



All of my machines came out of estates and closed shops so there was no paperwork. I've never even contemplated having the idea of fantasizing about a new lathe due to the expense, although there is a Chinese one that has my eye even though it's on something closer to a Geneva bed rather than a more sturdy WW bed. (I'm actually more interested in some of the accessories since this machine probably can't do anything more than what my 6mm can handle due to the bed design.) In any case, I have no clue what comes with them out of the factory.

Best regards,

Kludge


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## Circlip (Sep 6, 2008)

Sorry Fred, I'm " Old School " HSS tooling, not carbides. When you've done your "Cleaving " cuts, before you do the finishing cut you give the tool tip a quick hone with the tool still in the toolholder with a fine sharpening stone or one of those fancy DMT diamond hones to get a keen edge back on the tool.
 PS. if you can get hold of some carbon steel tool bits, they keep a good edge for most of our applications.
 Regards Ian


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## mklotz (Sep 6, 2008)

Bob's got the right idea. I always try to make the last two cuts the same depth so that the cut after the final measurement is almost identical to the cut that immediately preceded it.

A typical closing-in-on-final-size sequence looks like:

roughing to size cuts
0.010 DOC (that leaves approximately 0.020 to remove from diameter)
0.000 DOC (spring cut to establish where the tool is really cutting)
0.005 DOC (first finish cut)
measure with micrometer
0.00x DOC (final finish cut where 1<=x<=5)

(Note that my crosslide is calibrated directly - a 0.005 cut removes 0.010 from the diameter.)

Depending on the rigidity of your equipment, you may want to adjust those numbers slightly.


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## vederstein (Jun 3, 2018)

My lathe is old (80 years) and sloppy so when I got to do Precision fits, where I can I use reamers.  Then I use cold rolled stock for pistons. With a little bit of lapping usually everything is good.

...Ved


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## KJE (Jun 4, 2018)

An interesting topic at any level. Being a machinist and machineshop supervisor for over fourty years this topic has come up many times. The answer I have always given is be fussy where it counts and use the tolerances given where it doesn't ....to machine everything to a tight tolerance is just a total waste of time. People who think they are working to a .0001" tolerance in any shop that doesn't have stringent quality control, which includes regular calibration of measuring tools and temperature control is just kidding them selves. However working to .0005" is perfectly obtainable on most home shop machine tools with some practice.  A good understanding of fits and allowances is critical for any kind of success with model engineering and can be obtained by most home shop Hobbiests with time and experience. Enjoy your hobby!


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## mayhugh1 (Jun 4, 2018)

Since this is a hobby and not how I earn a living, I can afford to try to machine everything to within a thousandth. It's part of the satisfaction. When I don't achieve it, I try to figure out why so I have a better chance next time. I don't scrap parts unless they really won't work or they have a cosmetic defect that I can't live with. But, I'll be the first to admit I have a pretty low threshold with cosmetic defects. After doing this for a while, I found common sense things like keeping work-holding and tool-holding surfaces clean, periodically correcting tram, and cleaning collets with solvent tends to generally improve precision and repeatability. On a difficult or particularly important setup I also make it a point to not do any cutting until after the setup has been completed and allowed to sit a while - maybe hours or even overnight -  so I can think about it. - Terry


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## el gringo (Jun 4, 2018)

. I don't scrap parts unless they really won't work or they have a cosmetic defect that I can't live with. But said:
			
		

> Allowing myself some time, especially over nite, to rethink a setup has always helped in many ways...all your comments are right on Terry.


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## bazmak (Jun 4, 2018)

I have broached this question on a previous thread .The original has answered itself. Generally 1 thou is good enough
Most dims are acceptable to say +- 5 thou but a few need to be say 1/2 thou. Important dims can be sized to suit each other
If you make a cylinder to say 1" and it finishes at +2 thou then you make the piston to suit. A baseplt for mounting which
is unimportant can be say +- 10 thou or more.The general answer to your question is that the dim needs to be as accurate
as it needs to be. The advise i remember from my apprentice days are burned in my brain. Close tolerances are expensive
and the more tighter they are the more expensive and difficult to achieve. Choose the peg that best fits the hole. An example
i was given (dont know if its true),that Rolls Royce made the car bumper bars to 5 thou etc etc  Why when its not required
The answer is because they could but it all came out in the price of the car


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## TonyM (Jun 5, 2018)

I agree with Bazmak. 

For one offs, which is mostly what the majority of us produce, fit and function is more important than individual tolerances.


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## Hopper (Jun 5, 2018)

A quick touch with some fine grade emery paper at the end of the job is the absolver of many sins.


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## bazmak (Jun 6, 2018)

If im looking for a fine fit then a fine finish is needed so i always use fine grade emery to finish


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## Charles Lamont (Jun 6, 2018)

Very often, geometrical accuracy is far more important than dimensional.

It does not matter within a few thou how long a steam engine cylinder is, but if a flange is used to hold the cylinder, it is very important that the face is square to the bore. 

It is also highly desirable that the bore is parallel, even if the size is a bit off.

With a twin cylinder marine engine with a one-piece cylinder block, it does not matter exactly how tall the columns are, but they need to be accurately the *same* length.

It is important to understand sources of geometrical error within our machines and in our set-ups, and to plan machining sequences to minimise such errors where it matters most.


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## alchr (Jun 6, 2018)

Don’t forget to ask those sudo perfectionists what temperature they do their .0001 work at!


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## Schnupfhuhn (Jun 6, 2018)

Take a slip of paper. Take a micrometer or a good calipher with 0,01mm resolution (Tesa, Swissmade,  but still no match for a micrometer). Start with a part. Put down your aimed at dimension. Do the work, measure. Put the measure on paper. You´ll get a feeling what precison you can achieve without any worries pretty fast.

On average sized parts, I aim for 0,1mm on general dimensions and something around 0,02mm when it gets hairy. This equals an h7 tolerance field in ISO tolerances. If you and your lathe can´t hold that, something is wrong in your chain of tools, measuring equipment or your process...


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## machinejack (Jun 6, 2018)

Not really answering the main question. When I am bidding and making questimations on cost the more 0.0000 on the right side of the decimal the more it cost, but that is for making money. Now as to the hobby side. I will make  close to the final dimension but if I miss by a bit I adapt the mating part to fit.  It all goes to ones skill level. Perfection is  almost unattainable.  40 years tool and die experience and I still screw up. We are here to have fun.


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## ICEpeter (Jun 6, 2018)

Schnupfhuhn,
Arent you typically posting on Peter's CNC Ecke in DE?

Peter J.


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## Schnupfhuhn (Jun 6, 2018)

Main question:  " What do you consider to be an acceptable level of accuracy for model making? I have always strived for .001" as a standard where it counts, some times I make and ......"

My answer: see what you can achieve. Usually h7 is a good tolerace field for a lathe. You can loosen up on general tolerances to 5 times (0,1mm) the value. 

We never talked about bidding or contract work.


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## bazmak (Jun 6, 2018)

I did a post on making a 1" dia test bar.First down to 1 thou and then down to 10 microns
its not difficult but time consuming.I did it for an excercise.but general rule of thumb is
1 thou is good enough for most things and half a thou when needed,for model engineers
If people have better lathes and more accurate measuring equipment then go for gold if its needed


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## Hopper (Jun 7, 2018)

When I worked in machine shops -- when Noah was still a junior midshipman --  generally speaking jobs with tolerances of .001" and upwards were done on lathes and mills. Jobs to be done to tolerances of less than .001" were done on cylindrical grinders and surface grinders. In the home shop, with no precision grinders available, that translates to use a bit of emery paper below .001".


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## Cogsy (Jun 7, 2018)

Personally I think you get used to your measuring equipment. 1 thou equates to just over 0.025mm and on close fitting parts I like to hit close the nearest 0.01mm. I mostly use metric measuring equipment so that hundredth is easily measurable. I know in steel I'm going to be there a while with emery paper to remove 2-1/2 hundredths so I'll try a spring pass of two and see what that does.
Another trick I picked up is to stop and measure just before I get to where I'm headed, take a spring cut or two, measure accurately, then run a couple of grades of paper over the part and remeasure so I can account for surface finishing on my final dimension. Then it's relatively easy to hit very close to desired dimension without having to 'sneak up' on it as much.

For those that wonder about hitting close tolerances versus temperature changes, it really makes little difference. For example, relative to other common materials, aluminium expands/contracts quite a lot by changes in temperature, yet for every degree Celsius it changes only around 2.2x10-6 metres, per metre. So for a 1 metre (over 3 foot) long piece of ali raised in temperature by 10 degrees Celsius, it will grow in length by 0.00002m, or 0.02mm over 3 feet - virtually immeasurable at the part sizes we deal with.


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## alchr (Jun 7, 2018)

You might want to consider t3mperature with tolerances in .0001!!


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## Cogsy (Jun 7, 2018)

alchr said:


> You might want to consider t3mperature with tolerances in .0001!!


Not much though - my 3.3 foot chunk of ali detailed above will grow in length by almost exactly 0.0001" per degree Celsius increase in temp. but I don't work with 3.3 foot lengths, especially anywhere near that precisely. In the real world, the ali part I'm trying to measure might be 50 degrees hotter than ambient from cutting friction, (so about 0.005" longer over 3.3 foot) but if I'm measuring with a mic it's probably less than 2" in diameter, so the change is in the region of 1/20th of that. Total error from temperature ~0.00025", total error from surface finish + measuring error is far greater than that (at least for me). And if you really want to get finicky you just let the part cool off for a minute or two and most of the error goes away.


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## gbritnell (Jun 8, 2018)

What is an acceptable machining tolerance for model building? The short answer is it depends on what you're making. If it's something that requires a close tolerance, shaft fit, cylinder bore square or cylindricity, wrist pin fit or such then you make it fit like it should. If you're making a connecting rod and you machine the profile (outside shape and it's withing .005) what difference will it make. Now the main journal bore and the spacing between the bore and wrist pin hole that should be held to print although if the center to center distance varied by .002 who's going to know and it will never affect performance. The next variable that comes into play is the size or scale of what you're building. The smaller the model the closer you have to hit the numbers, .002 on something that's 3.00 diameter and .002 on something .300 diameter make a big difference in fit. Some builders will scrap a part if they don't hit a number right on the head. That's the level that they work to and are happy with. Most home shop tools, lathes and mills, depending on quality are capable of .0005 that is if the user knows the machine and how it cuts. Here again side cutting with an end mill will only get you so close. Try taking .0005 from the side of a block of steel in the mill. 
The next variable is what kind of measuring tools are we using to establish size, digital calipers versus digital micrometers, telescoping gauges compared to bore gauges.
What is done in a high tech shop with millions of dollars of tooling isn't quite the same as what is being done is someones basement or garage. 
The bottom line is strive for the best you can do and learn what dimensions are needed and what aren't. 
gbritnell


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## DanP (Jun 9, 2018)

Hi all
I'm not a machinist just a hobbyist.  I was always doing wood work until I got involved with Live steam Gauge 1 trains.  I inherited a Myford lathe with most attachments and tooling from my Uncle.  That started the game.  I picked up a very nice knee mill which belonged to a friend's father who passed away, it came with lots of tooling.  OK not ever having any training on machining I jumped right into the deep end.  First year all I accomplish was being able to make big pieces of metal small enough to fit in the garbage can.  But as time when on I was able to make progress.

OK I know the subject of this thread is accuracy.  Right from the start I knew I was not going to be able to meet people like Gail Graham's (a good friend) level but I would try to get as close as possible and as time passed I got better and better at my accuracy.  I guess what I am saying is keep doing your best and soon you will surprise yourself, the day that one piece fit's so perfectly you will know you are on your way.  The one thing I have never lost is the amount of fun I have with all of this and that's what is important to me.

I do not post on here often but I read as many of the posts as I can for the knowledge.


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## Jhawk (Jun 9, 2018)

This thread was started in 2008. Has a consensus been reached yet?


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## Ken I (Jun 9, 2018)

All good advice - do your best as a matter of good practice.

How good are your measuring instruments - especially bore gauging.

Do you have any slip gauges for reference.

Plan your manufacturing sequence so that if you make a bore first and it goes oversize you can correct the mating part accordingly and vice versa.

Its easier to ream a holes and adjust the shaft to suit etc. etc.

All my plans (posted on this site) are part numbered in the sequence I made them (or later wished I had) to this end.

Regards,
                 Ken


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## mnicholson (Jun 10, 2018)

Loose nut said:


> What do you consider to be an acceptable level of accuracy for model making? I have always strived for .001" as a standard where it counts, some times I make and ......
> 
> In the golden olden days of Model Engineering when the main measuring tool was a ruler marked out to 128 Th's and parts where made to fit each other, 5 or 6 thou was considered good but we now have much better tools today.
> 
> The pros. keep talking about finishing parts to a couple of tenths of a thou. which is beyond my capability so what is a good and reasonable level of accuracy to try and achieve.


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## mnicholson (Jun 10, 2018)

Loose nut said:


> What do you consider to be an acceptable level of accuracy for model making? I have always strived for .001" as a standard where it counts, some times I make and ......
> 
> In the golden olden days of Model Engineering when the main measuring tool was a ruler marked out to 128 Th's and parts where made to fit each other, 5 or 6 thou was considered good but we now have much better tools today.
> 
> The pros. keep talking about finishing parts to a couple of tenths of a thou. which is beyond my capability so what is a good and reasonable level of accuracy to try and achieve.


Good morning, Just had to chime in for the first time. I been a journeyman Toolmaker for +40 years, own a general machining and fab shop, and have been turning handles most of my 60 years.I have built many laminated injection molds for the military. These molds are required to be within .ooo1 or less for accumulative error purposes. If your shop is not held at a temperature of 68.8 degrees, your are not using a A+ granite surface plate, a .oooo5 indicator  and Johannsen blocks as a reference. Tenths are only  wishful words. .oo1 is fine for anything of most machining jobs. Thanks for listening. Mark


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## BryanK (Jun 10, 2018)

I think I have an analogy from my career as a surveyor that summarizes what the group is saying ( that tolerances depend on the job being done)

Surveyors tend to be very anal when it comes to measuring or setting out reference points for other people to use. 

     During my career I’ve worked on jobs that have required tolerances of +/- 0.001 meter to laying out cut/fill stakes for earth moving equipment building roads. Obviously if  I taken the time and care to layout the fill stakes with as when I was laying out the mounting bolts that needed to be accurate to .001m it would be a total waste of time.....now being a surveyor it could be very hard not to go overboard in making sure all was accurate especially so early in my career. But time is money...and not only my time but the time of the contractors and machinery if they could not work because they were waiting for me to do the layout, and typically to stay ahead of them meant laying out 500 to 1000 work points. On the other hand when laying out bolt patterns,  in a 8hr day I might only be able to layout for construction or verify the position of the finished location of a bolt pattern that would be for a highly specialized piece of equipment............
           We had a saying that summarizes my longwinded intro.
“We’re building a road, not a piano”
         In practical terms, when starting  a project we would be given an accuracy ratio to work with, a common ratio used for most work we did was 1:5000, for dirt work typically we would use a ratio of 1:1000, and high accuracy plant work it would be 1:20000 but could be as high as 1:100000. ( in normal people terms) 1:5000 is your tolerance would be (as an example) if your plan says 1 inch your finished piece should not measure more or less then 0.005”.
  When starting a job when we  put in our control points that everything was measured from at a 1:10000 accuracy to the main control points used for the whole project. Then from there we laid out our work, we made sure our work was within that 1:5000 guideline. Then we could be confident that then the end product
 would all work together.
        How I have incorporated this in machining is the main piece that incorporates the other pieces I will take great care to insure it is really close to plan measurement. Then depending on how close. The tolerances need to be for the finished product I decide realistically what an acceptable tolerance is to make the end product work well and then measure those to +/- of that.


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