# How Accurate Do I Need To Be or [What Are My Tolerances Today Boss]



## Whyemier (Aug 23, 2014)

How accurate are we in our shops and how accurate do we need to be?  I ask because I often see posts on machining sites discussing staying within .001 - .0005.  Is it always necessary? I understand personal best and pride in our work which would drive us to get the most precision we can.  And more power to the guys who always works to those tolerances.  

41 years ago I started work in a small 'Jobbers Shop' We did a lot of work with punch presses and made dies for the machines.  Those were machined, hardened and ground to close tolerance.  Had to be for the consistency of product and workability of the dies.

Later I worked in a shipyard (Jacksonville shipyard) for a while.  There it was an attitude of get it done fast and so it works.  They would weld it in or torque it down, beat it with a hammer to fit.  The ships had to be turned around quickly because it cost money for them to sit in the dry dock. At times we worked to greater tolerances when turning bearings etc.  Not that this always held true, many of the boats and ships that came our way were worn beyond belief and you did the 'best that you could.'

Then I worked in 'Job' shops where the boss would tell you, "Get it done fast and don't worry about it." or "Take your time and on that one work for precision".  

I have talked to machinist (real machinist compared to anything I ever was) who worked in aerospace and held tolerances of .00001 or tighter.  I know I never had that kind of skill.

I later got out of machine work and for many years after did more office and engineering work. So much of whatever skill I may have had I no longer do.  

These days I work to .001-.003 for my tolerance.  For much of the work I do around here for myself and friends that is more than enough.   To me it is a hobby which is satisfying much more to see the part I machined being use than for the extreme precision I might...or might not...be able to hold.


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## hvontres (Aug 23, 2014)

For my personal stuff, i usually use the "Gesinte" tolerancing system, i.e "this gesinte that"  Other than that, I think it really depends on how much time you feel you want to spend on any given project. Obviously, the end use is really what dictates the kinks of fits we need in the end. And the beauty of making our own stuff is that since we ususally are making both halves of an assembly, we can sometimes recover from a boo-boo by adjusting the other part.


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## wnec65 (Aug 24, 2014)

When you run a business and you are trying to make a profit holding tight tolerances cost more.  A good design will loosen tolerances on features that are not critical to keep the manufacturing costs down.


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## Rbeckett (Aug 24, 2014)

Im with Henry.  If it  Gazinta the other part without slop and looks decent then that is good enough to suit me.  It all depend on what you are doing really.  If you are selling a product to the public the .001  is pretty decent but if it is a high stress complex part for a high dollar machine then I would take my time and try to get .0005 or better.  But that isn't really hobby type of work and is worth a lot more dollars when you are done too.  Generally speaking if you are holding .001 to .003 for hobby work and the fits are not critical then that is close enough to not fuss with too much because if you do fuss with it too much it will end up undersize and have a sloppy end result that will be a big disappointment and a waste of time and material.  Dot ask me how many parts I have thrown out because I spent too  much time trying to perfect them... Needless to say I have some spare material in my junk bucket in case I need a smaller diameter part for the lathe or a smaller chunk for the mill.

Bob


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## chuckorlando (Aug 24, 2014)

I try to hit the number. If it say's .125 I try to hit .125. Provided it has a +/- of equal forgivness. If it say's .125 + .010 I would aim for the middle of that so .130 which leaves me .005 on either side.

I understand the older heads get it in range and it's good. But I think if your just learning, learn to hit the number.

Another thing to keep in mind is compound error. If you have 10 evenly spaced holes each with .005 +/- and your not paying attention you have a potential for .050 of error at the last hole, yet ever hole is "with in spec" from each other.


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## Whyemier (Aug 24, 2014)

hvontres said:


> For my personal stuff, i usually use the "Gesinte" tolerancing system, i.e "this gesinte that"  Other than that, I think it really depends on how much time you feel you want to spend on any given project. Obviously, the end use is really what dictates the kinks of fits we need in the end. And the beauty of making our own stuff is that since we ususally are making both halves of an assembly, we can sometimes recover from a boo-boo by adjusting the other part.



BTW hvontres, I was born in Oceanside.  That was many moons ago (64 years) when my father was in the Marines doing a tour in Korea. Never been back of course.  Always thought I would like to just to see where I began.


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## billdeme (Aug 25, 2014)

It really all depends on what you are machining. You need to look at what that particular feature is for. If it a close ground fit, +/- .001 isn't unheard of. If it's an ovl of say a bolt +/- .03 would be acceptable.


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## astjp2 (Aug 26, 2014)

Tolerances are based on form, fit and function.  We bend sheet metal brackets and hold .001 with a press brake on one part and +-.030 on others, boring holes +.000/-.0005 or better, just general milling +- .010 unless it is a mated part then it get much tighter.  Hole tolerance in composite structures have a .002 tolerance. We do over $200 million of production per year as a major supplier for my company.  Its all about the engineering needs, we have hole tolerances in key board brackets that are .0005 tolerances on the lightening holes, Really ?  Use some common sense and there is a general rule of thumb based on an ISO standard that I cant remember....Tim


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## toolholder (Sep 2, 2014)

In going to school for an associates in drafting and design I have to say the most useful class I have taken is GD&T, geometric dimensioning & tolerancing. It is blueprint reading on steroids, you really wouldn't believe how crazy the world of measurement is until you take this class. Lucky for me the instructor was a former Boeing engineer, also had owned his own machine shop, not an easy class by any means but totally worth taking.


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## gjmontll (Sep 2, 2014)

My story on accuracy/precision/tolerance: 

When I was 18, my summer job was working in the machine shop at a electronics equipment manufacturer. Mostly "semi-production" tasks; this was the first place I ever used a mill, then never used one again until starting into this hobby 5 years ago.  
Anyhow, one Friday afternoon, some guy came into the shop and gave me two pieces of plastic, similar to what is used for name tags. We used them to fill blank spots in racked equipment. "I need two pieces, each 1" by 3"; and  these are the last two pieces we have in the plant, so don't mess them up.  (Too heavy a cut with the mill would chip the edges.)
I clamped the first piece into the Bridgeport and carefully started trimming the plastic. ... 
Some time later, he returned and asked, "Aren't you done yet?"
Still cranking away on the mill, I said, "Another 0.005 off this side and the first piece will be done, then I'll start on the second one."
I'll always remember his reply: "Thousandths? Plus or minus 1/4" would have been okay."
I wonder if he learned the importance of specifying tolerances...

Greg


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## Whyemier (Sep 4, 2014)

Thanks for the input.  

My question related to what I am.  Shipyard, barnyard, backyard, under the shade tree machinist. :secrethandshake: There were times when the boss did say, "Use your scale, just scale (1/32"-1/64" or less) it that will be close enough.":boss:.  So I'd scale it and put the outside/inside calipers on it and get it done.  Other times I would use the micrometers and verniers and check it every way from Sunday and it better be 'held' or else.  

So I'll still scale and use calipers, with a test fit or two in between if that is what the project calls for, :makingdecision:to a close 'scale/rule' fit.  Other times I will hold the best tolerance I can (e.g. the recent 2C collet block I machined).  It all works though I do admit to probably having some bad habits picked up over the years.:sorry2:


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## Fabrickator (Sep 4, 2014)

My machining background started with engine building.  When I was building motorcycles, Harleys and stuff, it was +/- .001 or .002".

Once I moved on to working in Top Fuel Drag Racing (Keith Black nitro-blown Chrysler hemis) I had to hold even tighter tolerances to pacify the customers.  Even though they would blow them up through the timing lights almost every run, they demanded perfect fitting parts as a starting point in the process of elimination as to *why* it happened. 

For myself and my hobbies, I try to hold size within .001-.002" on most parts other than a cylinder bore/piston fit which is important to be just right.


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## stupoty (Sep 5, 2014)

wnec65 said:


> When you run a business and you are trying to make a profit holding tight tolerances cost more.  A good design will loosen tolerances on features that are not critical to keep the manufacturing costs down.



Designing to achivable tolerances, definetly a good idea 

its all a learnng curve though, some things need real high accuracy and some don't.  Also theirs a lot to practice and apropriate work holding methods for the job at hand. 

 E.g. My milling vice has lugs that fit quite well in the t stots , so i push it to the frount of the mill whilst i tighten it down thats the wide tolerance alowed situation, if it requires more acuracy i can dial it in using an indicator.  Usint the locating lugs is much quicker ;-)

Stuart


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## Falcon67 (Sep 5, 2014)

I got to spend 20 years in manufacturing, and during that time we went from electro-mechanical to mostly digital.  Back in the pulleys, gears bushings and such days I got to see a lot of blueprints.  Most of items were +/- .005 on machine parts, depending on application.  Hole locations could be that or +/- .010.  Where you had turning shafts between two side frames, the holes might have to be +.000 / -.005 or better with bushing tolerances down to .0005.  Length on aluminum frames, pieces door castings and such could be in the +/- .030 range.  

So I generally figure that if I can hit within .005 of target on most things, I'm doing pretty good.  I don't own any reamers yet, so I haven't had a call for that close tolerance.  I would like to get into some cylinder head work, and on that stuff - especially performance items - I'll be wanting concentricy between guide and seats cuts to be as sub .001 as possible.

I get the Top Fuel thing - and LOL.  I have seen some slap together crap run big numbers.  Precision is always good, and I check everything before putting it in a motor.  But some folks would be surprised at how much you can get out-of-whack and still run hard.  My last 351C made 500 HP.  Car ran two seasons, spent the last season oiling the water.  Car ran - at our altitude of 1800' (DA usually 3500~4500) - mid 7.20s at 95 in the 1/8 mile.  Last race of the 2nd season (about 400+ passes on the motor) car ran 7.28 at 95 just like the race before, and the race before that, and the Friday test-n-tune bunch of laps, etc. Then next pass it could not get past the 60' cone without falling on it's face.  Pulled the motor and it had 5 split cylinders, three broken outer valve springs and the crank had .110 of end play because it had eaten the thrust bearing face and machined a groove in the thrust face of the crank.  The reason it would only go 60' is because the thrust was finally gone enough to allow a crank throw to rub the center main casting in the block and slow the motor.  Except for flat out nose up, you could drive it anywhere and not know anything was wrong.  Oil clean like new.  Yes, I bought a cutter after that and now cut open my oil filters to see if a motor is chewing on things.


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## SG51Buss (Sep 10, 2014)

Long ago, I was pouring over some blueprints, and saw something unusual.
At the bottom left of the sheet, in the typical corporate standard boilerplate notes section indicating materials and tolerances, was this additional note:


8. Pound to fit, paint to match.


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## Falcon67 (Sep 10, 2014)

That's a variation on a Bugs Bunny quote from a book I had as a kid.  It had a diagram in the front with all kinds of things in it, like "washing machine motor thrown in for confusion".  On one part was a note, "hammer to fit, bend to suit".


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## SG51Buss (Sep 10, 2014)

Falcon67 said:


> That's a variation on a Bugs Bunny quote... "hammer to fit, bend to suit".



Fantastic!  Thanx, Falcon.  That's been bugging me for near on 30 years.  One of the draftsmen snuck that in there, we never knew who or when.  Found it on some of the other older sheets.

Guess he wanted to know if anybody was paying attention...:nuts:


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## Falcon67 (Sep 10, 2014)

Hahaha - there was another one I remembered later, another personal favorite: Shaft and bushing labeled "Free running press fit"


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