Tolerance

[robwilk]

I am starting building the wobbler EJS . On the drawings after some of the dimensions it says H7 i believe this to be a tolerance level my question is this is there a chart of these different tolerance's or a way of understanding them because it is a new thing to me. Any help would be great . Thanks

Rob......

 

[Tin Falcon]

OK shooting in the dark here . Not sure what a EJS Wobbler is or what part of the drawing the H-7 referees to but Google to the rescue look here: http://www.wisetool.com/fit.htm
if this info is not what you need try to be more specific and I or someone else will take another shot at a correct answer.
Tin

 

[peatoluser]

Just ream the hole. H7 is the tolerance for a reamer (well, one made up to a standard rather than down to a price). on some good qualityy ones you can see this marked next to the size

yours

peter

 

[robwilk]

That is just what i was after thank you Tin.
The plans are here if you want to take a look.
http://www.homemodelenginemachinist.com/index.php?action=tpmod;dl=item223


Rob......

 

[robwilk]

Pete having seen a chart i would agree with you to just ream the hole because i definitely cant work to Microns I strugle with millimetres some days.

Thanks

Rob......

 

[Tin Falcon]

I see said the blind man LOL.
like i have said before in the home shop. The actual size or tolerance in most cases is not as important as how mating parts fit together.
Tin

 

[Ken I]

Going slightly off topic and at the risk of throwing gas on a fire.....

As a lifelong designer I fully understand and appreciate the use of standardised geometric tollerancing - but it drives me nuts.

As a designer I might put H7 next to a hole but then I will also put the tolerance or the statement "ream" next to it.

This doesn't take much effort but without it, every person using the drawing thereafter has to refer to tables (I certainly don't remember them all) which to my mind is a collosal time wasting exercise.

I regularly get drawings so infested with the stuff, that what should be a simple read exercise turns into an hour of poring through a machinery handbook to find out what it all means.

AAAAARRRRRGGGGHHHH !!!!

You will regularly see such drawings scribbled full of anotations on the tolerancing. In my book, if the user can't make the part from the drawing, then as a designer you have failed.

Ken

 

[JorgensenSteam]

Amen on what Ken said.

 

[dsquire]

Going slightly off topic and at the risk of throwing gas on a fire.....

As a lifelong designer I fully understand and appreciate the use of standardised geometric tollerancing - but it drives me nuts.

As a designer I might put H7 next to a hole but then I will also put the tolerance or the statement "ream" next to it.

This doesn't take much effort but without it, every person using the drawing thereafter has to refer to tables (I certainly don't remember them all) which to my mind is a collosal time wasting exercise.

I regularly get drawings so infested with the stuff, that what should be a simple read exercise turns into an hour of poring through a machinery handbook to find out what it all means.

AAAAARRRRRGGGGHHHH !!!!

You will regularly see such drawings scribbled full of anotations on the tolerancing. In my book, if the user can't make the part from the drawing, then as a designer you have failed.

Ken

I fully agree with what you have said Ken. :bow: :bow:

Cheers

Don

 

[Tin Falcon]

I agree ken what is easier for the guy building. an annotation like clearance drill for say 1/4-20 or just specify the size.
I know from experience that a tap drill for a 1/4 -20 is # 7 but it is easier if such things do not have to be looked up on a chart.
also I have seen drawings with no tolerance. Like a shaft size of 1/4 mating to a fly wheel with the same 1/4 hole there is no such thing unless you are using a bit of loc tight either you have a running fit and a bit of clearance or a press fit with a bit of interference.
Assuming builder knowledge is just not fair. Yes tolerances and drawing is a form of communications and there has to be an assumed like frame of reference and symbols understating of line meanings etc. but one can not expect mind reading or a research project on the other end to get the true meaning.
Tin

 

[Brian Rupnow]

H7 tolerance is for a "sliding fit".--exactly as said, thats a standard reamer.

 

[peatoluser]

I agree with Ken. putting tolerances on a drawing- especialy for home shop machinists- can sometimes be counter productive. as Tin says 'it's how mating parts fit' that's important to us.
or as somebody said 'it's not that the axles of my loco are all different sizes, they are merely bespoke to their individual wheels'
I think I remember a write up in 'model engineer' where Tubal Cain gave a dimension as so many inches + 1/64" on the grounds that had he given the decimal equivalent this would have caused some concernation amongst builders about the accuracy involved, whereas all he was specifying was that there would be enough clearance for the parts not to bind.

the best advice i can give to any beginner is to always machine a shaft/piston to fit a bore, not the other way round. On the grounds it can be dificult to accurately measure a bore (how many of us don't have internal micrometers but relay on vernier calipers to measure hole size, with all the vageries using them can bring- if you look at the literature that comes with some, the accuracy is sometimes only gaurenteed to +or- 0.002" )
whereas you can always shave a touch off a shaft even if you use a file or some fine grade wet and dry.

that's why i've always admired the writings of Tubal cain or Rudy Kouhoupt. They knew how to communicate to the newbie when a part is critical or when there is some leeway in size allowed

yours
peter

 

[Brian Rupnow]

Ken1---I like your answer. I too have been a mechanical designer all my life, and I live about as far away from you as I can get and still be on planet Earth. I too understand tolerances, and use them, and they drive me absolutely crazy!!! I like the way things were when I started out in 1965-----Light press fit, Hard press fit, Sliding fit, and Clearance fit. Entire fields of science have been applied to tolerancing fits. Damn few use them, even less understand them, and unless you are mass manufacturing rifles with interchangeable parts or hi speed engines, very few need them.----Brian

 

[bp]

Just to throw another slant on the discussion, by the way I agree pretty much with most of what has been said.

However, when I was a draftsman we had to prepare drawings to be used by anybody, our workshop or any subcontractor for instance. So we were specifically prohibited to put such things as "Ream", or "drill and tap M3 x 0.5" . The argument being that we didn't give a hoot how the feature was achieved, just that it was achieved.
cheers
Bill Pudney

 

[kvom]

This year I took a 2 quarter self-study course on blueprint reading at the local votech school. What I gained from the course is that overstating tolerances is probably worse than understating. Putting a dimension in a drawing implies a tolerance unless otherwise overridden, and it seems some engineers often don't understand that when doing drawings. As an example, a drawing might state that dimension x.xxx are +/- .005 but they'll use 4-decimal dimensions instead.

Almost all the models people make here are pretty flexible on tolerances; as said earlier, if the parts fit they're probably OK. I've been frustrated initially with the lack of any tolerance info from Kozo in my locomotive build. However, after being told that a steam locomotive needs to be built "half worn out" I was able to relax a bit. When an engine runs with steam vs. air, a lot of parts heat up and expand, so anything built too tight will jam up.

 

[Ken I]

O.K. just testing the water.

You might have noticed that plans I have posted on the site are typically "untolleranced - make allowances for limits and fits" - mostly because that's the way we operate building models.

Since most of my professional output is for one-off machine building excercises they are generally tolleranced or fit specified. I don't bother will all the fancy tollerancing for a one off - I want the toolmaker to pick up the drawing and make it.

My customers on the other hand send me (production) drawings festooned with incomprehensible stuff - it a PITB having to wade through it all for say quoting on a machine to assemble two parts - you certainly can't ignore it - you might be missing something of importance.

My point was if its important put it on the drawing (I will concede that some things are more easilly stated in "code").

And Brian - your point is well taken there are a lot of "engineers" out there applying ridiculous tollerances when they know not what they do.

Microns are as big as footballs to some people.

I have a grad student playing around with one of my robots (accuracy 0.02mm) for a surgical aplication - he wants to know if he can work to 0.001mm but hasn't yet given me a reason why - I suspect he thinks its a big deal - I can't imagine anyone wielding a knife more accurately ???.

Regards,
Ken

 

[Tin Falcon]

As far as overstating and overusing tolerances I have heard it said precision is expensive and not necessarily a good value.
Tin

 

[mzetati]

http://www.misumiusa.com/CategoryImages/Metric_2006_pdf/METRIC1835-1836.pdf

Capital letter tolerances (G6, H7, etcc) are for holes, lower case letters (g5, h8, ..) for shafts.
The declared tolerance on the nominal diameter, both on the the hole and the shaft, ensures the kind of fit You're going to have when coupling them.

Marcello

 

[MachineTom]

On a similar note, a set of plans for the Corliss I am building, had less that 5 parts with any tolerance, and all sizes were in fractions of an inch. Yeah, that 17/64 counter bore is real common-not, 9/32 can be found, now in the tool drawer. Piston, 1 1/4, cylinder bore 1 1/4, I'l make that work, but a few more would be nice. Drawings were done in 1978, based on a model done in 1947. A buddy of mine who worked as a draftsmen in the 50's said he never saw fractions on a machined part ever. In the shop manual the spec's for my 1934 pontiac, the crankshaft journals were listed as 1 11/32 diameter, as well as other fractions, but these were not intended as blueprints they were just specs.

 

[Maryak]

Tom,

As I understand it, the common standard for dimensioning imperial drawings at that time was to use fractions down to the level appearing on a common scale, i.e. 1/64"

Smaller sizes and tolerances were expressed using decimal fractions.

Cumbersome to say the least. Your buddy was ahead of his time and I bet every machinist thanked him.

Best Regards
Bob