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dim tolerances

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MICHAL MANIA24/02/2014 12:25:41
3 forum posts


I have just bought the latest model eng special edition with LYnx 15cc SOHC engine design. I would like to make one but I can not find any DIM tolerances on the drawings for any of dimensions. could anybody help me with this please? many thanks Mike.

JasonB24/02/2014 12:57:30
22011 forum posts
2539 photos
1 articles

You wil not find many Model Engineering drawings that give much in the way of Tolerances.

On the lynx if you generally work to 0.05mm or better for the larger components like crank case and when it comes to smaller items 0.02mm or better though often its best to make one to fit the other rather than try to apply industry standards or machine to tollerances that a lot of modelers don't have the equipment to measure to anyway. The drawings do note critical sizes where they say such things as "to fit bearings"

MICHAL MANIA27/02/2014 08:42:21
3 forum posts


many thanks for Your respond! Is good to know how it works in modelling world. I have some engineering experience so I think , I can sort the problem now. Anyway, I hope I will be able to rely on your help in the future. once again thank you very much.

Paul Janes27/02/2014 11:36:42
16 forum posts
2 photos

Hi Michael,

Many of the model drawings are not prepared by qualified draftsmen or engineers and often do not contain tolerances. My biggest complaint with model drawings is that they are not always correct. Often mating parts do not match or are dimensioned incorrectly. Double check all dimensions before you attempt to machine them. (I work as a design draftsman so am very critical of the quality of drawings) To produce workshop quality drawings costs thousands of dollars in drafting time which is not available for hobby projects. Do a web search for "Limits and Fits" which will give you tolerances for different applications. Can be a bit mind boggling at first if you do not have any engineering background. Fitting a shaft to a hole is normally on a Hole basis and the shaft machined to fit if you are making both parts but if using precision round bar off the shelf use a Shaft basis fit. Horses for courses as they say. Each application may be different but it is good practice to use the correct fit, not just make one part fit another.

Good luck with your project, Paul.

jason udall27/02/2014 13:42:32
2031 forum posts
41 photos
Bang on Paul.
I asked long ago where are the tols. on drawings...

"Don't use 'em" in model engineering. .every thing is the best fit possible... was the gist of the reply...

Fits though. .a different issue..ok old hands can infeer from application..but guidance from the originator would help newbies. .
And I still say some clearances are not immediately apparent and as such tolerences would be informative
JasonB27/02/2014 14:32:45
22011 forum posts
2539 photos
1 articles
Posted by jason udall on 27/02/2014 13:42:32:

Fits though. .a different issue..ok old hands can infeer from application..but guidance from the originator would help newbies. .

Thats provided that guidance can be understood by a newbie. Take the Lynx valve guide as an example the drawings just show the hole as 3mm, would a newbie know what it meant if H7 was written next to the 3mm? OK they could Google it and then worry about how to measure their hole to make sure it was 0 to +12 microns

Far better to follow the originators guidance in the text which says "Ream 3mm" which would give the right tollerance as the majority of reamers are H7. Usually on ME subjects you can gain far more info on fits etc from the text than you can the drawing.


Edited By JasonB on 27/02/2014 14:35:44

jason udall27/02/2014 18:03:15
2031 forum posts
41 photos
And the label "ream 3.00 mm" would have been so hard?.
Ok to me ream means they mean 0.01
drill means 0.1
But whats wrong with writing this on the drawing?

Clive Foster27/02/2014 20:28:40
2990 forum posts
105 photos

Once upon a time, in the days when measuring gear of the standard we now take for granted was both rare and too costly for the individual worker, there was a fairly agreed industrial descriptive terminology for half a dozen or so generally useful fits whether loose, sliding, light press and so on. As Michael says such descriptive terms are in general much clearer to the hobby and occasional machinists than modern terminolgy as they give a pretty clear indication as to purpose from which a fairly good idea of how tight it needs to be can be gleaned. Especially if you have someone to show you physical examples of each although a few examples from common items is nearly as good.

Obviously there is no way to get samples to everyone but it ought to be possible to put some "Model Engineering standard" tolerance numbers to the descriptive fits suitable for "home workshop budget" measurement gear and issue these as a separate resouce. Whether paper or web. I guess an A4 page would be more that enough space to print a short description of the purpose of each fit, tolerances to use and a read across to the modern equivalent term. Back side of the sheet could have a very basic introduction to the proper terminology and a listing of what the normal size of a standard drill, reamer or whatever corresponds to. I suspect the looser end of the scale will need an addition or two to cover the essential slackness that prevents the motion work of a small locomotive from binding up when the suspension moves or going round curves. Rattling good and really rattling good fits maybe.

Naturally anything that didn't fit the standard document would have to be called out specifically by the designer. In practice such such specials are pretty much the only thing called out anyway so in practice there would be no great change. I suspect that such adocument would make life much easier at both designer and builder ends of the job.



Edited By Clive Foster on 27/02/2014 20:29:25

Paul Janes28/02/2014 09:02:47
16 forum posts
2 photos

It does not take much more effort to add tolerances. A simple method which was used in the past was to dimension with a different number of decimal places to specify the importance of the dimension.

The allowable tolerance is half of the last digit. eg.

Dimension.. Tolerance

50............. 49.5 - 50.5

50.0......... 49.95 - 50.05.

50.00....... 49.995 - 50.005


Edited By Paul Janes on 28/02/2014 09:04:51

Edited By Paul Janes on 28/02/2014 09:05:25


Edited By JasonB on 01/03/2014 09:52:46

fizzy28/02/2014 09:16:07
1836 forum posts
120 photos

I think it needs pointing out that if you follow the drawings to the letter it simply wont run! This has been my experience thus far.

Michael Gilligan28/02/2014 10:00:07
19599 forum posts
997 photos


I agree that's a very practical system -- as far as it goes

Unfortunately, it doesn't cover the [often important] "one-way" tolerances

... such as +0.000 / -0.005

Also [and I mean no offence];

May I suggest you check the accuracy of your examples.


Steambuff28/02/2014 11:21:57
522 forum posts
7 photos

Paul's example also does not help when the dimensions are Imperial.


roy entwistle28/02/2014 12:43:45
1459 forum posts

Steambuff It does but not if fractional


JasonB28/02/2014 12:52:05
22011 forum posts
2539 photos
1 articles

If you want teh same sort of thing for Imperial then this is whats printed on all the drawings that come with Model Engine Builder mag.


They also note on the drawings what holes should be reamed.

I do have some ME drawings that Show H & h tolerances and also the surface finish by a series of VVV marks, most of which are of German orgin.

Edited By JasonB on 28/02/2014 12:55:01

ian cable28/02/2014 13:58:55
40 forum posts

a few years ago back in the 1930s or40s a famous locomotive engineer (cant remember which one) who had come from the aircraft industry, designed a locomotive that would not run,the problem was he had used aircraft tolerences .we all like to work as close as we can and make a nice job but lets not take the fun out of it by people thinking that if they don't hit the prescribed measurement the product they make is not good which, when you read some of the post from a newbsy they think they have got to produce something exact instead of something that just works. think theres to much emphasis on tolerance and not enough on machining procedures which would be more helpful to a lot of people that unable think outside the box not every one has an adaptable brain but It should not stop them making there bits ian c

Roderick Jenkins28/02/2014 14:05:35
2137 forum posts
586 photos
Posted by JasonB on 28/02/2014 12:52:05:


Also, in ME drawings from the likes of LBSC or Tubal Cain, if fraction dimensions are given (e.g 1 1/2", 7/32" or 3/4" then the implied tolerance is +/- 1/64" i.e a rule(er) measurement.


Mark C28/02/2014 14:34:42
707 forum posts
1 photos

Mike's question was possibly worded wrongly in my view. The question should not be "why are there no tolerances" but what is the "design intent". This turns the answers from a specific argument about tolerance schemes to a more useful discussion of how best to convey the designers thinking when the item was originally designed/drawn/sketched etc.

Designing professionally, I have to accommodate and understand how my designs will be understood. I am expected to produce workable drawings that are fully detailed, including tolerances (the drawing carries frame a standard tolerance range based on dimension precision and anything outside this is treated individually) but I try to avoid putting tight tolerances on as it pushes the cost of manufacture up. As mentioned earlier, I also sometimes put on geometric tolerances and surface finish/lay condition labels along with weld symbols and a host of other information but only when absolutely necessary. A far simpler system is to perhaps add a few notes on a sheet attached to the drawing with some background information (as is often included in construction series) covering the operation and what is important to ensuring it works. This ties in with Michael Williams post above.

Perhaps the only workable solution (although it would be costly to implement) would be professionally drawn designs to satisfy those who are used to them and a cover note to explain the design in a more user friendly (for those who do not have or want the investment in time to read a fully detailed drawing) manner explaining the designers thinking and requirements.


ANDY CAWLEY28/02/2014 15:19:50
185 forum posts
48 photos

There always seems to be a confusion amongst folk who are good with spanners etc. but who are not trained engineers about what is meant by tolerances. Most talk of tolerance when they mean clearance.

Tolerance is the range of dimension permissible on a completed part as specified on a drawing.

Clearance is the gap between two mating parts. It can be positive meaning the two parts can move against each other. Negative means that the two parts interfere with each other and are therefore locked against each other.

The larger the clearance the free-er the two parts are to move against each other. The greater the interference the tighter the two parts are locked together.

Given that tolerances are not generally shown on model engineering drawings then an understanding of the clearance between the two mating parts is the knowledge that the newcomer to metal work needs to learn. (For the benefit of new comers this clearance is known as the "fit" of two components.

In my opinion the most useful guide to this is in "The Model Engineers Hand Book by Tubal Cain" . (Page 5.18 is the grubbiest page in my working copy of the third edition plus I have copies of the page around my work areas.)

He lists 12 classes of fit and puts numbers to them making the allowance necessary for the relationship of clearance to diameter. He goes from shrink fit through to large clearance fit via drive, slide, close run and easy run to name a few.

When all is said and done for one off production such as in our projects our main object is for things to work and the way they work is down to the correct clearance.

Nicholas Farr28/02/2014 18:08:18
3148 forum posts
1434 photos

Hi Andy, I'm not quite sure I understand your way of interpreting what a clearance is. To my mine it means there is a distinctive all round distance between two parts, i.e. in theory this means that a shaft for instance, will enter a hole without it ever touching the inside of hole. Of course this does not always happen, as with drilling a clearance hole for a bolt, invariabaly the bolt will touch the hole at one piont.

To my mine there is no such thing as a negative clearance, something either clears or it does not clear. On page 5.18 there are only two clearance fits mentioned, Small clearance and Large clearance. All the other fits that are listed are touching by a defined amount.

Regards Nick.

Edited By Nicholas Farr on 28/02/2014 18:09:21

Paul Janes01/03/2014 09:16:38
16 forum posts
2 photos

We only get what we pay for. The set of drawings for my traction engine cost less then $100. If they were to a professional standard they would cost $ thousands. Too expensive for a model maker (or me) to buy. They do however contain most of the data needed to create a working set of drawings. I am fortunate that I work in engineering as a design draftsman using Solidworks to model all of the parts and then create fully detailed workshop drawings. For those not so skilled I would suggest that they join a model club who have members with an engineering background. Most will be happy to advise and help.

(Note. My previous post about allowable tolerances was a practice for the length and width of components. Shafts, bearings, etc are dimensioned with suitable limits and fits)

Regards, Paul.

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