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[willray]

Hi Willray,

I agree with your comments but would add that the true, practical, reason for not “over dimensioning” is the tolerance problem:

Imagine a 100mm +/- 0.1mm long item with a hole 20mm +/- 0.1mm form one end - what is the “missing” 80mm dimension tolerance?

Ian

Greetings IanN,

As you've described the part and feature, it's not over dimensioned Were it over dimensioned, the its tolerance problem would be a subset of the "you get different dimensions (and tolerances) depending on which way you work the numbers around the part" problem.

 

[willray]

For the benefit of math challenged and anyone else interested I have prepared a spreadsheet to calculate that .0627 dimension as well as 2 different ways to measure the mouth diameter. If anyone is interested I'll attach it in my next post.

Please do. I think the fact that the .225 countersink diameter is the specified dimension, and what ought to be the thing measured for compliance, is perhaps overlooked by many readers because they don't immediately see a way to measure that dimension.

 

[easymike29]

Please do. I think the fact that the .225 countersink diameter is the specified dimension, and what ought to be the thing measured for compliance, is perhaps overlooked by many readers because they don't immediately see a way to measure that dimension.

My pleasure

Eugene

 

[brotherbear]

Thank you very much, Eugene!

 

[Richard Hed]

Hi Willray,

I agree with your comments but would add that the true, practical, reason for not “over dimensioning” is the tolerance problem:

Imagine a 100mm +/- 0.1mm long item with a hole 20mm +/- 0.1mm form one end - what is the “missing” 80mm dimension tolerance?

Ian

There is always stack up too unless you fix it.

 

[easymike29]

Thank you very much, Eugene!

You're welcome Brotherbear. If you have any suggestions for improving it let me know.

Eugene

 

[L98fiero]

Please do. I think the fact that the .225 countersink diameter is the specified dimension, and what ought to be the thing measured for compliance, is perhaps overlooked by many readers because they don't immediately see a way to measure that dimension.

The easy and, what my drafting instructor taught, appropriate way is to tolerance the important dimensions. If you're doing it in CAD, most if not all programs will allow you to set the default number of trailing zeros and you can put on the title block how to determine the general tolerances based on the number of zeros. Once you have that established, put special tolerancing instructions on the important ones. I've seen too many times when I have gotten drawings where everything is dimensioned to 3 decimal places, which generally means +/-0.005, even on 3" thick flame cut parts, and then you get someone with a CMM checking those parts!

 

[wthomas]

Hi Ian:
In Most system with the two dimensions at the limit would double the tolerance on the 80 mm dim. That would be the case if the
two limits (1) the bar long by 0.1 mm and the 20mm dim. short by 0.1 mm for a dimension of 80.2 mm and (2) the bar short by 0.1 mm
and the 20mm dim. long by 0.1 for a dimension of 79.8 mm. Few people would catch the difference because they would just check the
given dimension but a CMM would show it.
Also, just to show you that CAD work can be in error if the machine is not set to high number of places,here is a story told to me by a
toolmaker I know who became a checker because he was Good at his work. When he was checking a large diameter,around 5 ft., indexing
plate that had about 25 to 30 station on it he found one station at the final position was off location. He was doing the calculation to eight
places to find the error on the CAD print. When he went to the CAD Tool Designer, he had set the computer software to only six places not
thinking about the small error adding up that many times.
By the way did you laugh also, about the countersink dimension? 90% of the time in the shop they just drop a screw in the first hole to
be sure it is deep enough.
Stuck at home in Mich. USA
Bill Thomas

 

[IanN]

Hi Ian:
In Most system with the two dimensions at the limit would double the tolerance on the 80 mm dim. That would be the case if the
two limits (1) the bar long by 0.1 mm and the 20mm dim. short by 0.1 mm for a dimension of 80.2 mm and (2) the bar short by 0.1 mm
and the 20mm dim. long by 0.1 for a dimension of 79.8 mm. Few people would catch the difference because they would just check the
given dimension but a CMM would show it.
Also, just to show you that CAD work can be in error if the machine is not set to high number of places,here is a story told to me by a
toolmaker I know who became a checker because he was Good at his work. When he was checking a large diameter,around 5 ft., indexing
plate that had about 25 to 30 station on it he found one station at the final position was off location. He was doing the calculation to eight
places to find the error on the CAD print. When he went to the CAD Tool Designer, he had set the computer software to only six places not
thinking about the small error adding up that many times.
By the way did you laugh also, about the countersink dimension? 90% of the time in the shop they just drop a screw in the first hole to
be sure it is deep enough.
Stuck at home in Mich. USA
Bill Thomas

Hello Bill,

Thank you for humouring me and posting calculated tolerance for the 80mm dimension - I know you already know my response and it is kind of you to act as the “stooge”

If the drawing shows a dimension of 80mm +/- 0.2 from the second end, then it is permissible to use this dimension on the drawing to locate the hole.

So let’s assume the work piece is 0.1 short (99.9mm) and the measured 80mm dim. is 0.2 long, all in spec if checked from the 80mm end, but measured from the 20mm end the 20+/-0.1 dim. would measure 19.7mm - out of spec.

It is impossible to correctly and unambiguously “over dimension” a drawing with assigned tolerances.

Now apply this logic to the countersunk hole: You cannot specify three interrelated dimensions with tolerances (angle, diameter and depth of the countersink). In the case of the original drawing the diameter and angle are specified.

As you correctly stated (voice of experience?) a sample with largest diameter and smallest angle would be the “gauge” to use

Thanks for sparring with me!

Ian

 

[wthomas]

Hi Ian:
Would 70 years of machining experience be enough for me to qualify? I had worked in more shops
when I graduated from high school (3) then the leader-foreman in the last shop I worked in last year.
I like to do the whole run from design, build, and runoff of a tool (hand tool, golf tool, die, mold,) or machine. Largest run was 7,000 special
hand assembly tools for on the Auto Dealers in 9 months. (OOPS) I designed the clear coat / tinted coat paint system for the Ford plant in
Novi, Michigan 1992 that run for a number of years. The Lord only knows how many cars the system painted and how long it lasted running
60 cars an hour.
If you would like to see something I made while working for Buick Engineering Google "10,000 Miles in 5000 Minutes" Video. I machined
the special parts for refueling at 90 mile/hour. Also, machined the roll bars for both cars. I did both jobs on a Hyd. Tracer Lathe before the
time of NC.
Post your questions, I will try an answer them!
Bill Thomas