Decimals, Fractions & Rounding 1

[vonsteimel]

Greetings,
Wanted to see what the opinions were on a couple frequently debated topics here at my workplace.

1. Decimals. Decimal equivelants of fractions are often used on engineering drawings. When you get a drawing that says cut to drill a hole at 0.19. Will you drill it at 0.19 or 0.1875?

2. Do you know all your fractions down to the 1/16ths? both 2 & 3 place decimals?

3. Would it be any simpler if drawings were given to you in fractions rather than decimals?

4. Do you scribe locations using caliper jaws or measure and use an actual scribe when working with soft steel or aluminum?

5. Do you do much locating/scribing using tape measure?

Any input will be greatly appreciated!
Thanks,

VS

 

[BrianPetersen]

1. Bad drawing practice. There should be a tolerance - either explicit or implied - on every dimension. If there is no explicit tolerance specified then there should be a default tolerance on the dimension and somewhere in the drawing package, it should state what the default tolerances are. If the 3/16 drill bit is within the implied tolerance zone of 0.19 then it's fair game. If it isn't ... it isn't!

2. Yes, unfortunately.

3. Absolutely not. Fractions implies old English units. Metric doesn't use fractions. Stuff that I deal with is designed metric nowadays and you just don't do fractions on metric drawings.

4. Only if building something to tape-measure accuracy as opposed to CNC-mill accuracy.

5. Only if building something to tape-measure accuracy as opposed to CNC-mill accuracy.

 

[JohnRBaker]

And here I thought that this was going to be another round of debating whether you round UP or DOWN when the last digit is a '5'.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

 

[JStephen]

1. Decimals. Decimal equivelants of fractions are often used on engineering drawings. When you get a drawing that says cut to drill a hole at 0.19. Will you drill it at 0.19 or 0.1875?
- I don't drill the hole, I do the drawing. I'll either show it as 0.1875" or 3/16", not 0.19".

2. Do you know all your fractions down to the 1/16ths? both 2 & 3 place decimals? Yes.

3. Would it be any simpler if drawings were given to you in fractions rather than decimals?
I'm not given the drawings, I give the drawings. Typically, we show linear dimensions in feet, inches, and 16ths, show plate thickness as decimal inches. With AutoCAD, you can dimension in cubits or furlongs, doesn't make any difference.

4. Do you scribe locations using caliper jaws or measure and use an actual scribe when working with soft steel or aluminum?
I don't do the shop work, the shop guys do that.

5. Do you do much locating/scribing using tape measure?
I don't do the shop work, but yes, tape measures are used frequently.

 

[fegenbush]

1. Decimals. Decimal equivelants of fractions are often used on engineering drawings. When you get a drawing that says cut to drill a hole at 0.19. Will you drill it at 0.19 or 0.1875?

Since I do mostly structural drawings now, the drawing will show fractional inch measurements. Mechanical drawings will show the appropriate number of decimal places or fractional based on the desired tolerance from the standard tolerance block that are on my mechanical drawings.

2. Do you know all your fractions down to the 1/16ths? both 2 & 3 place decimals?

3/16 is 0.1875 which is 4 decimal places. I am familiar with and regularly use down to 1/32nds in fraction and decimal form. I also work in decimal feet as necessary.

3. Would it be any simpler if drawings were given to you in fractions rather than decimals?

It depends on what you are trying to convey with the drawings. I suit the drawing to the job. Building a house has different tolerances than making the cabinets.

4. Do you scribe locations using caliper jaws or measure and use an actual scribe when working with soft steel or aluminum?

I use the caliper jaws to make a mark on the marking blue layer then go over with a scribe, prick punch, or center punch as necessary

5. Do you do much locating/scribing using tape measure?

It depends on the nature of the job. Some jobs require a scribe line, some require a sharp pencil or soapstone, while others may only require a magnum sharpie marker. I no longer do any layout work (other than hobby work), but I have done my fair share.

 

[ornerynorsk]

The decimal must be taken to the place where title block tolerances will allow function at either end of the tolerance spectrum. The drawing must reflect the correct design intent.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[Jboggs]

I've been designing machines for over 30 years and have developed my own standards of practice (the hard way). First - you're talking imperial inch measurements, so metric units aren't an issue.

Generally, fractions imply greater tolerance than decimals. In my mind they can be laid out with a tape measure and marked with a dull pencil. Also, I once had a machinist ask me to PLEASE dimension the size of drilled holes with the same units he would see in his drill index. If I want him to drill a 7/16" hole, dimension it as 7/16", not .44". That made sense to me (and helps prevent errors) so I try to do that.

The most common practice for default decimal tolerances I have seen is this:
fractions = +/- 1/32
x.xx (two place) = +/- .01
x.xxx (three place) = +/- .005

You don't do this for this long without learning all your decimal conversions, at least down to 16th's.

All experienced designers and machinists know when they see .19 on a drawing that it means 3/16" is the nominal intended dimension and the tolerance is .010. Literally it would mean .1875 +/- .010. You don't dimension it as .1875 because the four place decimal implies a VERY tight tolerance.

My basic principle is this: I use the simplest possible method to accurately communicate the real need with the least chance of error. If I don't require a tight tolerance, I try not to imply one. If I do require a tight tolerance I make sure it is clearly stated.

There is no hard-and-fast rule about using either fractions or decimals exclusively. Both are appropriate for their intended purposes. For example, I'll dimension a wooden frame using fractions. A steel weldment with some machined surfaces would be a decimal default. My typical part drawing might have both fractions and decimals on it - again depending on the end purpose.

One last thing - I try to base my decimal display on the real required tolerances. For example, most stock sizes and cut lengths are relatively rough and can be shown in two-place decimals. However, two parts with matching hole patterns would require that the hole-to-hole dimensions be shown in three-place decimals.

 

[SnTMan]

And whatever you do, somebody, somewhere will be unhappy

Regards,

Mike

 

[KENAT]

vonsteimel , what if any drawing standards does your company work to - that will answer some of your questions more definitively than a bunch of responses here from people in different industries with different levels of experience ...

For instance ASME Y14.5M-1994 lists millimeter and decimal inch dimensioning - it doesn't address fractional.

1. Decimals. Decimal equivelants of fractions are often used on engineering drawings. When you get a drawing that says cut to drill a hole at 0.19. Will you drill it at 0.19 or 0.1875?

Well, I rarely do any making of actual parts but I would first consult the tolerance on the .19 dimensions, then take into account my process capability and then I'd select the appropriate drill bit - be it 3/16 or maybe a number 12 or 11... depending on the factors mentioned and drill bit availability. Despite what some above imply there is not a universal standard tolerance block that I'm aware of which can be assumed to be in play. In fact on drilled holes it's common to see the values from old AND10387 or similar used rather than just referring to tolerance block.

2. Do you know all your fractions down to the 1/16ths? both 2 & 3 place decimals?

No, I'm OK but I won't claim to immediately know all of them off the top of my head though I can work it out pretty quick.


As to the other questions, like I said I'm not a machinist.

If you really care about the answers I'd be asking machinists and their associated manufacturing/production engineers - not necessarily most of the folk in this forum.

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[drawoh]

...

Generally, fractions imply greater tolerance than decimals. In my mind they can be laid out with a tape measure and marked with a dull pencil. Also, I once had a machinist ask me to PLEASE dimension the size of drilled holes with the same units he would see in his drill index. If I want him to drill a 7/16" hole, dimension it as 7/16", not .44". That made sense to me (and helps prevent errors) so I try to do that.

...

My Machinery's Handbook, Edition 26, has a table showing all the drill types mixed together and sorted by size. For example, if I want to drill a Ø.200" hole, I need a Ø5.1mm[ ]drill, or a #8[ ]drill. I wonder why it took so long for someone to think of this.

Don't forget that drills cut slightly oversize. If I am really serious about that Ø.200", the I will be tempted to use a #9[ ]or a Ø5.0mm[ ]drill.

--
JHG

 

[fegenbush]

drawoh,

If you are really serious about 0.200" diameter, you should use a #10 drill (or smaller) and ream the hole, but I digress...

I agree that calling out drill sizes on shop drawings is ideal *IF* the preparer of the drawings is familiar with the subject. If not, it can have some costly consequences.

KENAT,

The tolerance block that I am familiar with is the one that I put in the title block of every drawing. It has a specific note that the tolerances in the block are in force unless noted otherwise.

 

[KENAT]

"The tolerance block that I am familiar with is the one that I put in the title block of every drawing."

So if the .19 is specified on your drawing great, but from the information given so far you don't know that your standard tol block and the OP's standard tol block are the same.

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[dvd]

To my way of thinking the word "tolerance" in the open setup world is what is displayed (briefly) to someone who can't make parts to size. Most open setup shop hands try to hit all dimensions and work around over/undersize problems.

If dimensions are listed as fractions it implies using a tape/rule to measure.
I disagree with scribing lines off of a tape - use a layout rule.