Why do designers do it?

[portlandron]

Please note I am not picking on this particular designer because I have seen others do the same thing.

What's with the dimensions/sizes some of them put on their designs?

Been working on a water pump that is part of a Sterling Engine.

It suggest the pump body be reamed and gives the final dimension of .376. Will I have 3/8 inch reams which are .375 why not just call for .375 and turn the pump washers a little smaller?

Also on the same pump it calls for pump rod of .156, granted if you were to polish a 5/32" (.1562) it would probably come out close to .156 but why not just call for a 5/16" rod. This rod goes through a gland nut that calls for a .158 through hole. Let's see there are #22 drills @ .1570 or #21 @ .1590 but can't find any .158 drills.

For experienced builders these are not major problems, more an announce, but for a newbie building their first engine this can and would be confusing.

So much for venting now back to building.

 

[Jasonb]

Its all a case of rounding up and how may decimal places or significant figures. You have even done what you are moaning about in your own post.

5/32 is 0.15625, you expressed it as 0.1562 , the designer chose to use 0.156"

There is also the convention that the number of decimal places is related to how tight the tollerance on the part should be.

J

 

[Deanofid]

This is a good reason to become familiar with the prints being used before making the first cut on a
new project. You will catch things like this often, and can make allowances for the tooling available
in your own shop.

I agree with your point about the new builder. New guy, or old head, we need to review before we start.
Solve tooling problems before we come to them.

 

[kvom]

Some people complain when drawings have fractions, others when they don't. ;D

In the .376" case, it's likely an indication that you need a .001 clearance for the piston or rod.

If a drawing has a .156" dimension with a +/- .001 tolerance, then it's clear that a 5/32 rod will work. FWIW, my current build has lots of 5/32 pins, and the #22 drill works just fine.

One issue with plans is that even when tolerances are explicitly given, a beginning machinist may not know how to read the plan correctly.

 

[Tin Falcon]

I have seen Rudy Kouhoupt use .12 instead of 1/8 or .125. sometimes close counts . typical the more decimal places the tighter the tolerance. There are just many ways of expressing the same thing whether in words drawings or dimensions. different designers are comfortable with different ways and the same designer may change his preferred ways over time.
It is all part of gaining experience and climbing the learning curve.
Tin

 

[ShedBoy]

Hit the nail oon the head there Tin. Studying plans and realising what sort of fit is going to be needed is essential especially when looking at some older plans which give you a dimension only, it gets even worse when you only have arangement picture with some basic measurements not individual parts. When I first started out I was driving myself insane before reading up on fits and tolerances. Imperial is really hard for a metric generation like myself.

Brock

 

[rleete]

I do it just to piss people off.

 

[Tin Falcon]

technical different ways of expressing a dimension imply different tolerances . I think that has been said.
1/4" would imply +- 1/64 Fractions
.25 +- .010 2 decimal places
.250 +- .005 3 decimal paces
Mating parts
.250 +.002 -.000 for inside

,250 +000 - .002 for inside

Unless otherwise specified.
So hope this clears any confusion.

All designers do not know and use this but it is convention. I worked for a model shop . the guys father was an architect. so he thought he knew how to draw and design. I was told to make parts the dimensions were given in fractions I used standard tolerance and was told my parts were no good. Sometimes you have to be a mind reader
Tin

 

[John S]

Please note I am not picking on this particular designer because I have seen others do the same thing.

Let's see there are #22 drills @ .1570 or #21 @ .1590 but can't find any .158 drills.

And I don't suppose that either drill will make a hole to those tolerances, welcome to the real world

John S.

 

[jonesie]

doing it to pi** people off. is ok but in the real world it is costly. i have been a moldmaker for 35+yrs and the job gets quoted according to the tolerances. the tighter the tol. means likely higher costs. it might mean the difference between a drilled and reamed hole or a hole that gets jig ground, big difference in time and cost.in the real world if you want to spend more money put the clearance holes for screws to .0000 then they can be bored instead of just drill 1/32 over or.03125.with the use of cad just change your settings or pi** people off and waste time and money. just my thoughts. jonesie

 

[steamer]

"Why do designers do it?"

Why indeed!

Well, Communication is the key isn't it.

"An Engineer's job is to provide documentation" That is a true statement.

If in doubt, or is critical, the size tolerance should be specified explicitly on the print.

.375 diameter +/-.001 for example.

Then the designer needs to keep in mind the standard tooling involved in the machine shops. Standard over and under reamers, end mill lengths..ect.

And oh by the way...it better not cost too much or take too long to draw up.

To say nothing of the lack of marketing input as to what the customer wants.

Then that pesky thing called science and physics have to accounted for...
Some of which can not be calculated with any accuracy and requires tests to be performed.

And it had better work the first time!...

Otherwise you, the designer, are a scumbum!.....

I've heard this story before....

I would comment that all these things are things we as Designers/Engineers need to pay heed to.

But

I would also suggest that Designers and Engineers are humans too.

There are literally thousands of decisions and details that can not be isolated from one another that need to be balanced to make a good design.

There are economic factors. Material costs...and end use realities that must be factored in.

So lets not cast too many stones....and read through the drawings before we cut. OK

That would be common sense now wouldn't it?

Lets not build Silo's or cast aspersions. That has never worked. For those that insist, I can assure you there are forums that have masterd that art and will provide a more satisfying experience.

Work together.

Dave

 

[jonesie]

well said. just need to take the time to know what each part does, and what the end result needs to be but for the newbie it takes a little more time to sort, but as a hobby what is time if you are having fun. jonesie

 

[Tin Falcon]

Precision is expensive and not always a good value!!!
Tin

 

[Tin Falcon]

Let's see there are #22 drills @ .1570 or #21 @ .1590 but can't find any .158 drills.

Drilling is a roughing operation. if you need tight tolerances of holes drill undersize and ream .
Tin

 

[steamer]

If you have 100K into a part. Ignorance is not bliss!

 

[shred]

A lot of it is CAD programs automatically slap the same tolerances on any dimension, critical or not, and/or round numbers to whatever the number of decimal places specified is. So 5/32 always comes out .156 and 3/16 comes out .188 or .187, depending on how they round.

Kind of a funny story, a friend sketched out an extension to his machine shop on his CAD system normally used for little CNC parts. He gave the drawings to his builder who came back a little later on and said "I think I'm going to have trouble holding half a thou on the framing" ;D ;D

 

[Tin Falcon]

Shred : I think you are right . with cad yoy pretty much have to set global dimensions on each part so you get either fractional . 2 decimal or three decimal. but you can change any and all if you like.
Tin

 

[steamer]

A good cad package will let the Designer /Engineer control all the dimensions to what ever they want....but they have to specify.

If you leave everything at 3 decimal places, that is what you'll get......right or wrong.

Dave

 

[rleete]

doing it to pi** people off. is ok but in the real world it is costly. i have been a moldmaker for 35+yrs and the job gets quoted according to the tolerances.

In case you were wondering, I posted that as a snide remark to the OP. If I came on here and posted a thread "Why do machinists do it?", and painted all machinists with the same broad brush, people would jump down my throat. And rightly so.

I'm a designer. And, a damn good one judging from how I'm paid. Making a blanket statement about all designers is akin to making a statement about a race or a religion. It's insulting and more than a bit ignorant. It's wrong, plain and simple. That not one of the moderators saw it as such shows the bias on this, and other machinist forums.

Be careful before making any blanket statement about any group.

 

[steamer]

Rleete,

You miss my point entirely.

I too am and Engineer and designer. I too have heard it all...and I too have been on both sides of the print.

I would also say I'm pretty good based on my paycheck and the success of my products over the years

I've been dealing with being broad brushed by those who have to make the parts for far too long.... nearly 3 decades.

And its Bull)**@ !

That isn't to say it wasn't always deserved. I've seen some lulu designers....I know you have too.
As a result, the guys turning the crank get the short end...

My point of my post is just that . The trash talk is non productive. From either side.

It will only get better if there is communication. That is how this Moderator feels about it.

Nuf said.