Tolerances and Clearances

[kenw]

"....trying to determine why Toyota transmissions lasted longer than Ford units even though both were assembled with parts in spec..."

the common assumption is that parts built "to spec" are somehow good parts. my 30 years in manufacturing and design proves otherwise.... being "in spec" guarantees nothing.

The guy on the floor can't build a silk purse from a sow's ear.

My motto: If it can be assembled wrong, it's most likely a bad design.

 

[Shannow]

Funny when you see the way that the 1955 small block was built, allowing high tolerances but using select fit to achieve desing clearances, while Japanese engines rely on tolerance control, allowing interchangeability.

Sometime it would be nice to be able to get a 0.001", 0.002", or 0.003" oversized piston.

 

[Steve S]

This subject is clearly intolerable.

 

[Steve S]

quote:

---

Originally posted by Shannow:
Funny when you see the way that the 1955 small block was built, allowing high tolerances but using select fit to achieve desing clearances, while Japanese engines rely on tolerance control, allowing interchangeability.

Sometime it would be nice to be able to get a 0.001", 0.002", or 0.003" oversized piston.

---

If the machinist would keep his work within the recommended tolerances we would not need the above mentioned sizes.

 

[Steve S]

quote:

---

Originally posted by kenw:


My motto: If it can be assembled wrong, it's most likely a bad design.

---

An excellent motto.

 

[XS650]

quote:

---

Originally posted by Kestas:
To add some confusion, the bearing industry sometimes refers to preload as 'negative clearance'.

---

That would be because it is negative clearance if it's based on dimensional control.

 

[1sttruck]

When finding inadequate controls in a tolerance stack we would often call it 'negative margin'; some of the parts have to be better than perfect in order for the assembly to work :^)

**************************
XS650 said:

Originally posted by Kestas: To add some confusion, the bearing industry sometimes refers to preload as 'negative clearance'.

That would be because it is negative clearance if it's based on dimensional control.
**************************

 

[Jaybird]

quote:

---

Originally posted by Steve S:

quote:

---

Originally posted by Shannow:
Sometime it would be nice to be able to get a 0.001", 0.002", or 0.003" oversized piston.

---

If the machinist would keep his work within the recommended tolerances we would not need the above mentioned sizes.

---

I think that when we are talking mass production, as in the auto or motorcycle industry, we aren't considering what the machinist can hold, tolerance wise. But rather the tolerence that the automation tooling can achieve...with repeatablity.
The tooling can be desinged and built to hold the tight tolerances, but to have repeatablity, the machinery must have the same care and precision given maintenance wise, as was put into the intital design and build.

Of course the proper lubricants and application are crucial to the success of repeatable precision.

BTW...small bore motorcycle pistons do come in variable sizes, to account for the factory tolerence window.

 

[Steve S]

I didn't know that, The first M/C I owned was a Yamaha 125 twin I think it was a 1968 .I replace a lot of pistons in it, and only remember certain oversizes. That was a long time ago so it is a blur.

 

[hone eagle]

Jaybird hit right on the head-mass production-the perfect piston bore match has been known for 50,60 years or more its just doing it in 40 or 50 sec. thats tough.
Just for discussion sake the engine plant I work in used to have piston sizes from "1 to 6" then "a to f",now we are held to 2 sizes only and all in microns(havent used thou in over ten years),now a size change that triggers a piston grade change is 13 microns,and thats a lot less than a the 10 thou we used to use.It is also a interesting fact that 10 degrees of coolant temp change is one size-in the materials we use.

 

[1sttruck]

Selective assembly by matching parts is a traditional way of meeting assembly tolerances with lower precision parts. The decision may be due to lack of capability, but more often it's a cost decision as the assembly tolerances could require very expensive parts.

 

[kenw]

a machinist can hold (either in one shot or mass production) any tolerance you are willing to pay for.

You want super tight tolerances? Fine, he'll get close and polish to the final dimension. Don't bug him for being slow and prepare to pay for it. His time and the machining center costs the same $$$ per hour regardless of how much he takes in one pass.

 

[Dr Cpk]

Sounds like my kinda board!

Eliminating special cause my game...reducing process variation my fame...