Tolarance for machining

[Yash16]

Dear All,

A simple question but always I ger confusion with this.

If I given a tolarance on shaft Dia. 10 -0.030 & hole Dia. 10 +0.050/+0.030 then the machinist will make the job at higer tolerence of hole and lesser of shaft?

Yash

 

[ornerynorsk]

It is customary to maintain dimensions as close to the nominal dimension as reasonable. That being said, you may fully expect to receive a part that falls anywhere within the specified tolerance range. Reading between the lines on your post, the dimensions need to be specified so that the part you are designing will actually work in its intended application, regardless of where it falls within your specified tolerance range.

 

[KENAT]

Probably not.

The part is acceptable anywhere within the tolerance range.

As such, machinists typically aim for somewhere in the middle of the tolerance range to allow for manufacturing capability tolerance.

So, your shafts will probably tend to be around 9.985 & your holes around 10.015.

This will vary slightly depending on manufacturing process. For instance if there is a standard tool size toward one limit, they may use that standard tool.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

KENAT,

On a one-off job, a machinist's pride is at stake. If the tolerances are fairly loose, a lot of them will make the features as close to maximum material condition as possible.

JHG

 

[KENAT]

There are all kinds of situations, manufacturing processes etc. where there will be a trend, I agree Drawoh.

However, I guess my point is you shouldn't rely on it when specking the tolerances. You should specify the tolerances required by function - as ornerynorsk mentions - so that any part in those tolerances is good.

If the Op really wants the shaft nearer 10 then he should specify 10-.001 or something, and accept the consequences.

I realize it's not quite this black and white sometimes but trying to rely on general trends rather than explicit requirements can be risky.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MintJulep]

You can't count on the hole and the shaft being made by the same machinist, or even the same machine shop.

 

[SnTMan]

"hole Dia. 10 +0.050/+0.030"

I hope not

 

[SteveMartin]

My experience so far has been that manually controlled machining will skew closer to MMC on average. Basically the machinist is taking away material slowly until the part is within acceptable tolerance and then stopping. Computer controlled maching on the other hand is generally programmed to the middle of the tolerance band and expected that the variation will be centered plus/minus thus resulting in the most possible good parts. The comment above toward favoring standard tool sizes is important too.

However, I wouldn't assume any of this for designing the tolerancing to a part even if I knew everything possible about how the part is to be machined. Assume the part will come in at the worst extremes of tolerance - because those conditions still have to work.