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Author: Subject: A question for those in high precision Engineering

posted on 20/10/22 at 10:57 PM Reply With Quote
A question for those in high precision Engineering

Let's test the knowledge of LCB, you guys know everything!

I have a shaft with a diameter 3.16mm tolerance G7/f6 with its interfacing hole.

a proposal of widening the tolerance by 4um and reducing the max dia by 8um. The 4um slips it to a f7 but the 8um? This slips outside an ISO. I'm not happy with that, right?

Am I missing something.

I know the info has no context, this is enough though.

[Edited on 20/10/22 by bi22le]

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posted on 21/10/22 at 08:00 AM Reply With Quote
Don't necessarily need context, but I'm not sure exactly what you're asking.

Bear with me, the morning coffee hasn't yet taken effect, but this is as I see it.

The f6 shaft is -10/-18 micrometres, the G7 hole is +16/+4 micrometres. Together, these provide a maximum clearance of 34 micrometres and a minimum of 14 micrometres.

When you mention widening the tolerance of by 4 micrometres, I assume you mean the shaft only? And yep, that would take it to f7, which is -10/-22 micrometres.

When you mention increasing the maximum diameter by 8 micrometres; what does this refer to - I assume you mean the hole, as the shaft now aligns to f7? This would give +24/+4 micrometres tolerance, which is kinda close to an F7 (+22/+10) but doesn't align to ISO.

Resultant min/max clearance from these changes would be 14/46 micrometres, so a bit slacker on max clearance than the 14/34 originally spec'ed.

Again, I can't judge as I don't know (nor really need to know) what it's for, but it comes down to the design intent of the joint and whether that additional play may have negative consequences.

In terms of deviating from the ISO standards, I try not to do it - it usually just causes confusion, and may require special tooling be made. But if there's a good reason that can be justified, you shouldn't feel bound to stick to them blindly.

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posted on 21/10/22 at 09:05 AM Reply With Quote
I can't comment/advise on this in any way as it's not something I'm familiar with or in my area of knowledge.

Curiosity strikes me to ask though, are you able to share what the shaft and hole are for, materials, purpose etc?

Understand if it's irrelavant to your question and you'd rather not get into that side of things.

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posted on 21/10/22 at 04:17 PM Reply With Quote
Without context, the question is pretty moot. If the design requires it, there's no reason not to deviate from ISO fits. We'd at least need to understand the justification for the tolerance changes.

If you're not working with nominal sizes (unless your 3.16mm is undersize 1/8" ) , it'll drive custom tooling anyway unless it's all single pointed.

edit: to remove winky smiley on 1/8"

[Edited on 21/10/22 by indykid]

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