Is the ISO fit G8/k5 combination allowed?

[Yuyu28]

My apologies if this is not the correct forum to post this.

I have a bushing of 8mm nominal OD that requires an ISO tolerance of k5 (8 +0.007/+0.001) to fit into a bearing; this is a given data. I am designing another clearance hole to house the bushing outside the bearing, and I am doubting between using a custom size tolerance range (for example, 8.02 +/-0.01), or the closest ISO tolerance for holes that can fit the bushing without interference. This would be a G8 tolerance (8 +0.027/+0.005), but I have never seen a G8/k5 hole/shaft combination. For that matter, neither G8 nor k5 seem to be common choices of fit tolerances.

So, what would be the preferred choice of size tolerance for a manufacturer:
a) 8.02 +/-0.01, or
b) 8 G8?

 

[dgallup]

If you use the G8 you can have a little interference at the max material condition. Is that OK? Tolerances all come down to design intent, only you know what that is. The manufacturer will pick a process that is capable of your requirement. Your choices will have cost implications.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Yuyu28]

dgallup,

I intend to design the hole with a clearance fit for the bushing. You're right that there's some interference at MMC; I can then reformulate the question with an ISO tolerance of F8 (8 +0.035/+0.013), but the combination F8/k5 still seems uncommon.

My question is focused on finding what a general manufacturer can do easier/cheaper for the same result (clearance of the hole). I don't know exactly which manufacturer my company will hire, so I have to make general assumptions in this regard.

 

[juergenwt]

For manufacturing use 8H7 (+15) for the hole. H7 reamers are mostly standard and easily available.
H7 is just a little finer fit than H8. You will not know the difference.
For the OD use 8f7 (8 -13-28). This will give you a +13+43 play between parts for a "FINE" running fit.
A H8 hole would just give it (maybe) a little more clearance but would likely require the purchase of special H8 reamers when you could just as well use H7 standard reamers.
Look here:
8 H7 0+15

8 H8 0+22

 

[pmarc]

It kind of amazes me how often people dealing with ISO limits and fits tend to forget that size is not the only thing to look at.

In this case, there has been no discussion up to this point that both holes should be controlled for mutual location (coaxiality) because this will also have an impact on the clearances in the assembly.

Also, if the documentation is per ISO, there has been no mention that in order to be sure that the features will assemble with clearances as intended and calculated, the envelope principle will have to be invoked on the drawings.

Just my $0.02.