[PSUBS-MAILIST] Jon's new penetrators

Mon Feb 15 15:04:47 EST 2016

I should add that precision metrology opens up a whole can of worms with respect to reportable accuracies. Where I work, we have a calibration lab which is temperature and humidity controlled, and any instruments going in for calibration must remain in the room for 24 hours prior to any calibration measurements being taken, in order to equalize with the room and keep consistent records of performance against the calibration standards. The moment you take, for example, a caliper, out of the cal lab and onto the shop floor, you have already introduced error into your measurement before even seeing your part. Such errors must be accounted for when checking supplied parts for correct adherence to drawing dimensions when tolerances are tight. I have definitely seen significant variations in measurements of a part immediately after delivery (exposed to cold on a transport truck), versus after sitting in the shop for a day or two to warm up. This may not matter for your parts and required
tolerances, but you should at least know that it doesn't matter for your application, versus just guessing.

It is of paramount importance in design not to specify tolerances which are tighter than actually required to put the part / assembly into service, because this can easily drive manufacturing costs into unreasonable territory quickly.  By the same token, specifying tolerances which are too loose can result in unusable parts or assemblies with reliability problems. Often, optimizing tolerances, taking into account tolerance buildup in a large assembly, is the most time consuming part of design.

Sean

On February 15, 2016 12:34:17 PM MST, "Sean T. Stevenson via Personal_Submersibles" <personal_submersibles at psubs.org> wrote:
>When fits really matter, as in high precision running machinery or
>critical transitional fits, a prudent designer will actually specify
>the part temperature at which the drawing annotations apply. For
>example, 20°C (normal / typical room temperature), or my preference,
>0°C (standard temperature / icewater bath), which allows a machinist to
>quickly bring all parts to a common reference temperature for
>measurement.  Even if that note is not used for manufacture, it
>provides a basis for rejection of the part during QC measurements when
>it is supplied close to a tolerance limit.
>
>Sean
>
>
>On February 15, 2016 12:14:45 PM MST, "Sean T. Stevenson via
>Personal_Submersibles" <personal_submersibles at psubs.org> wrote:
>>I typically specify fits using the ISO notation including tolerance
>>limits. For a penetrator, you aren't really concerned about location,
>>but your diameters will matter because of the O-ring sealing
>>tolerances. In that case, as a part fit I might specify something like
>>an H9/d9 fit, which you can look up the tolerances for on the basis of
>>the nominal size - certainly this would be sufficient for the
>tolerance
>>on the shaft diameter, but you might be forced to control more tightly
>>on the hole diameter in order to hold the minimum acceptable O-ring
>>compression in conjunction with the depth of the groove(s) on the
>>shaft. The selected fit class will also dictate the minimum / maximum
>>extrusion gap, which you may have to control. Are you using the Parker
>>O-ring guide as a reference? That should specify the required
>>tolerances. Just select a corresponding hole/shaft fit class that
>makes
>>it work out. As for temperature variation, I think you will find it is
>>negligible, but you can l!
>> ook up
>>the coefficient of thermal expansion for both materials and use your
>>maximum anticipated temperature range to calculate the delta - keep in
>>mind that both sides will experience approximately similar
>temperatures
>>in service, so you need only concern yourself with the dimensional
>>change of the completed assembly between temperature extremes. If it
>>turns out to matter, you rein in your tolerance limits accordingly,
>but
>>I suspect it will be a non-issue.
>>
>>Sean
>>
>>
>>On February 15, 2016 11:42:33 AM MST, Jon Wallace via
>>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>>
>>>I found a reference to it online.  Since these are non-rotating parts
>>>it 
>>>looks like Class 3 or 4 would work, but I suspect the snugness at any
>
>>>given moment of time will depend upon temperature with very close 
>>>tolerances.
>>>
>>>Jon
>>>
>>>
>>>On 2/15/2016 10:25 AM, Sean T. Stevenson via Personal_Submersibles
>>>wrote:
>>>>
>>>> Jon, do you have a copy of the Machinery's Handbook? Standard fits
>>>and 
>>>> tolerances for hole based and shaft based fits in nominal sizes in 
>>>> both inch and metric are covered. I can look it up for you on
>>Tuesday
>>>
>>>> if you like. Regardless, get a copy of that book if you don't have
>>>one.
>>>>
>>>> Sean
>>>>
>>>>
>>>
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>>
>>
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