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"Drilled" hole tolerance

D

dcrace

Aluminum
Joined
Jun 26, 2002
Location
U.S.A.
Ok.. been an ongoing conflict here at the shop about hole sizes. One of our guys here that inspects parts has a hole tolerance chart he uses for tolerance (attached for reference). Typically if a hole size is stated on the print we use the title block tolerance. Does the tolerance change based on if the hole dimension says "Drill Ø7/32" or if it is called out as Ø7/32.. Keep in mind, tolerance in title block for fractions is ± 1/64"

Thank you for any information.

Drill.jpg
 
It should always be on the print. If it has a tolerance callout for fractions then that is the tolerance. If that is not what they want then they should change the print. IF it's just the inspector wanting this he needs to be informed that parts are made to print, and asked where he got his chart from.
 
Don't know what the standards are but I'd say big difference between telling the machinist to use a 7/32" drill and actually calling out a 7/32" hole
 
Is this an old company? A lot of old prints used to call for fractional hole sizes with no decimal, or the fraction followed by some form of decimal tolerance (Ex: 1/4" +.001/-.0005). When those tolerances were not specified, it was because the tolerance was low, or the machinists were expected to know their fits and be familiar enough with the product to know if a dimension was nominal, under or over sized.

If his chart is something that the company gave him, it's essentially an extension of the print(s) that everyone should have access to. If it's something HE came up with... he should let it go or share it with you so everyone's on the same page. Even if he sees it that his dimensions are some kind of industry standard, everyone needs access to said standard because there are LOTS of standards out there and they are all "right" for how they are meant to be used. Either way, you can't make a part without all the needed information, and those parts can't be inspected with higher tolerances than the guys are making them to (unless the company just likes to make scrap parts). Inspection is about due diligence, not playing "who's micrometer is more exact."
 
A bunch of work I do has the standard tolerance block, and then a little chart that is similar to the one you posted. And it says Holes noted "drill". This isn't rocket science here. If the hole is noted "drill" it falls under the tolerance in the drill tolerance block.. If it is very clearly a hole that will be drilled, but not noted "drill".. It falls under the blanket tolerance.

Of course, a LOT of prints have a note to use some blah blah blah spec to interpret the drawing.. YOU NEED TO PULL THAT SPEC!!!!!!!!!!! You NEED to READ that SPEC!!!!! The note "Drill" could mean nothing, or it could mean something.


Of course the obvious answer here is to ask the inspector how he is determining the tolerance, which ones fall under his chart and WHY, and which holes fall under the blanket tolerance.
 
I liked to hold grinding closer than the print spec. Often the technique would make that as easy as being loose, so little or no time lost.
Drills with a pre-drill hole usually drill .0002 larger than the micrometer size...as long as the drill or the part does not get hot.
Drills can have .0003 to .001 back taper per inch so a shortened drill should be measured.
Drill starting their own hole can cut way big due to an off-center point.
 
Many of my prints specify that a fractional is also 1/64. 1/64 is in the tolerance block, that is what it is.

Fractions on technical prints [other than tap callouts] mean IDGAF.

Holding it to closer than the tightest standard tolerance is silly

Yes it is wise not to run under, but that chart is over tight.


wasting time fussing, wasting time checking, and oh gawd wasting time rejecting
 
Qt [Holding it to closer than the tightest standard tolerance is silly]
Agree that is the rule of thumb..but knowing a good customer is overly fussy and holding closer than spec will keep that customer happy can be good for the pocket book.
 
gustafson said:
Yes it is wise not to run under, but that chart is over tight.
Click to expand...

How does one get a hole or in fit under the size of a drill tip actual?
Done it more than once but confusing.
Drill a hole and then can not hand fit the same drill through it.
But,but,but it just went though that hole. Why don't you go in now on the bench?.
Happy Gilmore (5/10) Best Movie Quote - Are You Too Good For Your Home? (1996) - YouTube

Poasted here as a confusing question or big smile.
Darn thing made the hole, it should slide right through. But sometimes it does not.
Worse yet when using a pin to check and way under.
Bob
 
CarbideBob said:
How does one get a hole or in fit under the size of a drill tip actual?
Done it more than once but confusing.
Drill a hole and then can not hand fit the same drill through it.
But,but,but it just went though that hole. Why don't you go in now on the bench?.
Happy Gilmore (5/10) Best Movie Quote - Are You Too Good For Your Home? (1996) - YouTube

Poasted here as a confusing question or big smile.
Darn thing made the hole, it should slide right through. But sometimes it does not.
Worse yet when using a pin to check and way under.
Bob
Click to expand...

Then it should have been spec'ed differently

I get a group of parts, the first thing I do is look at them and figure out how to make them not work

Plenty of times .500 bore and a matching .500 boss both +- .005

Point is that is bad engineering.

Make holes over not under, but the listed rules of thumb are not fit for inspection, unless you have a reason to know different
 
BTW, what is a 7/32 drilled hole?

It can be about one thing

A clearance for a #10 screw

.218-.016-.005 positional tolerance=.197

Holding it closer is costing time and money
 
All I know is that a non toleranced "drilled hole" cannot always be considered a UOS because they may be controlled by specs of the issuer of the print.

Take the case of Sikorsky Aircraft specs as an example:

Part has 2 holes.
Hole #1 callout: "1/8 drilled hole"
Hole #2 callout: .125 dia through

UOS tolerance in title block:
.xxx - +/-.005
Fractional - +/-.015

What do you think your tolerance is for Hole #1 and Hole #2 ?
 
As for 'Drilled' vs diameter XXX. The company I work for does not like to put machining operations on a print. That limits what manufacturing processes can be used. So, it might limit or exclude suppliers from making parts. Thus, increasing the price of the part. Also, have to watch in dimensioning parts based on production process vs design need. Also, some manufactures have their own 'standards' that might not be provided on the print. I gets fun if they don't provide that information.
Also, as noted a drill can drill smaller than its diameter. I was told that is impossible. I've done it many of times, though. If the drill is dull, then it will heat up the material. So, the material expands when being drilled. Then it contracts when it cools down.

I'd add that some parts do require a specific process. It might effect the integrity of the part. Say, milled vs grinding. They would leave a different surface structure. So, the part might crack.
 
Thanks for everyone's input... it is a print from the 60's and definitely only specified "Drill 7/32" as thats how they drilled it in a fixture.. clearance for a #10 screw. This employee.. I call him our "YouTube Engineer" watches a bunch of videos and took a night class at the local high-school. Has good intentions for sure but lacks practical hands-on knowledge (Seems to be the norm these days).
We will stick to our tried and true method of following the tolerance block UOS, or if there is a particular customer spec we will follow that.
 
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