LOCATIONAL HOLES

[engiNERD77]

Just out of curiosity, because I've seen it done many ways and it seems to be draftsman's preference. How do you call out a press fit hole for a dowel pin when you want to ensure the pin sticks out say a 1/4 inch. We would want the machinist to drill thru but to maybe ream to a certain depth depending on length of dowel and how much stick out we need. Is there a standard for this?

 

[drawoh]

engiNERD77,

I show a side view of the dowel pin with a toleranced dimension showing the height that I want. I call up dowel pins on my drawing, and I expect the machine shop to install them.



--
JHG

 

[KENAT]

Ditto drawoh - I may add a note about 'HOLE MAY/SHALL NOT BREAK THRU' or similar if for some reason I care.

i.e. dimension the finished part treating it as an inseparable assembly.

If you need to install the dowel pin for some reason then if you're only interested in precision reaming to certain depth with the initial hole continuing through I'd probably treat it as a counterbored hole, with depth of cbore given as a min - unless you deliberately want a stop to the pin.

e.g.

Ø.120 +.004-.001 THRU
C'BORE Ø.1245 +-.0004 DEPTH .19 MIN

(made the tols up on the spot didn't do the press fit calcs;-), and having brain fart on matching decimals)

Posting guidelines faq731-376

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

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

 

[JNieman]

I dimension it as if it were a blind hole, with the projection dimension toleranced accordingly. I add a note to the hole, something to the effect of "pilot drill thru" or "vent hole as required" but you have to keep in mind your intended audience.

If you think the person may use a hammer to pound a dowel into a blind, press fit hole, you can get a shattered dowel, or a high speed projectile, if the pressure builds up enough. Saying the vent hole is "as required" may be taken as an optional feature when it should be mandatory. I say "as required" to only indicate that I don't really care how big it is, so long as the dowel is functional as shown in the drawing.

Hell, if all the important machining is done from the opposite side, they may desire an OVERSIZED counterbore to a certain depth, then drill and ream the rest of the way for the press fit. I try to leave it open _when it suits the design_ to let people make the part as cheaply/quickly as possible, within my requirements. But... without a supplier or shop relationship you're comfortable with, you have to go the 'legal eagle' route like KENAT specified quite clearly.