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Clearance Holes

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stevehuckss396:

--- Quote from: vcutajar on January 06, 2014, 05:39:47 PM ---Hi guys

I have another silly question which might be obvious to everybody except myself.  Bear in mind that when I started this hobby I did not have an engineering or machining background.

Today I was drilling the clearance holes in the rear cylinder cover of the Corliss and was thinking of what type of fastening I should use for this model.  Studs or bolts.  I am sort of leaning on using studs but the last time I tried using homemade studs (on the Kiwi) the experience was not positive.  I could not line up the two halves of the Kiwi crankcase with my studs.  When I used threaded rod instead I had no problem at all to line them up.

Which brings me to my question which portrays my level of ignorance.  What should be the clearance hole for a 3mm stud??

I have always assumed it to be 3mm but now I am having doubts about it.

Regards

Vince

--- End quote ---


I don't do mm's except the candy coated ones but I typically measure the fastener and go .004 or so bigger on the drill bit.

#0-80 = .060 - 1/16 drill
#2-56 = .086 - .089 0r #43 bit
#4-40 = .112 - .116 or #32 bit

Can't remember the last time I used a #6 or above. I would suspect the alignment on the Die may have been off if the part wouldn't go on the studs. Misaligned tap might have the stud coming out crooked. Did you thread the studs in the lathe with a tailstock holder for the die? If not try one they are easy to make and do a real nice job holding the die straight.

vcutajar:
Thanks for all the replies.

So, as a general rule, if the studs are made from 3mm bar stock, the clearance hole should not be drilled with a 3mm drill but with something wider, like say a 3.2mm.

Vince

Jo:
Vince,

The guys are saying make sure that the holes are accurately lined up first  ;).

As for the size there are great long tables to help engineers get an appropriate clearance for any given diameter of shaft or stud: A 3mm +/-0mm dowel, is not a clearance fit in a 3mm +/-0mm hole. I personally drill them out to the correct size and then use a watchmaker's broach to take a fraction off to give me the clearance fit. If you don't have one of those then having first drilled it to the 3mm, try opening it up with a 3.1mm... a 3.2mm is a big first jump and they could be loose...

As Jason mentioned commercial studding is undersized which is why it fitted: if you started with 3mm bar and cut the thread the chances are that the burrs have raised above the 3mm diameter and that is why it is not fitting. A gentle touch with a needle file will take away those sharp oversized edges.

Jo

BaronJ:

--- Quote from: smfr on January 06, 2014, 06:30:22 PM ---Or you could clamp the parts together and and drill tapping size through the cover into cylinder, then open out the holes in the cylinder cover to clearance size. Don't forget to mark the cylinder cover if necessary so it goes back in the same orientation. Also, don't make the (easy) mistake of drilling clearance size into the cylinder  :hammerbash:

Simon

--- End quote ---

That is the technique I generally use if possible.  It saves having to make a replacement part because you've drilled a hole in the wrong place.  In my case often because I've lost count of the number of dial marks.

Ramon Wilson:
Hi Guys - just like to clarify why I said to drill the clearance holes first - in effect spotting the hole for the tapping drill.

It is always good practice to take out the top thread of any tapped hole. It is much better to do this before tapping as if done afterward  that small countersink, for that's what it is, will follow the thread and go eccentric. Taking out that first thread definitely makes the tapping easier and subsequently aids inserting the bolts later especially if they are in awkward to get at places.

Secondly and probably far more important, if opening out pre drilled tapping holes, particularly on brass or bronze there's a very good possibility that the part will snatch and pull up the drill if this op is carried out on the drill press, which in all probablilty it will be if tackling it this way. It wouldn't be the first time a tapped hole has been ruined in this fashion that's for sure  ;) - yes, you can stone the drill cutting edges but why do that only to have to resharpen.

When spotting through like this if the drill depth stop is set not only will the spotting depth be consistent but the tapping drill will centre nicely too - it can then be drilled to depth without fear of a mishap.

Only passing on a lifetimes habit - that's not to infer anyone else's method is wrong.

Good luck Vince however you go about it.

Regards - Ramon

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