Author Topic: Clearance Holes  (Read 10964 times)

Offline vcutajar

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Clearance Holes
« 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

Offline peatoluser

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Re: Clearance Holes
« Reply #1 on: January 06, 2014, 06:08:19 PM »
I generally use a drill 0.2mm larger than the nominal thread diameter for small sizes, but there's no hard and fast rules about clearance sizes. A cylinder cover with 10 studs might need larger clearance holes just to be able to fit it over them without it binding, whereas a cover with just 4 studs, 0.2mm might be just fine.
interestingly, commercial counter bores , for socket head screws often have a guide dia. 0.5mm bigger than the nominal thread dia. , so clearance hole has to be at least 5.5, 6.5 etc. 

if your studs are binding in the holes and commercial rod isn't, it sounds like the threads are not square with the stud dia.

yours

peter

Offline Jasonb

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Re: Clearance Holes
« Reply #2 on: January 06, 2014, 06:18:50 PM »
Or more likely the commercial stud is undersize as it often is.

J

Offline Ramon

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Re: Clearance Holes
« Reply #3 on: January 06, 2014, 06:20:15 PM »
Yes, agreed, (edit - with Peter) and you'll probably find Vince that your studding is slightly under 3mm dia too which might be another reason that your studs were binding on the Kiwi.

As Peter says the actual clearance isn't that important unless its a 'fitted' bolt. Anywhere between 0.2 - 0.4 should suffice on most things.

Just in case - If you don't have an accurate means of dividing both parts separately then drill the cover with the clearance hole first and use as a drill jig to spot the holes to a depth such that the lip of the drill takes out approximately one thread - much neater and easier to start the tap. I would still do this if you drill the tapping holes separately - it's always good practice to take out that first 'thread' before tapping and not after  ;)

Regards - Ramon
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Offline smfr

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Re: Clearance Holes
« Reply #4 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

Offline stevehuckss396

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Re: Clearance Holes
« Reply #5 on: January 06, 2014, 08:43:57 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


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.
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Offline vcutajar

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Re: Clearance Holes
« Reply #6 on: January 06, 2014, 08:54:49 PM »
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

Online Jo

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Re: Clearance Holes
« Reply #7 on: January 06, 2014, 09:19:31 PM »
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
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Offline Baron

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Re: Clearance Holes
« Reply #8 on: January 06, 2014, 10:07:10 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

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.

Best Regards:  Baron.

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Offline Ramon

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Re: Clearance Holes
« Reply #9 on: January 06, 2014, 11:57:43 PM »
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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Offline Tin Falcon

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Re: Clearance Holes
« Reply #10 on: January 07, 2014, 12:44:39 AM »

Offline Hugh Currin

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Re: Clearance Holes
« Reply #11 on: January 07, 2014, 12:59:03 AM »
Crew:

Let me then ask a really stupid question. If nothing else it will prove Vince's "silly question" is a brilliant inquiry. I also hope it doesn't draw the thread off into a black hole. Please forgive me if my question is too stupid. With this preface:

What is the purpose of a threaded stud, and what do they look like? Are they fully threaded or, by definition, do they have an un-threaded portion in the middle? Are they more akin to a bolt or a shoulder bolt? i.e. Do they mainly hold two parts together or do they locate the parts?

If they simply hold two parts together, like a bolt, the clearance should be like a bolt? That's a fairly "loose" fit and there are tables for clearance holes. I think there are specs for close and free fits, but both leave some wiggle room. (Google "bolt clearance holes").

But if they act as shoulder bolts for location, like locating pins, the clearance is much tighter and dependant on the use. I don't think this is valid unless a counter bore is made into the threaded hole part, so the un-threaded portion of the stud extends partially into this counter bore. This would accurately locate the stud. Then, if the clearance hole in the second part is also a "good" fit to the un-threaded portion of the stud accurate location can be achieved. For this condition I'd think the tolerances for all the parts, and hole locations for a bolt pattern, need to be know to run numbers. I think this is what Jo was eluding to. For our use, I'd think the start small and increase till it fits is best.

So, please inform a beginner. What does a "threaded stud" look like. And are they for accurate location or just hold things together like a bolt?

Thank you for your patience.

Hugh
Hugh

Offline tel

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Re: Clearance Holes
« Reply #12 on: January 07, 2014, 06:54:21 AM »
If a stud wasn't threaded it would be a dowel.
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Offline Ramon

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Re: Clearance Holes
« Reply #13 on: January 07, 2014, 08:17:20 AM »
Hi Hugh,

A stud is a length of rod usually of standard diameter threaded at both ends with right hand threads. Usually one end - the end that goes into the casting - is threaded shorter than than the other end. I don't actually know what the defined  technical reason is over using them instead of bolts but obviously once fitted and nutted up a lock nut can be used if required. Covers on full-size engines are usually fitted with studs and not bolts.

Threaded rod is not an acceptable way (full size) of getting round the problem and is frowned upon. It can be used in a model particularly for very small sizes when holding material for studs for threading becomes difficult.

Unless a stud is 'fitted' ie the diameter accurately fits the hole for purpose of alignment then their sole purpose is that of a fastener. Clearance holes are usually the norm, end covers usually having a register to ensure alignment.

Here's a pic of some made for a steam chest - incidentally the bolt holding the cover is temporary


Hope that helps you understand a bit more - I'm sure others can add to it.

Regards - Ramon
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Offline Farmboy

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Re: Clearance Holes
« Reply #14 on: January 07, 2014, 09:46:53 AM »
In my experience, on older full-size machinery, studs were often coarse threaded (Whitworth/UNC) on the end screwed into the casting but fine (BSF/UNF) where the nut screws on. I always assumed this was the reason for their preference over bolts; fine threads not being generally used in castings.

I don't recall ever seeing a stud used for precise location, which is the job of a dowel.

Mike.

P.S. And it is much easier to locate the cylinder head on studs before fitting the nuts instead of wrestling with half a hundredweight of cast iron while trying to line up bolt holes  ;)
« Last Edit: January 07, 2014, 09:59:01 AM by Farmboy »