Author Topic: Clearance Holes  (Read 10961 times)

Offline Baron

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Re: Clearance Holes
« Reply #30 on: January 07, 2014, 08:55:31 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.

I countersink before tapping the hole.  I use "Weldon" type countersink tools.

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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.

I've been caught several times with the drill grabbing the work particularly with larger drills 5mm or so.  Smaller drills tend to break.
A few years ago I had this happen when I was drilling disks made from 6mm aluminium plate.  Someone came into the workshop unexpectedly.  My attention broke for a fraction of a second and I eased the pressure on the drill.  The drill grabbed and the disk pulled out of the jig and spun round catching the first finger on my left hand, neatly slicing the top half of my finger off from the second knuckle right down to the nail.  Fortunately it missed the nail itself.  (Shudder)...

 
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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

Some good tips.  Its nice to learn others techniques.

 
Best Regards:  Baron.

I donít regret the things Iíve done, I regret the things I didnít do when I had the chance.

Offline tel

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Re: Clearance Holes
« Reply #31 on: January 07, 2014, 09:01:14 PM »
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the first bit of thread is not to full depth.

Erm, if that was the case the nut wouldn't go on?
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Offline Baron

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Re: Clearance Holes
« Reply #32 on: January 07, 2014, 09:03:40 PM »
I think most people buy the screws and cut them to the length required which if it were a bolt may remove too much thread.

:facepalm2: You might as well use threaded rod  :ShakeHead:

Jo

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Best Regards:  Baron.

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Offline tel

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Re: Clearance Holes
« Reply #33 on: January 07, 2014, 09:11:32 PM »
Realistically, you can make a proper stud in not much more time than you need to cut off and clean up a machine screw or bit of studding.
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Offline Tennessee Whiskey

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Re: Clearance Holes
« Reply #34 on: January 07, 2014, 09:32:18 PM »
Well in redneck terms; it's a bit of rod with threads on both ends and none in the near about middle,  ain't rocket science. :thinking: :old: :lolb:

Whiskey

Offline Hugh Currin

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Re: Clearance Holes
« Reply #35 on: January 07, 2014, 09:40:15 PM »
The thread is moving fast, and this now read a little out of date. But I'm gonna post it anyway.

The consensus is studs aren't normally used for precise location. This makes sense to me. So, the clearance holes for a stud should be the same as for a bolt. For a 3mm stud/bolt the clearance holes should thus be 3.15mm (close fit) or 3.3mm (standard fit) [google "clearance holes metric"]. I usually start with a close fit and open it up only if needed.

Why use studs? Already mentioned is the ease of assembly. I had a VW that used bolts to hold the wheels on, a pain even with those "small" tires. Also, battering of the threads may be a reason. The shank of a bolt can't extend down to the threaded hole, or you couldn't tighten it. A stud can be threaded up to the shank, up to the un-threaded portion. Thus, if the bolted-on part is jostled about it won't bung up the threads, just bear on the shank.

Also, critical threaded fasteners are put under significant tension to press the connected parts tightly together. Think of holding a head on an engine, the bolts must hold the head tightly to the cylinder. This leads to torquing the bolt near, or into, yield (permanent deformation). I don't fully understand why, but the "nut" yields before the "bolt" (if similar materials). If a bolt is used, the "nut" is your casting and hard to replace. If a stud is used, the nut is easily replaced. Some machines use studs and specify replacing the nuts upon each reassembly. This is irrelevant unless the tension is carefully controlled (torque wrench).

None of these seem overwhelming reasons for a model engine. I suspect the main reason for studs is in building scale models. And as Marv says, all aspects can be critical. I marvel at some of the beautiful scale builds here, but I doubt I'll ever have that much patience. I love seeing them though.

Thanks for all the great info and insight.

PS. Whiskey, I think they really are used on rockets. We'd have to check with Morton-Thiokol to be sure though. :-)

Hugh
Hugh

Offline mklotz

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Re: Clearance Holes
« Reply #36 on: January 07, 2014, 10:17:24 PM »
Marv
"Anality" has now been officially added to the MEM lexicon.

I'm flattered.  Even more so now that your research reveals that anality is a genuine word found in several dictionaries.  I had no idea.

Nevertheless, the success of this word-coining has emboldened me.  Given the opportunities afforded by this forum, I'm sure I can come up with some really stunning additions to the local lexicon.
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Offline tel

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Re: Clearance Holes
« Reply #37 on: January 08, 2014, 01:51:54 AM »
Marv
"Anality" has now been officially added to the MEM lexicon.

I'm flattered.  Even more so now that your research reveals that anality is a genuine word found in several dictionaries.  I had no idea.

Nevertheless, the success of this word-coining has emboldened me.  Given the opportunities afforded by this forum, I'm sure I can come up with some really stunning additions to the local lexicon.

Hey Marv, we got a whole country full of 'em here that I would be willing to let you use (for a modest licensing fee, of course)
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Offline peatoluser

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Re: Clearance Holes
« Reply #38 on: January 08, 2014, 05:48:20 PM »
I've always believed that  - in full size practice anyway - studs where preferred to bolts in high load situations because when you tightened a bolt you put a twisting load as well as an axial load on the bolt and all this was taken by the threads in the hole and bolt. whereas when a stud is used , the load is purely axial and shared by the threads at both ends. The bit I don't understand is, I believe, that if you reduce the plain part of a stud to the core diameter of the threads it's actually stronger.
I'm no engineer, so if I'm wrong I really would like to be corrected.
personally , I've always thought the biggest factor in tightening a bolt and/or nut is the bloke in overalls with a lump hammer in one hand and flogging spanner in the other, and not the choice of fastener!

peter

Offline Admiral_dk

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Re: Clearance Holes
« Reply #39 on: January 08, 2014, 08:59:14 PM »
Ever since my teens, I've hated studs - the thread on the exhaust studs on for instance a Puch mopeds always disappeared after a few years  >:(  :wallbang:  :cussing:

But if you could get them out and put bolts in instead => no more trouble  :cartwheel:  and the same has been the case on many of the motorcycles I've worked on  ;D

That said - I would prefer studs on very heavy equipment ..... as others has stated, it makes it easier to assemble.

Offline tel

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Re: Clearance Holes
« Reply #40 on: January 08, 2014, 10:50:19 PM »
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Ever since my teens, I've hated studs - the thread on the exhaust studs on for instance a Puch mopeds always disappeared after a few years

 :Lol: Blame the material, not the stud! Make a set out of stainless or bronze and they will last forever and a bit.
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Offline Steam Haulage

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Re: Clearance Holes
« Reply #41 on: January 09, 2014, 07:59:43 AM »
Whlle we're talking about strength and accuracy as well as reproducing full size practice in a model engine has anybody considered the use of these? http://www.helicoil.com.sg/.

I have worked in a shop where Helicoils are used in aero engines. You will see on the linked site that there is more than a handful of specs. for various applications.

Jerry

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Offline Ian S C

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Re: Clearance Holes
« Reply #42 on: January 09, 2014, 01:03:42 PM »
We used Helicoils mainly on exhaust studs on aircraft engines, mostly Continental IO 470/ IO 520.
One person in NZ had an engine failure because of a Helicoil,  he rethreaded the sparkplug hole with a stainless coil, instead of a bronze one as specified,(single cylinder Robin engine, on an Ultra Light aircraft), cooked the sparkplug through lack of cooling.   Ian S C

Offline Steam Haulage

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Re: Clearance Holes
« Reply #43 on: January 09, 2014, 01:38:03 PM »
Ah, spark plugs! Helicoils come in StSt and Bronze. (follow the link above.)
Although I didn't specify, (my mistake) the ones I meant were of the Suck-Squeeze-Bang-Blow type. In some of these engines flanges are connected using 91+ studs or some other odd number with an H-C or equivalent in every one. It's quite possible you have been conveyed using them perhaps for several thousand miles.

Jerry
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Offline Admiral_dk

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Re: Clearance Holes
« Reply #44 on: January 09, 2014, 04:46:50 PM »
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Blame the material, not the stud! Make a set out of stainless or bronze and they will last forever and a bit.

That's right Tel - I can only whole heartedly agree the that originals where crap and the material was to blame, but stainless might not have worked with the rest (or it might), not all materials work equally well together. But the fact was that we had easy access to cheap bolts that never failed and the studs always did, no matter what brand of bike.
I suspect the one of the main reasons where that we always have our roads covered in salt in wintertime => a stud + nut will be covered inside out in a corrosive solution that destroys them  :hellno: But using a bolt + washer => that almost nothing of the salt-solution would enter the thread in the cylinder = no trouble  :D

I've repaired a number of threads with Helicoils, but I can't see them do studs as a repair - sorry.

Ian - thanks for the warning about stainless Helicoils in sparkplug applications  :o