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doorknob
01-03-2011, 10:16 AM
I would like to drill holes for socket-head cap screws to permit the head of the screw to be recessed so that it is flush with the surface of the material that I am drilling.

I believe that one tool designed for making such holes is a counterbore bit.

For example, I'm considering getting the set of HSS counterbore bits shown at http://cgi.ebay.com/7-PCS-HSS-COUNTERBORE-SET-6-8-10-1-4-5-16-3-8-1-2-/330339490670?pt=LH_DefaultDomain_0&hash=item4ce9c49b6e#ht_1662wt_802

Although I have a general idea of their proper function, I'm not completely certain how to use them.

Do I drill a through clearance hole in one operation and then use the counterbore bit in a second drilling operation, using the small "pilot" end of the bit to center the bit before plunging in with the 3-flute drilling/reaming portion of the bit?

I believe that I have seen counterbore bits that have cutting edges for the pilot section as well as for the socket-head-cutting section. The photo in the eBay listing link above is a little unclear - are the edges of the pilot section used in those bits smoothly-ground or do they have cutting edges?

Is there an alternative to using these bits to do counterboring for socket-head cap screws? For example, do they make reamers with pilots, or are these bits essentially just reamers with pilots?

These will work well in 6061-T6 aluminum, right? Guidance on what rpm to use with these bits in aluminum would be appreciated. Do I need to use a cutting fluid or can I get away with using them dry? Are these things foolproof, or are there tricks of the trade that will prevent me from messing things up?

I haven't ever used a reamer before (other than one of those hand-held pointy things). I suppose that the answers to my questions are buried somewhere in my Machinery's Handbook, but I'm lazy, sorry...

The smallest bit in this set is for a #6 screw. I'd like to find a counterbore bit for a #4 screw and also for a #2 if possible - any pointers would be appreciated (my search capabilities seem to be failing me).

That's probably enough questions for now, thanks... :)

dp
01-03-2011, 10:31 AM
This operation, more than most any other, brings out the bungie nature of my small mill. I've tried using end mills to face drill, spot mill, and counterbore and each time I seem to end up with a bent or broken mill. And I use collets.

What I do now is drill the central hole then drill part of the counterbore, then use a boring head to face the seat of the counterbore. And the reason I do this rather than use counterbore drills is I'm cheap! :)

The boring head is adjustable and does a good job regardless of the size needed (admittedly I've never needed a counterbore smaller than 1/4"). And that means I don't have to spend any money on counterbors of various sizes.

However - if I had to drill a lot of holes with counterbore, to save time I would buy the needed counterbore drill for the job. Until I get a more rigid mill and do some tests of spot drilling with mills, I'm done with that.

Mcgyver
01-03-2011, 10:41 AM
Maybe this'll get me 'posted' at the boys metal working club, but more often than not I just drill the counter bore. I know its not technically the correct way, that there's reasons why a flat bottom hole works better, but its not often its critical. when it does matters use a counter bore after dilling the proper hole for the pilot. Use at your lowest speed - its a big wide cut. Squirt of coolant wouldn't hurt either. Make them from drill rod if you don't want to buy them but it would only take a couple to cover most common sizes/ needs

brian Rupnow
01-03-2011, 10:44 AM
I use them all the time. First drill a clearance hole completely through the peice you are working on. (the clearance hole should match the diameter of the small end of the counterbore tool.) Without changing anything on the set-up position, remove the drill and put the counterbore tool in the chuck. Without turning the mill on, use the Z axis feed to lower the c'bore tool into the clearance hole untill the larger diameter stops against the surface of the part being c'bored. Set your Z axis feed readout to zero. Back the c'bore off a bit, turn the machine on, then advance it, untill the readout informs you that you have reached the required depth of c'bore. Remove c'bore tool, reinstall clearance drill, and then advance to next hole to be drilled and c'bored. this is kind of a slow, pain in the a$$ way to do it, but if you want you can drill all of the clearance holes first, then try and get the c'bore tool lined up with all the drilled holes "after the fact" which is even worse.---Brian

kf2qd
01-03-2011, 10:45 AM
First you drill the body drill sized hole for the screw you are using - that should match the size of the pilot on the end of the counterbore. Then you feed the counterbore the depth you want and you have a counterbored hole. If you have multiple holes you can set a depth stop on the spindle to have them all come out the same depth. Should work in most materials using the same feeds and speeds as a twist drill.

38_Cal
01-03-2011, 10:48 AM
Most counterbores I've had to cut have been small sizes...#6, #8 and #10. I have "official" counterbores for the two smallest, bigger than that I drill with a regular drill bit, then use a drill that I've hand ground to a flat bottom profile, essentially an un-piloted counterbore that starts in the previously drilled hole. Since I'm only cleaning out the drill point angle, it works for me.

David

becksmachine
01-03-2011, 11:23 AM
I don't know what the cost difference would be, but the interchangeable pilot version is more versatile as compared to the solid pilot version shown in your link.

Dave

Rosco-P
01-03-2011, 11:27 AM
but if you want you can drill all of the clearance holes first, then try and get the c'bore tool lined up with all the drilled holes "after the fact" which is even worse.---Brian

That's why counterbores have pilots. If you have the part loosely clamped in the vise, the part will self-align on the pilot, at which point, you can tighten the vise and counterbore to depth. Counterboring is routinely done on a drill press or gang drill, doesn't have to be done on a mill.

OP might consider buying one of these books: http://cgi.ebay.com/Technology-Machine-Tools-/380304211442?pt=US_Texbook_Education&hash=item588be5bdf2
Can't beat $5.07 for a $50 college level machine shop text. Much easier to have a reference to tools, machines and techniques at hand, instead of having to rely on getting a speedy answer from a BBS, every time a question comes to mind.

GadgetBuilder
01-03-2011, 11:36 AM
For occasional use I make counter bores from drill rod. They work well and are easy to re-sharpen if you make the guide pin an easy press fit.

Directions are here: http://www.onlineclockbuilding.com/

Go to "Articles" in his menu, then get the pdf on "Pilot Cutter".

Use as you describe, where the initial hole should clear the pilot pin size. Use WD40 or similar for aluminum.

John

SGW
01-03-2011, 11:42 AM
The few times I've needed a counterbore, I've used an appropriate-diameter end mill, or made the counterbore from drill rod. I think I do have a couple of "real" counterbores, somewhere. If you use an end mill, it may have to be an odd size (something-32nds or something-64ths), but I figure an end mill, even an uncommon size, is more generally useful than a single-purpose counterbore.

If your counterbore has a pilot, allow a few thou at least for clearance so there is no danger of the pilot seizing in the hole.

doorknob
01-03-2011, 01:23 PM
Lots of good info here, thanks guys...

Although I've taken two machine tool technology classes at a local school, they didn't go into nearly enough detail on topics such as this one, and I'm lacking in practical application knowledge and experience for the things that they did cover. I appreciate the book suggestion and will look into it...

doorknob
01-03-2011, 01:31 PM
Enco does list some counterbore bits for #2 and #4 screws, but they have two flutes instead of three, and they are advertised as being for fillister head screws instead of for socket head cap screws.

I believe that both fillister head screws and socket head cap screws have a round head. So my first guess as to the possible difference is that the standard diameter for the heads of #2 and #4 fillister head screws must be different from the diameter of standard socket head cap screws, hence different bits for different screws.

Am I missing any other possible difference?

The bits are here:

http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMKD&PMPXNO=942634&PMAKA=368-0002

and

http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMKD&PMPXNO=942636&PMAKA=368-0004

Erik Brewster
01-03-2011, 01:53 PM
I use a 1/4-20 sized counterbore quite a bit in aluminum. I just drill the clearance hole (9/32"?) and then counterbore on the drill press. Just set the depth stop before counterboring and have at it. I would use 1/2 of drill speeds as a guess, but honestly, I don't even calculate it - I just use whatever the drill press was set at. I've never had so much as a chatter.

brian Rupnow
01-03-2011, 02:53 PM
[QUOTE=Rosco-P][I]If you have the part loosely clamped in the vise, ------------I see a tremendous potential for disaster!!!

lbhsbz
01-03-2011, 02:53 PM
I just bought a set of these....and I don't like them.

The "clearance hole" needs to be a bit on the large size to accept the counterbore pilot. For example, I use a 3/8 (.375") clearance hole for a 3/8" fastener. The counterbore cutter for a 3/8 socket head cap screw uses a 13/32 or 7/16" pilot. When I'm putting effort into doing something with some level of precision, it pisses me off to have a bolt flopping around in the hole like that.

in a side note, a quick way to counterbore multiple holes to exactly the same depth is to drop of piece of roundstock in the hole before counterboring...the roundstock will act as a stop so you don't have to pay attention to measuring anything.

brian Rupnow
01-03-2011, 02:55 PM
...Damned double post!!!

Mcgyver
01-03-2011, 04:00 PM
[QUOTE=Rosco-P][I]If you have the part loosely clamped in the vise, ------------I see a tremendous potential for disaster!!!

I took his note to mean, at least i hope it meant, that the part is held in the vise which is floating, unfixed on the table. SOP if the cut is small enough. I can't ever see one wanting the parts loosely clamped in the vise :eek:

Boucher
01-03-2011, 04:47 PM
I have some old good quality counterbores that work great. ie the Pilot is the correct size for the clearance hole. The newer ones that I have are like lbhsbz says have too large of a pilot. I intend to turn them down or chunk them if that proves too dificult.

Carld
01-03-2011, 05:34 PM
You have to check the charts but for most bolts the drill size is 1/32" bigger than the bolt size. If you measure the pilot size of a counterbore cutter you will find that to be true.

For a 3/8" bolt you use a 13/32" drill and the drill will cut oversize enough to make the pilot run freely. You DON'T use a 3/8" drill then use a counterbore cutter for a 3/8" bolt. The pilot will bind in the hole.

when drilling and counterboring you don't drill all the holes and then go back and cut the counterbores. Even with a DRO to relocate over the hole it is not the best way. That has issues of alignment and wastes time. You drill the hole and change to the counterbore cutter and cut the counterbore without moving the table then go to the next hole.

brian Rupnow
01-03-2011, 07:11 PM
You have to check the charts but for most bolts the drill size is 1/32" bigger than the bolt size. If you measure the pilot size of a counterbore cutter you will find that to be true.

For a 3/8" bolt you use a 13/32" drill and the drill will cut oversize enough to make the pilot run freely. You DON'T use a 3/8" drill then use a counterbore cutter for a 3/8" bolt. The pilot will bind in the hole.

when drilling and counterboring you don't drill all the holes and then go back and cut the counterbores. Even with a DRO to relocate over the hole it is not the best way. That has issues of alignment and wastes time. You drill the hole and change to the counterbore cutter and cut the counterbore without moving the table then go to the next hole.
Well CarlD---I'm glad that someone agrees with me-----Brian

Mcgyver
01-03-2011, 07:37 PM
when drilling and counterboring you don't drill all the holes and then go back and cut the counterbores. Even with a DRO to relocate over the hole it is not the best way. That has issues of alignment and wastes time. .

I agree a tooling change is probably quicker than coming back to the position....but I'm going to call you on making alignment for conterboring an issue on mill. There's more than enough flex in the counterbore even if you're a thou or two out and counterboring is hardly precision stuff.

Brian, the confusion is two different things are being discussed, if the work is fixed in position (mill or drill) by all means drill then counterbore without disturbing the setting, but so much of this work is done via centre pop marks on the drill press (from layout work or transfer punch) in which case, when safe, the work is not clamped but floats either in a vise or on the table (note i said when safe, when in doubt clamp). In those cases the pilot will absolutely do its job and cause counterbore to follow the drilled hole.

oldtiffie
01-03-2011, 08:04 PM
My metric set of counter-bore cutters are a PITA as the counter-bore(d) diameter is very close to the diameter of the head of a standard metric hexagon-headed socket screw.

Screws as not meant to be dowels of "fitted bolts/screws". They are primarily for clamping - not location - and the holes for the heads and shanks of bolts/screws should be larger than the nominal sizes to provide clearance and to allow some clearance and "float" during assembly.

(See Machinery's Handbook for "Clearance drills/holes").

I do what I've always done - drill a clearance hole (say 1/64">1/32">1/16" - depending on the bolt size and the job) - for the shank of the bolt. Then I use those as pilots for the drill (say 118*) that is a clearance size for the head of the bolt so that its depth - at the pilot hole - is just over the length of the head of the bolt/screw. I use a flat-ended (included angle = 180* ie "flat") drill same size as the previous head-clearance drill and set my pedestal drill depth to just over the length of the bolt, drill on low speed, lots of cutting/tapping oil and I clean out the bottom of the (now flat) bottom of the hole.

Job done.

Those counter-bore sets are not much use if you need to have a washer under the head of the bolt.

Grinding (free-hand) a "flat" counter-bore facing drill is no harder than grinding a trepanning drill/bit for drilling holes in thin/sheet metal.

Dan Dubeau
01-03-2011, 08:12 PM
Brian, the confusion is two different things are being discussed, if the work is fixed in position (mill or drill) by all means drill then counterbore without disturbing the setting, but so much of this work is done via centre pop marks on the drill press (from layout work or transfer punch) in which case, when safe, the work is not clamped but floats either in a vise or on the table (note i said when safe, when in doubt clamp). In those cases the pilot will absolutely do its job and cause counterbore to follow the drilled hole.

I agree %100

Quicky piloted counterbores can also be made from old drills of the appropriate size. chuck the drill up in a spindex, and grind a pilot with a surface grinder. a little stone work for the cutting edge, and clearance, and you're got a counterbore.

alu
01-03-2011, 08:19 PM
I too have one of the enco couterbore sets (I think it has counterbores for #4,6,8, and #10)

I found the pilot too large since I use the 'close' fit clearance hole for my fasteners usually. For instance a standard clearance hole for #8 is .177" but I prefer the .1695" hole size.

What would be the best way to turn down the pilot end? Either that or I might just cut the pilot off the tool. I just use closely sized endmills most of the time now anyways.

Regarding drilling all holes first and then using the counterbore tool (or endmill) after, that's the way I do it when using the mill with the DRO. If you change the tool in and out constantly, do you have to always re-touch off the tool and change your depth stop or zero your dro? Or do you just shove the tool as deep as it can go in the chuck (if you're using a chuck) so that you have consistent depth?

dp
01-03-2011, 08:33 PM
Y'all are reminding why I came up with using my boring head for counterbores. Here's my setup:

http://metalworkingathome.com/images/counter-bore-mill.jpg

The mill is smaller than the counterbore and is offset from 0/0 so it cuts a larger diameter using just one flute.

MaxxLagg
01-03-2011, 09:22 PM
And for those that don't know or haven't figured it out yet; the depth is the same as the pilot diameter, i.e., for a 1/4 socket-head cap screw the pilot it 9/32 diameter. The depth of the counterbore is also 9/32. This is a constant and it works for metric too.

J Tiers
01-03-2011, 09:47 PM
Well, ain't this a fine kettle of eels?

Som of yez hate them, some don't...... there's no pleasing these serpents.....

First, interchangeable pilot types are the best..... if you don't like the pilot size, MAKE ANOTHER. It's not hard.

The only thing I have against counterbores, which I happily use in the drill press, is that they do have a tendency to chatter sometimes. that's life.

I dunno about that self-aligning deal, I'm with CarlD, drill then counterbore. It's much easier on everything. But if you DO have to go back, just drop the pilot into the hole before you clamp down the part to "pick up" the alignment.

Tiffie's point about screws not being dowels is correct, but sometimes the "standard" clearance just isn't what you want, in which case having the ability to make and pop-in another pilot is great.

Use? Get set up so that the pilot is in position to drop into the clearance hole, start the motor, and drill to desired depth. Done.

oldtiffie
01-03-2011, 10:30 PM
http://www.engineershandbook.com/Tables/taphole.htm

http://www.ligo.caltech.edu/docs/T/T030118-01.pdf

http://www.google.com.au/#hl=en&source=hp&biw=1255&bih=469&q=bolt+clearance+holes&aq=6&aqi=g10&aql=&oq=bolt+cl&gs_rfai=&fp=42c33c885e5e233

becksmachine
01-03-2011, 10:35 PM
Well, ain't this a fine kettle of eels?

Som of yez hate them, some don't...... there's no pleasing these serpents.....

First, interchangeable pilot types are the best..... if you don't like the pilot size, MAKE ANOTHER. It's not hard.

The only thing I have against counterbores, which I happily use in the drill press, is that they do have a tendency to chatter sometimes. that's life.

I dunno about that self-aligning deal, I'm with CarlD, drill then counterbore. It's much easier on everything. But if you DO have to go back, just drop the pilot into the hole before you clamp down the part to "pick up" the alignment.

Tiffie's point about screws not being dowels is correct, but sometimes the "standard" clearance just isn't what you want, in which case having the ability to make and pop-in another pilot is great.

Use? Get set up so that the pilot is in position to drop into the clearance hole, start the motor, and drill to desired depth. Done.

Yes, what he said!
:D


Dave

oldtiffie
01-03-2011, 10:46 PM
Take the guess-work out of it.

http://www.stanford.edu/~jwodin/holes.html

http://www.ligo.caltech.edu/docs/T/T030118-01.pdf

dalee100
01-03-2011, 10:49 PM
Hi,

I used 'em and abused 'em, made them and broken them. I've ground too big pilots down and I've even sleeved one or two to make the pilot bigger.

But honestly, a high degree of accuracy isn't something that needs to be expected from a counterbore. Like oldtiffie said, bolts aren't a high accuracy locating device. So it's OK to allow some extra room for clearance.

I mostly use them in a drill press. And unless I only have one or two holes to counterbore or the part is simply too big maneuver easily, I do all the drilling first and then come back and do the counterbores. The pilot will locate the tool well enough to do the job right. That's what it's there on the end of the tool for. If I have 300 holes to drill and counterbore, I'm not going to do 300 tool changes. I'm only going to do two. This is how it's done for production runs. Time is money and tool changes waste both. So spend less time fretting about things that really aren't that critical. And spend more of it making chips.:)

dalee

Carld
01-03-2011, 11:05 PM
If I am using a drill press I may drill all the holes and then counterbore them as it is easier that way because of everything moving around and the quill stop on a drill press is not very accurate.

If in a mill I drill and counterbore each hole at a time. I have the button type stop on my quill so I set the counterbore in the hole and then use the allen bolt head to set the depth allowing a little slop so the head is below the surface.

It's just the best way I have found to do it and as I have said there are many ways to do something and everyone has their favorite.

I have made my own counterbore cutters out of a drill bit by grinding the end down to the size needed and then relieving the flats to do the counterbore cutting. If you have a surface grinder and spin indexer it's an easy job.

A counterbore cutter without a pilot will walk all over the work so be careful about removing the pilot. It's important to have the pilot a few thousandths under the hole size.

rustamd
01-03-2011, 11:09 PM
I preffer what is called Drill-Tap Counter bore, mostly get them from McMaster, look on McMaster page 2457, or for example PN: 29445A17

http://images2.mcmaster.com/Contents/gfx/small/29445a15p1s.png?ver=27007814

Leaves beautiful finish and does it chatter free.

toolmaker76
01-04-2011, 02:27 AM
Back in the olden days, before CNC was widely available, drill presses were used for drilling much more than they are now, and it would not be thought of to do any drilling at all on a milling machine (ties up the milling machine unnecessarily) unless something had to be super precise (made to be an interchangeable part).

The journeyman would lay out his blocks to be drilled and prick punched them, at which point the apprentice went to the drill press (think heavy duty) with them. I remember working on a stack of blocks at the drill press for days at a time. First you drilled your holes, smallest to largest (so if you made a mistake, you could always drill it larger). Then you went back with the appropriate counterbore and counterbored the all the screw holes- then you chamfered everything, and went back and tapped any other holes and reamed any dowel holes (in the tool steel blocks headed for heat treat- cold roll was reamed after it was installed). I remember spending weeks at a time on the drill press. Standard screw clearance was + 1/32" and the counterbore matched it, and also gave the screw head the same amount of clearance.

It wasn't until CNC became widely available that people did the majority of their drilling on the mill, because one of the benefits of CNC was easily interchangeable parts, or the ability to make mating parts that were able to be assembled once the parts were finished. Then most shops had an almost equal number of drill presses as mills- I have worked in shops recently that didn't even have a drill press.

I have a whole cabinet of transfer punches, buttons, and screws that haven't seen much use in the last 20 years for the same reason. In those days you laid out your holes (scribing lines) drilled them, and then transfered those holes to their mating parts. Don't see much call for a "Chicago screw" aka "Texas screw" these days either, for the same reason.

In any case, a counter bore is a throwback to those days. It was really meant for drill press work- not to say you can't do it in the mill, but it is designed to be self aligning to your clearance hole already in the block (block in a vise that was not bolted to the table). You could set the depth once and do a couple hundred holes with it.

macona
01-04-2011, 02:43 AM
I preffer what is called Drill-Tap Counter bore, mostly get them from McMaster, look on McMaster page 2457, or for example PN: 29445A17

http://images2.mcmaster.com/Contents/gfx/small/29445a15p1s.png?ver=27007814

Leaves beautiful finish and does it chatter free.


We use those at work. They do work nice, but two things. One they dont come in a #4, and two the clearance hole is a bit larger than I like.

luthor
01-04-2011, 05:24 AM
[QUOTE=oldtiffie]Screws as not meant to be dowels of "fitted bolts/screws". They are primarily for clamping - not location - and the holes for the heads and shanks of bolts/screws should be larger than the nominal sizes to provide clearance and to allow some clearance and "float" during assembly.

The designers at Boeing, Airbus, Pratt and Whitney and Rolls Royce etc would disagree with statement as this is exactly what close tolerance aircraft screws and bolts are used for.
I have fitted many close tolerance screws and bolts into aircraft, aircraft engines, landing gear and other aircraft components where the screw or bolt not only held things together but also acted as a "dowel" for location.

oldtiffie
01-04-2011, 05:49 AM
Those are the equivalent of the "fitted bolts" used in portal frames and the like - by Boiler-Maker Welders and in yachts etc. etc.

The bolts and screws we are talking about here are the common screws and bolts used in machine (including HSM) shops and used in the ordinary every-day way - nothing "special" at all.

coldformer
01-04-2011, 06:24 AM
Those counter-bore sets are not much use if you need to have a washer under the head of the bolt.
a washer in a counterbore? why?

oldtiffie
01-04-2011, 06:38 AM
Spring or "Shake-proof" washers and the like?

Black_Moons
01-04-2011, 08:24 AM
Iv seen spring washers under the heads of SHCS in counterbored holes before.

Carld
01-04-2011, 09:29 AM
There are washers and lock washers available that are no larger than the head diameter of an Allen bolt.

Ian B
01-04-2011, 09:36 AM
What's the application here? For a lot of jobs, simply using a normally ground twist drill to give the required clearance for the head would be sufficient. It would mean that the cap screw only makes contact on the outer rim of the head, but it may be good enough.

Ian

dalee100
01-04-2011, 11:26 AM
[QUOTE=oldtiffie]Screws as not meant to be dowels of "fitted bolts/screws". They are primarily for clamping - not location - and the holes for the heads and shanks of bolts/screws should be larger than the nominal sizes to provide clearance and to allow some clearance and "float" during assembly.

The designers at Boeing, Airbus, Pratt and Whitney and Rolls Royce etc would disagree with statement as this is exactly what close tolerance aircraft screws and bolts are used for.
I have fitted many close tolerance screws and bolts into aircraft, aircraft engines, landing gear and other aircraft components where the screw or bolt not only held things together but also acted as a "dowel" for location.

Hi,

It is perhaps more a matter of degree of locational accuracy. Standard clearances for bolts can be good enough to locate many, perhaps the majority of mating parts in the world, while holding them together. And they are used a such everyday. Those close tolerance fasteners used in aircraft and other assemblies are definitely better than standard. But bolsters and recesses can locate even closer yet. As can pins and bushings.

I suspect that aircraft designers developed and use those close tolerance bolts not because they are so good at locating or that there aren't better ways to do it. But rather because they are cheaper to design in and then cheaper to execute and use. While providing a "close enough" locational accuracy.

Again, as people who do machining, we often get too hung up on close tolerances. It seems to satisfy an OCD type streak that afflicts machinists. But a person really needs to step back and assess what accuracy level and tolerances for what features are really important. And clearance holes and counterbores aren't perhaps the best place to put a whole lot of effort as a general rule.

dalee

brian Rupnow
01-04-2011, 11:34 AM
I must agree with many others on here--The pilots on my store-bought counterbore tools are far larger than I like. A large clearance on the bolt hole makes for fast and easy assembly of mated parts, but it also makes things pretty "sloppy" in terms of squareness. If I had my preferences, I would see the pilots all smaller than what comes on "standard" counterbore tools. Since the pilot doesn't cut, only pilots, someday I may get industrious and set up a toolpost grinder (which I currently don't have) and take a few thou of the pilots on all my c'bore tools.

Mcgyver
01-04-2011, 11:50 AM
I must agree with many others on here--The pilots on my store-bought counterbore tools are far larger than I like. A large clearance on the bolt hole makes for fast and easy assembly of mated parts, but it also makes things pretty "sloppy" in terms of squareness.

I agree; its fine to say dowel the parts and bolts shouldn't be used to locate, but there's lots and lots of times with locating via bolts is plenty good enough. Since bolts are typically under nominal size, and twist drills drill a little larger, counterbores with pilots at nominal would be handy

dalee100
01-04-2011, 12:33 PM
I agree; its fine to say dowel the parts and bolts shouldn't be used to locate, but there's lots and lots of times with locating via bolts is plenty good enough. Since bolts are typically under nominal size, and twist drills drill a little larger, counterbores with pilots at nominal would be handy

Hi,

This true, and the standard clearances ARE good enough to do that kind of locating for the over-whelming part. And for those times when it isn't, there are other options that are available.

But if you must have smaller pilots, simply grind them down to fit your ideal. And you don't need a cylindrical grinder or even a tool post grinder to do the job. Heck, just chuck it in your lathe and take a dremel type tool and steady your hand on say, the tool post and have at it. The accuracy of the diameter of the pilot isn't that critical. If you get it anywhere within a 1/64" under the size you are shooting for, it's good enough. I've done it free-hand with a pedestal grinder. You aren't trying to locate off the head anyway, just the screw body itself.

You are making things far more complicated than they are or should be.

dalee

dp
01-04-2011, 12:39 PM
I have become a big fan of using tapered pins to locate parts. It gives me a reason to use up all my tapers and wear out my reamer, and it's got a lot of old school simplicity going for it.

Paul Alciatore
01-04-2011, 03:58 PM
I have an alternative way of doing counterbores. Instead of using a counterbore, I use the "bullet" style drills currently made by DeWalt.

These drills produce what is essentially a flat bottomed hole with a small pilot hole that is somewhat deeper, much like a center drill. The thing is, you use the bullet drill FIRST. Then you use your clearance sized drill.

I like to break the tips of the bullet drills by grinding a small 45* chamfer to reduce the stress riser in the hole at that point. These drills also usually produce a small radius between the flat bottom and the pilot hole and this is good as cap screws have a radius at this place also. This can save a step with a countersink if the clearance hole is very close to the screw size.

My sequence is:

1. Spoting drill (or center drill) to locate the hole
2. Bullet drill for the counter bore
3. Tap drill if the hole is going to be tapped
4. Clearance drill
5. Large size countersink to clean up the OD from the bullet drill

doorknob
01-04-2011, 06:40 PM
What's the application here? For a lot of jobs, simply using a normally ground twist drill to give the required clearance for the head would be sufficient. It would mean that the cap screw only makes contact on the outer rim of the head, but it may be good enough.

Ian


My immediate goal is to attach the base of a 1" length of 1.00" diameter round rod to a .125" thick flat member, using two (or possibly three, in a triangular pattern) #2-56 socket head cap screws to provide a strong mechanical attachment (and, to a lesser extent, to locate the rod in a specific position on the flat member).

The height of the screw head is not all that much less than the thickness of the flat member. If I were to use a normally ground twist drill to give clearance for the head, I'd be concerned that there would not be sufficient contact area underneath the screw head to give an adequately-strong mechanical connection. So, I'd prefer to drill a flat-bottomed counterbore hole for the screw head.

Mcruff
01-04-2011, 07:01 PM
[quote=Rosco-P][I]If you have the part loosely clamped in the vise, ------------I see a tremendous potential for disaster!!!
I don't, I have counterbored literally 10's of 1000's of holes and never had the part in a vise at all, I float them in. I do hold parts that I use a 1/2" capscrew or larger on but anything smaller I float them in. All you need is something on the left side, vise, stud clamped to table or something similar to capture the part so that it can't grab and spin
Standard counterbores are generally made in 3 flavors, on size pilot, 1/64" over screw size and 1/32" over screw size. I use 1/32" over only. Drill the hole, then stick the pilot in the hole run your counterbore around 1/3 to 1/2 the the drilling speed. To properly sink a CB you use the diameter of the screw plus the clearance, in other words for a 1/4" SHCS, the depth should be .281 deep, that .250 the screw body and .031 for the body clearance. This applies to all capscrews metric and standard. Use your quill stop and knee to set your depth.
I build injection molds for a living and every thing is counterbored, we've built 200lb molds that would have literally a 1000 screws in it. of varing size, from #5 to 3/8". Capscrews are not designed to locate parts thats what dowels and pockets are for, clearance is your friend. Once you counterbore the hole chamfer the top also, it looks much cleaner and more professional.

Evan
01-04-2011, 09:19 PM
The designers at Boeing, Airbus, Pratt and Whitney and Rolls Royce etc would disagree with statement as this is exactly what close tolerance aircraft screws and bolts are used for.
I have fitted many close tolerance screws and bolts into aircraft, aircraft engines, landing gear and other aircraft components where the screw or bolt not only held things together but also acted as a "dowel" for location

Close fitting non countersunk aircraft fasteners still do not provide absolute location. If the fastener can be inserted by hand it is still a clamp. The close fit is to minimize fretting when it moves, which it will. When a true hard location is required then a fastener such as the HI-Lok is used. A HI-Lok is designed to burnish the hole when it is driven in with a rivet gun or hammer. It is a mild interference fit. The locking collar is applied on the threaded end by screwing it on to the threads with a socket. The hex portion of the collar snaps off at a preset torque providing a predictable preload to the fastener. They are removed by drilling off the head. This ensures that they can only be used once.

Magnum164
01-04-2011, 09:35 PM
[QUOTE=oldtiffie]Screws as not meant to be dowels of "fitted bolts/screws". They are primarily for clamping - not location - and the holes for the heads and shanks of bolts/screws should be larger than the nominal sizes to provide clearance and to allow some clearance and "float" during assembly.

The designers at Boeing, Airbus, Pratt and Whitney and Rolls Royce etc would disagree with statement as this is exactly what close tolerance aircraft screws and bolts are used for.
I have fitted many close tolerance screws and bolts into aircraft, aircraft engines, landing gear and other aircraft components where the screw or bolt not only held things together but also acted as a "dowel" for location.

Yep I have to agree with the close aircraft tolerances. While I understand what luthor is saying about "float" for general applications. There are times when you need to limit that float and even control with GD&T.. I know, I am opening a can of worms there:)

Mcgyver
01-04-2011, 09:41 PM
You are making things far more complicated than they are or should be.



:confused: and i complicated things how?