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Can I bore or slot drill instead of counterbore?

Need to recess allen head bolts in new motor plate

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John Coates12/12/2010 20:57:44
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Am going 3 phase on the lathe and need to make an adapter plate to mount the new motor, basically to position the feet and raise the spindle up 10mm
 
Now the adapter plate needs two sets of holes - one set for the new motor which will be 10 x 1.5mm metric (as I have the bolts and the tap) and the other to fit the adapter plate to the headstock plate which will be 5/16th x 18 imperial (will buy allen head socket bolts). As the new motor will sit on top of the imperial holes they will have to be recessed so will need counterboring.
 
Now looking at buying counterbores seems costly just for the one I need (7/16th) and very costly for a set. So I thought maybe I could use my boring head or else a slot drill. Anyone see a problem with this method?
 
regards John
dave manchester12/12/2010 21:01:35
2 forum posts
slot drill should be fine
KWIL12/12/2010 21:44:53
3681 forum posts
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Do not worry about it, I always use a slot drill for counterbores, the standard counterbores always use a larger guide pin than I use for the thread clearance diameter so are no good for me. Just set your depth stop so they are all the same!
John Coates12/12/2010 21:49:02
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Thanks dave and Kwil
 
Didn't think it would be a problem but doesn't hurt to ask in case I've missed something. Nothing worse than a buggered up work piece due to a newbie error
 
John 
Chris Trice12/12/2010 21:59:33
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Posted by KWIL on 12/12/2010 21:44:53:
Do not worry about it, I always use a slot drill for counterbores, the standard counterbores always use a larger guide pin than I use for the thread clearance diameter so are no good for me. Just set your depth stop so they are all the same!

I'm glad you said that. I just bought a good quality set but the nose diameter creates a far larger clearance than I was anticipating. I think I've only used them once. Up until now, it's always been a slot dril (or an end mill if the initial clearance hole has been drilled) but all set up on the milling machine. 

John Olsen12/12/2010 23:48:31
1294 forum posts
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You can counterbore with slot drills. You can also use what my Dad called a fishtail cutter...you take a spare drill of the right size, and grind the end square. Then you grind a little releif on each side so that you have cutting edges. You counterbore the hole first with the same size of drill, then the fishtail cutter will square off the bottom of the hole.
 
You can also use a D bit to do the squaring off.
 
Otherwise you can make a proper counterbore out of silver steel. The usual ones I make are made by drilling into the end of the rod the size you want for the pilot, and turn the outside to the desired counterbore size. Then set the rod up vertical and file cutting teeth around it with a triangular file. The trick is to make sure that you leave each tooth with the full cutting height, and yet don't leave any of the original flat face of the bar. This is not actually as hard as it sounds. Then harden and temper the tool, light straw for cutting. Make a suitable pilot to fit the hole, loctite it in place if it wants to fall out. This style of cutter does not have anywhere much for the chips to go, and is only carbon steel not HSS, so take it quiet and back it out frequently if going deep. You can make another style that only has two cutting edges if you want to cut a bit faster, but they are harder to describe.
 
regards
John
 
 
Nicholas Farr13/12/2010 00:35:49
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Hi John C, I have only ever used end mills to counterbore, having put the clearance hole through first. Have never had a problem, as Kwil says just set your requierd depth.
 
Regards Nick.
JasonB13/12/2010 07:34:37
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I thought that the reason the hole for allen head screws/bolts was a bit larger is that there is a slight radius where the head meets the shaft so you need to clear this if the head is to seat fully. But in most cases we are unlikely to be loading the fixings to that extent so a slot drill is fine.
 
Jason
Eric Cox13/12/2010 08:53:13
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557 forum posts
38 photos
Why not use countersunk set screws
Steve Garnett13/12/2010 09:49:55
837 forum posts
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Eugh! horrible things! And anyway, it's much more of a pain getting the countersinking correct than it is to do an accurate counterbore - however you do it. I've never run into a problem with the slight radius on the edge of the head/shaft myself, but I suspect that not having oversize holes is a good thing from this POV if you want the bolts to remain tight; when you do them up, they are probably going to lock or deform at that point, and then the only real issue is that of judging when to stop tightening them...
Nicholas Farr13/12/2010 10:13:28
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Hi, if you feel there may be a problem with the very small radius between the head and shank, then just touch the edge of the hole with a slightly larger drill or suitable size countersinker.
 
Regards Nick.

Edited By Nicholas Farr on 13/12/2010 10:25:28

Gordon W13/12/2010 10:24:38
2011 forum posts
All bolts / screws have a radius here. If the fit of bolt to hole is good then a c/sink must be used. For the use this is getting a good clearance would be ok. I just use a std. drill for this sort of duty, if worried about this crude system just turn a 120 deg chamfer on bolt head, easier than making a c/bore.
Ian S C13/12/2010 12:25:25
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7468 forum posts
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Steve,  i'v used countersunk allen head set screws, I reduced the diameter of the head, then with a drill of that diameter, and ground as a countersink, drilled the hole so that the bolt fitted flush with the surface of the work. Only thing is, countersunk screws hve a smaller hex than cap screws. Ian S C
Terryd13/12/2010 12:59:06
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1946 forum posts
179 photos
Hi There,
 
Re counterboring, I usually drill the pilot hole and without moving anything use a normal drill to create the counterbore just short of full depth and then us a home made D bit in the style suggested by G H Thomas to flatten the bottom of the hole.  I find this simple quick and cheap.  Ramon Wilson showed some of his home made counterbores on one of his threads and recently I saw a successful counterbore and spotfacing tool made from correct diameter silver steel.  The end was turned down to pilot size to form a guide spigot and then the main diameter was milled away for about 20 mm long each side of this spigot so that it looked a little like those spade bits the woodworkers use but with a spigot instead of a point.  each side was sharpened with a file and it was hardened, tempered and honed.  The user said it was successful and quick to use.
 
Terry

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