Counterboring and reaming

Either. 4 flute end mills can't mill down into solid material like slot drills but as you'll already have made the M4 hole this isn't an issue. You probably need more clearance than that though, especially as the mills might have a slight corner radius and if so the bolt won't seat properly. I'd say you want 1mm diametrical clearance AND a sharp edged mill to be safe.

Would the hole

No, although it might not be the same size.

Be very careful. Drills usually drill oversize, especially at high speed, and a 7.9mm could easily produce a hole bigger than 8mm if you run it fast. You should first drill to 7.5mm then drill to pre-ream size at a slow speed,

300 rpm or so to keep hole size in check. Double drilling we call it. I'd suggest 7.8mm for an 8mm ream. Reamers tend to rub if you don't leave enough stock for them to cut but I manage ok at 7.9mm for an 8mm ream most of the time.

Agree with Dave, 0.1 (0.004") is probably a bit small for reaming and I'd be inclined to leave 0.006-0.008" for the reamer to cut, so drilling to 7.8 would be better IMO.

Peter

use an 8mm endmill and it will be too tight

in my experience proper counter bores tools leave too much room around the head ...and make it look a rough job ..

use a 9 mm endmill if you can get one ...

or cut a drill bit flat .

all this has to be done without moving the machine, after you've drilled the through hole ...if you try otherwise ..it will look a mess ...off-centre etc

nobody reams counter-bores ...for this purpose ...since you cant see the sides anyway once the head is in.

all the best.markj

7.9 mm is too big - a 7.9 mm drill will most likely drill a hole which is 8 mm wide at some point. 7.5 mm would be better, and if I didn't have a 7.5 mm drill handy I'd use a 7.0 mm drill without qualms.

-- Peter Fairbrother

I'm sorry but reaming out 1mm with an 8mm reamer is going to mean the flutes will clog within seconds if the operation is being done horizontally. You 'might' get away with it on a mill vertically but reamers really aren't designed for removing so much material. The guidelines in the Dormer book are pretty clear. 0.2mm to 0.3mm stock removal up to 10mm reamers going up to1mm stock removal on reamers over 30mm diameter. I prefer lower limits for lathe use with 4 to 8 thou for small reamers and up to 20 thou for large ones.

OK, I'll abandon the 7.9mm drill (actually it would have been 5/16, which is even closer to 8mm!). I'll try with a 7.5mm drill if I can't find a 19/64 (or maybe borrow a letter N off a pal).

I'll be doing all this on a small milling machine - what sort of speed should I run the reamer at?

Thanks for all the replies!

Brendan

I'd

Well that explains why drilling 9.9mm to ream to 10mm in ally tends to go awry then...I'd better go get some x.8mm drills!

Thanks

Steve

Slow to very slow, but push it in as fast as feels right.

And Dave makes a good point, watch for swarf clogging the flutes, it makes the hole oversize (as does overuse of the reamer).

-- Peter Fairbrother

And lock the X and Y before coming down with the cutter. You could drill 9mm first then use the cutter to produce the flat bottom

Commercial counterbores are IMHO useless for model type work. Far too big a counterbore to look neat and the pilot will be too big for your expected size for the screw. I made three counterbore tools the other day from old drills. They do not look that pretty but do produce the required counterbores for M3 M4 and M5

Is it easy to do? Presumably it involves rather more than just grinding the end of the drill off?

Brendan

Irrespective of size, if you're going to use a slot drill or end mill I'd do the counterbore with a drill first then just bottom it out with the end mill. An end mill or slot drill will have no centering for use in a drill press. If you have a milling machine then fair enough, but if not they'll skate about horribly.

A proper counterbore drill has a pilot to guide it in.

Richard

Not too difficult. I rough ground the pilots on the off-hand grinder then put the drill into a collet on the lathe. Fabricated a holder for the "Dremel" type tool and fitted a diamond cutoff blade. Ground the pilot to size moving the carriage to a fixed stop to create the shoulder. Set the compound over at about 10 degrees. Rotated lathe spindle to set drill cutting edge vertical. Wound compound into create cutting edge, noted reading. Backed off, rotated drill so other edge was vertical. Compound in to noted reading. Removed drill and tool and tickled the clearance around the cutting area. Tested in ally, created counterbore. As I said they are not pretty but they work. BTW the diamond cutoff wheel was one of 5 in a set of diamond burrs I bought for £2.50 in a shop closing sale. Well pleased.

PS The lathe bed carriage etc was well covered with wet newspaper.

Hi Brendan

If you are looking to do any amount it may be worth buying a "Ready made" counterbore. I have used PJ tooling before and he can make them to your spec/size. This usually involves grinding a drill or slot drill to suit. He has made me a few which have been used at work on a regular basis and have been great. he is not too expensive and I think a 9.00mm counterbore was about £7.00 inc p&p.

He also sharpens cutters at a good price and does a good job.

New website looks very grand but I think he also attends some of the model eng. shows.

No connection other than a satisfied customer.

Good luck

Paul M

why do you want to put an 8mm head in an 8mm counter-bore

its not going to work

no way ... you need clearance

and the cap-heads bolts are not perfect ..

some guys have given you the right answers

and some have given answers without thinking what you are trying too do.

like i said before you need 9mm

all the best.markj

Just measured the heads on two different lots of M4 capheads, one HT and one stainless - one is 6.68 mm across, one is 6.67 mm. I don't know whether that is an "official" standard or not.

But both should easily fit in an 8mm counterbore.

-- Peter Fairbrother

Just found BS EN ISO 4762 BS 3643-2 , according to which the head diameter of an M4 cap head should be between 6.64 and 7.00 mm.

Then I dont know peter

he said in his first post the heads were 8mm across.

if they are ......they could also be not only 8.01 -02 or 8.1 even, heads ,but off centre heads

all the best.markj

Provided you can drill straight - if not you might need 10, 11, or even a slot! :-))

Steve

I ended up using a 7mm slot-drill (newly purchased) to form the counterbore. The M4 bolt head fitted very snugly, but easily, into the resulting hole.

Brendan