Drilling straight

Hi guys

Despite ensuring everything is level I cannot seem to drill a hole so that it is exactly in the right place on the reverse side. Ive got 56 holes to drill in 6x6mm square brass stock ,no room for error as each hole has to be tapped for M3 bolts.I m using 2.5mm cobalt drill ,brand new,.

I'm obviously doing something wrong can anyone point me in the right direction

Thank you

Andy

It may be a new drill but is it sharpened correctly If the cutting lips are not equal the drill will be loaded one way causing runout and to be fair 2.5mm drills are fairly flimsy. I don't know how fast the drill is rotating but speed is everything with small drills.

Shorter drills are stiffer and easier to control

(don't ever throw broken drills away. The day will come....)

Edited By Ady1 on 08/11/2022 09:36:38

If you're drilling into brass you may need to consider the cutting edge of your drill. It may need the rake taking down a bit ( do a forum search on this - plenty of suggestions / opinions) as it's a new drill it will probably be 'optimised' for ferrous materials. As Bernard says, the cutting lips must be absolutley equal (as near as practical on such a small size) and your cutting speed needs to be as high as possible.

Best of luck

Are you picking up a punch mark or similar, or going straight in? Most drills are ground with a chisel-point - short straight edge at the very tip - and have a tendency to wander slightly as they engage with the work. If the hole does not start truly axially, it will never be straight. You can buy (or even grind yourself!) "split point" drills which do self-centre better, or you can make a tiny starting mark with a spot drill to guide that crucial initial engagement. I normally buy split-point drills anyway, but if I want to really make sure, I also spot-drill - but that is on a mill with DRO so easy to go along the row and do all the holes with the spot-drill then split-point drill to minimise tool-changing.

Andy, How are you holding the workpiece?

Are you using a hand drill? With the item held in a workshop vise.

Drilling is a bit of a black art and relief angles like split point stuff can make an amazing difference

The high precision chaps tend to use precision drill grinders

Thank you all for your replies they are most helpfulsomeone asked how I was holding the work well I've tried it all ways , clamped in milling machine, clamped in movable vice on pillar drill so as to enable the drill to find the punch hole

What grade is the brass? For machining I think CZ121 is best IIRC.

Cobalt drill overkill. Buy a new high quality HSS STUB drill or at least have as little projection from the chuck as possible. Use a lens to check that cutting edges are symmetrical.

Centrepinch one block, clamp in vice against a stop

centre with a sticky pin. — Plastercine or bluetac with a sharp nail in it . Switch on, not too fast and use a rule to straighten it.
position vice and work, clamp it down

centredrill then drill (spot drill is better)

then no marking out for the rest of the batch, all accurate to set position

if it’s still bad it’s the drill not the method so in that case Back off the point of the drill as suggested above

if you have an edge finder this is better than the sticky pin

As mentioned, HSS is fine.

Branded stuff like Dormer are always a pleasure to use compared to "others"

You haven't clearly defined your requirement and problem. How thick? are these items 6mm cubes? Are the holes in the centre?

If you clamp some pieces of bar on the mill table to form a right angle into which you can set the block and clamp it then you can align the drill chuck without measuring each one.
Drill with centre drill so very short and rigid just a couple of mm in.
Flip block over and it will instantly align to do the same from the other side.
Do all blocks
Then put in longer drill and finish off right through.
beware as your drill meets the hole from the other side it will snatch badly in brass but will be pulled back into line with the second hole.

No mention, so far, of drilling chuck. Don’t forget that, as a possible risk of wandering. A collet chuck would/should be better than anything less than a high quality drill chuck.

If I want a straight hole, I often use a slot mill - at least for a few mm.

Any and/or all of the above suggestions could be worth checking out. Pecking, to start might also be better than plunging straight through. For precision I would most certainly not be holding it free-hand under a pillar drill. It would be held securely on the mill table, for me.

Edited By not done it yet on 08/11/2022 11:56:16

+1

I suspect the drill isn't ground symmetrically, I have a box-set of cheap small drills that are all over the place; I always check them with a lens, and about 1 in 3 are reasonable. The others are a good way of making wonky oversized holes!

Because I have loads of them, I'd start with a centre drill. This starts a deep support hole for the main drill that helps keep it straight. I'd run the drill as fast as possible in my mill, and proceed in a series of short pecks, not too hard because drills can bend. The purpose of pecking is to stop swarf building up: it can jamb in a flute and force the drill off axis, or blunt an edge by mincing at the bottom of the hole. Definitely an advantage to use a stub drill rather than an ordinary one, and to keep grip it deep in the chuck.

The metal may also be problematic. Not all Brasses cut nicely. I have a couple of rods bought from a DIY store that always cause trouble.

If the drill and the metal are both iffy, an ordinary job will go seriously pear shaped!

Dave

Start the hole with a spotting drill so that the 2.5mm drill is not deflected at the very beginning. A correctly ground short series drill running at the correct RPM and feed rate is the way to go. A twist drill is like a twisted ribbon and is easily deflected by pushing it too fast and a jobber or long series drill is more flexible than a short series drill. Peck drill to remove debris as you go as well. Be prepared to replace the drill if it is not cutting well.

Martin C

Edited By JasonB on 08/11/2022 13:01:25

Holes seem to go straighter in the lathe when the drill is stationary and the job rotates than in a drill press or cheap mill. Can you hold your pieces of brass in a four jaw chuck and drill the holes, or clamped to a face plate, or held in a special jig/fixture on the faceplate? Not sure how long your pieces are, or if it's one hole per piece or multiple holes in a longer piece of 6 x 6. But there ought to be a way. You could tap your thread in at the same setting too, using a spring tailstock centre to hold the tap square.

Probably worthwhile to make a little jig up so that you can drop your squares into a captive space where you know the start hole is in the centre.of your square blocks.

If you are using a Jacobs then insert the drill as far as it will go just leaving enough protruding to clear the workpiece. Tighten the chuck from all of the 3 key positions. Witdraw rfequently to clear swarf.

Personally I would stone the edges for brass as well.

regards Martin

6mm deep is only 2.5 diameters. That should be fairly straightforward.

it’s likely the problem is at the start of the hole. Once a drill wanders it is very unlikely to self correct.

56 holes is worth a little effort in jigs.

A top hat drill bush carefully made - bore the hole to a tight sliding fit on the drill, face the end in the same setup to ensure squareness and the harden fully, maybe temper to straw. Clamp this to the bar and it will ensure the start can’t wander away.
You don’t say the spacing between holes, but the drill bush can also have a pin to drop into the previous hole to give you simple accurate spacing.

I would be tempted to purchase a good quality solid carbide drill - not for the hardness, but for the stiffness.

Dave