One for the engineers.
I'm trying to drill 12mm holes through 19mm square tubing to make my w/bone bracket jig.
I've done a 2.5mm pilot hole.
The drill is in a stand- keep intending to buy a pillar drill but not got round to it yet!
The bit is a HSS, Black&Decker branded 'Piranha' sold by B&Q and is the more expensive of the two they sell.
The tip is a 124° with 'points' on the tips so it cuts out a disc a while after it's started drilling at the centre.
I'm using the drill on the lower of the two speeds- I think it's about 800-1000 RPM.
Anyway, the problem is that having started cutting ok, once the top of the hole gets to about 6mm wide it just gives up and won't cut anymore. If
I keep trying to drill, it just 'hammers' the hole and if I increase the pressure will migrate sideways.
The end result is the hole looks like I've tried to push something through instead of drill it!
I realise I've probably missed a few basic principles of drilling a decent hole so hopefully one of you engineering types could give me a few
pointers please.
Thanks very much,
James
P.S. Looking at the bit closely the 'tips' appear a bit blunt and slightly blued although the chisel looks ok.
James,
Drilling in sheet is always a pain (and the side of 19mm square is just a small bit of 1.6mm sheet!). It tends to snatch as the bit goes through the
work. You also get an odd-shaped hole (usually triangular) as the bit rattles around due to lack of anything to locate it.
I would do it by using a range of bits up to the finished size. This may mean using 3, 4 or even 5 bits to get to the finished size. To check what
you're doing, you could scribe a series circles before you start drilling, from the centre-punch dent. These indicate whether you're
drifting off, giving you a chance to fix it before it's too late.
Note that I would only do this if I wanted the hole to be really accurately placed - otherwise I'd just use a pilot and then the finished
size.
The posh way to do it would be to drill out to just below the target size, and then ream it to the finished size - but I wouldn't bother most of
the time!
Or you could consider a stepped hole drill, designed especially for sheet metal work - bit expensive at around £25, but brilliant if you have lots of
different-sized holes to drill in sheet around 1 - 2 mm thick.
good luck!
David
I bought one of those Piranha drill bits seeing it had some kind of guarantee on it. Three holes later it was f**cked so I'd suggest getting it
replaced might be your first port of call.
Chris
try using some oil or wd40 when drilling to lubricate the cutting edge of the drill, if nothing else might help stop the bit going blue with heat. is
the tube g-clamped to some wood or a bench to stop it waggling about?
see you sunday,
Ned.
When you say points, do you mean it looks like a wood bit?if so a) bin it b) they dont want a pilot hole.
If you want a nice hole mark it out on both sides ( dont drill through both together the bottom hole will wander) center punch, drill 6mm @about 1500
rpm then 11.5mm @about 500 and ream through both sides together 12mm at 200. Good hss drills will be about £3 each and the reamer about £20. Well you
did ask
having spent the last year drilling holes in harrier's, when you start getting around the 10mm + mark drill speed become quite critical. If you
drill too fast the bit "chatters" and makes a hell of a mess, I discovered this whilst drilling cable holes on a sheet of 1.2mm ali, f**ked
up the metal, cost me a round of beers and about £500 for the navy.
If you have a hand drill try that, pilot drill first then whack the 12mm through, hard work but should do the trick -
Chris
Buy a pillar drill and some proper hss bits.
I got a cheap one from screwfix and it runs on its lowest speed all the time. I put a 4mm drill through the punch mark first then whatever size I want
through that. Use some cutting fluid or failing that a spot of oil and clean the swarf up regularly. Don't swing on the handle too hard or
you'll blue the tip.
yours, Pete.
James, I'll bring a bit of cutting oil Sunday 
Jason
I got a CNC 12mm drill bit and drilled ok through 25mm box. I think those B+D Fish drills are designed for hand drills and yep they are crap.
B&Q cheap bench drill on the slowest setting with plenty of oil, the job well clamped and gently does it. (gets the wife every time)
My blacksmith lent me an amazing bit of kit - lovingly called the 'glove chewer'. It's a Bosch thing reving to 25k powering a small
inch long bit with a flat end and very fine twisted sharp edges in it.
Excellent for opening out holes that were drilled wrong. I had drilled 10m holes through 5 mm steel plate about 3mm out. This thing just chewed
through it like butter. Great for opening out large holes in thin ali too as I haven't got a hole cutter.
Don't know what's it's called but I know the bits alone are £12 each! It's my new favourite tool.
this sounds like the tool called a rotozip....
atb
steve
http://www.rotozip.com/indexusa.phtml
[Edited on 3/6/03 by stephen_gusterson]
Thanks for all the replies everyone.
And thanks to Deckman (Jason) for the oil.
Having taken back the second blunted drill bit to B&Q I replaced it with a normal tipped one.
I then did all the holes by working my way up the bit sizes in about 4/5 increments so thank you for all the advice.
The cheaper B&Q bit was far superior to the more expensive one! Typical!
Cheers,
James
I always find that centre punching the plate then using loads of cutting compound (grease like stuff, about £6 a tin from engineering supplies) does the trick. Easier to clean up than oil as well.
quote:
Originally posted by stephen_gusterson
this sounds like the tool called a rotozip....
atb
steve
http://www.rotozip.com/indexusa.phtml
[Edited on 3/6/03 by stephen_gusterson]