Thread: How's this technique for drilling mild steel?

I've now got some new Gühring M35 drill bits, and the two sizes that I use mostly for mild steel (gal angle) are 6mm and 10mm, and for these two sizes I have 135° TiN coat M35 bits. I have some XDP-2000 cutting fluid, and have a squirt bottle with 1:10 fluid:water solution.

Given that I have previously not had great success with mild steel drilling (never used fluid, have blunted Cobalt bits too quickly, too much burring which was probably due to too much pressure when breaking through) I thought a quick check of my proposed technique might be in order.

My process will be:

• Using a 10mm 135° TiN coat M35 bit, drilling through 5mm mild steel
• Hold the job in a drill press metal working vise
• Give the drill tip and target a squirt of fluid
• Drill press set to 500rpm
• Use only enough pressure to make decent progress and back off when approaching break through
• the DP is a Nova Voyager, so I can set it to stop drilling at about 6mm, which will be just 1mm through the 5mm MS, and then I can start it up again with less pressure to try to get a cleaner hole

For the first one, I'll try it unsupported underneath to see how the breakthrough goes. Then I'll deburr both sides.

Does that sound reasonable?

Suggest using a centre punch
Line up drill tip so it sits nicely into the CP hole
and
Spot the hole with the rotating bit about 1mm in depth
Then add the coolant/lube/oil.
AND
maybe add some more oil half way through the cut.

Thanks Bob, I just received some new centre punches so I'll use them.

Originally Posted by FenceFurniture

...

• Use only enough pressure to make decent progress and back off when approaching break through
• ... and then I can start it up again with less pressure to try to get a cleaner hole

If the material has any tendancy at all to work harden, a minimal pressure approach may cause problems. I would be drilling with a firm feed to avoid that before backing off at the end of the hole. As for coolant, if the chips are coming out bright (no oxide colouring), then things are not getting too hot

Michael

I would be tempted to apply a couple of drops of straight cutting oil too your holes, after you have made your dimple in your material.

I would also suggest that you break your swarf coming off the drill as soon as it gets to the chuck or 100 mm ( pecking ).

If you are chamfering your drilled holes to remove burs don't worry about the dags on the underside too much.

With your 10 mm holes, even though it's not really necessary I might be advisable to drill a pilot hole first.

Thanks for your responses.

Originally Posted by Michael G

If the material has any tendancy at all to work harden, a minimal pressure approach may cause problems.

I've read that Stainless has a strong tendency for work hardening, but I'll be mostly drilling mild steel – I take your point though – don't faff around, but get on with it.

Originally Posted by pipeclay

With your 10 mm holes, even though it's not really necessary I might be advisable to drill a pilot hole first.

The other 135° TiN coat drill I have is 6mm – suitable for pilot holes or too large?

6 mm should be fine, it just depends on if you get chatter when using the 10, if so you may need to go a little smaller.

Originally Posted by FenceFurniture

I'll be mostly drilling mild steel – I take your point though – don't faff around, but get on with it.

The easiest way to determine your feed for this is by the swarf. A fresh, sharp 10mm drill should happily produce nice continuous thick ribbons of swarf, that are somewhat difficult to break by hand (don't cut yourself), and only break by momentarily pausing the feed. Paper thin flimsy ribbons means a slow feed, and thin flat triangular chips means the drill chattering (can be caused by a lack of feed pressure, but also by not being centred on an existing hole, chipped drill bits, loose setup, wrong grind etc...). It should also just make a consistent smooth hissing sound as it cuts, squealing and/or chatter is usually a sign of either too much rpm or not enough feed pressure.

With a 10mm drill and no pilot hole, I would have to say with the average drill press you'd have to be fairly trying to pull the lever hard enough to overdo it (unless you're a fairly weighty individual), especially if you're bothering with lubricant (I don't normally bother unless I'm going 20+mm deep). If you're attentive, and you're using a heavy feed pressure, you'll normally notice as you get towards the bottom the swarf starts to change colour, and the sound changes, which is your clue you're getting close to breaking out. I quite often find the very last swarf out of the hole is going blue, and more like chips due to the rapid heat build up in the thin section remaining. But the reality is that in my experience, a good quality drill bit in a rigidly clamped setup through mild steel really shouldn't be damaged upon break through even without taking that much care. Throw in a weld seam or something at the exit, and it's a different story, of course.

Once you get a feel and an ear for it, you'll easily be able to adapt for any size drill bit, and be able to identify too high rpm/not enough feed. The feed pressure is often much more than you'd think when you look at the size of the drill bit, provided of course you're using a machine of some sort that keeps everything square. I'm actually tempted to do an experiment, I think I have a small handful of 6.5mm drill bits somewhere. I have a feeling that if I fed as hard as I could into mild steel, the drill would slip in the chuck or stall the spindle on the Bridgeport before I broke a 6.5mm drill, and it should just take a bigger bite all the way to that point. The handful of drill bits I have snapped in a machine over the years have normally been from swarf jamming in the flutes, or drilling horrible things like weld buildups with hard inclusions.

The only caveat to all the above, is that if you go straight to full feed the moment you contact the surface, the drill will want to walk out the side, even with a decent centre punch. So as Bob says, use light pressure or a spotting drill (very short drill bit used for this purpose) to get the tip started, and you can usually start feeding harder when the cutting lips are engaged about 50% of their length. The other big variable is that if you DO pilot the hole, much of the resistance mentioned above comes from the tip working (as the surface speed is far lower at the tip than out at the edges), so your feed pressure will often need to be backed down a significant amount. 6mm for a 10mm hole is about the point where I find trying to be too keen with the feed seems prone to lodge work hardened swarf underneath the drill, and the cutting edges don't appreciate recutting it. Again, it's a matter of watching the swarf flow out of the hole, and modulating your pressure to accommodate. I'd prefer around a 4mm or 1/8 pilot for a 10mm if I had to use one, but 6mm will indeed work.

Others may disagree with my approach of course, all I can say is I have a 15+ year old set of TiN cobalt bits (135 degree split points) I bought off the Snap-On truck as my primary set, and the vast majority of those have never been sharpened, despite LOTS of use over the years. The handful that I've had to touch up have usually lost a corner when enlarging a hole with the cordless drill - a great way to create chatter IME, as it's often awkward to be able to apply enough feed pressure, and not easy to keep the drill from kicking over slightly from side to side. I also have a cheaper set of black oxide bits that are a bit more fragile, and a larger percentage of them have needed sharpening, but again, the damage is normally done in the cordless drill. Given a rigid setup, and some form of machine tool (drill press counts), an ear for the appropriate rpm and an eye for the swarf coming off, drill bit should last a very long time in the average home shop.

I found I had about 5 spare new 6.5mm drills in a box, so I chucked up one I've been using in the Bridgeport for some destructive testing. The first specimen was a piece of 100x100x8 gal angle from somewhere or other, second was a bit of maybe 1 1/2 x 1/4 i grabbed because I thought the first piece was kind of crumbly.

This particular drill is a TiN 135 degree split point made by Savage. No idea where they come from normally, I got supplied them by work in a couple of sizes at one point, but they seem to be very good drills. Well worth buying if they're a reasonable price.

What did I learn? First thing I learned was it's amazing just how fast you can punch a hole through a piece of steel. I'd be guessing under two seconds through 8mm of steel, and maybe even under one second.

Second thing I learned, is that much like a lathe tool when you push it harder, the drill transitions from creating long spirals of swarf to breaking chips.

chips.jpg

Said drill bit after 6 holes at speeds of 500, 750 and 1000 rpm along with some of the chips produced. Drill is none the worse for wear, there are a couple of bits of swarf attached to it there, but it's just fine. Keeping in mind, I was honking on the quill lever with two hands, there was NO backing off at the end of the hole, nor was there any lubricant used. I wanted to create the worst possible conditions I could. I actually thought I'd broken the bit breaking through on the second hole, as something clanged off the shed wall quite some distance away, but it seems that the last chip or two comes out with some serious energy behind it, and gets thrown far further and faster than the typical spiral of swarf - happened a few more times. Those couple of chips sitting there measure just over 1mm in thickness.

My conclusion is that indeed, in the average home drill press, you aren't going to be able to break or damage a 1/4" or bigger drill by feed pressure alone. A setup that moves will break it (wobbly table on the drill press?), a weld, inclusion or void could, running at far too high a speed or with a blunt drill could, but just by feed I don't think so. And given that most drill presses don't have the 2HP that Fence and I have, I wouldn't be surprised if they just stall trying it, as I managed to get the Bridgeport working a little bit even on the 6.5mm.

Having discovered a couple of good brand 10mm drills in the drawer, I then tried a reasonably cheap plain old HSS 10mm 118 degree drill bit that has seen plenty of service, and has been touched up lightly on the belt grinder at some point. Again, no pilot (just floating the tip in to half the cutting lip engagement), no lube, and hanging off it with two hands at 500 and 750rpm. Results, no real difference to the 6.5mm. Increased feed pressure led to breaking chips rather than spirals. At 750rpm it was hard to get that much pressure on it before being through, as the drill just bored straight down, where at 500rpm it presented more resistance. Unsurprising given the effect on the surface feet/minute at the tip.

What I did find quite interesting was the backsides of the holes:

holes.jpg

These have not been touched, they're exactly how the drill left them. Only very slightly more burr than the entry side. Seems to me there could be a strong argument here for NOT backing off the feed pressure when breaking through - the one lump sitting there just flicked off with a screwdriver, and looked the same as the rest. I've seen much more heavy burring on holes where I HAVE backed off, that's required a powered countersink to deburr. Something I'm going to consider going forward.

End result, about 12 holes with each drill, and I'm not down any drills. A fun experiment, and as always, educational. I like trying to push cutting tools to their limit, as I usually find out I'm on the cautious end of the envelope, and the drill/lathe tool/endmill etc can handle far more than I thought. In this case, over the years I've developed a reasonably firm hand for drilling anyway, but I hadn't realised just how much abuse a drill can take. HSS is an impressive material...

Of course, there is no need to be quite so brutal, and it has to shorten drill life somewhat, but the takeaway should be don't be afraid to give a drill more of a heave than you might have thought necessary - you just might be killling it with kindness instead by excess rubbing.

The other thing I was reminded of is just how variable 'mild steel' is. The gal piece in particular was hard to actually get a continuous spiral going, just seemed to be more crumbly in texture. When it did produce spirals, they were quite torn and brittle so broke easily. Cutting oil may have helped a little bit here, I guess. In comparison, I have various flat bar kicking around which is nominally mild steel, and that usually produces resilient springs that you can step on and they just embed in your boot, instead of crumpling flat.

Thanks for that write up Jekyll – very interesting indeed.

Originally Posted by FenceFurniture

... but I'll be mostly drilling mild steel ...

Trouble with mild steel is that it is a base level mystery metal. It's meant to be a low carbon steel, but if the mill was running short of 1004 (or what ever they use), they probably would not hesitate to throw in something with a higher carbon content to make quota.

Michael

Originally Posted by Michael G

Trouble with mild steel is that it is a base level mystery metal. It's meant to be a low carbon steel, but if the mill was running short of 1004 (or what ever they use), they probably would not hesitate to throw in something with a higher carbon content to make quota.

Michael

I doubt that this happens these days Michael.
I was a 3rd year apprentice in the late 60's and was cutting 75 x 20 mm flat( Allegedly Mild Steel) docking them using a huge belt driven Asquith iron worker.The flat bar was the basis for gulley grates found commonly in street gutters.

The material was not mild steel as the colour coding indicated.

The 10 ton machine let out a crack like a cannon shot.Loud enough to bring the office staff racing into the workshop to find out what happened. The old Asquith was cracked completely through its throat section and ruined.

The boss got a brand new ironworker some time later after the legals were settled.

It is for this reason I believe the steel companies being risk averse to lawsuits would be pretty careful about making sure the product was kosher.
I have often thought back and wondered how I would have fared if I was at the hole punching stage. I reckon I could have been struck by that punch had it broke.

I am with Jekyl and Hyde on this one.

If observed carefully the swarf tell can you what you should know about the set up process for the drilling operation.

It is more about what you should know about the basic drilling process than the need to use a fancy high priced drill bit.

Grahame

Originally Posted by Grahame Collins

It is more about what you should know about the basic drilling process than the need to use a fancy high priced drill bit.

Not sure what to make of that Grahame. I posted the thread because I clearly admit to not knowing much at all about the basic drilling process in metal, and so am keen to learn. I purchased a couple of quality 130° M35 TiN bits because I thought they would be appropriate for repeated use, and because the 130° angle is a less aggressive cut.

Hi Guys,

One thing that is missed here is that between the amount of pressure applied to a drill, the pressure causes the flutes unwind ! This can cause jamming and the hole wandering off particularly if it is a deep hole. Obviously this applied less to a larger diameter drill.

As a first year apprentice in 1961 the company I worked for had a scheme to help us buy tools of trade, so along with the usual engineering tools I bought sets of fractional, number and letter drills. I still have 95% of those drills in daily use because I look after them. The other 5% have been replaced because they simply got too short from grinding or damaged through spinning in pistol drill chucks. My mills, drills and lathe all have keyless chucks btw.

The point of saying this is that none of these drill are cobalt or pretty gold things sharpened at some specific angle but ordinary old high speed steel that are kept sharp (don't ask me what angle) and never used dry. Just a squirt bottle with soluble oil does but keep the drill lubricated & not just a couple of squirts. When breaking through the other side then slow your feed rate. Speed of the drill is important too but drilling a hole isn't a race so if you see blue shavings then slow the thing down.