Large drill bits - why so long?

[deema]

The reason you go up through the drill bits is cutting speed. The centre of the drill moves at a peripheral speed much slower than the tip. Each metal has the optimum cutting speed, ft/min or m/min that the cutting edge should removal material. Too fast or too slow can cause chatter and create poor finish and lots of vibration……which destroys the cutting edge.

So, you centre drill to ensure the start of the hole is precisely on centre. Centre drills have a different point to standard drills and are stubby and very rigid. Getting on centre is important to both reduce vibration and for accuracy. You then step up drill sizes, and adjust rotation speed accordingly. The larger the drill the slower you go. Technically you also adjust the feed rate, ie the amount of downward pressure you apply. The larger the drill the more pressure is required.

 

[guineafowl21]

I stopped stepping up in sizes after picking up tips from this bloke, a very accomplished machinist:



Extreme example with big kit, but around 8mins he just centre drills this tough steel bar, then goes straight in with a 50mm bit.

In previous videos, he’s answered comments on the subject, saying that drill bits are designed to cut the full width, at most needing a pilot drill the size of the chisel bit.

By stepping up, he said, you are not only wasting time, but also wearing your drills unevenly, just at the edge, and have more risk of going off-centre.

 

[Phill05]

One drill I have not seen talked about here is the spot drill it is more precise in drilling a starting hole or a through hole in not so thick material, a spot drill does much the same as a centre drill, but there are two differences.
One is that the spot drill has thinner tip than the centre drill does so you are not pushing hard into the metal. another reason is that the spotting bit does not have grooves or spirals that go all the way up the bit shaft.
This gives the bit more sturdiness and is less likely to wobble, example is a 12mm bit has a grove length of 25mm so would be ideal to do the job the OP wants.

 

[guineafowl21]

Spot drills are a good suggestion, as long as the flutes are long enough to clear the thickness of the material, or the bit might bind.

Centre drills are mainly intended for making the 60deg cone for a lathe centre. They’re often very short compared to a jobber drill and mean you have to adjust the table/bed height during the operation. Spot drills are a good choice if this is a problem.

 

[Limey Lurker]

If I'm drilling up to say 13mm, I'll centre drill first and then go up through sizes until I reach 13mm. Obviously the speed of the lathe/mill/drill needs to correspond to the bit size and material.

The sort of work that I'm used to, 13mm is the first pilot!

 

[deema]

@guineafowl21 I watched the video, very interesting, reminded me of the old John Cleese parody training videos Showing you exactly what not to do. I particularly loved the use of the emery cloth. What ever you do, please don’t see this chap as good or indeed safe practice.

 

[guineafowl21]

@guineafowl21 I watched the video, very interesting, reminded me of the old John Cleese parody training videos Showing you exactly what not to do. I particularly loved the use of the emery cloth. What ever you do, please don’t see this chap as good or indeed safe practice.

Interesting. I’m not near his or your level, but he seems pretty well regarded over on the MIG forum.

 

[Sideways]

This is an informative video.
There's a ton of explanation about material, coating, tip and helix angles, etc.
Interesting to me was the point made 9 ish minutes in about keeping two drill diameters of flutes clear above the surface for best chip clearance.
The explanation about jobber drills being poor at self centring because of their cheaper to grind geometry is also made.

 

[deema]

I rank myself as amateur, there are a whole load of far more knowledgeable people than me. If I caught you holding with your fingers emery cloth in a lathe that would be the last minute you worked in the shop. If the emery cloth catches, (what he was doing is exactly how a friction band clutch works) it will literally pull his hand into the work and remove something if not his whole hand!

You would check (clock) for run out and concentricity of any work piece mounted especially when on a steady. You would face the piece before you centre drilled it, if the face isn’t square and true when pushing the drill into it the reactive force will try to push the centre of the drill off centre, you are effective trying to drill a cone (any camber rotates creating a cone that the fixed drill sees). You defo would pilot before trying to shove a 50mm drill into it. The nose of the drill isn’t cutting and tries to find the easiest path, so the hole will be over sized and not central, you can easily break the drill that way…yes even a 50mm drill….or push the part out of concentricity. Anyone doing accurate work won’t be using a vernier for final checks, and would wait for the part to cool before taking a final cut.

Another of my favourite moments is him spraying his lathe bed ways with aluminium oxide, emmm, that will help them wear just wonderfully. Just go to a person who is proud of their lathe and use a file or thin belt sander over their lathe……you will be doing an impression of Hussain Bolt.

 

[flh801978]

Deema. What did you find bad practice in the video?
….Just seen your comments whilst i was posting…

 

[J-G]

I rank myself as amateur, there are a whole load of far more knowledgeable people than me. If I caught you holding with your fingers emery cloth in a lathe that would be the last minute you worked in the shop. If the emery cloth catches, (what he was doing is exactly how a friction band clutch works) it will literally pull his hand into the work and remove something if not his whole hand!

You would check (clock) for run out and concentricity of any work piece mounted especially when on a steady. You would face the piece before you centre drilled it, if the face isn’t square and true when pushing the drill into it the reactive force will try to push the centre of the drill off centre, you are effective trying to drill a cone (any camber rotates creating a cone that the fixed drill sees). You defo would pilot before trying to shove a 50mm drill into it. The nose of the drill isn’t cutting and tries to find the easiest path, so the hole will be over sized and not central, you can easily break the drill that way…yes even a 50mm drill….or push the part out of concentricity. Anyone doing accurate work won’t be using a vernier for final checks, and would wait for the part to cool before taking a final cut.

Another of my favourite moments is him spraying his lathe bed ways with aluminium oxide, emmm, that will help them wear just wonderfully. Just go to a person who is proud of their lathe and use a file or thin belt sander over their lathe……you will be doing an impression of Hussain Bolt.

I watched that video before reading your response @deema and was mentally making exactly the same comments as I did so!

There is no way that either the hole or the thread would ever be concentric with the OD - which he didn't machine nor 'true up' by checking with a dial gauge. He holds the billet in a chuck with hard jaws rather than soft jaws which had been machined absolutely concentric with the lathe bed - but thats just his first basic error.

It also looked to me that when he was drilling the 50mm initial (ostensibly) hole that the swarf was no where near as wide as I would expect, so it is possible that he did in fact drill a smaller hole but omitted it from the video.

To some extent your point about the initial hole being off centre is probably a red herring since he did at least bore it out properly with a solid boring bar - still doesn't make it concentric with the OD though.

When he machines the tapered section, you can clearly hear that the OD is not concentric.

At least he did drill & tap a new locking grub screw hole - but with a pistol dril !!! ???

 

[stuart little]

I rank myself as amateur, there are a whole load of far more knowledgeable people than me. If I caught you holding with your fingers emery cloth in a lathe that would be the last minute you worked in the shop. If the emery cloth catches, (what he was doing is exactly how a friction band clutch works) it will literally pull his hand into the work and remove something if not his whole hand!

You would check (clock) for run out and concentricity of any work piece mounted especially when on a steady. You would face the piece before you centre drilled it, if the face isn’t square and true when pushing the drill into it the reactive force will try to push the centre of the drill off centre, you are effective trying to drill a cone (any camber rotates creating a cone that the fixed drill sees). You defo would pilot before trying to shove a 50mm drill into it. The nose of the drill isn’t cutting and tries to find the easiest path, so the hole will be over sized and not central, you can easily break the drill that way…yes even a 50mm drill….or push the part out of concentricity. Anyone doing accurate work won’t be using a vernier for final checks, and would wait for the part to cool before taking a final cut.

Another of my favourite moments is him spraying his lathe bed ways with aluminium oxide, emmm, that will help them wear just wonderfully. Just go to a person who is proud of their lathe and use a file or thin belt sander over their lathe……you will be doing an impression of Hussain Bolt.

Emery is ok as long as you don't hold both ends in the same hand! He used chuck jaws with brass pads.

 

[deema]

@stuart little How does that work? I know @Sideways created a solution at his company, but I don’t understand how holding it with one hand is OK? The friction increases and I would have thought increased the chance of a grab pulling your hand in.

 

[stuart little]

@stuart little How does that work? I know @Sideways created a solution at his company, but I don’t understand how holding it with one hand is OK? The friction increases and I would have thought increased the chance of a grab pulling your hand in.

Read it again, I said; "-as long as you DO NOT hold both ends in the same hand".

 

[deema]

Read it again, I said; "-as long as you DO NOT hold both ends in the same hand".

My bad, apologise. Same question though, one or two hands makes no difference, it’s still a slip clutch arrangement. I believe @Sideways has a gruesome story of what happens.

 

[guineafowl21]

Lightly held between finger and thumb of each hand, so if it grabs it’ll just slip out of your grip, is what I’ve been shown.

 

[Phil Pascoe]

I hold rags, wire wool etc. with one end in the palm of the right hand held by finger pressure from the left hand. The material just flies off if it catches, it's not wrapped around anything.

 

[stuart little]

Lightly held between finger and thumb of each hand, so if it grabs it’ll just slip out of your grip, is what I’ve been shown.

That's what I said, it's always worked for me, I KNOW 'cos I can still count to 10!

 

[Sideways]

It's called "avulsion" when a finger, hand or limb is pulled until it's torn off.
The ring finger is especially vulnerable to this.
Fingers have no muscles in them and can last longer before they are too degraded to have any chance of being reattached successfully.
Hands have plenty of musculature and you only have a window of a few hours to get into an operating theatre to stand a chance. A hand pulled off by being caught in a lathe is a major amputation. Not something you want to be carried into A&E with on strike day....

The majority of even small metalworking lathes use gears, not belts, to reduce the motor speed and amplify torque. Whether they have a clutch or not, they can generate more than enough force to break you without stalling the motor or prising the work out of the chuck.

Wood lathes are bad enough. Metal lathes are dangerous. Keep your fingers out.

 

[Sideways]

I'm not a fan of sanding on a metal lathe. A great way to ruin it, but not everyone cares.
If you must, then make up something like this or simpler to hold your emery cloth in the toolpost...