Thread: Chipping drill bits!! Frustrating!!

Are you backing the work up with something? Unless using a drill press, I find it a lot easier to have a piece of scrap timber* underneath to stop the bit doing the breakthrough grab and pull.

*A sawhorse counts as scrap, doesn't it????

- have you heard of RPM = 300 X cutting speed
diametre

to work out your rpm to set your drill press, multiply 300 by the cutting speed of the material ( 30m/min for steel, 20m.min for stainless ) and divide this by the diametre of the drill

example, a 20mm drill in mild steel 30m/min

300 X 30 = 9000

9000 divided by 20 = 450 rpm

dont stress if you drill press doesn't have 450 rpm on it, just go the next speed under it.

- be careful using a pilot drill, many people dont realise that you dont need a pilot drill for even use to a 50mm drill, but drilling a pilot hole sure makes it easier

the size of a pilot drill is equal to the thickness of the web of a drill, im not sure how well you know the parts of a standard jobber twist drill, but this is the solid section going down through the middle of the drill

for a rough estimate, use a pilot drill that is about 1/5 the diametre of the larger drill ( eg for a 20mm drill, use a 4mm pilot drill )

- when sharpening drills, dont let them go blue

- you say that you ease the feed off going though the end of the hole, i would increase your feed and use PLENTY of coolant, not cutting oil. reason being, when breaking though ( and when using too large a pilot drill ) only the ends of the lips of the drill are cutting and this will burn out a drill really quickly and soften it, therefore it will chip off

- i've never used a drill docter or sharpener etc, i always sharpen by hand. when drilling, you have sharpened it correctly if your shavings come out of the hole the same of each side ( not curling on one side and poor little chips out the other side )

just a couple of tips i find useful when at work ( i work in a shop with 30 machinists, where everybody uses the same drills from a big set of drawers we have, so when you get one out it always needs sharpening as nobody puts them away when they are in decent condition )

The TAFE machining teacher advised me not to drill the pilot hole too large . He said you want the drill to be cutting on most of the width of its edges , not just on the outside perimeter edge only as this will stress the drill bit . This was news to me as I had always drilled out holes progressively in steps . MIKE

As Mike suggests, the pilot hole should not be too big - a maximum of 25% of the final drill size - the idea is to drill a hole just big enough so that the web of the larger drill does not have to cut (as that's the inefficient part).
The other suggestion is sharpening - how hot are the bits getting when you sharpen? If you are sharpening aggressively and then dip them in water to cool them, you might be doing things to the metallurgy that make them more prone to cracking (either micro cracks or changing the temper). Try and keep the drills as cool as possible.

Michael

Hi Mike,
Your right their, it's usually recommended to only use a pilot big enough to clear the web on the larger drill. But with home shop machines being lower horse power than industrial, it's common for people to go in steps.
I remember seeing something year ago about never going over half the larger drill diameter for a pilot, and I stick with that.
The only time I break that rule is when I am drilling a precision hole for a press fit etc, I will drill slightly under size then enlarge it slightly with the next number/letter drill bit.

Dave

Thanks guys! All good advice!

Master Splinter a backup is a great idea. My saw horses are a regularly used like that for more carpentry work and were designed around standard 4x2 pine for ease of replacement. Will have a play with that idea.

The fabricator, great reply! Just what I was looking for! I did not know about the 9000 and 6000 rule! Great tip! I'm goin to call that one 'the rule of thumb' so idont forget it

And the 1/5th pilot hole size I think is another area where I have been going wrong. I will have a look a that one too.

I know about burning HSS and have always been a above careful with that but was always told not to quench a hot HSS as it can cause micro cracking? High carbon like 1095 is ok to quench to keep it cool but HSS is better not to heat up to much. Is that correct advice?

Actually, I was just spoilt by my Lady and mum for my birthday. Was just given a lovely lathe and a milling machine so will look at the feed speeds a little closer using the mill now.

Thanks guys

I have been usin this forum for years now.. Always put me back on the right path!

Hi Ben,

Have you tried the "rag trick"?

You get a piece of folded rag, place it over the hole and drill through the rag, it stops that grabbing as you break through the other side.

The other thing I haven't seen anyone mention is using plenty of cutting lubricant always helps drill bits stay cool and last longer.

Regards
Ray

Originally Posted by morrisman

The TAFE machining teacher advised me not to drill the pilot hole too large . He said you want the drill to be cutting on most of the width of its edges , not just on the outside perimeter edge only as this will stress the drill bit . This was news to me as I had always drilled out holes progressively in steps . MIKE

Thats good advice, pilot drilling is important for large bits but anything under 10mm dont bother unless your drill has not guts

If you are chipping the corners you can slightly round them back so they are stronger but again its not always advised and when you step drill they go supper fast down a hole!

Your problem is that you have far too much front clearance on the end of the drill. Reduce this to the amount as shown on my previous post/s on drill sharpening and you will have no more problems. Do a search for my posts on drill sharpening and if you cant find it, let me know and I will re-post the entire article. I should do this anyway as so many people have been asking how to sharpen a drill by hand.

Kody

Thanks Kody. I'm going to take a look now!