Centre drilling with lathes...

Ok, so to drill a nice centre hole, I realise I need some centre drills - but what size set do I need (I how many bits). It's just to start off a pilot hole, so what with all these sets with a gazillion sizes?

There seems to be a few 5 piece sets...with various permutations of sizes (eg 1mm-3.2mm, or 1mm - 4mm). Can anyone recommend me a good starter centre drill set? (ie what siizes should be in there)

Also, I've noticed that standard drills 'wobble' when drilling holes from the tailstock - so does a center drill solve all the potential problems associated with this wobble (ie can I assume that since the pilot hole is bang on the money that a longer drill will proceed through the material more or less on target) Why don't they make stubbier drills to try & eradicate some of the wobble seen with standard drills?

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Originally Posted by HankMcSpank

Ok, so to drill a nice centre hole, I realise I need some centre drills - but what size set do I need (I how many bits). It's just to start off a pilot hole, so what with all these sets with a gazillion sizes?

There seems to be a few 5 piece sets...with various permutations of sizes (eg 1mm-3.2mm, or 1mm - 4mm). Can anyone recommend me a good starter centre drill set? (ie what siizes should be in there)

Also, I've noticed that standard drills 'wobble' when drilling holes from the tailstock - so does a center drill solve all the potential problems associated with this wobble (ie can I assume that since the pilot hole is bang on the money that a longer drill will proceed through the material more or less on target) Why don't they make stubbier drills to try & eradicate some of the wobble seen with standard drills?

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Its not the stubbiness, its the fact that centre drills have a long lead spiral compared to conventional drills.

I have a 4 drill set (BS1 - 4 #8932484X1A) from RDGTools which are fine for £5 as long as you dont abuse them too much

Centre drilling is actually an old process to support a bar with a centre [ hence the name ] for support.
They are the shape they are because the short pointy bit at the start was put there to hold white lead paste which is an extreme lubricant when they used dead centres.

With the general use of revolving centres that start diameter doesn't need to be as long as it used to be.
It only has to be long enough to ensure that the point doesn't foul the bottom of the hole.

It's usually this extended point that breaks off and ruins a job. I grind new centre drills to about half their length as soon as I get them. Now some will say that reduces it's life by 50% but if you break one it reduces it's life by 100%

Times move on and industry has realised that on CNC machining centres, centre drilling first [ with the chance of one breaking ] then drilling wastes a lot of time so they use spotting drills to start the hole followed by the deeper drill

Often they just used stub drills with modified geometry to do the hole in one pass.

We can use this same procedure in the home shop, stub drill as usually dearer that their counterpart, the jobbers drill, but it's rare to break one and when drilling from the tailstock if you pause just as it make contact for a few seconds then it will centre itself up and start the hole perfectly.

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Quote:

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Originally Posted by HankMcSpank

Also, I've noticed that standard drills 'wobble' when drilling holes from the tailstock - so does a center drill solve all the potential problems associated with this wobble

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A Slocombe centre will do it darnedest to align itself with the centre of rotation, even if your tailstock is a tadge out of alignment. Don't know how they do that but I'm sure it's very clever.

To get a larger drill to start on centre (no wobble) make sure you open the hole out a tadge bigger than required diameter using the wider part of the Slocombe.

There are a lot of crap Slocombes out there and they can be a booger to clear when they snap off. I try not to use anything except Dormer but I am superstitious. Once bitten, twice shy.

OK I think I've learnt something here...

So a Slocombe, aka, centre drill is really intended to create holes for a shaft centre support for further machining on a lathe, but because of their narrow tip often get used instead of...

spotting drills... which have a precisely ground 90degree tip and are used to create a pilot hole for a bigger jobbing or twist drill.

Stub drills are conventional but shorter stiffer drills that are less likely to wobble or mistrack and can be used as spotting drills to start a twist or jobbing drill for deeper holes...

Did I get that right? Stub drills seem to be cheap (Dormer ones are £1 - £5), while spotting drills seem to be expensive (£10 - £40)

So what is the best practice, say, to:

drill a 6mm dia 15mm deep hole centrally in a 20mm steel shaft on the lathe?

drill a 6mm dia 30mm deep hole in a 50mm piece of ali in the mill?

assuming ownership of a complete set of 1 - 10mm in .1mm HSS twist drills...

I know I'm a bit fuddy-duddy and old-hat with my Slocombes but I do like them for starting a Jobber.

You get that 60 degree cone to align the bit PLUS a perfectly concentric pilot hole to clear the awkward drill centre.

The Jobber goes in real easy to the bottom of the pilot by which time you have a much better hold on centre.

I'm not going to argue the why's and wherefore's, but I think they are neat :beer: