drilling brass?

ping...cue ricochet sounds as another M2.5 drill bit whizzes past ear and off wall behind me...

I decided to have a go at iimproving backlash on the cross-slide of the lathe by drilling two holes through the nut parallel to the axis of the thread, tapping them M3, drilling M3 clearance 1/2 the length and cutting a slot across the nut 1/2way along...

well i never got past step 1... the HSS M2.5 drill hardly made a dent before snapping... and the second fared little better...

I thought brass was about the same as ali to work...clearly not... so where am i going wrong...

Off to order another job lot of M2.5 drills!

Is this old brass?

I used to have terrible trouble with brass and it's wonderful tendency to snatch, but not for many years, thought maybe they'd improved the formula and the world had standardised on CZ something or other.

To drill it you mustn't have any slack in the system, it mustn't be able to pull the bit forwards and dig in. That was it's ambition in life.

Lube might help, you could soften it up, possible not a good idea if it's a bearing face. Treat as per copper, apply soap, heat 'til it goes black. Quench or air cool makes no difference, only way to harden brass is to wait a few years.

Edit: Forgot, it does work harden

Um, err, good luck :whistling:

old as in 100 years give or take 5 - the lathe was made in 1914!

i've ordered some stubbies....

Irving

The cutting edge of the drill needs to have a negative cutting angle. Grind a flat on the cutting edge. This way the drill can't snatch.

Sorry Peter you'll have to explain that more

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Originally Posted by Peter Griffin

The cutting edge of the drill needs to have a negative cutting angle. Grind a flat on the cutting edge. This way the drill can't snatch.

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That could work. But obviously we aren't explaining it very well.

The problem is probably 'engineering speak'.

Brass 'snatches' where other metals 'dig in'. It has a word all of it's own, probably because it does it so well.

If Irving, thinks 'dig in' rather than 'snatch' then the 90 degree, or more, rake angle might make more sense.

I understood the snatching bit..:smile:. it was the negative rake angle I needed clarification on... I dont do drills - when they get blunt I replace them, when they break I replace them. I have never sharpened a drill or taken a grinder to one.. but I found this description helpful. A picture paints a thousand words and eventually found one that mentions drill and rake angle in the same sentence... but I am still not clear what it should look like when I am done.. or how I am going to do this on a 2.5mm drill... I suppose the die grinder held in the vice and a small wheel as my big griinder is...too big!

Attachment 1481

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Originally Posted by irving2008

A picture paints a thousand words

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Here is a 1.5" drill with a 90 degree rake angle :smile:

You could do it with a diamond hone easy peasy

Edit: From your picture that would be a 0 degree rake. I'd call it a 90 but they're probably right :heehee:

Attachment 1483

You just need to remove the sharp cutting edge, use oil stone or diamond file.
Only problem then is they are not much use for steel!
Mind you if you keep breaking them you will have enough for a separate brass set.
I always thought the cz bass was worse for snactching than plain brass.
peter

hmmm I wonder then if I could repurpose the broken end, its still got about 20mm of fluting on it... i'll need to regrind the point but I reckon I could manage that using the diagram... well I can't do any worse I suppose....

hi irving
i usually take the sharpness off of a drill by holding a sheet of wet and dry on a flat surface with a little light oil on it and rub the cutting edge untill the sharpness is off of it,
i also push the drill bit into some soft soap for lubrication,may seem a bit mad but it seems to work for me in brass.
Tom

Irving

Thats what I mean't the cetting edge needs to be removed as in Robin's picture.

Irving,

I was thinking about you method of backlash removal; It might be worth arranging things so the bulk of the thread is pushed or pulled towards the handle of the cross slide, rather away from it, since most turning is external and the cutting force will be in that direction.

Bill,

The plan was that the screws would be on the side away from the handle so they can be adjusted from the outside, and this would be the shorter 1/3 of the nut (approx) as per diagram

Attachment 1487

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Originally Posted by irving2008

Bill,

The plan was that the screws would be on the side away from the handle so they can be adjusted from the outside, and this would be the shorter 1/3 of the nut (approx) as per diagram

Attachment 1487

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So, your adjustment screw will squeeze the threads? If so, it means that while outside turning the force will be taken via the adjustment screws to the cross-slide. I modified my Southbend in a similar way except, the adjustment screw were threaded into the split portion (I simply sawed part of the way through the nut) , so the adjustment separated the parts IYSWIM.

i see what you mean, and i was going to do it that way, but i was told there was a risk of the nut splitting if i did this (being old brass) - the amount of backlash is significant... it might need quite a gap!

What I had planned to do is saw it with a fine saw, then reassemble and check the backlash. then mill a small amount off the fixed portion such that when the screws are done up its a little overtight.. then shim it back. Its only until I can make the new nuts and screws so only has to last another, ummm, year or so... these have lasted 100y already!

You could shim it apart just as easily as closed. That would make it nice and stable.

Yes of course I could.... i was looking at it this way... the thread is 8tpi, 3mm. the kerf on the saw is 1mm+. The backlash is 2mm (yes, almost a turn). So I could shim it out a lot, or mill it down and shim it back out a little... but obviously I'll try the former first....

Just had a thought.
Mentioning no names, I bought some cheap Jobber drilll bits on ebay, supposedly 'quality' items.
After the first 2 sizes snapped on the first hole attempted the rest went in the bin.
Maybe your trouble is naff drillls and what you really need is a Dormer?

Strangely I nearly bought some cheaper jobber drills, then changed my mind and got some dormer ones off eBay... twice the price, but probably last more than twice as long. The current drills are from a drill set from one of the usual sources...

OK, I give up ... what's the difference between a 'jobber' and a Dormer' :confused: ... I think a 'jobber' is supposed to be L =10 X dia (when new) but any more and I am lost ... If I want to drill a 7mm hole I grab a 7mm drill bit ... Yes I know my lack of engineering knowledge makes me a Neanderthal knuckle dragger ..

SKF Dormer make quality jobber bits.

If you really want a 7mm hole grab a 6.7mm bit.

Don't start with the 6.8 because it's probably blunt being the M8 threading size :smile:

Threading size???? I normally use a smooth sided pointy bolt and a hammer or super glue to hold things together or a hammer or a few grams of PE :naughty: to take things apart. Mind you my subject matter is almost always organic in origin.

PS ... Black Maskers was always the first option for fixing things ... The whole of the MOD is held together with Maskers ...

Attachment 6508I thought you might be interested in some special purpose drills
From left to right
45 degree hole
next two for corner drilling
middle one for approximation drilling
three on the right are straightened flute, for when you just want to unwind

Dont ask me how they happened, they aren't mine. Mine just break !http://www.mycncuk.com/forums/images/icons/icon6.png