Rake angle on Cutting Tools…..memory tips???

[Chris TickTockParticipant @christicktock]

Thanks Guys for some useful posts here. I will have a good read later when i have more time.

Chris

[Mick B1Participant @mickb1]

Posted by Martin Johnson 1 on 21/11/2019 16:37:54:

If the O.P. is working in industry trying to make a profit or bonus by turning things out as quickly as possible, then rake angles, clearances etc. all matter.

If as I suspect he is working in a home shop. They really don't. I turn just about any material with the same set of tools – all ground by eye with a bit of clearance on the front and whichever side face is cutting. Put a few degrees of rake on the top and that does the job. Only exception is some brass which is better without the rake on the top – even then, most of the time I can't be arsed to change the tool.

My qualifications for such a gung ho approach? 50 years model engineering, 40 years as a chartered mechanical engineer.

Martin

I think that's about it. Most of it's fairly straightforward once you understand how the cut actually occurs, and a lot of the muck and magic's just showing-off. I've got most of the years Martin mentions, except for a very long and rather frustrating diversion into MRP systems…

[Neil WyattModerator @neilwyatt]

I don't think anyone has answered the original question, which was quite straight forward,

If you think of a 'typical' tool which has a top that slopes up towards the work as 'normal' and 'positive rake', then understanding other arrangements is simple:

'Zero rake' is when that slope is reduced to noting i.e. the tool is flat on top.

By elimination, then, when the slope then changes so the tool slopes down to the work, that's negative rake.

As mentioned over and again the exact angle is not always critical, as so many other factors affect the cut as well, often a change in one can be balanced by a change in another.

Neil

[Anonymous]

Posted by JasonB on 22/11/2019 07:02:14:

Unlike some here I have never modified a drill bit for brass or used specific ones made for the job, just use the same ones that I do for every other metal, may just take a bit more care when enlarging a hole but that's all. So just like the replies to your question it can matter but is not really that critical for the average guy in his shed.

That's not my experience. Must mean I'm below average; and my workshop is in a garage, not a shed.

Brass and bronze can and do snatch a drill, to the extent of pulling out the Morse taper in the tailstock. For small drills, say less than 6mm, it doesn't seem to matter. But I run fairly heavy machine tools. Above 6mm if one goes up in small increments, say 1mm, there is not generally a problem. But I'm idle and I can't be a*rsed to keep changing the drill. I've bought a few slow spiral drills specifically for brass up to about 12mm. They work well and allow one to open out a hole in one pass with a sensible feedrate, around 10-20 thou per rev, if one can wind the handle that fast. Snatching seems to be worst when a pilot drill has been used. So I try and drill holes in one pass. I expect that the force needed to push the chisel edge into the work counteracts the force on the lips pulling the drill into the work. Above 12mm I'll bore if possible. Like wot the books say I tried stoning the lips on a standard drill. Didn't work for me; whereas the slow spiral drills do work without having to be careful.

On the manual lathe I use CCMT inserts for brass and bronze. For HSS specials, and on the repetition lathe, I'll use zero rake if grinding from scratch, but otherwise don't sweat about it. The only time I have a problem with swarf is when it birdsnests and/or gets trapped between the tool and work. Of the metals aluminium alloy is by far the worst for birdsnesting.

Andrew

[Ian PParticipant @ianp]

Posted by Neil Wyatt on 22/11/2019 11:09:58:

I don't think anyone has answered the original question, which was quite straight forward,

Neil

Maybe I am the only one but I found the original question far from straightforward so hard to understand.

Chris seems to be asking for an easy method of remembering the angles, maybe something like a mnemonic.

In one of my replies I did ask Chris to clarify but got now response, later though he did mention some of the replies were polite which I took to mean that mine wasn't.

Ian P

Ian P

[VicParticipant @vic]

Your reply was fine Ian. I too found the original question was a bit lacking in information but hopefully Neil has got it right and it will suffice.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 22/11/2019 11:09:58:

I don't think anyone has answered the original question, which was quite straight forward,

[…]

.

Is that really all there was to the opening question, Neil ?

[ definitions of the three terms ]

I thought Chris was looking for a way to remember appropriate rake angles. … but perhaps that was just my inference.

MichaelG.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 22/11/2019 15:18:18:

Posted by Neil Wyatt on 22/11/2019 11:09:58:

I don't think anyone has answered the original question, which was quite straight forward,

[…]

.

Is that really all there was to the opening question, Neil ?

[ definitions of the three terms ]

I thought Chris was looking for a way to remember appropriate rake angles. … but perhaps that was just my inference.

MichaelG.

I suppose there was a degree of ambiguity. When starting out I confused positive and negative rather than not understanding the relatively simple relationship that (in broad terms) ductile=sharp brittle=blunt. I also find things like R/H and L/H with regard to tools needs a bit of thought despite it being obvious which to use for a particular job…

[Michael GilliganParticipant @michaelgilligan61133]

This is worth watching: **LINK**

https://youtu.be/0b34pkLjIuM

MichaelG.

.

Edit: Here’s his website:

https://www.dmt-llc.com/metalcutting-training-ron-d-davis

Edited By Michael Gilligan on 22/11/2019 22:33:39

[Chris TickTockParticipant @christicktock]

Posted by Michael Gilligan on 22/11/2019 22:26:38:

This is worth watching: **LINK**

https://youtu.be/0b34pkLjIuM

MichaelG.

.

Edit: Here’s his website:

**LINK**

Edited By Michael Gilligan on 22/11/2019 22:33:39

Yes thanks Michael, I was hoping for some really easy way but for me I use a crappy aid (which is why I was looking to see if someone had a better memory aid) POSIN as the cutting edge will be inclined into the metal, remembering this means the opposite Negative will incline away or outwards. It's the best I can do at the moment.

Having poked around and read the posts here it comes down to generally for the small lathe home user most often rake angles on lathe cutters not that important but this may not apply when you get into machining very hard metal (possibly very soft too). If you had a whole machine shop using the wrong cutters could be expensive whereas not so for the occasional home user.

Chris

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