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

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.

Like others I don't grind tools specifically for different metals, the most I am likely to do is use a **GT for non ferrous and **MT for ferrous but even then I will use **GT on steel and iron. With HSS then I'll just give the top a few degrees and use it on everything. I don't even have a fancy tool rest just the poor basic one that came with the 6" bench grinder so all grinding done by eye and not measured. So a tool that I have ground up to a special shape for use on say brass will also get used on steel or iron if I need the same shape or the profile altered and the top surface left untouched.

When it comes to milling I had always used the same geometry milling cutters but have taken to using specific ones for non ferrous in some cases but don't have duplicates of all even if the sizes and shapes were available but can still cut metal without problems.

The other advantage of the higher rake **GT inserts when used on brass and the harder bronzes is that you don't get covered in a shower of fine chips as the swarf comes on in curls. Can't see much wrong with the finish from a CCMT with it's positive geometry compared to HSS. Notice how the insert tooling also throws the swarf away from the surface being cut so that there is less chance of it affecting the finish.

Edited By JasonB on 22/11/2019 07:32:40

Edited By JasonB on 22/11/2019 07:35:02

I use TNMG inserts (negative) on all materials I use - stainless, brass, steel, alu, and they cut well and with a good finish. Most of the times the finish comes out like a burnished mirror finish. Down side is I cannot machine very small diameters as the tool creates quite a lot of side pressure, and it does not give a good finish with spring passes - but what I mean to say is, it all depends on the application - maybe rake angles come to play most when turning small parts etc but not much for larger work - my limited experience opinion.

So I would experiment with tool geometry, materials, speeds and feeds and find what suits my application best, as probably no book will have your exact conditions (type of lathe/tools/etc) and work requirements. I opted for TNMG inserts because as a beginner I was going to ruin a lot of inserts until I get a good feel of my lathe, so having 6 cutting edges on one insert and the negative insert being more robust, it made a good compromise.

With the bit of experience I have picked up so far I dont get hung up on the angles and when I grind a HSS tool I just go with what looks about right and it always seems to work. I am starting to lean more towards insert tooling though, I have MT inserts for general turning and my parting is an insert tool plus I am about to get some GT inserts for non ferrous.

One thing that did jump out at me on Jasons link and the LMS link to recommended angles for different metals was the 35 degree back rake for aluminium, the tools I have ground and used have nothing like that probably around 15 degrees not that I have measured but they have seemed to work ok.

Edited By Ron Laden on 22/11/2019 08:57:39

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Nice demonstration, Jason

MichaelG.

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

Chris

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...

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

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

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

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.

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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...

This is worth watching: **LINK**

https://youtu.be/0b34pkLjIuM

MichaelG.

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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

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