Rake angles

I

Izzy

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I wanted to make my own insert tooling for my lathe and before I start I'd like to know the effect rake angle has when it's positive, negative or neutral I've tried doing some research but couldn't really find a solid answer.
The inserts I happen to have, have a 25° clearance angle so I was thinking of doing a 10°-15° rake angle which would leave me 10-15° clearance. What is everyone's advice? I can provide any Info y'all might need I Wana get this done right the first time!
 
Rake... and "back rake"... which one?
Here's a thread on back rake: http://www.practicalmachinist.com/vb/general-archive/tool-back-rake-80591/

mechanics-of-metal-cutting-5-728.jpg
 
Thanks for the diagram being a newbe this is the best diagram explaining the angles I have seen.
 
I guess I should be more specific with my question, good threads and good info but not quite what I was looking for.
I'm going to be making a boring bar, a turning tool and a facing tool the lathe I have came with a whole bunch of 60° triangle inserts that have a flat face and 25° clearance angle but no tool to hold them so I'd like to make use of them. I'm thinking 10° rake but don't Wana over do the rake angle does 10° sound good? And for the boring bar what does everyone recommend I use, round stock or square stock? What's everyone else use at home? I'm still trying to come up with a design so I'm open to ideas...
 
My boring bars were purchased so the manufacturer has already done the angle-of-the-dangle machining.

The bars are round stock with a flat machined on the top. I recommend this approach. It allows me to clamp the bar in the correct position in the toolholder and a consistent position when I need to re-position the bar for various depth of boring.
 
With that style of inserts, you generally have a neutral top rake for the insert. The problem you run into is the smaller the bar is, the more negative rake you have to put in the bar so the insert does not drag on the bottom side of the insert.

Go to some of the manufactures website that sell small insert boring bars and study the pocket geometry for the inserts offered. Some manufactures give angle information, some do not. Check other manufactures, be suprise to the details you can get to make a boring bar to. I would suggest using screw down inserts over the old generic ones like TPG's for boring bars. The screws are available off of eBone for reasonable prices.
 
I honestly wouldn't have thought to check manufacturers websites! And honestly I'm only trying to make these inserts work because I have hundreds of them. My main reason for asking this question was because I made a facing tool to hold these at a neutral rake as a tester and I was getting quite a bit of chatter. I just used 3/4" cold rolled steel with an angle milled in and 6-32 screw to hold it. Tool was on center and just chattered horribly. I don't Wana spend a bunch of time on a boring bar and have it chatter like crazy too. I noticed the boring bars at work have a negative rake too why is that? Just better for boring or something more?
 
Hi Izzy,

Couple of comments - which may (or may not help) you. I use inserted tooling for some things (parting and screw-cutting) - basically because it's easier to get the specific tool shapes I need. But for most stuff I still use HSS. Whilst there are clearly differences between inserts and HSS - I'm going to assume that the basics remain the same.

When I grind a HSS tool - the "rake" used will vary depending on the type of material I'm going to cut. Not sure you mentioned what this was but I'll assume you want to machine some form of steel. You should therefore have some side and back rake - the combination of these two giving you the 'top' rake. How much rake you give is really also a compromise - as sharper tools tend to have shorter lives. You can look up rake angles for HSS tools elsewhere - but I only use a zero (neutral) rake when machining brass.

Remember that inserted tools were intended originally for industrial use and hobby machines are not as rigid as large commercial ones - so whilst negative rakes might get used in commerce they will generally not be much use to you in the home workshop - unless you are trying to machine something very sticky - like phos. bronze - and even then I'd only use them if I had real problems...

Hope this helps.

Regards,

IanT
 
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