Post By stephen thomas
This may be a dumb question.The carbide inserts seem to be divided into those with a positive and those with a negative rake. Yet all the ones Ive seen seem to have a flat upper surface which does not slope in any direction and they sit horizontally in the toolholder. I thus can not see where the rake angle is. I would expect a positve rake insert to slope away from the tip and a negative rake to slope down towards the cutting tip but I dont see that in the ones I have or pics of those in catalogues.
Your from New York... so you know how to shovel snow...
You push the snow shovel in and under the snow and lift, that is a positive rake shovel.
You turn the shovel downsideup and reach out with it up aginst the riser of the steps and pull the shovel back to you, that's negative rake...
The "positive rake" takes a little imagination to find it.
On a lathe, IF you include the feed "helix" with a zero rake tool top face, the net result is "positive", not much, but positive.
The negative rake inserts are used with an obvious tip into the work.
In the past carbide inserts were fragile enough that they were NOT used if the "cutting" edge was more acute than about 80 - 85^.
Micrograin is kinda changing that.
Standard carbide inserts will NOT take the sharp edge than HSS will.
Where best sharp edges are needed, go HSS.
Experience says that M-42 and T-15 steels are usually MUCH more economical per sharp edge than carbides.
HSS is even more economical if you rig your tooling to use either- -
1. A flat top bit tilted to a positive rake and only sharpen the end.
2. Use the end of the bit as the tool top face.
Joel, if you look at a TPG insert you will see
that the upper surface is "flat" which is to
say, there are no formed in surface features
that give postive rake, if the bottom surface
of the insert is mounted perfectly horizontal.
However if you put the TPG insert into a holder
that is designed for it, there will be side
rake added in which make them effectively
positive rake. It's the tool holder that
creates the correct geometry for inserts like
This having been said, there are other types
of inserts that will give postive rake when
they are mounted dead flat. These have obvious
features which seem to give a 'rim' around
You can tell the negative rake plain inserts
because they have basically zero side clearance.
The side clearance happens with those when the
front edge droops a bit. On the TPG positive
rake inserts, you will see a fairly large
front clearance all the way around - that has
to be there or the edge would rub when the front
is tipped up to give positive rake.
First off, what's the exact part number for
the inserts you are working with?
Are you using a particular brand of toolholder
If you like I could show you the valentite
holders I use, with TPG221 inserts.
GaryE - I am from Minnesota originally, and I can shovel snow too, but I am not sure how your analogy describes positive and negative rake. Doesn't rake deal with the angle of the cutting edge at the point of the cut?
If so, I am not sure how the lift a scoop and downslice a scoop relate. Can you help a midwest snow shoveler translate that a bit?
You've been in New Mexico to long...
Lets start with a flat pc of plywood for a shovel...
your holding one end of it and the other end is layng on the sidewalk, pushing the plywood lets the front edge of the snowshovel contact the sidewalk first and peel the snow from it. That is POSITIVE RAKE.
You cant do it but lets pretend your strong enough to actually hold your end of the plywood and fling the other end out in front of you. Now pull the plywood shovel so that you keep the edge on the sidewalk and drag it to you. Sorta like spreading wallboard compound. you cant do can you, nobody can, but if you could because the plsywood was held by a very rigid machine, that would be Negative Rake.
I am using a SCLC toolholder and CCMT insert both from McMaster Carr. Their pictures at the beginning of the insert section shows pics of pos and neg rake inserts and it looks like it is the tool holder which seems to determine the rake angle.I would like to see pics of what you use
Aside from an insert with flat top, like the TPG for instance, other positive rake inserts actually have an angle down and back from the edge ... but for only a VERY short distance.
To actually see it, other than in manufacturer's pictures, you'd have to cross section an insert, probably with a diamond saw, in order to place the angle in plain view.
The primary reason for negative rake inserts is a stronger edge but that comes at multiple prices ... higher hp required at the spindle, a more robust machine and lastly, you're not going to do any fine finishing with a negative rake.
Unsolicited carbide comment ... when operating at proper speed and feed, a "flow" develops over the edge of the insert. It is not really a cutting edge but more like the snow plow mentioned earlier, moving along at full speed it develops a flow over the blade
you're not going to do any fine finishing with a negative rake."
i'll have to disagree here. perhaps you can't
"dust tenths" as some say w/ negative rake tools ,
but rake alone is not the only factor in fine
finishes. many(most in steel ) factory turned commercial
products have fine finishes ,produced w/neg geometry tooling.
i run neg rake tooling almost exclusively (steel)on my
flat belt drive south bend (w/ 5-1/2" motor sheave). when i ran a harrison
13" ...mostly tnmg 233 and 322 . with mirror finishes. sure, you need a REALISTIC finish pass of .020 or more for finish and accuracy...but that's part of the trade.
pos rake tools are useful in many cases-like super light cuts , aluminum ,plastics , "dusting"
a few thou , small dia , thin parts...etc.
novice machinists , i have personally found
(teaching mtt), are often misguided as to the
applications of insert geometry. lots of misleading
advice thrown around (conjecture) without regard
for application.(which should be the #1 consideration)
With a flat face insert, there are still 3 "axes" of rake. 1.) Axial: looking at the side of the tool stood shank vertical, does the insert lean front or back at the top, or neutral? 2.) Radial, (looking straight on at the end of the tool, does the line described by the cutting edge of the bottom of the flat insert intersect above center of the axis of rotation of the tool (negative), below (positive) or neutral? 3.) "lead" angle: Looking at a shadow of the tool held vertically, does the insert cut a square shoulder, or an angled shoulder on the work? I can't think of any undercut leads, except dovetails. Generally the lead angle is "negative" though typically described as a positive angle of 90deg or less. E.g., a 75 deg lead angle will leave an angle at the edge of the cut path of with that included angle, or 15deg off vertical.
Tools have a combination of all three, designed to accomplish a certain package of "benefits" for a given application. A moderate lead angle "softens" the cut by spreading the impact of an interupted cut or edge entry over a larger area of edge and creating a slicing effect (shear) So a "negative" lead, creates the effect of a positive cut. Too much may give a lifting effect that overpowers the available rigity of smaller machines. OTOH, the tool may have a more universal application, if it creates a 90deg edge, and sacrifices some smoothness of operation.
edited: Obviously my description best fits rotary cutting tools such as end mills/face mills, etc. But the 3 axes of orientation of a flat faced cutting edge to the work applies as well to tools held stationary while the work moves, such as planer or lathe tooling.
Negative inserts have 0 rake. When you put a neg. insert on a table you have no rake on the sides, the sides are perpendicular to the top &/or bottom. They are stronger when cutting but also require more HP - they push the material off. Positive inserts, in your case CCMT, has 7 degrees of clearance under the cutting edge, that's the second C. These have more of a shearing action when cutting & require less HP & exert less pressure. No matter what, speed still kills the insert.
What you are speaking of is 'clearance', not rake. The rake is the angle on the top of the insert, when mounted in a holder. If the insert tilts down, it's considered a negative rake insert, HOWEVER; most negative rake inserts have a positive cutting action, because of the geometry molded into them, with the chipbreaker. This is why excellent finishes can be obtained with negative rake inserts. Most positive rake insert tools actually hold the insert horizontally, the insert geometry produces the positive rake. A negative rake insert can be flipped in the holder, thus, doubling the cutting edges; a positive rake insert cannot.
Negative inserts have 0 rake. When you put a neg. insert on a table you have no rake on the sides, the sides are perpendicular to the top &/or bottom.
RAS and toolrul are both right -- negative inserts have 0° Relief Angle (straight sides), and are set in the tool pocket with the tip tilted down to get the negative rake angle. Because the sides of the negative rake inserts are straight, they're stronger than positive inserts, which have a 3 - 30° relief angle under the cutting edge.
You can't always count on the chipbreaker for positive rake -- a lot of the chipbreakers don't start until well past the cutting edge. If you look at the **MG, MH, and MM series chipbreakers (TNMG, CNMG,...) the top edge of the insert is flat for the first 0.05 - 0.06" before the chipbreaker starts. So unless you take a really big cut, those inserts have an effective 0° top rake.
So flat top inserts that are given a rake angle by tilting their resting pockets aren't meant for heavy cuts, since the further out from the cutting tip - the more below center it is cutting? Does this mean an insert in this configuration will have a higher spindle load than one with a built-in chip breaker, so that it is always cutting on center at any position in the width of cut?
Entoffice no question is dumb,
The way material shears off or fractures is the reason for different cutting edge rake angles.
Mild steel and aluminum shear off the part and flow over the bit more like saving butter and so like a positive rake angle of the edge going up to cutting action like a knife blade, and a very sharp edge.
Cast iron (and harder steel) don’t flow down they holds back against the cutting tool's pressure and then break away more like shaving ice so they likes a flat top or even better a negative or having the cutting edge angled down to the cutting action.
Each material and each different hardness of that material have a specific preference of rake angle.
Cast iron actually likes a slight (or even up to 20 degrees) negative and a honed edge rather than a sharp edge, and the hardness of carbide and its strength make it best to take the high pressure of machining cast iron, rather than high speed steel. Cast iron also likes a thick chunky insert or brased piece of carbide to absorb more heat, as the heat often dose as much or more damage to the bit than the cutting action. Holding pressure also requires a strong beefy tool and much machine strength.The change to indexable and adujustable tooling has cost manufacturing dearly in some kinds of tooling (Yes for some operations it is good to best) because of the nature of machining cast.
Page 4 of this link gives a really nice visual aspect to all of this discussion. Keep in mind that a "negative" insert can in fact act like a "positive" with the addition of a pressed in chip groove which will mimic the positive cutting actions when the tool itself is in fact not. The link also explains many other things not addressed here for those interested in some of the more rudimentary basics. http://legacy.secotools.com/upload/n...ep/Turning.pdf
Above all comments/replies were really helpful. Thanks
How much back rake for a triangle/square insert (i.e not primarily used for grooving) for common materials? Hard/Soft Steels, Aluminium, Stainless?