Is secondary clearance needed on the end of end mills?
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    Default Is secondary clearance needed on the end of end mills?

    I am practicing regrinding end mills. Is secondary clearance necessary because once the primary clearance gets past the material nothing touches the material. I see the secondary clearance that the manufacturers put on the end mill and realize it must be necessary or it would not be there. Advice-opinions, or experiences welcomed?

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    End or side?
    You say end so will it ever be asked to plunge?
    If so the cut material is coming up.
    A straight primary alone alone simply won't clear as it goes around the arc under the flute base.
    Sort of the same thing in facet drill flute grinds. Without the second the back hits or you have to make the only front one really, really steep. To the point that is just about impossible to grind without hitting the next front guy.
    The angle of grind is not the rotation of the tool so you run out of clear very fast.
    It's hard to describe but if you only put on the primary with no second or other and then go to use it you will find this trailing back corner clear problem.
    You don't grind the primary angle radially, you grind it straight. The tool works in rotation, not straight and it's the back ass before the next flute starts that is the problem.
    I hope any of this makes sense..........It's a circle being cut and the grind is a line and plane.
    Bob
    (PS. rereading this post makes it hard to understand for me and I model this stuff all the time so.... talk)

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    Quote Originally Posted by CarbideBob View Post
    End or side?
    You say end so will it ever be asked to plunge?
    If so the cut material is coming up.
    A straight primary alone alone simply won't clear as it goes around the arc under the flute base.
    Sort of the same thing in facet drill flute grinds. Without the second the back hits or you have to make the only front one really, really steep. To the point that is just about impossible to grind without hitting the next front guy.
    The angle of grind is not the rotation of the tool so you run out of clear very fast.
    It's hard to describe but if you only put on the primary with no second or other and then go to use it you will find this trailing back corner clear problem.
    You don't grind the primary angle radially, you grind it straight. The tool works in rotation, not straight and it's the back ass before the next flute starts that is the problem.
    I hope any of this makes sense..........It's a circle being cut and the grind is a line and plane.
    Bob
    (PS. rereading this post makes it hard to understand for me and I model this stuff all the time so.... talk)
    i get the idea for plunge cutting
    but for cutting sideways ?
    i have trouble visualising how it would interfere with the cut.

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    Q:[ Is secondary clearance needed on the end of end mills?] YES
    The primary at perhaps 5 or 7* (and more)just ground off the edge likely will leave an area below the cutting edge and before the next tooth that will not clear the part..One could radial relief the end by incremental rolling upwards for a distance but may hit the next flute doing that. Adding the secondary is quicker..and allows plenty of chip clearance.

    A four flute or more might clear to the next tooth but would not give enough chip exit room..

    For plunge cutting more clearance is desirable perhaps 9 to 12 (and more)for the primary and yes having a secondary.

    The gash/cut-in helps the chip to curl a bit and then exit.
    Last edited by michiganbuck; 10-16-2019 at 10:42 AM.

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    Would some moderator ban that spamming Skytech? He is infesting several sub forums on PM.

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    It can be done without a secondary but the primary will either need to be ground as a radius that falls away from the cutting edge (pretty much impossible for all practical purposes with one grinder pass on the cutter periphery - but possible on the end cutting edges) or the angle will have to be high enough that the back side of the grind clears the path cut by the front side - which will usually create a very weak edge.

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    I usually grind a secondary relief first so as to reduce the amount of surface that needs to be ground for the primary. Typically, the secondary relief is almost incidentally formed as I try to grind a form of 'split point' to get rid of the big dumb web at the center of the tool. I use a saucer wheel (diamond or CBN) to plunge/gash the center web into some form of cutting faces so the tool can plunge a little if need be.

    Disclaimer: I only do this by hand to get a rougher back into some sort of working state. I'm not into regrinding finishers much. But cutting the end off of an endmill is something I do quite often. Then some sort of end geometry is worth having, even if only one flute is cutting. Hey, you pay extra for that, it's called a wiper

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    I made a drawing to try to illustrate why the secondary clearance is needed:



    On the left it shows a side view of an end mill. The outer corner of one cutting edge is shown on the center line and a line representing the primary clearance grind is labeled "Primary Clearance". That line is shown as a solid line for the part that remains after the secondary clearance is ground and the remainder of that original line is shown as a dotted line to the OD of the cutter.

    Just above that "Primary Clearance" line is a line that represents the actual, helical path taken by the outer corner of the cutting edge if it follows the downward ramp that the primary clearance would cut with the maximum feed rate. This helical line is labeled "Ramp". The drawing on the right is my method for determining where this helical path would intersect the OD of the cutter. That, right hand drawing represents 1/4 (90 degrees) of the OD of the cutter FLATTENED OUT in the plane of the drawing. As labeled, this drawing is 1/4 of the cutter's full circumference in width. The helical path of the outer corner of the cutting edge becomes a straight line in this view and it is drawn at the same angle as it is in the left hand drawing so it is an accurate representation. The dotted line with arrows from the right hand drawing back to the left hand drawing transfers the point that this Ramp reaches in that 90 degrees back to the side view drawing on the left where it determines the end point of the helical line labeled "Ramp".

    At this point is is best to magnify the view of the left hand drawing. Nothing is changed, it is just a magnified view.



    As you can see, that "Ramp" line intersects the 90 degree point of the OD at a point that is significantly higher than the primary clearance grind shown by the dotted line below it. This means that if the cutter is fed down into the work at a rate that matches the primary clearance angle (the maximum feed rate) then the tail end of the flute which is about 90 degrees behind the cutting edge would interfere with the ramp being cut by the primary clearance. This occurs to the largest degree at the outer edge of the milling cutter and the amount of interference decreases as you move toward the center line.

    Finally, above both of the above lines you can see a third line that represents the secondary grind. It is a solid line and is labeled "Secondary Clearance". As you can see, this secondary clearance grind completely eliminates the interference caused by the primary clearance.

    These drawings are accurate but the angles used are probably a bit larger than real life clearance angles. This was done to make the point as clear as possible. It does not change the conclusion. Yes, the secondary clearance grind IS NECESSARY. And this is true in almost every end milling style cutter that is ever expected to plunge down into the work, even if only the outer edges of the cutter are doing the cutting for that axial plunge. The only time that this secondary clearance would not be needed is for a cutter that is used exclusively for side cutting.

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    Some milling cutters have a small number of flutes. On cutters with two flutes, those flutes will reach the 90 degree point that I assumed in the drawings above. If the secondary clearance is not as aggressive as I illustrated, it can be necessary to add a third clearance facet. This can also happen when the designer of the cutter is trying to make the tooth behind the cutting edge as strong as possible.

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    Thanks All, learned something from this.

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    Quote Originally Posted by EPAIII View Post

    *Clear and concise explanation with pictures*
    Thank you! I was pretty close to understanding the points made by text alone, but a picture truly is worth a thousand words. I appreciate the effort you took to make those illustrations.

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    You are quite welcome. I am glad it helped.



    Quote Originally Posted by BoxcarPete View Post
    Thank you! I was pretty close to understanding the points made by text alone, but a picture truly is worth a thousand words. I appreciate the effort you took to make those illustrations.

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    Good to have drawings or photos at your grinding station to look at when doing ends. Very often the cut-in or gashing to center is done by hand and looking at a end drawing can help you do that better. the out most corners are functional and need be all the same. The center cutting need be thinned or made like a split point... or only one flute to center with having a dish.
    Best to finger on the same flute being sharpened very close to the front edge....not off an index.

    I like a double hump finger, so the higher hump is registered at the OD and the lower hump touching the tooth face close to the end. this keeps grinding pressure form pushing the lip down.

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    Some guys must ramp into a cut pretty aggressively if they are actually going to crash into the primary relief. In fact, the primary relief itself would still be a problem with such aggressive ramping.

    I ramp in most of the time at between 1 to 2 degrees, but the primary relief is a lot more than that.

    Note that a twist drill does not have a secondary relief (when sharpened on an Oliver drill grinder )

    I'll maintain that the secondary relief is primarily concerned with creating a chip flow space at the center of the tool because it has nothing to contribute to the cutting action.


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