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Sharpening countersinks

rpchristian

Aluminum
Joined
Jan 11, 2012
Location
Sheffield, England
Hello there,

We have a small tool and cutter grinding machine lying around mostly unused, and I'd like to try sharpening our counter-sinks. Does anyone have any advice on how to do this? I can't seems to find any info, and my efforts so far have been fruitless: the counter-sink comes out looking good, but it won't cut at all. I'm obviously doing something fundamentally wrong. Any help greatly appreciated.

All best,
Richard
 
Richard........ off the top of my head I am guessing you have no relief behind the cutting edge...... axial or radial. IE did you just circle grind the counter sink..... ie zero relief?

There are fixtures specific to this task (Weldon, Harig, Richleau and etc) Lacking same you put an appropriate back off (about 6 degrees for steel, maybe 12 degrees for aluminum leaving a 0.015" land.... followed by a radial grind to the next flute...... or all the way round back to the cutting edge on a uni-flute.
 
What of the many styles of countersinks are you asking about? Single-flute, 3 flute, six-flute, etc? Hard to suggest a solution if the cutter type and size is unknown. A photo or link to a photo from a catalogue could help if ytou cannot post a picture from your phone.

Denis
 
Countersink flutes need to have a cutting angle like an endmill, that is, not a conical surface. I've sharpened one like an endmill with a cutting edge and a back relief and all it did was grab and lock into the work. I've noticed that single flute countersinks, those with a hole in the side, are cam ground, they have a constantly decreasing diameter. How to do this is beyond my humble experience. Multi-flute cutters must have the same profile so they cut but don't hog in and lock, but the width of the flutes gets larger near the top of the cutter. I saw one made by a toolmaker that had a carbide insert in a slot. Basically, my co-workers and I have tried everything but nobody seems to know the proper way. Cutter grinding experts know, because I had two from New Process Gear that have been resharpened.
 
This won't provide quite the same continuous relief on a single flute, but the instructions might give some ideas.

Op Manual

Basically you need one of many fixtures that take a cam. Either single lip, or multi-lip, cams per the # of flutes on the tool. The polychoke will do multiple flutes easily with short cam sections. For single lip, the operator has to get the amount of cam desired in the first 15 - 30° (guesstimate) and then constant relief from there, or step over a notch and ad some more cam relief in steps.

Here's another style, bought from a PM'r. It will do 2,3,4 &6 flutes all in one set up. The amount of cam is completely adjustable, but it is a little complex and I've never used it yet. It also will only do cam relief in short sections, not continuous the whole way 'round the tool.

smt_R-Amachineindex4.jpg


I have thought of merely putting a short leadscrew on a cheap chinese spindex to get continuous cam, but have not needed it bad enough yet....

smt
 
It would be easier to grind the face(s) of the countersink rather than the relief, I did a three flute last week by hand on a wheel dressed to a fairly sharp angle, it would be better done in a jig or cutter grinder workhead but I was in a hurry and it made the difference between just rubbing, and making a nice chamfer on hole(wasn't for CSK screw).

I would have thought that a tap grinding fixture could do the relief,but I have never tried that.
 
Richard,
You will need to sharpen it just like a drill bit. I would take it and put it into an existing countersink, and it will become apparent that you do not have relief on it. Drill sharpening rigs use a cam to make the relief as the drill rotates, neat to watch. Think of it like a drill, and it will work well.
Joe
 
This won't provide quite the same continuous relief on a single flute, but the instructions might give some ideas.




I have thought of merely putting a short leadscrew on a cheap chinese spindex to get continuous cam, but have not needed it bad enough yet....

smt

I did exactly that. I used a 20 TPI screw to get the desired relief. Worked out well. Quite a bit of work, but an interestinbg design project. You do not need the boss with the ten holes on the top of the spindle housing for this application so I sawed it off and cleaned it up wiht a file. Sometimes that boss gets in the way otherwise. Various "lead screws" can be used for varying relief. Has other applications as well if no lead screw is used and is a simple spinner.

I will likely power mine similar to the compound powerfeed for spinning various cylindrical items not using the lead screw.

Denis
 
My buddy has a Darex countersink sharpening fixture and I took my dull single lip countersink over and sharpened it up clean. Now it's dead sharp and won't cut worth a damn. And the Darex machine does have a cam. I think there's more to the story somehow. Either that or we didn't understand the (crappy, one-sentence) instructions.

So for now I'm back to buying new ones when my old ones get dull. Sigh.

metalmagpie
 
My buddy has a Darex countersink sharpening fixture and I took my dull single lip countersink over and sharpened it up clean. Now it's dead sharp and won't cut worth a damn. And the Darex machine does have a cam. I think there's more to the story somehow. Either that or we didn't understand the (crappy, one-sentence) instructions.

So for now I'm back to buying new ones when my old ones get dull. Sigh.

metalmagpie

I have the Darex tap and countersink grinder and I have successfully sharpened single and multi-flute countersinks. I have not run it for a few years, so next time I will have to study the instructions again. But there are three things to set: angle, number of flutes and starting point (where you locate the tool in the collet). If you do not get the starting point set correctly, you can end up with no or even negative back relief and no cutting.

Larry
 
Could it be that the nature of a countersink is exactly determined by the nature of the relief.

I have tried it too, countersink, dull to the point it would raise a bigger burr than it removed. Just grind it like a "generic cutting tool" . Grab and chatter. Horrible. I am fairly certain it might have worked in the mill, but no way in a 80 year old totally clapped out drill press.

Thinking about turning tools. The relief angle determines the maximum feed possible without rubbing. A brass finishing tool can do with amost no relief. A threading tool for coarse threads will need a ton of extra relief in the direction it advances.

Woodturning with manual tools relies on exactly that same effect. The relief is made to slightly rub on the workpiece before pivoting the edge into the wood. The amount of rubbing controls the cut and prevents the tool from digging in, especially with the sharper and more unstable ones like the skew.

There are also woord turning tools (modern ones) consisting of a cutter and a chip width limiter, which is again a piece of metal riding the wood and preventing dig in.

Now all you need to know is the exact angle which will allow feeding to happen, but not self feed. That might be the secret behind countersink grinds, their relief is minimal and even with a lot of pressure they dont cut especially agressively...
 
Could it be that the nature of a countersink is exactly determined by the nature of the relief.

I have tried it too, countersink, dull to the point it would raise a bigger burr than it removed. Just grind it like a "generic cutting tool" . Grab and chatter. Horrible. I am fairly certain it might have worked in the mill, but no way in a 80 year old totally clapped out drill press.

Thinking about turning tools. The relief angle determines the maximum feed possible without rubbing. A brass finishing tool can do with amost no relief. A threading tool for coarse threads will need a ton of extra relief in the direction it advances.

Woodturning with manual tools relies on exactly that same effect. The relief is made to slightly rub on the workpiece before pivoting the edge into the wood. The amount of rubbing controls the cut and prevents the tool from digging in, especially with the sharper and more unstable ones like the skew.

There are also woord turning tools (modern ones) consisting of a cutter and a chip width limiter, which is again a piece of metal riding the wood and preventing dig in.

Now all you need to know is the exact angle which will allow feeding to happen, but not self feed. That might be the secret behind countersink grinds, their relief is minimal and even with a lot of pressure they dont cut especially agressively...
It was not hard to determine the factory grind. Just chuck a new single flute in your lathe or mill. Set a DTI in contact with the face and turn the cutter 270 degrees. Read "runout". That is how I determined the pitch of my lead screw for sharpening. But based on Heavy's patent idea, a purpose-made eccentric collet might be an easier solution to achieve the desired runout.


Denis

Thinking a bit more about this, you could use a standard 3/4" collet and make an eccentric 3/4" OD bushing and put that around the 1/2" shank of the countersink. Prevent the shaft from rotating in the bushing and put the now eccentrically bushed countersink in your spindex. Orient it as shown in the patent drawing. I think it would work fine. The patented idea uses a lead screw just like the idea SMT mentioned and is what I did with a spindex. It is just that the eccentric should be an easier still way to do the job.
 
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I am doubtful of the eccentric bushing versus a cam. To sharpen a zero flute, you need a sudden change in height of the fixture, and you must position the tool so that as soon as the finger is riding the lowest part of the cam, the grinding wheel is on the cutting edge. Clearance must steadily increase all around until the wheel gets to the hole, at which point, the cam makes the sudden transition again.

An eccentric will never give you this clear line of demarcation with significant difference in the height of the cutting edge.
 
I am doubtful of the eccentric bushing versus a cam. To sharpen a zero flute, you need a sudden change in height of the fixture, and you must position the tool so that as soon as the finger is riding the lowest part of the cam, the grinding wheel is on the cutting edge. Clearance must steadily increase all around until the wheel gets to the hole, at which point, the cam makes the sudden transition again.

An eccentric will never give you this clear line of demarcation with significant difference in the height of the cutting edge.

Dang it, you are right, Hu. I was going to hop on and correct the post before someone nailed me on this. But you are so right. Back to the lead screw idea I am afraid. It does work.

Should have drawn this out rather than just thought about it---too good to be true.

Denis
 
I was told that the basic geometry of a single flute countersink is a cone tilted at a small angle with respect to the axis of revolution and arranged so that the point is slightly the opposite side of the axis to the base. Taking a 90 degree slice out of the cone was said to produce a cutting edge with appropriate relief. So far as I can visualise the combined effect of tilt and offset produces essentially the same spiral relief as the screw used by Denis. If this is the case grinding the face of the cutting flute and leaving the quasi conical surface alone should sharpen the countersink.

Seemed to work for me when I ran a slipstone up and down one of mine a few times as advised by the guy who told me about the cone derived shape. Any significant grinding would need a similar amount taken off the trailing edge of the cut out to ensure that only the cutting edge contacts a circular, or conical hole. I imagine that the trailing edge clearance acts to dynamically control cut depth.

Clive
 
I don't know what you guys are so worried about. Buy single-flute csks, and hand sharpen 'em on the bench grinder. Works well. The hand is a marvlous self-guiding powered cam. <puts on flame-proof suit>
 
Spec fab, real men can hand grind three fluters, just need a nicely dressed wheel (takes a bit of luck or exceptional skill to get them to cut even on all three flutes though) Best practised on the crappiest CSK you can find (MSC own branded sets of 6! There ground with flutes with a sharp corner, so when CSK'ing anything ductile just instantly jam up, nice CSK's have a nice radius in the root of the flute to roll the chip!) just grind and see what happens, rubs you just back it off a bit more, grabs grind the cutting edge back a nats, couple of goes sub 5 minutes you will have something workable again. Might not be the best csk to use for the bolts attaching the nozzles to say a Saturn 5, but plenty good enough to break the edges on holes drilled in black mild steel flat for gate pivots!

I normally work on the theory of having a "best" countersink for actual csking, then a few runners up for just chamfering - de burring edges. Have contemplated trying to make a fancy jig to grind them cam stye with a adjustable angle cam, but a decent countersink will last a long time, and when its times up it will after a quick hand grind do just fine in a de-burring only kinda roll.
 
We never did get an answer to the type of countersink question. Different types need different techniques.

The O flute types can be sharpened using a small stone on a Dremel INSIDE THE HOLE. But this is limited in the number of times you can do this before the hole is too big. And the rake angle increases with each such sharpening. And I have purchased O flute countersinks that were not ground properly in the first place and sharpening them this way is useless. You have to completely regrind the outside to the proper shape (see below).

The single flute CSs can be sharpened by hand. I have done this with new ones that did not cut well. I used a slow wheel with a water bath of the type that is sold for sharpening knives and chisel blades. The slow cutting action gives better control and you can follow the original relief from the cutting edge all the way around to the other side of the flute. It takes a number of passes, but it works well and you get a nice sharp edge with the proper relief. You need to be careful to start the pass with the rake face square to the surface of the wheel and the rotation of the CS should start almost immediately after first contact with the wheel. If you start before this point you can destroy the relief. Holding the CS by hand in this process allows it to conform to the wheel along a straight line parallel to the wheel's axis of rotation. The CS axis should intersect the wheel's axis at all times during this process. The ones I sharpened this way cut far better afterward than when they were factory new.

The above technique also works with the O flute CSs that are improperly ground at the factory (or on good ones that are just dull). You start at the middle of the hole and go in the reverse direction to create the relief. Each pass starts at the same place and the initial ones are just a few degrees of rotation. Each successive pass goes a bit further towards the other side of the hole until you finally reach it. Some layout die or magic marker on the CS can help you see exactly where you are grinding. If the area just behind the cutting edge is too low initially, you should make several sets of passes that stop at the high point instead of going all the way to the cutting edge. Each successive set of passes would go a bit further to approach the edge with the proper relief.

The above technique requires a bit of skill, but it is easy to learn. Do not try it on a high speed wheel as you will almost surely go too deep. You need the fine control that a slow speed wheel provides.

As for multiple flute CSs, you probably need a T&C grinder and a knowledge of the proper angles to set to produce the proper clearance on each flute. Just as a drill bit that is ground with facets can need a secondary clearance facet behind the primary one, a CS edge can have proper clearance just behind the cutting edge, but further back that flat facet may protrude outside/above the the cone of the cut and prevent the edge from penetrating. The facets may need to be at a greater angle than one might think just from observation. Some math is probably in order here. Sorry about that!
 
I was told that the basic geometry of a single flute countersink is a cone tilted at a small angle with respect to the axis of revolution and arranged so that the point is slightly the opposite side of the axis to the base. Taking a 90 degree slice out of the cone was said to produce a cutting edge with appropriate relief. So far as I can visualise the combined effect of tilt and offset produces essentially the same spiral relief as the screw used by Denis. If this is the case grinding the face of the cutting flute and leaving the quasi conical surface alone should sharpen the countersink.

Seemed to work for me when I ran a slipstone up and down one of mine a few times as advised by the guy who told me about the cone derived shape. Any significant grinding would need a similar amount taken off the trailing edge of the cut out to ensure that only the cutting edge contacts a circular, or conical hole. I imagine that the trailing edge clearance acts to dynamically control cut depth.

Clive


It is not a tilted cone, it is a combination of a cone and a helix/spiral. As you rotate past the cutting edge, the angle of the cone, as measured to the axis, constantly decreases. This creates the clearance. So you can not generate this shape by tilting any cone with a constant angle. The action used to create this is much the same as that used to create "conical" tips on drill bits. The angle must change while the tool is rotated across it's face.
 








 
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