Sharpening Technology and Methods for CNC Tooling

Sharpening spiral cutters for CNC equipment is a very complex undertaking. Here is a detailed discussion with full explanations.July 29, 2012

Is anyone sharpening their own tools in house either on an occasional basis or on a regular basis? My understanding is that sharpened tools are simply turned down in diameter. Is this correct? I am curious about abrasives, lubricants, etc. Any relevant information would be useful. I am aware it is likely not cost effective for most people but wish to explore this in principle at least.

Forum Responses
(CNC Forum)
From contributor H:
Since this is on the CNC forum I assume you are talking about router bits vs. profile knives? I suspect you could set up to sharpen straight flute bits fairly easily. However properly sharpening spiral bits is a very expensive proposition.

From the original questioner:
Yes, bits, not profile knives. I am not sure if a spiral bit is turned down from the outside or if it is touched on the inside of the flute. It would be a complicated "indexed" procedure in the latter case and an examination by eye leads me to believe they generally are only turned down. I also have some tapered spiral bits, which my regular sharpener cannot do nor can Onsrud.

From contributor R:
I believe that most router bits are sharpened on the inside surface not the outside diameter. Even if it is a straight carbide bit it is taken off the face of the tooth not the edge.

From contributor H:
Contributor R is right. Sharpening takes place on the cutting face. From what I've seen, spiral bits are done both ways. It is my understanding the "right" way is to do the face. This obviously requires machines that will follow the spiral.

From the original questioner:
That explains a few things. Not quite sure how they go about sharpening a compression bit on a production basis though.

From contributor B:
I work for a sharpening center in the northwest and our specialty is CNC router tools. Typically across the board bits are sharpened on the O.D. (Outside Diameter). This is the quickest and less expensive way to sharpen. The diamond wheels we use are very expensive along with the machine. You can sharpen on the inside of the Flute but it is twice as expensive and twice the amount of time on the machine. But you do take off about half the amount of carbide vs. an O.D. grind. On Straight Flutes they are usually all sharpened on the inside of the flute.

From contributor J:
When you sharpen bits rather than replace them, doesn't that alter the dimensions of the milled piece especially if you're sharpening from O.D.? Also, profiled router bits are almost never sharpened along their profile but always at the face, true? Wouldn't it be less expensive to just replace straight or spiral flute bits rather than sharpen them and have to reconfigure programming? Is it even possible to sharpen diamond bits?

From contributor R:
Most of our bits are used on a CNC so we adjust tool diameter in file and the part size stays the same. The cost to resharpen a spiral is roughly 1/4 to 1/3 the price of new and you can resharper a bit multiple times, so for us it is definitely worth it to sharpen bits. Yes it is possible to sharpen diamond bits depending on the type of bit. For our custom diamond bits we can get several sharpenings out of the bits depending on how badly beat up the bits are. It must be done with special equipment.

From contributor U:
We sell sharpening machines to the vast majority of tool manufacturers, resharpeners, and in-house tool rooms all across America. Contributor B is on point. The "right" way to sharpen a CNC spiral bit is to grind the "inside Flute" which is the face and when the O.D. relief angle gets small enough, lightly grind the O.D. as well. However, the machines that can "flute" grind are expensive. Our most popular is an UTMA LC35e-CNC and its $100,000. You can also re-sharpen on the big 5-axis machines that make the tools, but they're over $200,000.

In the real world, probably 50% of the regrind shops are still sharpening only the O.D. because itís faster, easier, and less expensive. You can do this on a manual tool grinder with a finger follower for under $10k used. The problem is you lose the diameter of your tool much faster and you lose hook angle and gullet on the flute. They still cut well, you just can't get as many resharpenings this way.

Yes, for straight tools, resharpening on the face is easy inexpensive and the proper method. Yes, you can resharpen diamond CNC router bits. We sell a machine called the Lach-Mini that resharpens diamond but itís even more expensive (over $200,000).

Rule of Thumb: until you're annual sharpening costs equal or exceed the price of buying a sharpening machine ($100k per year for CNC compression spirals, $35k per year for carbide saw blades, $12k per year for carbide inserts, etc), you're better off sending them to the professional grinding shops. They have large investments in machinery and a lot of experience sharpening tools.

From contributor G:
This is a very good thread. To do your servicing in-house, you should have 50 plus tools a day to make it cost effective. A finishing tool allows to do the outside of the flute, which provides proper relief then we do the inside. The OD will change. Never take a tool under 10% of the diameter. Tool life would be reduced. Use a firm that understands what you require.

From the original questioner:
I did find out in the meantime that most spirals are sharpened from the outside, like Contributor B and Contributor G have mentioned. Like Contributor R, I use tool compensation at the machine and program nominal tool dimensions, so diameter changes on the fly are not an issue.

Here's the thing: Some guys have touch tool. Really simple attachment and some manufacturers include the feature on the machine. Others have aggregates or saws, and others have lathe attachments bolted onto the router. I knew of one guy who put an air drill on his machine for drilling holes to save wear on his spindle. This is complex stuff at times, but worth the effort.

Wouldn't it be possible to have a bit dresser onboard the machine to do simple grinding? I am not talking something thousands of dollars, but more like just a simple Delta or Grizzly bench grinder bolted to the side of the machine, perhaps with the shaft mounted vertically. With the correct grinding wheel and a pretty simple program it could dress a bit in a minute or two, I would think.

I know there are abrasives that professionals use that are expensive and very effective. CNC guys spend a lot of money on compression spirals that last a long time and are genuinely worth the money because we cut all day long with them. Sharpening guys have specialized machinery and abrasives that work well for them on a production basis. In reality though if I cut five sheets a of MDF a month I could use a 1/4 inch steel straight bit for $20 and be done with it nearly as well.

My silicon carbide gray wheel attached to my Delta bench grinder will remove a surprising amount of material from the end of a chipped carbide compression spiral pretty quickly. Guaranteed it is not the best abrasive for carbide and for a little more money I could likely do ten times better. What would be the best relatively inexpensive (less than $100) type of wheel that will dry grind carbide on an occasional basis? What grit does a professional use for turning down carbide bits in diameter?

From contributor T:
This is a great thread but I think there is a disconnect. There have been several references to sharpening the OD and turning the OD down, however this is not actually what happens. The grind has to follow the helix of the cutter and it is not diametrical. It is not as simple as just turning or grinding to a smaller diameter. The relief on the backside of the cutting edge has to be less than the cutting diameter otherwise it would drag as it cut. I realized as I was typing this how hard it was to explain! This is not a straight forward turning/grinding recessing process.

From the original questioner:
Thanks for that clarification, it makes all the difference and explains why more people do not sharpen their own bits. Even if I could radially index the bit it would not on its own solve the problem as tooling just got a lot more complex than a grinding wheel, and would need to approach at various angles for a compression bit.

From contributor H:
Most all of the responses are correct. When sharpening a straight flute carbide-tipped bit, the face is sharpened. This is the best way on those bits. On spirals bits the diameter or primary land is usually sharpened. You must remove the wear land and replace with the proper clearance angles in order for the tool to cut correctly. I will say that 2-3 OD sharpens is usually the norm. More than that and the tool will heel or burn.

From contributor W:
If sharpening an industrial cutting tool was as easy as mounting a bench grinder to the side of your router, I'm sure it would be a standard feature and I would be out of a job! We have a very large investment in grinding equipment, coolant systems, diamond wheels, inspection equipment, and numerous other support functions, all of which you have access to for, in my opinion, for a fairly inexpensive cost.

I understand your thought process on trying to reduce costs and looking at every aspect of your business where you are sending money to others - we do the same thing. Again, I would suggest you make sure you are getting the most out your sharpening budget (not all cutting tool services are created equally) and spend that money and effort you have suggested toward trying to do your own sharpening on other areas of your business where you may get a better return on your investment.