Performance of Resharpened Bits

CNC router bits should take several resharpenings before performance drops off. October 18, 2005

We recently ran two jobs on our CNC router that turned into a large pile of wasted material... We are cutting .75" 2 sided white melamine (flake core). We cut with a Thermwood C50 - Twin 15hp spindles. We traditionally use a down spiral 1/2" bit , 18,000 at the spindle with feed rates between 300 and 600 IPM.

Twice now we used a resharpened bit and got major chipping after the second or third sheet. First bit was a compression, second a down spiral. Chipping occurs at the face (we do not cut through). In both cases, the bits were brand "new" from sharpening. First and second sheets cut well, then major chipping starts occurring (same feed rates, collection etc.)

I don't suppose it could be a spindle runout problem, since it would happen immediately. Bit does not seem to overheat (can be touched right after machining). Collets are clean, as well as shanks. I'm starting to think our supplier does not do that nice a job sharpening those bits. Any thoughts?

Forum Responses
(CNC Forum)
From contributor T:
I have not yet had cutters sharpened for our CNC. I was recently discussing this with a fellow at Onsrud. He indicated to me that when they resharpen a cutter, the relief of the cutter changes even on the first resharpen and that we won't get the same performance (cut quality and longevity) from the cutter that we will from a new one. So far, I've chosen not to go that route...

From contributor I:
Quality tooling, in most cases, is certainly able to be sharpened. The fellow from Onsrud is correct in saying that the geometry of a tool will change when it is sharpened. These tools are designed to specifications that will affect the feed rate that can be achieved, along with the quality of cut. It is also a fact that sharpened tooling will have a shorter edge life than a new tool will.

The length of time a sharpened tool should last is certainly longer than the few sheets that you mention. There are some factors that may be contributing to the short life that you are seeing. First, and probably the most important, is the tool one that supports being re-sharpened? If you are using high quality tooling to start with, chances are that the answer is yes. Second, is the tooling being sharpened on the correct sharpening equipment and to the appropriate standards as specified by the manufacturer of said tooling?

Third, are you running the tooling at the correct feed and speed? In the majority of cases, the feed and speed will not be the same as it is for the same tool as it was new. Because of the geometry change that the tool experiences after sharpening, the feed rate is generally reduced.

When purchasing CNC tooling, the old motto holds true - you get what you pay for. If you are spending the bucks to buy high quality tooling, such as Onsrud tools, you will get more than a good tool. I have nothing to gain by expressing my opinion, but I have found that the Onsrud staff and the support that is offered by them is second to none. I feel that the price I pay for these tools does not stop at the delivery of the package of tooling to my shop, but rather I have a vast network of resources to turn to when I need help, advice, and guidance. Yes, I have had problems with Onsrud tools. Some were my fault, and some were the fault of Onsrud, but in every case the customer support available from Onsrud has always been top shelf, a fact that justifies why I pay for quality tooling. I am not saying that Onsrud is the only game in town. Find a tooling manufacturer that can provide the support that is needed to accompany their product. Poor quality tooling will cost you more in the end, when things like pre-mature spindle wear are calculated into the cost of your tooling. Poor quality tooling will also cost you more in trashed parts, as you refer to in your original post. In the end, the cost per part is what you should be basing your tool purchases on and I hope that I have shown that this cost goes beyond the cost of a new tool purchase price.

From contributor J:
We had our bits sharpened by the manufacturer and they never lasted more than 30% that of a new bit. When the cost of sharpening is more than 30% the cost of a new bit, it becomes simple - toss them out.

From the original questioner:
Thanks for the input. This seems to confirm what I thought - toss the bits. I must acknowledge that the bit I ran probably saw several sharpenings. I think my tool comp was -.075 on a 1/2" bit and looking at the tool, it seemed that the angle at the bottom was not as aggressive as normal. We do have the manufacturer sharpen the tooling and when I asked them, they claim they sharpen the tools on the same equipment they make the new tool on. We've had good luck with their tooling but will try Onsrud to see if we get better tool life. For now, we'll try one sharpening only. I think second or maybe third sharpening might be the problem.

From contributor L:
Out of interest, how many sheets are you getting from the Onsrud cutters? We have never run anything else but Onsrud. Our main cutter, when nesting kitchens and cutting melamine colour and white on both MDF and particleboard, is the 3/8 inch Onsrud. We are cutting over 900 sheets per cutter. Our worst has been about 550 sheets. These are counted sheets, not just guessed. We ramp down and change the cutter at the first signs of chipping, as it breaks through the bottom of the board. The rest of the cut remains perfect, except for about half an inch where it breaks the bottom surface.

We have never tried them reshapened, though. I guess that is still to be tried one day. We nest everything in the kitchen cabinets, cutting mortise and tenons for joints and the toe spacers as well. Often our sheets (8' x 4') will have between 8 and 15, counting the drawer backs, rails and other small items. Feed speed is set at 37m per min.

From contributor S:
We use new cutters on our melamine and then get them resharpened at the manufacturer. After that, I use them for plywood, MDF and solid wood jobs. We use mostly Onsrud tooling. For what I do, they have delivered the best performance and longevity. I'll typically resharpen once or twice at the maximum. In plywood, I get 60% to 75% of the life of a new tool, so it makes sense for what we do. Different ships, different long splices.

From contributor B:
We re-sharpen our tooling all of the time. The re-sharpened cutters last almost as long as a new cutter. We run feed rates as high as 1500 IPM in 1" particleboard with laminate, using 1/2" compression cutters at 18,000 rpm cutting through the board. No chipping on the laminate. A cutter is good until the diameter is reduced by 6-7 percent. Then the changed tool geometry is a problem. Maybe the people you have sharpening your tooling are not grinding the tools correctly, keeping factory specs as close as possible. Maybe cutters are run too long before being re-sharpened. Do not get me wrong - nothing is as good as a new cutter, but re-sharpening makes perfect sense with the right people sharpening your tools.

From contributor G:
Is it worth getting your solid carbide tooling serviced? Yes! Most firms that have had their tool serviced and do not get good results never have them serviced again. We receive these tools for evaluation and we see a number of errors, from incorrect primary, secondary and relief angles, to taking off more carbide on the compression on either the up or down, leaving a line, etc.

When we service the tools (1/2" OD), they will come back to you smaller. They will perform as new, until they get to a .465 OD. At that point, you will notice a decrease in tool life, but not before.

Don't throw out the old tool - save them and Courmatt will provide you with our recycle credit program. Any piece of solid carbide, whether it's an end mill style or an insert, we will purchase from you. The cost of carbide has risen 300% in 2 years, so anything we save (which we sell to recyclers or we service and give to woodworking schools) will save mother earth from additional mining.

I do believe you can get the same performance from a service tool. If you don't believe me, send some of your tools to us and we will service them. The only thing you are going to lose is your tooling cost per part.

From contributor J:
A quick question for Courmatt: What effect does the bit being overheated before sharpening have on performance after sharpening? Do you have a way to tell besides the color?

From contributor G:
Depending on how the tool was manufactured… Cycle times are important when manufacturing anything, but we can not speed up the process in manufacturing. If we do, a couple of things could happen. One tool may have a micro fracture and the tool will break. Or the carbide will flake off if it's an inferior grade of carbide. When we have serviced some tools over the years (600 grit wheel), we noticed the carbide flakes off during the sharpening of these tools.

When a tool heats up during the cutting operation, it does not change the structure of the tool and/or its performance once it has been serviced. The tool and chips will heat up to 130-160F during its cutting cycle. If your tool and/or chips are hotter than that, reduce your RPMs. I know you are not going to have a device to measure the temperature, but you should be able to handle the tool right after the cut cycle. If it's too hot to handle again, reduce RPMs. Remember, make chips, not dust.

From contributor W:
I buy most of our bits from Vortex Tooling for our CNC. I will have all of our 1/2" bits resharpened by them one time. We see no difference in the performance of the tooling from new to resharpened. The diameter of the bit comes back at .48" after being sharpened.