Tool Wear

Every shop is as different as the people in them. Machine shops face an ever changing diversity of challenges and forces that drive their businesses down many roads. Machinists are generally a very resourceful bunch. We enjoy a good challenge. The flip side of that is the more challenging the job is to machine, the slimmer margin of error and sometimes profit there is.

Many pieces of the puzzle exist in the overall makeup of a machining job. Material, equipment selection and condition, coolant, fixturing, programming, QC methods, tool holders and last and most important: cutting tools.

Engineers and programmers are experts at selecting and applying tooling. Machinists are trained to recognize what type of tool can perform a given function and give their everyday efforts and skills to making sure they are put to the test in the most efficient ways.

Once the tools are selected and applied, all of the offsets are tweaked in and the job is under way the machinists are responsible from that point on to assure the parts are coming off the work center within quality specs. But that’s not the end of the story. Tools will wear out eventually. This depends on many factors. Machinists need to be able to read the signs of good cutting tool performance, and bad.

The signs vary from job to job and material to material. Bad finish, out of tolerance features, chatter, burrs, poor chip formation, and increased spindle load are usually signs that tool wear is beginning to be a problem. Deciding when to change out a cutter is a decision that is made with many years of experience making chips under one’s belt. Machinists will often consult with supervisors and managers to determine what level of wear is acceptable before changing a cutting tool. Driving a tool to catastrophic failure is never a good choice. Who wants to scrap a part and go home feeling like a kicked dog?

Pulling tooling too early may guarantee a good part but the tooling manager and boss will not be happy if the tool expenditure is way too high and it ends up eating all of the profit. You could end up going home that night with your tool box riding along in your truck bed.

Finding the perfect balance can be elusive but if done right, profitable.

What is the perfect balance? The best answer is: It varies.

As mentioned before, shops are as different as people. Some shops will tell you to crank it up until the tool breaks then back it off one click. Others baby their cutting tools so much it is hard to tell it has been used or will pull tooling after a set number of parts no matter what the wear is. Finding out what the atmosphere is step one.   Step two is finding out how your tools are performing for each specific function. For most milling and drilling applications, a simple burr test can tell you if your tools are still sharp or not. Sharp tools will not usually leave a heavy burr unless the material is a harder alloy. Noise levels can tell you a lot. Spindle load when roughing is also a pretty good indicator.

The best and most reliable way, however is to visually inspect the cutting tools after they come out of a cut. Inspecting the wear characteristics of the tool will tell the machinist the whole story of how that tool was running. Keeping track of how parts the tool has run is a critical step to determining your ideal tool change interval. Once a consistent interval has been determined, tools can then be managed.

We like to see tools that have even wear on the flanks and O.D.’s with the tips worn but not shattered or burnt. The flutes of end mills will show signs of usage but not be chipped.

Nobody wins in a catastrophic tool failure event. Bosses freak out, and in addition you may need to change your underwear sooner than expected. The kicked dog feeling will not go away without going home and actually kicking the dog. (Disclaimer: not advisable)

Watching for the signs and then pulling the tool at the correct point each time will give positive results on every front.

Now, finally on to re-sharpening.

Re-sharpening or otherwise known as re-conditioning a cutting tool is a process where the tool is put into a cutter grinding machine, either CNC or manual, and a grinding wheel is applied to remove the worn areas. Care is taken to measure the amount of stock removal so as to remove just enough to recover the sharp clean edge without reducing the tool diameter to excess.

Often, this number is approximately 10% of the original tool diameter. So, a 1.00” diameter end mill can be ground down to .900” diameter and still perform similar to new.

Below this 10% threshold a choice can be made to recycle the tool or remanufacture it. The reason is that the flute form changes with the reduction of diameter. The shallower flute form will not allow chips to flow as they were intended to thus reducing performance. Clearances will change as well and rubbing could occur unless feed rates are reduced.

(Hint: Reducing feed rates is generally frowned upon. Owners see it as a threat to their super-yacht diesel fuel fund. )

Re-manufacturing consists of grinding down the flutes with a fluting wheel to recover the original flute depth and hook, then regrinding the periphery. This can take a tool down to the next nominal diameter spec.

So of we take a the .900” end mill as above and remanufacture it to an .875 spec mill, it can then be re-used and re-ground many times more as a 7/8” spec end mill and so on. (Mills smaller than 7/16” generally speaking, are not a good candidate for re-manufacturing.)

Careful selection of clearance angles and, hook and rake are entered into the control data so that the tool will perform at or in some cases better than new.

Remanufacturing a cutting tool is an added expense over a simple re-grinding process but often it can still be very economical over buying a new mill.

Cutting off damaged ends and grinding several passes of an end mill to remove excessive wear is sometimes necessary but it usually comes with an extra cost. Here at KV we believe in no added costs when removing damaged portions of tooling and restoring them “within reason”. Excessive regrind charges are sometimes a necessary evil.

The other cost to consider is that the excessively ground end mill now has had its life shortened even more since it can now no longer be reground, say 5 times. It now can only be re-ground 2 more times before recycling.

A good tool management system finds the balance between productive machining and reasonable tool change intervals to allow the most out of both machine productivity and tool regrind cycles.
Shops that train their machinists to find this balance will certainly reap it in profits, retain customers and employees and have more life to enjoy. Without having to kick their dogs.