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Step 3: Leverage the advantages of grinding over Swiss turning
Should you use a tool grinder to produce rotary instruments when you can make them on a swissturn?
The answer is most definitely – Yes!
A swissturn is the most accurate style of chip making machine you can buy for small round parts.
As most industrial tool makers have now discovered, the advantages of grinding over machining are considerable and form compelling reasons for you to switch rotary instrument manufacture over to tool grinders. In this installment we’ll discuss just some of them;
Better than swissturn style work holding and support
The idea of a swissturn that makes it accurate is the bushing that supports the bar stock right up close to the cutting tool. CNC tool grinders now have travelling steady rests that perform a similar work-supporting function, but they do it much better. ANCA’s is called ‘the P axis’. Housed in a rigid casting, the P axis is a CNC controlled axis that traverses along the length of the tool, automatically following the grind point. The work holding on the P axis can be a half bushing, V or the renowned Arobotec auto diameter compensating steady.
ANCA’s travelling steady (P Axis)
Let’s say you were making a 2.2mm (.09”) drill on a swissturn out of 4.5mm (.176”) bar stock. The guide bushing would support the stock while you turn the OD. But then, when the secondary live tooling was being used to machine the flute, the guide bushing would no longer be providing support, allowing the part to flex under machining loads. This problem obviously gets worse for long thin drills like bone drills.
P Axis with Arobotec Steady
But on a tool grinder equipped with a travelling steady rest and Arobotec, the drill is perfectly supported at any OD, not just the OD of the stock, like on a swissturn. The grinding forces during flute grinding are directly above the steady rest, so deflections are eliminated. The end result is consistent flute depths and margins, even on very long, skinny parts like bone drills.
Another benefit of grinding is that on a tool grinder, it’s possible to use un-ground stock as well as ground stock. In most cases, a swissturn requires that centerless ground bar stock be used to avoid fouling or galling. Out of round stock on a swissturn will show up as uneven flute depths because swiss turning improves but doesn’t completely eliminate out of roundness.
Add to this, the limited headstock stroke of some swiss models and the case for grinding starts to really stack up.
High speed flute grinding
The starting point to low cycle times in instrument manufacture is usually high performance fluting. The majority of your metal removal after blank preparation is in the flute grinding stage so it makes sense to flute as fast as possible. The spindle of a modern tool grinder is significantly more powerful and robust than the live tooling on a swissturn.
For example, ANCA’s TX series machines have a 37kw (49hp) induction spindle motor, featuring high torque at high RPM; which is very well suited to grinding stainless steel where high surface speed is needed. When setup properly, a 200mm (8”) wheel on a purpose built tool grinder will flute much faster than a 5mm (0.2”) ballnose cutter on a swissturn with power as little as 0.5kw (0.7hp).
Using special coolant fixtures and long grit aspect ratio, vitrified bond, Aluminium Oxide wheels, flute grinding feedrates as high as 4,000 mm/min (160”/min) are possible on stainless steel bone drills.
High speed flute grinding of a medical reamer
Sharper edges that cut with less force and heat build-up
Machining is suitable for all kinds of components, but you intuitively know that if you want a quality edge that must cut something, it should be ground. A surgeon wouldn’t use a scalpel without a ground edge, so why should he use a bone drill without ground edges?
Ground edges are sharper than machined edges and a sharper edge will cut with less force and transfer less heat to the surgery site.
With grinding, you can choose the grit size of your wheel to match your surface Ra and edge sharpness requirements. Sparkout and reverse direction passes help give that extra bit of polish, edge and de-buring if you need it. And the low Ra of a ground edge will be more resistant to chipping than a machined edge.
Electro-polishing is a popular finishing technique for rotary instruments but you need to be careful how far you take it. Too much polishing can dull the cutting edges of the instrument. Because grinding produces better surface finishes, sharper edges and less burring than machining, ground instruments will usually need less electro-polishing. This saves time and improves instrument quality.
In parts 1 and 2, we talked about how a high helix angle is desirable for rotary instruments, but how it complicates the flute shape and lip design. Grinding gives you more control over flute shape than machining does. Here’s how;
There are two ways to attack the job of machining a flute on a swissturn. The most common way is to bring a small ballnose endmill in perpendicular to the flute surface. But this gives you very little control over the flute shape; the only flute shapes you can produce are those that can be swept out by a sphere (the ball). With this technique, you probably have to fiddle with the lip relief and the point angle to get a lip that cuts properly. In many cases, particularly high helix on steep pointed drills, you may not be able to get a good lip at all.
Model of a swissturn machining a flute on a bone drill
The other way to go about machining a flute is to use a larger T-slot or keyway cutter oriented at right angles to the flute surface. With this technique, you can vary the shape of the flute by pivoting the cutter relative to the helix angle, offsetting the cutter, using a custom cutter with a specific profile shape or a combination of all three. But choosing the right combination is complex and standard CAD/CAM software doesn’t really help.
This is where tool design software that comes with high end tool and cutter grinders comes in. Tool grinders assume the second method; ie: a (relatively) large grinding wheel oriented at (approximately) right angles to the flute surface. The software does all the math for you automatically. It gives you the ability to specify your desired flute shape and it will calculate and even dress the correct wheel shape for you.
Model of a tool grinder grinding a bone drill flute using a large diameter,
custom wheel (also shown is the travelling steady with Arobotec)
A couple of features in recent ANCA software releases take you even further; they calculate optimal grinding settings that allow you to use a standard shaped wheel in many cases without the need to dress a custom wheel shape.
So with grinding, you have more control over the flute shape, which in turn, gives you a better lip and hence a better performing rotary instrument.
Quicker, easier setup and programming
Compare the huge size of the industrial tool market to the much smaller size of the rotary instrument market. Combine this with what we’ve covered so far about the similarities in the geometry of these jobs and it’s easy to see why usability of tool grinders has advanced significantly over the usability of swissturns for this type of application. You can quickly and easily setup a modern tool grinder to produce a drill without complex trig or even any direct G-code programming.
They have Graphical User Interfaces (GUIs) where you just punch in the key dimensions on the print, they simulate the job automatically in 3D and generate and run the grinding program automatically for you.
ANCA’s Drill Wizard lets you define a new drill in 6 simple steps
With a tool grinder, you can work directly at the machine, entering the dimensions of the next job into the GUI while the machine grinds the previous job, or you can work from the comfort of an off-line simulator PC then upload the job to the machine for grinding.
The size and diverse needs of the industrial tool market have spawned a bewildering variety of tool shapes and sizes. Look in a rotary tooling catalog and you’ll see thousands of designs; drills, mills, saws, face cutters, profile tools, insert tips, combination tools, burs, routers, reamers … The list is long, diverse and ever growing. If there’s one thing that makes a tool grinder stand out from the CNC machine tool pack, it’s flexibility; particularly flexibility of the application and CNC software.
The geometry of spiral tools and the grinding software required to produce them is incredibly specialized. It’s designed from the ground up around very complex, specialized mathematics. This is why standard CAD/CAM, which is built on more generic mathematics, is a poor choice for designing rotary tooling (or instruments).
A tool grinder’s software must balance two opposing needs. On the one hand, it must be very easy to use by non-programmers, but on the other, it must be ultra-flexible so it can adapt to new tool designs as they emerge.
For instrument designers, the flexibility of a modern tool grinder will give you the freedom to design combination tools, new tip designs and new flute shapes as well as designs you never thought possible before. ANCA’s introduction of the TG7 tool grinder in 1991 was a turning point for the industry. The TG7 was the first tool grinder with a CNC designed from the ground up, specifically for tool grinding. And it showed, as our competition quickly followed suit. Ever since, flexibility has been a key theme in all our developments and ANCA holds an enviable reputation as arguably the world’s most flexible tool grinder.
A complex left-hand-helix, right-hand-cutting
(industrial) step drill designed using tool segments
Better process control
The more operations you can do on one machine the easier it is to hold tolerance for the entire batch and the less it costs overall. If you own a swissturn with live tooling, you already know this, but did you know that tool grinders now also have significant multi-operation and process control features?
As an example, a recent application has an ANCA performing unmanned grinding of complete dental drills including the shank and tip. The robotic loader loads the blank and the machine grinds the latch end. The robot then removes and flips the part, loading it back into a stepped collet. The machine then grinds the flutes and drill point. Lastly, the robot returns the completed part to the pallet, shank end down.
Because tool grinders sharpen tools as well as make them, probing has been integral to the process for many years. This experience has evolved to the point that tool grinders now incorporate probing features right into the manufacturing process. Features like ruby probing of web thickness, laser probing of OD, automatic wheel dressing and Statistical Process Control (SPC) help you run large batches without operator intervention.
Where the batches are smaller, typical for medical applications, you can sequence multiple small batches into one super-batch and run the entire super-batch, unmanned. The machine will swap out wheels as needed, schedule dressing when needed and keep an eye on the quality and compensate for wheel wear using probing.
Reduced blank preparation costs
On ANCA’s flagship model, the TXcell, you can even do blanket grinding to eliminate the need for blank preparation on a swissturn. Normally, instrument blanks are turned to size on a swissturn from centerless ground stock, then heat treated. Heat treatment can introduce warping, requiring laborious hand straightening of each blank before the flutes and tip can be ground.
But with blanket grinding, you start with heat treated, centerless ground stock and finish with completed instruments in a single chucking. The TXcell’s integrated robot loads long stock, usually 300mm (12”) or more, the machine then peels or cylindrically grinds the stock to the correct OD size, including any back-taper, performs any shank end work, then grinds the drill features.
Blanket grinding – from heat treated blank to finished instruments in one chucking
PLUS post process operations – eg: finish polishing, laser marking …
After this, the robot grasps the drill and the machine parts the completed drill off from the stock. The robot then grasps the remaining stock and slides it out along the collet and the process repeats, giving you multiple, completed drills from each blank. Since the loader can handle hundreds of blanks, that’s lights-out manufacturing on a grand scale.
So far, we’ve discussed good practice in rotary instrument design and the benefits of grinding over machining. In the next installment (Select a material and grinding wheel to suit your bone drill), we look at different instrument materials and how best to grind them.
||Step 1: Apply some fresh techniques from industrial cutting tool design
||Step 2: Optimize instrument geometry to match the specifics of bone structure
||Step 3: Leverage the advantages of grinding over Swiss turning
||Step 4: Select a material and grinding wheel to suit your bone drill
||Step 5: Drill your way deep into the medical components market
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