There has recently been an initiative from within the medical industry to couple bone screw applications with square and spline drives, as well as hexagonal shapes. To do this, it’s necessary to use materials such as titanium, cobalt, and several high-temperature alloys, and that makes it fairly difficult to impart the desired shape to the head of the screw.
One of the best methods for accomplishing that is through rotary broaching, which is sometimes referred to as wobble broaching. A rotary broach is capable of obtaining such non-standard shapes, and it’s a process that has been used for years on automatic lathes.
Rotary broaching has become the default choice for CNC lathes, Swiss machines, and a good number of machining centers because it’s capable of eliminating any kind of secondary operation which would otherwise be necessary.
Rotary versus Conventional Broaching
These two broaching methods are quite different in the way they accomplish their tasks. Conventional broaching will have a sequence of polygon forms or other forms which increase in size and which get shoved through a hole until the perfect form size has been attained. Rotary broaching achieves the same thing in a single pass by cutting the entire form, one corner at a time, and this will generally negate the need for any kind of secondary operation.
This works extremely well on either vertical or horizontal spindle machines like mills, or lathes. Rotary broaching is a precision method for creating internal polygon forms at the fastest possible rate. An entire operation can be completed within seconds, and the precision rate is extremely accurate. This is why rotary broaching has come to be an extremely popular method for producing high-quality precision components.
It is especially popular in industries that require these kinds of components, for instance, plumbing, aerospace, medical, and automotive.
The Basic Principle of Rotary Broaching
Both internal and external polygonal profiles are shaped to a single point, as opposed to being form-punched, and the operation can be performed very quickly while the spindle is under rotation. Rotary broach manufacturers claim that the cutting action only requires 80% of the force that form punching would, when an optimal feed rate is being employed. The reduced stress from this approach adds to the life of the tool, and can significantly lower machine maintenance costs over an extended period of time.
The broach holder revolves in the spindle while both the part and the tool remain stationary, and this is directly opposite of the way it will be positioned on a lathe, where the part is rotating synchronously with the tool, and the holder is fixed in the specific position. This operation takes mere seconds, and it eliminates the need for any kind of secondary operation, which allows the part to be produced completely with a single setup.
There are two functions that are served by the broaching holder, the first of which is to hold the broach tool in a free-spinning bearing and placing it at a 1° angle with respect to the workpiece centerline. The facing of the broach tool is at the apex of this 1° angle, and it will be situated on the same centerline as the piece being worked on. When the tool comes into direct contact with the part which is spinning, it also starts to rotate synchronously like a gear, due to the principle of friction.
When the tool gets thrust into a pre-drilled hole, a wobble effect forces the leading edge to rotate in and out of the cut in the same manner that a cam would. When the tool gets advanced forward in a kind of wobbling motion, every tooth will cut the same groove while it rotates in and out of the lead position. This is very much like a wobbling coin, in that a single point is always touching at any moment, thereby significantly reducing the necessary force required to form the shape.
How to Rotary Broach a Hole
Here are some tips on how a hole can be broached to high-quality standards:
- Preparation– the hole needs to be prepared with a chamfer between 60° and 90°, and which is slightly larger than the largest dimension of the broach tool.
- Hole drilling– when internal broaching is being done, the hole has to be drilled about 1% bigger than the diameter of a hex shape. This percentage can be lowered when the material is being free-cut, and it can be increased as machinability decreases. The hole needs to be drilled as deep as possible, so as to allow room for chips to accumulate, and an ideal depth would be 1.3 to 1.5 times the profile length.
- Recommended speed– the 1° offset permits high-speed applications between 1500 rpm and 3000 rpm. The broach’s cutting edge will have a tendency to dig into the part face when the tool contacts the rotating material, and at higher speeds, this mark will be more impactful and will cause tool life to suffer.
- Feed rate– the feed rate will usually depend on the characteristics of the material. For instance, when using mild steel, it’s recommended that somewhere between .0012 and .0024 per revolution is used.
- Coolant– rotary broaching will work best with either a cutting oil or a water-soluble coolant, and the larger size of the pre-drilled hole will allow ample room for coolant to leak out when the broach tool is thrust in.
- Centering the brooch– in order to get a really high-quality broached hole and to preserve the life of your tools, the tool needs to be centered, and the workpiece must be within .0008 inches or less. If you fail to use proper centering, the shape will end up being eccentric, and there could be increased dig marks that appear.
- Adjusting center on the machine– when you adjust the centerline of the machine, you’ll have the advantage of correcting any misalignment between the turret bore and the machine spindle.
- Adjusting centering off-line– you can adjust the position by using the adjusting screws provided, and once this operation is complete you can use the clamping screws to fix the head in position.