A Guide To The Blind Broaching Process

The process of blind broaching derives its name from the fact that during the machining process, a broaching tool is unable to pass completely through the part being machined. This means that during the approach, you’ll be obliged to machine into a blind cavity. This usually occurs because the design of the part being machined prohibits any cutting which allows a complete pass-through. There are some very simple types of blind broaching, such as punching a square shape into a round hole.

There are also some much more complex kinds of blind broaching which include many different tools and several passes that are necessary to complete the process. A typical example of blind broaching process is cutting an internal spline or hexagon into a headless setscrew. As you might expect, it calls for some serious expertise to design any tools which will be used in the blind broaching process, and only the most experienced craftsman is usually this adept and capable of such design work.

Types of Blind Broaching Services

In many cases, the only possible way of cutting shapes, slots, and forms on some components is to use the blind broaching process and depending on the size and shape of the component, it may very well be that a specialized broaching tool must be designed and manufactured. A process that is sometimes used instead of blind broaching is known as plunge EDM, a.k.a. sinker EDM, ram EDM, or conventional EDM. All of these names refer to the same process, which involves the erosion of materials in the pathway of the EDM tool by introducing electrical sparks or discharges.

These discharges can produce heat in the range of 8,000° to 20,000°, and that’s how they are capable of eroding the desired material. Blind broaching is a much faster process than plunge EDM, and it’s also far more cost-effective, even if a broaching tool has to be designed and developed for a specific process.

Guided rams will facilitate very precise cutting tolerances so that precision cuts can be made when necessary, and this is often the case for delicate equipment needed for detailed work. Some blind broaching services must be implemented with multiple-stage broaches, so as to produce some really complex internal forms. There are essentially no limitations on these kinds of processes since any number of passes can be conducted in succession, to achieve the desired result.

Blind Hole Broaches versus Blind Keyway Broaches

It is possible to still have a blind keyway even if you have a through bore because a keyway is considered blind wherever it stops in the middle of the material. To use certain broaching tools, it will be necessary to have a blind keyway at a cross-hole relief, or in an appropriate groove, but the keyway must always end in the air. It’s possible to have a through keyway while still machining it out of bar stock, and that means that a blind keyway would be machined, and would still require adequate relief space.

Broaching a Blind Keyway

You should be aware that gravity is not on your side when you’re trying to cut a blind keyway in a mill, so this is something you have to overcome with precise cutting and attention to detail. You should also be aware that any material you cut into can pile up into your relief area, and that could cause a sudden stoppage in the process, with the possibility of damage to the machined part. Your best bet is to use a cross-hole relief when using the mill, because this will allow the coolant to flush chips out on either side, preventing any kind of buildup.

When you have a simple groove relief, it would be possible for chips to fall into the groove, and that too will result in an accumulation that can potentially cause a crash with your equipment. It is not recommended that you taper out of a blind keyway or radius out of it, because both these approaches are likely to result in a crash. The best approach is to always have an adequate cross-hole relief area or an adequate groove in position while machining.

Recommendations for Relief

Try to always have a cross-hole which is larger than the keyway, because if your groove relief is not radially deeper than the keyway itself, it becomes very possible to bottom out and experience an equipment crash. Your relief process should involve programming the insert cutting-edge to stop at about .08 inches beyond the keyway itself, so it becomes impossible to decelerate while in the keyway and create a taper. Once you pass beyond the .08 inches, it will be necessary to have space for your chips to drop away without exerting any kind of pressure on the face of the insert.

Rotary Broaching

This process is sometimes referred to as wobble broaching, but either way, it’s a very easy and quick way to broach an internal hex, square, or keyway in a blind hole. This is much different from conventional broaching which requires that the broach be pulled through the length of a part, which means there has to be a through-hole. Rotary broaching allows you to machine in forms which have a polygon shape, into a blind hole on the end of any workpiece.

This entire process can be done in just a few seconds on any lathe or mill. When doing this type of broaching, you will need to drill slightly deeper than the form depth which you are attempting to broach. This will provide room for chips to accumulate at the bottom of your blind hole, and it would be a good idea to include an undercut so that these chips can easily fall away.

If it’s not possible to pre-drill deeper than the form, you will need to broach a flat-bottom blind hole. Using an orientation brake, you can broach halfway down the flat-bottom blind hole, while retracting the broach from the hole. Then you can re-enter the hole with your pre-drill to remove any accumulation of chips, and re-enter with your aligned broach so that you can broach another half. This process will have to be repeated until you reach the flat bottom.