The broach process has been much-maligned in recent years due to a few myths and misconceptions. This article will try to reintroduce the validity of the process to the newcomer, but it will also give insight into the simple basics that are essential to success. It has been written in a very real and practical way, with the intention of helping the “guy or gal” on the shop floor keep the parts rolling.
In addition, I’d like to point out that there will be no answers to some questions. The idea and the realization lost on some management is that, sometimes, “heck happens.” Each part is unique in the broaching process. Different length of cut/hardness, wall thickness, and element requirements dictate its predictability and life cycle. And unlike other machining processes, there is rarely a second chance. Once the broach enters the cut, there is little or no opportunity to change the outcome on that particular piece.
What is Broaching?
Broaching is a precise, economical, and dependable process to remove a specified amount of stock in a very short time, usually seconds. Typically there is a roughing section, a semi-finish, and a finish section. In most cases the first tooth is just as important as the last, as most broaching is generated rather than full-form cutting. It is important to sharpen all teeth to maintain continuity. If any one tooth on the broach is damaged, there is a good chance that size or surface finish will be compromised.
Broaching is a fixed cut per tooth process, and the user must stay within the design limits of the broach, the fixture, and the machine. There is a misconception that all broaches are standard, which would probably be true if all parts were standard, but just because you have a 12T 32/64 30` spline broach does not mean that it will work for your job. You cannot exceed the chip capacity or tearing or breakage can occur, and it is important to remove chips from the broach after every pull. You must make sure that the elements are correct, such as major diameter, fillet radius, and circular space width. (Table 1)
Things to Keep in Mind
Always engage at least two teeth in your work at all times. This will reduce breakage and stabilize the cut. Most broaches are part-specific and must be within a known range to avoid problems. Most broach engineers will design with a row of three to four teeth engaged, but high part hardness or small broach diameter will effect those numbers. The reasons for this involve the physical capacity in the tooth gullet and broach breakage by exceeding the tensile strength of the broaches weakest point.
The most common problem in broaching–besides size and surface finish — is true position/squareness or concentricity. The key elements most overlooked are part/blank quality and the fixture. The parts must be held square and be of good repeatable quality in order to achieve a good position from broaching. No matter how good the broach or the operator is, they can rarely overcome bad parts at the start. The basic rule of thumb is “the better the parts in, the better the parts out.”
For example, if the face of the part is out of square to the pilot hole by .004, there is very little to be done to improve it. This is especially critical on small broaches where the broach will try to find “the line of least resistance” and follow the hole.
When to Choose Broaching
Let us say as an example that an internal spline broach costs $1,000 and you have 50 pieces to broach. Basic math gives us an amortized per piece cost of $20 each. When that same broach cost is spread over 5,000 pieces, the cost becomes 20 cents each and 10,000 pieces becomes 10 cents each. These are very doable numbers in most normal cases, if done properly. Therefore part process, care, and application become very important. The importance of keeping the broach sharp cannot be overstated. But this is not to say that you must have large quantities to justify broaching. The run time on the average pull broach part is about five to 10 seconds. There are very, very few machining operations that can make that same claim, while holding a precise size and good surface finish.
Once size is established, the process is like a cookie cutter: every part is the same within tenths, and very dependable. Allowances can be made to allow for H.T. growth or shrinkage on a constant basis, allowing for a far higher yield than, say, shaping. Therefore one of the unrealized values is a high part per process ratio (i.e., less parts lost to scrap).
Tearing is the most common broach problem, and it is usually caused by one of two reasons: part hardness is too soft–causing the edge or edges from either to overrun–or physical damage is not allowing the chips to flow correctly. The cutting face must be free of any damage or mushrooming or tearing will continue. (Figure 1)
With all of this said, the single-most important element to successful broaching (assuming that the fundamentals are adhered to) is consistent broach maintenance. As an example, if you bought a new car and didn’t change the oil until 15,000 miles, then you should assume that the engine will not get the life it would’ve had you done the proper maintenance. If you don’t keep the proper air pressure in your tires and rotate them once in awhile, then you can expect your performance to decrease and operating costs to increase. The same is true with a broach tool. Keep it tuned up and it will run, and run, and run.
Gauling occurs for the following reasons:
• Part hardness is too soft
• Part moves in the fixture
• Broach is bent
• The alignment of the broach puller, fixture, and part are not correct
• The broach does not have adequate relief
If a slight amount of gaul does appear on the flank of the tooth (normally the surface finish of the part will deteriorate), the gaul can be removed with an Indian Knife Stone–being very careful not to go to the top edge or the face of the tooth, as that will make the problem worse. The broach should be resharpened and restraightened. (Figure 2)
This question is akin to asking someone’s favorite song: Everybody is different, and the same is true with broaching. Every part is unique and has a different answer. The factors that determine life are the same for any other machining operation. The length of cut in this material, and at that hardness, all play a role. The most common reasons for shorter life is handling damage and incorrect or lack of resharpening. A few garden-variety examples would be cutting mild steel, plastic, or aluminum, etc., in which you could go thousands of parts per sharpening. Cutting “middle of the road” steels such as 4140, 9310, or mild stainless, would allow for hundreds per sharpen. And on the high alloy side–such as titanium, Incolen, high Rockwell, or if the broach is made with a material such as T15–you could be in the dozens per sharpening range or less.
Speeds and Feeds
There are no real formulas to determine the correct speed, any more then there is for turning aluminum. Sure, you can use a basic surface ft/min., and you can reduce RPM on a six-inch diameter and increase on a 1/4 diameter. But remember that the broach is a fixed cut per tooth. The key requirement is to find the speed that allows the chip to curl into itself. (Note: some materials, such as brass or cast iron, will not curl easily, if at all). Different materials, Rockwell, and lengths of cuts will determine speed. (Another note: if the cut is squealing, then the broach could break). So start slow, find the speed, and make the broach sing.
Last but not least is the realization that some parts should be broached by the company that designed the tools. They possess the experience and ability to fine-tune the process, allowing for less downtime and better parts, and without question they will be able to save you money. To me, this just makes sense.
Here at The Broach Masters, for nearly 28 years we have broached some of the most high-precision and critical parts in the industry: from the internal ring gears on the Mars Rover, to numerous projects for the nation’s leading manufacturers. As we see it, there is no substitute for quality, and there is no excuse for the lack of it.