Mr. Bone Screw

Dear Shop Doc,

I’m going to be rotary broaching a 9/64” hex in titanium. The hole is a blind hole about .160” deep. I’m worried about hydraulic pressure building up during the broaching operation. What are my options?

Mr. Bone Screw

Dear Mr. Bone Screw,

You have a few options available. But first, let’s talk about the hole. Be absolutely sure to drill it deeper than the broaching depth. You will need to leave room for chips, fluid, etc. The deeper the hole, the more room is available for swarf to get out of the way.

The first option has to do with your pilot hole. A pilot hole drilled to size will create significantly more pressure than one drilled oversize. The oversize hole allows air and fluid to escape. This larger pre-drill diameter also reduces the size of the chip while broaching. The chips are also sure to be separated. A standard 9/64” hex has a dimension across the flats of .1425”. Drilling the pilot hole about 3 percent larger, requires a drill size of .147”.

The following tooling options are all intended to reduce pressure while broaching. Most of these options are commonly available in the marketplace.

• Spun, Ground Diameter – Eliminating the sharp corner from the broach reduces chip size and depth and strengthens the broach at the corners. Rotary broach failure is often a result of chipping at the corners.
• Broach Pressure Relief Holes – Small holes added to the center of the broach and used in conjunction with secondary holes drilled in a cross direction allow the fluid and air to escape.
• Broach Holder Relief Holes – A relief hole in the broach holder allows air and fluid to escape completely. Air and fluid are pushed through a center relief hole in the broach, and out of the holder through its relief hole. This option is currently available from Polygon Solutions and reduces cost of broaches requiring two vent holes.

Here are a couple tips to make sure you get the most out of your tooling. Make sure the broach is aligned with the pilot hole. This may seem obvious, but many machines can be off center by more than a few thousandths. Most broach holders have end play built into them. The broach will follow the hole. But double check it anyway; this is a common troubleshooting problem. Also, if you’re going to be broaching titanium, upgrade to a premium broach material, such as M-42 or PM T-15. These broach materials are very hard and include elements like cobalt to enhance their strength.

As you can see, all of the information here has to do with reducing the forces required while broaching and strengthening the broach. Hopefully these tips will relieve some of the pressure created when trying out a new machining operation. If you’re still uneasy, run the broach in aluminum to get a feel for the set-up.

Peter Bagwell is a Rotary Broach Product Engineer at Polygon Solutions. He is a frequent contributor to Today’s Machining World’s Shop Doc column and is also an Essential Oil enthusiast. To contact him, go to www.polygonsolutions.com.

I have recently been asked if my shop does “micro” machining. I’ve done some work on small parts recently, but I’m not exactly sure what is meant by “micro.” Any thoughts?

Small Beginnings

Dear Small Beginnings,

One of the problems with the term “micro” is that it is often used to defne a very small portion of a wide array of categories. Maybe you’ve been to a microbrewery or have a computer that uses a microprocessor. In each case, the prefx or adjective “micro” defnes a small-scale or very small feature of the original term. To date, the term is loosely used in machining to refer either to the exact measurement of the parts, such as in microns, or to a small range of work, in the neighborhood of 1 mm or less.

In May 2010, I posed a similar question to exhibitors and attendees at MM Live—the Micro and Precision Manufacturing Event for North America, in Cincinnati, Ohio. As an exhibitor myself, I thought micro meant sizes under .050”, as this was the smallest tool in our catalog and very near to the 1 mm dimension. I often referred to parts from this diameter up to .500” diameter as Swiss, so everything smaller I considered micro. A large number of attendees defined micro as being smaller than a certain dimension.

Some said micro meant parts smaller than 8 mm or .250”, or 1 mm. Kyocera’s booth advertised a .250” dimension on their sign. However, when I asked them about it, they explained that although they make a wide variety of small tools, the ones they considered to be micro sized were really those .125” or smaller.

Photo from JMMedical

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I have recently been asked if my shop does “micro” machining. I’ve done some work on small parts recently, but I’m not exactly sure what is meant by “micro.” Any thoughts?

Small Beginnings

Dear Small Beginnings,

One of the problems with the term “micro” is that it is often used to defne a very small portion of a wide array of categories. Maybe you’ve been to a microbrewery or have a computer that uses a microprocessor. In each case, the prefx or adjective “micro” defnes a small-scale or very small feature of the original term. To date, the term is loosely used in machining to refer either to the exact measurement of the parts, such as in microns, or to a small range of work, in the neighborhood of 1 mm or less.

In May 2010, I posed a similar question to exhibitors and attendees at MM Live—the Micro and Precision Manufacturing Event for North America, in Cincinnati, Ohio. As an exhibitor myself, I thought micro meant sizes under .050”, as this was the smallest tool in our catalog and very near to the 1 mm dimension. I often referred to parts from this diameter up to .500” diameter as Swiss, so everything smaller I considered micro. A large number of attendees defined micro as being smaller than a certain dimension.

Some said micro meant parts smaller than 8 mm or .250”, or 1 mm. Kyocera’s booth advertised a .250” dimension on their sign. However, when I asked them about it, they explained that although they make a wide variety of small tools, the ones they considered to be micro sized were really those .125” or smaller.

Photo from JMMedical

A few attendees believed the term “micro” referred to parts that were smaller than the human eye can see. The MM Live show had a wide of variety of these parts on display, and it seemed that every other booth had a microscope or magnifier of some type to help you see their wares. One of the most intriguing answers to define micro came from an exhibitor at Makuta Technics Inc. He said they use the term micro not to refer to a part’s size, but more exclusively to the feature’s size. You may have a part not considered to be a micro machined part, but if the features and tolerances are small enough, it may require what is commonly known as micro machining. This can lead to a lengthy discussion about tolerance, and if a part with +/-.001” variance can be classified the same as a part with +/-.000010”.

I believe micro machining refers to parts with an overall size or feature in the neighborhood of 1 mm. The features are not as large as common Swiss machined parts, nor as small as a nanometer (one billionth of a meter), but you will still need some form of glasses to make out the details. My suggestion is to not split hairs, but just state the size of the features you are comfortable making. The number of people who agree on the definition of the term “micro” is very small.

Peter Bagwell

Spinning out of Control

Dear Spin Doctor,

The solution to removing the twist from your form is easier to find when you understand the nature of the problem. The sides of the broach include a relief angle greater than the angle of the rotary broach holder so it will not interfere with the part. The broach is held in the holder at a one degree angle. The rotary broach is designed to cut a form into the part using a cutting edge with contact points that are constantly changing. The center of the cutting edge is always kept in line with the axis of the part. As the contact point continually changes, separate chips develop in each corner of the form. As these chips increase in size, pressure is absorbed by the broach tool and tool holder. This resistance against the broach holder spindle and bearings may cause the broach to drag slightly against the material being broached. The sides of the broach cannot hold it straight because they have a greater relief angle for clearance and sometimes a spiral will develop along the length (depth) of the form.

At first you may have noticed that the form appeared smaller at the bottom. What you are really seeing is the sides of the form following this spiral path. Although there may be a slight twist, the part may still be within specification. Technicians will often recommend that you broach to the high side of your tolerance for this reason.

Work piece material can also affect this condition. Some materials could be too tough or too hard for the capabilities of the tool holder. Your material (400 series stainless) is difficult to broach, and may result in poor tool life. The combination of a dulling tool and hard material increases the thrust required to broach which increases drag thus increasing the spiraling of the form. However, at this small size and form, I’m hopeful that there are a few things you can do to try to reduce or eliminate the spiral.

First of all, good broaching practice is to check your tool holder and broach to make sure they are on center. If not, re-center the tool holder. If you are using an adjustment free model, make adjustments on the machine to assure that the broach is on center with the part. It is also good to check and make sure the pre-drilled hole is on center. Next, anything you can do to reduce the pressure caused by chip accumulation will help. Check your pre-drill diameter. Can you make it larger? The recommended pre-drill for a hexagon is 1.035 times the across-the-flat dimension. The standard quarter inch broach is likely .253 inches, and the pre-drill should therefore be .261 inches. If your customer will allow it, make the pre-drilled hole larger. This will reduce the required thrust. Have you checked your speed and feed to compare them to the recommended settings? If your tool is moving too slow, the chip may not curl over as readily as is necessary and this could result in added pressure. Increase the feed rate to improve the chip flow.

Finally, if the above recommendations do not help or are not practical, reverse the direction of the spindle at half of the depth. This will drag the spiral in the opposite direction and can reduce the overall deviation by half. Hopefully, these suggestions will help you make a turn in the right direction.

Peter Bagwell
Slater Tools Inc.

Peter Bagwell is an engineer at Slater Tools Inc., which specializes in rotary broaching tools. For more information go to www.slatertools.com