I’m using a quarter inch hexagon broach to create a quarter inch deep form in 400 series stainless steel. However, the form is twisted, somehow spiraling from one end to the other. I don’t see any type of adjustment available on the broach holder. How can I get rid of the twist?
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.
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