ER Collet Conundrum

ER Collet Conundrum: Optimal Collet Performance

Recently we worked a project for a customer that required some fairly large threads to be machined into the ID of the workpiece which was made from 316 Stainless Steel. Between the size, accuracy, and surface finish requirements of the work, it was obvious that tapping wouldn’t be feasible for this application. The choice now boiled down to single point threading or thread milling.

Given the stringy nature of the chips from cutting 316, plus the cycle time penalty of single point threading, we decided to thread mill. When the job was set up, the thread mill chattered horribly. No amount of fooling with speeds and feeds, or taking additional passes solved the problem. Finally acceptable threads were machined by milling the thread in a single point manner, spiraling in from the end of the part, cutting one thread lead at a time. We cut very nice threads but the process did not achieve our productivity goals.

We shipped the sample parts and the customer was sufficiently interested that we needed to solve the chatter problem. A different engineer began working on the machine and while experimenting with the thread mill, it broke. He found that it had pulled out of the ER collet. The ER collet was technically the right size. The “inch” size shank of the tool was less than 1mm smaller than the ER collet nominal size. The collet nut was tightened down plenty tight. Tight enough where there really should be no question that it was holding onto the shank.

The engineer decided to get an “inch size” ER collet that matched the shank of the tool. One he made the switch, installed a new thread mill, the chatter went away and he was able to mill the thread in the normal manner of milling the entire thread length in one revolution.

Most old timers remember that ER collets used to be called Schaublin ESX collets. Along the way, different manufacturers started offering them and renamed them as they went. Southwick & Meister calls them “MPC” collets, with MPC denoting “Multi Purpose Collet”. Most manufacturers settled on “ER” as the name, with “ER” denoting “Extended Range.”

The “extended range” bit is what causes trouble in milling applications. A typical ER16 or ER20 collet above 3mm has a gripping range of 1mm. A 10mm collet will grip from a 9 mm to a 10 mm tool shank size. A 3/8” diameter end mill is 9.525 mm. A 9mm ER16 collet is too small. Never use a collet that is too small, even if it’s only by a couple of thousandths of an inch. Going over the size of the collet causes the tool to run out and severely compromises the collet’s ability to grip the tool shank tightly. Instead use the next larger size collet, in this case 10mm. The problem is that the 10mm collet is ground to have a 10mm inside diameter. As the collet is squeezed down, the fit and grip to the tool shank isn’t as good at the 9.525mm diameter as it would be on a 10mm shank.

In drilling, which is the application that ER collets were originally developed to address, the extended range of the collet doesn’t affect the ability to hold the drill bit reasonably well. The forces in drilling that act on the collet are the feed force which is the force pushing the drill into the collet along the drill’s axis, and there is rotational force which is the force trying to “twist” the drill in the collet.

In milling you have the same rotational or twisting force as in drilling, but it’s coupled with the pounding action of the milling cutter. As each tooth of the milling cutter enters the cut it imparts a shock along with the rotational cutting force. In addition to that, the feed force is pushing on the side of the tool rather than down the center of its axis. Obviously the further the tool is sticking out of the collet, the greater the feed force will lever against the collet. ER collet chuck manufacturers realize this problem and many of them offer ER chuck nuts with a bearing mounted inside the nut. This allows you to tighten the nut with greater force without twisting the ER collet.

A worst case scenario would be something like a key cutter with a 1.0” cutter diameter with a 3/8” shank being held in a 10mm ER16 collet. The 1.0” diameter of the cutter would provide additional leverage in the rotational direction, like a breaker bar on a nut. The teeth on the cutter would apply impact to that leverage, plus the large feed force created by that type of cutter would be levering against the collet, trying to pry it out of its seat in the taper of the milling spindle. This type of cutter is certain to chatter when held in a 10 mm collet. If you switch to a 3/8” diameter ER collet, the inside diameter of the collet fits the shank tighter and since the collet isn’t being collapsed as far, the tapers on the collet fit the taper in the spindle and nut better. Ideally in this situation you should use a “UP” or Ultra Precision 3/8” collet along with the low friction nut to achieve the highest degree of rigidity and rotational accuracy. Having less run out on the key cutter will also help smooth out the pounding action of the cutter and as a result the cutter will be far less likely to chatter.

What if you could eliminate the ER collet connection altogether? Ingersoll IMC in Rockford, IL has answered that question with their Chip-Surfer line of milling tools and key cutters. The Chip-Surfer tool has screw on tips. The shank is solid steel shaped like an ER collet.

The screw on tip of the Chip-Surfer has a true dual surface contact. At the end of the thread there is a taper which mates to a taper in the shank, and the flange face is ground and also mates with a precise surface on the face of the shank.

In use I have found these tools to be a remarkable improvement over conventional end mills being held in ER collet chucks. In one case we milled precipitation hardened stainless steel heat treated to 48 Rc and had exceptional metal removal rates, tool life and best of all, no chatter.

The other advantage to this tool is the ease of replacing the tips when they are dull. The operator simply unscrews the worn tip, screws on a new tip and hits cycle start. He doesn’t have to touch off the tool and the very next part should be in tolerance.

The next time you have a job with milling, rather than grab whatever ER collet is close to the tool shank and throwing it in the machine, give some thought to improving the way you hold the end mill and see if you can’t improve productivity, tool life, and surface finish using these tips.