OmniTurn Technical Support - Himts and Tips Pages
Taming the Dunham Collet Closer

The Dunham collet closer found on most OmniTurn machines is a reliable and accurate device, but does need some care in its use. This FAQ will explore what to do and what not to do when using one.

The Dunham Collet Closer is rated at 4000rpm at 125psi. At this speed and this amount of air pressure, the closer WILL run HOT. The rotating seals generate a great deal of friction and in time, the seals will go bad, and the holding power will be greatly reduced. The guts of the DCC run on a pair of double race ball bearings, which are pretty tough, but they can be damaged over time in a couple of ways. (More on this later) OmniTurn has been building all their machines in the past several years with a collet closer bearing lubricator system. This lubricator sets a air/oil mix into the bearing races, lubricating the bearings, and displacing any coolent that finds its way inside. You will notice that on a new machine or new bearings, that the existing green grease is displaced by the air/oil mix over the first couple weeks of use. This is normal and other than nasty looking, is working the way it was intended.


The factory recommends DTE Lite oil be used in the lubricator, and that the drip seen in the sight glass drip once every 45 seconds (approx). This provides an economical balance between oil usage and bearing life. The leading cause of DCC failure is failure of the operator to check and fill the lubricator reservoir. This MUST be done daily, or as needed. Each shift should check and fill the bowl. The draw tube does not go to the bottom of the bowl, so it should be topped off at the beginning of each shift. The operator should also check to make sure that it is oiling properly. Simply run the spindle, and check the sight glass for a drip every 45 seconds or so. If the lubricator fails, stop the machine Immediately and replace the lubricator.

Air Pressure

When using ANY collet closer, try to use as little air pressure as possible. Many OmniTurn machines have a separate regulator for adjusting this holding air pressure. You simply need set the regulator for the least amount of air that doesn’t let the part slip. This puts a minimum load on the seals, and keeps the heat generated to as low as possible. Bearing and seal life will be greatly improved. Suggested starting pressures are 30psi and up.

The formula for computing draw force on the collet closer is:
PSI x 19.9 = draw force. Example at 120 psi the draw force is 2380 pounds.
The formula for computing Push force is:
PSI x 16.2 = push force. Example at 120 psi the push force is 1202 pounds.

This is fairly important to know, if you are using expanding or standard collets, and are holding fragile or hollow work pieces. If you do not have a separate regulator for the collet closer pressure, and you have to change holding pressures often, we recommend you install a separate regulator.

This regulator may be installed in the Close air line which will allow full air pressure to open the regulator, and allow low holding forces on the part.

Spindle Liners

The second common cause of DCC failure is failure to use a spindle liner when bar feeding long whippy stock. If the stock constantly whips and beats the inside of the drawtube, shock is transmitted to the bearings, a ball will go flat, the bearing will fail shortly. By simply turning a nylon or plastic plug to fit the back of the drawtube, with a hole in the center around the stock diameter, this "spindle liner" will prevent damage to the bearing and really enhance bearing life. This is really important on High RPM machining operations.

Excessive heat/noise/vibration

If the DCC develops bad bearings or is running hot, or there is excessive vibration felt at the body of the closer, it must be checked out and repaired.

Bad bearings may be diagnosed by simply placing a stethoscope or wooden stick to the body of the collect closer on either end, and listening to the bearings while the spindle is turning at around 250-500 rpm. Do this with the collet open, then closed. The sound is unmistakable.

The DCC will tend to run hot, almost too hot to touch, at high speeds and high air pressure. If the bearings fail (usually the rear one but not always) it will also heat the DCC very hot.

If extreme vibration is felt at the body of the DCC, it could mean the bearings are bad, or the collet closer is running out of alignment. Check the bearings as above and replace as needed. To check run out of the collet closer, (which MUST be done every time the collet closer is removed) place a dial test indicator on the neck of the closer, between the body of the DCC and the adapter flange. Rotate the spindle by hand and look at your dial test indicator. Run out should be no more than .0005. If it is more than that, you must loosen up the adapter flange, and tap the collet closer side of the flange until the run out is .0005 or less. (the smaller the better). Retighten, recheck and test again. Run the spindle at normal speeds and see if the vibration has been reduced. If excessive vibration is still present, contact OmniTurn for assistance.

If you have a DCC that does not use the 4 bolt flange, but screws directly to the spindle, there are set screws arraigned around the adapter which are loosened or tightened to adjust the run out. Often you may have to back the adapter off bottom of the spindle a partial turn to get run out within specs. Otherwise everything is the same.

Bearing and seal kits are readily available from OmniTurn and are quite inexpensive.

MRW 4-25-00