Discovering Three Points about Open Value ‘n’ of Swiss Collet

WHAT IS THE OPENING VALUE OF COLLET?

Swiss style Collets and Draw bushings are made to be elastic.Therefore,the bore size will bigger then required clamping size while not be worked.
For example,while required I.D.10mm hole size , the hole size will be 10.3-10.4mm while not be clamped.
But this doesn’t mean that the collets can clamp the rods within this range of diameters.Only specified size can be clamped.

Now let’s set the opening value to ‘n’

What is the typical opening value(n)?
Can the ‘n’ be customized according to requirements?
What is the upper limit of the ‘n’?

1、Standard Opening Value

The typical opening value will have different for collet types and collet hole size,for regularly collet and bushings, the ref.size mostly will be as per below:

Main(headstock) collet: ≈0.3-0.4mm
Sub collet: ≈0.1-0.2mm
Guide bushing: ≈0.1mm

2、Opening Value Customized

The Opening value can be customized as specific required. However two issue that need attention:

one is the fit on problem with the collet sleeve nut
The other is the clamping force, the opening value is inversely proportional to the clamping force.

3、Opening Value Upper Limit

It depend on the nut size. Gernerally speaking, collet nut inner holder size will bigger 1mm then the outer size of collet.
Therefore, while the opening value is too big, like Citizen 20 type machine collet.
If opening value required to 0.7mm,the outer size would bigger 1mm on the end, then might not fit on the collet nut.