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Dear massed wisdom of this group, how would you measure the head gap of a floppy disk r/w head?
By counting pixels on a microscope pic I've arrived at the more or less believable 0.35 mils, or 0.009mm . One thing's for sure, it's tiny.
But that number is ten times smaller than the one reference I found that says it is 80 microns, or 0.08mm.

The usual way to measure such small distances is using
an optical microscope that has an eyepiece with micrometer
controlled moving reference lines. Zero the micrometer
with the reference lines at some known distance... usually
overlapping, and then adjust them to match the head gap.
The micrometer reading gets multiplied by the optics to
arrive at the actual gap.

Absent one of those special oculars, you can use a finely
ruled ruler, placed in the same frame as the gap, and
extrapolate the size of one of the lines to the gap.

If you have some fine wire, that you can measure with a
micrometer, you can compare it to the gap...

When the sizes get even smaller, a scanning electron microscope
can easily perform the same duty.

-Chuck Harris

snapdiode via Groups.Io wrote:

Hi snapdiode,
You probably know the density of the media that your drive reads and writes already but it is worth noting that the read/write head gap depends on the magnetic coercivity of the coating on the diskette, and that depends on the capacity of the diskette (360KB, 720KB, 1.44MB, 2.88MB, etc.).

The greater the storage capacity of the diskette the narrower the tracks have to be. Narrower tracks require magnetic material with higher coercivity to be coated on the diskette. In order to read and write these narrower tracks the magnetic field in the gap had to be higher which meant a thinner gap and the width of the gap had to be reduced to squeeze more tracks in.

In summary, the gap width depends on the data density of the floppy so your measurement and the reference may be referring to two different density heads.

Dennis Tillman W7PF

On Tue, Dec 24, 2019 at 04:50 AM, snapdiode wrote:

Dear massed wisdom of this group, how would you measure the head gap of a floppy disk r/w head?
By counting pixels on a microscope pic I've arrived at the more or less believable 0.35 mils, or 0.009mm . One thing's for sure, it's tiny.
But that number is ten times smaller than the one reference I found that says it is 80 microns, or 0.08mm.

As a very crude guide the gap width is something like the bit length in the tracks. Now (total track length)/(total bit capacity) is in the order of 10 um for an 8 inch floppy. I wouldn't thrust that 80 um.

Ahhh, the units in my reference are microINCHES. I see. 80 microninches is two microns. And they are talking about "future" media from the perspective of 1980.
OK, so 10 microns gives .39 mils.
This makes sense!
Thanks!
You wouldn't happen to know how these things were built, do you? :)
Happy Holidays everyone!

The easiest way is to make the core in halves, and to
create the gap with a known thickness spacer where the
gap should be.

Another way is to make the core "V" shaped where the gap
should be, and carefully grind/lap away the core material
until the gap presents itself.

These were methods used in the early tape recorders... I
would imagine that similar methods were used in the disk
heads.

-Chuck Harris

snapdiode via Groups.Io wrote: