Grinding of Cots

Instructions for correct grinding

Improved yarn quality is a substantial competitive advantage for today’s spinning mills. Accotex cots are standard front cots accepted by the machine manufacturers, on both the regular and the compact spinning frames. Improving yarn values (evenness, thin places, and thick places) require a high-quality soft cot; the performance of this cot however depends largely on the proper preparation of the cot surface.

Common information

For a good performance the cot needs the correct surface roughness. In special applications UV treatment of the cots surfaces can be suitable.

Dressing

Independent of the type of grinding machine, we suggest ensuring the following:

The grinding stone is dressed and maintained properly.
The stone should be dressed with a diamond after every 1500 top rollers.
Afterwards the stone surface has to be polished with a medium sandpaper to avoid grinder cuts – cuts in top to bottom direction.

Cleaning

The stone must be cleaned every 15 minutes, using either compressed air or a soft metal brush
New cots will be delivered with a certain wall thickness tolerance.

Diameter

After mounting all new cots must be buffed to the same outer diameter in a first grinding step. The cot can then be ground to nominal dimensions in a further step.

Cot surface after grinding

The cot surface after grinding is clean and free of grinding dust and grinder cuts.

Correct surface roughness

The cot has the correct surface roughness (Ra-value) after grinding

Measuring device

Use a measuring device which is calibrated

Measuring

When the surface is cold, three single values should be measured around the circumference of the cot and should be averaged.

Grinding instructions

With each grinding it is necessary to remove at least 0.3 mm in diameter to get a fresh rubber layer on the cot surface. If less material is removed surface cracks can occur after several grinding cycles.
The freshly buffed cots should relax in the spinning climate for approx. 24h before they will be used.

Time for grinding

Grind as fast as possible and as long as necessary.
On a new installation of a spinning machine all new top rollers should be ground after approx. 500h running time.
The lower grinding limit is 27.0 mm. It may be higher depending on the pressure arm generation.
If in doubt, read the operating instructions for the spinning machine.

Grinding Machine: Wide-Stone Type

Semi automatic or fully automatic grinding machine:

Stone grain 80 or 120
Stone porosity 14 or 10
Cot diameter to be reduced by 0.3 mm for each grinding

The grinding results that should be achieved are:

Type	Removel (OD)	Targeted roughness [µRa] (cold surface)
Ring spinning	0.3 mm	0.8 – 1.0
Air-Jet spinning	0.3 mm	0.6 – 0.8

Grinding intervals:

Hardness	Type	Recommend grinding interval
Soft cots	Ring spinning	1 - 3 months
Medium cots	Ring spinning	1 - 3 months
Hard cots	Ring spinning	4 - 5 months
Soft cots	Air-Jet spinning	7 - 15 days
Hard cots	Air-Jet spinning	4 - 5 months

Precise grinding

1. Measure the outer diameter of the cot before grinding

2. We recommend starting with the following parameters:

For grinding stones with a grain of 120 and a porosity of 10:
- Total contact time* is used 8s for soft and medium soft cots
- 3s grinding time5s polishing time
- We recommend to chose the feed 1s/ 0.1 mm material removal
For grinding stones with a grain of 80 and a porosity of 14:
- Total contact time* 10s for soft and medium soft cots
- 3s grinding time
- 7s polishing time
- We recommend to chose the feed 1s/ 0,1 mm material removal

*Total contact time=grinding time+polishing time

Measuring after grinding

3. Measure the outer diameter of the cot after grinding; it should be
0.3 mm smaller now, measured on the cold surface

In the case of automatic measurement, check the measured values
regularly with a caliper gauge. We recommend the option "grinding
to size”.

4. If 0.3 mm diameter reduction has been achieved, please wait for the
cot surface to cool down and measure the Ra-value of the surface.

The total contact time might have to be adjusted, in case a different stone type or stone condition is used
In this case, please proceed as follows:
- If the surface is smoother than Ra 0.8 μm, shorten the total contact time by reducing just the polishing time
- If the surface is rougher than Ra 1.0 μm, lengthen the total contact time by extending just the polishing time.

Grinding Machine: Narrow-Stone Type