My cut length accuracy is poor. How can I improve it?

Our latest Mk.6 range of Servo-Torq® rotary cutters are the most accurate machines we have ever made. Even our older designs can hold market leading tolerances.

However, our cutters are often handling a product which is both flexible & unstable. This means that even the most accurate machine may appear to be cutting inaccurately if not set-up correctly.

The tighter the length accuracy you are trying to hold, the more everything needs to be "perfect" to obtain the optimum results.

Due to the wide variety of material types, product shapes & sizes that our Servo-Torq® rotary cutters handle, we can only give general advice here. However ...

Product slip & stretch are by far the two most common causes of cut length inaccuracy!

The reason for this is that the infeeder belts are usually used to measure the cut length. If the extrusion slips or stretches as it moves through the belts, it will almost inevitably be cut to an inaccurate length.

The points listed below are the main things to check if you are achieving an unacceptable cut length tolerance range from your machine:

  • The extrusion is slipping in the caterpillar infeeder belts:

The extruded product must be firmly clamped between the upper and lower caterpillar belts.

When setting-up the machine, check that it is not possible to pull the extrusion out from between the belts. If it is, increase the nip pressure/belt grip.

In addition, check that the extrudate is not wet or slippery. This can be a particular problem in the rubber industry where anti-tak or Sterate solutions are used.

If the extrusion is slipping between the belts, install an air wiper or similar at the exit of the cooling bath or conveyor to dry the product as far as possible before it enters the infeeder belts.

Once the nip pressure/belt grip is set correctly do not keep adjusting it during a run.  Any changes in nip pressure or belt grip will immediately influence cut length accuracy.

Make sure the belt surface is clean & dry. Any water, grease or similar should be removed as this may cause the extrudate to slip between the belts.

Make a note of the belt grip settings so they can be replicated again during the next run of similar sized product.

  • The extrudate is being stretched:

The extrusion must be under a constant tension as it enters the caterpillar belts.

This is particularly critical for materials that might stretch easily, e.g. uncured rubbers, foam or sponge materials, silicone rubbers & small diameter flexible plastics, e.g. flexible PVC, TPU or similar.  Ideally, the tension on these types of materials should be as low as possible.

If necessary, create a loop of material before the cutting machine. This will allow the product to relax before it enters the infeed belts. This will ensure that the pre-cut product moves through the belts under minimal stretch & tension. For best results use an ultra-sonic sonar device to control the loop.

  • The caterpillar infeeder belt speed is varying:

The infeeder belts must be set to run at a constant linespeed to achieve best results. 

Do not adjust the caterpillar belt speed unnecessarily.  Leave it set to a stable speed. 

Any changes in belt speed will immeadiately influence the cut length accuracy in a negative way.

  • The cutter guide bush set-up is incorrect:

The guide bush bore should be a reasonable tight fit to the product diameter. If too loose, it may allow the product to move from side-to-side. This will cause an out-of-square cut end, which will seriously reduce the cut length accuracy you can achieve.

However, if the bush bore too tight, it will cause product snags as the extrusion is pushed through the inlet guide bush by the caterpillar infeeder belts.

For plastics a general rule is to allow between 0.5 mm & 1 mm all-round clearance on product top-limit. For rubbers & non-polymer extrusions you will need to allow more clearance; 2 mm or more. 

For very flexible materials, the distance between the exit of the caterpillar belts & the inlet guide bush lead-in should be kept to an absolute minimum.

It is also important that the knife blade brushes the guide bush face during cutting. Too much gap between the guide bushes will cause an out-of-square cut end. This out-of-squareness will clearly make holding a tight length tolerance almost impossible.

There are more recommendations on cutter guide bush set-up in our FAQ on cut quality:

  • The cut pieces are not leaving the exit guide bush correctly:

Any build-up of cut lengths in the exit cutter guide bush will create a blockage for the on-coming extrusion.  This will affect the accuracy of the oncoming length & will probably cause the extrusion to snag or jam in the inlet bush.

For longer cut lengths, make sure they have some support when they leave the exit bush. This particularly applies to flexible materials such as rubber & some plastics. The best solution is to have an exit conveyor belt to support the extrusion & carry the cut lengths away. Ideally you want to open a gap up between the recent cut length & the oncoming extrusion. To achieve this we recommend the conveyor belt speed is set to at least +10% over the extrusion linespeed.

For shorter cut lengths, make sure they are free to exit the guide bush once they have been cut.  Do this by reducing the length of the exit bush1, putting an internal cone into the exit bush1 or using compressed air to blow the cut pieces out of the bush.

1 If you do either of these steps please ensure that the exit bush length remains of sufficient to act as a guard to the knife blade. If it doesn't then you MUST fit a custom guard over the exit area of the machine to protect the operator from the risk of serious injury!

  • The cutting machine & caterpillar infeeder should be set-up correctly & properly maintained :

Make sure that the caterpillar belts are in good condition.  Worn, damaged or very dirty infeeder belts should be replaced with new belts.

Make sure the knife blade is sharp. Make sure that you are using the correct blade shape & thickness for the product you intend to cut.

There are more details on our knife blades in these FAQs:

Make sure all gearboxes, pulleys & shafts etc. are not excessively worn or loose. Also that the machine is lubricated in the correct way. Please see the Safety & Maintenance section of the FAQs for more information.


For more advice please contact our Tech Team via email:

Downloads (PDF):

> Tech datasheet # 52 - Getting the best results from your machine.

> Cutting Hints.