Calibrate a Cone issue on a Rotating Laser

August 3, 2019
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Topcon Adjustable Prism Rotating laser coning error cone issue

Calibrate a Cone Issue on a Rotating Laser

This post will concentrate on calibrating out a cone issue on a rotating laser.  So first of all what is a cone or coning issue?

To understand what a cone issue is on a rotating laser, it best to know how a rotating or rotary laser works.  Very simply these types of laser produce a laser dot, this is shot upwards to the rotating head.  The head contains a prism which diverts either all the laser light or a proportion of it at 90 degrees.  So now we have a laser dot shooting out of the side of the head.  When the head is rotated we have a horizontal plane, which when calibrated correctly provides a consistent level at any point around the laser and at any distance away from the laser.

Rotating laser prism diverting laser by 90 degrees

This is all dependant on the accuracy of the prism slightly more than 90 degrees and you will get a laser that increases in height as you move away from the laser.  Slightly less and the laser height will decrease in height.  This is a cone or coning error.  named because if you visualise it there is either an upward or downward cone.

Rotating Laser Coning error

Cone issue on a rotating laser

So how do you know if you have a coning issue or not?  First of all these errors are fairly rare as on a good quality laser the prism is of high quality and is mounted in a secure manner.  Some manufacturers have a lens to allow the fine-tuning of this prism and some do not.  In the photos below you see rotating lasers manufactured with an exposed prism and without.  Another image shows a  laser featuring a prism with a cover containing the adjustment lens.  A prism or lens can be dislodged after a particularly heavy drop.

Rotating laser prism uncovered and covered coning

Image shows the same Rotary laser first with prism uncovered then with the cover attached.

Hilti Rotating laser not adjustable prism

An expensive Rotary laser with exposed prism with no adjustment possible.

Fukuda rotating laser not adjustable prim cone

Inexpensive Rotary Laser with a prism cover but no adjustment lens.

Topcon Adjustable Prism Rotating laser coning error cone issue

Close up of adjustment lens on Rotating Laser.

Diagnosing a Cone Error

A professional laser level calibrator will pick up this sort of error whilst they are calibrating the laser.  They will note that the further away the laser is tested it’s heigh will consistently increase (or decrease).  At home or on the job site it is more difficult to pick up.  One way to test is to level a site with the laser at one end of it.  So, in this case, we have knocked pegs in the ground at various distance from the laser and set the level according to the laser.

Try to be precise as possible by holding the receiver and staff absolutely vertically each time using the bubble vials on the receiver or staff.  Then we move the laser to the centre of the site and check the heights are consistent.  We are assuming for this test that the laser is perfectly calibrated for level.  Therefore, any error that we pick up is a coning one and not a calibration one.  If you find a significant and gradually increasing difference to the original heights set then you may have a coning error.

Fixing the coning issue

Fixing a cone issue is not something that you easily achieve without the correct equipment.  The principle is, however, pretty straight forward.  If your laser has an exposed prism without an adjustment lens then you are out of luck.  The only fix, if this is the case, is a new prism and head assembly. If you have an adjustment lens then rotating the lens will deviate the beam either up or down depending on the direction of rotation.  In this manner, the prism can be dialled in to be exactly 90 degrees.

Topcon Adjustable Prism Rotating laser coning error cone issue

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