Ornamental Turning Book of Knowledge
Amplitude Adjuster

Amplitude Adjuster Terminology

An Amplitude Adjuster is used to reduce or increase the amplitude of the Rosette. A simple, linear representation of a wave's amplitude is shown in the picture to the right. (A rosette would implement this about the spindle's axis.)

Some reasons why you would use an amplitude adjuster include:

  • an amplitude of 0.070 inch may be fine for a bowl, but that would be mountainous on a pen.

  • that same amplitude may be too small for a collar on a larger form (depending on what the artist's preference is).

So, the amplitude adjuster extends the capabilities for a given rosette. For example, if you use an amplitude adjuster, then won’t have to buy a new 24-bump rosette with a different amplitude for a given operation, when you only want to adjust the amplitude.

The amplitude adjuster I made for my MDF rose engine lathe is outlined on this web page link. The video below shows it in action.

Just getting started?
This can be quite useful. Do consider it for one of your first add-on parts.


How it Works

There are two types of amplitude adjusters : horizontal, and vertical. The horizontal one is simpler, but cannot be easily implemented on an MDF Rose Engine due to space constraints. The concepts below apply to that one though.

My recommendation is that you not try to model this too much when it comes to use. The math never seems to really work out for me, and there are a host of variables that should be considered when making one.

What I have found to be the better approach for using one is to setup a Dial Indicator attached to a Noga Arm, and have it indicate the amplitude of the spindle as seen from the business end of the lathe (where the item is held). This is far more exact, and you can get a better understanding of what happens when using the various settings for the Amplitude Adjuster touch.


The terminology for this is in the diagrams below.

Fundamentally, a vertical Amplitude Adjuster is a linked set of two levers. If you want to better understand how linked levers work, there is a nice Wikipedia page about linkages.




First Lever - a Class 1 Lever



Class 1 Lever
image courtesy Ron Kurtus' School for Champions



Lever Number 1

The first lever in use here is a Class 1 lever. (That is shown diagrammatically to the left, and more information about lever classes is at Ron Kurtus' School for Champions.)

The key here is not to think in terms of load reduction, but changes in the load's distance of movement.

The Amplitude Adjuster's touch (green) is affixed to the lathe's body, and acts as a fulcrum for the lever.

The rotation of the rosette (orange) will impart motion to the Amplitude Adjuster (the lever), moving the effort arm part to the Amplitude Adjuster.

The result is motion of the load arm in the opposite direction. (One more reminder for this discussion : we don't care about the movement of the rosette, only the movement of the spindle.)

The Amplitude Adjuster's touch (green) can be moved up or down, changing the ration of the lengths of the effort arm and the load arm.

The calculation for how that changes the movement is :

ΔxLoad Arm = ΔxEffort Arm Load Arm Length

Effort Arm Length

So, as the Amplitude Adjuster touch is moved down, the load arm is made shorter (and the effort arm longer), decreasing the effective amplitude of the rosette. And, as it is moved up, the opposite happens..

Side Note : If the Amplitude Adjuster touch (green) is moved above the axis of rotation of the. rosette (orange), then this becomes a Class 3 lever. And the math for that is below.




Second Lever - a Class 3 Lever



Class 3 Lever
image courtesy Ron Kurtus' School for Champions



Lever Number 2

The second lever in use here is a Class 3 lever. (That is shown diagrammatically to the left, and more information about lever classes is at Ron Kurtus' School for Champions.)

Again, the key here is not to think in terms of load reduction, but changes in the load's motion.

The headstock's pivot point acts as a fulcrum for this lever.

The Amplitude Adjuster lever (the blue, "L"-shaped piece above) is attached to the headstock midway between the pivot point and the spindle. That is shown as the Amplitude Adjuster Mount Point in the picture above on the right.

The movement of the bottom of Amplitude Adjuster, left and right, imparts motion to the headstock at the Amplitude Adjuster Mount Point. But, this movement is amplified by the time it gets to the spindle. The calculation for this is :

ΔxSpindle = ΔxAmplitude Adjuster Lever Load Arm Length

Effort Arm Length

Now here's the real difference between the horizontal Amplitude Adjuster and the vertical one. For the horizontal Amplitude Adjuster, there is no difference between the length of the effort and load arms. So this part can be ignored!

The Amplitude Adjuster that Al Collins implemented on the MADE lathe, is a horizontal Amplitude Adjuster (using a design from Fred Armbruster). The video on that web page shows well how this works.


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Disclaimer : eMail comments to me at OTBookOfKnowledge @ Gmail.com. The process of woodturning involves the use of tools, machinery and materials which could cause injury or be a health hazard unless proper precautions are taken, including the wearing of appropriate protective equipment.