Amplitude Adjuster

Amplitude Adjuster Terminology

A rosette's amplitude is the measure of the movement that the rosette imparts upon the spindle. It is shown in the picture to the right.

At times, that amplitude may be too great or too small for what the artist wants to achieve. These 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.

An Amplitude Adjuster is used to reduce or increase the amplitude of the Rosette. (Reducing the amplitude is the most common use; it is rare to increase it.)

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-lobe rosette with a different amplitude for each 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.

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.

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, among other reasons. The concepts below apply to that one though.


Amplitude Adjustment

The chart to the right shows the effects Tom Johanson measured on his vertical amplitude adjuster. It is a slightly different design than the one I built, but the curve is the same.

  • Y axis - the vertical distance (in inches) that the amplitude adjuster Rubber (the green part above) is positioned from the spindle axis. In this case, a movement above the spindle axis is a positive number; below is a negative number.
    • Positive (+) numbers are distances above the spindle axis, away from the headstock pivot point.
    • Zero is positioning the amplitude adjuster Rubber at the spindle axis. At this point, there is no amplitude adjustment (e.g., 100%).
    • Negative (-) numbers are distances below the spindle axis, towards the headstock pivot point.

  • X axis - the effective amplitude (as a % of the rosette's amplitude)

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 rubber.

Thusly, as the AA Touch / Rubber is lowered, the amplitude is reduced.

Just getting started?
This can be quite useful. Do consider it for one of your first additions.

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. And a further explanation of that activity is below.


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 rubber (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 resulting movement of the spindle.)

The amplitude adjuster's rubber (green) can be moved up or down, changing the ratio of the lengths of the effort arm and the load arm.

The calculation for how the length of each of these segments of the lever's arms changes the movement is :\[\Delta x_{AA Lever} = \Delta x_{Rosette} \left(\frac{Load Arm_{AA Lever}}{Effort Arm_{AA Lever}} \right)\]

So, as the amplitude adjuster rubber 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 rubber (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 :\[\Delta x_{Spindle} = \Delta x_{AA Lever} \left(\frac{Load Arm_{Headstock}}{Effort Arm_{Headstock}} \right)\]

And since the ratio of the Load Arm's length to the Effort Arm's length is fixed for a given MDF Rose Engine's headstock (HS), we can use the ratio,
\[HSratio = \left(\frac{Load Arm_{Headstock}}{Effort Arm_{Headstock}} \right)\]

Arriving at this generalized formula,
\[\Delta x_{Spindle} = HSratio \cdot \Delta x_{Rosette} \cdot \left(\frac{Load Arm_{AA Lever}}{Effort Arm_{AA Lever}} \right)\]


One additional complication is to be noted here. As the headstock rocks back and forth, and the rosette also moves the lever arm in and out, the ratio of the two levers changes dynamically. So it is not a static set of ratios. It is far more complicated that it seems at first glance.


Now here's the real difference between the horizontal amplitude adjuster and the vertical one: For the horizontal amplitude adjuster, there is no second lever arm.


About this site
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.

Formulas are displayed on this page using MathJax.