Amplitude Adjuster
Overview
Vertical Amplitude Adjuster Terminology
A rosette's amplitude is the measure of the movement that the rosette imparts upon the spindle. 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" 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 you will not have to buy a new 24-lobe rosette with a different amplitude for each operation, when you only want to adjust the amplitude.
Dynamic Amplitude Adjuster
on an MDF Rose Engine 2.0
One enhancement implemented on the MDF Rose Engine 2.0 is a dynamic amplitude adjuster. This allows the amount of amplitude adjustment to be dynamically changed as the piece is being cut.
This is used with some other device such as a curvilinear slide.
There are two use cases, and they apply to the turning of tapered pieces.
1. The artist desires that the rosette's design stay consistent across the length of the taper. An example of this is to use the puffy-4 rosette to make a square piece, and keep the sides square despite the changing diameter.
2. The artist desires that the rosette's design change across the length of the taper. An example of this is using an F-4 rosette to make the post for a ring stand. The upper part of the post will need a very low amplitude to not be to "pointy", but much of the design for that rosette will be lost. Towards the bottom of that post, the amplitude can be increased as the diameter is increased, displaying more of the rosette's design.
Examples of this device in use
The amplitude adjuster I made for my MDF rose engine lathe is outlined on this web page link.
This video below shows it in action.
The amplitude adjuster which Al Collins implemented on his MADE lathe, is a horizontal amplitude adjuster (using a design from Fred Armbruster). The video on that web page shows well how this works.
Usage Notes
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.
Vertical Amplitude Adjustment
The chart to the right shows the effects Tom Johanson measured on his vertical amplitude adjuster.
- 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)
Tom's amplitude adjuster is based on a slightly different design than the one I built, but the curve is the same. You can notice that the amplitude adjustment is not a straight line; rather, it is curved.
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Ray Simmons noted that a rubber with a large radius effectively works as an amplitude adjuster by smoothing out the ups and down of the lobes.
How it works
An amplitude adjuster is a lever. By adjusting the pivot point (fulcrum), the effect of the rosette is amplified (usually in a negative way).
Vertical Amplitude Adjusters with Vertical AA Rubber Movement
The calculation for how the length of each of these segments of the lever's arms changes the movement is:
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HLAL |
= Headstock Lever Arm Length |
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AALAL |
= AA Lever Arm Length |
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RDD |
= Rubber Distance Down |
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AA% = |
HLAL * (AALAL - RDD) |
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AALAL * (HLAL - RDD) |
or
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RDD = |
HLAL * AALAL * (1-AA%) |
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HLAL * (AALAL * AA%) |
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Based on those equations, the AA Lever Arm must not be at the axis of the headstock's rotation (i.e., the AA Lever Arm Length ≠ Headstock Lever Arm Length).
Vertical Amplitude Adjusters with Horizontal AA Rubber Movement
Jean-Claude Charpignon developed this type of AA for his fixed tool machine.
(Still working on the math for this one ...)
Notes on making one
Some lessons learnt include:
- The pivot point for the amplitude adjuster's lever arm must not be the axis of rotation for the headstock (i.e., Headstock Lever Arm Length must be different from the AA Lever Arm Length).
- The rosette rubber shapes are easy to model in Fusion 360 and 3D print.
- A 3D-printed rosette rubber works well.
Vertical Amplitude Adjusters
- The impact to the achieved amplitude adjustment is minimal
- It is hard to achieve low amplitudes. About 25% is the least I could achieve.
Dynamic Amplitude Adjusters
- When implementing a dynamic amplitude adjuster, add one or more limit switches. The AA is implemented on the side of the machine away from the operator, so limit switches can help ensure the movement of the rubber does not crash against the limits of movement.
The instructions for making one for the MDF Rose Engine 2.0 are in the MDF Rose Engine 2.0 Library.
More Information
Published Articles
- Amplitude Adjusters by Al Collins. Al explains the history of amplitude adjusters and how they work, then surveys the current crop of these very useful rose engine accessories.
Ornamental Turners International Newsletter, Volume 27, No. 1 - May, 2020, pg. 16 - Amplitude Adjuster for an MDF Rose Engine by Rich Colvin.
Ornamental Turners International Newsletter, Volume 26, No. 2 - Winter, 2019, pg. 23
Web Sites
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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. |