Archive | December 2016

Variable Stroke Pt. 1

I’ve been experimenting with variable stroke linkages for a while, with most of my new prototypes being 3d printed. These linkages have two inputs – the crankshaft and the stroke amplitude input – so I’ve been using gears to drive the two from one input.

Traditional gear manufacture involves using off-the-shelf tools to cut gears from blanks, and proper use of the tools will give accurate geometry. But 3d printing the gears requires an exact model of the involute tooth surface, which is a bit more tricky. Solidworks includes a “gear” template, but this is just a placeholder, not an accurate model. Some models can be found online, but these are almost always in imperial units. I wanted a parametric model of a precise involute cylindrical gear, using the metric standard, so I made my own.

There are 5 key numbers that define a given gear:

  • m – module
  • z – number of teeth
  • \alpha – pressure angle
  • \psi – helix angle
  • x – profile shift

I’ll discuss the last two in a later post. For now we can assume they are equal to zero, so that the model is only dependent on the first three.

A metric gear has two primary tooth size measurements aside from m: h_a – the addendum height, and c – the root clearance. These are shown below in a sketch of an “unrolled” tooth. Typically, h_a = 1.0 and c = 0.25. Four important dimensions associated with the gear are:

  • D_p = m \cdot z – the pitch diameter
  • D_a = D_p + 2 \cdot h_a – the tip diameter
  • D_f = D_p - 2 (h_a + c) – the root diameter
  • D_b = D_p \cdot cos\alpha – the base circle diameter

Involute gears use a tooth profile following an involute curve – this is by definition the curve obtained by unrolling a string from a circle. The “string” is always tangent to it’s “base circle” and perpendicular to the curve obtained, as shown below for a base circle of radius r. The position of point O is (x_O, y_O) = (r cos\theta, r sin\theta). Point Q on the involute is then:

    \begin{align*} (x_Q, y_Q) &= (x_O + l sin\theta, y_O - l cos\theta) \\ &= r(cos\theta + \theta sin\theta, sin\theta - \theta cos\theta) \end{align*}

This equation can be used directly in Solidworks as an equation curve defining the tooth face, however it only defines one side of one tooth. Ideally the modelled tooth will be symmetric about the x-axis, so the involute needs to be offset (rotated around the base circle) by the appropriate amount. We also need to know when to start and end the curve.

The rotated involute curve can be formulated by multiplying the involute equation above by a rotation matrix R(\gamma), which rotates the curve counter-clockwise about the origin by \gamma radians. The generic equation for an involute of a base circle with radius r_b, rotated by angle \gamma is:

    \begin{align*} \begin{bmatrix} x_\theta \\ y_\theta \end{bmatrix} &= \begin{bmatrix} cos\gamma & -sin\gamma \\ sin\gamma & cos\gamma \end{bmatrix} \cdot \begin{bmatrix} cos\theta + \theta sin\theta \\ sin\theta - \theta cos\theta \end{bmatrix} \\ &= r_b \cdot \begin{bmatrix} cos(\theta+\gamma) + \theta sin(\theta+\gamma) \\ sin(\theta+\gamma) - \theta cos(\theta+\gamma) \end{bmatrix} \end{align*}

The involute curve needs to be rotated such that the tooth axis lies on the x-axis, as depicted below. This angle \beta can be separated into two components:

  • The half-tooth-width angle \phi = \frac{2\pi}{z} \cdot \frac{1}{4} = \frac{\pi}{2 \cdot z}
  • The involute angle inv(\alpha) = tan\alpha-\alpha

So \beta = \frac{\pi}{2 \cdot z} + tan\alpha - \alpha, and the involute should be rotated clockwise by this angle, so \gamma=-\beta. The equation for this curve is:

    \begin{equation*} \begin{bmatrix} x_\theta \\ y_\theta \end{bmatrix} = \frac{D_b}{2} \cdot \begin{bmatrix} cos(\theta-\beta) + \theta sin(\theta-\beta) \\ sin(\theta-\beta) - \theta cos(\theta-\beta) \end{bmatrix} \end{equation*}

For the opposite of the tooth, the involute “string” is beign unrolled clockwise: simply take \theta\rightarrow-\theta. The curve is then rotated counter-clockwise, so take \beta\rightarrow-\beta.

What are the limits of the curve? From the above diagram, the distance between the origin and Q is \frac{D_Q}{2}. Then l = \sqrt{\frac{D_Q}{2}^2 - \frac{D_b}{2}^2}. But we also have l = \frac{D_b}{2}\cdot\theta, so the angle \theta where point Q is at a diameter D_Q is given by:

    \begin{align*} \theta_Q &= \frac{l}{(\frac{D_b}{2})} \\ &= \frac{\sqrt{D_Q^2-D_b^2}}{D_b} \end{align*}

The maximum point of the curve (at the tip) is then given by \theta_a = \frac{\sqrt{D_a^2-D_b^2}}{D_b}.
The minimum point (at the root) is given by \theta_f = \frac{\sqrt{D_f^2-D_b^2}}{D_b}.
The tooth profile is then formed by tracing the involute curve (x_\theta, y_\theta) between these limits. Except for some gears (z <\approx 40 – see below) the root is actually below the base circle – and the involute is not defined below the base circle.

In this case I model the tooth flank as a straight, radial line. In reality the tooth flank is sometimes undercut, and I will go into this in another post.

3d Printer

Earlier this year I decided to get a 3d printer to help with prototyping in robotics & various other hobbies. I wasn’t particularly wowed by any of the consumer-grade options available, and it seemed like sourcing individual parts for a full DIY design would take more effort than it was worth, so I went with a kit!

20170213_182920

My requirements were pretty simple:

  • E3D v6 hotend
  • Heated glass bed
  • >150 mm printing range in all dimensions
  • Easily expanded

There were a few options satisfying these, in the end I chose a MTW MiniMax as the best value for money.

IMG_20151231_100138

Parts!

 

Mechanical systems assembled

Mechanical systems assembled

It took a few afternoons to get it up & running. I started printing with PLA as most people recommend it as the best all-rounder material, but I’ve since found PETG to be stronger, more stable and easier to print.

After (slowly) printing a fan bracket I printed a case for a raspberry pi, which would act as a print server and allow me to not be tethered to the printer during a job.

Print server

Print server

Since then I’ve added a whole series of modifications, improving print quality and ease of use:

  • Y-axis cable chain (from MTW) – keeping things tidy
  • Rubber foot mounts (from MTW) – to reduce noise & vibration
  • Easy bed levellers (by beerglut) – the original bed levellers needed a screwdriver to adjust, these use thumbwheels instead
  • 360 degree print fan shroud (by beerglut) – for more uniform print cooling
  • Extruder fan mount – to cool the extruder gear & prevent it from warping PETG before the hotend
  • Sprung Z-homing screw – to reduce backlash present in the original design
  • Filament spool mount – intended to reduce the load on the extruder motor by mounting the spool on a bearing-supported roller, but this removed all tension from the spool and just resulted in tangled filament
  • Power supply cover & switch – the kit as-built includes only minimal protection from the mains wiring at the back, and needs to be turned on/off at the wall – so I installed a fused switch with a C14 socket to make it a bit safer & easier