Saturday, February 14, 2009

90:1 Gear Ratio

These are the parts machined to produce a 90:1 gear ratio with a 10 TPI worm and 90 tooth spur. These items form part of a rotary axis assembly used to position antennas for accurate EM pattern measurements. The spurs are made of delrin and the worm is a 10 TPI ACME threaded rod. The previous post shows how the spurs were cut.

The axis are driven by 1.8 degree steppers drives which operate in 1/2 step mode making 400 steps per revolution. With the 90:1 ratio each degree is traversed with 100 steps. This is lots for the antenna positioner.


Nope this is not furniture making but it is a great setup exercise along the road to making gears for clock mechansi

Friday, February 13, 2009

Gear Cutting in Code

On the road to setting up for making clock gears I CNC enabled my bench-top mini-mill. The idea of machining in G-Code motived me to add stepper motors to the X, Y and Z axis of the mill and then later to a rotary table. The entire system is run from the open-source EMC2 program. www.linuxcnc.org

This set up below allows for the rotary A axis to rotate the plastic gear blank into position and then a cutter to be depthed via the y axis into the blank. This project is a 90 tooth spur gear that is first gashed every 4 degrees with a 1/2 inch 45 degree dovetail cutter (first video)

The actual teeth are "hobbed" by a 10 tpi ACME thread tap that the blank is pushed against. The pre-gashing of the 90 teeth ensures that the correctly sized blank properly engages the hobb as it cuts. The jacobs chuck on the rotary table is loosened so the blank will rotate and advance properly as the hobb cuts.

Monday, February 9, 2009

TV Table - Practical Woodworking


My wife doesn't re-arrange furniture - she just puts in requests for new pieces. Back to the shop it is then. I think I heard her say "nothing fancy dear" as I put on my shop apron.

We needed a TV table to hold the heaviest TV I have ever owned and probably the last CRT TV I will buy as well. The TV was to come out of the living room and into a bedroom.


The table is built from 2x10 Douglas Fir I picked up from the local hardware store. Nothing fancy - I just picked through the pile for the clearest planks they had. Back in the shop I ripped, jointed then planed the wood to a reasonable dimension. Most pieces are 1.5 x 4.5 so when glued up became 4.5 inches square.

The shape of the top is a trapezoid so as to fit into the corner of a room better. Under $25 in lumber, two weekends, and left over varathane to finish. The hardest part was keeping the shop warm enough (December) so the glue and finish could set.