Benchtop Hobbing Machine - The BHM

[PaulL]

Next job is mitre/bevel gears with an involute cutter. Its all in the software!

Got a GitHub?

 

[Dan Dubeau]

Impressive build.

 

[MotoRider]

Well done!

 

[phaxtris]

Excellent project!

 

[Don Sipes]

The BHM - a benchtop hobbing and gear cutting machine.

This project has been brewing for a couple of years and its now in a working, but not pretty stage, so I thought I would share a few details.

Gear hobbing is a highly efficient and accurate machining process used to create teeth on gears, splines, and sprockets. The multi-toothed cutting tool, called a hob, looks somewhat like a worm or a screw with cutting edges.

Here's how it works:

1. The Setup:
   • A gear blank (the workpiece that will become the gear) is securely mounted on a rotating spindle in a hobbing machine.
   • The hob is mounted on a separate, synchronized spindle. The angle of the hob relative to the gear blank is precisely set based on the desired gear characteristics - specifically, the helix angle of the gear and the lead angle of the hob. When cutting spur gears, the gear helix is zero.
2. The Synchronized Dance:
   • Both the gear blank and the hob rotate in a precisely timed relationship. As they spin, the hob is slowly fed axially across the face of the gear blank.
   • Think of it like the hob "rolling" with the gear blank as if they were already a meshed gear set.
3. The Cutting Action:
   • Each cutting tooth on the hob (called a "gash") takes a small chip out of the gear blank.
   • Because the hob's teeth are arranged in a helical path and the hob itself is rotating and feeding, the successive cuts progressively generate the desired tooth profile on the gear blank. The shape of the hob's teeth directly determines the shape of the gear teeth being cut (typically an involute profile).
4. Continuous Generation:
   • The magic of hobbing is that it's a continuous generating process. The teeth are formed one after another in a continuous motion, rather than indexing and cutting one tooth space at a time (like in milling with an involute cutter). This makes hobbing fast and accurate for producing large quantities of gears.
   • Because it is a generating process, exact tooth profiles for any number of teeth can be produced with just one hob. Involute cutters by comparison, are an average shape that is suitable for a specific range of teeth, e.g. a #4 cutter makes 21 to 25 tooth gears.

The BHM is designed to produce spur, helical and bevel gears - however we must use an involute cutter and an indexed action to produce bevel, miter and crown gears using the shift & rotate method.

The machine is built from barstock, with a 16" x 16 " footprint and 20" high. It weighs approximately 150 lbs.

There are the three linear axes, Z for depth of cut, Y for the work advance through the hob and X for transverse shift to utilize all of the hob length when it becomes worn.

When cutting helical gears, the gear helix angle is set manually with a quadrant plate.
When cutting bevel gears, the work axis can be tilted to match the various bevel angles.

The hob can cut either above or below the work, with the cutting forces directed towards the chuck.

The workhead spindle and the hob spindle are very precisely synchronised to generate the correct tooth profile. The hob spindle is driven by a large NEMA 23 servomotor, geared down 3:2 and the workhead is geared down 3:1 for increased torque. When cutting, the hob runs about 120 to 200 RPM and the gear blank at approximately spindle RPM divided by the number of teeth.

Originally I had intended to use a standard CNC control board to drive the steppers and servos. However AFAIK there is no precision spindle coordination that can be controlled using g-code, so I developed a custom controller based on the Teensy 4.1, and my own software and firmware to coordinate the 5 motors. The software is a continuous development (read 'neverending") process which is the cause of my grey hair.

The HMI (Human Machine Interface) is an 800x480 colour touch screen, and a custom 24 key keyboard.

Here's a few pictures of the machine at this stage and some finished gears. If there's a way to upload videos, I'll post those too.

View attachment 64649View attachment 64650View attachment 64651
Setting the gear helix angle on the quadrant plate, and setting the tilt to zero using a 123 block
View attachment 64647View attachment 64646

The undercarriage showing the X ballscrew and the hob helix adjustment mechanism.
The micrometer allows 1' (1 minute) adjustment.
View attachment 64645

Cutting a 60T M0.6 helical gear and a similar spur gear. Left/Right helical and a pair of spur gears made from Delrin.
Small gears like these take about 2 minutes to hob.
View attachment 64643View attachment 64642View attachment 64644

Custom electronics. Under the grey part are the 2M542 stepper drivers. There's a 24V 300W PSU plus a 5v and a 12v BEC.
View attachment 64641View attachment 64648

Stay tuned...

Very nice.

 

[jcdammeyer]

Amazing project worthy of a Gingery Style Book on the construction and programming.

 

[Arbutus]

So.....

Brass m0.6 gears are not a problem but cutting a steel (1045) m0.8 gear was a real shakedown test. Literally.

This is an m0.8 25 deg steel helical being cut just before I abandoned the job.

I had it set to a full depth of cut (1.8mm) with a very tiny feed of 0.25mm/work revolution. That doesn't sound like much but it sure made a lot of noise. About 3mm in and I could see that wasn't going to work. The Z (depth of cut) feed control is a work in progress - the goal is to set the DoC per pass so that the MRR (material removal rate) is reasonably constant. The plan is for the DoC to take a deeper bite on the first passes when just the cutting tip is engaged and reduce the DoC per pass as the shoulder of the tooth starts to cut. Probably overkill, but it will speed up the process and produce a smoother tooth profile.

The vertical column is fitted to a sleeve and base which can rotate +/- 4 degrees to set the hob helix angle. I was relying on the cotter clamp to keep the column in the correct hob helix position, but the offset cutting forces are too much for this, and there is some movement under high loads, so I will need to add another method of securing the column.

Much worse than that though, the spindle head should only move vertically - but again under high loads there is some rotational movement. So back to the drawing board for the column arrangement. I would like to cut a slot in the column - about 4mm deep and 12mm wide. On the inside of the head bore, I would fit a bronze key to mate smoothly with the slot. This should prevent rotation and allow only vertical movement.

Also, since the vibration shook out some of the setscrews that secured the column to its base, those will be replaced with pins and loctite.

More soon....

 

[Arbutus]

And the root cause of the vibration under high load.....

These things.



I used these couplers on the X and Y axes. They are fine for light duty rotary transmission - but axially, they are springs!

Under the light loads when cutting plastic and brass, any axial movement was not noticed, but when cutting steel it got ugly. As the cutter does it's thing there are radial and axial forces which moved the workhead in the X and Y directions.

Stay tuned...

 

[johnnielsen]

Beautiful work.