Re: how to cut worm gear on hobbing machine

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Posted by BillS on August 05, 2011 at 14:35:03:

In Reply to: how to cut worm gear on hobbing machine posted by kasiktsi on January 19, 2009 at 13:53:09:

Ah... the No. 12. Had one years ago and cut a lot of throated worm gears on it.
Three things to remember:
Never touch change gears with machine running!
All cutting is done with the work slide.
Feed table is only used to position the cutter.

Also, I'm assuming that you do NOT have a cutter with same OD, Starts, and Hand as the worm that runs with this gear.

Here is the step-by-step procedure that I've used:

Pick a single start hob cutter with same NDP as gear. If it is RH then you don't need an additional idler in the index. A LH cutter will work, but you need to add an idler to the index. Also, the cutter diameter should not be smaller than sample worm diameter unless your sample worm is multistart. In that case your hob is skewed to match sample worm angle, so it will cut a larger effective radius than its actual cutter radius.

Set index gears for number of teeth as though it's a spur gear.

If you plan to throat the gear using the hob, set table angle to zero.

Otherwise set the cutter table to correct angle - either add or subtract the hob cutter angle from desired worm gear angle - this may need a small correction later.

Mount the work piece.

Raise workpiece slide until cutter clears the OD.

move the cutter under the OD until it approximately centers under the workpiece.

Start machine.

Move the workpiece slide down until cutter barely touches OD, then raise slide. You don't want to cut around completely. Let workpiece rotate around until you can easily view the shallow marks.

Stop machine.

Next you must disable feed by swinging the feed change gear arm out of mesh with other change gears.
Engage the feed worm drive to keep cutter forces from moving the table.
To do this, turn the feed change gear on the worm shaft by hand (which should turn freely) until you are able to engage feed worm with gear. This lets you adjust cutter table by hand-turning the feed change gear (you will not be able to use the normal feed hand wheel). This will make precise cutter positioning easy.

Put a dial indicator on the slide to indicate cutter table movement. Zero the dial.

Measure each edge of mark to its closest face, and subtract the two measurements.
Rotate the feed change gear by hand to move cutter table half of this difference in the direction that centers the cutter. Zero the dial indicator.
Take backlash out of feed worm screw as you complete the move to zero.

Be sure machine is stopped before continuing!

If you want to use the hob to throat the workpiece, you need to put index ratio out of sync.
One way: swap a driver with a driven.
(I also have used a smaller dia cutter that was just larger than the root dia of the sample worm, and swung the cutter table to zero degrees).

Start machine and lower workpiece slide. Do this until you get to throated depth (NOT WORKING DEPTH!!). Let work revolve a number of times, until you get a good throat.

Stop machine.

Change index back to correct gearing for cutting spur gear.

If cutter angle was not set for correct worm gear angle, set it now.

Using workpiece slide, drop slowly (maybe .010 - .020 per revolution) until you get almost to depth. Have patience with your cutting feed rate. At conclusion, let workpiece make 1-2 rounds.

stop machine.

Take your sample worm and mesh with gear while still on machine. Sight or measure as best you can to see if the angle is correct. If not, make an adjustment to cutter table angle.

When you are happy with the angle, finish cutting (almost) to depth.

Take gear off and check angle, depth, and contact.

If you need to put gear back on machine,

Leave gear loose on arbor and slowly drop work slide to full depth as you rock the workpiece back and forth by hand. You will feel the slack diminish as you get closer to depth.

When slack is just barely gone, tighten workpiece on arbor.

Move workpiece up out of mesh with cutter. Now is the time to make an angle adjustment if necessary.

Or drop to depth to take a deeper cut if necessary.

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