C3 Mini-Lathe Metalworking Beginners
Making a telescope worm wheel and gear set on the mini-lathe. Part 12, Hobbing the worm wheel
Now for the good bit, using the hob to cut the teeth in the gashed worm wheel blank - finally it will become a real worm wheel and not a worm wheel blank!
The general process here is to mount the worm wheel on the cross slide of the lathe using some sort of homemade turntable. This fixing needs to be made before you can hob any gears, and must be extremely rigid. Making a suitable worm gear lathe turntable will be quite an investment of time, but the accuracy of this part dictates the quality of your finished worm wheels.
On my lathe I've made up a housing for some bearings I had spare, removed the compound slide completely, and bolted my contraption to the cross slide. The whole thing is devised to ensure that a 0.5 inch think worm wheel is held exactly at the center height of the lathe. I'm not going into detail of this here, it depends on what bearings you have available. If you've got this far in my guide, then I judge you more than capable of devising something.
The Worm wheel blank must be firmly mounted to the turntable. It is vital that the gear teeth are cut concentric to the bore in the worm wheel. Once the worm wheel is bolted onto the turn table, it is very difficult to indicate on the central bore of the worm wheel. You may recall we went to a lot of trouble to ensure that the outer diameter and inner bore of the worm wheel were concentric. We now rely on this and indicate on the outer diameter. With the turntable bolted to the cross slide, and the indicator base on the cross slide, you shouldn't be able to wobble the worm wheel with your hand.
Indicate on the outer diameter of the worm wheel until it is concentric to the rotation of the turntable to better than 1 thousandth of a inch. We are going to be pressing the worm wheel into the gear hob with force from the cross slide - what we do NOT want is this force pushing the worm wheel out of concentricity with the turntable - it needs to be very firmly bolted onto the turntable.
It is also sensible to keep a dial indicator on the outer diameter at all times during the hobbing operation. The indicator should not move during each revolution of the worm wheel - if it does, then it indicates that the worm wheel has shifted in its mounting on the turntable, and we should stop and adjust the mounting of the worm wheel before continuing.
That said, it is impossible to get this complete correct, so on the final construction, you should allow the worm gear to move a bit - but do not worry about that now - on with the hobbing.
We have the worm wheel blank mounted on a turntable on the cross slide of the mini lathe, and the worm wheel is turning completely concentric with this turntable. We have the worm gear hob on the lathe, completely concentric with the rotation of the lathe spindle. So now it is the easy bit. Use a lot of cutting fluid. Advance the cross slide into the worm hob until the teeth start to engage with the gash marks. Do not advance the cross slide too hard, just enough that the worm hob can draw the worm wheel around by the gash marks. It is normal to help the worm wheel around during these early cuts, apply constant rotational pressure to help keep the worm hob engaged with the gash marks. After a few revolutions, the worm hob starts to cut into the worm wheel and you no longer have to help it.
The carriage of the lathe does not have to be locked - indeed, you can move it around to use the whole of the hob, not just one tooth! Keep advancing the cross slide to push the worm wheel into the hob until you feel it starting to bite, and then leave the whole thing to go around for 5 or 10 minutes. It takes 3 or 4 hours to hob the worm, so be patient, and keep tightening the wheel into the hob bit by bit.
It is hard to describe exactly how this is done, like most operations on the mini-lathe, you have to operate by feel and instinct rather than by following a manual. Slowly, the gash marks will be turned into gear teeth. The gash isn't really there to aid material removal, they are mainly there to help the worm and hob index properly and get you the right number of teeth on the worm wheel.
As the lathe is running, it should sound fairly smooth with no clunks. A bit of scratching is ok, and squeaks need oiling. Actually, if you don't hear a scratching or cutting noise, it probably means not much cutting is being done. If the hob makes a disturbing clonking and bang noise every so often, this is bad. You've got a mismatched worm wheel outer diameter and worm gear pitch, and the thing is jumping over a tooth. You can continue, but you are probably just going to end up with a toothless worm wheel. Go to jail, do not pass go, and do not collect 200 pounds!
Do not be tempted to sharpen the teeth of the worm hob by grinding with the dremel. All that will happen is dust from the grinder will get into the teeth on the aluminium worm wheel, and slowly grind down the hob, ruining the whole process.
How deep do we cut the teeth. This page tells us how deep to cut, on my 7 inch nominal worm gear I should be cutting about 1mm deep. If you move the worm wheel away from the hob, and turn it by hand with the lathe turned off, as you advance the cross slide you will feel and hear the first contact between worm and wheel. Note the position on the cross slide dial. This is the zero position. Once the cross slide has been advanced by 1 mm (a whole turn) you have the final cut depth. This stage is only reached often a few hours of slowing increasing the cut depth. It is wise to leave the whole thing going round and round at the final cut depth for several tens of minutes to get a good cut. It might even be worth pushing the worm wheel in a bit harder to ensure the teeth are fully and consistently cut.
Once finished, count the teeth and inspect them. If all has gone well, you will have 360 well formed teeth. If things have gone poorly, you will have a mess. It will either be perfect or useless, there isn't much middle ground.
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