suitable gear material for backgear - SMALL lathe and homemade cutters

Folks,
A couple of questions if you don't mind... Scenario - I'm building some backgears for a Taig lathe (small lathe the size of a sherline - chuck is nominally 2.5-3" in diameter) and
will be cutting the gears with a silver steel (Drill rod) parallel hob
The bull wheel will be around 3.5" in diameter at most and the mating spur wheel will be around 1" (up to 1.5") The face of the wheels will be in the 10-12mm (3/8" - 1/2") range and the lathe uses a 350W (0.5 hp) motor off a 1/8" gates v-belt
What would be a suitable material for the bull wheel which can be cut using a homemade cutter? materials I have available include: mild steel, aluminium, nylon, brass (if I can get the foundry up running again)
I'm worried about using the home made cutter on mild steel due to reading about burning the cutter - or am I worrying for no good reason? Aluminium, brass and nylon are easier to cut, but I keep reading that aluminum should not be used for gears becuase it's deemd high friction, and would gall...
any tips or suggestions? any machines out there of similar size with backgears - what are those gears made of?
Thanks, Des
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    O.K. What diameter hob? How hardened? And is this truly a hob? (Looks like an Acme thread which has been interrupted, and requires being geared to the workpiece at the proper ratio for the number of teeth desired on the workpiece.
    If you mean a single path multi toothed cutter like a conventional milling cutter, this is a gear tooth cutter, not a hob, and is easier to use. You need only the ability to turn it at the right speed, and an indexing head to position the workpiece at the proper number of locations for the teeth. (However, these are made in sets, and which one is used from the set is a function of how many teeth you are cutting. A true hob will cut the right shape for all pitches, while the gear tooth cutter will only cut a range of teeth to a close approximation of the needed shape. The one for the smallest gear (unlikely for a bull gear) covers only two or three tooth counts. The opposite extreme cuts something like from 180 teeth to a straight rack gear.

    Hmm ... the usual way to do a back gear is the bull gear, another smaller gear on the other end of the pulley (the pulley is free to turn on the spindle, while the bull gear is locked to the spindle), and the back gear itself, mounted on a shaft which is in an offset mount so it can be rotated into mesh at both ends or out of mesh and the lock pin for direct drive pushed in for the higher speeds. So you have two stages of gear reduction. Working just from memory, the total gear ratio is typically about 6:1.

    Well ... my Clausing 12x24" uses cast iron for the bull gear (starting from a rough casting, with only the necessary parts machined center grip to spindle, the gear teeth and sides, the sliding bushing for the lock pin for direct drive.
    The back gear could be of similar material. The gear on the end of the pulley might be better made from steel.

    Of those, mild steel would be the best -- but cast iron (even if you have to buy it) will be easier to machine once you get past the skin. (Turn off the skin, turn to proper diameter, and then use the hob to cut the teeth.

    Assuming that you have hardened and tempered it properly.
    Take the diameter of the hob, look up the proper SFM for carbon steel, and calculate the appropriate RPM for that hob. Then back off by about 20% to make up for possible error in the hardening and tempering process.

    Aluminum on aluminum is deadly from galling. Aluminum meshing with stainless steel or brass is better -- but I don't think that the aluminum would be strong enough for the task.

    My thoughts mostly above.

    Well ... the 6" Atlas/Craftsman has a lot of pot metal (zinc alloy) in its gears -- perhaps even the bull gear and back gear. That is about twice the size of the Taig. But they used pot metal for reasons of price for the completed lathe, not because it is a good choice. It is just easy to cast the gears ready to run. :-)
    I still suggest cast iron for strength and ease of fabrication. Forged steel would be better, but overkill for a Taig, I think.
    Good Luck,         DoN.
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DoN,
thanks for the reply... the cutter cuts one full tooth, plus parts of the 2 teeth either side of it. It's not "threaded" like a true hob, so gearing isn't used, but the profile is a rack profile - thereby allowing the same cutter to be used for all tooth counts.
There is a good article describing the parallel hob in Model mechanics - may 1979 by Rex Tingey and i've seen it around more recently as well.
The drill rod (silver steel in my neck of the woods) will be machined up, and then hardened and tempered as best I can in a home workshop.
I'll see if I can scare up some cast iron for the bull wheel, or default to mild steel if I can't obtain it.
thanks, Des

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-The drill rod (silver steel in my neck of the woods) will be machined -up, and then hardened and tempered as best I can in a home workshop.
-I'll see if I can scare up some cast iron for the bull wheel, or -default to mild steel if I can't obtain it. -thanks, -Des
You could make one (or 2?) from aluminum and then risk the cutter on a free-machining steel. I bought some "1018 CRS Shafting" from a bearing dealer that machines quite easily, nearly like 12L14. This stuff, without the key slot: http://www.keystonemfg.com/trukeyoverview.php /
In my experience the "water anneal" makes flame-cut hot rolled steel machine well. Heat red, cool until color disappears, quench. I don't bother to do it unless the metal had hard spots and the first cut left a rough finish.
Scrap hydraulic cylinder rod also machines well, after removing the outer case. I got some short rod stock cut-offs from a local cylinder manufacturer's scrap bin. Compare Nitralloy's machinability to low and medium carbon steels: http://www.a-i-t.com/machinability-rating-chart
Here's a similar method with a single-point rack shaped cutter ground from a lathe bit, which cuts slower than a hob but was quick to make accurately on my surface grinder and can spin at HSS instead of carbon steel speed. https://picasaweb.google.com/KB1DAL/HomeMadeMachines#5285710370886636434 The teeth are ragged because the cutting force overpowers the index's weak small-diameter spindle lock.
I roughed out the blank with a faster-cutting standard gear cutter: https://picasaweb.google.com/KB1DAL/HomeMadeMachines#5285710360947850418
jsw
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Des Bromilow wrote:

Aluminum is a terrible choice for gears with any load. If anodized, they are OK for very light loads. Nylon would probably work at this load. Brass would be fine, and steel would be too, just a little harder to machine.
Jon
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