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

On 2012-06-27, Des Bromilow wrote:
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

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.

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")

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.

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?

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.

materials I have available include:
mild steel, aluminium, nylon, brass (if I can get the foundry up
running again)

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.

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?

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.

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...

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.

any tips or suggestions?

My thoughts mostly above.

any machines out there of similar size with backgears - what are those
gears made of?

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



On Jun 27, 1:15*pm, "DoN. Nichols" wrote:
On 2012-06-27, Des Bromilow wrote:


* * * * 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.

-"Des Bromilow" wrote
-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/...10370886636434
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/...10360947850418

jsw

Des Bromilow wrote:


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)
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