Re: barber colman #12 gear hobber questions

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Posted by Jim Pollock on September 24, 2009 at 21:23:25:

In Reply to: Re: barber colman #12 gear hobber questions posted by Dan Mc on September 24, 2009 at 13:35:49:

Thanks guys for all the help. I haven't had alot of time to play with the hobber but I did get the flywheel tightened up and I cut another gear. This gear was 6 dp and it had 35 teeth and was made of 8620. The feed was still too fast but this gear cut much better than the first one. I left about .100" for the finish cut. It still seems to me that I am running the hob spindle too fast. This hob was 3 1/2" in dia. and I was running it at about 120 rpm. This is the slowest change gears that I have for the speeds. How fast should I run a 3 1/2" hob in 8620 steel? I am using sulfur based cutting oil. I did find some more change gears for my feeds but not until after I made this gear so I should be able to slow down the feeds on the next gear.

I have another question for you guys. I needed to carburize and harden this gear as it is a transmission gear for a pulling tractor. I have an electric kiln that is really made to fire pottery. I put this gear in a steel pan and packed around the gear with charcoal that I had smashed up into powder. I then put this steel pan in the kiln and placed a steel plate over the pan and put a few firebicks on top to hold it down. I left the kiln on for about 8 hours and the temperature got up to about 1675 degrees Fahrenheit. I then quenched the gear and then tempered it in my home oven at 350 degrees. This gear came out perfect. It has no scaling on it whatsoever. It just has the same smooth black finish that an allen head bolt has. The surface is about 62 Rc and the hardness went about .030" deep. The only problem is that the gear grew about .018" on both the O.D. and the I. D. Is this normal when you carburize and harden a gear?

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