i was not factoring the time to make a hob since this in my view is not needed, first off i have a bunch of hobs, i addition there are hobs on ebay on a regular base and there are several companies that trade or rent hobs fairly inexpensive.
No the hobber doesnt have a bowl feeder, my definition of automatic and yours differ, automatic in my vocabulary means that once the machine is setup all you have to do is to change your blanks, pull a lever and use a gauge to check your finished parts every once in awhile.
I grew up and went to college in germany, the second operation lathe is a unknown animal there, most all lathes have threading capabilities the only difference is how its done, some usually the cheap ones require change gears while the more expensive ones have gearheads you just have to switch some levers.
Even with making blanks 5hours is far fetched, anytime i made blanks i usually ended up machining a cylinder that was 10-12" in legth, machine it to excact diameter then put it on the gear hobber and hob the gears, then put it back on the lathe , part the gears and do the secondary machining, next step was to grind the flanks and induction harden the flanks( i dont remember anymore what came first the grinding or hardening) on some larger gears the gear was packed in some toxic stuff and put into an oven for hours to harden
Generally plastic always turns out rough, over the years i have experienced two methods to reduce this,- a sharp almost razorlike cutting tool is understood- the first method is to use a forced airstream to cool and blow away the chips, part of the problem when machining plastic is that the material starts ro melt and then the chips "cake" together and adhere to the edges, removing the chips and cooling are the most effective ways, another method that i actually discovered for teflon is to use liquid nitrogen to virtually "deepfreeze" the plastic, the only disadvantage is that you have to machine fairly quick and you have to consider the shrinkage of the material when taking measurements
I know it was a lucky purchase, i was chasing gear hobbers down for years and usually anytime i found one that was reasonable it turned out to be without change gears, i know what a good set of change gears costs and i know the cost of a hob, new and used as well as the cost of sharpening them, i have been involved in machine shop operations more than i wanted to but if your brother starts a business and he doesnt have $ to hire somebody the family has to pitch in :) so altough im a EE i know almost anything there is to know about machining
Chuck Harris <cfharris@...> wrote:
lothar baier wrote:
i agree on the fact that hobbs are expensive but i have about 120 hobs that came with the machine,
That was a very lucky purchase. The hobs alone, if they are sharp and
any good, are worth easily 10 to 50 times what you paid for your machine.
Now, do you have any 48 dp hobs in your collection? If not, you will
be making, or buying some before you finish this project.
.... Any lathe has
threading capabilities
Begging your pardon, but that is not true. Most lathes used in
industry are second operation machines, and they can only cut a
thread if you have a die already made. A hob is way out of their
league.
The only other lathes used these days are CNC lathe centers. Manual
machines show up only in small model shops, and hobby machine shops.
.... and you need a lathe anyways to make the disks you cut your gears out of,
I included that in my guesstimate of the time to make a single gear.
Did you?
.... you wont need any
heat treating of the hob since you are only cutting plastic,
Plastics, such as delrin and nylon seem soft and buttery, but they are never-the-less
abrasive when you cut them. An unhardened tool steel hob will make quite a few gears,
but the sharp edge will be gone very early on. Plastics such as nylon and delrin
cut with a very ragged edge unless the tool is scary sharp.
If you are only going to make a couple for yourself, sure, you can take short cuts.
If you are going to make them for others, you don't dare.
a good quality toolsteel will do for that Apparantly you
havent been much around machining, i really dont know where you get the 5hours from,
Ever tried making a hob? I have made a few, and 5 hours is a good estimate for
one hardened and ground tool steel hob made on manual machines. If you have ten
identical hobs to make, you can take advantage of some economy of scale, but to
make one hob, will take you about 5 hours.
setup of your hobber takes about
1hr max and that includes calculating your index and feed gears, the cutting process in itself is depending on the
ammount of teeths on your gear and the feedrate you can run, if you use forced air cooling on delrin or PVC you can
generally run a good rate so the overall time to cut is about 10minutes 30minutes the most ! i have cut gears on
hobbing machines in the past helping out in my brothers shop, those were gears used in CASE harvesters and they only
took about 1hr/piece to hob, keep min mind that they were much larger in size ! There is no such thing as a manual
hobber either, all hobbers are automatic,
Really now? So, you have a stock feeder that puts the new blank into
your hobber, and removes the old? You just walk away, and the bucket
gets filled with gears?
No? Then you have a manually operated hobber. Someone has to be in
attendance during the entire operation.
-Chuck Harris
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