What's new
What's new

9" model A gearing to make hobs??

iwananew10K

Diamond
Joined
Sep 12, 2010
Location
moscow,ohio
this is closely realted to another thread and i didn`t want to hijack it.

i would like to try making a gear hob, have a 9" model A , would like to make hobs in 16,18,20 DP, all 14.5 PA.

i need to figure out what gearing combo will give a lead of .1963" (16 DP, very close to 5 tpi) , a lead of .1745" ( 18 DP ), and a lead of .1571 ( 20 DP )

i can make different stud gears easily enough, i just don`t know how to figure what ones to make.
i think my brain exploded.
 
DP worms and hobs require pi in the geartrain, but it is an irrational number, so most lathes use 22/7 as a compromise. This gives 0.04% error. We can actually do better than that, if we are willing to swap gears a little. I'll post something this evening.

allan
 
So the magic number here is to turn the gearbox input 1.570800 times faster than the normal 20/56 gears used on the 9A/10K. This will cause the 8, 9, and 10 tpi gearbox positions to cut 16, 18, and 20 DP worms respectively. The closest simple replacement gear is a 23 tooth stud gear and a 41 tooth gearbox input gear. This gives a ratio with 0.004% error, 10 times better than lathes which use the 22/7 approximation. It is possible that there is an even more accurate compound gear train, but it won't be any simpler than these two gears.

allan
 
iwananew10K,

On a SB9A you can set the gear box to A5 with the standard stud gear and QCB drive gear and get 5.75 tpi.
This will cut a 18.0642 dp hob which is quite capable of producing any size change gear you might want.
Setting the gear box to C3 with the 20 tooth and 56 tooth gears swapped will cut a 16.0285 dp hob as shown.
I do not know of any way to cut a hob close to 20 dp without additional change gears.
If you are going to cut gears, the 23 & 41 tooth are the best choices available that still fit with the gear cover on.
 

Attachments

  • Hob.jpg
    Hob.jpg
    53.1 KB · Views: 901
thanks lbender!

great looking hob! that`s exactly what i was thinking, did you freehob with it or set up an actual hobber with your lathe or mill?
 
Curiously, 20DP translates to 4mm pitch within .25%. Maybe there are possibilities with working in mixed imperial/metric... (discovered this when looking at making hobs with a Hardinge or Myford lathe)
 
The hob pictured is only relieved at the tips with no side relief. This works well for free hobbing, but not for an actual hobbing machine. The lack of side relief does help the hob follow the path the tip cuts, critical to free hobbing. However, it can't be resharpened as a form relieved hob can be, so it is limited to non-production use.

One important point about making the hobs. These are usually cut from low machinability steel with a form tool which is large for a SB9A. It is painfully easy to break teeth from the drive train cutting the lower dp hobs. Go easy on cutting depth and seriously consider cutting only one flank at a time to reduce the load on the lead screw.
 
good suggestion lbender! i`ve cut a 1" x 5 tpi acme on my 9A before so i know what you mean!

i ended up roughing it out with a grooving tool set to the helix angle before i used the form tool, even then it was kind of touchy.

i may just try to accurately set the compound at 14.5" and use a tool with only 1 cutting edge, and then just skim it with the form tool.

lots to consider!
FWIW- the gears i plan to make will be from delrin so i might try one of the hardening compounds( Kasenite, Cherry Red, etc. ) and use a mild steel to make the hob.
 
hi lbender,
i did play around a little but to actually try it for real i need to get some bigger 01(i have 3/4") - i cut a worm in some 3/4" 12l14 just to see if i could get the blank to rotate with a hob that small- the results were not promising. :)

i ended up making single tooth cutters by the button method to get me by for now.
i have not given up on the hobbing though. i`ll still need to do it for making some metric translation gears.
i don`t want to have to index those tooth by tooth!
 
iwananew10K,
Unless you have some type of gearing to force the blank to rotate in syncronization with the hob, the blank pretty much has to be gashed before free hobbing. The good news is that the gashes don't have to accurately spaced to the arc second, hobbing will correct spacing. A quick method is to print a radial array of lines equal to the desired tooth count from any CAD program. Attach to your blank and gash on each line. Gashing not only insures the proper tooth count, but leaves less material for the hob to remove.

A slitting saw less than than the tooth tip thickness makes an acceptable gashing tool, but I use a gash cut to the pressure angle to leave a minimum of material to hob.
 








 
Back
Top