unwanted helix angle on spur gear 2

[Luslugger]

I am making a 16 dp 44 tooth gear on a Barber C #12. I have set the hob slide to 1 degree 47 minutes as is printed on the right hand hob. When I cut towards the spindle I do not get a zero helix. The rough cut had a helix of about .020 per inch. The finish cut did not have the same helix. is was closer to zero but not zero. This machine is new to us. We have some some worn keys but nothing that we think can cause this. We are beginners so your answers for this may be simple. I would also say we have the book for this machine and have the 4 change gears that the book recommends for 44 teeth. We counted the teeth on each gear.

 

[gearcutter]

This looks more like a tapered profile as a result of varying depth.
I hate to tell you this.............these horizontal machines are awful to try to have to work with, particularly if they are the longer ones. The idea of having a horizontal, heavy and long over-arm support for the tailstock is a really bad design as you are relying on both ends of the support to raise and lower evenly when setting the depth of cut.
Releasing the end clamps causes the over-arm to sink on the tailstock end. This is where the tapered cut is generally created as it causes the end of the job to sink as well.
Having said that, your picture is showing the cut to be deeper at the headstock end.
The first thing I'd check is that your centers are running parallel to the bed ways.

 

[mfgenggear]

In addition what was said by gear cutter

make sure your Index gear ratio is exact.
follow the formula supplied in the manual.

on a spur gear
the tool spindle must be swiveled to cut a straight lead.
the lead angle marked on the hob must be compensated on the hob spindle

the hob swivel setting depend on the lead angle of the hob.
if its a right or left hand hob the manual will state which direction to turn the swivel.

make sure when mounting all detail are clean & mounted within .0005" or better.

make sure centers or tooling is indicated with in ,0005" or better.

indicted the hob within .0005" while the machine is running
in manual setup mode.

 

[Luslugger]

The hob run out is .0008.
I do not understand which direction to swivel the hob. We think we have it done as the manual shows. I do not understand how hob swivel would create a helix. If wrong swivel, right or left, I would expect the tooth to be too thin when the whole depth was reached.
the machine has cut a straight line within a few thousandths, It has also made a helix of about .o2o with the same change gears.

 

[unclesyd]

gearcutter and mfgenggear,

What is you opinion on the finish he achieving?


To me it looks like something is moving oh so very slightly.

 

[mfgenggear]

luslugger

if your problem has not been solved then
look at your manual which shows illustrations of the hob swivel.

My 16-10 handbook states this

looking from the operator view (in front of the machine)
conventional hobbing
Right hand hob turn swivel right
left hand hob turn swivel left

Clmb Hobbing
Right Hand hob turn swivel right
Left Hand hob turn swivel left

the object is to compensate of the lead of the hob.
to cut a straight lead.

it is possible the dog or collet (work holding is slipping)
like said by Unclesyd. make sure work clamping is tight.

double verify the index ratio & the mounting of the change gears, the machine constant of the machine should be in the
door cover of the change gears. (****Important*******)

The blanks must be accurate, if not the gears will not not cut correctly.

MfgEnggear

 

[Luslugger]

by index constant, do you mean a single lead worm on the spindle head or somethingelse? I am thinking that the loose mechanical fits and the angle on the hob slide would make the tooth thinner than required, but not change the helix.
We adjusted the worm gear, we can see the helix is still there. We checked the hob bearings and they were great. We have about .002-.004 end play in the hobbing spindle. We took the play out of the keyways with new keys.

I have not made one good part with this hob. Tomorrow I am going to put on a hob that has made good parts on a machine in another shop. If the helix is zero then I will know it is the hob. If the helix is there it must be the gear train between the hob and head.

 

[gearcutter]

The angle that the hob head is set to, cutter run-out or job run-out has no relevance (while all are important) to the problem you are describing.............you are correct when you question the issue of the hob head angle.

The only way a hobbing machine can cut a helical profile is if the fixed indexing constant is changed, or in other words, the relationship between the rotation of cutter and job is different to the standard relationship calculated for the number of teeth in the job if it were a spur gear.

Does this machine have a differential?

Have you been able to confirm if the machine is in fact cutting a tapered tooth profile or a helix?

unclesyd's observation regarding the finish may be an indication of other issues but it looks to me like you are using a course feed rate.........what is the feed rate you are using?

 

[mfgenggear]

Luslugger

Buy or make a ground straight arbor. “Cylindricity of .0005 max.”
With 60 deg included centers.
Place the arbor between the tailstock & headstock
Place an indicator on the hob carriage and indicate the length of the arbor.
This will tell you how much taper error there is.
Shim or fix the tailstock as necessary.

Now the hob swivel combined with tail stock error will wipe out half the tooth of the spline or gear. Causing the taper and thinning as shown on your Op. fix both.

Now the Index change & feed gears must be calculated for a spur gear. if you don't +/- 1 will cut a helical.

Spur Gear for a 16-10, 6-16, 6-20
is as follows (for example only) non-differential
Index Gear Ratio=KT/N
Feed gear Ratio = F/M

K=Machine Constant
T=# of threads of Hob (# of Starts)
N=# of teeth of gear
F=feed in inches of revolution of work spindle
M=Machine feed constant

Machine Index constant
Triple thread worm=12
Single thread worm=24

You must know the exact machine constant for your machine.
Also For differential machine it must be disengaged.

 

[Luslugger]

No differential. The work between centers is straight within .004 over more than 12 inches, horizontal and vertical plane. The tooth form is making a helix.

 

[mfgenggear]

would you please post your actual Index & feed change gears used.

post the machine constant & the index & feed formula from your hand book.

post the hob data, # of starts, RH or LH, lead angle.

are you making sure the part is not slipping during hobbing.
what is being used at the head stock. collet or dog.

is the electronics workin properly?
what is the feed your trying to use.

try an other hob.

 

[Luslugger]

We tried another hob today that has made straight parts before and the part today looks straight. Tommorrow we will indicate and know in thousands if it is staight. Since this is probably a combination of problems, the hob itself is a problem.

 

[Luslugger]

We put on a 20 dp hob used on another machine and it cut straight. The 16dp hob I have has defects in the way it was manufactured. I am procuring a hob made in USA.
I will start over and see if any problems are left using the 20 dp hob.

bob