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Setting Up a Gear Hobber (when all you know is the name of the machine)
This article details the process of setting up a gear hobber, when the only information available is the name of the machine: no manual, no change gears and no English (it is a Japanese machine). For anyone interested, the hobber is located at the Penafrancia Sugar Mill, Pili, Camarines Sur, Philippines. This is about 300 miles South of Manila. The Mill is visible, cloud cover allowing, on Google Map, 13Deg-32.487’ / 123Deg-18.184’. This article follows the ‘logic flow’ in "Meshing with Gears" for setting up a gear hobber in the "Frequently Asked Questions" section. It uses the same terminology. The article starts at the beginning with the hobber installed on a foundation but not connected to the mains. It ends with the operator in possession of all essential information required to set up the hobber for spur, helical and herringbone gears. Most of the ‘finer’ points are glossed over (to be covered in later articles, I hope). IMPORTANT: Safe operation of any machine to which this information might be applied is essential. It is the responsibility of the reader to exercise care and caution.
Some of the information in this article is generic and can be applied to any gear hobber, but many of the specifics pertain to a differential vertical gear hobber. The details of the Seiwa hobber in the following illustrations can be found at the "Meshing with Gears" site. The process of getting the Seiwa hobber up and running was a difficult, tedious process. This article is built on the learning experience. Many of the missteps have been corrected or eliminated. As with most ‘engineering’, the manual is written after the fact and so is this article and with the above in mind, we are at the starting gate
What you will need:
A machined true shaft. 4" to 6" in diameter
by 12" to 18" long. One end of the shaft
mounted on the work table of the hobber
(a welded flange, machined true is the
the other end should have a true center drill
tracing paper and drafting instruments
2" wide Mylar packing tape (optional),
spray paint (optional), chalk (optional)
16-ga to 20-ga music wire; about 12"
(This will become the 'Scribe')
IMPORTANT! Remove all change gears from the hobber.
Check for preferred direction of rotation on the main motor. If there is no clear indication on the direction of rotation, connect the machine to the mains, observing the required voltage. The machine will surely be 3-phase.
Check (carefully) all oil levels, fill as needed, with the correct type.
Clean or change filters as needed and lubricate all spot locations.
Locate the main oil pump. Check/clean the sump screen, make certain the inlet is well below the oil level in the sump.
For this next step you should be prepared to check current in at least one of the electical lines.
Start the machine and quickly check line current. Does it match the name plate on the motor?
If you pass the first power-up 'smoke' test, check the cutting oil flow. If the machine is running backwards, the output of the oil pump will be very low, even zero. In this case, swap any two of the electrical supply lines to the machine and check the oil flow again. If the oil flow is deemed adequate, the machine is rotating in the correct direction. Don't continue until correct main motor rotation is verified!
Check if the bed (ways) were leveled when the machine was installed and check all anchor locations for ‘soft-foot’. If you choose to post-pone leveling at this time, make sure that machine is correctly leveled and supported before sustained operation.
Step 1: Identify the INDEX and DIFFERENTIAL sections: One of the first questions to answer is: Where are the hobber's change gears and how are they arranged?
(typical machine designs place the index gear set at the end of the machine. -Ed.)
We start with a simple formula most commonly used to define the arrangement of four change gears.
A, B, C, and D represent each location and number of teeth for each change gear:
Notice in first picture that 'A' meshes with 'B', and 'C' meshes with 'D'. The second picture shows a variation in that the idler shaft has only one gear; so it becomes both 'B' and 'C' on machine and in the formula.
A C Ratio = X B D
Identifying each gear in a set begins with 'A'. With all gear trains unmeshed (i.e. swing arms with idlers moved clear), a quick power on/off should reveal the 'A-Gear' shaft by rotating under power.
(If none of the change gear shafts rotate, stop and find the problem - is there a pair of larger pitch gears that couple the motor to the machine? These are used to control machine speed but might be missing - Ed)
'D-Gear' shafts are always the final driven (fixed) shaft. 'B' and 'C' are always mounted together on the swing arm.
(Not mentioned here is a reversing idler that is used on the swing arm of some machines, but is not required on the Seiwa. -Ed.)
So without any labels to rely on, which is the INDEX section and which is the DIFFERENTIAL section? To answer this question, turn on the hobber (remember, no change gears should be mounted). The ‘A-Gear’ shaft of the INDEX section will rotate - this identifies the INDEX section. By default, this identifies the other section with four change gears as the DIFFERENTIAL section.
Change Gears at
Front of Machine
Change Gears at
Left End of Machine
Step 2: Confirm the INDEX and DIFFERENTIAL sections: Mount a set of index change gears in the INDEX cabinet with 'A' identified in step 1. For example, use: 30-T on ‘A’ 60-T on ‘D’ and use any size intermediate idler that will mesh with ‘A’ and ‘D’.
WARNING: After mounting index change gears, use a wrench to rotate shaft 'A' with power OFF. You want to see if there is any seizure or binding. For example it is possible to place an OVERSIZE gear in any location and lock up the gear train with disastrous consequences! ALWAYS!!! rotate the gear train by hand (with the aid of wrench) before turning on the power.
Now, turn on the hobber. If the INDEX section has been correctly identified, the hob arbor will turn and the work table will turn.
The ‘A-Gear’ shaft of the DIFFERENTIAL section might also turn at this time. There MUST be a method for engaging and disengaging the ‘differential’. On the Seiwa, a lever controls differential engagement and hob table feed.
Hob table feed and differential action are linked together for helical gear generation as they must be.
When cutting spur gears the link between feed and differential must be disabled and the differential locked out. For the Seiwa, with the machine running, there is a lever that can be shifted right or left.
In one position, the ‘A-Gear’ shaft of the DIFFERENTIAL section will turn as the hob table feeds up (or down). This position is used to cut HELICAL gears.
In the other position, the DIFFERENTIAL ‘A-Gear’ shaft stops regardless of table feed. This position is used to cut SPUR gears.
You MUST locate this control (see levers on top of front cabinet below). (Please note that machine designs vary in how differential is engaged/disengaged. -Ed.)
Front View of Machine
Differential and Feed
Change Gear Cabinet
with Control Levers
The lockout sleeve does two things: (1) Removes any final 'D' gear that may have been mounted with a previously used differential gear set. (2) Prevents any travel or 'creep' in the differential mechanism by locking the shaft to the machine cabinet. For the next step you want to disable the differential by applying the lockout device. Disengage the hob table feed and the ‘differential’ with the control identified in Step-2 above. In other words, setup hobber to cut a SPUR gear.
Step 3: Locate the ‘differential’ lockout:
There will be a mechanism (dog or sleeve) for locking out the ‘differential’ action of the hobber. This is necessary to cut spur gears.
Gear Cabinet at
Front of Machine
Change Gear ('D')
shaft at Lower Left
on Gearshaft 'D'
to lock Shaft
to Cut Spur Gear
Step 4: Determine the INDEX CONSTANT: With the machine setup as described above, place gears on the ‘A’ and ‘D’ shafts on the INDEX section. These gears MUST have exactly the same number of teeth. The actual number does not matter, say 40-T on ‘A’ and ‘D’ index shafts and meshed with a suitable idler. Place ‘witness marks’ on the work table and the hob arbor. Next you are going to turn the ‘index’ gear train by hand and count the exact number of turns of the hob arbor to produce exactly one turn of the work table (remember the 'witness marks'?). This number, the number of turns of the hob arbor, is equal to the hobber's INDEX CONSTANT. Be sure and count the turns of the hob arbor, NOT the flywheel. Like many hobbers, the Seiwa has a flywheel coupled to the hob arbor through a gear train whose ratio is not 1:1. You might be tempted to count the turns of the flywheel but you'd be wrong. (Most machines have an integer value for index (12, 24, 36, etc). There are some machines with decimal value for index constant (12.5 for instance). -Ed.)
Step 5: Verify the INDEX CONSTANT: Mount and dial in the gear blank you have selected. Mount two gears, with EXACTLY, the same number of teeth on the A-shaft and the D-shaft. With reference to the WARNING in Step-2, look closely at picture below.
Index Reversing Gearshaft (closeup)
You can see that the retaining nut and keeper on the reversing idler are missing. If the nut and keeper were in place, they would lock the gear train, with disastrous results. Again, ALWAYS rotate the gear train by hand before applying power.
(What's it for? if you want to use a LH hob cutter, that's what it's for... Ed.)
Decide how you are going to ‘coat’ gear blank. First picture shows tape as an option. Ordinary blackboard chalk works just fine. The pinion blank used for these illustrations has been painted. The point of the scriber is blunt so the scribe marks will show up in the pictures, but you may want a sharper scribe. Bring the hob table near the top of your gear blank and bring the blank near the scriber. If possible, before starting the hobber set the RPM to the lowest speed. It may not be possible to adjust the RPM of the hob unless the hobber is running. Now, start the hobber. Do not be surprised if the work table turns rapidly. Reduce the hob RPM to its lowest value. Very slowly advance the gear blank into the scriber. The intention is to lightly scribe the coating on the gear blank without removing metal.
on Hob Shaft
Scribe is Springy
loop in Foreground
Once you have a set of scribe marks, count them. The number should match the result in Step-4. Again, this is your INDEX Constant.
Scribe marks visible on coating.
Marks should be parallel to work axis
(So far the INDEX ratio used is 1:1. This may be a problem on your machine especially if the workpiece spins too fast due to "gearing up". Excessive gearing up puts a large torque load on the index train. This should be a cause for concern if manually rotating the index by hand is excessively difficult. If this is the case, then mount an INDEX gear set that represents 1:2 (or 1:5) ratio. This will result in twice the scribe marks (or 5 times the scribe marks) respectively. For example, we might mount A/D as 24/48 (a 1:2 ratio). Instead of24 scribe marks we would expect to count 48 marks to confirm a 24 INDEX. -Ed.)
Step 6: Practice ‘Scribing’ Spur Gears: If you are lucky, your INDEX CONSTANT will be 18 or more. This allows you to put a gear on the A-shaft with the same number of teeth as the index constant and automatically cut number of teeth equal to change gear teeth on the D-shaft.
As a more general example we can mount a 24-T change gear on the A-shaft and put the change gear on the D-shaft that is equal in teeth to whatever gear we want to scribe (with suitable idler that meshes with both). For example, we put a 77-T change gear on D-shaft to get the following picture:
Two things should be obvious now: (1) The index gear set must be EXACTLY equal to:
77 Scribe marks overlaying original 24.
Marks should be parallel to work axis
as table feed progresses and differential
is locked out.
(2) the index decimal ratio is whatever it is and is not important as long as (1) is true.
INDEX CONSTANT INDEX RATIO = A/B x C/D = HOB CUTTER STARTS X TEETH TO CUT
Experiment with ‘Scribing’ different gears, using both the direct method (Index Constant Gear on A-shaft) and the A/B X C/D gear train. (the scribe is representative of Fly Cutting, a slow process of generating a gear when a hob cutter is not available but a hobbing machine with axial cutter infeed is. -Ed.) This is a good time to familiarize yourself with all of the controls on your hobber.
Step 7: First Approximation of the LEAD: Set up your index section for a 30-T to 40-T gear Disengage the differential lockout by removing the sleeve from the 'D' shaft. Place a set of gears on the differential section with A-shaft equal to about 40-T and D-shaft equal to about 50-T. Mesh A-shaft and D-shaft with an idler. Set the hob table to 0-degrees as shown below:
With a freshly coated gear blank (paint, chalk, tape) turn on your hobber and bring the scriber near the blank; slowly advance the work table until the scriber cuts the coating. Now, engage the hob table feed. Set the feed rate to about 75% of the maximum feed rate of your hobber. You will see a pattern of scribe marks start to form as shown in the next picture. The starting/ending points of the scribed lines will form a helix angle. Note: if the helix angle is too large, say more than 30-degrees, or too small, say less than 20-degrees. Adjust the gear train ratio on the differential section until you get a helix angle between 20 and 30 degrees. Keep adjusting the angle of the hob table until it closely matches the helix angle being scribed. Once matched you will see sharp scribe lines at each tooth location. Now, with a freshly coated blank, scribe your blank for a face width of at least 6-inches, 9-inches would be better,
Wrap a piece of tracing paper tightly around the blank.
Orient (by means of baselines on the tracing paper) the tracing paper parallel and perpendicular to the axis of the gear blank. Mark the top and bottom of a single helix scribe line,
Paper is being oriented with
edges lined up with edges of
Be sure to mark the top and bottom of the same helix line. It is easy to be off by one line. Remove the tracing paper. Knowing the diameter of the blank and with trigonometry from the tracing paper, you can compute the LEAD of the gear.
Top and Bottom of a helical
mark are marked on tracing
Step 8: Determine if you have a TYPE A or TYPE B hobber: Change the index section to ‘cut’ (scribe) a gear with, if possible exactly double the number of teeth as in Step 8. If you used 30-T for Step 8, change your index section to scribe 60-T. Now, scribe a new gear right over the top of the gear you scribed in Step 7. If you have a TYPE ‘A’ hobber the helix lines will remain parallel over the full face width of the gear blank,
If you have a TYPE ‘B’ hobber the helix lines will NOT be parallel and will likely cross as shown below:
If Lead angle for
30-T and 60-T
is same then
Machine is Type A.
The Seiwa is a Type A machine. The picture above is a simulation as if it were a Type B.
If Lead angle for
30-T and 60-T
is different then
Machine is Type B.
Step 9: First Approximation of the DIFFERENTIAL CONSTANT: By now you have: (1) the LEAD of your gear blank (from Step 7); and, (2) the TYPE of your hobber, A or B (from Step 8). With this information, refer to FAQ Topics Constants and formulas or Basic differential formula to determine the correct lead formula (A or B) and to compare your DIFFERENTIAL CONSTANT to other known constants.
Step 10: Experiment ‘Scribing’ Different Helical Gears: Using your first approximation of the DIFFERENTIAL CONSTANT, experiment scribing helical gears. Try scribing a herringbone gear on the same face:
(How did he do that? you ask.
Most likely he reversed the rotation
of DIFFERENTIAL without changing
the ratio or anything else...
but there is another way, too! -Ed.)
Step 11: Accurate Measurement of the DIFFERENTIAL CONSTANT: Note: By now you should be familiar with your hobber and the following instructions are based on that assumption. Also, the workpiece, hob cutter, and scribe may be removed. You are going to make two exacting measurements: (1) the vertical travel of the hob table and (2) the angular displacement (rotation) of the work table Set the Index Section for about 40-T Set the Differential Section for about 30-degrees. Here are the gears used:
A = 68 / B = 58 X C = 60 / D = 79
Diff. Change Gear Set = 68/58 X 60/79 = 0.8904408555 (differential ratio)
Bring the hob table down within 25% of its maximum downward travel Now, with the differential engaged and the hob table feed disengaged, use the rapid transverse of the hob table and raise the hob table about 1-inch. Measure the location of the hob table from some fixed reference point:
set up and scribe a precision line on the circumference of the hob table. The following pictures show the steps used:
Now, using the rapid transverse of the hob table raise the hob table to produce approximately 45-degrees rotation of the work table. Note: As the work table approaches 45-degrees rotation, kill the power switch to the hobber. The objective is to have the entire gear train ‘coast’ to a stop under load (the hob table is going up). This will maintain the backlash ‘set’ on all of the gears throughout the entire gear train of the hobber. If the hob table travels down, friction would stop it but inertia would cause the ‘set’ of the backlash to shift and give an erroneous reading. With everything at rest, scribe a second line on the circumference of the hob table Next, measure the vertical travel of the hob table. With the calipers, measure the chord between the two scribe marks. Use optical magnification to help make an exact (+/- 0.001") measurement of the chordal length. Now with the chord, the work table diameter and the hob table travel, you can accurately compute the lead of the current set up. Repeat the DIFFERENTIAL CONSTANT computation and you will have an accurate value for the DIFFERENTIAL CONSTANT. Of course, you will want to repeat this procedure several times to statistically improve the accuracy of your DIFFERENTIAL CONSTANT. Averaging a few trials, the differential constant works out to be 0.313270583 (the number of decimal places might be due to the calculator display rather than the statistical accuracy -Ed). Note: if you can beg, borrow or steal a digital scale to measure the hob table travel, life will be a lot easier.
fix ground parallel to machine
base close to work table
Closeup showing end
ground cutting tool used
to scribe work table
Cutting tool in position
to scribe work table
on work table
Step 12: Alternate Method for Measuring the DIFFERENTIAL CONSTANT: set up the hobber as in Step 11 but mount a precision rotary table or dividing head on the work table. With the same zeroing procedure for the hob table, mount a dial indicator for that after the hob table has been elevated. You can rotate the stopper on the rotary table back to the same location as determined by the reading of the dial indicator. Now, you can read the degrees rotated by the work table vs. the distance traveled by the hob table and make the computations as in Step 11. Note: For Steps 11 and 12, it is critical you ‘think’ through all of the backlash issues to ensure an accurate result. The hob table must be going up as mentioned above. If you use the scribe-on-the-circumference method, be sure and block out your scribe lines before each trial.
(Anyone approaching hobbing for the first time, or a new and unknown machine, can certainly appreciate this excellent treatment of discovery. Also, it might be said that the process of discovery is a journey - not a destination. There are so many topics beyond the scope of this article (Using LH and RH cutters, climb versus conventional hobbing, using axial hob cutter travel, cutting very large prime gears, adapting to various hob machine designs (non-differential machines for instance), extending range of change gear sets with additional change gear pairs - these are certainly topics for future articles. Also, the reader is encouraged to read through hobbing topics in the FAQ page and to ask questions on the forum. - Ed.)
Time allowing, I will write more articles: Some machining tips for getting your gear blanks ready for the hobber and for after the hobber as well. A special article "Does you Gear Hobber need a Friction Brake". Something I discovered when making very large bull gears on the Seiwa. Regards, August Lehman Information and photos contained in this article have been generously provided by: August Lehman, Managing Director Lehman Associates Manilla, Phillipines Website and contact information
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