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Thread: Cutting Helical metric gears
08-24-2011, 01:24 AM #1
Cutting Helical metric gears
I'm re posting this plea for help.
I'm still having a great deal of trouble in setting up the milling machine to cut the helical gears
I've come to realize that I had lost some of my mental comprehension to do calculations that were out line in the book "Theses On Milling "
by Cincinnati that was recommended to me by Johnoder.
The information I need to verified is the outside diameter of the gear blanks, the LEAD angle ( to set the transposition gears) and the milling machine table angle.
The metric gears are 2.5 MODULE
21 pinion tooth count
67 large gear tooth count
The large gear helix is 20 degrees for sure..
The man who needs the gears is a small family owned business.
The machine has been down for over 3 weeks.
Parts are not available here in the United States.
I told the customer I was having trouble setting the lead on my universal mill..
and I was seeking help to solve the problem..
He couldn't wait on me any longer and was desperate.
The customer found a machine shop in Tampa some 45 miles away who said that he could cut the gear (pinon)
but the machine shop didn't have the gear cutter or the proper material..
Trying to help the customer out ..I provided him with the gear cutter and the 4150 steel.
The gear was machined and he returned it to me for the heat treating .
He said that the machinist had cut it on his Bridgeport..
The pinion was cut straight on a 20 degree angle with no helix.!
Yesterday morning the customer and that machinist who cut the gear, tried to assemble the gear box and ran into trouble..as I suspected..
after 6 hour struggling ..they force the assembly together and "popped" the bearings apart.
This mixer has been down going on 4 weeks .
This man is going to loose his family business and his home ..
He's 60 years old..can you imagine starting over again at that age..
In these hard times I got to try to help this man out..
Johnoder had given me advice and I'm very much appreciated.
It's not my intention to abuse this site or badger any member
If I did this ..I apologize .
For me ..not being not able to concentrate a little frightening..I don't want my bride to know.
Again ,the information I need to verified is the outside diameter of the gear blanks, the LEAD angle ( to set the transposition gears) and the milling machine table angle.
I be willing to compensate anyone for their efforts if willing to help in this matter...
Please let me know..
08-24-2011, 01:52 AM #2
What is left of the old gears? Are there simply a few broken teeth on either one, or are there some good teeth remaining?
Reason I ask, is that if you have a single good tooth to indicate along, you can make a trial setup on the mill, and indicate the tooth face with the trial setup. You'd be looking for zero change in the indicator reading as you crank the table along (in a trial cut mode), to demonstrate that you had nailed the correct ratio.
Admittedly, it takes a lot of work to set up each gear train for trials, so its best to estimate as close as you can to save time. I've done this a time or two, and it takes hours to get ready to do the cutting, and a compartively few minutes to do the cutting.
The OD of a helical gear cannot be determined without knowing the exact helix angle, so tooth numbers alone is not sufficient to get the correct angle. You say the large gear is 20 degrees for sure, but how did you measure that? I'd say the margin for error with eyeballing the tangent is probably on the order of 1/2 to 1 degree or more.
Lay the two gears on the table, and try to estimate the center distance. Remember that there will be some backlash, so if the gears are tight together, that is probably a little too short of a center distance. In helical gearing, the center distance could absolutely be any number under the sun, there is no 'standard' except for the mathematics. That is not to say that the manufacturer of the gearbox would not have used some logical center distance, probably some round metric number, but that is just a guess.
Once you have that, you can pretend that you've got one large gear with 88 teeth, and the PD is twice the center distance. Knowing those numbers, then you can work out the helix angle. Remember that a straight cut spur gear has the maximum number of teeth (for a given module) that will fit in that pitch circle. Helical gears cut on that same pitch circle, will have one, or two fewer teeth, because the effect of twisting the tooth by the helix angle increases the circular pitch width.
You are probably familiar with the milling machine leads tables in Machinery's handbook. You'll need those tables, of course. And you need to know the lead of your milling machine table screw, because the tables are set up for a 4 tpi screw so you will need to apply a correction factor if your table screw has 5 tpi, etc.
Anyways, get back to us with the best estimate of the center distance between the gears. Take a caliper measurement between the adjacent walls of the bores, and give us the bore sizes as well. We should be able to narrow down the lead calculation.
Once you have the helix angle determined, then the table lead for each gear can be figured out. The actual setting of the universal table on the mill is not that critical, once you've got the helix angle calculated, just set it to that, and good enough. It is the spiral milling setup that creates the real helix angle of the gear, and the table swivel angle merely provides cutter clearance.
08-24-2011, 02:01 AM #3
Surely there must be companies in the US that specialise in making one off gears in a hurry and have a great deal of competence to do so. The only trick is to find them.
I know exactly where to go in Europe. They even quote the surcharge for rush jobs on line.
08-24-2011, 02:16 AM #4
Here is a gear co. to ask for help.
Rush Gears inc. -- gears delrin gears plastic gears bevel gears spiral gears worm worm gears spur gears helical gears phenolic gears stainless steel gears cast iron gears steel gears miter gears metric gears hardened gears heat treat reverse engineer
Been in your shoes, wish I had called them.
08-24-2011, 02:47 AM #5
Get the welder, the lathe and another gear box.
Or the gears from another gear box, and see if it is possible to juggle the centerline distances to match the donor gears. ?
08-24-2011, 03:20 AM #6
And whoever you might contact is going to need the info that I outlined. It is one thing to design and build a helical gear set (comparatively easy), and another thing to reverse engineer it under less than ideal conditions. Some gear cases are a bitch to measure centerline distances without the help of a height gauge and a surface plate. And dough mixers typically have sort of an integral gearbox which makes it all that much more difficult to take good measurements.
There might be machine shops around that could do the job, but if they charge the price of the mixer, then they don't get the job. This is where the OP, as a small shop owner can donate his time (if he so wishes) to figure the job out. Not saying I would, but if you want to, then you want to
08-24-2011, 03:48 AM #7
Like I emailed and PMed Robbie, the center distance dimension comes first.
Would be good to put that here just as soon as you have a good accurate decimal number.
08-24-2011, 04:49 AM #8
My go to guy when I had any problems on gears we Bruce Siler at Lamount Gear in Pa.now named RUSH Gears
He sold his part in the business and went to the Government building .50 Cal rifles for the Secret Service in the 80's then quit.
I lost contact with him when he went out west to the big sky country somewhere.
I can not find the center distance inside the mixer ..
The gear box top where the pinion gear is mounted on the drive motor has no reference point for me to start with..
I know that the center distance is critical to made a accurate helical gear set.
There is no teeth whatsoever left on the pinion ..the large gear is about 30% worn on the gear face
The last resort was to make a pinion close enough to mesh with the worn larger gear to get back running while Rush Gear is making a new set.
On the flank of the large gear where it was not worn bad .
When was on the mill between centers I was able to indicate by trial and error was able to determine the the helix was 20 degrees exact by moving the table back and fore with a parallel wedge between the gear teeth according to the table setting on this universal mill before I connecting the gear train
Since the intention was to make both gears LH/RH I didn't think much if it was off a little.
08-24-2011, 04:56 AM #9
I just read the PM
I have not received any Email from anyone as of yet.
The correct email is <firstname.lastname@example.org>
08-24-2011, 06:11 AM #10
A few photos of the gearbox might help. There may be a way you can lay out the location of the top gear via intersecting circles based on dowel pin centers, or lay out the geometry of some locating boss. There must be some way in which the gear centers are defined.
These gears are on parallel shafts, right, not crossed shafts?
08-24-2011, 06:13 AM #11
Robbie without a good solid centers dimension you will be just shooting in the dark.
If you want to go in that direction, that is fine, but the info below, based on the little info given, is worth exactly what you paid for it.
A 20 degree helix angle 67 tooth helical that is 2.5 Module will have the following:
It will be 183.574mm or 7.2273 O.D
It will have a pitch circle circumference of 560mm or 22.0472". Dividing this by the tangent of 20 degrees gives the length of lead as 60.5742"
A 20 degree helix angle 21 tooth helical that is 2.5 Module will have the following:
It will be 61.192mm or 2.4091" O.D
It will have a pitch circle circumference of 175.522mm or 6.9103". Dividing this by the tangent of 20 degrees gives a lead length 18.9859"
08-24-2011, 06:18 AM #12
As far as setting up your mill for the lead- remember that the helix angle will be the same for both gears, but the lead will be different if the two gears have different numbers of teeth. If you can get an accurate center distance, I can calculate the parameters you need to cut the gears. I might even be convinced to cut the gears for you if it comes down to that.
08-24-2011, 11:58 AM #13
ambitious mill project
re: I'm re posting this plea for help.
Contact me if you're serious about getting this done correctly, vs a rush job. Use email at link to our site.
08-24-2011, 12:49 PM #14
Taking you advice I went out to the batch mixer with a large pair of veneer caliper to find out the diameter of the bore where the drive motor is bolted thru the gear box side plate and measure the distance between the motor and the bearing bore where the large gear is mounted
The owner and the machinist were in the process of putting back together the mixer and didn't want to take apart what they had done
They refuse to listen to good advice and not let me take the measurements.
The owner had called me and thanked me for the efforts
He told me that the mixer ran but was noisy..
The other machinist said no telling how long it could last a few hours or a few weeks.
When it goes down again the opportunity to retrieve the center to center dimensions that are needed to make a proper set of gears.
Follow the formula in the book and I archive similar results.
What kept tripped me up was that pitch circle circumference calculations .
It just didn't look right to me .. in being bigger then the outside diameter of the gear blank..
I still don't understand it...
I'm going to try out what you helped me with and setup the mill and try to make a set of gears.
I know that what the other guy had done won't last.
I really appreciate your efforts and patience with me.
Dan from Oakland and 10 fingers
Thank you for your offer..
Practical Machinist Forum has a lot of talented and generous individuals.
I Thank You All.
08-24-2011, 01:06 PM #15What kept tripped me up was that pitch circle circumference calculations .
It just didn't look right to me .. in being bigger then the outside diameter of the gear blank..
I still don't understand it...
08-24-2011, 01:39 PM #16
08-25-2011, 07:12 AM #17
I have made a few of these and found that that despite all your efforts with the maths the practical problems on the miller make the job a nightmare, The backlash in the gear trains and the physical problems of clearance make a mess of the low tooth count gears.
I ended up buying a hobber to make helicals do so on a regular basis however my machines are just not big enought for your job.
My advice is to go down the route of geting a professional to do the job, just think of the hours you have wasted fiddling with it up to now.