Model Engine Maker

Help! => Machines, Tools and Fixtures => Topic started by: gbritnell on August 11, 2018, 01:17:29 AM

Title: Helical gear cutting
Post by: gbritnell on August 11, 2018, 01:17:29 AM
First off let me give total credit for the design and development of the helical gear cutting fixture to Chuck Fellows.
When first published he was making gears for a hit and miss engine. As his drawings were published on HMEM and some of the readers on this forum said they had to log into that forum to see threads I will copy them to this thread. For those of you that visit both forums just do a search for: helical gear cutter by cfellows.

The fixture itself is fairly simple. It consists of a body, spindle, index plate and cam follower. The body is bored to a nice fit for whatever size spindle you choose. Mine is .50 drill rod. I have cut some fairly large helical gears for model work with no chatter with this size spindle. On the holding end is a reamed hole with set screw for holding arbors to mount the gear blanks. (mine is .375) On the working end of the spindle is a boss, 1.00 diameter with a knurled flange with holes that correspond with the rows of holes in the index plate. This shouldered boss is free to rotate on the spindle. Attached to the spindle and is the index plate. This corresponds with the number of teeth you will be cutting. A pin locates the index plate to the shouldered boss. Onto the shouldered boss you mount a cylinder created by cutting a triangle of thin metal (I make mine from brass) (easier to form) This cylinder represents the lead of the helix of the gear. The cylinder runs against the cam follower to give the helical advance when cutting the gear teeth.
Here are some pictures of my fixture and some of the gears I made for my Fordillac flathead V-8 engine.
First the fixture and cutting a gear.

Title: Re: Helical gear cutting
Post by: gbritnell on August 11, 2018, 01:18:55 AM
This photo is of the cutter. It's made from W-1 drill rod and has the required profile for cutting the pitch and helix of the desired gear.
Title: Re: Helical gear cutting
Post by: gbritnell on August 11, 2018, 01:22:25 AM
Here's a picture of the brass template. I cut it on the bandsaw then milled it to the proper angle. I'm checking it with my sine plate and indicator. The second photo shows the annealed template being rolled around a rod to form the cylindrical shape.
Title: Re: Helical gear cutting
Post by: gbritnell on August 11, 2018, 01:26:48 AM
With helical gears if you cut two of the same gears they will operate at right angles to each other. If you need gears that have parallel axis then you need to cut a left and a right gear which requires two templates.
For my flathead engine I needed 2 parallel gears and and 4 right angle gears.
Title: Re: Helical gear cutting
Post by: gbritnell on August 11, 2018, 01:29:45 AM
Here is a video clip of cutting a helical gear for my inline 300 six engine.
Title: Re: Helical gear cutting
Post by: gbritnell on August 11, 2018, 01:34:42 AM
And finally here are the PDF's that Chuck Fellows created to make the fixture and calculate the template angles.
Another fellow modeler, Marv Klotz, developed a program to do the major calculations for creating the templates. If you Google Marv Klotz you will find numerous programs that he developed to aid fellow builders
I would be remiss if I didn't mention Mr. Don Darbonne. He also came up with a spreadsheet for both spur and helical calculations.
Title: Re: Helical gear cutting
Post by: steam guy willy on August 11, 2018, 02:06:29 AM
That is an impressive and basically quite simple and easy jig to make and use, However there is a lot of careful and accurate measuring and setting up to do. when you look at some gear cutting machines they look very complex and using lots of parts !! this tool however has just one moving part !! so simplicity is the answer. great stuff  ,I really am impressed.
Title: Re: Helical gear cutting
Post by: nj111 on August 11, 2018, 08:00:53 AM
That's innovative and fascinating. I shall be making one of those. Many thanks for sharing. Nick
Title: Re: Helical gear cutting
Post by: jadge on August 11, 2018, 08:52:54 AM
That's a neat way of cutting small helical gears, and one system can deal with a wide range of helix angles.  :ThumbsUp:  Some of the gears look a bit odd though; I would have expected the root and crest widths to be pretty much the same.

Title: Re: Helical gear cutting
Post by: PJPickard on August 11, 2018, 12:53:14 PM
Thanks for posting this!
Title: Re: Helical gear cutting
Post by: b.lindsey on August 11, 2018, 02:06:25 PM
George, thanks for collecting all of this information in one thread. I am making the topic "sticky" for the time being so it won't fall down the list until everyone that has an interest in the subject has a chance to read it

Title: Re: Helical gear cutting
Post by: gbritnell on August 11, 2018, 03:10:46 PM
The reason the crests look so wide is because of the angle of the helix. You'll have to take my word that the gears mesh nicely.
Title: Re: Helical gear cutting
Post by: nj111 on August 11, 2018, 03:43:51 PM
Andrew, I was typing this and George beat me to it with a very concise answer! -
I'm not an expert  but I do have a couple of old Sunderland gear planers in the shop at home and sometimes I need to cut a helical gear on them.
I was thinking maybe the answer to your question is because the helix angle has an effect on the pitch required to generate the tooth.  I'll try to explain as best I can:
If you view a helical tooth looking square onto on either side face of the gear rather than considering it's cross section you will see a wider tooth than normal for any given helix angle. The tooth is the same height but it's wider / fatter to look at.  The greater the helix angle, the wider / more distorted the tooth appears to be - from that viewpoint.
On a gear planer there are two gearboxes to set up to produce the part , one for the number of teeth and the other for the tooth size i.e. DP or Module.
When you set up the pitch for a helical gear  you have to allow for this extra width. In other words you have to set the machine as if you were cutting a larger tooth pitch - but you are not.
For example I had to make a MOD 2.5 gear with what appears to be the most odd helix angle of 36.869 degrees. At first I thought - why ever did they choose such an odd helix angle? - but the answer was actually straightforward because cos 36.89 is a dead 0.8  Following the calculations through it meant that to cut those MOD 2.5 teeth at that particular helix angle I had to use a MOD 2.5 cutter but set the machine to index as if it were a MOD 3.125 tooth.  (quite simply 3.125 x 0.8 = 2.5) All nice and easy round numbers to work with. The gear came out perfectly - but it's kind of hard to get your head around.
I assume that is the reason some odd looking helix angles are encountered , as actually during the calculations they will in fact relate to dead sizes of pitch available on traditional gear cutting machinery, and perhaps this is also a factor in what you observe above. - I'm not 100% sure but it could be a factor? Nick
Title: Re: Helical gear cutting
Post by: tangler on August 11, 2018, 05:56:55 PM
Chuck's fixture is a brilliantly simple way to set up the long leads necessary for cutting helical gears.  There is a refinement for achieving a more accurate tooth form for the helical gear.  Most treatises on cutting helical gears with Brown and Sharpe type milling cutters say that the selection of the tooth number for the cutter should be modified by dividing the tooth number required by the cube of the cosine of the helix angle - so for a 20 tooth 32DP 45 degree helical gear you would use a cutter meant for 57 teeth.  Elsewhere, Andrew (jadge) has gone through the math(s) to show why this works.  We model engine makers mostly use helical gears for cam shaft timing which is a pretty undemanding low power application so extreme accuracy of the tooth form is not required.


Title: Re: Helical gear cutting
Post by: mcostello on August 11, 2018, 08:38:47 PM
Title: Re: Helical gear cutting
Post by: jadge on January 22, 2019, 11:23:29 AM
I understand that the tooth form is rotated by the helix angle. But I would have expected the gap form to undergo exactly the same transformation. For a spur gear the width of the tooth and the space are the same, at the PCD. I don't understand why this wouldn't be true for a helical gear as both shapes undergo the same transformation. Of course the widths are only equal at the PCD. I've looked at the helical gears I made, and it's difficult to tell. But if the space is different then an identical gear, of the opposite hand, but with the same tooth width would have a lot of backlash, or not fit at all. I'm going to have to put this on the list of the things to work through.  :)

Title: Re: Helical gear cutting
Post by: kvom on January 22, 2019, 01:30:31 PM
If one makes a matching gear by cutting it in two, does the changed tooth form cancel out?
Title: Re: Helical gear cutting
Post by: gbritnell on January 22, 2019, 02:13:20 PM
Hi Andrew,
 I'm certainly no gear expert, I just have read and studied making gears for quite awhile so I'll try to explain the helical gear conundrum as best I can.
First of all you are correct in stating that the tooth and space of a spur gear are equal at the pitch circle. So let's take that a little further. Gear teeth are formed by the involute curve, that is no matter what number of teeth are on the mating pair they have a smooth rolling transition as they mesh, from rack to 13 teeth. There are 8 cutters per pitch to create the proper involute curve for each tooth count. Now I have never tried it but I'm assuming that from your statement of equal tooth width and spacing if you cut 2 gears with the same tooth count but used different numbered cutters on them the width and space would be the same at the PD. The difference is they wouldn't roll together smoothly because the mating tooth profiles is incorrect. I hope that makes sense.
 Now onto helical gears. Here again going back to what you stated, tooth width/spacing being the same, helical gears are the same as spur gears in that respect. Now I will try and explain what is happening. Let's say that you set up to cut a helical path on a shaft but for the sake of explanation you use a woodruff key cutter to cut the groove. Given the diameter of the cutting head and the helical path it is traveling the sides of the groove will be splayed out. (wider at the top) The only way to cut an exact profile would be to use an end mill that was ground to the tooth profile and the cutting action was directly at the point of contact. This is impractical so involute cutters are used.
Now we know that each involute cutter in the set has a different profile and that cutting a helical path on a shaft with a large diameter cutter will produce a splayed slot we choose a cutter from the set that compensates for the splayed cut. There is a formula for finding NTCS (number of teeth for cutter selection) which is NTCS=NT x cube of the secant of the helix angle. There are charts that simplify that math.
The bottom line is the tooth and spacing for a given pitch are equal at the PD. it's just that you have to use the proper cutter to get the correct tooth profile for a given tooth count.
 I have two free books that I downloaded years ago, one is a U.S. Navy machinists book the other is A Treatise on Milling by the Cincinnati machine company. Each of them cover gear cutting to the point that your head starts swimming but the basics are there and for the hobbyists it helps to explain how to cut gears more effectively.
Title: Re: Helical gear cutting
Post by: Don1966 on January 22, 2019, 10:21:41 PM
George always refeshing to see some of your reviews on gears. I have attached the spread sheet for those who dont have it for helix gears. It calculates Chucks temple for you and the cutter needed to cut your gear. I have a compliation of gear calculation sheet if anyone is interested just PM me and I will email it to you. The forum wont let me upload it because it is to large a file.

Regardsd Don