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Supporting => Tooling & Machines => Topic started by: donrecardo on October 23, 2014, 04:32:16 PM

Title: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 23, 2014, 04:32:16 PM
Hi
I have been chatting to Chuck Felows for the last week or so and he has been kind enough to answer
all my questions about his amazing Helical Gear cutting fixture

Once I had it fairly clear in my head as to how it works  I set about building it and he said I should let you guys know how it went as
he likes some of the small differences I made

As mine will be set up slightly different to Chucks ( I mount mine on a rotatable vertical slide on the lathe) and because I didnt have exactly the same sizes of material
I had to make mine slightly different though basically the same as Chucks , I will try to attach a picture at the end of this post

The only part I had any worries over was bending the template around the hub, 3mm thick sounds hard work to bend
I felt it might be a better idea to use a piece of tubing which is already the right shape to fit on the template hub with no bending involved
the tubing will of course need the correct slope machined on it .
As the template hub is 0.375" thick the tubes minimum height must be the same
as I never intend to cut gears more than an inch thick  the tube never needs be more than 0.375 + 1.00 "
I set about making a spreadsheet that allows you to input 3 numbers ( the ones in red ) and these set the

DP of the cutter
The number of gear teeth
The helix angle

The spread sheet works out the height to machine the tube to at each of its 360 degrees of rotation
It also tells you the depth to set the cut for the gear tooth and the size of the gear blank

You dont need to machine all 360 degrees  , only enough to give you 1.00" above the minumum ( 0.375") even less if you only cut thin gears
For example  on a 24 dp,  20 tooth gear,  45 degree helix angle  the tube heights will go from 0.375  ( min height )
and at 98 degrees around the tube its height will be 1.383" and as this is enough to cut a 1.0" thick gear you can level off the rest of the tube at this height
but  also leave a landing area at the start at 0.375 to allow the width of the bearing to sit in

I found that I didnt need to machine in 1 degree steps , every two degrees made a smooth enough slope and halved the time to machine it .

So you put your tube in the rotary table horizontally the the bed of the miller and just wind it round 2 derees at a time whilst bringing the X axis up to cut the tube to the desired heights   

I will now attempt to add the picture and the spread sheet  but bear with me if I fail, I havent attached a picture or file before
and of course , all credit goes to Chuck without whose idea for the fixture in the first place wouldnt have made this possible

Don

Title: Re: Chucks Helical Gear Cutting fixture
Post by: cfellows on October 23, 2014, 05:38:55 PM
Good Job, Don.  The tubing template looks a lot cleaner.  I also like that you have extended the template farther out from the supporting mandrel so there is no interference with the guide bearing.  Mounting the guide bearing bar on the front of the fixture seems to improve the overall looks as well.

Chuck
Title: Re: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 23, 2014, 05:47:55 PM
Thanks for the kind words Chuck, and of course for designing the fixture in the frst place
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Don1966 on October 23, 2014, 07:14:35 PM
Hi Don, I have make and used Chucks fixture and it works great. I do like the way you mounted it to the vertical slide. I may just remake the block to fit mine. Right now it mounts to the vise on the vertical slide.
Your data sheet is in a format I am not firmular with. Do you have it in Microsoft Excel?

Don
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Stuart on October 23, 2014, 07:25:27 PM
Don

any .xls format will do fine

we all do not use a windoze pc a small number of us use mac's and therfor do not have the required program

but our spread app is numbers which can import .xls

Stuart
Title: Re: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 23, 2014, 07:28:11 PM
Here it is in Excel I forgot to change it over as I normaly use planmaker

Don
Title: Re: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 23, 2014, 07:47:06 PM
Hi Don, I have make and used Chucks fixture and it works great. I do like the way you mounted it to the vertical slide. I may just remake the block to fit mine. Right now it mounts to the vise on the vertical slide.
Your data sheet is in a format I am not firmular with. Do you have it in Microsoft Excel?

Don

I just made a long T nut for it so it fits the slide
Title: Re: Chucks Helical Gear Cutting fixture
Post by: kuhncw on October 23, 2014, 10:42:27 PM
Nicely done, Don. 

Why does the bar that carries the roller extend beyond the bearing or mounting block?

Regards,

Chuck
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Stuart on October 24, 2014, 08:07:05 AM
Don

Thank you for posting the new file

Stuart
Title: Re: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 24, 2014, 09:23:22 AM
Nicely done, Don. 

Why does the bar that carries the roller extend beyond the bearing or mounting block?

Regards,

Chuck

It needs to extend by at least the length of the template to allow the bearing  to move forwards to the bottom of the template

I put a bearing at each end just as Chuck Fellows did so if required you can fit the spindle from the other end of the block to make it
fit your setup better
Title: Re: Chucks Helical Gear Cutting fixture
Post by: kuhncw on October 24, 2014, 04:11:45 PM
Thanks, Don.

I didn't word my question well. It was the end toward the chuck I was wondering about and you've cleared it up.

Chuck
Title: Re: Chucks Helical Gear Cutting fixture
Post by: tinman13kup on October 25, 2014, 04:50:48 PM
I found this thread doing an internet search for a helical cutting jig, and looking at it gave made the little wheels in my head start spinning.

  Instead of the ramp. why couldn't you use a regular old twist drillbit? What I'm envisioning is the shank end of the bit attached into the workholder, and then bronze fingers coming through the body of the fixture into the opposing flutes of the bit.
  The other end of the bit (cutting end, pointed back to the handle) would need a bushing and the handle attached to that. I was thinking a hex-headed bushing, which could slide inside of a regular ol deepwell socket mounted to the handle body. The handle body would be a cylinder with dial increments around the outside, and have a stepdown  shoulder that fits into the tool body. A few setscrews into a recessed groove in the stepdown would keep it fixed to the body, but able to turn.

What I'm picturing in this is the ability to minimize any backlash, and be able to change the pitch by changing out the drillbits. I've just been mulling this over in my head the last day while putting a new clutch in my truck. Is this idea feasible or am I all washed up?
Title: Re: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 25, 2014, 06:54:59 PM
The idea of the sloping ramp is because the angle of slope is critical for the helix angle to come out right.

I doubt you would be able to find a twist drill with just the right slope
Title: Re: Chucks Helical Gear Cutting fixture
Post by: tinman13kup on October 25, 2014, 11:07:29 PM
Don, perhaps that is so when attempting to match a gear to an existing gear. When making the whole set, I would think they would mesh just fine. Still, it might be worth checking out since there are all kinds of fractional bits out there in right and left twists, and don't forget the metrics. If I'm thinking right, a bit of equal diameter to the gear should yield a 45 degree cut. Larger bits would lower the angle, smaller would increase the twist
Title: Re: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 26, 2014, 12:12:50 PM
Yes , I take your point about if you are not trying to match to an existing gear then the helix angle might not be so important but

If you are hoping to run the two gears parallel to each other you need to make one right hand and one left hand helix so
you would need both left and right hand drills , are you able to get those in the exact fractional size  both L/Hand R/H to give
the required slope ?

If you want to run the gears at 90 degrees to each other  or some other precise angle then the helix angle is again going to be very critical

As the twist drills are going to be of different diameters and their flutes different widths  then the holder for the drill and what ever runs in the flute
will also need to be different for each size . That seems like a lot of parts to have to make , where as in the system I use the only thing that has to change is the
template , which is a bit of tube milled to any precise slope I want using the data from my spread sheet

But give your idea a try and let us know how it goes , who knows you may be on to a winner 

Don
Title: Re: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 26, 2014, 12:20:41 PM
. If I'm thinking right, a bit of equal diameter to the gear should yield a 45 degree cut.

That should be fun then if you want a 16DP gear with 80 teeth , can you get 5.125" drill bits in LH and RH 
and if so , wont they be rather expensive

Even a 32DP gear with only 30 teeth would be 1" in diameter and thats not a large gear , but is still an expensive drill bit
Title: Re: Chucks Helical Gear Cutting fixture
Post by: tangler on October 26, 2014, 02:13:43 PM
Nice idea but.....

The lead required to cut a specific gear depends on the Number of Teeth, the Diametrical Pitch and the Helix Angle.  Only a pair of 45 degree gears running at right angles with the same number of teeth (1:1) will need the same lead.  All other situations will need a different lead for each gear.  I've just a had a look at my set of of 1/16"-1/2"  x 1/64" drill set;  The lead for the 1/2" drill is 3" as is  31/64, 15/32, 29/64, and 7/16.  I didn't measure them all but the lead on 3/8" is 2.3" and 1/4" is 1.8".  There does not appear to be systematic progression of lead with drill diameter.  I imagine the manufacturer sets up his machine to give a certain lead and then flutes a series of drill blanks of similar diameters at this setting. For a pair of 45 degree crossed helical gears to give a 2:1 ratio as is often used for cam shaft drives, the lead of one gear will be precisely half of the other gear.  If you can fiddle the design of one gear to fit in with the lead available from a drill, it is seems unlikely that you will find a drill to give exactly half this lead.

Slow and fast spiral drills are available apart from the standard jobbers but I think one would need a very large selection from which to choose a drill with the correct lead.

Shame really :(

Rod
Title: Re: Chucks Helical Gear Cutting fixture
Post by: tinman13kup on October 26, 2014, 02:45:30 PM
Well dang, you had to go and mess up a perfectly crazy idea.  That lets me get back to thinking about a cold fusion reactor ;).
Title: Re: Chucks Helical Gear Cutting fixture
Post by: donrecardo on October 26, 2014, 07:23:51 PM
Never give up on thinking up new ways to do things ,
now and again they actually work quite well   :ThumbsUp:

Don
Title: Re: Chucks Helical Gear Cutting fixture - Question about the math
Post by: stefan-k on May 14, 2020, 11:54:06 AM
Hi all

I´m just building the fixture to make Helical gears and i have a question about the math in the excel sheet that Donrecardo createt.
If i look to the numbers of a Helical gear with lets say: 24 DP , 12 teeth and 60 degree angle, how can it be that the Pitch diameter is 1 inch in that case and the Blank diameter is only 0,583 inch.???
I don`t understand that. Afaik shouldn`t the Blank diameter always be a little bigger than the Pitch diameter?

If someone is still out there, please enlighten me.

Stefan

Title: Re: Chucks Helical Gear Cutting fixture
Post by: Jo on May 14, 2020, 12:04:13 PM
Welcome to the forum Stefan, Would you like to introduce yourself over here: https://www.modelenginemaker.com/index.php/board,3.0.html

I seem to recall seeing the calculations in Model Engineer in about 1946  :thinking: If no one else can find the answer for you I will have to see if I can find it  :noidea:

Jo
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 14, 2020, 01:24:42 PM
One of our members and contributors, Don1966, who had responded to this thread earlier on has made spread sheets for both helical and spur gears. I opened his program and put the numbers in that you stated and the O.D. came out as 1.083. The O.D. is very easy to calculate. 1\24=.04166 x 2= .0833. Add this to the P.D. of 1.00 and you get 1.083. This is the same calculation for spur gears.
I don't know if Don's spread sheet is in the files section or not but I used it in making all my helical gears and the numbers were fine.
gbritnell
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 14, 2020, 01:31:36 PM
I just did a search in the plans and drawings section and here's Don's spread sheet and fixture information.
http://www.modelenginemaker.com/index.php/topic,4871.msg92744.html#msg92744
gbritnell
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 14, 2020, 02:35:53 PM
I built Chuck's fixture but rather than setting it up in the lathe I used it on my milling machine. Here is a video of me using it to cut a helical gear.
gbritnell
https://www.youtube.com/watch?v=blaZ5tz0_6E
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Don1966 on May 14, 2020, 04:35:01 PM
The file is in plans a drawings but it’s not the correct one As George has pointed out. I had tried to upload the file with revisions and it was too large for the forum. So the one there is not the complete spread sheet. If anyone wants the spreadsheet PM me with your email address and I will gladly forward it to you. And your email address is safe with me it will not be shared.


Regards Don
Title: Re: Chucks Helical Gear Cutting fixture
Post by: stefan-k on May 14, 2020, 06:35:56 PM
Hello Jo , gbritnell and Don

Thanks Jo for welcoming me..!
I´m surprised about the short response time.  :) Thanks for helping me out of this Problem. I guess i found the mistake by comparing the Spreadsheet from Don to the one from donrecardo. There was a wrong formula in the cell that calculates the blank diameter. I corrected the formula after gbrittnell pointed me in the right direction and now both sheets produce the same numbers. I already finished building the fixture and the next challenge will be making the template. I plan to use my fixture in the mill, like gbrittnell in his videos, because i don`t have a height adjustable milling attachment for the lathe. I like to try both ways to make them, the one by cutting a piece of sheet metal and the other one by milling a pipe to the desired shape. I´m curious how the first pair of Helical gear will come out.  I will post some pictures. For now Thanks very much, i´ll keep you updatet.

Stefan
Title: Re: Chucks Helical Gear Cutting fixture
Post by: awake on May 14, 2020, 08:09:31 PM
Stefan, welcome to the forum. We love to see pictures - looking forward to seeing your fixture and your first gears!
Title: Re: Chucks Helical Gear Cutting fixture
Post by: crueby on May 14, 2020, 08:26:31 PM
I built Chuck's fixture but rather than setting it up in the lathe I used it on my milling machine. Here is a video of me using it to cut a helical gear.
gbritnell
[youtube1]https://youtu.be/blaZ5tz0_6E[/youtube1]
Fascinating to see it in action - thanks for sharing that video. For different helix angles, would the angled piece at the bottom of the fixture that the wheel rides on be changed out, or is there some other adjustment for that?
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Craig DeShong on May 14, 2020, 08:33:51 PM
Yes you need to make a new ‘angle piece’ for the fixture for about every gear you make.  You also need to make a different ‘indexing wheel’ for each gear with a different number of teeth.
Title: Re: Chucks Helical Gear Cutting fixture
Post by: crueby on May 14, 2020, 08:39:50 PM
Yes you need to make a new ‘angle piece’ for the fixture for about every gear you make.  You also need to make a different ‘indexing wheel’ for each gear with a different number of teeth.
Thanks!
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 15, 2020, 12:23:04 AM
Hi Chris,
Helical gears are basically spur gears with the teeth at an angle. That's an over simplification but here's why I said it.  A spur gear and a helical gear can have the same diameter pitch (DP) The Addendum and Dedendum are the same. This is based on the spur gear calculations.
For most pairs of helical gears the helix angle is 45 degrees. If both gears are the same hand (twist goes the same direction) then the gears will operate at 90 degrees to each other. If one is left and and one is right then they can be paired like spur gears. (parallel axis)
If one looks at Chucks information they will see how the helix applies to different diameters. A gear having a 45 degree helical angle and 1.00 pitch diameter will have a lead calculated on how far a point moves along the axis from -0- degrees twisting around the PD until that point comes back to the same -0- degrees, or 360 degrees.
Helical gears can be cut with involute gear cutters. The difference between spur and helical is that for a spur gear with a given number of teeth you would use a number X involute cutter (8 cutters to cut the complete range of teeth from 12 to rack) When cutting the same amount of teeth on a helical gear you wouldn't use the same cutter because with the cutter at an angle to the axis it cuts a wider swath through the gear blank so there needs to be compensation so that the gear tooth spacing and profile would match a spur gear.
When you want to change the ratio between 2 spur gears you just change the tooth count on each gear. With a helical gear just by changing the helix angle will change the tooth count for a given diametral pitch.
This is a simple explanation of the two types of gears but once you have gone through the calculations it all makes sense.
gbritnell
Title: Re: Chucks Helical Gear Cutting fixture
Post by: crueby on May 15, 2020, 12:49:32 AM
Very interesting, thanks. Makes me want to cut some to get first hand interaction to really get an understanding. That fixture of Chuck's is makeable and usable without CNC, right?
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Craig DeShong on May 15, 2020, 01:28:45 AM
Also, you need to wrap the ‘angle piece’ clockwise or counter clockwise around the fixture depending on whether you want a right hand or left hand gear.

I use mine on my vertical mill, not CNC,  you need to experiment with some throw-always to get the procedure down.  It’s easier than you might think to use, but it’s very repetitive in use and let your mind wander for a second or two and you will probably end up with a ruined gear.
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Don1966 on May 15, 2020, 01:44:20 AM
Love the explanation George and to all who uses the spreadsheet it calculates the cutter to use for you. The helix gear calculation sheet can be found here... http://www.modelenginemaker.com/index.php/topic,4871.0.html with it is Chuck Fellows helix fixture PDF. THE SHEET ALSO CALCULATES THE TEMPLATE FOR CHUCKS FIXTURE.

REGARDS Don
Title: Re: Chucks Helical Gear Cutting fixture
Post by: jadge on May 15, 2020, 09:10:22 AM
Cutting a helical gear with an involute cutter is inherently inaccurate as the helix angle of the gear varies across the tooth depth. A cutter running at a fixed angle will cut at that angle over the whole tooth depth. Of course, in practice it often doesn't matter and the resultant gears are fine. One of the Brown and Sharpe books on gear cutting gives some advice on tweaks to improve the accuracy, usually by setting the cutter at slightly less than the helix angle.

None of the above is anything to do with the cutter number to be used. The cutter number changes from that used for a spur gear (helical gear with a helix angle of zero) because the teeth on the normal diametric pitch are on an ellipse, not a circle. The local radius of the ellipse is larger than the radius of the blank. So the equivalent number of teeth is larger than the number of teeth on the actual gear. The cutter number is selected according to the equivalent number of teeth, not the actual number. The equivlant number of teeth is the actual number of teeth divided by the cosine of the helix angle cubed:

N' = N/cos3(A)

Cutting a helical gear the old school way using an involute cutter with a universal mill and dividing head:

(https://listerengine.com/coppermine/albums/userpics/10022/normal_Helical_Gears_MEM_1.JPG)

Andrew
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Jo on May 15, 2020, 09:14:17 AM
None of the above is anything to do with the cutter number to be used. The cutter number changes from that used for a spur gear (helical gear with a helix angle of zero) because the teeth on the normal diametric pitch are on an ellipse, not a circle. The local radius of the ellipse is larger than the radius of the blank. So the equivalent number of teeth is larger than the number of teeth on the actual gear. The cutter number is selected according to the equivalent number of teeth, not the actual number. The equivlant number of teeth is the actual number of teeth divided by the cosine of the helix angle cubed:

N' = N/cos3(A)

The cheats amongst us look it up on a chart  :-X

Jo
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Zephyrin on May 15, 2020, 09:28:38 AM
I did the Chuck Fellows setup to cut tiny helical gears,(0.5 Module), and it worked a treat...
I simply used the tool post for setting the height of the blank, but the useful height range is limited under the lathe mandrel as compared to the mill setup of George Britnell.

https://www.youtube.com/watch?v=wr6H7kfVFp8
https://www.youtube.com/watch?v=WKbYn27H9HM
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 15, 2020, 02:07:45 PM
I have always been fascinated with making gears. As silly as that might sound I just find that to make them and have them work is very satisfying. I bought the book, Gears and Gear Cutting by Ivan Law, I downloaded scans of several books that are public domain on the internet. One is the Cincinnati-Milicron 100 years book, another is the Navy Machine Repairmans book and a third is The Treatise on Milling.
All of them have extensive coverage of gear making from spur to worms.
I cut 90% of my spur gears and generally with home made cutters.
My first association with making helical gears came way back in the late 80's when I was building my 302 V-8 engine. While designing the cam, block and distributor I knew that the distributor and cam had to run at the same speed but couldn't get my head around why the gears were 2 different sizes. They had the same tooth count but the P.D. was different. It was then that I started studying what helical gears were.
When Chuck Fellows came up with his home shop fixture I knew I had to make one and try it out. The first application was when I was building my Ford 300 inline 6 cylinder engine. I wanted to replicate the engine as much as possible which meant placing the distributor mid block. This negated the use of miter gears but as with the full sized engine would require a pair of helicals. Fortunately the gears are at right angles to each other so only one radial template was needed.
https://www.youtube.com/watch?v=B6AFWrWV7vU
The next time I made helicals was for my flathead engine. I tried to replicate the flathead as much as possible which meant making all the internal gearing helical. There are 3 gears which drive the oil pump, the 2 timing gears and then the distributor drive gears.
This meant making several different helical templates. Whenever the P.D. changed it needed a template. Whenever the gears needed to run parallel or at 90 degrees this required separate templates. This also necessitated making a couple of different index plates.
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 15, 2020, 02:11:33 PM
Not wanting to modify or make fixturing for my lathe I used my helical fixture in my mill vise.
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 15, 2020, 02:20:35 PM
This is the gear set for the oil pump drive on the flathead engine. On the right is the gear that goes on the camshaft. In the center (brass gear) is the intermediate gear which provides the spacing to get over to the oil pump centerline. The gear on the left is the right angle helical which mounts to the oil pump drive shaft.
The next picture is one of the home-made cutters that I used for cutting the gears. The profile of the cutter was created using one of Don's worksheets.

Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 15, 2020, 02:28:08 PM
The last set of pictures is the helical gearing that I used for the Maudslay marine oscillating steam engine. This was another of Chuck's many projects. When the drawings were published I scaled them down by half and when compete I thought that to add a little more interest to the engine I would make a tiny governor, not a working one but just the spinning mechanism.
Another set of helical templates were needed because these gears have 2 different helical angles.
Thanks Chuck for all you contributed to the model engineering community. You are missed!
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 15, 2020, 02:31:12 PM
When you get down this small with a governor there's just not enough weight and mass to make it fully functional, at least not at slow speeds.
https://www.youtube.com/watch?v=1EKaYGyOmEU
Title: Re: Chucks Helical Gear Cutting fixture
Post by: crueby on May 15, 2020, 02:55:08 PM
With Chuck's fixture, it looks like you need to keep the moving center portion pushed up against the guide wheel as you turn it to make the cut, correct? Or is there some other bit that keeps it in place? The feed rate must take some 'feel' to get right.
Title: Re: Chucks Helical Gear Cutting fixture
Post by: gbritnell on May 15, 2020, 03:25:58 PM
Chuck had a spring on his fixture to keep constant load against the roller bearing. I decided to try it without it and just make sure that I always has pressure against the wheel. It worked fine. The only thing you have to remember is to conventional cut otherwise the cutter will pull the blank into it.
gbritnell
Title: Re: Chucks Helical Gear Cutting fixture
Post by: stefan-k on May 16, 2020, 08:03:59 PM
So, with the help of all the nice guys here i managed to finish my fixture and cut the very first Helical gear with it. Attached you find some pics of my Fixture and the Helical gear i just cut. Next gear will be the matching one and then we`ll find out if they run together. This gear has a 34 DP or Modul 0,75 , 12 teeth and 60° Helix angle. The matching one will have 34 DP, 24 teeth and 30° angle. At the moment i have only 34 DP involute gear cutters at Hand. Means I´m a little limited in making gears.
How can i make my own gear cutters? specially for small gears with less than 12 teeth.? the involute ones are a little pricey......

Stefan
Title: Re: Chucks Helical Gear Cutting fixture
Post by: michelko on May 16, 2020, 08:21:17 PM
Hi Stefan,
http://metallmodellbau.de/GEAR-CUTTING.php
Maybe you find this helpfull? Not sure if the hob method works for helical gears.
Michael
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Don1966 on May 17, 2020, 12:16:34 AM
Stefan the gear sheets I sent you has the information to make your own cutters and gear hob. It has tabs to select which gear your making alone with making cutters and Hobbs. If you have never made cutters get the book “ Gears and gear cutting “ by Ivan Law. It cost no more than 8 dollars on amazon. money well spent to understanding gears and making gear cutters to cut your own gear. By the way nice looking gear.

Regards Don
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Craig DeShong on May 17, 2020, 12:29:32 AM
Chuck had a spring on his fixture to keep constant load against the roller bearing. I decided to try it without it and just make sure that I always has pressure against the wheel. It worked fine. The only thing you have to remember is to conventional cut otherwise the cutter will pull the blank into it.
gbritnell

Like George I omitted the spring.  Just remember to keep pressure against the wheel (as George says) and you’ll have no problems.
Title: Re: Chucks Helical Gear Cutting fixture
Post by: stefan-k on May 17, 2020, 07:50:52 AM
Stefan the gear sheets I sent you has the information to make your own cutters and gear hob. It has tabs to select which gear your making alone with making cutters and Hobbs. If you have never made cutters get the book “ Gears and gear cutting “ by Ivan Law.

Regards Don

Thanks Don, i just found it and already ordered the book.

Regards Stefan
Title: Re: Chucks Helical Gear Cutting fixture
Post by: stefan-k on May 17, 2020, 07:54:23 AM
Hi Stefan,
http://metallmodellbau.de/GEAR-CUTTING.php
Maybe you find this helpfull? Not sure if the hob method works for helical gears.
Michael

Hi Michael

Just took a look at the site. It´s interesting! Keeping an eye on it......
BTW: Funny! Two Germans talking english to each other........ :Lol:
Title: Re: Chucks Helical Gear Cutting fixture
Post by: jadge on May 17, 2020, 08:47:34 AM
I expect the 'hob' method would work for a helical gear, but the indexing and offsetting would be interesting. Here's a link to a thread on another forum discussing the method:

https://www.model-engineer.co.uk/forums/postings.asp?th=155945 (https://www.model-engineer.co.uk/forums/postings.asp?th=155945)

Personally I wouldn't use the hob method. I'd either make an involute cutter or just machine the gear direct on the CNC mill.

Andrew
Title: Re: Chucks Helical Gear Cutting fixture
Post by: Mcgyver on May 17, 2020, 12:38:27 PM
Personally I wouldn't use the hob method. I'd either make an involute cutter or just machine the gear direct on the CNC mill.


whats shown there isn't a hob as I suspect you know as you put it quotes.  I call them faceted gear cutters.  They work for undemanding applications, however with smaller teeth counts quality breaks down.  Their advantage is only one cutter is needed, they're quick cheap and home made....but you will get a better profile with a involute cutter for spur or helical...which of course is a reasonable approximation of what you'll get with a hob (generation = most accurate)

Chucks creation is really something, I admire the simplicity and how well it functions on what many/most would view as a really challenging home shop task; helical gears.  Good stuff, part of his legacy.
Title: Re: Chucks Helical Gear Cutting fixture
Post by: jadge on May 18, 2020, 09:20:44 AM

whats shown there isn't a hob as I suspect you know as you put it quotes.  I call them faceted gear cutters.

That's an elegant description. In the linked thread it was discussed that the method gets less accurate as the number of teeth decreases. Even some of the gears with large numbers of teeth looked to have a poor tooth form. Although one needs to bear in mind that the gears were physically small.

Conventional hobbing still produces a series of facets, just many more of them. For ordinary gears the hobbed finish is fine. But for precision gears the gears were often shaved and ground after hobbing. Two big advantages of hobbing are that only one cutter is needed and no time consuming and error prone indexing is needed. A disadvantage is that hobbing can create undercutting of the tooth even when it isn't needed for clearance of the mating gear.

Standard involute cutters are only correct for the lowest number of teeth they're listed to cut. However an involute cutter designed and made for a specific number of teeth may well be more accurate than a hobbed gear. Depending of course on how accurately the involute curve is formed.

From a practical viewpoint I doubt many of us would be able to measure the differences discussed above. In the commercial world there are many tweaks and twiddles applied to the tooth form to promote smooth running of high speed and highly loaded gears.

Andrew
Title: Re: Chucks Helical Gear Cutting fixture
Post by: john mills on May 18, 2020, 01:20:11 PM
The advantage of Hobbing is it generates the tooth shape when you talk of under cutting when low numbers of
teeth that can be adjusted by increasing the size  the centre to centre difference can be maintained by reducing the size of the matting gear or increase the size of the matting gear and adjust the centre to centre distance to
suit  .I had to cut gears with 5 teeth there was no undercut.but the sizes did not work out with the formulas i could find un til i found old machinery hand book where it was explained and  the formulas to work  out so no undercut. which did match the drawing sizes.when you hob gears you can alter the sizes quite a lot . 
                   
              John
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