Author Topic: Cutting Spur Gears -- A neat technique  (Read 4448 times)

Offline xo18thfa

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Cutting Spur Gears -- A neat technique
« on: February 22, 2017, 01:24:34 AM »
Several years ago, I did a series on cutting spur gears using the methods in the “Workshop Practice Series” book #17, “Gears and Gear Cutting” by Ivan Law:



Mr Law's method works great.  You can follow his techniques without deviation.  However, recently I found another method to cut gears that approximates involute tooth using only one cutter to form gears of any size within the diametral pitch.  The method is described by "Helicron" at his website.

http://www.helicron.net/workshop/gearcutting/

I followed Helicron's technique with good results.

First step is to make the cutter tool.  My lathe is not rigid enough to make the cutter as Helicron shows.  Instead mine is a single point cutter with most of the blank removed before forming the cutting teeth.



The steel for the cutter is alloy “O1”. O1 is the most widely used tool steel and is available from McMaster Carr and other suppliers. O1 is very easy to work with and harden. Shape and form the tool to the desired size. Heat with a torch to 1400 degrees F, which is cherry red heat in room light. Plunge in oil to quench. I used some leftover salad oil from the kitchen. Leave in the oil quench about 30 seconds, it cools down slowly. Quenched tool steel is too brittle at this point and needs tempering. To temper, suspend the tool in a small pot of salad oil on the kitchen stove. Heat to 350 degrees F using a candy thermometer to check the temperature. Leave it in about 10 minutes, pull out and let air dry. The O1 steel comes out very hard and will cut just about anything.

For an upcoming project I want to cut some 48 diametral pitch gears with 52, and 16 teeth.  First, the 52-tooth gear.  These are from 1/8” thick flat brass bar.  First, make a shank from length of 1/2" diameter steel in the lathe cut and turn a 5/16” diameter boss. Thread the end to accept a nut. Gang up four pieces of brass stock on the shank.



Turn the ganged stock to the final diameter for the gear.  In this case 1.125"



To cut the gears I used my little, homemade Harold Hall dividing head.  It is a “direct indexing” head that uses the change gears from the Atlas 6” lathe for indexing.  The spindle hole is tapered with #2 Morse Taper (2MT). A 2MT collet holds cinches stock tightly in the dividing head be means of a draw bar.



Secure the 52-tooth lathe change gear to the spindle of the dividing head. Mount the dividing head to the mill table. Use a 1/2" 2MT collet to chuck the gear blanks.  Chuck the gear cutter in the mill spindle and use a height measure device to set the gear cutter height.



Adjust the depth of cut and start cutting. depth of cut on 48DP gears is 0.045".  Advance the dividing head spindle one notch on the indexing gear.



Finish the 52 tooth gears by soft soldering an appropriate hub.  Drill and tap for a set screw.



Next time we will cut the 16 tooth gear and do something with the results.
Bob Sorenson
Las Vegas, Nevada, USA

Offline bertie_bassett

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Re: Cutting Spur Gears -- A neat technique
« Reply #1 on: February 22, 2017, 01:23:41 PM »
nice work

that method is one im planning on using again in the near future to make change gears for my lathe.

last time i tried it didnt quite work, think my cutter wasnt very accurate. and had issues with working out the depth of cut

couple of questions:

a multi tooth cutter would have been easier on your lathe surely? its just a series of v's at the correct spacing. cuting just one tooth as you have must have been quite clunky as it went round and id have thought harder on your lathe?

also from memory you only need about 5 teeth, anymore dosnt help as they dont do any cutting ( unless your cutting a gear with hundreds of teeth.
a competent engineer uses the tools and knowledge available, to get a challenging job done.

 An incompetent "engineer" tells his boss that the existing equipment "can't do the job" and to get another machine

Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #2 on: February 22, 2017, 02:57:05 PM »
There used to be a series of videos on YouTube by a guy called Hobbynut, showing how he used a very similar technique to make gears on his Sherline lathe & mill. I think I downloaded them somewhere, no idea where they'd be now, I'm pretty sure they've gone now. I can't remember if his cutter required indexing, or if it was a free-running hob, though.

Even so, liked the write up on Helicron's site, that definitely gives me the confidence to have another go at gear cutting!
Cheers!
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Offline John Stevenson

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Re: Cutting Spur Gears -- A neat technique
« Reply #3 on: February 22, 2017, 03:20:59 PM »
Yes those Hobbynut videos were very informative but as you say they have gone now.

There is a DVD by Jose Rodriguez on gear cutting doing the rounds, about £30 a pop.

Don't bother it's absolute $hite
He's using some home made mill made from licorice root and drinking straws, the video is out of focus and any close up could be a shop of anything [ probably blancmange ]

But the biggest fook up is he gets Pi wrong all the way thru all his calculations so all his calculations are off.

Since when is Pi 3 416 ??
John Stevenson

Offline PekkaNF

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Re: Cutting Spur Gears -- A neat technique
« Reply #4 on: February 22, 2017, 04:13:15 PM »
....
Since when is Pi 3 416 ??

Imperial PI? Metric is standard :lol:

Pekka

Offline sparky961

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Re: Cutting Spur Gears -- A neat technique
« Reply #5 on: February 22, 2017, 07:33:20 PM »
Quenched tool steel is too brittle at this point and needs tempering.

In practice I've found that no tempering is needed when working with brass or other soft metals like aluminum.

In fact, for a one-off tool that's going to see little use you can often get away without tempering the tool for most materials found in the home shop.

Offline xo18thfa

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Re: Cutting Spur Gears -- A neat technique
« Reply #6 on: February 22, 2017, 10:15:19 PM »
nice work

that method is one im planning on using again in the near future to make change gears for my lathe.

last time i tried it didnt quite work, think my cutter wasnt very accurate. and had issues with working out the depth of cut

couple of questions:

a multi tooth cutter would have been easier on your lathe surely? its just a series of v's at the correct spacing. cuting just one tooth as you have must have been quite clunky as it went round and id have thought harder on your lathe?

also from memory you only need about 5 teeth, anymore dosnt help as they dont do any cutting ( unless your cutting a gear with hundreds of teeth.

I tried the multi tooth cutter using helicron's method.  My little lathe started to chatter a lot and the attempt failed.  The single point tool bumped too, but less.  I'll re-visit the tool making method again.  Yes, 5 or 6 teeth are enough.  I did not know what to expect.

According to Mr Law (in his book), depth of cut = 2.157/DP (DP is diametral pitch)

Thanks, Bob
Bob Sorenson
Las Vegas, Nevada, USA

Offline xo18thfa

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Re: Cutting Spur Gears -- A neat technique
« Reply #7 on: February 22, 2017, 10:19:31 PM »
Quenched tool steel is too brittle at this point and needs tempering.

In practice I've found that no tempering is needed when working with brass or other soft metals like aluminum.

In fact, for a one-off tool that's going to see little use you can often get away without tempering the tool for most materials found in the home shop.

Good to know.  That saves a step.  Wife gets upset when I use her cooking oil anyway
Bob Sorenson
Las Vegas, Nevada, USA

Offline Lew_Merrick_PE

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Re: Cutting Spur Gears -- A neat technique
« Reply #8 on: February 23, 2017, 11:57:05 AM »
Quenched tool steel is too brittle at this point and needs tempering.

In practice I've found that no tempering is needed when working with brass or other soft metals like aluminum.

In fact, for a one-off tool that's going to see little use you can often get away without tempering the tool for most materials found in the home shop.

There are two steps to heat treating steel.  1) Hardening and 2) tempering.  The first (hardening) takes the surface of the steel to a Body-Centered-Cubic crystalline form (which is why "hard steel" grows in size).  And tempering "relieves the stresses" generated by such a transformation which makes it more resistant to breakage.

Offline sparky961

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Re: Cutting Spur Gears -- A neat technique
« Reply #9 on: February 23, 2017, 07:31:37 PM »
There are two steps to heat treating steel.  1) Hardening and 2) tempering.  The first (hardening) takes the surface of the steel to a Body-Centered-Cubic crystalline form (which is why "hard steel" grows in size).  And tempering "relieves the stresses" generated by such a transformation which makes it more resistant to breakage.

No arguments here.  What you've said is correct.  Though I doubt most home shop machinists care what form of crystal the steel is, as long as it does the job.

What I was pointing out was merely a shortcut that you can often get away with - kinda like sex without birth control.

I didn't think it necessary to add the obvious: "Don't come crying to me if your tool shatters" (or if the baby is crying).

Offline PekkaNF

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Re: Cutting Spur Gears -- A neat technique
« Reply #10 on: February 24, 2017, 08:07:07 AM »
Often tool steels temper ok, without complete cycling...Ie. Hardening, but you don't let it cool completely, just when water does not boil anymore, lift it out and let residual heat to do tempering. Some guy's at the knife making class were pretty good at this.

I did the two step process and first blade went fine, second had wery thin blade and I got bit bluish near tang and straw on the tip. I was aiming in-between, but it became ok knife. Not perfect, but not dog either.

Pekka

Offline xo18thfa

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Re: Cutting Spur Gears -- A neat technique
« Reply #11 on: February 24, 2017, 09:27:00 PM »
The 16-tooth pinion cuts in the same general manner as the 52’s. The notable difference is that the hub is integral, rather then hub less.

Change out the 52 tooth indexing gear on the dividing head and replace it with a 32 tooth.  During machining, skip every other tooth to get 16 divisions.

A 16 tooth, 48DP gear has a rough stock diameter of exactly 3/8” which is convenient. Use a 3/8” collet to chuck some 3/8” stock.  Cut as before.



With the dividing head set up I did some 32 tooth gears too.



Chuck the 16 tooth gear in the lathe and turn a hub. Tap for some set screws and it’s done.



These gears are going into a scaled up Ernest Glaser “Cracker”



They mesh perfectly. The chassis rolls smooth as silk.



Observations:

- Helicron's method is a great way for cutting gears. Simple to follow, simple to do.  A single cutter rather then a set of 7 or 8 is so much better.

- My lathe is not rigid enough to make the cutter blank as he shows.  So I went with the single tooth cutter.  Perhaps feeding the lathe bit in at an angle would be better then plunging straight in as I did.

- Overall grade: A-
Bob Sorenson
Las Vegas, Nevada, USA

Offline sparky961

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Re: Cutting Spur Gears -- A neat technique
« Reply #12 on: February 24, 2017, 10:21:07 PM »
Nice work.  I definitely like the idea of using a rack-shaped cutter over the use of a specially shaped single point tool.  Are you able to get an extreme close-up picture of the resulting tooth profile?

As for the lathe not being rigid enough to cut the cull diameter, this is possible - even likely.  But, I wonder if you couldn't get the result you were looking for either by turning the speed (RPM) way down.  Feed as quickly as your lathe will allow with a sharp tool.  Another possibility is to rough out the grooves a bit first with another tool then come and finish using the above parameters.

... Or go CNC and peck at it using a few plunges.  Too finicky for manual, though some might attempt it.

Offline PekkaNF

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Re: Cutting Spur Gears -- A neat technique
« Reply #13 on: February 25, 2017, 03:57:40 AM »
Those gears look very nice. Very good work and very nice info.

That method looks worth trying...hmm..where do I need some gears?

Pekka

Offline krv3000

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Re: Cutting Spur Gears -- A neat technique
« Reply #14 on: February 25, 2017, 02:29:16 PM »
I see a project for me one of them dividing heads

Offline xo18thfa

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Re: Cutting Spur Gears -- A neat technique
« Reply #15 on: February 26, 2017, 08:54:39 PM »
Thanks for the kind words.

Here are the best pics I could get with this camera.  Holding these gears up to the light with a commercial gear shows they are pretty close.  You can see the flat cuts of the "approximate involute" but it does not seem to affect the performance.  I have a set of these in a "Cracker" loco that's been run hard and no appearance of wear.





I like this method.  Going to try 32 DP gear next.

The dividing head is Harold Hall's design.  It has a #2 Morse Taper spindle which is not a good idea. Morse Tapers do not quick release.  They go in tight and need a hammer blow on the draw bar to loosen.  I have a set of castings for George Thomas' "Versatile Dividing Head".  Going to set that one up with a straight shank ER-25 collet.  I think that will work better.  It will also be direct dividing with change gears or plates.
Bob Sorenson
Las Vegas, Nevada, USA

Offline awemawson

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Re: Cutting Spur Gears -- A neat technique
« Reply #16 on: February 27, 2017, 03:01:16 AM »
Very nicely done  :thumbup:

I'm tempted to have a try just for the fun of it despite not at the moment needing any gears.
Andrew Mawson
East Sussex

Offline Biggles

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Re: Cutting Spur Gears -- A neat technique
« Reply #17 on: February 27, 2017, 01:10:52 PM »
Propar Job!  :)

Offline sparky961

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Re: Cutting Spur Gears -- A neat technique
« Reply #18 on: February 27, 2017, 09:25:57 PM »
Again, very nice. I'm in the same boat as others though... don't need any gears at the moment.

Offline SeaCat

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Re: Cutting Spur Gears -- A neat technique
« Reply #19 on: April 22, 2017, 06:26:59 PM »
Bob, very nice gears and a very nice application you have made for them.  A very useful method of gear making which I have noted down - I have some gears to make sometime soon including an internal gear and this looks like solving the problem of the making of the spur gears at least.

Yes, I can see that the gears have a series of straight lines on the teeth - had the been steel gears I would have suggested you make a "running in board", where the gears are mounted on spindles on a temporary board and one gear driven to drive the rest and run for ten minutes or so with grinding paste to grind the gears into one another, which would take off the rough edges, such as they are, not very rough really.  But being brass this would not work as the grinding paste would only inbed itself into the brass for every more.  Perhaps if they are run with plenty of lub they will bed themselves in, if you are worried about it.  Not sure I would be much!

Chris

Offline fatal-exception

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Re: Cutting Spur Gears -- A neat technique
« Reply #20 on: May 05, 2017, 06:27:29 PM »
I spent hours last weekend making a cutter as pictured by the OP for 20DP gears. It did not work at all for cutting the actual gears. The cutter cut just fine, but the geometry of the finished teeth was, well, non existent. The teeth are pretty much triangular with no involute to them at all. I think I will try to replicate what this cutter does in a solidworks model. I should be able to see if this was doomed from the start, or it's a scale issue.

Has anyone else tried this method with a larger pitch tooth? The gears made at 48dp look good. Wish I could say the same.  :coffee:

Now I've cheated and bought an 8 piece cutter set off e-bay for $100. Completely against the spirit of Mad Modder...oh well, I gave it a shot.

Offline Joules

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Re: Cutting Spur Gears -- A neat technique
« Reply #21 on: May 06, 2017, 12:08:11 AM »
Well done, you gave it a go.  :clap:   And have new found respect for a fellow modder. 
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline PekkaNF

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Re: Cutting Spur Gears -- A neat technique
« Reply #22 on: May 06, 2017, 02:34:25 AM »
I spent hours last weekend making a cutter as pictured by the OP for 20DP gears. It did not work at all for cutting the actual gears. The cutter cut just fine, but the geometry of the finished teeth was, well, non existent. The teeth are pretty much triangular with no involute to them at all. I think I will try to replicate what this cutter does in a solidworks model. I should be able to see if this was doomed from the start, or it's a scale issue.

Has anyone else tried this method with a larger pitch tooth? The gears made at 48dp look good. Wish I could say the same.  :coffee:

Now I've cheated and bought an 8 piece cutter set off e-bay for $100. Completely against the spirit of Mad Modder...oh well, I gave it a shot.

What do you think went wrong? I never tried this method, but (to electrical engineer) it looks like if you get the cutter profile correct, lace it to centreline of the "disc" and have depth set right it should produce nearly correct tooth form.

Is it possible dwell on this one more? I see this a learning experience.

Pekka

Offline John Stevenson

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Re: Cutting Spur Gears -- A neat technique
« Reply #23 on: May 06, 2017, 04:32:16 AM »
Quenched tool steel is too brittle at this point and needs tempering.

In practice I've found that no tempering is needed when working with brass or other soft metals like aluminum.

In fact, for a one-off tool that's going to see little use you can often get away without tempering the tool for most materials found in the home shop.

Very true.

True story based on real world experience.
Few years ago I used to make proper form cutters using a full sized Eureka attachment, 1" bore model.
These were made out of gauge plate which is the flat version of silver steel or drill rod as known in America.

These were machined and then sent away for professional hardening and tempering. Cost wasn't a problem, what was a problem was the profile had to be exact.

When they came back the faces of the cutter were ground, one face one way, the other face the opposite way so all the burrs were thrown ff the cutter as believe it or not these also provide clearance.

Then fitted to the machine, basically a wood router they worked very well but the life before regrinds wasn't as good as using HSS and was a bit of a disappointment.
On one set because we were pushed for time we self hardened the cutters but didn't temper them, just ground them and put them to work.

This set cut far better and lasted longer. We determined that the heat generated from grinding was enough to take the brittle edge off the file actual tips that did the cutting.
After that it was make, harden, grind and use.
John Stevenson

Offline fatal-exception

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Re: Cutting Spur Gears -- A neat technique
« Reply #24 on: May 17, 2017, 06:43:04 PM »
So I had a chance this afternoon to simulate this cutter, cutting a gear in Solidworks. I setup up the DP, PA, OD, and tooth count as variables and used them to do an extruded cut on the outside of a simple cylinder, which is driven by the OD variable. Then a circular array of this cut is done 'Tooth count' times to form the gear.




Here's the larger gear with more teeth.


Maybe I wrote off this method a bit hastily. Oddly enough, in theory, this should work, even with 20DP and larger gears. The only place it gets a bit whacky is with low tooth counts, the crest of the tooth takes on a more triangular shape.

Honestly, the gears don't look all that great, but when put together in an assembly and constrained at the correct distance apart, they mesh and have no unwanted interference during the contact time, so again, they should work.


Now I have to go home and figure out why my real world example didn't work. Maybe I was a bit too sloppy setting up the DOC....

Paul

Offline philf

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Re: Cutting Spur Gears -- A neat technique
« Reply #25 on: May 18, 2017, 12:33:03 PM »
Paul,

Your cutter profile looks all wrong to me and would explain why the resulting gear teeth look thin.

The cutter teeth should resemble a rack profile and the tooth/space should be equal at the pitch circle line and not at the tip of the tooth which yours looks.

Phil.
« Last Edit: May 18, 2017, 01:08:03 PM by philf »
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Offline BillTodd

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Re: Cutting Spur Gears -- A neat technique
« Reply #26 on: May 19, 2017, 03:52:21 AM »
Quote
Then a circular array of this cut is done 'Tooth count' times to form the gear.
That's not how a gear hob works.

your cutter needs to be a symmetrical rack form and (in your reference frame) it needs to move along the tangent as it is copied around the blank (as it it were rolling around the circumference).

If you need a gear model (reasonable accuracy, good enough for printing) I have a couple, mod & dp,  based on a modified button method (uses elliptical buttons for a better involute match). mod version attached

Bill
« Last Edit: May 19, 2017, 04:16:27 AM by BillTodd »
Bill

Offline RussellT

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Re: Cutting Spur Gears -- A neat technique
« Reply #27 on: May 19, 2017, 05:07:18 AM »
Hi Paul

I've just been looking at your first diagram.  The figures you have used for pitch and depth of cut are exactly as described on Helicron's web page linked at the beginning of this thread but what isn't particularly clear on his page is that these figures are for a 20 degree pressure angle.  If you change the angle in your solidworks model I think you will find it works.

I have tried this method of gear cutting - my first attempt at gear cutting to test my home made dividing head - and the teeth when examined closely have three distinct facets on each side rather than the two seen in your model.

Russell

Offline BillTodd

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Re: Cutting Spur Gears -- A neat technique
« Reply #28 on: May 19, 2017, 11:13:02 AM »
Using the straight hob  method described you'll have a different number of facets depending on the numbercof teeth engaging with the hob, larger gears (more teeth)  will have more facets as they'll be flatter against the hob

Have a look on you tube for a method using a straight hob on a small cnc mill that indexes both the gear blank and hob to get many more facets for a closer involute form.

bill
Bill

Offline RussellT

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Re: Cutting Spur Gears -- A neat technique
« Reply #29 on: May 19, 2017, 11:35:29 AM »
The original post describes using a 5 tooth cutter central to the gear blank.  Under those circumstances it should only give three facets using this method.  If the gear were small enough there might only be two facets but there were three on the 16 tooth gear I tried.

Russell

Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #30 on: May 20, 2017, 05:05:09 AM »
OK, so in lieu of actual machines, I broke out SolidWorks as well  :lol:

Unfortunately, I just can't get mine to work properly  :scratch:

Please look at the 3 attached pictures:

The first picture shows my "cutter" lined up with a gear blank. The dimensions are all taken from Helicron's site, namely: 4.32mm maximum depth-of-cut (assuming pointy-tipped cutter). 1/2 way point between (2.16mm) is the contact point (gap & tooth width equal), so I assume this is my PCD. Actual DOC (assuming pointy cutter) is 3.41mm (again, taken from Helicron's site). And finally I've lopped off the top to 1.4mm above the PCD (red circled dimension).

This results in the "gear" as seen in the 2nd picture. Looks to me like the teeth are too skinny, have an odd "dink" where the tall cutter is catching just inside the PCD, and a strange "pit" at the bottom where the cutter is at max depth.

So... I reduced the cutter height-above-PCD to 1mm... and got the result in pic 3. The teeth still look too skinny, but at least that "dink" has gone away. Still got the weird pit though.

Obviously, I've mangled something somewhere. Can anyone set me back on the right track? The measurements are all based on Module 1 gear (as far as I understand Module...) & 20 degree pressure angle.

I've attached a ZIP containing the Solidworks (2013) part, so if someone fancies fixing it for me! Note that I'd like to use Helicron's method (i.e. simple 40-degree included angle pointy tool) to actually cut the cutter.

Thanks!
Cheers!
Ade.
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Offline BillTodd

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Re: Cutting Spur Gears -- A neat technique
« Reply #31 on: May 20, 2017, 05:23:11 AM »
Your rack still does not look quite right . The width of the V at the circumference of the blank should be at least equal to the flat at the top of the rack.  it looks like the flat is too wide given the depth of the cut.

Bill
Bill

Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #32 on: May 20, 2017, 05:48:05 AM »
Your rack still does not look quite right . The width of the V at the circumference of the blank should be at least equal to the flat at the top of the rack.  it looks like the flat is too wide given the depth of the cut.

Doh! You're quite right of course... I got the cutter blank diameter wrong - I was working with an outside diameter of 48mm, not a PCD of 48mm...

Re-set the diameter to 50.50mm (PCD + 2x 1.25mm), and it's looking better. I increased the cutter depth back to 1.4mm, as the tooth looked a bit shonky at 1mm.

Still got that "pit" at the bottom, is that because 1.4mm is too deep a cut? Or because it's only an approx tooth form?

Thanks for the prod anyway, made me realise what I'd done wrong  :thumbup:
Cheers!
Ade.
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Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #33 on: May 20, 2017, 06:22:11 AM »
Well... they mesh... more or less...

Might need a spot of virtual grinding paste in there to smooth things off a bit  :lol:

Cheers!
Ade.
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Offline BillTodd

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Re: Cutting Spur Gears -- A neat technique
« Reply #34 on: May 20, 2017, 11:34:42 AM »
They don't look too bad  :D

what happens if you do another cut with the rack and gear half a tooth around (if you see what i mean).?

Bill
Bill

Offline RussellT

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Re: Cutting Spur Gears -- A neat technique
« Reply #35 on: May 22, 2017, 06:36:00 AM »
Hi Ade

Helicron suggests the DOC below the PCD should be 1.25*MOD and you seem to have 1.4.  I haven't gone any further investigating but I think that might deal with the pit at the bottom of your teeth.

Russell

Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #36 on: May 22, 2017, 08:20:57 AM »
Hi Russell,

Every time I look at this again it makes my brain ache... For the pics I did above BTW, I did reduce the cut to 1.25mod (i.e. 1.25mm), it reduced the "pit" but didn't eliminate it.

So, attached is another pic of the cutter sketch.

The four dotted lines, starting with the top one are:
 - My maximum depth of cut (note the triangular peak marked 3.41 - that's the theoretical DOC if the cutter were infinitely sharp)
 - The next dotted line is the PCD (point where tooth & gap are equal, exactly half of the absolute maximum DOC, 4.32mm approx)
 - The third dotted line is the outer edge of the gear blank, and represents the point where I stop cutting
 - The fourth and final dotted line is the bottom of the cutter (=bottom of the rack).

Now... according to Helicron's site, the total DOC should be 3.41mm based on a 2.16 + 1.25 cut. But I'm not cutting as deep as 2.16, is that a problem??

Thing is, if I go full depth (i.e. the full (theoretical) 3.41mm cut, I end up with vampire teeth (pic 2)...?

 :scratch: :scratch: :scratch:

A bit more information:
Gear blank is 50.5mm diameter (increased to 52.32mm to account for the increased DOC)
I'm "cutting" a module 1 gear, so 48 teeth.
20 degrees pressure angle.
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
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Offline AdeV

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Re: Cutting Spur Gears -- A neat technique
« Reply #37 on: May 22, 2017, 08:22:45 AM »
Ignore that last one... my DOC's gone all wonky...

No... don't ignore it... the DOC is actually 3.41mm from the base dotted line to where I've trimmed off my cutter.

Last modification....! Do ignore it, I've got my blank WAY too big. I'd missed the bit where the OD of the blank is N(o of teeth) + 2 in mm... i.e. 50mm.

I've still got a "pit" though. I can only assume it must be because it's an approximation rather than an actual involute.
« Last Edit: May 22, 2017, 08:49:06 AM by AdeV »
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
Skype: adev73

Offline RussellT

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Re: Cutting Spur Gears -- A neat technique
« Reply #38 on: May 22, 2017, 02:24:16 PM »
Hi Ade

Don't forget there are two DOCs here.  The first one is for making the cutter - 3.41*MOD as you show.  The next one is the DOC when cutting the gear blank which according to Helicron is 2.25*MOD or 2.4*MOD for fine gears.

It may not help but I attach the spreadsheet I used to calculate the numbers when I tried this method - I wanted to make some 14.5deg PA gears. :smart:

Russell