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Thread: Making a gear mill cutter
08-15-2011, 11:34 PM #1
Making a gear mill cutter
Does anyone know of any literature on making a gear mill cutter? I recognize that there are 8 cutters for a given diametral pitch to cover a range for a number of teeth.
I can easily create the correct involute profile on my CAD, but am curious on how to cut this profile into a tool using basic equipment (i.e., non-CNC equipment; just using standard tool room engine lathe & universal milling machine).
I know I could make a virtual hob, but without a direct differential drive I cannot hob the gear, plus I would have to take multiple passes and reset the angular position of the work piece and cutter to approximate the correct profile. This would generate the involute out of straight lines. This sounds to me like a lot of work and there's probably a better way.
One thought I had was for a gear mill cutter was to approaximate the involute with a radius. This would give me approx a 0.003 error in profile for one of my test cases. However, if I remember correctly, out of the cutter set, each cutter has the actual profile for either the lowest or highest number of teeth; I can't remember which one.
In any event, this tells me that somehow they cut or grind the correct profile. Tis brings me back to my original question; how do they do that?
I hope someone can help me on this.
08-16-2011, 12:41 AM #2
The book Gears and Gear Cutting by Ivan Law, #17 in the Workshop Practice Series printed by Nexus Special Interests has good instructions for making gear cutters at this level. He approximates using circles, but gives you the calculations and workshop setups to easily produce them. This is a single point form tool for use in a holder like a fly cutter.
More recently John Stevenson wrote up a procedure on one of the discussion groups about using a tapered endmill to produce the cutter. The tapered endmill has a range of radii so you can position it for the size you need, plus it's producing the required clearance automatically.
If you get way enthusiastic about gear cutters, the Ivan Law book has a chapter at the back about building a device to actually make circular form-relieved cutters.
Lots of good options.
S_W_Bausch liked this post
08-16-2011, 02:06 AM #3
Good question, I'll be watching for the answer too.
Obviously with the CAD profile, a template could be accurately made, which would work with a tracer lathe, so maybe you are close enough with your tool room lathe?
My choice in design might be a pantograph style, with a say, 4:1 ratio, to "magnify" accuracy, though it runs the risk of multiple points of "working clearance" in which to multiply error.
Though much harder to accurately grind the template profile, a 1:1 ratio could provide great accuracy if executed and employed well. Simply a ridgid, vertical post from a hard point on the carriage, the template ridgidly suspended in it's path, the post turned/ground to match the radii of a specific R or "button", round insert, set precisely to center. A set could be produced for a range of radii, to suit larger and smaller gears.
The beauty of this system would be the ability to lock the compound gib, thereby providing an absolutely ridgid duplication of the template. IE, a rock solid link between guide pin and tool, with the compound clearance closed.
Making the assumption that the template mount could become needlessly complex to provide very ridgid, lateral adjustment for, I think that I'd use a mandrel to hold the to-size blank between threaded collars, mount between centers and then locate the collars to center the blank on the template, using a lock nut against one of the collars, so that it's opposite would be the one that was removed, thereby holding-place for additional blanks.
signed Bob, who is mightily plagued with the Rube Goldberg affliction....... and whose old 17" LeBlond came without the taper attachment but does have the 1/2-13 attachments holes bored and tapped behind the back way on which to ridgidly mount a template bridge.
Just cuz" I couldn't find a way to hastily delete my post in embarrassment..... EDIT, and there ya' go, Goldberg made me fergit' the complex profile relief, which of course John Stevenson brilliantly didn't..... My one stab at duplicated profile turning, which had a pleasing outcome, wasn't for a cutting tool.....
08-18-2011, 02:54 AM #4
Thanks for the suggestions.
I've looked around a little bit more, but only found one site that said they machined the profile of a gear mill cutter by using a radius type tool/cutter.
I don't know how they can say that the lowest number of teeth for a given gear number size cutter would be the theoretically correct profile????? It may be close, but the profile they are making is a full radius not an involute.
I did have one interesting find. I came across a book titled: "Practical Treatise on Gearing", by the Brown and Sharpe Manufacturing company, 1896. I haven't read it yest, but at first glance...no info on how to make a gear mill cutter.
08-18-2011, 03:16 AM #5
I don't know how it is done either. I mean I can imagine how the relieving attachment on the lathe bobs in and out to make the cutter (I've done that much with a shop made contraption) but the actual full formed profile of the relieving tool I made from comparison to a sample gear. So that was cheating
And even at that, the cutting action of a full profile cutter chomping away on a relieving lathe is not exactly 'nice and smooth'. There's got to be some kind of mechanical linkage that would allow one to profile cut the involute cutter with a simple fixed radius lathe tool.
08-18-2011, 03:18 AM #6
have a look here:
Ivan Law's book covers making circular B&S type cutters on a lathe. It's quite a complex process and requires you to make special tooling. The big issue is cutting the relief on each tooth.
08-18-2011, 03:20 AM #7
08-18-2011, 03:30 AM #8In any event, this tells me that somehow they cut or grind the correct profile. Tis brings me back to my original question; how do they do that?
A profile was established and made into a shaper tool. The shaper tool was used in the shaper to create a form type lathe tool, and after hardening and lapping to final shape, used in the lathe with a relieving attachment to produce a form relieved milling cutter for gears.
That is one way.
There are template type grinding wheel dressers. Engis, later owned by Pratt & Whitney, made them in many sizes and styles. All were guided by an oversize template - 10:1 on the one I own. The template is just a flat piece of metal made in the mill and checked for accuracy with whatever equipment was needed, such as optical comparators. These will dress an extremely accurate profile in a grinding wheel which can be used to form the lathe tool for use in the lathe with a relieving attachment.
That is the same process with an improvement in execution.
08-18-2011, 10:45 AM #9
I may get crucified for saying this but I have cut lots of gears using hss lathe bits in fly cutter heads I used the old gear as a template for grinding the tool. Take it slow and use your dividing head and you can get decent results.
08-18-2011, 11:18 AM #10
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08-18-2011, 03:21 PM #11
I have access to Mastercam with gear tooth software. I have thought about making a template out of 1/8 inch thick sheet brass. I would machine the male contour (on a cnc mill) of the space between the teeth of the correct profile of the gear that I wanted to cut with a fly-cutter tool. I would mill a slot 1/16 inch deep with and end mill of the size of the width of the HSS fly cutter tool, this slot would be aligned with the shape of the space between the teeth of the gear tooth profile.
Once the template is made, the top of the tool bit would be covered with Dykem blue, and the profile scribed on the top of the tool bit. This would allow you to rough out the shape of the tool on a pedestal grinder wheel. The finish could then be further refined by laying the template on top of the tool bit between grinding sessions.
I have cut many gears with a fly-cutter, using an existing gear as a template for making the fly-cutter. The process can be speeded up by roughing out with a arbor mounted slotting cutter, and using the fly-cutter for finishing. The process of generating the tooth form with gear generating software would allow you to grind the tool to the exact theoretical form of the particular cutter for a given DP and number to teeth. Should be more exact than a set of 8 cutters for a particular DP. All of those cutters are an approximation of the exact form only.
08-18-2011, 09:56 PM #12
Using a fly cutter seems to be a reasonable approach for one-off. To get the profile on the actual cutter bit, Law used profile cutters made from disks of HSS, mounted to a cutter holder that held the cutter disk at an angle. Apparently this approximates the proper involute form pretty well.
For cutting a multi-tooth gear cutting wheel, Law also tells the story of seeing an ad in an old newspaper, and looking (with his friend, Professor Chaddock, famous for designing the Quorn cutter grinder) for more info. Finally, they just decided to try and make the relief mechanism from pic in the ad. It's a pretty cool little setup. The device has it's spindle mounted in the lathe. The spindle has an eccentric, and an arm on that eccentric which moves the form tools (two disks as above, with a cutter for the top of the tooth) in and out to give relief.
Or you could buy used gear cutters on ebay, or, I suppose, new ones.
Or you could buy a used gear hobbing machine on ebay, but I guess you then have to find the hobs.
08-19-2011, 04:40 AM #13
10-30-2011, 11:15 PM #14
Regarding tutorials on gear-making, I have a copy of Ivan Law's book, and it's a great book. If you buy one, don't ever lend it out.
It's like that bottle of DeOxit....you will never get it back.
After that book, it wouldn't be a bad idea to purchase a machinist textbook from 100 years or so, when they would make their own taps, cutters, reamers, drills, etc. Oh, and gears.
Ivan discusses a simplified technique of cutting bevel gears, and the machinist's textbooks from the 1900's to the 1930's will also cover gear-making technique.
Unless you have some Gleason machinery sitting around, "simplified" gear-making may be all you need.
10-31-2011, 01:30 AM #15
This is Charles Lessig's Eureka gear cutter relieving tool from the Ivan Law book. It is modified to relieve a hob rather than a cutter.
Charles Lessig's Hob Reliever Eureka Balzer Gear Cutting - YouTube
Here is a thread where he describes it:
Here's a thread on how to make the cutters that are relieved by the Eureka.
10-31-2011, 04:20 AM #16
10-31-2011, 05:54 AM #17
10-31-2011, 10:43 AM #18
For a tool to cut the "shape" you need, you could approach it like John Parsons did in 1946.
Parsons is the "Father" of NC/CNC.
He was milling airfoil sections for helicopter blades on a Bridgeport Mill.
He plotted the points he needed and then had two men "hanging" on the the X and Y axes.
He then told each one in separate steps to move to the next point. He blew the wheels off the inspectors, because they had not seen such accuracy. It's doable!
I say all this because I thought it was wonderful. It may not be something you need to do, but it's there if you need it.
10-31-2011, 01:32 PM #19
I agree that Ivan Laws book is the best place to start, the idea of the button method of making the lathe tool for turning the cutter is also very clever.
The fly cuttting method does work provided you get the meat out first with a slitting saw, the actual cutter is best made by chucking a piece of round HSS in a pistol drill and with the drill switched on shape it by eye on a pedistal grinder using a mating gear of roughly the right tooth count as a gauge. When the hss fits the tooth gap properly you then remove half the diameter of the rod to leave a perfect tool. better still buy the cutters on ebay.