22 DP 42 tooth worm gear

I am working on restoring my 1918 Wade 8A Precision Toolroom Lathe and I found that the worm gear which powers the longitudinal feed is
extremely worn and needs to be replaced. A steel worm rides with key rides on the feedscrew, and is in good shape. The mating worm gear, however, is brass and is extremely worn. The outer teeth are worn to the sharp points, and the internal taper that forms a slip clutch to engage the longitudinal feed is worn such that the clutch no longer functions.
Replacing this gear would be no problem if it was a standard 24 DP 40 tooth brass worm gear, which is $37 from Boston Gear. However, according to my best calculations, I have a 22 DP, 42 tooth gear. I can't simply change to a standard, readily available gear because the center distances between the feedscrew and the clutch mechanism is fixed by the lathe's apron.
The options now are basically to have a custom gear made to match the worn gear and the mating worm, or to attempt to make a replacement myself. I have a Bridgeport with a large universal dividing head, and I've made straight spur gears before. The problem is that I haven't found any 22DP involute gear cutters, so I'd have to start with making either a single point tool or a multitooth gear cutter first. Then, because its a worm gear rather than a straight spur, each tooth is actually concave to the OD. Thus, instead of simply feeding the gear across the cutter for each tooth, you have to plunge the cutter into the gear on the centerline.
Any ideas? woodworker88
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On Sun, 20 Jul 2008 22:47:02 -0700 (PDT), woodworker88

I've made worm gears by making a cutter to match the worm in size and shape and then using it to cut the gear. You need to design it to cut both the top and bottom of the tooth to get the radius. I've made both a multi tooth cutter by turning a "worm" with clearance and then milling teeth, and a single point by grinding a large lathe tool into a fly cutter type tool to machine both sides of the groove between the teeth and the top of one tooth.
You will have to set the dividing head at an angle to match the pitch of the worm though.
Bruce-in-Bangkok (correct Address is bpaige125atgmaildotcom)
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This was a perennial subject in The Model Engineer, a British magazine. Always someone having to cut odd worms and wheels in there for various projects. Might be your local library would have back issues. Two ways of going about it with the average equipment in the home workshop. One is to make a flycutter and a brass blank, cut the teeth out one at a time using whatever means for indexing at hand. Some of the writers used the lathe for indexing and had a separately driven cutter head mounted on the cross slide, offset at the correct angle, some used the lathe for powering the cutter and held the blank in an indexing fixture on the cross slide. Blank was usually held between centers on a mandrel. The other method was to make a hobbing cutter from "silver steel"(drill rod) same as the dimensions of the worm, gashing that and backing off the teeth by whatever means available, then setting that up to cut the blank. A little more work, but the resulting teeth would probably better formed. Basically, the hobbing cutter is mostly doing an Acme-style threading job. Pick your poison! Going to take some effort no matter what you do.
Stan
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On Sun, 20 Jul 2008 22:47:02 -0700 (PDT), woodworker88
<snip>

==========Before you do anything, review _Gears and Gear Cutting_ by Ivan Law. This is #17 in the Workshop Practice series [SI model books) from the UK. All of these books that I have seen have been very good. # ISBN-10: 0852429118 # ISBN-13: 978-0852429112
It appears that you are about to discover why your lathe has the capability to cut DP and module threads in addition to inch and metric, and how to cut an Acme thread.
While thre are several suppliers of this book in the US two sources are (Amazon.com product link shortened)16658833&sr=1-1 http://www.campbelltools.com/workshopseries.htm (about 3/4 down the page on the left) also Good luck on an interesting project. Let the group know how you make out and post some pictures to the dropbox.
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
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Thanks all very much for the advice. I talked to another machinist at work today and we decided that a single point flycutter seems to be the best bet. I have that book on order as of 15 minutes ago.
Thanks Michael
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On Mon, 21 Jul 2008 14:14:51 -0700 (PDT), woodworker88

Greetings Michael, I think you'll find that making a worm and using that to make the worm gear will be the easiest and best way. The book you ordered will make that obvious to you. I have made worm gears using a tap as the hob too. Using a high spiral tap is the best way to go. The tap, acting as the hob, pulls the worm gear around so no indexing head is needed. The method I used was to hold the tap shank in a collet and the gear blank is mounted horizontally on a holder which in turn is clamped in an Aloris toolholder. The gear blank is set on the spindle centerline and is free to rotate. Then the gear blank is fed into the spinning tap with the cross slide. Since you are making a brass worm gear you might be able to get away with using an un-hardened steel hob. ERS
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Mike: please post pictures and keep us up to speed as you progress through this project. thanks
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Will do
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Measure carefully as a lot of gears, especially worm gears, are not made to diametrical pitch sizes.
Don Young
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On Mon, 21 Jul 2008 21:06:11 -0500, "Don Young"

=======Because DP measures number of teeth per diameter in one sense, and the circumference and the diameter are related by PI [3.1416....] with a worm/wormgear set you can either have a DP [or module] with a whole number like 20 or 22 DPI but a worm with an "odd" thread, or you can have a worm with a standard whole TPI but the worm gear will have a funky DP/module. This is the reason for the strange thread pitches that the DP/module gears/settings for your lathe produces so that you can make worms that will mesh with standard DP/module gears. If you have a genuine acme thread lead screw the gear will have a 14-1/2 degree PA, but this is not a given.
Law's book goes into this in considerable detail and how to calculate a change gear setup for a DP/module thread [have lots of change gears]
I continue to be amazed at the quality/sophistication of work exhibited by these old machines.
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
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On Jul 21, 8:56 pm, F. George McDuffee <gmcduf...@mcduffee- associates.us> wrote:

I see that makes sense.
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On Mon, 21 Jul 2008 11:55:18 -0500, F. George McDuffee

Thanks Unka' George. I have been meaning to order the Gear book for a long time now. Your mention of it prompted me to go to Amazon and order it and a couple of others too.
Errol Groff
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woodworker88 wrote:

Tilt the head (or nod it, depending on your set-up)to the helix angle of the teeth, and cut the teeth with a single tooth cutter. If the worm that drives the gear is small enough.
You may have to make up a cutter to make a cutter, sort of thing, where you make one tool, to make the one you need. That would be making a radius cutter out of HSS, to make the appropriate radii on the gear tooth cutter (of high carbons tock, then harden), then use that to gut the teeth.
Dimensions for the cutter can be got from any of several different sources, or calculated, or you can just grind a cutter thats a decent fit on an unworn portion of the gear, if any.
You have the tools. The hard part is getting your head around the process. The cutting is boring (other than the fear :-P) as the job is just the same thing, 42 times.
Since it is brass, you could also free hob the gear, by making a section of threaded rod, 22DP (does your lathe do DP threads? Got access to one that does? Got a few spare change gears?), from mild steel, or something a little bit tougher, and gash the teeth on the blank with the indexing head, then hob the form onto the gear.
Cheers Trevor Jones
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For the most part, Ivan Law's book tells how to make gear cutters to use for cutting gears; not just how to cut gears. John Stevenson also has a nice paper on the subject. Both yield extremely accurate shapes, probably a lot better than one could achieve with only a bench grinder working on wheel's edge. Basically the cutters are formed (cut) with the curves of standard end mills.
Recently, I needed a 56 DP, 25 tooth gear to mate with the the Sherline threading attachment. Law's book didn't go that small. The process was outlined in Jan/Feb 2008 Home Shop Machinist using a 3/32" end mill. The cutter was milled in tool steel, then heat treated.
Bob Swinney
woodworker88 wrote:

Tilt the head (or nod it, depending on your set-up)to the helix angle of the teeth, and cut the teeth with a single tooth cutter. If the worm that drives the gear is small enough.
You may have to make up a cutter to make a cutter, sort of thing, where you make one tool, to make the one you need. That would be making a radius cutter out of HSS, to make the appropriate radii on the gear tooth cutter (of high carbons tock, then harden), then use that to gut the teeth.
Dimensions for the cutter can be got from any of several different sources, or calculated, or you can just grind a cutter thats a decent fit on an unworn portion of the gear, if any.
You have the tools. The hard part is getting your head around the process. The cutting is boring (other than the fear :-P) as the job is just the same thing, 42 times.
Since it is brass, you could also free hob the gear, by making a section of threaded rod, 22DP (does your lathe do DP threads? Got access to one that does? Got a few spare change gears?), from mild steel, or something a little bit tougher, and gash the teeth on the blank with the indexing head, then hob the form onto the gear.
Cheers Trevor Jones
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Basically the cutters are formed (cut) with the curves of standard end mills.

Yeah this sounds like the process. Use a commercial tool to make a homemade tool to make the gear.
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