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BadDog
08-05-2006, 03:03 AM
I found this (http://www.homeshopmachinist.net/bbs/showthread.php?t=13609&highlight=worm+gear) thread with Evan's hob and gear. I also found some other threads with similar or less info. But I'm afraid I need more help...

I found out tonight that my Rockwell lathe power feed problem was due to a marginal brass clutch hub which is driven by a worm, which in turn is driven off the keyway of the lead screw. I figure there's about as much chance I'll find that part as there is in me winning the lottery, which is not much considering I don't buy tickets... And if I'm going to get THAT lucky, I would just as soon save it for the big lottery win. :D So I strongly suspect I'll be making one...

That brass hub looks somewhat like a bell, bell, and I don't think it will be too difficult other than the precise inner cone shape for the other part of the clutch, but I’m sure that will seat itself quickly enough if I’m off a tiny bit (or there is always lapping).

Anyway, the problem is the worm wheel teeth on the ID of that bell rim. It has 36 teeth and I suppose I need to make a hob based on the worm driver TPI and such. Ok, I think I can manage that. But to get an exact and even set of teeth (no half teeth) it appears I need to “gash it” precisely to help guide the hob. How is this done? Set rotary table on an angle and use something like a slitting saw or thin mill cutter?

Any help (maybe links to worm wheel making in depth write-ups?) would be appreciated.

Mike Burdick
08-05-2006, 04:44 AM
Here's one write-up....

http://www.atmsite.org/contrib/JSAPP/wormgear/wormgear.html

Evan
08-05-2006, 10:04 AM
This is the fixture I used for gashing the worm wheel blank.

http://vts.bc.ca/pics/fixture1.jpg

It mounts on the cross slide and a fly cutter with a single point tool is used to cut the gashes. The fixture is mounted with a shim on one side to provide the correct helix angle for the teeth.

CCWKen
08-05-2006, 01:08 PM
Got a picture of the part? I've got some sort of clutched assembly with a handle that is NOS. I'm not sure what it's for though--Could be a South Bend. It was in a bunch of parts I got at auction. The gear looks like it's driven by a worm (large).

nheng
08-05-2006, 01:15 PM
... And if I'm going to get THAT lucky, I would just as soon save it for the big lottery win. :D

I've wondered how many others save themselves for the "Big" win ;)

If you decide you want to buy a part, you might try Joe at Plazamachinery in VT. He frequently parts out various machines. added - noticed that he currently has a number of parts listed for the 11" lathe but didn't see that specific one.

Den

LastOldDog
08-05-2006, 02:45 PM
This is the fixture I used for gashing the worm wheel blank.

http://vts.bc.ca/pics/fixture1.jpg

It mounts on the cross slide and a fly cutter with a single point tool is used to cut the gashes. The fixture is mounted with a shim on one side to provide the correct helix angle for the teeth.

Evan, nicely done. I suspect the spacing of the two pivot holes in the pawl as perhaps an increment of the spacing of the two threaded holes in the base plate hence providing 'split' locations, or four in this case. Clever ! This will accomodate several sizes of 'index' plates (gears).

An inspiratation of Pierre Vernier's contributions perhaps.
Lloyd

Evan
08-05-2006, 02:52 PM
[qiote]I suspect the spacing of the two pivot holes in the pawl as perhaps an increment of the spacing of the two threaded holes in the base plate hence providing 'split' locations,[/quote]

Yep.

[added]

The brass nut on top is cut from hex stock and has a threaded shaft with a dead center on the end that bears in the top of the shaft the blank rides on. I would have to photograph it further to really explain it but it is also used during the hobbing. By replacing the fly cutter with the hob in the lathe and disengaging the pawl perfect alignment is maintained to hob the teeth.

BadDog
08-05-2006, 02:56 PM
Thanks to all...

I also seem to remember someone figured out (and posted on one of the boards?) how to single point a worm gear using a boring bar between centers. Hmmm...


Evan: That fixture seems to require a gear with the correct number of teeth to use for indexing. In any case, I’ll probably just set it up on my rotary table as my Hartford Super Spacer has less than 36 divisions (24?). And of course, the rotab only indexes on 15* (like the Hartford) and not the 10* I need. <sigh>

Ken: I’ll get a picture shortly. But I’m sure it’s not what I need, that would be “winning the lottery” and I’ve never even won a raffle. ;)

Den: I keep trying to “save for the big win”, but it just doesn’t happen. :D And Joe was the first person I contacted. He’s got several previously good aprons he’s pulled apart for just that part.

Apparently these Rockwells will “eat” that worm gear if you let the carriage get “too stiff” and use the power feeds much. The shop I got it from NEVER cleaned anything and the table was VERY stiff from accumulated black tar like junk. I think the mess came from the mix of water based coolant and sulfur oil that they routinely swapped as needed (coolant tanks were 5 gal buckets with quick connects so easy to move/swap) without ever cleaning anything. When I looked at the machine, I was amazed at how filthy it looked, but how little wear there was on everything (critical) that I checked. But the carriage would barely move with the hand crank due to the mess on the ways, and I just figured that was because, as they said, it’s been used almost exclusively with the turret for many years (probably because the power feed was out?).

BadDog
08-05-2006, 03:12 PM
Here are the pics. First a side view showing the brass hub and it's internal cone. Second a view from above with a scale for size ref.

http://www.members.cox.net/thebaddog/rockwell/worm1.jpg
http://www.members.cox.net/thebaddog/rockwell/worm2.jpg

Evan
08-05-2006, 03:48 PM
The gashing doesn't need to be perfectly spaced. Index 30 degrees and do every third tooth. Remount the part and do the in between teeth. Do that again. Then hob it.

CCWKen
08-05-2006, 07:57 PM
Well, close but no cigar. :(

Here's what I have:

http://img.photobucket.com/albums/0903/CCWKen/Loads/2006_0805Shop0012.jpg

http://img.photobucket.com/albums/0903/CCWKen/Loads/2006_0805Shop0011.jpg

BadDog
08-05-2006, 08:29 PM
Shoot! And that looks pristine! So, no surprise, of course it's not the one I need... ;) Thanks for trying.

Looks like I may have a few more options. Talked to several people today and there are some reasonable priced gear cutters locally who can either cut it themselves or make a profile cutter for me realatively(?) cheaply. I'll get actual numbers on Monday. I've also been offered the loan of a proper indexing head to cut the teeth.

lazlo
08-05-2006, 09:15 PM
there are some reasonable priced gear cutters locally who can either cut it themselves or make a profile cutter for me realatively(?) cheaply.

I'd be interested to see what a machine shop will charge to cut that gear...

Have you looked at the Boston Gear and Stock Drive Products catalogs to see if they have that worm?
You could machine the brass hub seperately and braze it together, or even easier, machine off the worm
on the current hub and pin or braze a new worm onto it.

Just a thought...

Evan
08-05-2006, 09:22 PM
The only thing you need to know about the profile is the pressure angle if you intend to hob it. That can be deduced from the worm or even simpler, just make a single point tool to match the worm and use that to make the hob. The tooth profile of the hob is straight sided, same as an acme thread, and will generate the correct profile on the wheel as it is hobbed.

BadDog
08-05-2006, 09:27 PM
Yeah, I checked Boston Gear today. They don't show a single worm gear (that I saw) that even had 36 teeth, nothing even close to what I need.

I had considered machining off the gear ring and creating a "register" to locate another gear if one could be found, but it's a moot point if a gear can't be located. Plus there is not really enough material in the hub to provide a decent register to locate the gear during brazing unless the ID of the gear is turned so large that there is little material left at the tooth root. To get back that "meat", the inner cone would need to be turned down a bit at the major diameter. Once the local shops open again on Monday, I'll do some more checking, but I think that the result would be marginal.

JCHannum
08-05-2006, 10:03 PM
Duplicate the worm, and use that to make a hob. Carbon steel, case hardened with Kasenite or other case hardening compound will work just fine in cutting the brass gear as Sir John indicates. I have done a couple myself.

The worm quite likely is a profile very close to an Acme thread, if not an Acme. Simply grind a cutter to match that profile and thread the blank.

Once you have that, you can start a cottage industry making replacement gears for those less talented.

BadDog
08-05-2006, 10:21 PM
That "duplicate the worm" is basically exactly what I had in mind. But I've got a few more questions to throw out.

I've had several people suggest that a profile simple profile cutter can be made/used. Either single point, or looking like a profiled Woodruff Key cutter. I can see that it might not result in a “perfect” profile, but it seems to work and require a lot less effort. In fact, I had one seemingly very knowledgeable guy tell me that there is no way he would put the effort into creating a hobb unless he was making multiples. So, what’s the story on using a profile cutter to cut the wheel? If I’ve got to “gash” the blank anyway, why not cut the full tooth and then let the bronze self “lap” to the steel worm for any tiny inconsistency?

In that same vein, I think this part is Bronze, but I'm also told that brass will work fine for this part in a HSM atmosphere for a very long time. Thoughts? I've got a chunk of appropriately sized brass that I'm thinking of doing a test prototype on just to see how it all works out this weekend rather than waiting till Monday to get Bronze and/or check with local gear cutters/suppliers for other options...

Further more, since it will only cut a few bronze worm gears, does the hobb (should I go that route) really need to be hardened, case or otherwise?

John Stevenson
08-06-2006, 03:59 AM
BD,
To cut each tooth with a single point tool you will need to tilt the blank to the helix angle of the worm so the single point tool follows the correct path, secondly you will also need to bring the tool to the correct shape, i.e. an involute because the blank will be fixed, not rotating and so it can't generate the correct form.

The accuracy needed when gashing isn't important as you don't go full depth and the 'hobbing' process straightens out any errors.

The easiest way to do this in the home shop is to rough gash the wheel, make a hob and then free hob the blank by the time honoured methods above.

The hardest part is copying the worm. If they have cut corners when making it they will have cut a worm based on a thread, i.e. it's measured in Teeth per Inch, easily checked with a thread gauge.

If they have followed gearcutting practice they will have based it on Diametrical Pitch which isn't a solid TPI. So this is the first thing you need to know.

In making a hob you copy the worm exactly except for making the thread a little deeper for clearance .
So starting off with a HSS single point tool, ground on the bench grinder to 29 degrees, like an Acme, you cut a new piece of worm on a length of drill rod.
Once cut, gash a couple of flutes into it with the edges radial, watch out for direction of rotation, get it the correct hand, and then harden it.
Just heat to cherry red and quench, Don't bother with tempering as it's cutting in brass or bronze and you need a sharp edge.
In fact when you grind the front face the heat of the wheel will actually draw the temper in the teeth to about the right degree.

That's the hob done.

Now for the blank, turn up so that it can spin on the toolpost bolt or a jig as outlined in other peoples posts above.

Now you need to gash it, again some good ideas above but let me add the Stevo tried and tested dirty method {TM}

On a CAD system draw a ladder with 37 rungs, the width of the blank and Pi times D in length. You can even tilt the rungs to match the helix angle if you want, print this and cut out.
Wrap it round the blank and #1 and #37 should overlap, if not play with scale command and get a good fit.

Now glue this to the blank, with couple of saw blades in the same hacksaw frame to increase width, if needed, hold the blank in the vice and saw the gashes, not going too deep.
Transfer to the lathe and free hob the blank to depth using the hob.

Crude method but it works, it's quick and you don't need any specialist equipment like indexing heads.


Pay a lot of attention whilst doing this to the helix angles and hand of the hob, very easy to finish up with the wrong hand.

BadDog
08-07-2006, 12:24 PM
Thanks for the great description.

BTW, nobody commented on Brass vs. Bronze. And looking at the original, I'm starting to wonder if it IS Brass. It seems very yellow for Bronze...

BadDog
08-13-2006, 10:11 PM
Well, still don't know brass or bronze on the original gear, but I've produced the clutch with gear blank from Brass using my 9x20 Griz. Turned out rather well I think.

Unfortunately, I've now wasted a fair few hours trying to make a hob. This thing is roughly an acme thread profile at very close to 5 threads per inch according to my simple (not gauge) measurements. Best I could do was a borrowed "gear tooth gauge" matched on a gauge labeled "16", whatever the devil that indicates. Unfortunately, my lathe only goes down to 8 threads per inch without acquiring extra non-standard change gears. So I went down town to borrow time on a Moriseki or Leblond belonging to a friend, only to find out that the "5" on threading was metric, and both those big boys could only go down to 8 as equipped. We futzed around for a while and finally gave up...

Now I'm left with a quandary. I'm not even sure my 9x20 can cut an the profile in tool steel that size since it pretty much requires a plunge cut at 1.5" diameter. I'm starting to think I should just pay to have it cut, but that really goes against my "grain"... :(

Any suggestions before I go down tomorrow and see about having a gear shop do the cutting?

lazlo
08-14-2006, 10:10 AM
Thanks for the great description.

BTW, nobody commented on Brass vs. Bronze. And looking at the original, I'm starting to wonder if it IS Brass. It seems very yellow for Bronze...

BadDog, if you're going to go through all the trouble to cut this worm, I would use modern bearing bronze like SAE 660. It's pretty expensive at McMaster, but you can find drops on Ebay dirt cheap:

SAE 660 has excellent machining properties, good hardness, strength and wear resistance with excellent antifriction qualities. The alloy is not subject to dezincification and has reasonable corrosion resistance to seawater and brine making it suitable for pump and valve components.

SAE 660 is suitable for bearings, bushings having medium loads and speeds with adequate lubrications.



Unfortunately, I've now wasted a fair few hours trying to make a hob.

I wouldn't call that wasted -- you learned a lot making the hob, right?

Evan
08-14-2006, 11:40 AM
Why does it have to have the same number of TPI as the original? This isn't the threading drive is it?

BadDog
08-14-2006, 12:30 PM
Lazlo:
Your right, it wasn't wasted as I learned that I really LOVE running a big Moriseki. OMG! Now THAT is a lathe. I feel like a guy that was married to one of the more attractive farm girls from his local village. All is well and good, never realizing that she's really rather plain and not very bright. Then I went to the big city where I had a one night stand with a truly fantastic "playmate" who is both beautiful and intelligent. Home life will just never be the same... :o

Unfortunately, I did not get to make the hob, so no, I didn't learn anything "making the hob". :(

I've made a fair number of bearings/bushings from 660, but couldn't find a piece big enough at any of my local sources. The brass was "free" to try my hand at a prototype. That would have been perfect if I could make my own hob or otherwise cut the gear myself since I could always remake from bronze as needed, and it cost nothing but some time while gaining experience if I scrap my first gear. However, since it now appears I may be force to pay to have it cut, things have changed. Problem is, I can't be sure WHAT the original is made from. Will 660 bronze, being a "bearing" material and running *IN* oil work effectively as a "clutch" with the inner cone of cast iron (I think, may be steel?) I’ve drug along so long on this now, maybe the local suppliers have some stock now, if I need it.

Evan:
No, it's not the threading drive, only power feed. It has to be the same number of teeth because the worm is good and I don’t want to make one. Large internal bore, internal keyway, threaded on one end with a bearing, and then of course the worm itself. I have no shaper or other way to cut a very precise internal keyway some 4”+ in length, and filing is not too appealing. Based on how it’s wearing, it may well be hardened (case or otherwise). Could I make it if I had to? Sure. Though I would have to make a hand driven shaper or something for the keyway. But I would rather not as that just introduces another place thing for me to have to hope I get right on something I want to “just work” after I “finish” it. Working on the tool is not my interest, it’s a required investment to get to the things I’m interested in. Changing the gear will also change the feed rates, and since the problem is that I can’t cut below 8 threads per inch, then all my feeds would move slower by almost half. So I would say goodbye to roughing cuts under power, though it would have some glacially slow finishing cuts around 0.001 per rev...

John Stevenson
08-14-2006, 12:43 PM
BD,
all isn't lost let me get this stupid broken varispeed out the way and I'll reply tonight.

.

BadDog
08-14-2006, 12:54 PM
Sure John, all help is appreciated. :D

Hmmm, you know, since this is NOT the threading feed, I think I can cut 5 threads per inch on the Rockwell... Now to verify that, I have to put my crappy scored up cross slide back together along with the apron minus the power feed. Then run down to the shop where I was at yesterday to get the piece of tool steel we were going to make the hob out of. Turn it here, then go back down town to heat treat it. Then disassemble the carriage and cross slide again to mill, grind, and scrape it (along with Turcite or Moglice?) to fix the scoring, then reassemble again with the repaired ways and gear...

<sigh>

Most likely to find out that I did something wrong and have to do something AGAIN, but at least it's a plan of action and moving forward...

lazlo
08-14-2006, 02:01 PM
I've made a fair number of bearings/bushings from 660, but couldn't find a piece big enough at any of my local sources.

How big of a piece do you need? I can check and see what I have...

I've got a bunch of aluminum bronze as well, but that's probably too hard...

Evan
08-14-2006, 02:06 PM
What's the OD and ID on that? I can send you a chunk of this if it will work.

http://vts.bc.ca/pics/bronze1.jpg

BadDog
08-14-2006, 02:06 PM
I'll have to measure again, but it's roughly 2.5" dia x 2" long with ID of exactly 1". I also need to check local sources again since it's been about 2 weeks since I first checked...

BadDog
08-14-2006, 02:16 PM
Just checked, they now have it in stock, but WOW it's gone up. $75 for a 13" piece cored 1", $78 not cored...

kap pullen
08-14-2006, 03:31 PM
Here is a thread from the pm website.

http://www.practicalmachinist.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=1;t=010449;p=0

These guys did these jobs with cnc, but with time and patience it could be dome on a manual lathe as well.

I would look for an acme tap, run it between centers and cut each tooth using the lead screw for feed.

Some type of indexing mechanism would have to be built to hold the gear.

This thread is a keeper.

Kap