Building a 120mm rotary table

[arnoldb]

Thank you Zee - I'd get fired in a production environment though; I work waay too slow for that :big:

Dean, thank you. Yes, I'll lap the mating surfaces - not to a mirror finish though; only till operation is smooth. The fine "grooves" from facing the top of the body will help with oil retention.
I do like single pointing; its extremely satisfying to end up with a good thread. Many people dread single pointing, but I think its one of those jobs where its mind over matter - and a couple of simple rules to follow. Maybe I must post a full write-up on the way I do it at some point; any takers out there ?

Thanks Phil ;D

Helder, thank you. It's an angular contact bearing. It is mounted the correct way around as shown but maybe counter intuitive without the rest of the assembly. The outer race will be pressed into the top of the base from the bottom (underside). When the table is then installed from the top, the nut is used to screw up against the inner race of the bearing to pre-load it - This should become clearer when I get to the point when I start to fit things together.

Today's little bits...

First thing, I wanted to get the top plate of the base bolted to the frame; I'd marked it out already for the bolt holes, so I just needed a way to make the whole lot stay together for drilling and so on. I sawed two longish strips of 20x5mm flat bar off a length I keep handy for incidental needs such as this. Both strips were drilled for clearance holes for some 6mm cap screws, and then the top plate and frame were bolted together like this:

The bottom strip of plate is through the mounting slots of the base.

Then I clamped the whole lot down square on the mill table; there is a clamp on the hidden side in the other groove of the mill table:

I forgot to put a piece of paper below it....

Then I started drilling all the holes. Even though I had laid out the hole locations, I decided to go for X and Y coordinates using the handwheel calibrations - as each hole had to be center drilled, then tap size, for clearance trough the top plate and finally counterbored for recessing the M5 cap screws I would be using. So I located the edges of the right front corner the primitive way; with a bit of 6mm silver steel chucked and a piece of paper. (Aside - I've had some great news today; I will be the owner of a proper edge finder soon ;D - THANKS MATE!) Then I started counting turns and reading handwheels while center drilling each hole, and jotting down the figures on a bit of paper. Here all the holes are drilled to tap size (4.2mm):

That lot was followed by a 5mm drill just deep enough to provide clearance through the top plate. The heads of the cap screws measured out at 8.4mm in diameter and just below 5mm high, so I used a 9mm mill to counterbore the clearance holes 5mm deep. My advanced lubrication delivery system is the orangey bottle in the right of the photo :big: :

With all the holes I needed to tap full of swarf from the counter boring, I used a drill chuck with a 4mm drill to manually clean out the holes. Just plonked the lot on its side, and by hand turned the drill in each hole to get the swarf out. Photo without the hand that should be turning:

Then I sat down on the bar stool I keep handy (my "working table" is a bit high), and tapped each hole. The 5 mm clearance holes in the top plate are excellent tap guides to keep things square when starting with the first tap, so nothing fancy required as guide. Just manual work ;D. I'm due for a new set of M5 taps though; I could feel these starting to struggle, but they have tapped many tens of holes in the last two years... All bolted together:

Things went well didn't it ??? - Well NO - If you look carefully at the left hand hole in the top row in the picture, you'll just see the mark where the end mill wandered when the lot came loose on the mill table. Remember I said I didn't add the piece of paper below when clamping ? :wall: Fortunately I could recover; and another mark on the table to serve as a reminder... I think I can still hear some bad words echoing around the shop; fortunately Shrek was out of earshot!

With the base finally together, I started on the gear. A slightly oversize chunk of the phosphor bronze getting sawn off:

Then faced and bored in the 3-jaw:

Next a section of the left-overs from that old big bolt was turned down for a press fit in the bore in the PB ring:

Then I sawed off the turned section of steel, and dumped it in my freezer on top of some ice for 10 minutes - it was nice and cold then ;D
It pressed into the ring in the bench vise without too much effort, enough to give a good press fit, but not so much that the PB ring would crack:

Some more turning, and I have the makings of a gear:

I need to double check some calculations now, and run through the needed steps in hobbing the gear in my mind, so updates might be a bit slow for the next couple of days. Think thrice - machine once...

Regards, Arnold

 

[Deanofid]

I don't see the end mill marks, Arnold, but I'm not looking for mistakes. I'm looking for success!

For the lapping bit, I didn't make mine shiny either. Just smooth, although it has a gray appearance. Doesn't matter, as it's out of sight under the table.

That's a nice chunk of bronze you have there. Will be good to see it being turned into a worm gear. Are you making the worm, too?

Dean

 

[keys]

Is the paper under the piece to keep the part from sipping? Is that something that should be done to the vice as well?

 

[arnoldb]

Dean thanks. I thought as much on the lapping; it should end up with a dull grey appearance. And Yes ;D I'll be making the worm as well as a hob.

Justin, yes the paper is to keep it from slipping. I don't think it will be needed below your vice, as it has a much bigger contact area; as long as you bolt it down well. It won't do any harm though; just squirt the paper with oil if you put it there for extended periods; to keep things from rusting where you can't see. These are just my thoughts though; some more experienced people might disagree with me; in that case I'd be happy to be put right.

Today's bit; Dean was reading my mind ;D. I need the hob before I can cut the gear, and while making the hob I can just as well make the worm. Attached is an Excel spreadsheet I set up quickly to do some calculations from information I drew off the Internet. An interesting thing I found was "crowning" of worm wheels to make sure that the worm/wheel combination can stay properly lubricated. The crowning entails cutting the gear teeth with a slightly bigger hob than the worm would be. Just for the heck of it (nothing like learning from testing) I'll be doing this.

There is a good amount of information on gear calculations here on HMEM as well as on the web, so I won't go into detail except for that my original planning was for a 72 tooth MOD 1 wheel with a 20 degree pressure angle. Screwcutting for MOD 1 is pretty close to 8 TPI - and bliss; my lathe has an 8 TPI leadscrew. 8 TPI calculates back to about MOD 1.01 - so that's fine with me - I'm lazy to set up weird & wonderful combinations on the change wheels, and this is one of the easiest to set up.

First up, I needed a toolbit with the correct shape; a point with a 40 degree included angle; clearance for helix angle, and so on. I marked out an 8mm bit of HSS for the angled tip using a protractor:

A couple of minutes (well, about 20) on the bench grinder, and then some manual work on the oilstone, and I ended up with this:

In the first of the above photos, you can clearly see the the bottom of the bit is at an angle to the top; not straight down. This is for adequate clearance of the helix angle; I did not calculate that or anything; just ground it on by gut feel. Also, you will notice I didn't touch the top of the toolbit with the grinder or oilstone; the original ink I marked out with is still on there.

A trial run to measure sizes on the worm-in-making from some 16mm silver steel. I added a lot of run-out space at the headstock side, as this is a fairly coarse thread and the space is needed to stop. I also have the topslide set parallel to the work and locked, as I might have to add some additional side-feed with it to make cutting easier if it becomes too difficult. (That's frowned on by screw-cutting purists, but have stood me in good stead on large threads like these in the past):

I stopped with the threads half-done for tonight. Just jotted down the necessary readings from the dials so I can continue from there when next I have shop time:

And yes, it's slow going; I don't have space to add tailstock support, so I go in 2.5 thou infeed steps with a repeat pass at the same depth after every 10 thou.

Regards, Arnold

View attachment GearCalc.xls

 

[Philjoe5]

Arnold,
Thanks for documenting the making of a hob and gear. :bow: Though I have no immediate need for it, it's something on my to-do list at some point. I've taken a seat right near the front of the class since I don't want to miss anything :

Cheers,
Phil

 

[Deanofid]

Good start on it, Arnold. Looking good!
Dean

 

[arnoldb]

Phil & Dean, Thank you ;D

Phil, I hope I don't set a bad example though; this is my very first attempt at something like this - I'm purely putting theory from a lot of research into practice. There might be gotcha's somewhere - if so, I'll be sure to post that as well.

I've had a slow couple of days in the shop; had to attend to domestic and social duties for a change...

This afternoon I finished off the worm; it was a simple process of finishing of the threading, drilling it through and reaming for an 8mm shaft. I then parted it off at length leaving a shoulder, in which I cross drilled and tapped for an M4 grub screw (set screw). All done:

Next I started on the hob - after honing the edges of the toolbit again. The threading process is exactly the same process as for the worm, but the hob is 1mm larger in diameter as I explained in a previous post. I didn't take photos of the threading done on it; except for a close-up of a bit of swarf that came off while I was turning it. Why? - the swarf shows that the toolbit was cutting properly on all sides and the front:

Today's progress. When I have shop time again, I'll mill teeth and some clearance into the hob:

Regards, Arnold

 

[Deanofid]

Good stuff, Arnold. That worm looks about as good as any I've seen from the factory. Very pro!

Dean

 

[Philjoe5]

Nice looking worm Arnold. :bow: Thanks for the update

Cheers,
Phil

 

[bearcar1]

Good heavens Arnold! I did not notice this thread, hmmm. Anyway, WOW! What a great start, I fully appreciate the work you have put into making that gear hob and cutting those threads, well done indeed!

BC1
Jim

 

[arnoldb]

Dean, Phil & Jim, thanks guys ;D
I'm normally very critical of my own work, but for once I even admitted to myself that I'm happy with how the worm turned out. Not perfect, but I'm happy.

This afternoon I milled the cutting teeth into the hob; no fancy setups for indexing; I just judged by eye for indexing, but I took pains to make sure I got the cutting edges dead on center, and triple-checked that I was cutting the teeth for right-hand rotation of the hob when in use. ( sometimes I learn from other's "oopses" )

Instead of milling clearance behind the "teeth", I just took a file to it; 5 minutes in all, and I had some clearance behind the teeth all round. I didn't want to take away too much for clearance, as the hob will be used to auto-rotate the gear blank when I eventually get to cut it. I didn't bother de-burring anything either; the heat treatment will get rid of some burrs, and once hardened, I'll give the cutting edges a once-over with the Dremel with a grinding stone to really sharpen them up - and should remove any left over bits of burr. In this photo you can see how I filed clearance to just behind the tips of the teeth - there's a little facet left just on top of the tooth cutting edge that I did not touch with the file:

Another view of the "profile" - as one can see, my "by-eye" indexing was not spot on :big: :

Regards, Arnold

 

[Artie]

Hey Arnold, intensley interested in how this is going to go.... just stripped a worm gear on my lathe so this is right up there in the 'how to' thanks mate looking great!

Rob

 

[Maryak]

Arnold,

Nice spiral hob. :bow: :bow:

After milling the teeth, I found the only way to get the profile correct and remove the milling burrs was to reset it up in the lathe and lightly skim the grooves or for you, lightly skim the thread. When I was happy with the profile then on to the heat treat.

Hope this helps

Best Regards
Bob

 

[arnoldb]

Thanks Rob - its a pleasure ;D

Bob, thank you ;D I thought about skimming the threads again, but I chickened out on that one... I've "picked up" partially turned threads in the past, but with this one I was hesitant, as I do not know how my lathe would handle a picked up and interrupted thread cut... I ended up cleaning the worst of the milling burs with a needle file - tooth by tooth :

Today's bit involved more thinking than working. Like mentioned, I cleaned the milling burrs off the hob, and then hardened it. This is a fairly big bit of metal, so I went outside and heated it with my butane torch (took quite a while to get it to temperature). When I thought it was about right, I heated it some more, to allow me enough time to turn the torch off first and then plunge in the oil bucket. This is for safety - I don't allow any flames (not even a lit cigarette) when I do an oil dunk, as the smoke coming off is potentially very explosive - and yes - I am positioned between a fire extinguisher and the dunk site to allow me to pick up the extinguisher while departing a possible fire. If you have the luxury of choice, rather use water hardening than oil hardening silver steel.
Once cooled, I carefully hand-ground the cutting faces with the Dremel and a small green grinding wheel. Everything feels nice and sharp, and the burrs are gone completely:

I'm not going to temper the hob; I think it will do just fine on the phosphor bronze wheel as-is.

Next I turned down and bored out the gear blank to size - well on the OD at least. I miscalculated the last feed while boring, and ended up with the hole 0.1mm over size :wall: Fortunately, I can still compensate for that mistake, as I haven't turned down the shaft yet for mounting the gear:

The gear is too small to hold in the chuck for gashing (well, either that or my slitting saws are too big :big, so I turned a mandrel for mounting it further away:

The mandrel was turned in my 3-jaw and never removed - the gear blank was just mounted on it, and the chuck taken off the lathe.

Then the thinking started... How was I going to mount my Myford dividing head on the mill scratch.gif The T slots are not compatible, I needed a way to tilt it (rather than tilting the mill head, which I spent a LOT of time recently tramming in) and also get adequate workspace. I ended up choosing my lathe's swiveling vertical slide to mount the DH on - that will allow me to set the angles needed. I set everything loosely in place on the mill table to see if I could get adequate clearances everywhere and be able to cut some chips without running out of travel:

The last challenge was mounting the vertical slide to the mill table. I nearly started cutting metal to make new T-nuts and so on, when I noticed the cross-slide extension I made for the lathe about 2 years ago. Some checking followed; and YES! - I can clamp it to the mill table to mount the vertical slide on. The completed assembly looks like the cobbled together solution that it is, but it should work:

That's where I stopped; I'll do the gashing when I'm fresh tomorrow morning; lot's of dividing to do.

Anybody with sharp eyes might have noticed I didn't turn a radius on the gear blank - I just noticed it now while posting - I'll either have to take the chuck back to the lathe to do it, or use an end mill to make it...

Regards, Arnold

 

[Deanofid]

Good thinking for your setup, Arnold. You give a lesson in using what you have!

After the blank is gashed, will the cutter rotate the blank around as it's cutting the teeth?

Dean

 

[arnoldb]

Thanks Dean ;D

After the blank is gashed, will the cutter rotate the blank around as it's cutting the teeth?

;D ;D ;D Yes that was the plan, and it DID ;D ;D ;D

I'm a lazybones; normally I sleep till 9 or 10 am on a Sunday. This morning I awoke at 6 am with the same feeling of anticipation as a 6 year old on his birthday, and just could not get back to sleep... Once in a rare while I get days like this, and I cherish them; things were going to be good.

When I heard all the neighbours were awake I headed for the shop.

The radius I forgot to turn yesterday was just over 10mm to a depth of 1mm - I used a 10mm slot mill to make it, hoping that the tendancy for slot mills to cut slightly bigger than their stated size would do the job. I just cranked the dividing head through 3 full 60 turn revolutions - first infeed cut 0.5mm deep, second as well, and for the third one left as-is to do a final cleanup. Light feeds, as the setup most definitely is not as rigid as one would want:

Next I set the whole lot to the needed 4.5 degree angle from horizontal. This meant my center reference to the blank was gone. Measuring things were difficult - well pretty much impossible - but fortunately I did zero the mill on all axes before changing the angle. I was able to calculate the new "center cutting spot" from those with a bit of trigonometry, and dialed in the differences as appropriate. I then spent more time checking that I did indeed change the angle in the correct direction - upwards vs downwards, and that I didn't make a mistake in my calculations. My dividing head is based on a 60 tooth worm, so some calculations and it turned out I needed to stop on every 35th hole on the 42 hole plate to make a 72 tooth gear.

I changed plates (rotten luck; of my 2 plates the wrong one was on :big and set the index fingers on the DH and started slitting. One round of 0.1 mm deep slits to check:

I even counted the "teeth" to make sure I had 72!

Satisfied with the above, I repeated a full rotation of slits to a depth of 2.5mm:

I have 2 slitting saws; the 1mm thick one I used above, and a 0.5mm thick one of the same dimensions (brand new; never been used before). When I checked visually with the hob against the slits, I wasn't sure that it would have enough depth to start auto-rotating the blank once I got to hobbing. So I added the second slitting saw to the mandrel - without removing the mandrel from the collet chuck - and then lifted the Z feed by 0.25mm to account for the "new" total thickness of 1.5mm. Then I ran a full revolution again slitting only 1.2mm deep with the thicker combined saw. If you look carefully, you can see the additional cut in the grooves:

With the blank now slit, I needed a way to hob it. I have a bunch of old bearings I get from a local auto-electrician for free - and I selected 2 of the same size with a slightly larger ID than the gear blank - but that were still "sort of OK". Then I dug around for more bits and bobs, and once found, I turned a mandrel to suit the bits 'n bobs, gear blank and bearings from some HRS rod.

The Bits 'n Bobs mentioned is a block of brown stuff... My metals are too precious to waste on a once-off use like this, so wood it will be. With the 4-jaw still occupied by the table-in-making, the wood "jumped" onto the face plate after some persuasion. I then started boring out a pocket to fit a bearing in - after center drilling and drilling a 6mm hole right through the block:

Once done, I flipped it around, and used the mentioned drilled hole to set the block on center again and re-attach it to the face plate:

I then bored a pocket for the other bearing, and also cleared out the center bit to allow the mandrel to pass through:

"Gotcha!"s and notes at this point...
1: A note: I made the mandrel so that it would pre-load the bearings while tightening down the gear blank on its end. I don't have a picture, but if anybody wants one, I'll be happy to make up a quick C-o-C of what I mean.
2: A Gotcha: When I drilled clearance for the center hole in the wood block, I forgot to make it big enough to clear the bearings' inner races. When I fit everything together and tightened up to pre-load, everything froze up. I ended up fashioning clearance for the inner races with the Dremel and a smallish routing bit. (Too much work to re-setup everything on the face plate!)
3: Another Note - the method I used is to make this "jig" is VERY crude; it worked for me - I think primarily because I did take the time to make sure both faces of the wood block were parallel, and one side (that was then marked as a reference side) was square to the faces.
4: A Gotcha: Cleaning wood off a well-oiled lathe is a bugger :big:

I then clamped the wood block with bearings et al on the mill, standing on the mentioned "reference" side. then with the hob in a collet, I advanced on the Y axis, turning the mill chuck by hand until I could see that everything would mesh, and the blank would auto-rotate. Then I set the mill to it's slowest speed, held my breath and started up. I nearly fainted from holding that breath; things were going really slowly :big:, but the wheel started revolving, and bits of swarf started appearing. I made a mark on the blank with a permanent marker so I could judge progress around, and I slowly started feeding in 0.1mm for every revolution of the blank. after about 5 passes, I stopped, selected the next higher speed on the mill, and started off again; a bit quicker:

I can scarcely describe the feeling of immense gratification (and joy!) of the proper profile slowly appearing on that wheel ;D
The end result - fresh off the mill:

Sorry; the photo is a bit out of focus; its pretty hard to try and take a photo of this!. As you can see, the profile is slightly offset to the right -but it will do for me for now.

The worm meshes with the wheel beyond my expectations:

I then chamfered the edges of the wheel:

Another note: From the above I obviously gashed the teeth too deeply with the slitting saw, but that was the only way I could ensure proper meshing for auto-rotation. I think this can be eliminated by using a much smaller slitting saw - or even better, a little cutter made up to the same OD as the hob with some taper on it's edges.

Having said that though; I'm over the moon - I've never ever made a gear, and for a first foray into gear cutting, this came out MUCH better than I expected - especially for a worm & wheel setup ;D ;D ;D ;D (Can anyone tell I'm happy :big

Regards, Arnold

 

[RobWilson]

Hi Arnold :bow: :bow: :bow: :bow: :bow: :bow: :bow: great job , turned out spot on Thm:

Regards Rob

 

[SBWHART]

Great stuff Arnold

I could see where you where going with the wheel its a method I've seen done many times before, never actually done it myself, but I was keeping my fingers crossed for you all the way.

Real nice work and very well shown.

Stew

 

[Maryak]

Arnold,

Congratulations that's a great way to worm yourself out of a difficult job :bow: :bow: :bow: :bow:

Best Regards
Bob

 

[Deanofid]

Splendid work, Arnold!
Watching your steps from the beginning of this gear cutting exercise a few days back, and to this completion, and knowing the direction you were headed, I had no doubts about your success, but still feel the excitement radiating from your shop.

As you can see, the profile is slightly offset to the right -but it will do for me for now.

Do you mean the cut is slightly more on one side of the blank? A very small amount, if that's what you're referring to. Won't make a whit of difference, anyway. The worm doesn't know.

I've never ever made a gear, and for a first foray into gear cutting...

No one makes a worm and wheel as their first gears, Arnold. I don't know what you do in "real life", but you are a natural born machinist. Such a canny fellow!

Really impressed, and happy for your accomplishment,

Dean