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View Full Version : Explain to me about dividing heads vs rotary tables please



brian Rupnow
09-16-2016, 12:44 PM
When I first started machining 6 or 7 years ago, I bought a rotary table which came with a set of divider plates. I mounted a 3 jaw chuck on it, and have since built numerous spur gears and used it for putting a radius on the corner of plates, connecting rods, etcetera. It is a good rotary table, in the sense that it has two machined mounting faces, so it can be mounted with the center of the 3 jaw chuck mounted either vertically or horizontally. Now I am seeing posts on this forum which talk about dividing heads. I don't know much about dividing heads, and I would like to know where they would be used and what makes them in any way "better" or "worse" than the set-up that I have. Can someone enlighten me please.---Brian

Errol Groff
09-16-2016, 12:54 PM
Here is a link to the manual for an Enco brand dividing head. Pretty good info in it.

http://neme-s.org/Shaper%20Books/Enco%20Dividing%20Head

Mcgyver
09-16-2016, 01:05 PM
A smallish RT that can sit on its side and takes plates can do pretty much anything you'd want....for typical work. There are a couple of reasons why separate is preferred, how much stock one places in these will of course vary. I have seperate and I like the no compromise.

My RT is 12" and only sits horizontal. On its side there would possibly be clearance issues in use. A large RT is a heck of lot nicer than a small one - that would be the most important consideration imo and biggest compromise of a smaller dia combined unit. Mine also has full flood coolant channeling around the the outside, something you may have to give up on the vertical/horizontal style, I like and use flood.

Next biggest compromise would be angles. Dividing heads pivot, and this is feature that does get used. Either that's a bunch of messing about or very expensive RT to be able to index with the work at an angle

Finally, for some dividing head work, differential, you need to gear the spindle....and for some other you need to hook the gear train up to the table feed (milling helixes, hobbing etc). These are specialized enough jobs that most/many would never call upon them in a lifetime, and their are electronic alternatives (ala Mr Stevenson) but if we' inventorying differences....

With cnc and digital division, there is less and less call for some of these tools and their most specialized capabilities. If I ever get my cnc going, not sure when I'd use the RT....have a vmc at work, and it makes an RT obsolete. With a cnc, I'd probably forgo the RT and stick with a dividing head.

Errol Groff
09-16-2016, 02:17 PM
At P&W Aircraft I did work on a 36" tilting RT. Is that what you meant by nicer? Ho Ho!

Errol

Mcgyver
09-16-2016, 02:22 PM
At P&W Aircraft I did work on a 36" tilting RT. Is that what you meant by nicer? Ho Ho!

Errol

what i was getting at was that in a home shop context, 12" is a lot more functional to work with than say 4 or 6 ......as an RT, ie bolting stuffing to it. I know tilting RT's are availabe but again would expect large dia tilting RT's to be less prevalent in the HS

brian Rupnow
09-16-2016, 03:40 PM
I THINK that probably the dividing head that can be tilted is how you would grind a relief onto a milling cutter or a tool like the George Britnell valve seat cutter. I'm not sure of that, I have a hard time getting my head around it, but I have never figured out a way to do it with my rotary table.

Mcgyver
09-16-2016, 04:21 PM
I THINK that probably the dividing head that can be tilted is how you would grind a relief onto a milling cutter or a tool like the George Britnell valve seat cutter.

lots of creative ways it could be done, but easiest and the tool for the job is a work head on a T&CG - shown top left of the pic below. It'll space the teeth and is a lot lighter than a dividing head and is to set angles in two directions. Can be used with a tooth rest and/or they'll usually have basic direct division like a spacer

http://www.cinmac.com/CutterToolGrinder/StandAccessor.jpg

Paul Alciatore
09-16-2016, 04:54 PM
Dividing heads and rotary tables both use the same, basic worm mechanism to divide the circle into specific angles or a given number of divisions. But they differ in the way they are designed for use.

Dividing heads generally assume you are trying to divide that circle into a desired number of equal divisions. So, they usually do not have any angular scales where you can move a given number of degrees. Instead, they almost always have "circle plates" which facilitate moving the part by the necessary angles to go from one position to the next in that equal division. So they are good for bolt hole circles, gears, and other features that require equal divisions around the circle. They can do other things, but that is where they shine.

Dividing heads generally hold the work with a chuck or between centers. They usually do not have a face plate style attachment and when fitted with one, it is generally small. Many dividing heads can also be adjusted to any angle above the table that they sit on. So you can cut facets or teeth at any angle to the axis of rotation. This makes them more versatile than RTs.

Rotary tables are intended to move in degrees, minutes, and seconds. So if you need to rotate a part by a given angle, they are the choice. They also have a table by default so parts that are more suited to being clamped down are at home here. I have a clamp set that I purchased just for use on my RT. They usually have a central hole that can be fitted with a center or a taper mounted chuck or a chuck can be clamped directly to the table (I do that).

Dividing a circle into equal intervals with a RT is more tedious and prone to error using the angle scales. But they can be fitted with the same style hole circle plates that are used on dividing heads for faster dividing operations. Usually they are limited to mounting at only two angles, vertical and horizontal, so cutting features at other angles is more of a challenge. But they are the first choice if you are trying to cut a arc feature or slot. They will also handle larger parts easier. In the horizontal position you can easily mount a part that is 25%, 50%, or even more larger than the table's diameter. It simply hangs over the edge. When mounted vertically this still applies but the amount of offset for that mounting surface limits the size of the parts as they will hit the table.

A tail stock can be used with either of them for longer parts or mandrels.

One further consideration, and some may disagree, but, in a given price range, RTs are generally larger and that can translate into greater angular accuracy.

firbikrhd1
09-16-2016, 06:42 PM
As an amateur my thoughts probably don't mean much, but I'll offer them up for criticism.

IMHO, a horizontal vertical rotary table with dividing plates, sector arms and a tailstock can do pretty much everything a semi-universal dividing head can do in horizontal position, and with about the same ease. Where a dividing head earns gains the advantage is in the ability to pivot at any angle from horizontal to vertical which makes cutting when angular divisions are needed easier than if a rotary table is used. For such cuts a rotary table could probably be used but would require the addition of an adjustable angle plate, however obtaining the correct angle would be more problematic and head room under the mill spindle is compromised. Such an application might be a bevel gear. A universal dividing head takes things to another level in that it allows spiral cuts to be made so spiral gears, spiral bevel gears and helical flutes can be cut because the universal dividing head can be geared to the movement of the mill table or universal mill table with an included set of change gears.

Surely some of the folks that make their living as machinists will chime in and correct any mis-impressions I have given.

brian Rupnow
09-16-2016, 08:49 PM
This is one of the amazing valve seat cutting tools, originally designed by George Britnell, for cutting the valve seats in model engines. I have made two or three of these, in varying sizes, to suit the engines I have built. there is supposed to be a back relief on the four cutting edges. I have stoned a relief on the cutters I make, because as far as I am able to tell, there is no way to machine that relief in with my conventional rotary table. I THINK that with a pivoting dividing head you can do this, but the geometry involved makes my head hurt. this is one of those situations where I think I'm right, but I just can't visualize it.
http://i307.photobucket.com/albums/nn294/BrianRupnow/CONTINUATION%20OF%20MAIN%20ALBUM1/ALBUM%20THREE/VALVE%20SEAT%20CUTTER-%20FOR%20.375%20VALVE%20WITH%20.125%20SHANK_zps6db ks9je.jpg (http://s307.photobucket.com/user/BrianRupnow/media/CONTINUATION%20OF%20MAIN%20ALBUM1/ALBUM%20THREE/VALVE%20SEAT%20CUTTER-%20FOR%20.375%20VALVE%20WITH%20.125%20SHANK_zps6db ks9je.jpg.html)

BCRider
09-16-2016, 09:29 PM
Brian, try to picture the following or even try for yourself with some scrap. Set up the three jaw for cutting such a tapered shape. Use a stop rod up from the rear of the head stock spindle so the work cannot seat deeper for the next operations. Now slip a shim of some thickness under one of the jaws so the shaft turns with an offset and recut a skim cut such that it affects only the "highest" 1/3 of the shape's runout. Without rotating the piece move the shim to the next jaw and repeat. Then the third jaw.

What you should have is your tapered shape with three curved facets to the conical portion that meet at three "high" lines. If you then cut the front clearance such that the cut ends just past these high lines and is deep enough that it removes the "rise" leading up to the cutting edge you're forming you will have your clearance actually machined in.

That make sense?

You can also do this with a four jaw but in that case you'd simply alter the jaws to produce the desired off set on each of the four "peaks" and zero out the two adjacent jaws.

JCHannum
09-16-2016, 10:06 PM
I have made similar cutters for valve seats and various form tools as well as taps and reamers. I use a 5C collet block for 2, 4 or 6 flutes depending on cutter. I mark the cutting edge with Dykem or magic marker. I file the back relief at about a 5* angle, leaving a couple of thousandths of marker visible. Harden, draw and touch up the cutting edge with a fine stone.

Paul Alciatore
09-17-2016, 01:49 AM
This job seems to call for the dividing head instead of the RT. I would:

1. Mount the dividing head on the mill table and set the angle of it's axis to produce the angle needed for the cutting edge. This may be complicated by the steps below and some 3D trig may be needed.

2. Mount the blank which has been turned to the outline and milled to produce the rake surfaces that you show in your drawing. In other words, everything except the clearance behind the cutting edge.

3. Instead of mounting the blank with the cutting edge exactly vertical, rotate it a few degrees in the direction it will rotate when in use so that the area a bit behind the cutting edge is at the top/center position.

4. Mill down to the cutting edge, creating the clearance behind the cutting edge. Index and do this for the other three(?) cutting edges.

5. You may have to rotate the cutter somewhat more and mill a secondary relief facet behind each cutting edge. And perhaps a third and even a fourth to continue that clearance to the next flute.

What I said about the angle in step 1 being somewhat complicated is because when you rotate the cutter by an additional amount, the angle of the already formed cutting edge will change. This can be worked out if you know the original, desired angle and the angle that you rotate it by. I could help you with that calculation if I had the specifics of the cutter. OR, you could just wing it. Set the axis angle to the desired angle and make careful trial cuts until you get close to the original cutting edge. Then make fine adjustments to the angle and keep cutting carefully until the cut line matches the original edge. Record your setting for future use.

It could be done with a RT, but getting that angle just right would be a lot more trouble. An adjustable angle support for the RT would be the best approach and it would need an adjustment range of over 45 degrees, depending on the angle of your cutter's edges. If I had the RT and the dividing head sitting there, I would grab the dividing head for this, in a flash.




This is one of the amazing valve seat cutting tools, originally designed by George Britnell, for cutting the valve seats in model engines. I have made two or three of these, in varying sizes, to suit the engines I have built. there is supposed to be a back relief on the four cutting edges. I have stoned a relief on the cutters I make, because as far as I am able to tell, there is no way to machine that relief in with my conventional rotary table. I THINK that with a pivoting dividing head you can do this, but the geometry involved makes my head hurt. this is one of those situations where I think I'm right, but I just can't visualize it.
http://i307.photobucket.com/albums/nn294/BrianRupnow/CONTINUATION%20OF%20MAIN%20ALBUM1/ALBUM%20THREE/VALVE%20SEAT%20CUTTER-%20FOR%20.375%20VALVE%20WITH%20.125%20SHANK_zps6db ks9je.jpg (http://s307.photobucket.com/user/BrianRupnow/media/CONTINUATION%20OF%20MAIN%20ALBUM1/ALBUM%20THREE/VALVE%20SEAT%20CUTTER-%20FOR%20.375%20VALVE%20WITH%20.125%20SHANK_zps6db ks9je.jpg.html)

Mcgyver
09-17-2016, 09:30 AM
This is one of the amazing valve seat cutting tools, originally designed by George Britnell, for cutting the valve seats in model engines. I have made two or three of these, in varying sizes, to suit the engines I have built. there is supposed to be a back relief on the four cutting edges. I have stoned a relief on the cutters I make, because as far as I am able to tell, there is no way to machine that relief in with my conventional rotary table.

I get what you are after now, how to do the relief behind the cutter.

The "proper" way is with a relieving attachment in lathe. Some of the great old lathes had them as accessories. This is why guys make the "Eureka" relieving tool - google it, there's some good videos of it inaction. After hardening, you grind the face (where the rake is) if the cutting tool to sharpen using the work head etc.

That's the way if you really want some quality made cutters.

However, the vast majority of us (I speculate), myself included, don't have relieving tools and are making cutters for one-of/low volume solutions. SOP is to leave a small land behind the cutting edge. This surface does not have relief and is necessary to define the cutting edge. It does rub against the work. You can file so its minimal, it has to be quite small or the tool won't cut well. While less than ideal, good work stills gets the done. The geometry is dead simple.....RT, dividing head or super low tech like below - a collet block. you could even turn the work slightly a put a few facets on for a stronger cutter. This will produce the cut outs shown in your rendering

http://i.imgur.com/9PzJzFn.jpg

http://i.imgur.com/dnbpbxL.jpg

Mcgyver
09-17-2016, 09:31 AM
duplicate post