View Full Version : Super-spacer vs dividing head - dumb question of the week

12-04-2005, 03:17 PM
I want to get good indexing capability, such as what one would use for gears. I also expect sometime in the future to either upgrade my Bridgeport to CNC, or sell it and get a CNC machine of similar size.

So, given that background, I'm in the market for either:

1. A dividing head/tailstock combo.
2. A "super-spacer"/tailstock combo.

Preferably, whatever I wind up with will have taper and/or nose threads that can work with some of the stuff I already have. (MT2 tapers, 5C collets, R8 collets, SB9 spindle nose) I may not get all that versatility, but the more the better.

I'm mostly hung up on the difference between a super-spacer and a dividing head. The DH uses those plates and turn-counting to get the correct angle, and the superspacer presumably uses dials calibrated in degrees and minutes, like a rotary table.

How are these two different in design and intended use? How is a super-spacer different from a horizontal rotary table? (a horizontal rotary table is one with the axis of rotation parallel to the floor, right?)

Also, I've read a little that rotary tables are falling out of favor in the CNC world. Is CNC curve and circle work that good? As in, can a typical CNC setup have no need for any kind of rotary table?

As always, thanks for any info.


Michael Moore
12-04-2005, 04:47 PM
Hi Mark,

I can address your last question, though keep in mind I've got little experience with CNC (last week I couldn't spell CNC, today I own one http://bbs.homeshopmachinist.net//smile.gif )

I had a nice Bridgeport clone and a reasonably expensive 10" Yuasa H/V rotary table. If I wanted to do a bolt circle with those I'd hump the rotab up on the mill, clamp it down, center and clamp the part to the rotab, align the mill to the rotab, and then start cranking the rotab and pulling the quill handle. It could easily be 30-40 minutes before any metal was cut, depending on how much fumbling I did.

With my CNC mill I put the part on the table, anywhere that will let the mill reach everywhere it has to reach. I can pick drilling operations, bolt hole circle, enter the number of holes, diameter, and direction to go from the starting point (typically the center point). I then specify what tool is being used (if it is standard the speeds and feeds are already loaded), show the mill where the 0,0 point is, and hit the start button. It doesn't take much longer to do the entries in the control that then it took to type this. That canned cycle can easily save 30 minutes, and if you have a lot of holes to drill it can save more as it will position itself a lot faster than you can crank. It also doesn't lose count of how many times the handle was turned. http://bbs.homeshopmachinist.net//smile.gif


has a 5.5M video demonstrating a CNC'd Bridgeport Series 1. They do the BHC on that, and watching that is what sold me on going with a CNC machine.

The accuracy of an interpolated curve/circle will depend on the particular machine. .001-3" on a 5-6" circle shouldn't be out of expectation on many machines, and I'd expect the high priced stuff can do that over much greater sizes. On 1-2" holes or bosses I generally can't see any out of roundness with a set of decent dial calipers.

If you are doing a bunch of bearing pockets you'd probably do the interpolation to .010" or so undersize, and then load a boring head and let it go back and make a pass with that to finish them. But for a lot of stuff the holes or bosses you make on the mill will be just fine. And if you want several different holes (done with the table, not a drill) or bosses on a part you'll need to reposition the work on a rotary table for each one of them. With the CNC you'll just give a new start location for a fresh pocket/frame/boss cycle, plug in the new sizes, and let it rip.

My late 1980s Tree 325 manual shows positioning accuracy on all three axes as +/- .0005"/10", repeatability as +/- .00015", and circular interpolation ("cutting accuracy 6" circle with good shop practice") of .003" T.I.R. I've changed over to encoders with about 4x the resolution, so I'm hoping that I'll be able to improve on that .003" figure.

As usual there will also be variances with the quality of the ball screws, amount of slop in the machine, etc.

On the other hand, if you just want to drill a quick hole you can have it done on the manual mill or drill press before the cold CNC machine finishes booting up. I can actually see myself eventually picking up a drill press or mill/drill for those quicky jobs. I would have liked to have kept my manual mill, but I didn't have the space.

But if the CNC mill is up and running, the more complicated the part is, the more time it is likely to save you.

Now if you want a fourth axis on the CNC mill you will still have use for a rotary table/dividing head etc. That can be either a manual or CNC rotab, depending on what you want, what you have, what your control will let you do, and so forth.


12-04-2005, 05:31 PM
<font face="Verdana, Arial" size="2">Originally posted by Wirecutter:
(a horizontal rotary table is one with the axis of rotation parallel to the floor, right?)</font>

No, axis of rotation is vertical. The surface of the table that you clamp to sits horizontally, like a dinner plate.

Super Spacer is a fancy indexer. It may have masking plates to give you common index positions - 2,3,4,6,8,12 positions might be typical. It's much quicker to use than a dividing head if you only need those divisions.

A dividing head is a much fancier device. It will give you a much larger set of positions but is more tedious to use.

Both of these are commonly (not always!) adjustable for angle, so that the rotary axis can be varied from horizontal to vertical.

[This message has been edited by sauer38h (edited 12-04-2005).]

12-04-2005, 07:19 PM
My understanding...

A super spacer has a limited number of possible divisions, as Saur38 notes. Even if it's got a vernier to get every degree, that's not going to be good enough for a lot of gears you might want to make. 46 teeth, for example, is a tooth every 7.826... degrees.
Now, a dividing head doesn't care about degrees, at all. It gives you various numbers of divisions, which is what you need for gear cutting. You put on the plate you need for 46 divisions, whatever it is, turn the crank, and you get 46 divisions, exactly, out of it.

A rotary table will work too, to the precision of the rotary table dial graduations. It's "analog." It doesn't have discrete steps, the way a super spacer does. you can increment by 7.826... degrees if you want to, or whatever that comes out it in degrees, minutes, and seconds.

12-05-2005, 10:07 AM
I own a rapid dex index tool. Also two rotary tables and three dividing heads. For simplicity and accuracy the dividing heads are the way to go. With the dividing head any division is possible with the plate sets.

12-05-2005, 11:08 AM
A rotary table can come in several sizes. The smaller sizes (under 10") generally come with a base casting that will allow you to mount it in either the horizontal or in the verticle positions on the machine surface. The larger tables are usually horizontal only and need to be wrestled(sp)onto either an adaptor from the manufacurer (think Bridgeport 15")or onto any angle plate or knee big enough for the plate. They can be had without a crank and thus need to be positioned by hand(Palmgren made one like this). They can also be had with an optional set of dividing plates for more versatility. Some are made to be driven from a power take-off from the mill they are on.

A super spacer is more of a production dividing device than a dividing head is. The standard is the 8-10" Hartford head. Equipted with a set of masking plates, it can be used to rapidly index from 2-24 divisions with repeatable results. This is ideal for drilling holes or milling flats and other geometric angles that need to be opposite one another on multiple parts.

The indexing head can be used as a spacer, but shines in its use to cut gears and parts with a helix in them. They can be manually turned, or attached to a power take-off on the mill. When used with a power feed they generally are on a universal horizontal mill and have a compound gearing set-up to drive them at the proper co-ordinated feed rate.

Most of these function can be duplicated on a CNC mill though a 4th axis with the appropiate table makes it a lot easier.

Hope this helps.

Paul Alciatore
12-05-2005, 03:06 PM
For a simple hole circle, you are probably better off with just the CNC routine. Faster, easier, less set-up.

The spacers are limited to the positions that are available. Most do one degree increments so they are not all that useful for gears. They can only do numbers that are factors of 360: 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 36, 40, 45, 60, 72, 90, 120, 180, 360. It's a pretty good list but many numbers are missing. If that works for you, then by all means...

Rotary tables are perhaps the most universal device. You can do divisions or any angle. They are not as quick to use as a dividing head because many of them do not have the plates for fast dividing. But many do have the plates available as an accessory. If you need odd angles, they are the only way to go. One thing I recommend if you are doing dividing on a RT is to use a program like Excel to make a table of the angles required for each setting. If properly done, it will avoid any cummulative error that can be caused by rounding.

Dividing heads are most useful for any whole number of divisions. Hence, very useful for gears. Thay also usually have a tilting adjustment so you can do bevil gears easier. And they are more compact than RTs. The plates and arm will allow fast, accurate movements from one position to the next and generally, there is no cumulative error. I said "generally" because for certain numbers of divisions you may have to use an approximate setting. This is the one drawback in that, like the spacers, only a limited set of divisions can be made with the standard circles. Of course, with the proper circle, any number of divisions at all could be done. But there are limitations on this also as most circles do not have room for 1000 holes. In any event, with a set of three circles, most popular numbers up to 100 can be done. The missing numbers will tend to be primes like 97.

Of course, there are expensive, compound dividing heads that use two circles to allow many more divisions to be performed.

The choice depends on what you expect to be doing, how many you expect to do, and on your pocketbook. I have a RT and plan to make some circles for it - someday. I would also like a gear hobbing machine. I am looking with great interest at John's device.

Paul A.