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Confused ....Advice needed rotary table vs dividing head

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Mike Poole16/02/2021 14:48:06
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I think letting errors accumulate is where we can start to get into trouble in the home workshop.

For many tasks either a rotary table or dividing head will suffice but a fully specified dividing head will be capable of much more than a rotary table. A rotary table with the dividing plate facility will do much of what most people need and can be pressed to do things that a dividing head would be first choice for in a tool room. If you are lucky enough to have a proper universal mill then a dividing head would be a great accessory to have but only if you envisage tasks like helical milling to be something you might want to do.

Mike

Tony Pratt 116/02/2021 14:54:21
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Posted by SillyOldDuffer on 16/02/2021 13:59:06:
Q1. Using this plate, what indexing ratio is needed to rotate the rotary table by 5.29°? (There are two candidates.)

Q2. What's the error, in degrees, of both indexing ratios?

Show working! If you happen to have a lookup table for a 40:1 ratio, please don't cheat by using it. The test is doing the maths.

Dave, is your 5.29 -- 5 point 29 degrees or 5 degrees 29 minutes?

Tony

Andrew Johnston16/02/2021 15:16:31
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Posted by Oily Rag on 16/02/2021 14:29:22:

I think we have hi-jacked Me.s thread enough!

Agreed - Andrew

Andrew Johnston16/02/2021 15:46:21
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Posted by Mike Poole on 16/02/2021 14:48:06:

A rotary table with the dividing plate facility will do much of what most people need and can be pressed to do things that a dividing head would be first choice for in a tool room.

Therein lies the problem, we don't know what the OP wants to make. So advice can only be offered on the basis of what we make, not what the OP wants to make.

In general gear cutting is easier on a dividing head:

6DP Main Gear

Although a rotary table can be used; in this case the gear was too big to fit under the spindle:

final drive gear cutting.jpg

Of course it helps if the number of teeth is an integer divisor of 360, as it was in the above case (72 teeth).

However, as Mike says, there are some things for which one definitely needs a dividing head:

helical_gear_cutting.jpg

My dividing head has a reversible index plate behind the chuck, so I also use it for simple indexing work such as squares and hexagons.

Conversely a rotary table is best when radial cuts are needed:

tapping_rear_hubs.jpg

Or for large parts that will not easily fit on a dividing head:

spotting_drill.jpg

The centre hole on my rotary table is 1" diameter and parallel. In my view that's more useful that a Morse taper as it makes it dead easy to make mandrels or alignment pegs. After buying my rotary table I also bought a cheap 3-jaw chuck, but have never fitted it and never will. I simply don't need it, especially as I use the dividing head for indexing. In the past I've used the rotary table for rounding the ends of rods, but it's a right royal PITA. It takes care to set up the part on the axis of the rotary table. If an arc is needed rather than a full circle it is only too easy to go a gnats whatsit past the desired point. It might only leave a shallow witness mark, but it is surprisingly difficult to remove it. Lots of filing and an end that no longer looks semi-circular. I prefer filing buttons for rounding the ends of rods and clevises.

In summary the OP needs to decide what he wants to make and can then decide which is better, rotary table or dividing head.

Andrew

Me.16/02/2021 15:53:30
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Thanks Andrew - I should have said at the very beginning my aim is to make lots of ...... swarf ...... ! and just have fun trying different things - no real objectives just trying to get the things I need to progress.

I think I have decided on the SOBA 6" table with extra plates - I think this will suit my basic needs for now.

Let the Swarf making begin.....

Howard Lewis16/02/2021 17:17:33
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I know that I have a bit of a thing about this, having once wasted a week, until finding the error, and others!.

There were only a few, but I managed to pick one of the mistakes..

If your Soba 6" is a 90:1 ratio, it may be worth checking a few numbers on the chart. There is a corrected chart, if you start searchingthe Forum for HV6

My VERTEX HV 6 told me to use 6 turns and 30 holes on a 39 hole plate to get 13 divisions.

It was wrong! It should have been 6 turns and 36 holes on a 39 hole plate!,

Hopefully, the charts supplied with Tables will have been corrected by now.

Howard

Mike Poole16/02/2021 18:26:30
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Although a trustworthy table makes life very easy, being able to calculate from first principles is a good double check. Once you have cut metal it’s too late if a mistake is made.

Mike

JasonB16/02/2021 18:40:31
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Posted by SillyOldDuffer on 16/02/2021 13:59:06:

Q1. Using this plate, what indexing ratio is needed to rotate the rotary table by 5.29°? (There are two candidates.)

Q2. What's the error, in degrees, of both indexing ratios?

Dave,

1. You mention that using published tables is cheating. Do we assume you have rounded the 5.29deg as tables only tend to give No of divisions in which case 68 would be closest as 360deg is not divisible you 5.29deg at least not to a whole number

2. I had better get in before Andrew and ask if you will be doing the indexing as that may increase the likelyhood of an errorsmile p Sorry could not resist.

Howard Lewis16/02/2021 18:52:01
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Presumably the suppliers provide tables so that the user does not have to calculate every time. Having been correct every time., UNTIL then, I believed what the Chart said.

Having spent a day and a quarter setting up a spreadsheet, to cover all sets of holes on three Division Plates, i do now believe what MY chart says. The spreadsheet brought to light errors other than the one causing the immediate problem, and others as well as a few omissions.

Calculating is not my prime hobby by any stretch of the imagination. Cutting metal brings much more pleasure.

Howard

Emgee16/02/2021 21:59:38
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If an arc is needed rather than a full circle it is only too easy to go a gnats whatsit past the desired point. It might only leave a shallow witness mark, but it is surprisingly difficult to remove it. Lots of filing and an end that no longer looks semi-circular. I prefer filing buttons for rounding the ends of rods and clevises.

Andrew

I agree it is a pain if you overtravel, I usually add 0.20mm to the radius to be cut to avoid the tool mark you describe.

Using the dividing head as set-up on your mill looks like a bit of a pain with the controls at the back of the head, especially if you are using the sector arms.

Emgee

SillyOldDuffer17/02/2021 09:32:26
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Posted by Tony Pratt 1 on 16/02/2021 14:54:21:
Posted by SillyOldDuffer on 16/02/2021 13:59:06:
Q1. Using this plate, what indexing ratio is needed to rotate the rotary table by 5.29°? (There are two candidates.)

Q2. What's the error, in degrees, of both indexing ratios?

Show working! If you happen to have a lookup table for a 40:1 ratio, please don't cheat by using it. The test is doing the maths.

Dave, is your 5.29 -- 5 point 29 degrees or 5 degrees 29 minutes?

Tony

Hi Tony, 5 point 29 degrees - I'm not malicious!

Dave

SillyOldDuffer17/02/2021 09:57:42
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Posted by JasonB on 16/02/2021 18:40:31:
Posted by SillyOldDuffer on 16/02/2021 13:59:06:

Q1. Using this plate, what indexing ratio is needed to rotate the rotary table by 5.29°? (There are two candidates.)

Q2. What's the error, in degrees, of both indexing ratios?

Dave,

1. You mention that using published tables is cheating. Do we assume you have rounded the 5.29deg as tables only tend to give No of divisions in which case 68 would be closest as 360deg is not divisible you 5.29deg at least not to a whole number

2. I had better get in before Andrew and ask if you will be doing the indexing as that may increase the likelyhood of an errorsmile p Sorry could not resist.

1. Not rounded, and not a whole number. The point is to explore the maths behind a non-trivial example.

Fortunately for me it's rarely necessary to dig into the maths, though it's hard to avoid in design work. Most jobs on my rotary table involve straightforward obvious angles, and the HV6 table covers a lot of common cases. The maths behind index plates is interesting because it also covers approximating gear ratios for unusual threads, working with index plates that don't have an exactly suitable circle, milling spirals, and producing feed-rates. Also answering questions like, 'On a given lathe what's the smallest set of change gears that can provide close approximations of both imperial and metric threads?'

2. Who me make a mistake? Never happened...

surprisecryingblush

Dave

Me.17/02/2021 10:12:09
127 forum posts
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cool......

Edited By Me. on 17/02/2021 10:36:02

Me.17/02/2021 10:35:27
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I was sure it was 42 - now I'm not.....

Andrew Johnston17/02/2021 11:19:56
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Posted by SillyOldDuffer on 17/02/2021 09:57:42:

.......'On a given lathe what's the smallest set of change gears that can provide close approximations of both imperial and metric threads?'

None, because with an imperial or metric leadscrew one or other set of threads will be exact and the other an approximation, not both approximations. smile

Andrew

Michael Gilligan17/02/2021 22:07:08
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Posted by SillyOldDuffer on 16/02/2021 13:59:06:

[…]

Not everyone is good at maths, which I think can be demonstrated by this relevant question: Assuming a Rotary Table has a 40:1 worm and an indexing plate with hole circles from 30 to 60, each circle in steps of one, thus:

indexplate.jpg

Q1. Using this plate, what indexing ratio is needed to rotate the rotary table by 5.29°? (There are two candidates.)

Q2. What's the error, in degrees, of both indexing ratios?

Show working! If you happen to have a lookup table for a 40:1 ratio, please don't cheat by using it. The test is doing the maths.

-----------------------------

.

Please, Sir ... will this do ?

[ sorry, I didn’t bother completing Q2 ]

The gist of the working is to first multiply by 100, so that we’re working with an integer target.

Then find how many hundredths of a degree each hole represents on each circle.

Then find how many holes are required for 529 hundredths.

Quick & Dirty spreadsheet looks like this:

.

50b95c5a-7626-4596-afa7-55ae3fbb0a65.jpeg

.

Row 10 looks pretty good to me ...

and I’ve sanity-checked the formulae by substituting some easier angles in B2

MichaelG.

.

Edited to correct a typo, and to mention that if anyone needs more detail, just ask.

Edited By Michael Gilligan on 17/02/2021 22:24:35

Me.18/02/2021 08:23:04
127 forum posts
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Phew - thanks for that.

I'm never going to be sure I will ever have to rotate my table/Head....(when i get one) by 5.29 degrees, but when I do I will know how to work out which plate hole combo I will need to use.

Me.18/02/2021 08:33:24
127 forum posts
18 photos

Is it just me (as in Me) - why when you look at this picture the line a 12 o'clock is straight up and down but when you turn the picture through 45 degrees the new 12 o'clock line is not.....

Have I discovered a floor in the dividing disc design or is it just an Illusion....... Spooky....

And also - why the line at the 6 o'clock position has half the amount of holes when you look at the picture as posted - Anyone have Pythagoras's phone number I think hes missed a trick with this.

Me.18/02/2021 08:39:27
127 forum posts
18 photos

PS - I do know why but when someone that doesn't know what the picture is of, then it does confuse very quickly.

SillyOldDuffer18/02/2021 13:48:31
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Posted by Me. on 18/02/2021 08:33:24:

... why when you look at this picture the line a 12 o'clock is straight up and down but when you turn the picture through 45 degrees the new 12 o'clock line is not...

Have I discovered a floor in the dividing disc design or is it just an Illusion...

And also - why the line at the 6 o'clock position has half the amount of holes when you look at the picture as posted...

Illusion, or at least the brain detecting patterns other than the one intended. Here's a real Index-plate showing similar effects:

dsc06403.jpg

It's because the number of holes in each circle increases by one, so they stagger around the plate, aligning in various ways. This plate gives index holes stepping at:

Holes Degrees
15 24
16 22.5
17 21.18
18 20
19 18.95
20 18

Index plates are used with a plunger and clock-hand indicator and are set up from a table like this:

hv6index.jpg

PDF Version Here.

Indexers provide a relatively easy way of keeping count when a dividing head or rotary table is turned repeatedly through an angle. The HV6's Basic Indexing Table covers the common case, which is when a circle is divided into an equal number of angles, say to drill holes on a PCD, putting flats on a work piece, or making gears.

For example, to drill 7 holes in a PCD, angle approx 51.43°, fit Index Plate 'B', select hole ring 21, do 12 full turns of the handle, ie 48°, then move 18 holes along the 21 hole ring to add 3.43°, total 51.43°. The clock-hands are set to indicate 18 hole movements so the operator doesn't have to keep count of them, and can be moved indicate the next target. The operator has to count 12, and woe if he miscounts 11 or 13, or selects the wrong index hole. A microcontroller is much easier to drive.

Another point, the 'Basic Index Table for HV6' is just that, basic! It doesn't help when the angle needed isn't one of the listed divisions, as is the case with my 5.29° question. The table's shortcoming may not be a problem if you can do the maths and identify a reasonable approximation.

Microcontrollers are popular because they don't rely on an integer only index plate to keep track and can get close to pretty much any angle required. Best of all, the operator just enters what he wants and presses 'go': he doesn't have to concentrate or understand how it works.

Haven't looked at Jason's answer yet, but I bet he's right!.

Dave

Edited By SillyOldDuffer on 18/02/2021 13:50:47

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