Rotary Table Choice 36:1 o 90:1

I am starting out with my recently acquired mill, and the (ever longer) list of projects contain several that would be easier (possible!) with a small rotary table.

The Mill (an SX1LP) seems to suit a 4" table. Reading the forum about such devices I think a simple H/V one would be fine. There seem to be two main choices (in my price bracket) 36:1 (10°/turn) ratio and 90:1 (4°/turn) ratio. The former are a bit cheaper, but the handwheel is on top in the vertical position while the 90:1 seem to have the handwheel horizontal in both orientations which would seem more useable on my small mill.

Thinking about the two raises a couple of questions.

Do the different ratios make one more or less useful ( both would be capable of being set to sufficient angular precision for anything I can visualise doing)?.

Which will be stronger? Does 90:1 mean more teeth (of the same size) on a bigger wormwheel, or many smaller teeth on the same size wheel. The former should mean the 90:1 will be be stronger (bigger radius hence lower forces for the same torque). The latter would seem to make the 90:1 weaker.

Comments gratefully received.

A 36:1 table turns more quickly than 90:1, making it faster to use, but less accurate. Reduced accuracy doesn't matter provided only simple angles will be needed. Stuff like hexagon heads.

However, new rotary table owners are soon likely to find jobs where extra accuracy is valuable! Gear cutting is the obvious example, but there are many others, such as cams, helices, drilling awkwardly spaced PCDs etc.

If gear cutting & chums are ever on the menu, a 90:1 table is better. But, oh dear, more money - gear cutting requires a set of division plates or a stepper motor with driver. Manually moving a table to cut 57 teeth ( 6.316° ) per step, soon reduces operators to a nervous wreck and mechanical help keeping track is pretty much essential. I'm not sure 4" tables can be had with division plates, and would recommend a 5" 90:1 table with plates if any advanced functions are on the cards.

Strong rotary tables cost big money. I suggest the hobby type and some expensive makes are unsuited to heavy work whatever the ratio. Brutally hacking out the inner arcs and spokes of several big traction engine wheels at top speed will trash them. Treated with mild respect for ordinary work either ratio should be fine.

Dave

Edited By SillyOldDuffer on 11/08/2020 17:47:30

Your question is very well put, Peter ... but unfortunately I don’t have a definitive answer.

Just posting this to welcome you, and to <bump> the question.

... which was unnecessary, because Dave answered !  

MichaelG.

Edited By Michael Gilligan on 11/08/2020 17:57:44

I'm actually quite liking using 72:1 and tend to pick that up in preference to my 90:1 table but not sure if there are any to fit your budget with that number of turns as the ARC ones are a bit more costly than the norm.

I find the 5deg per turn works out easier for a lot of common numbers that you are likely to want to do basic dividing/indexing without having to resort to plates.

If (and I repeat, if) you go down the route of adding dividing plates to it, you will almost surely have to calculate all the plate and hole numbers for the 36:1 ratio table.

In contrast, you can find many published/printed information tables for a 90:1 ratio device (though some have errors).

The maths is not hard but if it is not your desire to do the calculations, that might be a small point against the 36:1 device.

I've got the 90:1 - bearing in mind the low power and lack of rigidity of the SX1L (not a criticism - its great for what it is), the slower feed rate is probably desirable if you're using it to radius corners and the like - you do have to baby it a bit for that sort of thing.

With regard to the worm mechanism - it's pretty robust, you don't need to worry about that at all with a mill of this size.

As far as I can tell, you're on your own if you want to have a division plate set-up. I was part way through designing such a thing for mine when the need for a 63 tooth changewheel became urgent, so I went for the cheap and cheerful approach of printing a strip of paper with the appropriate divisions marked on it, and taping it round the circumference of the table. Eyeballing alignment with the index mark on the table worked fine - it may not be super precision, but my Imperial threads screw into existing fittings without a problem (and the 63-tooth method isn't perfect anyway).

The only slight irritation with this method is that the strip is 319.9 mm long for my table, so you have to print it at 33 degrees to fit it on a sheet of A4 - it would have been nice to have printed a whole lot of them on one sheet for different tooth numbers.

Does anyone else use "factors" for working out what divisions you can use a particular dividing head or rotary table for ?

I have done charts for all of mine with the standard hole plates, and some that allow different indexing plates to be used from other units, so I can mix and match between different ones.

The excel spreadsheet table shows every possible division that will be available for each combination.

Phil

Edited By Phil P on 11/08/2020 21:26:51

Your choice of 36:1 or 90:1 will depend on the type of work that you intend to do.

The 36:1 model would probably have bigger, stronger teeth and more suited for machining while rotating the table while the 90:1 model will be more accurate but will have smaller, weaker teeth. No problem if locked in stationary position but not as good while rotating.

Many of the smaller RT's have a worm wheel which can be easily damaged. As I found out the hard way. (this was an expensive Japanese made RT, not one of the cheaper ones)

Paul.

Edited By Paul Lousick on 11/08/2020 23:40:47

My take on RTs.

Modern, cheap items are not as robust as some older examples.

If machining curves, reduce any ‘free rotation’ to a minimum and under no circumstances climb mill.

’Built down-to-a-price’ items are not very robust.

I have two, both the same supplier (manufacturer) but manufacture of these machines was moved, from one country to another, and reports on here indicate that the original (older) supplier provided a superior product - an example of cost reduction versus longevity?

Buy the largest your mill will accommodate - I would have bought a bigger one, as a second purchase (but I only bought the second because I really only wanted the chuck attached and paid less for the whole lot than for a new chuck🙂 ).

Regarding turns ratios - mine are both 90 turn items and both now have dividing plates. The 72 turn (5 degree/turn) item seems good for most - so a good compromise between 36 and 90 versions for some.

I always calculate the (relatively) few settings I need. Simple maths to calculate the turns and holes required. It’s just not worth the risk of getting hold of a duff internet list of settings.

Summary: Buy cheap, buy twice. Don’t mill any more than absolutely necessary, with cheap machines. Bigger (within reason) is better.

36:1 gives a 10 degree increment for 1 rotation of the handle.

72: 1 gives 5 degrees increment for 1 rotation of the handle.

90:1 gives a 4 degree increment for 1 rotation of the handle..

So 90:1 gives better resolution, = greater accuracy, ( Moving one hole on a 40 hole division plate will move the table by 0.1 degree.

My HV6 is 90:1 and I am happy with it (Having had to correct the chart supplied with it, many years ago!)

HTH

Howard

1. A more important question may be the number of slots. I think 4 slots is much more useful than 3, which many of the cheaper tables have, as it is easier to fit a 4 jaw chuck which will allow more accurate centering and take up less vertical space than a 3 jaw. Also generally allows easier clamping.
2. What is the main application? If you will be doing actual curved milling then an RT is essential. But for pitch circles it may be better to put the cash towards a DRO system which these days have the ability to set out various hole patterns. Or a dividing head if making gears is more important.

I have 2 RTs, one 4" "Sharp" and the other a generic 6", both H/V. I have used each one precisely once "in anger" I think. My digital DH has seen much more use. In retrospect I wouldn't buy either again.

But you do also have CNC John and if like me that now gets used to do work that could have been done on the R/T but taken a lot longer.

Peter, I have the SX2P mill and apart from it having 25mm more height between table and spindle the table size and X and Y travel are the same as your SX1LP. I started out with a 4 inch rotary table (a cheapie dont go there) but I found it a bit too small. A 6 inch would be too big but came across a 5 inch which is what I went for, came as a set with chuck, tailstock and plates.

A picture below showing its size against the table which gives you some idea. With the 4 jaw chuck and backplate fitted and a Jacobs in the spindle there is only 45mm between the two chucks so I have had to resort to shortening one or two drills in the past. There is more space of course with collet mounted drills and cutters and without the 4 jaw there is 205mm between the rotary face and spindle.

Just thought I would mention it should you want a bit bigger rotary though a 4 inch version may well suit your work.

Ron

p.s. The 5 inch version is 90:1

Edited By Ron Laden on 12/08/2020 12:37:53

Edited By Ron Laden on 12/08/2020 12:52:21

We don’t know the ‘price bracket’, but I just looked at some that were offered on the net. A lot of cheap ones were from outside the EU (are deliveries from the EU still treated the same as prior to Brexit?) and even the cheapest would attract another £20 in costs. OP’s choice, of course, but I would not recommend a really cheap R/T from a very remote vendor. Any warranty would be as remote as the vendor, methinks.🙂

I have just picked a rusty M6 bolt from my stock and welded a 70mm dia. piece of steel to the end of it.

The piece of steel happens to have 200 equally-spaced marks around its perimeter.

This means it has a resolution of 0.005mm so it must be more accurate than my Mitutoyo micrometer as this only has a resolution of 0.01mm...

Thanks for all the advice - very helpful. The main projects I have on the (ever growing) list that need some sort of controlled rotation are a motor mount for the Taig lathe so that I can fit a DC speed controlled motor, and some calibrated dials (HH's Lining tool is on the timeline for that and the ratchet wheels will need the table). The motor mount needs curved adjustment slots but it will be in aluminium so less of a risk.

I have chosen to go with a 90:1 4 slot that comes with a set of index plates (for storage until the day comes - as it surely will). I was planning to get the Warco 110mm but it is currently out of stock. Axminster do the same thing but without the index plates but I found what looks to be the same deal as Warco elsewhere for a similar price.

Now to make an adapter to put the Taig chucks (M22 x 1.5 thread) onto the table. Why does every decision seem to add to the project list!

Thanks Paul for the warning. The reason I asked about relative strength was that the cheap 36:1 tables all seem to have the handle angles towards the centre while the 90:1 ones it is straight. That suggested the cheap tables had a smaller wormwheel, and I was wondering.

Thanks again for the advice

Edited By Peter Cook 6 on 12/08/2020 14:58:20

Division plates will be very useful.

Mine are used every time that the R T is used to produce accurately spaced marks, holes or gear teeth. (mine has been involved in cutting gears ranging from 12T to 100T, and graduating handwheels, etc.

For gear cutting, because of the loads involved the Tailstock is vital.

It may be worth checking that the chart supplied is accurate. I found out the hard way that there was an error relating to 13 divisions, trying to cut a 13T gear. Once it was clear that the error was not me, checking against a spreadsheet disclosed other errors or omissions, 8 in all. Admittedly, my RT dated from the 1980s, so hopefully the errors will have been corrected since then.

As quick check; 13 divisions on a 90:1 table need 6 turns and 36 holes on a 39 hole plate, per division.

HTH

Howard.

I'm of the same opinion, UK Consumer Protection only applies to items bought from commercial suppliers in the UK and EU. Importing privately means the purchaser takes on a shower of risks, including sellers not standing by their product. Good news is they often do, but it's voluntary - if they tell you buzz-off, that's it. The bigger the bargain the bigger the risk. Buying abroad is fine in my book but no tears please if it goes wrong!

NDIY mentions Brexit as if it's happened. Not yet. Today, 21 August, terms are still being negotiated with the EU plus another 21 International trading blocks, such as the USA. It's not going well. With 141 days to go 92% of British exports still aren't covered by a trade deal.

Midnight 31st December, 2020 is the actual exit. High probability given poor progress this close to the deadline it will be a hard exit, so could be rough.

Historic events unfolding and I'd love to be a fly on the wall in government. The risk of economic pain due to unfavourable Brexit deals was always high, and those risks were multiplied out of the blue by Covid. Coronavirus has triggered the worst British recession ever, and is doing similar financial damage to all our trading partners. Not a betting man, but I wouldn't be surprised to find the UK and EU obliged to delay Brexit to a more propitious time. Watch the news - silence often speaks volumes...

Shades of Hotel California, where the Eagles sing:

You can check-out any time you like
But you can never leave!

Bit like a rotary table, spin the handle and it goes around in circles.

Dave

Just an update for anyone interested - against advice I ordered table & index plates from a remote supplier (China!) but who shipped from the Czech Republic so no customs or duty (until 1st Jan). Table arrived by courier in less than a week. First impressions good. certainly looks more than adequate for my needs and mill.

On investigation it turned out to be a 110mm diameter 4 slot 72:1 table rather than the 90:1 I thought I was getting. So opened discussions with supplier. Very responsive - offered a swap, or a discount if I kept it. Based on Jason B's comments about usability I decided on the latter.

Only downside so far is I need a 25 hole plate to do 25/50/100 divisions and that is not one of the supplied circles. So yet another project gets added to the (ever lengthening) list!

That would have been useful when I had to machine a 72 tooth gear:

Andrew