Run out on a rotary table

I bought a rotary table a year or so ago (the price seemed rather good so I jumped). I've not used it much, but a friend asked me to make some pentagonal rod, which seemed like a damned good excuse.

Although I've made something with 5 sides it didn't look even, and it seemed that the rod was off-centre.

I checked the run out on the table with an 8 mm test bar (taken from a scanner in an MT2 collect. I got around .14 run out over 50mm. The actual milling was done nearer 100mm which would put it between .25 and .3 out.

The table came from Warco and my question is, 'can I fix it'?

I've a badly photocopied exploded parts list which is a little hard to read, but I thought I would seek the practical advice before I took it apart and buggered it up further...

Iain

You could mill a register for setting up purposes in the top of the morse taper using a carbide cutter. It would not help with the sort of set up shown running off the morse bore though. Can you mount a chuck onto a plate to set up independent of the table bore ?

Just a thought, but is it possible that the rod is bending under tool pressure? The forces applied during milling are quite high. It might be worth trying a tailstock.

Dave

I would check the runout on the morse taper insert first (how do I know?)

Or even make a steel or aluminium fabrication that aligns both your rotary table and tailstock that you can quickly clamp to your milling table without the need for tedious setup time, all you have to do then is make sure the plate is relatively square to the head. It could save thee a lot of time.

Michael W

Just a thought that are we taking about how parallel the probe is to the bar, or the rotary runout of the bar on the table. At 100mm you're bound to need a tailstock to keep things steady.

Maybe your table came from the same place as mine did ! Sounds like the run out is about the same.

I,m glad I came across your post,since I was considering buying a table from Warco,good price alright but good quality--------? I think we are both in the same boat. My hexagon was like your pentagon,right number of sides,but not too much in common. What did Warco have to say about it?

Why do we have to put up with such crap ?

I'm measuring the runout with a dial indicator on a ground bar in a collet.

Thanks all

There's no stress on the test bar so there is definately run out.

I was initially going to mount my 4 inch 4 jaw on the table, bit I pretty much gave up on that. I'd have to make a plate to mount the chuck on and then mount the plate, shim it or something and still manage to get the 4 jaw centred.

The milling actually looks to have worked quite well with the pentagon being fairly parallel - just not centred or aligned to the bar, so I'm not overly concerned about the need for a tailstock.

I want to do some gear milling with the table and this is just a bit too imprecise for that, I think.

If I take the thing apart can i tune it?

Iain

Because that's engineering for you.

.

1. To paraphrase Mr Stevenson [of this parish] ... it's "fit for purse". JS has already mentioned that a proper "fit for purpose" RT might easily cost you £3,200
2. To [allegedly] quote John Ruskin: "There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man's lawful prey. "

They are saying much the same thing.

Bear in mind that the 'intrinsic value of the materials' probably only represents a small percentage of the total value of a Rotary Table ... most of the added-value is in the making/testing/certifying, and that is where corners are generally cut.

I'm sure you knew this already ... but it's worth reminding ourselves occasionally.

MichaelG.

Edited By Michael Gilligan on 25/03/2017 20:22:42

Sorry, but I see it as a real shortcoming of the methodology. Rigidity is not considered. Blame is on the rotary table but no consideration of the possibility this run out is due to MT, the extension to a 4 jawa self centering chuck (with its own run out) and an unsupported bar being milled at a further extension from the rotary table, and no consideration as to the concentricity of the workpiece.

Try again, is my advice, but do find where the imprecision occurs - before blaming the rotary table!

Hi Ian,

We're slightly at cross-purposes. I'm sure you've measured the run-out correctly.

What I and others are suggesting is that the run-out isn't what's causing your pentagon to go adrift. Try this experiment. With the DTI in position push the end of the rod down with your finger. I think you'll be able to move it a lot more than 0.14mm!

It doesn't help that your metal rod is held by a small chuck plugged into the rotary table's centre hole rather than being bolted to it. There's a lot of leverage in the set-up and it won't take much to move it. A milling cutter applies a lot of force to the work and it's important to hold the job firmly.

Dave

Good morning all,

Just reading the latest post from that member with the bum rotary table. WELCOME TO THE CLUB !! I too had hopes of doing some gear cutting at some stage,but I hate to think what they might turn out like,since even the chart supplied with my table is full of errors.

You can get fairly good results by using the 4 jaw chuck,it,s just a bit slow to set up and you do need to make a mount for it,but that,s no big deal,even I could manage to make one,and that,s saying something ! I had already made one,a copy of the lathe spindle,just a simple disc with a threaded nose on it and it works quite well.

I agree it,s very disappointing to find that the table is of limited use,but it may be possible to correct it,I hope to have a go at mine soon,thanks to Neil. I saw the one being offered by Warco and was tempted,but I think I might just hold fire. I hope you get a better response than I did [at least they still sell them ]

What is the point of selling stuff like this? Perhaps if they want to sell through M E W,they should have to meet proper standards. The word gets around fairly fast in these circles and if professionals wont accept such junk,why should we? I know they will say that these things are made to a price,but if they dont work,the price is irrelevant. Just some food for thought. This is how junk boxes evolve !.

Edited By JasonB on 26/03/2017 14:55:05

Buy cheap, buy twice.

"Chinese made rotary table in runout shock horror"

not exactly a surprising headline to be fair........

The MT taper is an inserted piece, it presses out. That s where the error was on mine. Swapped the whole RT for another which was significantly better.

OH dear.

I thought I'd replied earlier on some things, but clearly didn't click 'send'.

My main point was that any stress on the bar would be consistent. That is it would apply to each of the facets as it was milled, making the bar pentagonally symetrical, even if not necessarily of the dimensions I expected. By carefully measuring and creeping up on the correct dimensions, I should be able to get a good pentagon (I didn't, but that's part of the learning experience - it was a struggle to get my mike in there so I would take the piece out, check it and put it back. Then attempt to alight it ). Albeit possibly slightly tapered.

I wrote to Warco this morning and I will let you know what the response is. If any.

I'l also check the run out on the table. If it's just the taper that's dodgy, I could try mounting something on the table proper.

Thanks as always.

Iain

Sorry to dissent Iain, but I don't think that the stress on that rod can be consistent. Cutting the flats weakens the rod and changes its shape. Therefore the stresses will be different each time you cut a new flat. And any bending will be worse than expected because there's plenty of leverage; that's why I suggested supporting it with a tailstock.

If the problem was being caused by run-out alone, then your "carefully measuring and creeping up on the correct dimensions" would work wouldn't it? I think that it doesn't because the rod moves differently under every cut.

In conclusion, as your method is on the right track and your Table isn't awful, it's more likely to be the rod.

I may be wrong. No improvement after firmly supporting the rod with a tailstock would put me straight back in my box!

Dave

Happy to have dissent, Dave. It's how I learn!

To cut to the quick - I don't have a tailstock (apart from on my lathe), so that's out!

What I should be able to do (if I'm sort of right and you're sort of wrong) is to mill a perfectly symmetrical pentagon which is no on the axis of the rod. If it ends up with significant (more than .1mm) asymmetry, I must blame the pressure of the cutter.

This is one of those jobs you probably need to do twice. Once to make mistakes and once to get it right.

However, there IS run out on the morse taper regardless of what my tool may be pressurising and I need to do something about it before I try and cut a gear (which I would like to sometime, just to round out my education). A tailstock may be part of the solution for that.

Cheers

Iain

.

Dave,

May I try using your logic to make an alternative suggestion ?

If the problem originates with variations in the bending of the bar, as it is being machined; then surely the resulting pentagon would also taper along its length.

Measuring the cross-section of the machined part should be a simple exercise, and be very informative.

MichaelG.