New Vertex RT-Disguised backlash-?

[Chris MateParticipant @chrismate31303]

Hi, after completeing and threading the holes(a lot) on the Disc I made clamps(Various) 8mm bolt size for use to RT Vetex-HV-6 model, 3x slots.

Stops made:
I then decided to make two adjustable sliding stops which slide on the outher edge of the disc, bolted in place to privide working between two stops, milling curved slots 8mm  in it.
It is with this operation, I realised there must be some backlash as cutter gets to end of cut and excit, where conventional milling feels like climb milling exciting the workpiece.

Backlash experience on this RT by me:

1-If I move the the table normally by cranking, I feel no backlash, it feels smooth, if connected a dial indicator I dont see any, but this is light load.
2-If I mill I feel the possibility of backlash in handle, under load with cutter, I can feel the difference between climb & conventional milling here.

3-This brings met to completing the stops and adjusting the table towards a stop, and if table stopped at 0 degrees as example by two stops, disc/table no movement, I can detect definite backlash.

4-The “Manual/Pamflet I got with it is quality printed, and it says under “ADJUSTING MESH OF WORMGEAR” (Theres zero axial play=Another type of adjustment), it says theres a screw A-Steel Ball-Screw B to adjust this. On inspection I found one screw(A-?), no steel ball, no screw B, if A is screwd out, I can see the conentric part turning inside.
Screw A=12mm long by 4mm allen.
Now whatever I try I cannot get this backlash adjusted out……I can adjust it where it feels uncomportable stiff to turn, but the backlash still theres.

Its not the end of the world, but just curious what you think. I am going to enquire as to where the ball & screw B is from seller.

Thanks for ideas……This backlash confuse me a bit….

[JasonBModerator @jasonb]

Screws and ball shown on the exploaded view, ball may be stuck in grease. Effectively it is a stop that adjusts where the eccentric stops when you turn the lever to swing the worm into mesh.

 

[Chris MateParticipant @chrismate31303]

Hi, If I loosen the T-Handle to release, If I feed(Try to) the dial with my hand and adjust that one screw in or out, I can see the dial moving, so there is only one grubscrew, no ball and screw B further down in. I can see the part unrestricted with light shine in 4m hole, so not sure if they change it on new one, or just a miss assembly, waiting for answer from dealer.

So the one screw(A) 12mm long does change the backlash to a point where you get no backlash under no load like against a stop, or cutting, it indeed feel very smooth and good cranked with no load, I can adjust where it feels stiff to turn, but I can still produce the backlash againt a stop-?.
Not sure if the main tapered bearings(maybe slight play, cannot feel anything by hand) can cause something like this-?

Its not like on a Lathe or A Mill where you can feel the backlash with no load, easy.

[DiogenesParticipant @diogenes]

There has to be some ‘space’ between the moving parts in any mechanism.

You cannot rotate a (300Euro) worm/wheel rotary table up to a stop in one direction, and then ‘back it out’ in the opposite direction without having some backlash at the point of reversal.

Maybe one reason why they don’t have stops as standard.

BUT.. ..the backlash can be minimal / you can use the locks to ‘damp’ it / withdraw the tool / rotate the table manually instead of by handle…

There are a number of ways of achieving a desired result, what is the operation that you are trying to perform – why does the backlash matter?

 

 

[Michael GilliganParticipant @michaelgilligan61133]

Diogenes makes an excellent point !

Without wishing to push this Topic too far astray from its spirit … I suggest dipping-into these pages:

https://sextantbook.com/category/chasing-tenths-of-an-arcminute/

The site is a master-work, and although a lot of it is not directly relevant to ‘Workshop’ rotary tables … all the fundamentals are there.

MichaelG.

 

[Michael Gilligan]

On Michael Gilligan Said:

Diogenes makes an excellent point !

Without wishing to push this Topic too far astray from its spirit … I suggest dipping-into these pages:

https://sextantbook.com/category/chasing-tenths-of-an-arcminute/

The site is a master-work, and although a lot of it is not directly relevant to ‘Workshop’ rotary tables … all the fundamentals are there.

MichaelG.

 

Hi, I understand backlash as I dealt with on mill as example, however as 1st time user of a rotary table HV-6(Small), I was a bit surprised at what I perceived it to be and what I experience as I was actually milling on it.

So I was milling a curved slot at the outer edge of the 200mm dict added to the 160mm table to have more space.
-My perceived backlash was theres nothing or very little, I cannot measure or feel it by turning the table without milling on it. On the mill theres no doubt.
-So imaginge you using a 4 Flute 8mm cutter to enter the workpiece from either side, with both entry points not square, IE the cutter 1st cut with one side a few mm, then it cuts the slot both sides till the other end is reached, then it leave the part by cutting again on one end few mm.
It was at either ends , no matter wich direction I turn the table, it gives the impression of limb Milling, slightly feeling it wants to run away(Feel in handle)=(Now you feel the backlash under load at play), and it does not matter from wich side you start.
-Milling slot in the part after cutter touches/cut both sides its very stable feels very good.

So I started looking into this as a result of above experience versus my initial perceived to be experience I though I was going to get.

xx-The interesting part for me was that I could adjust the backlash as the manual stipulate, and reach a point where its feels not right=Too stiff in turning, but at that point it still has backlash if forced with a stop.

xx-So it was the difference in measuring thebacklash without a stop with dial indicator & feel, versus a stop or locking the table.

Note:
I found some info where they actually test CNC milling tables by forcing it and then have a spec that permit X-Backlash repeatable, they crank it by X-Force to determine, and that figure is somehoe taken into account in machining.

Note:When I made the Aliminium disc and milling the outer edge of it with conventional milling, turning the table right into/againt the cut It feel cutting strong, no problems, but that was a different one sided 4-Flute cut than in this case entering exiting a workpiece for few milimeters.

I will post a photo later of how it was clamped down and you can see the slot milled.

[DiogenesParticipant @diogenes]

Ah, yes, I think I understand.

A cutter will give some (un)funny effects as it enters and leaves the work because incomplete engagement forces the tool sideways – a workpiece bigger than the table will really let you feel the ‘pull’, it should be controllable with care, sometimes it is prudent to slow the feedrate at awkward entry & exits.

Another issue with stops on a R/T is that the low gearing of the worm-drive mechanism provides enough force to overcome/stress the stops – it’s very difficult to achieve good repeatability because of the lack of direct feedback at the handwheel – you cannot feel whether you are just touching the stop, or ‘driving over it’ by 0.1mm.

To ‘work between stops’, I use a small manual table with no worm drive.

 

[SillyOldDufferModerator @sillyoldduffer]

Don’t expect too much of backlash adjusters on ordinary equipment.

My lathe cross-slide has a slot cut across the nut that can be tightened with a screw so the nut bends to grip the lead-screw on both sides.   The arrangement reduces backlash rather than eliminates it, and comes with a major disadvantage!   A tight slot minimises backlash at the cost of high friction and severe wear.   With this system,  it’s better to tolerate a little backlash, and compensate for it manually.   Constantly adjusting the slot for no backlash is more trouble than it’s worth.

There are anti-backlash mechanisms that work without causing wear and tear, but they aren’t simple or cheap!  None of my equipment  has a well-engineered anti-backlash mechanism.   Instead, I have to manage backlash myself.    It’s a nuisance that slows me down, rather than a serious problem: I seem to manage somehow!   Every so often I forget, causing ‘industrial language’ and bad tempered stomping off.  A glass of wine helps…

Dave

[PeteParticipant @pete41194]

Yes the backlash and clearances can be reduced, but I think your misunderstanding how the worms, worm wheels and complete rotary tables are made to meet the selling price. Secondly if one component moves against or within another, there has to be a certain amount of  allowable clearance or the assembly would lock up with a zero clearance condition. And everything manufactured always has an inevitable + – tolerance for size, straightness, angle and even surface finish.

Your not going to get finely ground and then high precision lapped worms and wheels that something like a Moore Tools R/T or Leitz optical R/T and many others have at the prices were paying. In fact with the few items I could check when my Vertex R/T was brand new such as the table flatness and run out numbers for it’s morse taper, that so called certificate of accuracy it came with is completely arbitrary and means nothing. So the angular accuracy of both the worm and wheel Vertex claims are to me also highly suspect. I guess there good enough for most of what were using them for, but expecting zero or at most a very few minutes of angle for back lash isn’t going to happen.

[Michael GilliganParticipant @michaelgilligan61133]

A quick observation, which is presumably relevant to the level of backlash that Chris is perceiving …

I don’t recall the actual dimensions, but the Vertex HV-6 and similar, have an internal worm gear which is relatively small for the size of the table: Any clearances in the worm drive will therefore be magnified by simple lever-ratios.

By comparison: the lovely little BCA ‘jig mill’ has its 180 tooth worm gear machined directly into the periphery of the table.

I wonder what it would cost [given CNC manufacturing] to make an attachable RT in the BCA-style ?

MichaelG.

[Howard LewisParticipant @howardlewis46836]

Two comments.

If you are worried about the cutter pulling the work, partially tihghten the locking clamps, to resist that tendency.

If you are cutting a slot, use a slot drill, rather than an end mill for the job, to minimise risk of the cutter or work being pulled.

The work or the cutter will try to move; how far will depend on all the clearances within the machine and the Rotary Table.

To allow the spindle to rotate, it must have clearance, so the spindle could move by that clearance. For the quill to move up and down, it must have clearance (Can be minimsed by clamping the movement)

So if all the clearances and backlashes add together, there can be substantial movement.

A couple of two thou clearances in the milling machine head, and another couple in the RT and you could have a six thou movement without anything being out of drawing spec.

The best that yiu can do is to clamp everything that does not have to move, and then hope that the machine, and clamping and workholding devices are all sufficiently rigid.

And the nachines that we are using, generally are either new, built down to a price, or ex industry with the wear from lots of hard use. Either way, it is unrealistic to expect the performance obtainable from from a new machine with a six figure price tag.

Howard

 

[Michael GilliganParticipant @michaelgilligan61133]

I have no desire to argue about this, Howard … but I would respectfully suggest the most of the clearances you mention could be ‘designed-out’ and by competent manufacture the improvement could be achieved.

Please don’t ask me to demonstrate any of this  … [a glance at my profile will explain why]

I have already mentioned that the BCA used a full-diameter worm-wheel, but it’s also worth noting the central bearing arrangement … This is not ‘Rocket Science’ it’s just good Engineering.

MichaelG.

.

Snippet from the user manual:

[Howard LewisParticipant @howardlewis46836]

Michael,

As you well know, for one part to move relative to another there HAS to be clearance, so they can’t be “designed out”. The clearances have to be there, and cutting forces will move one part relative to another, to take up that clearance.

And the clearances will add up.Of you say 0.002″ clearances, quill to head, quill to spindle, and then table to Rotary Table body, you can reach 0.006″possible movement.

I’ve spent too many years trying to convince folk that tolereances add up, yet with all parts to drawing. In one epic instance the build up of tolerances in each plane (+/- 0.005″)for a large number of parts could total nearly 0.750″!)

The only clearance (backlash) that can be removed is that between worm and wornwheel.

To prevent other clearances being closed up, the only way is to clamp the parts together. But one clearance has to be maintained for movement, if the intent is cut an arctuate slot.

Maybe, we should agree to differ!

Howard

[Howard Lewis]

On Howard Lewis Said:

[…]

Maybe, we should agree to differ!

Howard

Not for the first time, Howard

I wasn’t trying to suggest that ALL the clearance could be ‘designed out’ … simply that numbers you cited are only relevant to the fundamentally poor design that you were referencing.

Before patronising me any further, would you please do me the courtesy of considering the ‘tapered spigot’ arrangement that BCA used … Very much like the front bearing on my ML7R [or on the ground glass stopper of a  decanter] this is intrinsically self-locking, and is only kept free-running by the controlled insertion depth.

Tenths of a thou are quite sufficient clearance … if it’s built properly.

MichaelG.

[Tony Pratt 1Participant @tonypratt1]

We now have a months worth of comments on this thread, cutting to the chase as has been said ‘there will always be clearance/backlash’, to the not so mechanically sympathetic people reading my response the answer is ‘eliminate the clearance/backlash effect’ by always machining/rotating the rotary table in one direction. It’s as simple as that!

Tony

[Howard LewisParticipant @howardlewis46836]

For the sort of equipment that most of us can afford, clearances, and tolerances will be relatively large.

I studiously avoid climb milling .  If there seems to be any risk of the work “jumping”, the clamps are applied, partially, in an attempt to use as a backlash eliminator.

If I could afford top level industrial machines, backlash eliminators would solve my worries.

Michael,

I was not patronising you, merely disagreeing on the basis of forty years experience as a Development or Quality Engineer for world wide leaders in their field.

The size of the clearance can be minimised by design, at a cost, (You don’t get a Leica for the price of a Box Brownie) but cannot be entirely eliminated, if the device is to be able to function. (Unless the product can be designed with preloaded taper roller bearings at every point of rotation; but linear movements pose greater challenges).

The tapered spogot will remove clearance; (Just as we use Morse or R8 tapers) until you want one part to move relative to another.

Even then, the machine scantlings have to be sufficient to minimise deflection under load.

I think the argument is degenerating into about how many angels can dance on a pin head

Howard

 

[Michael GilliganParticipant @michaelgilligan61133]

We have agreed to differ … end of !

MichaelG.

[PeteParticipant @pete41194]

I’m not arguing at all, but that BCA was built from the start and sold as a very light weight “jig borer”. And from my collection of older M.E. magazines, they were when still being manufactured approximately 3 times what a brand new Myford Super 7 cost at the same time. So your point is hardly an apples to apples comparison to those built to a price point Vertex rotary tables. But I’d also highly agree that some basic part checks every so often and possible machine tool and finish grinding adjustments could seriously improve what were buying for probably not a whole extra added to the final selling price.

Moore, SIP ect built extremely accurate and almost totally back lash free rotary and optical dividing heads for use on there jig borers and grinders. That also took a massive amount of time, effort, expense, custom built machines, tooling as well as a highly skilled work force. So yes you could lower clearances and back lash to very low levels. But we all understand that can’t ever be fully eliminated. You also can’t do so at the prices almost any of us could afford. I’ve had my 6″ Vertex table apart for cleaning and re-lubrication a few times. Both it’s bronze worm wheel and steel worm appear to be hobbed and/or just machined for it’s worm. And certainly there not even finish ground in my example. So it leaves Vertex’s guarantee claims about being within 40 Seconds highly suspect to me. Luckily and for most of us, whatever there actual accuracy is, that generally works out as being close enough I guess.

 

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