Emco FB2 mill – weird Z position shift

[Y C LuiParticipant @yclui16187]

I was checking the tram of my Emco FB2 mill today and found something really puzzling. The mill seems to be unable to hold the Z axis position depsite the fact that both the quill and the Z axis were locked. At first I thought the DTI holder of the DTI itself has got something loose but after further checking, the problem seems to come from the quill. The Z position change is as much as 0.05 mm or 2 thou but the wierd thing is the shift depends on the direction of rotation of the spindle prior to taking measurement. I have tried to move the chuck vertically to see if there is any play but there is none. The small deflection of the DTI is just due to the relatively weak rigidity of the machine.

Video showing the issue

No related issues was seen in actual use of the machine. The error in the Z-axis direction is within 0.02 mm.

Any thoughts ?

Edited By Y C Lui on 06/02/2022 06:58:25

[Y C LuiParticipant @yclui16187]

[Kiwi BlokeParticipant @kiwibloke62605]

Interesting. Let's suspect the measurement system first – it's easier to play about with it, and metrology holds many traps for the unwary. I suggest that you replace the milling cutter with a rod which has a small rounded end (ideally), like a conventional DTI probe. Ensure the tip is as co-axial with the spindle as possible. Repeat the experiment. You could also do it under power, with no risk of damage to your indicator's contact pad.

The FB-2 quill is pretty simple. IIRC, thrust is reacted by a pair of deep-groove ball bearings (they might be angular contact), with the outer race of the pack retained in the quill by a screwed ring, and the inner by a collar screwed to the spindle shaft. I don't think there is any provision for adjustment, so the bearings ought to be a matched pair. Anyway, your test shows no significant axial free movement, so I suspect the measurement set-up, rather than the machine – at this stage.

[Y C LuiParticipant @yclui16187]

Here is the video with the tool changed to one with a ball end. The result is the same.

The video

I don't think the measurement system has anything to do with it because this was first noticed with an entirely different set up in which a DTI was connected to the spindle for checking the tram of the machine. It was the inconsistent tram measurement that led me to discover the issue.

Edited By Y C Lui on 06/02/2022 11:33:03

[blowlampParticipant @blowlamp]

It almost looks like something is unscrewing.

Can you isolate the measurement to just the components of the quill?

Martin.

[EmgeeParticipant @emgee]

The collet closing nut appears to be loose in the last video.that may have something to do with the axis changing.

Emgee

[Graham MeekParticipant @grahammeek88282]

The two matched deep groove ball bearing races were specially made for Emco. They have markings on to ensure that they are assembled correctly.

In the past I have replaced these with angular contact races, but with no real gain as regards machine performance.

Any preloading of these bearings can stall the machine on the highest speed.

The movement on the clock from applied external pressure is to be expected, this is a bench top milling machine remember.

Any checks for spindle play should be done with the clock fixed to the head of the machine, or with the Quill removed from the head, and held in the machine vice, with suitable protection. I doubt any play will be found.

There are a number of factors at play with this set up.

Is the top surface of the measuring device perfectly parallel to the table surface?

Is the end of the Cutter perfectly parallel to the table surface?

Is there any run-out on the collet?

All of these three defects can give spurious results.

Personally if I swing my clock over a 200 mm long parallel on the table, (tramming) and get zero deflection, then that is good enough for me.

Regards

Gray,

[Y C LuiParticipant @yclui16187]

Posted by blowlamp on 06/02/2022 12:04:09:

It almost looks like something is unscrewing.

Can you isolate the measurement to just the components of the quill?

Martin.

Not sure how it can be done ….. anyway, this is the construction of the spindle / quill of hte machine :

**LINK**

[Anonymous]

The test tells you nothing about the mill, as you don't know if the corners of the endmill are ground symmetrically.

Andrew

[blowlampParticipant @blowlamp]

Posted by Andrew Johnston on 06/02/2022 12:36:05:

The test tells you nothing about the mill, as you don't know if the corners of the endmill are ground symmetrically.

Andrew

You need to see his second video, Andrew.

Martin.

[Martin ConnellyParticipant @martinconnelly55370]

Is the spindle spring pushing the spindle down or up? Up is usual for drilling machines but it should be down for milling.

Martin C

[Tony Pratt 1Participant @tonypratt1]

Posted by Martin Connelly on 07/02/2022 07:52:45:

Is the spindle spring pushing the spindle down or up? Up is usual for drilling machines but it should be down for milling.

Martin C

Hi Martin, how to you come to that conclusion?

Tony

[Kiwi BlokeParticipant @kiwibloke62605]

Thanks for the second video. It's impressive and pleasing when someone acts so swiftly on advice given. Now I'm getting worried, because your measurement set-up does seem blameless. It would be even better to remove the possibility that the collet chuck is the culprit, but that seems a very long shot. Perhaps you should try again, turning the spindle from its upper end. It's certainly a puzzle.

It was probably back in the '80s when I was interested in obtaining a replacement spindle, that I discovered that Emco would not supply the component parts of the quill/spindle assembly separately. That probably explains why the exploded diagram, to which you provided a link, shows the quill/spindle as a complete assembly. The parts diagram I have in front of me is probably an earlier version, which shows the internals of the quill. The thrust bearing arrangement, which controls the spindle's axial position (z-axis) is as I described in my previous post. Unfortunately, it's not shown in your diagram.

I'm not sure what the spring (25) does. I think it acts between gear 12 and part 19 ('pressure washer' ), to reduce possible rattling of the gear on the spindle splines. Hopefully Graham Meek will be along to tell us more – he knows vastly more about these machines than I do. I think the gear is helically cut. This means that it will try to move in one direction along the spindle when torque is applied, and in the opposite direction when torque is applied in the opposite direction. Perhaps the gear's axial float is transmitted by the spring to the spindle, but it still doesn't explain why the spindle is free to move axially.

I have noticed that, in some gears, the spindle can be turned in one direction more easily than the other. I assume this is because of end-float in one set of helical gears, but I haven't gone hunting for an answer… Having seen your videos, I'll go and check my FB-2, when time allows. Perhaps, like the garage mechanics say, 'Oh, they all do that, Sir.'

It's beginning to look like one of the screwed bearing-locating collars is loose, but why you can't move the spindle by applying axial force is a puzzle.

Edited By Kiwi Bloke on 07/02/2022 10:08:58

Edited By Kiwi Bloke on 07/02/2022 10:09:39

[Graham MeekParticipant @grahammeek88282]

I have the official Emco works drawings covering the spindle. Its component parts and the assembly of those parts.

The Spindles are graded into three different sizes. As an example Class 1 is 30.023, +0.003, -0.006 mm, the Class 3 spindle is 30.033, + 0.005, – 0.003 mm. Class 2 being in the middle with similar tolerances.

Each Spindle is matched to another specially prepared Needle Roller bearing, made by INA NK 30×20 P6/ H32+26. Hence this is why Emco only supplied this as an assembly. When trying recently to get one of these bearings it was impossible. In the end I managed to track down a Japanese bearing with an extra roller and better tolerances. It is possible to get a new spindle but the owner will need to Lap the diameter to suit his bearing.

The two Special Ball races, FAG 6005 C3.K3 VSPK 10 kp, are secured in the Quill with Loctite 601, so even if the Ring nut came undone the bearings would, should stay put.

The problem I have with the original set-up above was touched on by Andrew.

If the two planes of contact are at a slight angle and complimentary to one another. Then when the cutter is rotated the error would be evident and double the displacement, or inclination.

Similarly the Ball acting on this inclined plane, although it appears to give only point contact. It is in fact touching off to the one side. If that ball is also running out, or eccentric in the collet. Then the contact point will move around on the top of the ball, and will not be directly under the spindle as it is assumed.

Unfortunately straight cut gears transmit the drive to the Quill. The Helical gear is the input drive gear to the gearbox from the motor. Turning the Spindle against rotation does tend to cause this gear to lock-up. As the helical gear is trying to lift the Sliding Gear and its spindle. Any additional force being applied manually to the Spindle to overcome this would affect this set-up.

It is also assumed that there is no side play in the measuring device. Which would allow the top to tilt slightly. There has to be a clearance here for the device to register freely. The unit is after all designed only to set the cutter height in a CNC application and is being asked to do something it was not designed to do.

Regards

Gray,

[Chris MateParticipant @chrismate31303]

Interesting..
 

Edited By Chris Mate on 07/02/2022 12:18:13

Edited By Chris Mate on 07/02/2022 12:19:01

[Chris MateParticipant @chrismate31303]

What is the tool/indicator set called you use-?
With the round point used, it moves when you change direction of spindle and then stable at new indication which ends up either one of two readings, made me think of the inside of the bearings….The play/space between balls in bearing and how much movement there is only when direction is changed(Not referring to other plays in a beasring)-? I was wondering what bearing frequency checker would reveal as a cause ?

Edited By Chris Mate on 07/02/2022 12:55:15

Edited By Chris Mate on 07/02/2022 12:56:18

[Y C LuiParticipant @yclui16187]

Posted by Graham Meek on 07/02/2022 11:39:38:

… the Ball acting on this inclined plane, although it appears to give only point contact. It is in fact touching off to the one side. If that ball is also running out, or eccentric in the collet. Then the contact point will move around on the top of the ball, and will not be directly under the spindle as it is assumed…..

Gray

What you suspect is very reasonable. I used the Z-axis tool setter in the video just because the dial is facing the camera at all time so the correlation between the needle movement and the rotation of the spindle can be seen clearly. Just to eliminate the possibility of inclined tool setter top + collet run out being the cause I have checked again with a totally different set up as shown in this new video.

The table of the mill is not perfectly level though. On the Y axis, the TIR seen in sweeping the DTI over a distance of 150 mm is 0.04mm ( are there any ways to eliminate it BTW ? ), On the X axis, the tram is close to perfect after adjusting the tilting of the head.

Edited By Y C Lui on 07/02/2022 14:01:14

[Y C LuiParticipant @yclui16187]

Posted by Chris Mate on 07/02/2022 12:53:37:

What is the tool/indicator set called you use-?
With the round point used, it moves when you change direction of spindle and then stable at new indication which ends up either one of two readings, made me think of the inside of the bearings….The play/space between balls in bearing and how much movement there is only when direction is changed(Not referring to other plays in a beasring)-? I was wondering what bearing frequency checker would reveal as a cause ?

Edited By Chris Mate on 07/02/2022 12:55:15

Edited By Chris Mate on 07/02/2022 12:56:18

The instrument is called "Z axis tool setter" which is used for setting the height of the tool tip above the mill table precisely so that you can have a reference point to locate the tool relative to the surface of the work piece along the Z axis. It's just a locating tool like edge finders but it works on the Z axis.

The shift of the spindle position along the Z axis is as much as 0.05 mm. Having this magnitude of free movement should guarantee bad finish in face milling but I am not seeing that. As of now I tend to believe that the shift is there by design although I can't figure out what the purpose is and how the bearing is constructed to achieve this.

Edited By Y C Lui on 07/02/2022 14:26:46

[ChrisLHParticipant @chrislh]

It seems to me that the (preloaded ?) ball thrust bearings locating the shaft in the z direction have some slight assymetry in their races causing the balls to track in a slightly different position depending on direction of rotation. Should perform OK though as the spindle usually only rotates in the same direction.

[Chris MateParticipant @chrismate31303]

Hi, put your lathe in freest neutral and do the same test there, see what you get-?

2nd thougt:It would be most interestring to know….
1-if theres a difference with new bearings after installation new mill-?
2-And bearings that run a while in th same direction, minimal in other direction if any-?
2-Bearing that runned both directions evenly -?

Edited By Chris Mate on 07/02/2022 15:00:00

[blowlampParticipant @blowlamp]

To isolate the components of the quill, you can attach the DTI base to the head casting.

Then locate the measuring finger on various parts of the quill, including those that rotate and in addition, also to those that should remain fixed.

Hopefully, this might help pinpoint what is happening.

Martin.

[Martin ConnellyParticipant @martinconnelly55370]

Tony, milling cutters get pulled down into the work. If you have the spring working against this it is like climb milling with backlash. The spring pulling up with drills doesn't matter, it allows you to let go of the handle to withdraw the drill, but if you are trying to mill at a fixed Z height and there is any chance of the spindle pulling down because the Z axis is not locked hard enough, if at all, you will get Z axis errors. The spring on a mill should be set to assist in lifting the spindle only, not to lift it without other input.

Discussion on Practical Machinist regarding spring bias.

Martin C

[Graham MeekParticipant @grahammeek88282]

Further to your last set-up I have tried the same set-up on my FB2, which has a brand new spindle and angular contact bearings instead of the Ball races.

Using a clock similar to the one shown in the last video, I had 0.03 mm deviation but in the opposite sense. Using my Mitutoyo clock in exactly the same way I had 0.02 mm deviation, but again in the opposite sense.

During these test I had a DTI resting on the spindle nose flange. While this face is not ground after heat treatment of the spindle. I had a consistent 0.01 mm deflection which ever way the spindle was rotated.

The next test was to use a Dial Gauge or "Drop Clock" to check this out. This was set up in my boring head to get the ball stylus running absolutely true on the machine centre-line. With this test I had 0.005 mm variation during the two opposite rotations.

As a last check I used the Dial Gauge held in the Machine vice and indicating on the end of the Spindle. I had the same 0.01 mm deflection I had earlier irrespective of which way the spindle is rotated.

All test were carried out using a long Allen key in the draw bar socket.

One problem I have learnt over the years in my profession is that the lever type DTI can give two different readings when presented to the work in different directions. It is due most probably to the clock construction.

This I believe is what is at play here.

Lastly, while the Lever clock has a good range of adjustment on the stylus. Its accuracy as a measuring device is limited. Most Manufacturers used to stipulate a maximum angle either side of the centre-line, or in-line position, after which the scale on the dial was meaning less.

Regards

Gray,

[Y C LuiParticipant @yclui16187]

Gray, Thanks for taking time to do the test. You are probably right because I am not seeing any problems in actual use of the machine. The finish of face milling is good and I am getting good accuracy ( within 0.01 mm ) in the dimension of the workpiece in the Z axis ( not so good for X & Y though ) so I think I will leave it as it is.

[Kiwi BlokeParticipant @kiwibloke62605]

Hmm. Following Graham Meek's words of wisdom, I think I was premature to consider the measurement set-up blameless. My theory about helical cut gears and end-thrust can also be safely ignored. I'd still like to understand what the spring is for, though.

I have just set a 0.01mm per div. plunger-type DTI against the end of the spindle, with the mag base on the milling head, quill locked. A quick and dirty test. Turning the spindle in each direction gives about 0.01 mm TIR, and I can't induce axial movement of the spindle by gorilla tactics.

I'm not familiar with tool-setting indicators, as used in the first video, so can't suggest how one might contribute direction-specific error. I don't buy the suggestions about indicator probes bearing on a surface that is not accurately at right angles to the spindle axis, in both orthogonal directions. An inclined plane would show up as a cyclic indicator deflection, in either direction of rotation, not a fixed deflection, as demonstrated. However, it's common experience to see a stable indicator deflection dependent on the direction of travel of the surface wrt the probe tip, for what should be fairly obvious reasons.

I suppose the pragmatic answer is to stop worrying, and register it as a curiosity, so long as you continue to have no problems with work produced on the machine.

Going off at a tangent… In an application like this, where the needle roller bearing and spindle are selected for desired (lack of) clearance, presumably a degree of radial preload is desirable. How then is the spindle assembled into the bearing? Will it slide in, or has it to be 'screwed' in? And if the needles tilt a little in service, perhaps because the shaft is not precisely parallel, will this induce axial deflection?

[Author]

Posts