Emco FB2 mill - weird Z position shift

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

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,

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.

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?

I think the spring is to keep the quill/spindle retracted into the head when the quill feed handle is let go. The lower end of the spring rests on parts #12 which is kept in place by the housing of the head. The upper end pushes parts #19 , which is attached to the upper end of the spindle upward. There is another coil spring in the quill feed mechanism so may be the question is why two springs are needed.

Oh, good grief! Of course it does! Thank you. What a monumental blind spot! Clearly, I'm losing it...

The small torsion spring fitted to the Quill feed pinion is to return the Drilling Depth Stop to zero. It has no part in returning the Quill, this is taken care of by Detail 25 above.

I did forget to respond to your query concerning the 0.04 mm error in the Y-axis. (You are not alone Kiwi Bloke when it comes to losing it). My machine too has a similar error but only 0.03 mm over the 200 mm parallel in the Y-axis. It was something I discussed with John Slater a while back.

While the error is present with the Head close to the table, it disappears with the head at the top of the column. Now there is a conundrum. I concluded, rightly, or wrongly, that the weight of the milling head induced a bending moment in the column. Thus as the head is raised from the table the error diminishes. There is more often than not a milling vice, or a rotary table on the table, and this does not include the drill or collet chuck protruding from the spindle. The milling head usually spends most of its time half way up the column.

Thus I concluded to leave well enough alone as it was probably what Emco intended. The fact that your machine has a similar reading leads me to believe this is the case.

Regards

Gray,

Hi, just further curious about this.

Say you turn a test piece in a lathe with with a round tip as had already been done, but now above the round tip you turn a 5-10mm thick quare shoulder on this test piece and insert it in thec spindle.

Now you add the set indicator at the bottom touching the round piece, and add another dial test indicator resting on this turned shoulder. Now my question is what does the 2nd indicator indicate as the lower set indicator is deviating as the rotation is reversed-? I am I right to assume if the dial test indicator show no deviation, nothig has happenned in the spindle as it was reversed by hand-?

Edited By Chris Mate on 09/02/2022 16:49:15

Edited By Chris Mate on 09/02/2022 16:49:49

Edited By Chris Mate on 09/02/2022 16:50:32

Hi Chris,

What you suggest amounts to the same test I did using a clock on the end of the spindle, while rotating the two lever clocks.

Admittedly the surface I was indicating off had not been ground. It did give a consistent High and Low spot, which was only 0.01 mm, and not the original 0.05 mm. Which for a post heat treatment surface was pretty good in my book.

Your test does however rely on the collet running dead true. Any run-out in the collet would make the flange wobble, even so your method should be a lot nearer than the original tests.

Regards

Gray,

Hi, If I understood the original test correctly, its only the difference indicated when the spindle changed direction in turning by hand, and that happened anywhere in one circle the spindle turn. So that mark/identify this argument, ignoring other forms of runout. This was an up/down movement not expected.

Now I would like to see if this particular induced up/down movement is from the spindle. So in my suggested method, if it shows on the tool set indicator as soon as direction is reversed any place in one circle, what would the 2nd dial test indicator indicate on a flat surface not on the outside but on the top side of a surface machined that can only reflect up/down movement of the spindle under correct circuimstances. If it needs to be grinded ok, then so be it for those that can do it. My idea is still measuring on a round but flat surface unlike the tool set indicater pointed upwards towards a roundy pointed surface.

Note:Maybe the idea I suggested must be cut in a lathe 1st, then placed in the spindle a re-cut truely with lathe cutters positioned to do that. I have seen another guy on youtube cutting on a mill like in a lathe, then you know it runds true to the spindle, and only the up/down movement in question should reveal or totally disappeear and changed to a new direction based on true cut to the spindle in question-?


I am I correct understanding it like this-?
 

Edited By Chris Mate on 09/02/2022 20:32:52

I'm a bit wary about contributing again, having made a fool of myself over the spring, a few posts back. The stupid thing is that I knew about the spring, a long time ago, but had forgotten and now got confused. It's an age thing, I suppose, and it's only going to get worse <sigh>.

Chris: yes, I think I understand what you're suggesting, however previous posts have suggested test set-ups which are, I believe, equivalent. Graham Meek and myself have 'clocked' the end of the spindle: it may not be absolutely 'flat', but if it's not quite flat, or if its surface is tilted wrt the spindle axis, any error due to these causes will show up as cyclic errors, or a pattern of deflection that repeats each revolution. That isn't what the original video shows, although, to be pedantic, one has to infer that from the spindle being turned approx. 180 degrees each time, rather than continuously. We could be seeing a cyclic error, but let's be generous to the OP and assume that's not the case.