How to use a round column mill

[iNfParticipant @inf]

Posted by Graham Meek on 05/07/2019 12:18:19:

I think the devil is in the detail. I know the Emco FB 2 uses a high specification material for the column. If the "clones" have not followed with this material then there is going to be differences.

Some of the round column belt driven mills I know have cast iron, or cast steel columns. This would be considerably cheaper to produce than something like the Emco column

Not being able to use my mill at the moment I took the feedscrew off to start adjusting the fit on the column and found this.

So it looks like Gray hit the nail on the head and the column is cast. It does seem reasonably substantial, the machined part is 6.5mm thick and the rough cast area about 10mm.

Dave

iNf

[BWParticipant @bw]

If all you want to do is change the resonant frequency could you fill the column with fine dry sand and give it a few months to compact down and then top it up again ? Maybe some sort of manual compaction as you fill the column might speed up the process ?

Bill

[Howard LewisParticipant @howardlewis46836]

If you have a problem with a resonance occurring in the running range, it needs to be tuned out, ie changed.

It can be tuned to a frequency below that of the running range, or above it.

To tune the resonant frequency upwards, the stiffness needs to be increased. Bracing, or thickening the column may go some way towards this. Although thickening may well increase the mass and have the opposite effect.

To lower it, the stiffness needs to be decreased, or the mass increased.

If you have a STRONG bench, pour molten lead into the column of your machine?

High frequencies can be absorbed , as in straight through silencers for motor cars. Low frequencies have to be lowered by increasing mass, or softening the suspension, such as low Shore hardness rubber mounts.

If you have ever seen a system when the exciting frequency approaches, passes through, and then retreats from the resonant frequency, you realise that the dynamic magnifier graph has a very sharp spike. 50 rpm either way and all will be quiet, but at the resonant frequency the amplitude of vibration increases at an almost incredible rate.

Nature gives nothing away, there is always a price, of some sort, to pay. Beware of what you wish for!

Howard

[Graham MeekParticipant @grahammeek88282]

Hi Dave,

Although the cast iron is reasonably thick, it needs to be given the duty it is being asked to do. While it is a good bearing material for machine tools and lovely in compression. In tension and torsion it is not so good, hence the thickness.

Another problem with cast iron is the nature of the material can change through the casting. Far better results would have been had if the part had been a spun cast tube.

This is one of the reasons I suggested a stud down the centre under tension. It would give additional strength to a cast iron tube by holding it in compression and thus balance out any bending moments due to machining and drilling. Plus any bending moment due to the weight of the overhanging milling head. It would also improve the torsional resistance.

By stud I am not thinking a piece of Screwfix 12 mm, what I had in mind was 25 mm diameter EN24T. It cannot be larger than this as it will not pass up the mandrel of my lathe. Some fine threads on the end would allow a reasonable torque to be applied, (yet to be worked out).

Adding this stud should also tune out some of the natural frequency mentioned above. One other thought that crossed my mind while thinking about this mod on the steel tube of the Emco, was to fill the void with oil under pressure. The pressure of the oil could be altered to increase the tension on the stud or lowered to reduce it. I would not advocate trying this on a cast iron tube.

Regards

Gray,

[AdrianRParticipant @adrianr18614]

Yes I think keeping the iron in compression is the best idea. Any internal pressure from either expanding filler (setting /thermal) or ramming is likely to burst the tube.

There is one possible problem of doing this on the ZX-16. From the drawings the tube mount to base is not the same as the FB2. On the ZX-16 it clamps to the top of the base casting, with the end of the tube directly in contact with the base. So to add the stud would mean drilling through the base casting and using a plate to spread the load

The vibration I have seen is not due to resonance. I have run the mill at a low speed with a large 2 flute and have seen the head get shunted sideways about 2mm

[Graham MeekParticipant @grahammeek88282]

Hi Adrian,

The column on the FB 2 is counterbored at each end to take the 60 degree cones to allow the machining of the outside diameter. The counterbore at the top end also has a floating plate retained by a circlip, to attach the feedscrew bracket. The stud idea uses these counterbores to both seal the column and take the stud plates. Nothing would protrude beyond the column ends.

Also on my mill I have made new Gib strips that almost within 0.03-0.05 mm fill the space available for them. Too many of the machines today have loads of space for the Gib strips to move around in. I am beginning to wonder if that is the root cause with your machine. Pushing with my feeble frame on the head of my FB2 I can only deflect a clock 0.02 mm with everything locked. You can see the one on the X-axis in the photo's above.

Regards

Gray,

[Clive FosterParticipant @clivefoster55965]

Hi Adrian

Your report of the head being shunted sideways in your last post pretty much confirms my opinion that you need to take a very close look at the fit of the head on the column and the action of the "close up the slit" clamping / adjustment systems. Especially as you have a key and keyway involved. Closing slit systems are simple can can be very effective but the design of a good system can be more subtle than you might expect. Small dimensional errors and clearance variations in the wrong places can seriously reduce the effectiveness.

For something like your milling head you ideally need three pretty much eqispaced, equiforce line contacts either all the way down the head or in two shorter sections top and bottom with the centre relieved a few thou for clearance. The idea of relieving the centre is to ensure that you don't end up with pretty much horizontal line contact round the middle which will naturally let it rock tiltwise. Eqiforce is essential because under intermittant loading the head can vibrate slightly more or less in the plane of the weakest clamp. If things don't snug down evenly you can easily end up with one contact being little more than a firm touch rather than a clamp. Where you have continuous or near continuous contact over significant areas you can also end up with what might be called a false contact that initially snugs in the wrong place giving a weak hold shifting to the proper, but actually a trifle loose, place under load. Very similar effect to fretting.

Having a keyway down the side makes the casting less stiff there so altering the stiffness of the casting. Hopefully this will just shfit the contact points a bit. However with a key strip and gibs inside the keyway its possible to end up with a situation where tightening clamps mostly bends the keyway a fraction so the key is doing most of the work and the contact areas on the column are more touch than grip. Vibration under intermittant loads will be almost certain if this happens.

I have disassembled systems where similar effects have sometimes been present. Small areas of wear and fretting showing where contact had actually occurred rather than where it was supposed to. Disassembly being undertaken to find out why some of the units wouldn't stay tight and needed regular re-torquing whilst others stayed up after assembly. As I recall it the problem was due to fairly casual tolerances and machining alignments. Field cure was to simply file or scrape off the high spots indicated by wear or fretting. A thou or so making all the difference.

Long split clamping systems on circular column or rods are not my favourite design. Were I to use such I'd always relieve the middle to localise forces and ensure things behave.

Clive

[Mark Gould 1Participant @markgould1]

Graham, that looks like a fantastic spndle lock on your FB-2. Do you still have any drawings for it? It’s one of the less popular things on this otherwise great little mill and together with powerfeed on Z the only thing I would like to add. If you have anything left and don’t mind me copying your design I would love to make that addition.

[Mark Gould 1Participant @markgould1]

Silly me, I used my mad Googles skills and found your website Graham!

[egaParticipant @ega]

Posted by Graham Meek on 05/07/2019 12:18:19:

…

Round columns for machine tools have been used for some time, I remember working on Jig Borer, an Oerlikon KC4, which had two round columns.

Does anyone know if there is some particular "magic" about a multi-column arrangement?

I seem to remember a two column toolpost design which claimed to be relatively very stiff. Unfortunately, I haven't the maths or physics to see why this might be so.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by ega on 01/11/2019 10:15:15:

Posted by Graham Meek on 05/07/2019 12:18:19:

…

Round columns for machine tools have been used for some time, I remember working on Jig Borer, an Oerlikon KC4, which had two round columns.

 

Does anyone know if there is some particular "magic" about a multi-column arrangement?

I seem to remember a two column toolpost design which claimed to be relatively very stiff. Unfortunately, I haven't the maths or physics to see why this might be so.

.

I think it is safe to say that the “magic” is cost-effectiveness. … Good stiffness and kinematics, with minimal custom-machining requirements.

At a first approximation, the maths ‘n’ physics comes down to comparing the polar moments of inertia of various arrangements.

The next step is ‘bean-counting’ the cost of manufacture.

MichaelG.

.

https://simple.wikipedia.org/wiki/Polar_moment_of_inertia

But don’t worry too much about the maths … many CAD packages will calculate it from a drawn object.

Edited By Michael Gilligan on 01/11/2019 10:47:40

[egaParticipant @ega]

Michael G:

Thank you. "simple.wikipedia" sounded just the thing for me until I saw the warning at the head of the entry!

I did look at it and noted the distinction between the effect of cross sectional shape and size as opposed to the material, a point made above.

The Barker bar bed lathe comes to mind; according to lathes.co.uk it "employed three steel bars arranged so as to provide a stiff "box-section" mounting for the carriage" The design was the subject of a provisional patent.

[VicParticipant @vic]

I nearly bought one of these but the table was a bit small.

Ixion

I was lead to believe they are very good for the type if you can find one in good condition.

[egaParticipant @ega]

Posted by BW on 07/07/2019 10:40:31:

… could you fill the column with fine dry sand …

Reminds me that one of the complaints about the Far Eastern machines was that the factory left large quantities of moulding sand in the column!

[Author]

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