Shimming Techniques

You said earlier the difference in shim thickness is only about 6 thou from bolt to bolt. And the bolts are, what, about six inches apart? So that's a taper of 1 thou per inch, or very slightly more if the bolts are a bit closer.

Not much to worry about there. The shim material and the base and column materials will either compress or distort enough to take up this kind of tiny gap.

If it were me, I would make up some L shaped shims with each leg of the L about an inch long. Then slide them into the gap at each corner, outboard of the bolts and cinch it all down. If I were being totally obsessive I might slide a bit of whatever shim fitted into the gaps mid-point between the bolts and tap it home to a tight fit.

I wouldn't use standard epoxy as it is too flexible and compressible unless the specialist stuff containing metal particles etc.

But I certainly wouldn't agonise over a taper of one thou or so across a shim on a roughly machined, highly flexible cheap Chinese hobby machine.

Thanks Hopper.

What you describe is pretty much exactly what I've got now. The width of the "L" shims is about 4 mm or 5 mm. Any more than that and it clashes with the bolts.

Each bolt looks to be within a machined rectangular pad of about 30 mm x 25 mm:

I

I can't put shims half way between the bolt centres, because it would be in the unmachined pocketed parts of the base.

If you imagine only the outer two edges of each pad being shimmed to a maximum of 5 mm from the edges, that's my current setup.

To me, it looks precarious, and that is why I wanted to get significantly more contact area.

Edited By Dr_GMJN on 18/12/2020 15:03:12

Sorry for butting in but. Is there any way you could have the whole assembly re machined to the correct spec. Or is that not an option.

Steve.

Steve - I suppose I could do, but I'd expect it would be quite expensive. As someone mentioned, there would be plenty of evaluation, setup and testing required, because I want the spindle axis perpendicular to the x/y table rather than the base itself (even than, moving the head in z might result in a horizontal shift of the tool due to column/spindle misalignment).

I managed to get it right by shimming, so I'm happy I know what to adjust etc, it's just that I'm not happy with the bearing area of the shims I ended up using.

Thanks.

Yes i thought it would be awkward to do. I do hope you find a way. When you mentioned JB weld or someone did. Have you looked at the different version they do. When i bought some ,there was different strengths. Also many years ago i had some 2 part resin that claimed to as hard as cast iron, it was called BELZONA.

Steve.

Why not cut 4 shims of the required thickness the same size as the pads on the base, with a clearance hole cut in the required place.

Slacken all four column mounting bolts, then remove one completely. Tilt the column to insert the appropriate shim, then replace the bolt loosley - repeat for the other 3 bolts. Tighten down the bolts & check the alignment - tweak the shims as required if it doesn't come good at the first attempt.

Out of interest, what do you have the machine mounted on ? That base casting looks rather "skinny", with a very narrow (front to back) column, so a less-than-substantial stand / bench would not help rigidity.

Nigel B.

One of the points I was hoping to get across in my earlier post was that when you do the maths you almost don't need to worry about the area. A shim is just a very short column. Very short columns have even smaller compression despite high stresses that might be created by reducing the area.

I know its counter intuitive, but that's the reality. All I would say is make sure the position of the shims gives a stable joint, and not one that has a plane/direction of weakness.

Gerry

I'm with Nigel B. on this one - if the shims are the same size as the pads, it can't be any worse than originally designed.

Rob

Nigel:

If the shims are constant thickness, and the same size as the pads, and the upper and lower faces of the joint aren't parallel (which they're not), then once I've got the column aligned, I'm effectively in exactly the same position in terms of shim area am I not?

It comes down to the impossible task of shimming a tapered gap with parallel shims. Hence the initial question about liquid shim.

Gerry:

I don't think it's about compression failure of the materials (if that's what you mean?) it's about the area in compression and its potential effect on stiffness that I'm concerned about. Surely if I've got four patches of c. 250 mm^2 under compression, when the total potential area available is four patches of c. 750 mm^2, there is going to be a corresponding drop in joint stiffness, or at least a potentially detrimental change in the resonant frequency of the system?

Thanks Steve - I think I got the lower strength version. I'll have to check.

Where I used to work we always used u shaped shims 5mm thick and machined them to suit. Easier than fiddling with shim steel.

the upper and lower faces of the joint aren't parallel (which they're not)

The one or both of the castings will flex when you pull the bolts down.

What is the machine mounted on ?

Nigel B.

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And hence my observation that I have used Loctite 638 very successfully in such circumstances.

... Now that I have seen the four-pad arrangement, I am even more confident that it would do the job nicely.

MichaelG.

The mill is mounted on a bench I made from fence posts and other spare bits of wood:



The top is two sheets of plywood glued and screwed together, giving a total of 35 mm depth. The mill base is mounted on an additional a plinth of 25 mm ply - mainly to raise the y-axis handweel to a practical height above the bench surface. The mill base is bolted through the entire thickness of the three layers of ply. Not sure why the photo rotated itself, but there you go:



Basically it's solid, and I don't see this as a primary issue anyway: it's a small footprint mill with a relatively stiff base. You could probably not even bolt it down and it would be fine. It's the column-base joint that's going to be the issue, not the base to table.

Re. the Loctite 638. I'm sure it would be OK, but why use retainer primarily designed for high shear-strength joints in cylindrical components in this application? Its *compressive* strength doesn't even seem to be listed on the Henkel datasheet. Surely I'd be better off using something like JB Weld, that at least has the primary mechanical property I'm looking for listed on its data sheet - and it's pretty high at that?

I assume that JB weld could be applied to the gap after allowing it to cure somewhat, so that it doesn't run everywhere, then tighten in two stages as I proposed previously?

Thanks all.

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I used 638 because we had it available ... and because I had predicted from its known properties, that it should work.

... the sharing of knowledge and experience seems appropriate to the forum, but I am quite content to leave you to find your own method.

MichaelG.

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Quoting from my post on 16th : 

Please note that my successful use of 638 [in its original formulation] as a ‘flat’ shim is clearly outside the product’s design-envelope ... 

Edited By Michael Gilligan on 19/12/2020 08:22:40

Just a thought but have you considered that even if you could get the column/base connection completely rigid that the chatter problem is coming from somewhere else. Could it be flex in the column itself, the head to column connection, the cut speed, feed, depth etc..?

I have only experienced chatter on my SX2P when I have got the set up wrong with the right settings it is fine.

With regards to the base/column fixing you know the shim sizes and positions to achieve alignment plus you know that you have a tapered gap across the shims. I think Jason's suggestion is a good one in applying epoxy and tighten the bolts to achieve alignment with the epoxy filling the tapered gaps, sounds good to me.

The department I was in at work often had to shim parts with anything from peelable shims to nominal 50mm wedge shims. parts were joined with your basic machine screws up to hydraulic (Pilgrim for example) nuts. The only real contact area between parts was in a ring around the hole about twice the diameter of the hole. You could tell this was the case because when parts were separated after some time the parts of the faces away from the holes would have signs of corrosion. We used to mount 50 tonne alternators on 4 pairs of 2 off Ø150mm screw adjustable machine supports with spherical bearing faces. I'll try to dig out a photo.

I think extra contact points between the column and the base would complicate shimming without adding to the stiffness of the joint.

Martin C

Edited By Martin Connelly on 19/12/2020 09:46:19

Thanks Michael,

Yes, I read your note before re. it being outside it’s intended use. That’s really why I was asking further. I’ve got some JB Weld already, so if I was to buy an alternative it would be something more intended for liquid shimming.

Alternator on Vibracon machine mounts. Vibracon technical page

Martin C

Edited By Martin Connelly on 19/12/2020 09:52:04

Thanks Ron.

Is your SX2P column shimmed? If so, what’s the contact area like?

I have considered that the chatter is caused by other things, but you may remember my early posts back in May/June where I was trying to figure out the issue. I tried all suggestions from tooling, to materials, to ensuring axes are locked, to adjusting gibs, to climb/conventional milling. I tried everything suggested but the issue remains. The column joint has always been at the back of my mind, because I know it’s not great.