Shimming Techniques

You could work it out in CAD, but the biggest issue will be cutting contours that are maybe 1 micron between heights

The time-honoured way to remedy this is to scrape the parts into alignment. What error are we talking about?

Works for square and rectangle. As long as you get shims round the bolts, the shims are nipped without a lot of load on the bolts, and then the bolts are tightened down properly I don't think you need worry about the bits in between. However you could always add bits to the centre of the edges (if you see what I mean) these should be the average of the 2 adjacent shims, again assuming the surfaces are flat. The problem is that if these are even a tad too thick they will stop the ones under the bolts seating properly, so I'd make them a smidgeon thinner than they should be and use the epoxy, anything will do, as someone else said it is very thin so very stiff. I just did the 4 corners on my Naerok and no further trouble.

I'd get the tramming right with the head at a useful working height, it might change as the head is moved up/down

Please see my posts on this brief thread from June-2020 : **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=166118

MichaelG.

[ Loctite 638 Appreciation Society ]

Michael what sort of gap did you use it on, I would have thought 5 thou or so too big for it to go off.

I think Loctite claim 638 is good to a gap of up to 0.25mm - 0.010"

Dont know how true that is. 

Edited By Ron Laden on 16/12/2020 07:57:39

Edited By Ron Laden on 16/12/2020 07:58:32

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It works fine at that sort of thickmess

... I have removed substantial ‘flakes’ of solid material after sepatating a test-piece.

MichaelG.

In that case it would be easier to let some Loctite wick into the gap once the column has been adjusted with shims and no need to rush to get it done before an epoxy goes off.

As Loctite 638 is a "High strength Retainer", could using it as a gap filler cause problems if dismantling is required in future ?

Loctite do another product "Quick Metal 660" that is claimed to fill gaps up to 0.5mm & one of it's uses is described as "retaining shims". At around £40 for 50ml, another expensive solution though.

Nigel B.

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As I stated in the other thread, Nigel ... its peel strength is relatively low : so the answer to your question is:

It depends on the mechanical design of your joint and where you put the 638

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

MichaelG.

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P.S. ___ For Jason: 638 is a high viscosity liquid, so unlikely to ‘wick’ successfully.

Edited By Michael Gilligan on 16/12/2020 09:48:22

Thanks all. So it's not a completely daft idea to use epoxy after all!

I think that since the compressive strength of the epoxy is much less than steel, the best way forward would be to:

1) Determine the torque I'm currently tightening the bolts to (using a torque wrench).

2) Put the epoxy under the column and replace the steel shims that gave perfect alignment.

3) Torque to within, say, 20 Nm of the previously determined torque value.

4) Check alignment.

5) Finally tighten by diagonally torqueing the bolts back to the original value (ie nominal +20 Nm), making fine adjustments if necessary to maintain alignment.

That way I'd know that the column was seated, aligned, and loaded against the shims. The final torqueing would give me some confidence that the bolts were loaded to a similar extent as they were before the epoxy was in place. It would also make sure that the epoxy was actually being compressed to a greater or lesser degree, which was the overall aim of the whole exercise

Question is how to apply the epoxy equally over the interface area, without it getting into the threads. Someone mentioned o-rings around the bolts, which might be a good option.

BTW the thickest shim in the worst-case corner is 0.013". This tapers to zero at the opposite corner, and approx. 0.006" under the other corners. As I said, these shims are very narrow (about 5mm wide), so this is why I want to get much more area in contact. Increasing the area by shoving the shims further under, always screws the alignment - believe me I tried so many combinations that I was worried about wearing the threads out!. I appreciate what Andrew says about column stiffness not influencing chatter much, but to me the amount of contact area I've currently got is pushing it in terms of what's acceptable.

The most intelligent comment here, gets ignored.

Concrete in pipes is used a lot in industry

Need to find a high quality version, then heat the pipe for a few days/weeks to expel water

I did mention I was a beginner. Short of dismantling the thing and taking it somewhere to be done, I can’t think me trying to scrape it into alignment would end in anything but a complete disaster.

It's such a slow process, it would be hard to seriously mess up, as long as you check it periodically, I can't imagine you'd go far wrong, scraping to remove material doesn't have to look pretty like bed ways, a small surface plate with blue printed onto the bottom of the column might reveal something obvious, inspection I would have thought would be first?

Pete, the issue - again - is conflicting advice. I've been told on other threads that scraping is a difficult task for the beginner, and to avoid it on critical or expensive parts.

You're now saying it's not difficult, so I'm left with gut feel. Having done a bit of research on the process, I doubt very much I'd be able to do it effectively on such an unwieldy and critical part.

On face value, your comment is undoubtedly correct in that scraping would be the established way to correct this issue, but in terms of practicality in the context of me asking how I can correct it myself, I'm afraid it's simply not an option.

There is a fundamental step missing from that video.

In order for the mill geometry to be aligned properly you need to align the column way not to the table but to the table's x and y ways. Using the table is fine so long as you first check that the table's surface IS parallel to the working ways. Doing this is easy - just put a parallel on the table surface and indicate the top of it with a dial whilst traversing the table in X, then turn the parallel and do the same check in Y.

Note that this isn't the only check that you should make if you're properly re-aligning the machine, but it is all you can do without dismantling it. It will at least confirm if the table top is parallel to it's direction of travel which is what you require if you're going to follow the video in your efforts. If you make the above checks and you find that your indicator moves, you'll have to decide what to do next to bring your errors to a minimum because if the top of the table isn't parallel to the direction of travel all you'll be doing is machining steps in your work.

Also note that you cannot use the spindle's rotation to do any of the above (in the traditional sense of 'tramming' ), that would be aligning the table to the spindle's axis, not the column. Aligning the spindle's axis would be the last step in aligning the machine.

Edited By Pete Rimmer on 17/12/2020 12:18:54

Scraping the base of the column would be the most accurate method, but seems to involve a LOT of work and effort.

(The heads of vertical mills are not usually particularly light! ) .

Knowing the out of square:

Remove head from column,

Remove column,

Scrape the base,

Reassemble,

Reclock.

Repeat until a satisfactory result is obtained.

Probably quicker and easier to shim and remeasure

Howard

Not sure I get that, just use a slightly thinner shim and get it well in, I made U shaped ones to slip round the bolts, square profile outside, and make them too long so that you can get them out again

A thinner shim pushed further in won’t do much to increase contact area. There’s only so much compliance the shims and bearing material has, and at the end of the day you’re trying to shim a tapered gap with parallel shims. Sure, there will be some local contact and compression, after which the load will literally taper off.