Shimming Techniques

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If your difficulty is that... “Its *compressive* strength doesn't even seem to be listed on the Henkel datasheet.” then, to a first reasonable approximation, you could use the figures for Perspex.

MichaelG.

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Edit: 

Maybe Duncan Webster could be persuaded to do the sums for a material thickness of [say] 0.1mm

Edited By Michael Gilligan on 19/12/2020 10:07:42

Thanks Martin.

I hear what your saying about adding area not making a difference to stiffness, but I’m thinking in this case I’m not really adding area, rather I’m trying to get back to something approaching a minimum design area.

If we assume the joint was actually designed rather than guessed (I know, I know), say it will still perform with 50% original contact area due to shimming - I’m currently nowhere near that 50%.

I always get back to thinking along these lines: taken to an extreme of assuming contact area doesn’t affect stiffness, it would mean that you could have four contacts of, say 4 mm^2. Would that really be as stiff as the same joint with four contact patches of c. 700 mm^2 area? OK it’s an extreme example, but at what point is the area not an extreme example.

The way I see it I need to Maximise the contact area while being able to maintain perpendicularity with the table.

Now I have seen what we are talking about, I have to wonder what makes you think that very flimsy casting won't pull into shape and sit nice and flat on larger shims once you tighten those bolts down?

We are talking only a few thou of variation across each shim, if you make each shim the size of one of those four pads, with a U cutout for the bolt . Each one of those bolts will exert up to about a ton and a half of clamping force. That's about ten times the force needed to pull that little casting into shape to match the surfaces of the shims.

Just whack the large flat shims in there that give you correct column alignment with the bolts tightened and carry on milling.

KISS -- Keep It Simple, Sam.

Yes I shimmed mine with cut pieces of feeler gauge and I guess I was lucky in that I have only needed to shim the two front pads.

My column was leaning forward and I found that a pair of 0.005" shims corrected that error, the shims (20mm long) where fitted along the Y on the outside of the bolts. I then found that the column was leaning left in X so changed the right hand shim to 0.002" which was too much and went for 0.003" this put both the X and Y quite close. You will know that tramming with shims is a lot of trial and error and I finally arrived at 0.003" front right and 0.005" front left but in a L shape running down the side in Y and across the front in X. The two rear pads are not shimmed at all.

Out of interest I just checked to see if things have settled and I have as near as damn 0.001" in Y across 135mm and 0.0015" across 175mm in X, I,m quite happy with that for the work I do.

Ron

Hopper, the whole reason for the thread is that when shims of reasonable area are fitted and the bolts tightened, the column deflects out of true. Much trial and error showed that the only reliable method required shims of marginal width. This is because it's a gap that's tapering diagonally, and the lowest corner is always going to press into its mating surface while leaving the opposite corner of its pad in fresh air.

Thanks Ron, yes shimming two adjacent corners was fine - to take out error in one axis. As soon as I try to correct for the other axis, that's when things get messy.

I know its very easy in theory, and adding another full shim to the third corner should be trivial with a bit of trial and error, but there was just no way it was having it with shims of reasonable area.

Here have a look at this site. This is the company i mentioned. They do liquid metal. They have a place in Harrogate.

Belzona

belzona polymetrics Claro Road. Harrogate. HG14AY. Tel 01423 567641.

Steve.

Get some spherical washer sets for the four bolts, shim them individually with u shaped shims and that will sort out any taper in the joints. You will raise the column by the thickness of the spherical washer sets but get the best joint stiffness possible. The limit on stiffness is the base casting and column fabrication not the contact area between the two parts. Shimming only sorts out the orientation of the parts and cannot be used to increase stiffness. Poor methods may introduce a slack joint but good methods cannot increase stiffness beyond the that of the design.

Mount the column with 3 spherical washers and no shims. Measure the gap for the fourth spherical washer. If it is larger than the remaining spherical washer then shim it to fit the fourth space. If it is less then shim one of the fitted washers to allow the fourth one to fit. You then have a starting point to measure and shim the column for a solid joint with no soft foot problems.

Martin C

Martin - I'm saying that the shims I've ended up fitting will probably have reduced the joint stiffness - I'm not hoping to increase it beyond what it would be for full contact, just wanting to get back to something like it was without shims.

Interesting idea about the spherical washers. I guess they'd have to be slightly wider ID than the bolts, since they have to move horizontally a small amount in order to orientate themselves parallel with each other? Maybe standard clearance would be enough for the amounts we're taking about. I will investigate sizes.

Thanks.

This is a spherical washer set, it's just an example of what I am talking about, you would need some to suit the bolts you have.

Ø13 bore spherical washer set.

Martin C

Basically it's solid it could be better, and I don't see this as a primary ssue anyway it could be a contributary factor: it's a small footprint mill with a relatively stiff flimsy base.

Fixed that for you.

You could probably not even bolt it down and it would be fine

Doubtful. Bolting down to a substantial foundation is a pre-requisite for many heavy machine tools to perform as designed - why do you think a lightweight hobby mill would be any different ?

It's the column-base joint that's going to be the issue, not the base to table is an assumption

If the plywood base has no bracing underneath, it could be that the mill is just sat on a diaphragm. If there is no brace, adding a 3"x2" stiffener fore & aft under the centreline of the mill base (long side down), glued & screwed could help, as could replacing the top raising section of plywood with a pressed concrete paving slab & bolting through the lot. Plywood is a lot more flexible than concrete.

Don't under estimate the value of a solid foundation - it makes a big difference to large machine tools with far more rigid castings than a hobby milling machine. If you dig around, you will find reports of users of similar style hobby machines to yours resorting to adding a substantial bracket from the top of the column to the building wall to stiffen up the machines.

Maybe look at some other aspects before fixating on complicated "work-arounds" for less-than-perfect construction aspects that I suspect will not yield the improvements in performance that you desire ?

Nigel B.

You said this in your OP!

I always ended up chasing my tail - it's all perfect apart from one shim in one corner, which when you put it in place and tighten the bolts upsets the rest.

All I would have done is put the dial gauge on the fourth bolt tab, read the extra deflection given by the tightened bolt and increased the shim by the same amount .

Then try a cut and see the difference rather than just assume it will be rubbish.

This is a sort of mind dump, no sums involved yet. A preloaded bolted joint with the faces in contact acts as tho the joint was not there, solid metal, until the load applied exceeds the preload. All of this preload is applied via the underside of the bolt. I've no idea what size the bolts are, but if they are M8 that works out at (13^2-8^2)*pi/4 = 82 sqmm (not allowing for a clearance hole which will make it smaller). If they are M10 it comes to 148 sq mm. As long as the shim area is greater than the bolt head area I think it should be OK, so at 5mm wide they need to be 16 or 29mm long, which ties in with the photo.

As Gerry Madden has said a thin shim will not affect the overall rigidity by very much at all. A 0.01" thick shim 1 sq in has the same spring rate as a piece of steel 100 sq " and 1" long. I'm still not happy that the shims are not under the bolts, doesn't feel right!

Duncan, if we’re talking about preload in the joint itself, and if I understood what you wrote correctly, the preload is not confined to the area under the bolt head. It depends on the joint, but load falls to zero some distance from the bolt axis - can’t remember the figures but it’s something like 2x to 4x bolt radius.

Now you're making it complicated. The load is applied to the area under the bolt head, but it then spreads out the deeper into the material you go. How far it spreads out is dependent on the thickness of whatever is being clamped and it's material properties. If there is a shim washer round the bolts the stress will spread out for some distance, then come back in again to the diameter of the shim. If we knew the size and thickness of the vertical column, and the position of the bolts, we could with a very complicated sum (or some FE) compare its stiffness to the assembly of bolt and shim, but as that is very short it doesn't matter anyway

Duncan - I see what you're getting at now, and that's all fine for a shim coaxial with the bolt, but what I have now is a shim offset from the axis. So instead of pure compression through and around the shimmed joint, I've also got a degree of bending and shear.

I think this is what you're alluding to with your comment about not being happy that the shims aren't under the bolts? This is why I wanted to use liquid shim - or metal shims that approximate the gap more closely, and getting back to something like pure compression, which I assume will give a stiffer joint.

Are we on the same page really or did you mean something else?

That's right. You will get some deflection under bolt pressure. So you need to adjust your shim thicknesses until the column comes into correct alignment when the bolts are tightened down.

"Leaving the opposite corner in fresh air" seems unlikely when the variation across one inch or so of pad is a couple of thou. Clamping pressure of the bolt will close up that few thou.

Have you used a feeler gauge to actually measure if there is a gap anywhere around the pads when the bolts are tightened fully down? You can't manage what you can't/haven't measured.

Hopper - yes, that's what I mean by trial and error. I just can't get it right with a full shim set. I just go round in circles.

I didn't mean that I'd left one corner in the air and simply torqued it down...

Then you are going to have to make your choice: Full shims and persevere until you get it right. Narrow shims and live with your doubts about stability, or liquid shim in addition to the narrow shims and call it good.

Your mill. Your choice.

Some interesting numbers here: **LINK**

https://www.bestech.com.au/wp-content/uploads/Modulus-of-Elasticity.pdf

... I love it that ‘Carbon nanotube, single-walled’ is listed amongst the “common Materials”

MichaelG.

Edited By Michael Gilligan on 20/12/2020 00:11:07