Shimming Techniques

[duncan webster 1Participant @duncanwebster1]

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

[Dr_GMJNParticipant @dr_gmjn]

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?

[HopperParticipant @hopper]

Posted by Dr_GMJN on 19/12/2020 10:52:01:

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.

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.

[Dr_GMJNParticipant @dr_gmjn]

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…

[HopperParticipant @hopper]

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.

[Michael GilliganParticipant @michaelgilligan61133]

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

[Dominic BramleyParticipant @dominicbramley60728]

If the primary objective is to eliminate chatter, I think I'd take out your existing shim, torque everything down and see if the chatter goes away.

If you can prove that the poor column contact is causing the chatter then you know it will be worth investing time in some of these more elaborate shimming methods. If the chatter is still there, then you may just be able to stick your shim back in and open up new lines of inquiry as to what the issue might be.

(full disclosure: I'm hoping you don't spend too much time on it – because I'm looking forward to the Princess Royal build )

Dom

[Stephen SpindlerParticipant @stephenspindler87715]

I've been following this thread with great interest as I have a very old RF-25 that I'm trying to accurize. A lot of good tips here and pleased to say after following them my old beast after a number of trial shims now reads 0.0003" – 0.0007" over a footprint of 5" square, more than good enough for my needs. Many thanks for your ideas folks and have a good Christmas break.

[Dr_GMJNParticipant @dr_gmjn]

Posted by Dominic Bramley on 20/12/2020 08:24:41:

If the primary objective is to eliminate chatter, I think I'd take out your existing shim, torque everything down and see if the chatter goes away.

If you can prove that the poor column contact is causing the chatter then you know it will be worth investing time in some of these more elaborate shimming methods. If the chatter is still there, then you may just be able to stick your shim back in and open up new lines of inquiry as to what the issue might be.

(full disclosure: I'm hoping you don't spend too much time on it – because I'm looking forward to the Princess Royal build )

Dom

Thanks Dom. Yes, I should have done a back-to-back comparison, but at the time I was shimming the column, I'd not really had chance to do much cutting, so didn't know there was any issue with chatter.

When I then started to do some cutting tests to get used to the machine, and the terrible vibrations and noises started on certain bits, I didn't want to touch the shims and risk messing up the alignment, so went through all the other possible causes like poor quality tooling, unrealistic expectations of performance, etc etc etc, but to no avail. By that stage I'd had enough and decided to just get on with building the 10V.

I had no issues at all with chatter during the build, but it was always at the back of my mind. Now I've finished the 10V, I want to get the lathe and mill in the best state I can before starting on the P.R. Trouble is I have no real interest in fettling machines at the moment – I bought them to make models, not fiddle with them, so motivation to get on with it is low. Factor in the cold weather and…meh.

Anyway, it will get sorted, and the next steam engine will get started – hopefully quite soon.

[Dr_GMJNParticipant @dr_gmjn]

I just re-discovered this article, which contains some decent info;

**LINK**

I might have one last try at metal shims, and perhaps try the wedge stack technique.

Does anyone know of a supplier of small horseshoe shims, in the thickness range 0.002” – 0.010”?

Thanks.

[JasonBModerator @jasonb]

Just get some shim or feeler gauge stock and go at it with some tin snips or even scissors.

[Martin ConnellyParticipant @martinconnelly55370]

Search for slotted shim or pre-cut slotted shim on the internet. You will find packs of 5 at a reasonable price, get a few packs in useful sizes for your needs.

Martin C

[mgnbukParticipant @mgnbuk]

At my previous employment we stopped using metal shim stock in favour of plastic

The thinner sizes are easily cut with sharp scissors, the larger using a craft knife. Holes cut cleanly with hole punches. No sharp edges or burrs to worry about.

Nigel B.

[HopperParticipant @hopper]

Posted by Dr_GMJN on 21/12/2020 08:44:14:

I just re-discovered this article, which contains some decent info;

**LINK**

I might have one last try at metal shims, and perhaps try the wedge stack technique.

Does anyone know of a supplier of small horseshoe shims, in the thickness range 0.002” – 0.010”?

Thanks.

The author in that link lost credibility with this sentence: "Thicker thicknesses (over .004" ) will require the acetylene torch or sawing, which is labor intensive and presents safety concerns".

Is he having a larf or wot? As Jason says, scissors will cut thinner shim, up to about 10 thou and common tin snips will easily cut up to .040" steel sheet and up to .062" with a bit of grunting.

Be sure to snip the corners off so they don't slice your fingers to the bone. And flatten the cut edges down with a hammer on a flat steel surface.

It definitely does help to follow his advice about having the size of shim written on it. Felt pen will do the job. Saves confusion in the heat of battle.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dr_GMJN on 21/12/2020 08:44:14:

I just re-discovered this article, which contains some decent info;

**LINK**

I might have one last try at metal shims, and perhaps try the wedge stack technique.

You should have endless hours of fun trying to scale most of that to your situation.

MichaelG.

[HopperParticipant @hopper]

Yes, I thought his shim stacking theory might not scale down well.

[JasonBModerator @jasonb]

Yep as I said several pages back any steps would be in microns or tenths.

simple to see if we said the base of the column was 6" and The Doc has 6thou of shim then that's 1 thou per 1" and if you are looking for 1" square shims then you are not going to be doing much in the way of stacking unless you can find them in 0.0001" increments.

What would be interesting to know is what sort of cuts were giving you chatter eg material speed depth & height of cut and what cutter. Maybe they were too big for the machine, the fact you did not get it while building the 10V suggests it can't be too much of an issue or have you simply learned what the machine is capable of and stayed below that limit?

 

Edited By JasonB on 21/12/2020 09:52:56

[Martin ConnellyParticipant @martinconnelly55370]

We always followed the max 3 shims rule at work. The part about trapping fingers reminded me of an event (back around 1986 I think) when a large pump was being lowered into position on its base. The fitter controlling it was talking to the crane operator…"down…, down…, down…, UP-UP-UP-UP". He had his fingers between the pad and the foot and they inevitably got squeezed, luckily without real damage.

Martin C

[Dr_GMJNParticipant @dr_gmjn]

Thanks all. I bought a load of shim steel at the time, and cut them into Us with scissors. I did cut the corners off, but the turned-up edges and over cuts (Especially in the bolt slot part) made working with them difficult. If they’re available and cheap I rather buy them pre-cut.

[Dr_GMJNParticipant @dr_gmjn]

I could only find one supplier in the UK of pre-cut slotted, shims of this size (slot for an M8 hole), and 'm not sure whether they'll supply a small amount or not. Anyway, they're probably shut for Christmas now, so it will have to wait until January.

I did use the mill yesterday to side mill a radius feature in the side of some brass rod (to fit against a cylindrical nut for a telescope mount). I'd removed the shims, and couldn't be bothered to tram it just for this job (no great accuracy required).

Even with no shims, and the bolts tightened straight to the bed, it still squealed like a pig irrespective of spindle speed and feed rate – I tried loads of combinations. The rod diameter was about 12mm – held horizontally in the vice – and I was going into the end of it with the side of a 12mm mill to a depth of about 5mm.

Anyway – I wish everyone a very happy and healthy Christmas. Thanks for all the advice over the past 7 months or so on the various forum sections. Cheers!

[JasonBModerator @jasonb]

That is actually quite a lot of cutter engagement so expect some chatter/squeal. If you worked out the cord length of that cut it would probably be 10-11mm so in effect you are taking a cut say 10.5mm wide and 10.5mm deep in one go with an area of 110mm2. Usual rule of thumb would be 1/4 diameter of cutter but that's a bit more than I would want to take with your size mill so lets say  2mm x 12mm which gives an area of 24mm2 .or about 25% of what you are taking.

For a scallop cut like that think about taking it out with plunge cuts moving say 1mm sideways per plunge.

Edited By JasonB on 24/12/2020 15:57:49

[Dr_GMJNParticipant @dr_gmjn]

Thanks Jason, so you mean down a bit, then into the side, out and down a bit more, then in again and so on?

What I found doing a similar thing with rectangular section (only feeding in y as well) was that once the already cut bit was a certain depth, it would seem to rub on the flank of the cutter and make a noise and vibration. So what I mean is the issue I’m trying to avoid seems to be there even with small cuts. If the workpiece is not too deep it’s not an issue, but if there’s a significant bit of the workpiece in contact, it eventually becomes a problem.

[JasonBModerator @jasonb]

Touch cutter onto side of work and lift, move work 1mm towards cutter and plunge downwards the lift cutter clear. Move 1mm sideways plunge down and then lift and so on. This should reduce the load and amount of cutter engagement a lot.

[Dr_GMJNParticipant @dr_gmjn]

Posted by JasonB on 24/12/2020 16:50:56:

Touch cutter onto side of work and lift, move work 1mm towards cutter and plunge downwards the lift cutter clear. Move 1mm sideways plunge down and then lift and so on. This should reduce the load and amount of cutter engagement a lot.

OK. So use the DRO to get a consistent depth, and lock all axes they aren’t moving for the cut?

[JasonBModerator @jasonb]

Yep

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