Magnetic Indicator Back

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Bob La Londe

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Nov 19, 2021, 10:51:34 AM11/19/21
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In the past I made a complicated (not that complicated) sliding bar
mount that bolted directly to my mid size lathe. On it I had an dial
indicator with a 2 inch range. It actually worked very well when I was
using it, and with the sliding bar I had 6 inches of working range (not
all at once). It was on my small lathe. The 8.5 x 18. Not the mini
lathe. I never progressed to that level of machining with the mini
lathe. The big (for me) 14x40 came with a 2 axis DRO. Its works well
enough and now I struggle to do manual machining without one. So much
so that hen I ordered a new knee mill for the shop I made sure to get
one with a DRO. I use all the time except for work stopped repetitive
operations like drilling hinge pins in a bunch of hinged molds.

Anyway, I have found a way to eliminate the primary operation I did on
the small lathe (radiusing alignment pins) and I am thinking a magnetic
back indicator might be useful. Well, for that machine and the old
turret lathe I picked up a few weeks ago. If for no other reason than
to help set the stops.

I can certainly make a magnetic back. I have a small inventory of rare
earth magnets I use for one particular type of mold I make. My concern
is this. Wouldn't a magnetic field in close proximity to the clockwork
for an extended period have the potential to magnetize the some parts of
the clockwork? I figure this has to have been answered by now given how
long dial indicators and magnets have been around. If so would simply
making an extra thick aluminum back of say an inch move the magnets far
enough away as to no longer be an issue?

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Joe Gwinn

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Nov 19, 2021, 11:51:16 AM11/19/21
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On Fri, 19 Nov 2021 08:51:26 -0700, Bob La Londe <no...@none.com99>
wrote:
One way to evade the issue, and increase holding force, is to make a
cup of mild steel into which the rare earth magnet is installed with a
resilient rubbery adhesive (so the magnet won't fracture over time).
This will focus the magnetic fields onto the open side of the cup.

Joe Gwinn

Bob La Londe

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Nov 19, 2021, 12:38:28 PM11/19/21
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That is not a bad idea. I'd need to look into how much the "iron"
reduces the effective magnetic field.

Typically when I use magnets for holding I have a light press fit and
high temp epoxy with a very slight air gap between the magnet and the
surface it is holding or holding to. Magnetic paper holders, hardware
holding, box closures etc. It does reduce the hold very slightly over a
surface to surface contact, but only a few thousandths is needed to
prevent impacts.

I'm not crazy about the rubber cup idea unless it is a very high shore
number to reduce likelihood of movement. Probably not a huge issue, but...

Joe Gwinn

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Nov 19, 2021, 5:05:29 PM11/19/21
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On Fri, 19 Nov 2021 10:38:22 -0700, Bob La Londe <no...@none.com99>
That can be done here as well.


>I'm not crazy about the rubber cup idea unless it is a very high shore
>number to reduce likelihood of movement. Probably not a huge issue, but...

It's not a rubber cup, it's a steel cup. It is not a press fit. There
should be a reasonably large clearance between magnet OD and cup ID,
the clearance being filled with slightly soft epoxy, needed to allow
for both the fragility of rare-earth magnets and the difference in
linear coefficients of thermal expansion.

.<https://amfmagnets.com/rare-earth-holding-magnets-round-hole.html>

.<https://www.magnetshop.com/neodymium-pot-magnets.html#1>


Joe Gwinn

Bob La Londe

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Nov 20, 2021, 2:19:34 PM11/20/21
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I was pointing out that I know how to set a magnet in aluminum and I do
it as part of fairly common processes in my shop. Some of those items
experience quite a lot more thermal range than the average shop
environment casting parts from media that ranges from 350F to 850F. The
magnet itself typically does not experience those temps directly (It
would lose its magnetism), but in use experiences a thermal range as the
aluminum dissipates heat. I really don't think the heat range of from
maybe 30F (worst possible case if I left the doors open at night here in
the desert) to 130-140F (when the doors are closed and its a hot summer
day) will cause an issue in a short period of years. I do believe that
impacts will cause the magnets to break and crumble because I have seen
it happen. I can simpley set two on my desk and let them pull together
to see that.

FYI: I buy the higher thermal rating magnets from K&J Magnetics. They
can still shatter if impacted, but they keep their magnetism longer in
thermal cycling environments.

I am quite aware that you were referring to two different things. I
even spoke about the two different things separately. Iron in regard to
bending the magnetic lines of force, and weakening the effective
magnetic field, and rubber due to its propensity to flex and even
sometimes flow under various forces. I was not so confused to believe
that the rubber and the steel (mostly iron) were the same part.

I even acknowledge your overall idea, and commented more that I needed
to investigate or was uncertain for those reasons about the exact
application in practice.

I go one step further and note that its possible that even if the rubber
does flex friction of the back body may prevent it from being an issue.
Still I would go with a very high shore hardness if I implemented a
rubber thermal growth compensation mechanism. Your ideas do merit
consideration. Just pointing out the things that appear obvious to me.

Jim Wilkins

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Nov 20, 2021, 5:20:21 PM11/20/21
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"Bob La Londe" wrote in message news:snbho1$1ak1$1...@gioia.aioe.org...
...
Your ideas do merit
consideration. Just pointing out the things that appear obvious to me.

-----------------

One that's obvious to me is that the magnet has to be retained in the cup
firmly enough that it doesn't pull out and stay attached to the work
surface. I think that means the steel cup bottom should be fairly thick and
quite flat and smooth inside and the potting compound should bond well to
the magnet.

I don't have a good suggestion for the compound because so many things I've
tried have failed. Actually my best results so far are from Gorilla-taping a
button magnet to the end of a same-diameter rod. It's not perfect but it
fails without breakage and is easily repaired.

Joe Gwinn

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Nov 20, 2021, 5:30:37 PM11/20/21
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There are epoxy types that are intended for attaching neo magnets to
steel or aluminum cups. I'd roughen the cup surfaces that will be
glue bonded. Everything must be *clean*, such that water does not
bead up. Hot TSP (the real kind, not the no-phosphorus kind) will
work.

Joe Gwinn

Bob La Londe

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Nov 21, 2021, 11:02:54 AM11/21/21
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I've got a buddy who makes (and sells) a specialty grinding accessory
that is held in place with magnets. Like me he has found that the right
press fit in aluminum works really well. Generally neither of us use an
adhesive. If I were to use an adhesive I would likely abrade the
coating on the magnet and the inside of the hole, and use something like
Devcon 2 ton epoxy. Alternatively if I was really concerned about
differing thermal expansion I might switch to something like Flex Coat
epoxy typically used in fishing rod building.

One thing to note is I've found polyester resin seems to stick to
aluminum better than epoxy. This is purely anecdotal. I have secured
parts to be machined to a sacrificial aluminum back plate with epoxy
before. I also make some aluminum molds for casting polyester resin
into a shape. I can break the bond with epoxy, but a fully cured
polyester bond is difficult to remove even with mild cleaners like
acetone, alcohol, or mineral spirits. On the aluminum polyester molds I
say in the description that the user MUST USE A MOLD RELEASE, and "If
you glue your mold shut I won't replace it."

I would note that the previous paragraph is based on anecdotal
observations. I have not deliberately tried to glue aluminum together
or to something else with a polyester resin. I seem to recall original
Gorilla glue is a polyester glue, but I could be mistaken. Of course
they now make a range of adhesives including their own claim to the best
cyanocrylate(s). LOL.

Bob La Londe

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Nov 21, 2021, 11:04:15 AM11/21/21
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Yes I know I mentioned bedding in epoxy in a previous post, but that's
only when there is a problem fit.

Joe Gwinn

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Nov 21, 2021, 11:44:32 AM11/21/21
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On Sun, 21 Nov 2021 09:04:10 -0700, Bob La Londe <no...@none.com99>
wrote:

>On 11/21/2021 9:02 AM, Bob La Londe wrote:
>> On 11/20/2021 3:19 PM, Jim Wilkins wrote:
>>> "Bob La Londe"  wrote in message news:snbho1$1ak1$1...@gioia.aioe.org...
>>> ...
>>> Your ideas do merit
>>> consideration.  Just pointing out the things that appear obvious to me.
>>>
>>> -----------------
>>>
>>> One that's obvious to me is that the magnet has to be retained in the
>>> cup firmly enough that it doesn't pull out and stay attached to the
>>> work surface. I think that means the steel cup bottom should be fairly
>>> thick and quite flat and smooth inside and the potting compound should
>>> bond well to the magnet.
>>>
>>> I don't have a good suggestion for the compound because so many things
>>> I've tried have failed. Actually my best results so far are from
>>> Gorilla-taping a button magnet to the end of a same-diameter rod. It's
>>> not perfect but it fails without breakage and is easily repaired.
>>>
>>
>> I've got a buddy who makes (and sells) a specialty grinding accessory
>> that is held in place with magnets.  Like me he has found that the right
>> press fit in aluminum works really well.  Generally neither of us use an
>> adhesive. 

Aluminum is pretty soft. There is no reason one cannot use a brass or
aluminum spacer between magnet and steel cup, with everything held
together with glue.

But beware too-thin glue layers between dissimilar materials - thermal
cycling may tear the glue bond. The thicker the bond gap, the less
the mechanical stress on the glue layer.


>> If I were to use an adhesive I would likely abrade the
>> coating on the magnet and the inside of the hole, and use something like
>> Devcon 2 ton epoxy.  Alternatively if I was really concerned about
>> differing thermal expansion I might switch to something like Flex Coat
>> epoxy typically used in fishing rod building.
>>
>> One thing to note is I've found polyester resin seems to stick to
>> aluminum better than epoxy.  This is purely anecdotal.  I have secured
>> parts to be machined to a sacrificial aluminum back plate with epoxy
>> before.  I also make some aluminum molds for casting polyester resin
>> into a shape.  I can break the bond with epoxy, but a fully cured
>> polyester bond is difficult to remove even with mild cleaners like
>> acetone, alcohol, or mineral spirits.  On the aluminum polyester molds I
>> say in the description that the user MUST USE A MOLD RELEASE, and "If
>> you glue your mold shut I won't replace it."

Epoxy is more sensitive to surface cleanliness than polyester, if I
recall. But polyester ought to be workable as well.


>> I would note that the previous paragraph is based on anecdotal
>> observations.  I have not deliberately tried to glue aluminum together
>> or to something else with a polyester resin.  I seem to recall original
>> Gorilla glue is a polyester glue, but I could be mistaken.  Of course
>> they now make a range of adhesives including their own claim to the best
>> cyanocrylate(s).  LOL.

Gorilla Glue is a polyurethane, if I recall. It does stay flexible,
but tends to foam badly with moisture. I'd read the data sheet.

Joe Gwinn

Clifford Heath

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Nov 21, 2021, 6:14:56 PM11/21/21
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On 22/11/21 3:44 am, Joe Gwinn wrote:
> On Sun, 21 Nov 2021 09:04:10 -0700, Bob La Londe <no...@none.com99> wrote:
>> On 11/21/2021 9:02 AM, Bob La Londe wrote:
>>> On 11/20/2021 3:19 PM, Jim Wilkins wrote:
>>>> "Bob La Londe"  wrote in message news:snbho1$1ak1$1...@gioia.aioe.org...
>>> I seem to recall original
>>> Gorilla glue is a polyester glue, but I could be mistaken.  Of course
>>> they now make a range of adhesives including their own claim to the best
>>> cyanocrylate(s).  LOL.
>
> Gorilla Glue is a polyurethane, if I recall. It does stay flexible,
> but tends to foam badly with moisture. I'd read the data sheet.

Yes, I believe that's correct. The foaming is triggered by moisture, and
is intended to improve its gap-filling behaviour.

Another PU glue that is/was available in the USA as an inexpensive
construction adhesive (gun tube) that doesn't foam as much is "PL Premium".

Clifford Heath
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