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Split emery mandrels for sanding - dangerous?


#1

When I was working in a fairly large jewellery workshop several years
ago there were a couple of accidents with people using split emery
mandrels. In both cases whilst the jeweller was using the mandrel to
do some clean up on a casting, one side of the mandrel bent outwards
and the flexi was thrown around wildly in their hand. The first
jeweller got an injury to his thumb that required stitches, the
second jeweller had the flexi swing up towards his eye (no safety
glasses) and ended up with an injury to his eye - he lost most of
the vision in that eye.

After the second accident all of the split manderels were collected
and sent back to the supplier, as it was assumed a faulty batch had
been received, and the supplier replaced them. My questions is - has
this ever happened to anyone else on the forum? I am now really
nervous about using these. I always wear my safety glasses when
using my flexi but am aware that I could still get injured if the one
side of the mandrel pushes out and I am left with an out of control
flexi swinging around everywhere.

RR Jackson


#2

RR Jackson,

My questions is - has this ever happened to anyone else on the
forum? 

I have been making my own split mandrels for 17 years, and have
never had this happen on a shopmade version. I use 1/8" brass round
rod, cut the desired length, and saw a 1/2" long groove in the end
with a 2/0 blade. Works like a charm!

Matthew Crawford
www.MatthewDesigns.com


#3

ha!

this has happened to me when i 1st began making jewelry. this is
what you do: cut the mandrel shaft length til it is about 1/2" long
to fit in your chuck. then the metal will not bend and there will be
no problem. i have been using such mandrels and using them in
teaching as well for a long time with no problem once you cut the
shaft. the head of the mandrel must sit close to the chuck jaws on
order to prevent the bending/breaking problem. then, you have a
wonderful tool that will last you until you lose it!

joanna


#4
In both cases whilst the jeweler was using the mandrel to do some
clean up on a casting, one side of the mandrel bent outwards and
the flexi was thrown around wildly in their hand.

I’ve bought split mandrels from Rio that had rather long stems that
I was uncomfortable with. So I sawed them to a shorter length.
Instead of a 2 inch stem they now have a 1 inch stem and stay nice
and secure in the flexi.

I hope that helps. I always wear safety glasses when working with the
flexi no matter what I’m doing.

pj
http://www.925studio.com/


#5

RR,

It sounds to me that too much pressure was being used to sand. I’ve
never seen this happen, although I have seen a bad batch of
sawblades.

Here are my tips for casting cleanups.

  1. Remove the nub end of the sprue with a carbide rotary file. Use
    at higher speeds with a light touch. A lower speed will cause the bit
    to catch and “bite”. D

  2. Change to the split mandrel with 320 grit. If you take a small o
    ring, place it on the end of the split mandrel securing the
    sandpaper. By dipping the casting in water, the dust particles are
    held in the water and don’t fly around. When using the sandpaper wet,
    it works more efficiently.

-k

Karen Christians
M E T A L W E R X
50 Guinan St.
Waltham, MA 02451
Ph. 781/891-3854 Fax 3857
http://www.metalwerx.com/
Jewelry/Metalarts School & Cooperative Studio


#6

I had that happen to me on the first time I used a new split mandrel.
Until I read your email, I honestly thought I had done something
wrong and yet the mandrel was perfectly centered in my flexshaft. It
destroyed a customer’s ring and I now have 2 thick knuckles that
probably had gotten small breaks in them! I have never used a split
mandrel since.

Judy Shaw, GJG (GIA)
Jasco Minerals


#7
When I was working in a fairly large jewellery workshop several
years ago there were a couple of accidents with people using split
emery mandrels one side of the mandrel bent outwards and the flexi
was thrown around wildly in their hand. 

yo freakstyle,

i have never had this happen, give the top of the mandrel a gentle
squeeze to give it an inward inclination or solder the end of it,
a very heavy duty straight sewing needle also works well, use
little drum sanders, cones, bullet shapes that are sold, they
come in all grits, it’s also good to have your material in a
finished state with burs or files, stones before you go in with a
light sandpaper, run the flex slow, get a speed control to keep
a top end on your pedal speed, keep the shank deep into the
chuck, better yet use the collet chuck, which buries it farther,
make sure chucks and collets are tightened well, dp


#8

We have had a few bent and broken split mandrels here. I make sure
students are wearing face masks when I see them using the split
mandrels.


#9

Hi RR, there is a description of how I like to make them at Ganoksin
here

Essentially, any such tool has to have the shank cut off and
shortened by one third before it is safe to use in a flex shaft. It
has to seat right in close to the jaws to avoid this problem. Same
with any large or heavy burr or flex shaft tool, unless the shank is
thicker than the standard 2.4mm.

best
Charles


#10
When I was working in a fairly large jewellery workshop several
years ago there were a couple of accidents with people using split
emery mandrels one side of the mandrel bent outwards and the flexi
was thrown around wildly in their hand.

I’ve had split sanding mandrels, and large cylinder burs bend over on
me- beating up, and ripping up my thumb. I have heard many, many
stories about mandrels failing like this. I use cylinder burs in my
classes to mill wax, and won’t let students leave the classroom with
them, unless they cut the shanks shorter - so that the heads of the
burs are close to the jaws of the handpiece. With a long thin shank,
and a large mass on the end of the shank, it does not take much
force to make the shanks bend over. I think it is irresponsible for
manufacturers to market these. My guess is that they don’t do much
product testing on them, and somehow don’t get customer feedback.

I recently started marketing a set of 3 split sanding mandrels- in 3
sizes (the largest makes sanding the inside of a ring much quicker).
These mandrels have shorter, heavier shanks. They also have a ridge
at the bottom of the slot- to keep the sand paper from sliding down
the mandrel. 3MAE was kind enough to stress test these for me- and i
am happy to say that they could not get them to fail at any speed and
amount of pressure (using a flex shaft). They are currently available
from Rio Grande, Gesswein, and soon to be available from Stuller and
Otto Frei. Have a great day and don’t forget to wear your safety
glasses!

Kate Wolf, Portland, Maine, hosting wicked good workshops by the bay
http://www.katewolfdesigns.com


#11

I have had split-mandrels bend when students where applying way too
much pressure to the tool or had not inserted it deeply enough into
the flex shaft chuck.

No injuries though.

Since those few occurrences, I have always ordered the split mandrels
with the heaviest shank or tang possible to avoid the possibility of
it bending.

Nanz Aalund
Associate Editor / Art Jewelry magazine
21027 Crossroads Circle / Waukesha WI 53187-1612
262.796.8776 ext.228


#12

I don’t believe anyone has mentioned this possibility. I use the
Foredom handpiece with the collets, my most frequently used
handpiece. Since it’s a collet you can slide the shaft of your split
mandrel as far into the collet as you’d like ( no shorting needed )
the same with burs etc.

This eliminates bending.

Caveat: most everything we use is dangerous if used inappropriately;
so, make sure brain is engaged before using tools.

Kevin Kelly


#13

The metal alloy, the heat treatment and manufacturing techniques
determine if the products for flexshafts are safe or not.

If you spin these at slow speed they do not bend. It is the
velocity, the metal and also the lopsided load that bend it right
away.

Most of these mandrels specially the ones from Grobet are made in
Germany. These are made on Swiss Screw Machines (Automatic) they spit
these out at high speed and are well balance plus the metal is strong
to begin with.

Recently there has been split mandrels made in India and they are
brass nickle plated. These are made on manual machines with cheap
labor. To begin with the metal is no good. The machines that they make
these with cannot turn out well balanced pieces. The slotting is not
done right and it is a disaster in waiting.

I would not blame the vendors alone. The market seems to be looking
for prices and most of the manufacturers in the third world countries
dont even know what you are using these split mandrels for or at what
speeds. We use to have 18,000 rpm and now with micro motors we are
spinning these at 30,000 rpm

Please Protect yourself with any of these German or Indian

Kenneth Singh
46 Jewelry Supply.