by Fred Holder
[Reprinted here from the April 1999 issue of More
Woodturning.]
Over the past several issues, I’ve mentioned the soft jaws from Midland Tool Manufacturing Company several times. The company designates them as Midland Pro-Jaws. I was impressed with these jaws, but hadn’t had an opportunity to actually use them. This month, that opportunity came about and this is my report on the results.
The normal metal jaws supplied with your four-jaw scroll
chuck, Nova, One-Way, Axminister, etc. only grip one diameter of
work correctly. Larger diameters and the corners of the arc dig
into the work and make marks. Smaller diameters and the jaws are
only touching at the center of the arc and are not gripping as
firmly as we would like. You simply get a 360 degree hold at one
diameter. The result can be slippage of the work in the chuck or
damage to the workpiece from corner dig in.
The Midland Polypropylene Pro-Jaws were designed to
eliminate this problem. You machine the jaws on your chuck to
fit the work at hand. If you do your job properly, you’ll
get a 360 degree grip that will hold perfectly and will not mar
your work. You will have machined the jaws to your chuck and to
your lathe spindle. You will have eliminated the error
tolerances necessary for the manufacturing processes that creep
into the original set of jaws (typically about .003 to .005
inches, sometimes not acceptable).
The Midland Pro-Jaws can be used either around the tenon
on the workpiece (outside holding) or can be expanded to fit
into a recess in the workpiece. In either case you machine the
holding surface to fit your needs.
My first test was to simply check the holding capability of the Pro-Jaws. As supplied, the jaws had a square hole at the center. I used this to grasp a piece of Madrone about 3/4” square (I didn’t actually measure it) and about three inches long. I then proceeded to turn a small goblet as shown in the preceding photograph.
The chuck held solidly while turning. There was no sign of slippage as I can sometimes get when using the Nova chuck with my 25 mm (1”) jaws on a square of this sort. I was impressed with the very solid hold that the jaws provided. Using the 25 mm jaws, one only has the points of the arc cutting into the square stock. In the first place it damages the stock and in the second place it does not provide a perfectly solid hold, because the jaws are digging into the wood, a softer spot will allow the wood to shift under pressure of cutting.
After this test, I would recommend these jaws for
holding small square stock that is large enough to be held by
the flat surfaces. I did not have a perfectly square piece of
wood that I could check with a dial indicator to verify that the
wood was being held perfectly on center; however, it was held
very solid.
Midland’s instruction sheet says to use a square end
scraper to do your machining on these jaws and they tell you how
to make one. I already have square end scrapers so I didn’t
make one.
They recommend a scraper that is about 1/2”
wide with an end that is 90 degrees to the side of the scraper
and that will cut on both sides and on the end. Recommended
bevel angle is about 70 degrees. They say you can make this
scraper out of a commercial blank or piece of scrap tool steel
or even an old file. Although I sometimes use old files to make
scrapers, I hesitate to recommend them because of the
brittleness of the material and the ease with which they can
break. They caution you to make a good handle for your tool,
don’t try to hold the piece of steel in your bare hand,
make a handle!
When scraping polypropylene, the tool rest must be as
close to the work as possible and you must always use a light
touch. Run your lathe at a speed between 1700 and 2000 rpm for
best cutting. I think mine was running at about 1100 rpm and I
experienced no problems in cutting.
Midland does caution you
to exert care when machining the Pro-Jaws so that you do not cut
so deep as to hit the screw heads. They say, while this will do
little more than dull your scraper, the exposed screws can mar
your work. Also, hitting the screws could be jarring to your
hands and to your mental coolness at the moment. They also note
that sufficient material must be left around the screw holes to
maintain the structural integrity of the jaws.
If you close the jaws down and machine the recess, it
will not grip the workpiece with sufficient strength. Midland
recommends gripping a dowel of suitable size so that the jaws
are not completely closed when gripping the dowel. You then
layout the size recess that you need to hold your workpiece. The
company recommends that you machine the recess slightly
undersize, about 1/64” to 1/32” less than the grip
diameter. Machine as necessary, remove the dowel from the chuck
and you are ready to mount your workpiece in the chuck.
I can see where these chuck jaws would be very useful to productions turners. Make up a special set of jaws for a job and file them away with the jigs and templates for the project. When another order is received for that product, simply install the jaws in your scroll chuck and you’re ready to go.
I don’t know of anyplace here in the United States
that is stocking these jaws from Midland, you can order direct
from the company. Use a credit card for telephone or fax orders,
I believe that they accept credit card orders. Check their web
site if you have internet access.
Their address is:
Midland Tool Manufacturing Company
Belle Eau Park,
Bilsthorpe, Newark
Nottinghamshire NG22 7TX
United
Kingdom
Web Site:
http://www.Wood-Turning.com/turners.html
E-Mail:
turnon@midlandtool.u-net.com
TEL: +44 (0) 1623 970411
FAX: +44 (0) 1623 871857
I was working on some chess pieces with a 1-1/2” base
that I needed to mount to machine a recess in the top and to cut
the notches around the top (these pieces were Kings). So I
blocked the jaws so that they were not completely closed and I
turned a recess that the chess piece would just fit into. I
didn’t follow the rule of making it a tiny bit smaller, but
it worked great anyway. You do have to clean up the screw holes
after machining because of the fact that the stuff does not cut
as clean over an edge as one would desire. I used a small pocket
knife. Midland provides a small length of polypropylene rod that
can be used to fill the holes before you start to machine. This
will make things cut more cleanly, but then you have a problem
of getting the plugs out of the holes without damaging the
machined surface. I chose not to use the rod to make plugs.
These jaws will be great for any woodturner with a 4 jaw scroll
chuck for which they are available. Currently jaws are available
for Nova chucks, One Way Stronghold and Bowl Chuck, the
Axminster Precision Chuck and the Axminster Carlton Chuck. The
jaw sets cost $22.00 plus $3.00 shipping and handling. A very
reasonable way to make up a dedicated set of jaws for a
particular repeatable task. I can see this being