I am almost finished rebuilding a Dumore series 44 TPG. Now I need help on
how best to operate it. For test grinding the material will be 4140
unhardened, if that works then I have some MT 2 and 3 centers to resurface.
What type of stone will be suitable for these materials? How important is
the SFPM of the work piece, does this have much impact on the surface
finish? The motor bearings have been replaced but not the spindle bearings,
does anyone here have experience replacing these?

Thanks for any and all help Jack

Jack Hayes wrote:

I am almost finished rebuilding a Dumore series 44 TPG. Now I need help on
how best to operate it. For test grinding the material will be 4140
unhardened, if that works then I have some MT 2 and 3 centers to resurface.
What type of stone will be suitable for these materials? How important is
the SFPM of the work piece, does this have much impact on the surface
finish? The motor bearings have been replaced but not the spindle bearings,
does anyone here have experience replacing these?

Thanks for any and all help Jack



If you are going to take on rebuilding a Dumore spindle I would suggest
you call Dumore and ask to talk to their service guy and request that he
fax you their internal procedure for this rebuild. Without that you are
really flying blind. I did this when I rebuilt the spindle of my K.O. Lee
surface grinder and I would have been hosed without it. - GWE

"Jack Hayes" wrote in message
...
I am almost finished rebuilding a Dumore series 44 TPG. Now I need help on
how best to operate it. For test grinding the material will be 4140
unhardened, if that works then I have some MT 2 and 3 centers to
resurface.
What type of stone will be suitable for these materials? How important is
the SFPM of the work piece, does this have much impact on the surface
finish? The motor bearings have been replaced but not the spindle
bearings,
does anyone here have experience replacing these?

Thanks for any and all help Jack

Use aluminum oxide wheels for grinding steel of any kind. Silicon carbide
wheels should not be used for any steel alloy. A green silicon carbide
wheel would be the choice for grinding a carbide center, however. The grit
you choose to use will affect the surface finish, but it also affects the
amount of heat generated while grinding. It's not a good idea to use
coolant on a lathe because it washes the swarf into places you can't clean,
so while grinding dry is a total PITA, that's the recommended procedure.
Be certain to mask your machine well. I am forced (occasionally) to grind
on my mill, which is a very poor compromise for a surface grinder, but it
does yield acceptable results. When I do, I cover the entire table, knee
and head such that any abrasive that falls can't get beneath the paper, and
when I remove the paper it stays trapped on the paper, which I roll up and
discard. That's important if you want to protect your machine from
serious damage, be it a lathe or a mill.

As to the speed of the item being ground, you should experiment, for it will
have a profound affect on the surface finish you'll get. It need not be
very fast, and should run the same direction as the wheel, although you may
find that you get better results with it running opposite. Cylindrical
grinders typically turn the part the same direction, so it's like watching a
lathe run in what we all would consider a forward direction, but the wheel
is on the back side. On your lathe, it won't be, so run your lathe in
reverse, if you have such a feature. I strongly suggest you run the
toolpost grinder such that it throws swarf down, not up, assuming you have
options as to direction of rotation. You likely do not. Toolpost
grinders are, at very best, a poor compromise for precision grinding, but at
least they allow resurfacing of items like the centers you described that
otherwise would not have been resurfaced.

The wheel selection for the soft 4140 likely won't serve nearly as well for
grinding the centers, although you can do both with the same wheel if the
wheel isn't too hard. Soft wheels work for grinding hardened items, but
break down too quickly for grinding soft items, so you fight finish
constantly and can experience poor wheel life when the wheel is too soft.
A wheel that's too hard will glaze and burn rapidly, so you'd have a hard
time with the centers if you choose a wheel that's too hard. I'd
recommend a wheel in the J or K range to start and see how it goes, maybe a
60 grit, although a 46 will cut cooler. The 60 will leave a better
surface finish, which would be a good idea for a center. To go harder,
move up the alphabet, maybe to an L or M. Vitrified bond is the best
choice.

You're highly unlikely to duplicate the type of finish that is capable from
a serious grinder, but the typical waviness you get with a toolpost grinder
that is in reasonable condition is usually very shallow, so it can be
polished out easily. If you're unhappy with the surface finish of the
grind, try polishing it when it's cleaned up by placing some fine abrasive
paper between two parallels and applying it to the work. The parallels
will prevent rounding and uneven polishing and will yield a very nice
finish, assuming it's not too chopped up by bad bearings.

What Grant said for bearing replacement. Precision bearings are very easy
to destroy in handling and assembly. They also require a proper setup in
order to run within their prescribed level of precision. Only a fool
would take apart a precision spindle without some kind of guidance. The
smallest thing can make a spindle run poorly, or to make noise, which in all
likelihood would be a sign of a damaged bearing. Get some good guidance
before tackling the spindle, if at all.

Be very careful about speed selection *BEFORE* turning on the grinder. If
you have the wrong setup with a large wheel, you could kill yourself very
easily. Wheels that are in perfectly good condition that are run too far
beyond their capable speed come apart with serious energy. If you're in the
line of fire, you could die, or wish the hell you would have if not.
Anyone that has grinding experience knows to *ALWAYS* stand aside when
starting up a wheel, regardless of size and speed. No need risking injury
when it can be so easily avoided. Give any wheel a ring test before
mounting, always use blotters, which help prevent cracking, and stand aside
for a minute after the wheel spools up to make sure it's going to hold.
Diamond dress your wheels, but if they cut too hot, you can always rough up
the surface a little with a dressing stick. The diamond dressing will get
it running true, and the hand dressing will diminish the amount of wheel in
contact at one time, making it cut slightly cooler, although at the cost of
surface finish.

If in doubt about the speed at which a wheel should be driven, any wheel,
calculate the surface speed and don't exceed 6,000 SFPM. Speed is
critical to performance in wheels. They behave at the prescribed hardness
at a given (safe) speed, but behave softer and softer as the surface speed
diminishes. That means that if you run a wheel too slowly, it will slough
away too easily, so try to keep the surface speed between 5,000 and 6,000
SFPM, or at the speed recommended on the blotter of wheels so equipped.
Remember, it's related to diameter, so a wheel that started life at 7"
diameter that is now reduced to 5" diameter is still quite useful, assuming
you run it faster than the blotter states.

Hope some of this helps----

Harold