How ToOperate a Tool Post Grinder

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
Reply to
Jack Hayes
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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
Reply to
Grant Erwin
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
Reply to
Harold and Susan Vordos

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