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brunneng
05-15-2003, 12:23 AM
I've been thinking about building a small tp grinder for my lathe but having never used one I just realised that I don't know if it matters which way they spin.

Should it rotate with the work or against it?
How fast should you spin the work? I'm guessing it varies with the material?

DR
05-15-2003, 12:35 AM
Rotate it against the workpiece rotation.

Slow rotation of the workpiece will be fine.

JCHannum
05-15-2003, 08:03 AM
They should rotate in opposite directions for best material removal.
TPG usually rotates down, requiring lathe to rotate reverse of normal so grinding dust will be directed downward. Grinding forces are low, so this is not normally a problem for threaded spindles. Take light cuts.
Cover ways, and clean and oil everything when done,

docsteve66
05-15-2003, 11:03 AM
brunneng: if you rotate the wrong way, you just speed up or slow down the stone. Its more like two gears (little or no slippage) in contact-one drives the tother. Grinding does not occur- at least that's what happened to me when my cobbled up TPG turned the wrong way! http://bbs.homeshopmachinist.net//smile.gif.

If I live long enough , I bet I make every mistake that can be made!!!! working on a long mistake filled life!
Steve

lynnl
05-15-2003, 01:14 PM
Thanks for that description Steve. That'll make it easier to keep in mind what is the right way. Don't yet have a TPG, but plan to make one.

Also, it's refreshing to see someone freely admitting his mistakes... Lord knows we all make enought of them!
Of course MOST on this BB do. But every now and then some oddball comes along who's incapable of that. ...too bad.

Peace to you!

Oso
05-15-2003, 01:36 PM
Well, I bought that opposite rotation thing too.

Then I tried the other way.

Slow rotation of work so both are going the same way at the contact point drastically improved my surface finish.

Before I was getting a faceted finish.

I don't know why, someone else may.

Others reported tha same improvement, so I can't be alone.

docsteve66
05-15-2003, 02:52 PM
OSO: I hoped to get by with the same direction rotation. I had cobbled up an air motor (gast) so probably the speed matched (the air motor was sped up or slowed to match the work). Might have a simular problem with a "universal" motor for same reasons, but I think you are correct that an induction motor would try to maintain its speed and might give a finer finsh. Did your "facets" also spiral around the work piece? And "they" say that holding the mouth right, incantations, chants have no place in metal work!!!!!

Lynn, I hope I have learned from every mistake, with that hope, I can addmit all mistakes and every one knows I am on the way to be educated. Must be a easier way to get educated though. BTW when I say learned from mistakes in no way do I think I learned THE proper way.
Steve

lynnl
05-15-2003, 03:22 PM
Steve, I know what you mean. I often find myself thinking "...well, that wasn't such a good idea", but then Just what is the good idea remains a mystery. Tho more usually a few mistakes start to reveal some useful insight. I've heard it said that one should always intentionally start w/the mistakes to get them outta the way asap. (as good an excuse as any I guess http://bbs.homeshopmachinist.net//biggrin.gif)

BTW, the TPG design I plan to make is that described by D.E. Johnson in a HSM series. I got one of the Ryobi trimmer/routers for that purpose, and most of the other materials, but it's still on a back burner. Anyone else made that? Thoughts or comments about it?

lynnl
05-15-2003, 03:31 PM
<double post deleted>

[This message has been edited by lynnl (edited 05-15-2003).]

brunneng
05-15-2003, 06:06 PM
I was wondering if it worked the same as "normal" milling vs climb milling. As in climb grinding takes more power and more robust setup but leaves a smoother finish? Maybe not.

WJHartson
05-15-2003, 06:57 PM
When grinding on a rotating piece the wheel should rotate in the opposite direction of the work. This allows the wheel to dress itself and expose new grit. The wheel must be balanced and trued to get a good surface finish. Speeds and feeds vary depending on what you are grinding and the finish you are trying to obtain. The type of grinding wheel used will also play a part in the finish you will obtain.

When they turn in the same direction the wheel won't dress itself and will tend to glaze over time and give a much finer finish.

To obtain a good surface finish with the wheel turning in the opposite direction, cut down on the infeed when you get near the finish diameter you want. You can make multiple passes without any infeed and let it spark out.

Coolent can be used but it is messy and the swarf will tend to cake on everything. Clean everything when you finish grinding and oil everything.

Hope this helps.

Joe

L Webb
05-15-2003, 08:27 PM
I agree with Joe.
Poor finish is usually chatter induced. I have three different size tool post grinders and use the one which fits the job.
The Dumore grinders work very well. Ground many punches and dies with them.
Les

darryl
05-15-2003, 08:58 PM
I have my tpg set up to come in from the rear of the lathe. That way, the lathe rotation stays the same, forward, and the tpg rotation is against the work. So the two sfpm add up, giving a faster action, and directing swarf downwards. I use a tray of sorts to keep the swarf off the ways, it rides with the carriage. I find that normally, a quite slow spindle speed is best. I believe this is best because the grinding action concentrates in a smaller area as milliseconds pass, giving a more efficient material removal, as opposed to faster workpiece rotation, where a few grains on the grinding wheel get to scratch the surface, which is then wisked out of the way by the faster rotation, changing the action to something more like sanding than grinding. Sorry for the long sentence. Anyway, the speed of the spindle changes the vibration characteristics of the lathe, so does the speed of the grinder, so I would exoect to find that certain combinations of speeds will produce poorer results than other combinations.
Having the work rotating with the grinder instead of against it will lower the relative speed of the grinding action, but it's doubtful that you would actually get the workpiece sfpm to even closely approach the sfpm of the grindstone, so what is most relevant is to choose speeds that give the best finish, within an acceptable time span. This is MHO, also it's based on experience with the tpg setup. I was surprised by the sometimes faceted surface created, and have attributed much of this to 60 hz vibrations from the lathe motor, which I have now dispensed with in favor of a dc motor. The tpg motor is also dc, but there remains the vibrations from imbalance of the armatures and the grinding disc or stone mounted. Experimenting with the running speeds will definitely lead to better finishes.


[This message has been edited by darryl (edited 05-15-2003).]

CCWKen
05-15-2003, 10:49 PM
I think everyone is on the right track. I know that when I've tried various combinations of direction and speed, I got the best finish with running opposite with the lathe at it slowest and TPG screeming! (Mine is homemade, air powered and uses a 1/8 x 3" wheel.)

WJHartson
05-16-2003, 12:29 AM
Chatter and pattern in the work piece can be caused by the way the part to be ground is rotated. The best way to reduce chatter is to mount the work piece between centers and turn it with a dog. A rigid setup is very important so that the work piece is not excited during grinding.
Pattern in the work piece can be caused by many things and is hard to see in some cases. If the wheel doesn't breakdown and expose a new grinding surface, ie the wheel starts to glaze, the surface will be affected and a pattern will be developed.

Grinding work and backup rolls for rolling mills is very similar to grinding with a tool post grinder but everything is much bigger and is more rigid. Balance and dressing of the grinding wheel is the most critical part of the operation. The rolls were supported between centers or resting on babbitted rests and driven by a dog type arrangement. The roll was turned slowly and the wheel was turned fairly fast. The finish was determined by the grit, bond type of the wheel and the traverse feed rate across the roll. The finer the finish the slower the feed rate and the number of passes taken without any infeed. Grinding was done in both directions.

There is a lot of art in grinding and some experimentation is required to get the desired results. It takes a lot of practice.

Hope this helps a little in understanding what happens during grinding.

Joe

Oso
05-16-2003, 12:44 AM
Doc,

The TPG is a small Dumore, universal motor with the 2" stone going about 10,000 rpm. (rated for 15,000+). I don't take but a few tenths at a time.

Spindle was only going about 100 rpm or less, with a 1" workpiece.

Surface speed of wheel is close to 60,000 inches per min. Work about 300 inches per minute. 60,000 + or - 300 is still 60,000.

So I don't think the rotation direction has much to do with the relative surface speeds here at least.

The pattern was pretty straight. At least not a spiral as is seen often from chatters at certain speeds. Not really rough, but visibly un-smooth, non-random pattern.

Going same direction, finish looked "ground", meaning visible fine random scratch pattern but very smooth overall. Like a good surface grinder leaves, or like a ball bearing race.

Another guy who operates grinders at work told me opposite for fastest removal, but same direction for best finish. That's what I found, so he must be right.

docsteve66
05-16-2003, 11:58 AM
Before I retired , I had a machine shop as part of my division. Those men were good with TPG, I watched many pieces of work (whilke I drank coffee) and they would remove facets and other weird things by various means- none of which neccessarialy would work on next job.

Touch hand on lathe motor, lay a stick on the work piece, lay hand on TPG, rag on part, change speeds- on and on. I think problems ,as said earlier by others, is vibrations at harmonics, natural resonances, little slop in bearings combining with work piece. Even the best use trial and error when the cheese gets binding.
Steve