Basic 4 jaw chuck question

[trg-s338]

Please educate a newbie on the 4 jaw and interpretation of inconsistent results.

Last indicated part was a CNC machined commercial grinder idler wheel from Beaumont Metals. As such, I am fairly confident that it is at the very least good to 0.002" TIR if not better. Chucked in my 4 jaw there is a facet on the side of the wheel that should be concentric with the diameter and the bore which has a bearing in it. I therefore decided to use this feature as the indicating surface. The outer diameter of the wheel has a severe crown that I want to machine flat or almost flat. That surface is also fairly rough so I am using the previously described smoothly finished facet as the indicating surface. Using a "7 jeweled" HF Chinese test indicator (all I got right now) marked 0.0005", I presume this to be imprecise to that claim. I generously give it 0.002 precision at best. Centering opposite jaws, I can get both sets of jaws within 0.001" on the Chinese test indicator reading. What puzzles me is that one set of opposite jaws (say #1 and #3) will read about 0.003" to 0.005" bigger than the other set (#2 and #4). This is a trend not only on this part but on all previous parts I have attempted to center. Granted that my test indicator is a POS, I should at least get consistently closer readings I would think.

Analyzing this dilemma, first suspect is my indicator. But how can I get very close to center between paired jaws but not so comparing the readings of the two sets. To my best theoretical visualization, if one can center the part between sets of jaws, I should get the same reading at both sets and all the way around when I spin the part. One might say that the part may be out of round. May be true of the other parts I've centered but this wheel is my best "round" part I can refer to since it has been CNC'ed and all. Could my 4 jaw be off center on its baseplate? Worse, could the spindle of my 9" Southebend be bent? I am staining my brain now trying to theorize and guess at what might be causing the inconsistency. Or perhaps and more likely, as a novice, there is something I am not doing correctly in my centering technique. I invite the kind folks here to postulate possibilities that might be causing my dilemma. Thanks for your time.

 

[Tony Wells]

One of the main advantages of a 4 jaw chuck is the ability to indicate a surface true regardless of many external factors. One thing that many people overlook is the necessity of having a face running true early in the indicating process. Imagine, for a moment, a grand exaggeration of an out of true face; viewed in a plane, the cross section is representative of a ellipse. An ellipse is symmetrical, so even though you are spinning your part in a true circle, as true as your spindle bearings, anyway, the surface presents an ellipse to the indicator. A symmetrical part will yield the readings you are seeing. A round part obviously would not.

I would not condemn the indicator. I would have to guess that your part is a fairly short, stubby part, almost a disc, from your description. That would require a face indication first, or pretty quickly. Use a soft mallet, like a rawhide or deadblow style hammer. Brass is ok if the material is fairly hard, and you don't have a severe grip on it. Be careful not to shock your indicator.

 

[trg-s338]

David U.
Thank you for your input. The part I speak of is an older model of an idler wheel from Beaumont that is no longer on their website. It is similar to the 6 inch drive wheel currently on there but has a bearing in the bore. It has an inner lip on the face about a 1/4" from the OD that I am gripping with the jaws. You correctly call the chamfer that I incorrectly called a facet. I will certainly keep the arbor idea in mind for future use. I may remove the bearing if I cannot resolve centering indication another way. I'll use the bearing ID to confirm center in the meantime. By setting jaws to 0.001 I mean the centering between a pair of opposing jaws read inline with those jaws, the variation in the two readings show to as low as 0.001. It is clear to me that the readings in centering a part are relative and not absolute readings on the test indicator. I will also get an accurate test shaft and check my chuck and spindle bearing condition as you suggest. The 4 jaw I have is an Ebay bought used Craftsman 6".

Tony Wells,
As a novice, it has completely evaded my mind that the slight wobble I noticed when the part is spinning does influence the readings off of that chamfer. I'll have to check that out first and foremost. Thinking back, I'll have to remachine my self made backplate on the chuck to insure perpendicularity to the axis of the spindle.

Mumbles,
That is a neat idea on getting the vertical face of the part perpendicular to the axis of the lathe spindle/bed.

Knudsen,
A decent American indicator is definitely next on my tool budget.

Thanks all. I'll recenter my part incorporating what I've learned here and see what I come up with and give feedback.

 

[trg-s338]

David U. and Mumbles,

I appreciate your patience and additional guidance. Being new to the hobby, I may make decisions that are very obvious to experienced folks as misguided and may result in an imprecise outcome. Thinking of your input, it does make all the sense to turn the wheel on a nice fitting mandrel that has also been turned on the same lathe and not removed from the chuck for while undisturbed. It is true to axis of the spindle and that precision cannot be bested by recentering a disturbed shaft with a dial indicator or an attempt to center the wheel mounted on the 4 jaw using said indicator.

I am, however, hesitant to dismount the bearing. I have a hydraulic press but I normally use sockets to match the diameters of bearing I am trying to press out. The bearings on this wheel are sealed and the outer race is thin. I am not confident that the beveled edge of a socket has enough surface to bear on. I suppose I could turn another piece of metal to match the race and use it to press on the bearing. Your point about the bearing "play" possibly affecting the concentricity is understood. I cannot feel play between the inner race and the wheel when I try to manipulate it so the "play" is smaller than I am capable of detecting. If I use Mumbles' tapered method leaving the bearing in place, I am inclined to think that whatever unconcentricity (is that a word?) results will be a tolerance close enough for the application, I would think. I am modifying this idler wheel into a drive wheel for my two wheel belt grinder project by attaching a 1/4 inch spacer and a drive belt pulley along side it. In my grinder design, the shaft on the drive wheel does not spin.

 

[Tony Wells]

I've repaired countless electric motor shafts by taking a piece of material oversized, turning a pin on the end of it with a 60 deg transition, drilling or boring a matching hole in the broken shaft (or sawing off a bad end), with a similar bevel, welding it all up, then with a steady rest, center drill the end, and turn it all back to original. I'd never try to match the concentricity with an on-size extension. That's just me though.

 

[Tony Wells]

Gotcha, Ed. Different deal on that. Have you considered doing the finish bore after the OD? Steady rest or swallow it up. Should be very close either way.