We regularly have customers send in their blocking arbors (chucks) to test with our blocking machines. This is a good idea so that we and they get a good idea of how the two will work in their system. By “system” I mean the combination of the lathe, blocker, and arbors. When we receive the blocking arbors, we look them over for general condition and precision, especially the diameter of the shank being held in the collet. You may recall that a standard diameter is ½” (12.7 mm). A tolerance of plus or minus .01 mm is acceptable for a collet close properly on the shank of the arbor.Other factors in the arbor is the presence of a flange and then the material.
Flanges vary in diameter and purpose. Also, there is a variety of materials in use from hardened metals, hard coated metals, plastics, etc. They all offer various advantages, but there are shortcomings, too.Recently, we received a set of front and back blocking arbors with the base curve (BC) blocked to the back arbors. Our task was to place the blocked BC in the lower collet of our blocking machine and the front arbor the upper collet. Upon examination of the set of arbors, the back arbors were plastic with a very fine finish and the front arbors were anodized aluminum with a very rough finish.
They both had a 25 mm diameter flange. I felt that the most important aspect was that the shank diameters were 12.6 mm. That is .1 mm (100 microns) smaller than the standard. I contacted the customer to see if they were using 12.6 mm collets; they insisted that we test the tools in the 12.7 mm collets.
We had a blocking machine set up with precision ground collet covers and 12.7 mm collets where the flange of the arbor is pulled against the collet cover. We also have a precision lathe spindle with a 12.7 mm collet and precision ground collet cover. We blocked about 10 sets of arbors after calibrating the system with the customer’s arbors. It is best to perform the calibration with the customer’s arbors to make the procedure reproducible in the customer’s lab. Even with the shank diameter discrepancy, surprisingly, we were able to achieve radial run-outs of less than .005 mm total indicate runout (TIR). There were some instances where we would see .010 mm TIR, but upon opening the lathe collet and making sure everything was clean and seated, the run-out was .005 TIR. These results are on average about 2-3 times better than a straight shank (no flange) arbor.So, the question was: How did we get such good precision when the shanks were so undersize? The answer is the presence of the flanges.
The flange is pulled tight against the collet cover which squares the tool to the axis of the blocking machine or the center of the lathe spindle. The collet only performs a centering function and need only contact the shank of the arbor at one place around the shank. Collets bend to accommodate the diameter of the shank. That is why it is good to have the shank within a few microns of the nominal size of the collet. In the case of the 12.6 mm diameter shank, the 12.7 mm collet will bend and close at its end so that that the blocking tool is held only at the front of the collet and the back of the tool will be free to wobble. The fact that we got excellent results with the collet covers supporting the flange of the tools demonstrates the value of the flange/collet cover arrangement.
A graphic example of the effectiveness of the flange/collet cover occurred a few days later when we were ready to ship the transfer-blocked arbors to the customer. I double-check the run-outs. I mounted a few transfer-blocked arbors in the lathe and took the measurements. I expect a few microns movement do the wax shrinkage, but to my shock, they were all over the place from .010 to .080 TIR; even with the same set of tools repeatedly placed the lathe spindle. It was then that I realized the collet cover had been removed from the lathe spindle. Clearly, the tool was being held by the end of the collet as it bent in to capture the tool, and the inner end of the tool was not being held, permitting it to be in any position between the 12.7-12.6 mm space. I removed the 12.7 collet, cleaned it and the spindle, installed the collet and collet cover and checked the radial runout of the collet with a 12.7 mm gage pin and then confirmed that the collet cover was dead square to the spindle (no tilt runout). I decided to block some more of the customer’s arbors and checked the runouts. Happily, the results were the same as the original in the .005 mm TIR range.
I have made comments in the past about the advantage of flanged tooling used in conjunction with collet covers. Until now, I had not tested the actual results of using an undersized shank. The outcome of this exercise proves the usefulness of a flange in conjunction with a collet cover.
Here are the specific requirements of the flanged tool/collet cover set up:
1. The flange must be turned or ground at the same time as the shank to ensure that it is square to the shank.
2. Provision must be made to ensure that there is no interference to the collet cover and/or collet at the junction of the flange to the shank. This can be accomplished by a 13 mm diameter opening in the collet cover or an undercut can be machined in at the junction of the shank and flange.
3. The collet cover or face of the lathe spindle must be square to the center line of the collet. The lathe collet cover can be pressed on and then face off to achieve squareness. If the lathe collet is recessed inside the spindle, a flanged tool with a large enough diameter to rest against the face of the spindle works very well.
4. The stiffness of the flange and collet cover should prevent axial movement of less than .001 mm. This is measured by lightly holding the tool in the collet and then, with a dial indicator against the flange, closing the collet. Make sure that the spindle does not move during the measurement.
5. A 12.7 mm diameter shank with a .010 mm maximum tolerance is recommended.
6. A ground or fine turned surface of the shank and flange is recommended.