Another: Lathe Test Bar

RJSakowski

RJSakowski

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Another Lathe TEST Bar

Anyone who regularly follows my postings on HM must realize by now that I am partial the Rollie's Dad's Method for lathe alignment, My weapon of choice is a 1" diameter bar, 15" long salvaged from an old printer. For those not familiar with the RDM method, it is based on the principle that a round bar mounted in the lathe spindle rotates around the spindle axis. If an indicator is brought up to the surface of the bar and the maximum and minimum position is measured, the average of those two measurements is the true position independent of any runout. This is convenient as the bar doesn't have to be straight. It just has to be round.

This means that I can chuck the bar in my 3 jaw. I mount a test indicator on my compound and adjust the tip to be horizontal with the spindle axis. Then I advance the cross feed until the indicator is zeroed. Next, I rotate the chuck and observe the minimum and maximum readings. For convenience, I adjust the cross feed to read zero half way between the maximum and minimum readings.

Next, I move the carriage to the far end of the bar, using care not to disturb the cross slide or compound. Again, I rotate the chuck, noting the maximum and minimum readings. The average of the two readings is my offset.

Some time ago, I realized that the same principle could be applied to tailstock alignment. The only difference that the tailstock center would engage the far end of the bar. If the bar was set up between centers, the bar could be rotated by hand. Ot also limits the measurement distance to the length of the test bar.

My preference is to use my 3 jaw chuck though as using the headstock center requires removing the chuck. A concern with using the chuck is that the chuck most likely has runout and mounting the test bar could introduce stress at the tailstock end with possible errors. To prevent this stress, machined a spherical bushing. The bushing is a slip fit on my test bar and is slit parallel to the axis so tightening the jaws will securely clamp the test bar. The bushing can be positioned at any point on the bar. The assembly is inserted in the chuck and the tailstock center is set to engage the far end of the bar. The chuck is then tightened. Measurement are then made as before.

The advantages of this setup are that the same test be can be used for both basic lathe alignment and tailstock alignment. There is no need for test cuts which greatly speeds up the alignment process. A perfectly straight test bar is not required, expanding the list of suitable bars and eliminating the requirement of verifying the bar truly straight.

For the spherical bushing, I selected a puece4 of 1-1/2" diameter steel of unknown composition. A spark test revealed that it was low carbon and it could be sawed. It came from a collection of drops that were known to contain exotic alloys like A, D and S series steels. The spherical profile was turned first. Then the work was drilled and bored for a slip fit for 1" I errored when cutting the relief on the chuck side of the bushing, cutting it to just under 1" so when I bored the hole, I stopped about .1" short for the last .100" of the diameter. When the hole was properly sized, I slowly increased the diameter of the last .1" until the bushing was parted off. To finish the parted side of the bushing, I mounted it on a 1" pin and used super glue to hold it. I then heated the bushing to break the bond and slipped the bushing off the pin.

I gave the bar a test run to see how it performed. The runout at the chuck was a pleasant surprise. It came in at .0012" TIR. I was really expecting more. Moving to the tailstock, the TIR was .0022" and the offset was .0007". Then I positioned the bushing midway on the bar with similar results. Before I make any correction to the tailstock, I will want to check the lathe ways as that adjustment will impact the tailstock setting.

Test Bar 4.JPG
This is the completed bushing and the test bar.

Test Bar 3 .JPG
And the bushing on the bar.

Test Bar 1 .JPG
Measuring the tailstock positions at near full bar length.

Test Bar 2 .JPG
Measuring the tailstock positions at mid bar length.
 
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RJ, I like this, will give it a try. It does remove a bunch of interfering variables. Now it would seem that for small tailstock alignment errors, which would be my case, very small, that just 3 jaw chucking would be adequate. Have you seen any evidence that this would or wouldn't work?
 
Winegrower said:
RJ, I like this, will give it a try. It does remove a bunch of interfering variables. Now it would seem that for small tailstock alignment errors, which would be my case, very small, that just 3 jaw chucking would be adequate. Have you seen any evidence that this would or wouldn't work?
Click to expand...
I have done that in the past. It worked OK but my 3 jaw has very little runout. For adjusting the tailstock, a thinner bar will work.
 
I haven't tried this because I don't have a test bar. I understand how this works to measure headstock alignment. The far end of the bar will wobble and the amount of the wobble when compared to the measurements at the chuck gives the amount of misalignment. But not for tailstock alignment. If the bar is held between centers the bar isn't going to wobble. The bar will spin on the centers and the DI reading won't change as the bar is spun. This of course assumes that the centers are truly in the center of the bar. Zero the DI at the headstock. Move it to the tailstock end. The reading on the DI should be the amount of the offset. If you hold the bar in a chuck then you have to rotate the chuck to find the halfway point in the chuck runout. Zero the DI at this point on the chuck. Move the DI to the tailstock end. The reading on the DI should be the amount of the offset.

Am I missing something here?
 
I have read at many machinist sites that the RDM has little value, that a millwright wouldn't use that method. I have no dogs in that fight so I have no opinion of my own to add. To clarify, the 2 collar method shows twist in bed and the RDM shows head alignment? What would you say the best order of operations are?
1) level bed
2) 2 collar test
3) RDM
?
I have done all the above, but it seems to me there probably is a best practice order of things?
 
GunsOfNavarone said:
I have read at many machinist sites that the RDM has little value, that a millwright wouldn't use that method. I have no dogs in that fight so I have no opinion of my own to add. To clarify, the 2 collar method shows twist in bed and the RDM shows head alignment? What would you say the best order of operations are?
1) level bed
2) 2 collar test
3) RDM
?
I have done all the above, but it seems to me there probably is a best practice order of things?
Click to expand...
The RDM method shows twist in the bed the same as the two collar method. It uses an indicator tip rather than a cutting edge. If there were no runout in the test bar, they would essentially be the same test. One could argue that the two collar test is the more accurate of the two as the micrometer used to make the measurements is capable of measurements to +/-.0001" whereas the RDM method uses a dial indicator or a dial test indicator. Most of these that are commonly used have either .001" or .0005" divisions. However, I have a .0001"/div. dial test indicator and a digital dial indicator which reads to .00005".

My order of alignment is:
1. level the bed
2. check headstock alignment for perpendicularity
3. check for bed twist (taper) with RDM method
4. verify taper adjustment with two collar test

If the RDM test is done correctly the RDM test and the two collar test will agree. I use the RDM test because it is faster. With the two collar test, the collars are cur in the bar and the difference in diameters are measured. Based on the results shims are added and the test is repeated.

With the RDM method, the average position is determined at the headstock and checked at the far end of the bar. Shims can be added and the far end readings made again to check the results. Based on the new readings, the shim pack can be adjusted again and the process repeated. Finally, the headstock end and far end readings are made once more to verify that the average positions are identical. Even using a .0005 indicator, it is possible to achieve tapers down in the low tenths.
 
Okay makes sense. That's what I was thinking. You would do this will a parallel against spindle and an indicator on the top slide?
 
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