Rollie's Dad's Method?

[gard]

When using the RDM method, it is essential that the bar be perfectly round at both test points.. It isn't necessary that the two points are the same diameter but if they differ, that has to be taken into account so it's simpler if the diameters are the same.. When averaging the readings at each point, it is essential that it be done algebraically; i.e if you read a maximum of +.005 and a minimum of -.001, the algebraic average is (.005 +(-.001))/2 = +.002. It is also essential that the dial indicator not be disturbed as you move from one position to the next.

The theory behind the RDM method is quite simple. When reading minimum and maximum at any position, the algebraic average. is located at the spindle axis. What we are trying to do when removing taper is to align the carriage so that the cutting tool is at the same distance from the spindle axis. When the two RDM averages are equal, we know that a cutting tool mounted on the carriage is equidistant from the spindle axis.

Personally, I find the RDM method to be reliable and quite useful in multiple ways. In addition to running the traditional test, it can be used to check alignment in the vertical plane.. It can be modified to align the tailstock for turning on centers, eliminating the need for a test bar.. I also use it for aligning the headstock on my CNC mill to the z axis ways. The method is nondestructive and quick to implement, allowing multiple successive adjustments and verification checks.

The bottom line, though is to use whatever method you are comfortable with. If you don't have confidence in a test method, you won't be comfortable with it.

I think this is an excellent summary of RDM. The test bar may be bent or slightly crooked in the jaws to a small or large extent and it really does not affect where the center of rotation is. You can have a TIR at the free end of 0.10" or 0.0001", the center of rotation is still the center. The center of rotation needs to be parallel to the tool path to cut a cylinder with no taper. There are a couple of ways to find the center of rotation, RDM is by averaging the maximum and minimum indicator readings. Tho old school, more traditional, slightly faster, no math way is to rotate the test bar and stop at exactly 1/2 way between the maximum and minimum readings at the free end of the bar, now run the carriage back to the chuck and you will be measuring taper. The test bar should be uniform diameter but it does not have to be perfectly straight or perfectly mounted in the chuck. This is described in the excellent book Machine Tool Reconditioning by Edward Connelly, considered to be the bible of machine tool rebuilding and measuring. The center of rotation can also be found by cutting a bar and measuring the taper but that takes way more time and can use up the bar.
Another way of thinking about it is assume you have a test bar that is bent or mounted crooked in the chuck, if you rotate it so that it is pointing exactly up or down at the end, the front face will be parallel to the center of rotation.

 

[daved20319b]

I don't assume that the headstock is perfectly aligned from the factory. What I am saying is don't mess with headstock alignment to correct turning a taper as a first effort. Turning a taper can be caused by a headstock misalignment and/or a a twist in the ways. It is more likely that the ways were twisted due to the lathe mounting method than the headstock is out of whack. You can certainly correct the taper problem by realigning the headstock and that is exactly what I did to correct a .004"/6" taper. Only now, I had a facing error of .004"/6". On my lathe at least, there is no way of adjusting a facing error other than through headstock alignment which is why I correct the facing problem first and then move on to addressing taper issues.

My lathe is on a welded steel stand constructed from 4" and 7" channel with a welded steel top plate. I have also designed it to have three point mounting. Three point mounting of the stand insures that I am not introducing a twist with the leveling feet. I level the lathe using the adjustable feet with a .0005"/10" precision level on the ways in both the longitudinal and transverse directions.

My next step is to check headstock alignment as indicated by perpendicularity of the cross slide to the spindle, the test being to check an identical point on the face plate both front and back. (This method is very similar to the method used to tram a mill head except the indicator is mounted on the cross slide and the spindle with the faceplate is rotated. ) While there is a possibility that the lathe ways are twisted , by fixing the carriage close to the headstock, the degree of twist will be minimal. With the headstock aligned thusly, I move on to checking for taper using the RDM method. Any errors are corrected by shimming the tailstock feet appropriately. When I am satisfied with that test, I perform the two taper test to verify the alignment.

Thread 'On the Level: Further Adventures with the G0602'

Aug 26, 2018

When I built the stand for my G0602 lathe, I made leveling adjusters for the four legs. They were simple in design; a 1/2-13 bolt and jam nut threaded into a plate at the bottom of each leg. However, in trying to precision level the lathe, I ran into some issues.

When adjusting the height of each leg, there was a tendency of the bolt to walk across the floor. Not a problem for a workbench but when you are leveling a lathe with a sensitive level (.0005"/10"), it can be a frustrating game of Whack a Mole. The second issue deals with the fact that three points determine a a plane...

• RJSakowski
• Replies: 10
• Forum: ASIAN LATHES & MILLS

• 

Regarding precision in alignment, it is a conventional practice to calibrate/adjust equipment with instruments at least four times more precise than your desired level of precision. While I may not be concerned about a .001"/1" taper for much of the work I am doing on the lathe, fitting a bearing or making an interference fit to a mating part requires extra precision. In the end, we use whatever means suits or needs and work style.

Thank you, RJ, concise and informative, as usual. I also liked the info on your stand, I'm hoping to do something similar in the future, but in the meantime, I can cobble on a center leg at the tailstock end, and use the existing feet as anti-tip devices like you did, and maybe add some reinforcement to the existing stand . I also hadn't considered the similarity to tramming a mill head, something I'm familiar with as I also have a mini-mill.

So all that said, I think I'm now at a point where everything is good enough for 99% of what I have planned for the near future. I will be installing my faceplate and checking that, as well as checking the headstock mounting bolts, I suspect they've never been checked, I know I've never done it in the nearly 15 years I've had this machine. Thanks again, to you and everyone that responded, I think ya'll have gotten me on the right track. Later.

Dave

 

[mac1911]

Following along as next spring as it warms up I will be back in the shop and checking my lathe even closer. Still a few more things to fix up.

 

[daved20319b]

Following along as next spring as it warms up I will be back in the shop and checking my lathe even closer. Still a few more things to fix up.

Winter is my typical shop time, 2' of snow on the ground and temps in the 20's (normally) means most outside activities are done for the season. I'm still not happy with my inability to get the twist out of this lathe, I'm debating with myself as to whether I just run it as is (for now), or doing something serious about it, starting with pulling the lathe off its stand. Doing so would make it a lot easier getting set up to bolt it down, and it would allow me to decide between reinforcing/stiffening the existing stand or just building an entirely new one. There's also a couple of other minor issues I could address at the same time. But hey, I'm already half way down this rabbit hole, might as well enjoy the ride. Later.

Dave

 

[daved20319b]

I don't assume that the headstock is perfectly aligned from the factory. What I am saying is don't mess with headstock alignment to correct turning a taper as a first effort. Turning a taper can be caused by a headstock misalignment and/or a a twist in the ways. It is more likely that the ways were twisted due to the lathe mounting method than the headstock is out of whack. You can certainly correct the taper problem by realigning the headstock and that is exactly what I did to correct a .004"/6" taper. Only now, I had a facing error of .004"/6". On my lathe at least, there is no way of adjusting a facing error other than through headstock alignment which is why I correct the facing problem first and then move on to addressing taper issues.

My lathe is on a welded steel stand constructed from 4" and 7" channel with a welded steel top plate. I have also designed it to have three point mounting. Three point mounting of the stand insures that I am not introducing a twist with the leveling feet. I level the lathe using the adjustable feet with a .0005"/10" precision level on the ways in both the longitudinal and transverse directions.

My next step is to check headstock alignment as indicated by perpendicularity of the cross slide to the spindle, the test being to check an identical point on the face plate both front and back. (This method is very similar to the method used to tram a mill head except the indicator is mounted on the cross slide and the spindle with the faceplate is rotated. ) While there is a possibility that the lathe ways are twisted , by fixing the carriage close to the headstock, the degree of twist will be minimal. With the headstock aligned thusly, I move on to checking for taper using the RDM method. Any errors are corrected by shimming the tailstock feet appropriately. When I am satisfied with that test, I perform the two taper test to verify the alignment.

Thread 'On the Level: Further Adventures with the G0602'

Aug 26, 2018

When I built the stand for my G0602 lathe, I made leveling adjusters for the four legs. They were simple in design; a 1/2-13 bolt and jam nut threaded into a plate at the bottom of each leg. However, in trying to precision level the lathe, I ran into some issues.

When adjusting the height of each leg, there was a tendency of the bolt to walk across the floor. Not a problem for a workbench but when you are leveling a lathe with a sensitive level (.0005"/10"), it can be a frustrating game of Whack a Mole. The second issue deals with the fact that three points determine a a plane...

• RJSakowski
• Replies: 10
• Forum: ASIAN LATHES & MILLS

• 

Regarding precision in alignment, it is a conventional practice to calibrate/adjust equipment with instruments at least four times more precise than your desired level of precision. While I may not be concerned about a .001"/1" taper for much of the work I am doing on the lathe, fitting a bearing or making an interference fit to a mating part requires extra precision. In the end, we use whatever means suits or needs and work style.

Couple more quick questions for you, RJ. First, with your 3 point stand, how do you eliminate twist at the tailstock end? Second, did you sweep and skim cut your faceplate? Reason I ask, I put mine on yesterday for the first time ever, and it's got about a 1-1.5 thou runout. And before you ask, yes, I pulled it and rechecked my spindle, no detectable runout with my .0005 DTI, not even a needle quiver. Thanks!

Dave

 

[RJSakowski]

Couple more quick questions for you, RJ. First, with your 3 point stand, how do you eliminate twist at the tailstock end? Second, did you sweep and skim cut your faceplate? Reason I ask, I put mine on yesterday for the first time ever, and it's got about a 1-1.5 thou runout. And before you ask, yes, I pulled it and rechecked my spindle, no detectable runout with my .0005 DTI, not even a needle quiver. Thanks!

Dave

1. I shim between the tailstock and the stand. The two outside feet at the tailstock end are adjusted to just barely touch.
2. I did skim cut my faceplate but it isn't really necessary with the method I used to check.