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MiniLathe Alignment

Last Modified: 05 July 2014 09:02 MST

Minilathe Alignment Setup

This page is for those who fail to recognize that a Homier is not a Hardinge and insist on tweaking their mini-lathe for improved accuracy, especially if this accuracy is beyond anything actually needed in practice. Surprisingly, the methods needed to improve a lathe's alignment are simple and the tools required are generally already available in the home shop. In fact, my little lathe as delivered was amazingly accurate, easily accurate enough for most uses; the only significant error in my minilathe concerned the tailstock. Read on for details of converting a minilathe from a Homier to a Hominge.

There are four major parts to aligning the mini-lathe:

  1. Align the tailstock parallel to the ways
  2. Align the spindle parallel to the ways
  3. Align the tailstock to the spindle horizontally and vertically
  4. Align the cross slide perpendicular to the ways (not covered here)

What follows is an outline of how I went about aligning my minilathe; there are other methods - here's why I use RDM (Rollie's Dad's Method). I did not proceed in the optimum order so read this complete section before deciding how to proceed with your machine and its unique alignment errors. I'm still learning but I believe the order given above is a better way to proceed than the one I muddled through. Overall alignment takes a while but it isn't an all or nothing thing -- do it one piece at a time and do the next part when time permits. To avoid surprises, correct the horizontal alignment of the tailstock to the headstock after each session since most every correction affects this; it is quick and easy to keep this correct, especially if you fit an alignment bar.

Prior to aligning anything, the carriage "V" should be checked for "ridges" since this can adversely affect alignment of the carriage with the prism and the prism (via the carriage) is the main reference for alignment. Although most 7x beds are not twisted/bent, you should verify this before alignment.

Minilathes are inspected prior to leaving the factory and some vendors include an inspection report with the lathe. This sample report shows the standards this vendor required be met; the procedure given below can improve accuracy considerably ("Radial pulsation" is also known as runout; error values in the report are in mm). Note that Item 5 in the report requires the tailstock be higher than the headstock by 0.03mm to 0.08mm(about 1 to 3 thou).

Some machinists turn a test bar to align their lathe, follow the link on the Wrathall site. The J-Latta variation of the two collars method works well according to that site (I haven't tried it); a 2" micrometer is needed to get full accuracy from this technique. If the test bar is bumped accidentally while installing shims then the test bar may need to be re-trued for which the 7x12 must be reassembled (motor, motor controller, etc.) or you could use RDM (see below)to accommodate any modest eccentricity introduced. J-Latta's underlying measurements are similar to RDM but the test bar is turned down to run true and be identical size at the two measurement points. Some time is required to true the test bar and get the collars to identical diameter, where RDM uses calculations to remove these artifacts from the measured values without machining the test bar. There are variations of the two collar method that work by re-truing both collars without moving the cross slide, then evaluating the difference in diameters with a micrometer; these variations are difficult to use on the 7x12 when adding shims because multiple trials are generally necessary to select the correct shims and the lathe must be reassembled (motor, controller, etc.) to re-true the bar for each trial. Plus the bar must be re-trued when fine tuning the alignment via headstock bolt torque. RDM can be used to evaluate the alignment of a lathe which cannot be powered up, a major advantage in some circumstances.


MiniLathe Spindle Alignment

Setup for Rollie's Dad's Method I used Rollie's Dad's Method (this RDM concept paper is also found in the 7x12 Yahoo Group's files) of aligning a lathe's spindle axis to the ways but stacked the deck to simplify its application. Study and compare the original RDM concept document to the less general execution scheme outlined below to understand how and why the "simplification" works.

At right is my setup: a piece of 1/2" drill rod extending 10.5" from the chuck, the compound removed and a DTI mounted in its place (a toolpost mounted indicator works nicely too). In order to achieve 10" between the left and right measurement points (not really necessary) I removed the threading dial. The bar holding the DTI is set parallel to the test rod and oriented so that the DTI finger can be easily adjusted in height without changing the horizontal position. The horizontal position of the DTI is adjusted via the cross slide.

NOTE: The 1/2" x 10" steel test bar is not rigid -- it bends slightly under its own weight. The error due to sag of a 1/2" test bar is small so if your goal is simply to eliminate taper due to a headstock/bed alignment error you can safely ignore this error. I calculate that my test bar sags about 7/10 at the far end. This won't affect horizontal measurements noticeably but it will affect vertical measurements, i.e. the headstock will appear to tilt down by about this amount in addition to any actual tilt. This error from sag can be reduced by using a larger diameter test rod and/or a hollow test rod (pipe). I used the formula for a uniformly loaded cantilever beam found here to calculate sag. The indicator force against the rod, about an ounce, adds an additional 2/10.


To stack the deck: 1) Use an accurately round test rod (drill rod, a shaft from a shock absorber, or a shaft from a line printer are good but can be improved) to eliminate variation in rod diameter from the RDM equation, 2) Use a straight test rod and adjust chucking so the far end has minimal runout (the DTI is already set up) - preferrably under a thou runout to minimize cosine errors, and 3) Adjust the DTI at the chuck end to deflect equally above and below zero (making this average zero) to eliminate this term from the RDM equation.

GadgetBuilder's Simplified RDM Procedure for the Minilathe

If this is your first trial of RDM it may be helpful to print John Wasser's RDM page plus copy the simplified version above into WordPad and print it for reference.

Once you muddle through the above, it becomes clear that with your trusty test rod and DTI you can measure a lathe's vertical or horizontal axis error in under a minute -- something I puzzled over for a long time without getting anywhere prior to discovering the RDM article. If you don't have an accurately ground test bar and a DTI you can still make the RDM measurement by working from the original article, it just takes a few minutes longer.

My Results Using the Simplified Technique and What I Did to Improve My MiniLathe's Alignment

Using the above approach I made measurements along the top and front of the test rod. The spindle was slightly off line, where the averaged rod position 10" from the chuck was 1.3 thou closer to the operator and 0.4 thou higher -- excellent for this inexpensive machine but clearly not in the Hardinge category. I heard from a site visitor whose errors were over 10 times as large as mine so not all mini-lathes have good accuracy out of the box.

After determining my spindle alignment error, I removed parts as needed until the headstock was off; the shafts and shift fork were lubricated but the gears were not -- my opinion is that plastic gears don't need lube. It took over an hour of trial and error work using aluminum foil as shims to improve the alignment; the headstock was removed and installed at least 6 times while selecting the best shim arrangement. The spindle now measures 0.3 thou high and 0.2 thou forward at the end of the rod 10" away from the chuck -- as good as I could do because the tightening torque on the headstock retention bolts causes changes larger than the observed errors plus there is some "noise" in the DTI reading from the test rod's surface finish. Shimming is an iterative process where RDM results guide thickness of shims and where to place them. You start out with a guess, evaluate the effect and take another guess, etc. Error decreases as you converge on the correct shim setup but what I found is that it gets shaky when you need to adjust by tenths. Eventually I settled for the values above -- others may have more skill and/or more patience.

To verify that the corrections I made improved things I trued a 1" OD steel round about 3" long by making very light cuts. The taper measured over this length was about a tenth, i.e. at the limit of my measuring ability.

A recurring misconception about RDM is that the accuracy of the chuck or the way the rod is held by the chuck (angle/offset) affects the result. In fact, RDM removes these errors by averaging them out as part of the calculation. The simplifications in the application of RDM suggested above may make it hard to see how this happens; refer to John Wasser's original document to gain a clearer understanding of this elegant technique. The original document suggests shimming the feet of the lathe to correct alignment -- this won't work for the 7x12 because it sits on rubber feet and isn't bolted to a stable surface so you must shim the headstock instead. RDM does assume that the prism/bed which guides the carriage along the ways is absolutely straight... with additional effort you can verify this.

Note that the original RDM assumed the headstock was accurately aligned with the ways and measured error was due to bed twist, correctable by shimming the feet. Here I assume the bed is not twisted and the error is due to misalignment of the headstock. I have seen one 7x machine with a twisted bed so anything is possible; it isn't difficult to check for twist before attempting alignment.


Some Practical Details

Inspect the contact area between headstock and bed, removing any burrs or defects which might prevent good contact. Chamfer the holes for the headstock bolts slightly where they mate to the bed and also the holes in the bed where they mate to the headstock; burrs here can affect the alignment.

I found the mating part of the headstock was deliberately (albeit crudely) ground so the part which fits over the prism makes contact at areas near the left and right ends of the headstock, plus there is a third contact point on the rear flat part of the ways. There is a bolt near each of these contact points. This clever design makes it reasonably straight forward to adjust.

Apply machinist blue or Sharpie pen ink to the headstock where it mates to the prism. Carefully place the headstock in position and slide it back and forth on the prism about 1/8", then remove it. The blue on the prism and the clean points in the headstock V will indicate the contact points. Verify that the contact points are near the bolt holes, preferably toward the ends of the headstock from the respective bolt holes.

The shims to shift the horizontal or vertical alignment should be placed at the indicated points. To adjust the horizontal alignment, apply a shim to one 45 part of the prism near one hole and to the opposite 45 on the other hole (I extended each shim down the side of the prism to the horizontal part of the bed and held it in place with a dab of grease). I used folded aluminum foil for a shim so it may have compressed a bit; I used one 3 thou and the other 2 thou to shift my horizontal alignment by 0.0011 at 10" -- this may help in estimating the shims needed.

To adjust the vertical alignment, apply a shim to both sides of the 45 on either the chuck side contact point (to angle the chuck upwards) or apply a shim to both sides of the gear side contact point (to angle the chuck downwards).

As the 3 headstock bolts are tightened, torque can be used to "fine tune" the last half thousandth or so measured 10" from the chuck.

I used a 22 inch long piece of 1/2" drill rod I had on hand, passing it through the chuck until 10.5" protruded, then tightening the chuck. In retrospect, this was a poor way to proceed because the rod picked up some small scratches in the process of passing it through the chuck; these scratches affect the DTI reading by causing jumps of up to 1/2 thou as the rod rotates. I should have cut off a foot of drill rod, dedicated it to RDM, not passed it through the chuck, and preserved it in a velvet lined box when not in use. Rollie's dad owns a car repair shop and uses old shock absorber rods -- better than drill rod because they're hardened and tough to scratch (and they're free!). Enco has hardened and ground rod but keeps it hidden as 505-3290 -- this rod is VERY scratch resistant but more expensive than drill rod.


MiniLathe Tailstock Alignment

Prior to aligning the tailstock, check the tailstock "V" for "ridges" since these can affect repeatability of tailstock error measurements as well as tailstock alignment. Verify contact between the tailstock base and the lathe bed, adapting the ink approach outlined for the headstock in Practical Details above. Also see my tailstock notes for more on the difficulties involved in precise tailstock alignment.

Tailstock Measurement and Shims The tailstock's angular alignment to the ways should be corrected prior to adjusting tailstock height and therefore prior to adjusting the headstock. Improving the angular alignment is quick but getting it near perfect takes time and fiddling because the results of shimming aren't completely repeatable, probably because things seat differently each time. To evaluate the angular alignment of the tailstock, extend the ram and lock it (to take up any slack). Mount the DTI on the carriage and indicate along the top of the ram to measure vertical alignment. Indicate along the front of the ram to measure horizontal alignment. My tailstock's angular alignment seemed fine initially but eventually I found the ram was pointing up and toward the front by 2 thou per 2 inches in each direction. Shims were installed as shown, a 3 thou shim across the rear of the tailstock to base joint and a 5 thou shim (about 1/2" long) on the front, forward of the center ridge in the base (I left the ends exposed for the picture). Shimming the tailstock is much easier than shimming the headstock: remove the tailstock from the lathe, loosen the bottom socket head screw and the rear screw a couple turns -- the base and top will separate enough so shims can be slid in. Re-tighten the rear screw and tap the base a couple times with a rubber mallet to ensure things are solidly seated, tighten the offset adjustment bar if fitted, tap again, tighten the socket head in the bottom, tap again. Install, check, and repeat as needed.

Once the angular alignment of the tailstock is correct then the angular alignment of the spindle should be completed prior to aligning the tailstock to the spindle horizontally and vertically as described below.

Mini-lathes are deliberately manufactured with the tailstock slightly higher than the headstock (see item 5 in this inspection report) to facilitate subsequent alignment. The implication of this didn't register with me initially so I thought I would need a mill to lower my tailstock by 2.5 thousandths. I reassembled my lathe after aligning the spindle and continued to ponder how to align the tailstock height.

Finally, the light came on: raising the headstock is equivalent to lowering the tailstock since the goal is to set them equal. I had to loosen the headstock again (not optimum) to correct the tailstock height error. Shimming the headstock is faster and easier than lapping the tailstock to match the headstock height; future wear may lower the tailstock which can be compensated by exchanging this shim for a thinner shim. My tailstock measured 0.0025 high so I added a 5 thou shim on the rear headstock support point (without disturbing the shims previously installed on the V), raising the chuck (in the center of the headstock) by half the thickness of this shim. This worked as anticipated: the height of the tailstock is now about 0.3 thou high while spindle angular alignment was unaffected. (Paper shims work but are not the best because they can be difficult to remove if re-shimming is needed later, best to use real shim stock or even pieces trimmed from a soda can.)

Setup for Tailstock MeasurementTailstock offset was measured by holding the mounting stud of the DTI in the chuck, then indicating on a dead center in the tailstock (easier and more accurate than indicating the inside of the tailstock taper). A minor difficulty with this technique is that a mirror is needed to read the DTI while the dial is facing down. The tailstock height error is half the difference between the indications taken at 12 o'clock and 6 o'clock; verify that the tailstock is higher than the headstock before proceeding to shim the headstock, of course. If the tailstock is lower than the spindle then the tailstock must be shimmed instead of the headstock - place equal thickness shims on both sides of the center section of the tailstock base to raise the tailstock. A more serious difficulty is that indicator weight causes an error, about a half thou in my setup. This can be evaluated by clamping the mounting stud to the blade of a square with the finger touching the wide part of the square, compare the readings with the indicator facing up and and then inverted (still clamped to the square). This makes the tailstock seem higher than it actually is by half this difference so adjust the result above appropriately.

While the DTI is set up, verify the horizontal alignment of the tailstock by taking readings at 9 o'clock and 3 o'clock. Any error here can be corrected using the adjustments provided. It can be tricky to adjust the tailstock because you must rely on the upper screw (under the crank) to hold the horizontal adjustment while you remove the tailstock to access the locking screw underneath. Then, the adjustment may shift slightly as you tighten the lock screw so it often takes a bit of trial and error -- I eventually added a bar and use shims to simplify accurately centering the tailstock horizontally. Theoretically the angular shims shouldn't interact with the horizontal shims -- but they do so if you change any tailstock shim check both angular and offset errors. This interaction may be due to seating errors since it doesn't seem completely repeatable or predictable -- I have about 0.0001 error per inch in residual angular error because this unpredictability prevents getting all settings to zero simultaneously.

My tailstock exhibited repeatability errors, e.g. the horizontal error would vary by up to 2 thou each time the camlock was released, the tailstock moved away and then back and the camlock re-locked. This bedeviled me for a long while before, while working on the carriage gibs, I found (and removed) a small ridge in the tailstock "V", probably caused by wear.

Some are surprised that DTI chucking errors (runout or anglular) don't affect aligning the tailstock to the headstock with this method. Note that the DTI's finger will rotate about the spindle's center of rotation regardless of how it is chucked; the measured error is the offset between this center of rotation and the tailstock's center. You can demonstrate that chucking errors don't matter by measuring the tailstock offset with this method, then offset the DTI's mounting stud by inserting a shim between the stud and one jaw and measure the tailstock offset again. With a thick shim you may have to adjust the angle of the DTI's finger but the offset will measure the same -- try it: the difference between the two readings (9 and 3 oclock) will be unchanged by addition of the shim.

Prior to alignment my lathe did not drill well from the tailstock -- the center drill would scribe a small circle when brought slowly into contact with work in the chuck, with increased pressure drilling would start but the drill would wobble; there was also a "scrapey" feel when drilling deep holes. Following alignment the drill starts nicely without wobble and is noticeably smoother in deep holes.


My Tailstock Notes

The tailstock is more difficult to align than the headstock (although access for shimming is easier). Measuring the headstock error and compensating for errors arising from chucking are handled nicely by RDM, assuming a good quality test bar. There are more sources of error/variability when aligning and/or using the tailstock including: clamping the tailstock to the ways, ram/taper concentricity, and chucking variability.

The method I used for tailstock angular alignment is flawed in that it assumes the exterior of the ram is accurately concentric with the interior taper. This alignment in my tailstock has an error of 1.5 thou per inch = .086 degree (which seems pretty precise when stated in degrees). I made a lever drilling attachment and while evaluating its accuracy ran into the error in the tailstock's taper alignment (made obvious by the increased distance from the taper to the end of the drill bit). Oddly, removing the rear shim inserted while aligning the ram to the ways aligned the taper correctly; hard to believe this was done deliberately during manufacture, but maybe.

It is instructive to check the tailstock chuck, indicating on a hardened and ground pin held in this chuck. This checks the overall error in the tailstock plus the arbor and the chuck itself -- my HF chuck isn't a precision gripping device although the basic alignment of chuck and arbor is good. Each time the hardened pin is gripped in this tailstock chuck the offset changes by up to 3 thou although the angular error isn't affected. My Jacobs Superchuck has less variation in offset, typically being under 1 thou. (An inexpensive source for a suitable hardened pin is the shaft from a discarded VCR's head motor.)

If the goal is to improve drilling performance from the tailstock then aligning the tailstock based on measurements on the ram leaves ram/taper concentricity and chuck idiosyncracies uncompensated. Using a hardened pin held in the chuck for alignment works but variability in how the pin is gripped by the chuck can add considerable error and is difficult to determine. Setting horizontal and vertical offset by measuring on the face of a dead center seems to be reasonably repeatable. One scheme would be to make h/v offset measurements with the ram retracted and extended, then take the difference to find the angular errors -- I haven't tried this (yet) but it suffers from being more complicated to explain and taking longer to make the measurements.

Reducing tailstock error is quick and easy and the improvement in operation is easily seen. Diminishing returns set in when attempting to achieve errors below a thou or so. The DTI is a double edged sword allowing measuring and correcting alignment errors but also showing residual and repeatability errors remaining after one's best effort. Perhaps it is best to be happy with the low hanging fruit :-)



Setting the DTI Vertical Position

"Carefully adjust the vertical position of the DTI for maximum deflection (center of the test rod's side)" seems easy enough to do -- and it is easy if you use a DTI mounted on a QCTP because then the tool height adjustment moves the DTI vertically. With the DTI on a mag base, ensuring the DTI moves vertically is more difficult -- the plane of movement is usually inclined slightly. Getting the DTI vertical position accurately on the center of the test bar based on needle deflection assumes the DTI moves vertically. Make this movement as near vertical as possible to minimize small errors that are difficult to analyze and quantify.



Checking the CS is Perpendicular to the Ways

To check whether the cross slide is perpendicular to the ways, mount an indicator on the CS either on the toolpost or using a magnetic base. Remove the chuck jaws and touch the face of the chuck near the periphery of the chuck face and at center height, i.e. on the near side of the chuck and set the indicator to zero. Lock the carriage so it won't move inadvertently as you move the CS. Mark the point touched with a Sharpie, then rotate the chuck so the marked point is on the far side and at the same height(you'll have to help the indicator finger across the chuck slots). Advance the CS until the indicator again touches the marked spot on the chuck. Any difference in reading indicates (pun) that CS travel is not perpendicular to the spindle axis. If the CS moves toward the headstock slightly as it is advanced then a faced surface will be slightly concave; if the CS moves away from the headstock as it is advanced then a faced surface will be convex. Generally a very slight concavity is desirable, perhaps a tenth or two per inch. In practice, this measurement is helpful but not completely reliable: often tool pressure increases as the tool approaches the center while facing (because sfpm drops) and this tends to make the result convex, depending on the work material, RPM, slide loosness, tool sharpness, depth of cut, etc. Bottom line: if it's close it's probably OK.

Correcting the CS travel to make it perpendicular requires modifying the CS dovetail (or the saddle "V" way) slightly which is beyond the scope of this document.



Checking Whether the Bed+Prism is Straight

Generally, the prism and bed of 7x machines is straight but occasionally a machine produces puzzling results when applying RDM. The concept behind RDM is to compensate various errors in measurement (bent test rod, chucking angle, etc.) mathematically by subtracting them out. This allows measuring the distance between the prism and the (theoretical) center of spindle rotation. If the bed is twisted or the prism curves then the result will be in error and attempting to reduce the measured errors by shimming may produce unexpected results.

Checking a 7x12 for bed twist requires a surface plate or other flat surface (mill table); remove the lathe's rubber feet, place it on the flat surface and verify all feet make contact with no rocking, where rocking likely indicates the bed is twisted. Vertical curvature of the bed can be detected with a straight edge. Assuming all is OK, verify the bed/prism straightness by taking additional RDM readings as suggested below (where I assume you're using the simplified RDM described earlier). On larger lathes where it is impractical to place the lathe on a surface plate use a sensitive machinist level to verify the bed isn't twisted. Correcting for bed twist or wear is beyond the scope of this document.

The (theoretical) center of spindle rotation is a straight line so if the bed/prism is not straight then the distance between the prism and the center of rotation will vary non-linearly - this sounds complicated but is straight forward to detect. To do this, record RDM type measurements at evenly spaced intervals along the test rod. For example, record a RDM type reading every 2 inches on a 10" long rod, i.e. 6 readings. Each RDM reading is the (min+max)/2 = distance change between prism and center of spindle rotation. Because these readings are taken at equal intervals, the change between adjacent readings should be equal. That is, the reading nearest the chuck is zero (set that way using the simplified method), so if the reading 2 inches from the chuck is 1/2 thou then 2 inches farther from the chuck it should be 1 thou, and readings should increase 1/2 thou for each additional 2 inches moved toward the tailstock. If the values change by differing amounts then the bed is twisted or worn or the prism is curved - use common sense in interpreting the variation found(changes of a tenth or two are likely measurement errors/noise). Correcting defects in the bed or prism is beyond the scope of this document.



Improving the RDM Test Bar

Improving spindle/bed alignment is an iterative process where initially the error may be large but reduces as the process proceeds. A piece of drill rod works fine when the error is relatively large but as the error gets smaller noise in the readings due to imperfections in the bar limits the ability to discern the error signal. The bar diameter may be slightly eccentric and the diameter may differ slightly between the measurment points. Plus, surface roughness adds to the difficulty in reading the indicator. It is possible to reduce the effect of all three by lapping the test bar.

Laps_Shop_Made The laps shown are made from 1.25" aluminum bored to size and slit. The lap holder is steel and includes a set screw to adjust the lap size and also to keep it from rotating in the holder. Lapping is a statistical process in which the lap and the work get rounder and in the limit the outcome would be a perfect circle. A little Clover valve lapping compound or dry abrasive+oil works well.

You can feel the eccentricity of items as lapping begins - the lap twists in your hand with each rotation. This eccentricity goes away pretty quickly because much of it initially is tooling marks rather than solid metal. In addition, running the lap axially causes the diameter to become constant over the lapped area, one can feel differences of less than 1/10 so it's easy to convert a bar that looks round and constant diameter to one that IS, within better limits than I can measure. As a fringe benefit, the surface noise as seen by the indicator is greatly reduced. Not generally a long process, it typically takes under 15 minutes if the bar is reasonably constant diameter with a decent finish to start.

Lapping is helpful when you're trying to get every bit of possible resolution from RDM - it improves the signal to noise ratio when the errors being measured are small.

Test bar length is a tradeoff where greater length amplifies the error signal (without adding noise) but results in more sag, so sag compensation plays a bigger part. Larger diameter is a help in reducing sag as is a tubular test rod - reducing weight reduces sag.



Why Do I Use RDM (Rollie's Dad's Method )?

The standard method of lathe alignment uses a test bar but I haven't found a detailed procedure for its use on the net. This overview of lathe alignment covers it somewhat but lost me in the final three sentences of section 16.3.5. Lots of interesting tests mentioned. The spec on the test bar is 4/10 maximum runout. This would prevent accurate alignment so my speculation is that they average it out -- which is RDM (except I suspect they don't say it that way).

The issue with test bars is they are expensive, typically $100 to $300 each for those with MT tapers; the 7x12 would need MT2 and MT3 test bars to test headstock and tailstock. It didn't seem reasonable to spend that amount to align a $300 lathe.

There are alternatives which don't require an expensive test bar: the "Two Collar Test" (TCT) and "Rollie's Dad's Method" (RDM). Descriptions I've found for each are: TCT in J. Latta's article, and RDM in John Wasser's paper.

Reading J. Latta's description, turning the outer collar sounds challenging. Some confirmation in a HSM forum post. Of more concern is that the minilathe headstock needs to be removed during alignment and it would be possible to accidentally bump the test bar in this process - this could require re-machining the two collars which in turn would mean reassembling the lathe, extending the time and effort needed considerably.

There's a lot of controversy about RDM in machinist forums, most based on misunderstanding Wasser's document. John Wasser's description of RDM explains the concept well but isn't a detailed description of its execution; it conflates the RDM measurement with the correction method. It also fails to identify headstock alignment and bed twist as possible sources of misalignment so it doesn't indicate how to identify which is causing the measured error. John's paper does explain the measurement error sources and how they can be removed mathematically but fails to point out how to eliminate them during setup to simplify calculations. The controversy arises when people fail to understand that the paper is about concept rather than a step by step execution plan - John assumed users of RDM would use common sense to connect the dots. Vociferous opponents of RDM seize on the fact that the document assumes bed twist is the problem and proceeds to suggest how to correct it. But the TCT is used in essentially the same way in this South Bend document see page 20.

When you separate out the emotion, all three headstock/bed alignment methods are the same: if you have a precision test bar then you use it. If you don't have a precision test bar, you simulate it. With TCT you turn two accurate test areas on a bar and use them to simulate the precision test bar. With RDM you remove the error sources using arithmetic thus simulating the precision test bar. However it's done, the goal remains the same: measure the horizontal and vertical deviation between spindle axis and bed ways.

So, I chose to use the RDM concept but used common sense in the setup (straight test bar of constant diameter) to eliminate most of the calculations involved, thus simplifying and speeding up the measurement part of alignment. Zeroing the indicator at the chuck end further reduces calculations. These simple changes reduce the measurements and calculations to one addition and dividing that result by 2. The downside to this simplification is that it makes it more difficult to see the underlying logic of RDM, as clearly described in John Wasser's paper. Unfortunately, this doesn't help the trial and error needed for correction (shimming).

Minimizing Runout: If you're willing to work at it you can lap a straight 1" round steel bar (preferrably a tube), use a 4 jaw and set the runout near the chuck to nil, then set the runout at the far end to 4/10 or less by tapping it in the right direction as you incrementally tighten the jaws. It takes a few minutes to do this but at that point you have the test bar aligned exactly on the spindle axis. This should perform just like a commercial test bar. Of course, you should use RDM to correct for any minor runout remaining, just like when using a real test bar. Or, you could chuck the lapped bar in a 3 jaw, accept a thou or so runout, push the high side on the far end as you tighten the chuck incrementally and get runout to a thou or so, then use RDM - my guess is there will be little difference in results.

From a different perspective, there is a noise floor set by things like indicator resolution, surface noise, bed wear, etc. and this floor limits the ability to discern the spindle/bed deviation because as one iterates toward perfect alignment, deviation is reduced to the same magnitude as the errors comprising the noise floor. The refinements to Wasser's RDM suggested here are simply ways of minimizing some obvious noise sources so the deviation signal is more easily and accurately extracted. Using RDM without these refinements is akin to using a commercial test bar that got a little rusty in storage: one wouldn't expect the best performance.

The minilathe isn't like most American lathes where the headstock is accurately aligned to the ways at the factory so bed twist and wear account for most spindle/bed alignment errors. Minilathes generally have beds that are new and straight but headstocks that aren't aligned to the ways. However, anything is possible (I've seen a 7x12 with bed twist) so one should verify that the bed isn't twisted prior to aligning the headstock to the ways.



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