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A cylindrical square may be inexpensively made by welding a 4" diameter steel tube with 1/4" to 3/8" wall thickness onto a base with a 10" diameter. The top of the tube should be capped with a plate equipped with a breather hole and a tapped hole for an eyebolt. Normalize the assembly after welding, and cylindrically grind the O.D. of the tube and the underside of the base. Perform all grinding in the same setup to maximize accuracy. If the bottom of the base is turned to relieve all but a 3/8" wide bearing circle at the largest diameter, the grinding of the base will be minimized. This square will be valuable to you in adjusting your machine at periodic intervals to maintain accuracy and minimize wear.
One of the most important things to recall about a cylindrical square is that when correctly used, it is never 'out of calibration'. A cylindrical square that is tapered, leaning, or both is just as usable (and just as accurate) as a 'perfect' square. This is because the square is calibrated at each use, and any variation may be easily allowed for.
Please Note: It is extremely important that the final motion of the machine's headstock be 'up' prior to taking any indicator readings. Thus, to take a reading at the bottom of the square: feed BELOW the point to be indicated, then back up to the desired point.
With the square on the top of the machine table, put the spindle back gears in neutral and position the head and table so that an indicator held in the spindle contacts the side of the square near the bottom end of travel.
Establish a 'zero' reading at the exact center of the square by moving the saddle back and forth to get the 'highest' reading, then set indicator 'zero'.
Traverse the head up the column and note the error and weather it is 'plus' or 'minus'. Save this indicator reading and call it reading "A". Make a light pencil mark on the base of the square corresponding to the path that the indicator traveled.
Without moving the cylinder square, re-position the indicator to read the exact opposite side of the square. Establish a 'zero' reading at the exact center of the square by moving the saddle back and forth to get the 'highest' reading, then setting indicator 'zero'.
Traverse the head up the column and note the error and weather it is 'plus' or 'minus'. Save this reading and call it reading 'B'.
Re-position the indicator to read the original side of the square. Rotate the square on the tabletop exactly 1/2 turn.
Establish a 'zero' reading at the exact center of the square by moving the saddle back and forth to get the 'highest' reading, then setting indicator 'zero'.
Traverse the head up the column and note the error and weather it is 'plus' or 'minus'. Save this reading and call it reading 'C'.
Insert the 3 readings taken above into the following formula and solve for 'T'. Please note the order of the calculation as indicated by the brackets, and perform the addition in an algebraic manner (taking into account he 'sign' of the readings). This is the amount of taper built into the cylindrical square due to manufacturing error.
T = (A + B) / 2
Insert the original readings taken above and the calculated value of 'T' into the following formula and solve for 'L'.
L = [(B-T) - (C-T)] / 2
Please note the order of the calculations as indicated by the brackets, and perform the addition in an algebraic manner (taking into account the 'sign' of the readings). This is the amount of 'Lean' built into the cylindrical square due to manufacturing error.
Insert the calculated value of 'T' and 'L' into the following formula and solve for 'L'.
D = (T + L)
Please note the order of the calculations as indicated by the brackets, and perform the addition in an algebraic manner (taking into account the 'sign' of the readings).
The machine tool may now be adjusted into perfect square by using the cylindrical square such that the indicator readings are set at 'zero' at the bottom of travel; and read 'D' at the top of travel. NOTE that this is ONLY true if the readings are taken along the track indicated by the pencil line made on the base of the square in the earlier step. All other paths up the side of the square are NOT calibrated and should not be used (unless, of course, you verify by this process that the square has no 'built-in' errors of taper or lean). Also, be certain to always take your readings centered at the 'high point' on the side of the square.
If the column is found to be out of square up to approximately 0.0015" in 30", it can be brought into square by slight adjustment of the leveling screws that are in the bed of the machine and located under the column area without affecting the rest of the machine's leveling. Generally, if more than approximately 0.002" adjustment is required, you will need to work back-and-forth between the leveling the main portion of the bed and the squaring of the column area, since each will slightly affect the other.
The cylindrical square can also be used (after calibration of the square and alignment of the column) to show the 'squareness' of the spindle to the column. This is a direct test of the ability of the machine to 'match mill'. Please note that the perpendicularity of the column to the tabletop, as established in the proceeding steps, has no bearing whatsoever on the ability of the machine to 'match mill', or 'blend' one cutter pass into another.
To check the spindle for 'squareness' to the column, mount an indicator at the end of a light, but rigid, 'tram' bar mounted at right angles to the spindle.
The 'tram' bar should be a single element of a structurally rigid shape (an 'I' beam or 'Channel' iron shape works well), and it must NOT be bent or welded, which could introduce stress. In the best of circumstances, it should be bolted to the end of the spindle using the tooling drive key bolts. Bolting it to an arbor that closely fits the spindle taper is the next best choice. Using a round rod, using a "Jacob's" Chuck, or introducing bends or welds into the tramming bar will usually produce inconsistent readings, but also will occasionally produce consistent readings that are simply incorrect.
Fasten an indicator to the tramming bar so that it contacts the side of the cylinder square that is facing the spindle.
Rotate the square on the tabletop to bring the calibrated line to face the spindle.
When the tram bar is rotated in the following steps, it is important to apply the rotation to the Spindle
SLEEVE; do NOT touch the spindle or the tramming bar, or attempt to use them to swing the bar between the two positions.
Swing the tram bar to the lower position and 'rock' it until the indicator contacts the exact center of the square as evidenced by a maximum indicator deflection. Zero the indicator at this point.
Swing the tram bar to the upper position and 'rock' it until the indicator contacts the exact center of the square as evidenced by a maximum indicator deflection. Read the indicator at this point.
The difference between the two readings (after making appropriate allowances for cylinder square taper or lean) is the degree of perpendicularity between the spindle and the column.
Note that allowance for the calibration error of the cylinder square, (value 'D' above) must be made, as well as any small error that results from not having adjusted the column perpendicularity to 'Zero'. (It is impractical when adjusting the column for perpendicularity to adjust any more than is really necessary to fall within acceptable tolerances, rather than adjusting for 'perfect' square). While there is nothing improper with this practice, this small error MUST be 'allowed for' when testing the spindle tram.
PLEASE NOTE:
It should be stressed that although a tram check of a cylinder square may provide useful information in the total alignment of a machine; it - alone - is not necessarily positive proof that match-milling problems will (or will not) exist, since several other machine variables can affect the capacity to match-mill. The ONLY true and final test of a machine's ability to match-mill is to actually perform a cutting test. Many variables enter into a tram check, and only the inspector's good judgment can prevent him from being led astray.