728VT really unhappy!

[davidpbest]

You said a couple of times during the video that your column is “good“. In fact, the column is out of tram by 1.5 thousandths of nod over the 3-5 inches of head travel, so it is not in tram. We could debate whether 1.5 thousand over the 3-5 inches you moved the head is “good enough“ but for the kind of work I do that is not accurate enough and would certainly be considered out of tram on something like a Tormach machine. You should also be checking tilt in addition to nod by indicating to the side-face of your 2-4-6 block. To correct the 1.5 thousandths nod error over the 3-4” inches of head travel you would need to shim the column to base connection.

Probably more concerning is the fact that the quill movement and the head movement on the column are not coplanar to each other - they diverge by 5 thousandths over the 3” travel of the quill. Imagine drilling through a 3” block of aluminum using the quill - the position of the top and bottom of the hole will differ by 0.005” front to back. Is that “good enough”? Not in my world, but you may be less demanding. I’ve never seen a specification from the factory as to what they think is acceptable on this head-versus-quill movement correlation. Again, check tilt in addition to nod. The correction for this quill nod travel error is to shim or scrape the connection between the head and column.

 

[Archer60x]

Is it $5k good? I dont know what to expect. I am leaning on the more experienced to tell me if my $5k investment was good or not.

My opinion of good is based on videos i have watched on youtube. I have seen people tram the head (tilt) not nod and they are happy with a couple thousandths. All i can say is my column is 1.5though over 6inches and that is better then my quill that is 5 though over 3 inches.

I did not buy a tormach. I cannot afford those.

I will be checking tilt tomorrow. Thanks for the feedback! This is what i needed.

Chad

 

[davidpbest]

Well, the good news is both of the tram errors that you’re seeing can be adjusted out of the machine pretty simply. It’s not rocket science to shim the column. Getting the tram error out of the quill movement is a little more vexing, but still within the realm of what can be done in a home shop with patience and some trial and error shimming the head to column interface.

Should you expect better tram alignment coming out of the factory than what you’re seeing on a $5000 benchtop mill? Well that’s probably a matter for conjecture. Like I said, I have not seen specifications from the factory, or Precision Matthews for that matter on what they consider acceptable in this regard “out of the box“. If it were me receiving this mill, I would be just fine with the column tram, and expect to make my own changes for tighter tolerances by shimming the column. That said, I would view the quill movement deviation of 0.005” over 3 inches of travel unacceptable and worthy of immediate attention and correction.

 

[RJSakowski]

Maybe I'm nit-picking, if so please forgive - but . . . If I take your explanation word-for-word, using this method you will know if the quill movement and the spindle axis are aligned. You do NOT know if the quill/spindle up/down movement is aligned with the head movement up/down. Assume for the moment that your "rigid bar" indicates zero TIR with the quill extended - you indicate to the bar, rotate the spindle, and see zero variation. Now retract the quill and repeat the same test and you get zero variation when the spindle is rotated. All you know is that the spindle/collet combination have zero TIR and that the spindle axis is parallel in both X and Y to the movement of the quill. You do NOT know if retracting the quill caused the "rigid bar" to move off the CL of it's original position unless you also look at how it was displaced. In order to know the quill/spindle combination move parallel to the head movement, you have to examine the displacement of the rigid bar in both X and Y, not just that the quill and spindle axis move parallel to each other.

Your method CAN measure that the quill/spindle movement agrees with the movement of the head up/down if you measure the displacement of the rigid bar with quill movement up/down - not just that the average of the rotational differences - and then compare that displacement to the displacement when the head is moved up/down.

The RDM method works fine on a lathe since you have only one axis being repositioned (the cross slide saddle moving along the ways). With a benchtop mill, there are two sliding elements (quill and head) that must conform to to parallel movement, which is why my personal preference is to use a machinist square or 2-4-6 block on the table as the reference surface to measure against. Even the inexpensive Chinese 2-4-6 blocks are sufficiently accurate to test quill and head conformity and to make the necessary adjustments to the column-to-base and head-to-column modifications.

The first step is to check the spindle to quill alignment as described above. Assuming that it is good, you move on to checking the spindle to column alignment. If it is not aligned and assuming that you can't correct it, you will still know how much it is out and in what direction. The second check of spindle to column alignment will give you the algebraic sum of the spindle to quill and quill to column alignment. If you subtract the first, you will have the quill to column alignment.

Regarding the use of RDM on a mill, if the contact point is on a line through the spindle axis and parallel to the y axis, you will measure misalignment in the y direction. if it is is on a line through the spindle axis and parallel to the x axis, you will measure misalignment in the x direction. (BTW, you can measure vertical misalignment on a lathe by placing the contact point in a vertical plane through the spindle axis.)


With the RDM method, runout is immaterial. That's the beauty of it.

As an example, assume a .5" diameter bar mounted in the collet. Set up the test indicator so the stylus point is on a line parallel to the y axis and passing through the spindle and just proud of the collet with the quill extended. .This particular bar has .005" runout at the collet as indicated by the test indicator. Adjust the position in the y direction so the average reading is zero (+ and - .0025"). Now retract the quill for some convenient distance, say 5" and repeat the measurements. The runout is now .010" with +.002" and -.008" readings. The average reading is -.003" which means that the spindle axis is now .003" further from the test indicator than previously. and the spindle axis is out of parallel in the y direction by .003"/5". Repeat this with the test indicator stylus point is on a line parallel to the x axis and passing through the spindle to determine out of parallel in the x direction. Any nonparallel between the spindle and the quill is most likely non correctable in practicality as it would require a reboring of the bearing seats but is worth knowing for reference.

This is repeated for the spindle to column by locking the quill and moving the head on the column. Any non parallel condition would be corrected by rotation or shimming of the head.

I initially used a square to determine perpendicularity. The are two potential problems with this. One is the square may not be square.. There are ways to check this but they can be beyond the resources and/or capability of the hobby machinist. The second and potentially greater issue is that in using a square, you are assuming that the surface of the bed is flat and that it is parallel to the ways. An older mill may have a dished surface due to wear. I swept the bed on my Tormach with a head mounted indicator some years back and found that the surface varied in a non linear fashion in both x and y. The major portion of this was due to the center of mass of the table being cantilever when the table moved to extremes and partially corrected by adjusting the gibs. There potentially remained some residual though as a CNC mill has to be free to move. It would be difficult to parse the cause of the remaining variation.

@RJSakowski , your modified RDM is a good test well described. Would you please elaborate on what comprises a "rigid bar known to be good"? And provide some information on where to find one?

I also need to do this with my lathe.

The requirements for a rigid bar aren't as stringent as one used for the two collar test as there are no cutting forces involved. The force created by a test indicator is on the order of a few ounces. Nevertheless I prefer to use a more stout bar. Roundness is a more stringent requirement. I use number of different diameter rods ranging from 5/8" to 1" depending on the situation . All of my bars were salvaged from printers and copiers. Another source would be shafts from small hydraulic cylinders.

A quick check would be to measure with a micrometer. Another way would be rotating in v blocks while measuring the height with a test indicator or dial indicator. The two methods differ in that the measuring points are directly opposite each other with the former and there are geometric shapes which will measure the same separation but are not round. The latter method has contact points which are separated by 90º, and 135º. I would use both to check for roundness.. Having a uniform diameter is a nice requirement but not totally necessary as one can compensate for different diameters.

 

[davidpbest]

As an example, assume a .5" diameter bar mounted in the collet. Set up the test indicator so the stylus point is on a line parallel to the y axis and passing through the spindle and just proud of the collet with the quill extended. .This particular bar has .005" runout at the collet as indicated by the test indicator. Adjust the position in the y direction so the average reading is zero (+ and - .0025"). Now retract the quill for some convenient distance, say 5" and repeat the measurements. The runout is now .010" with +.002" and -.008" readings. The average reading is -.003" which means that the spindle axis is now .003" further from the test indicator than previously. and the spindle axis is out of parallel in the y direction by .003"/5". Repeat this with the test indicator stylus point is on a line parallel to the x axis and passing through the spindle to determine out of parallel in the x direction. Any nonparallel between the spindle and the quill is most likely non correctable in practicality as it would require a reboring of the bearing seats but is worth knowing for reference.

This test will confirm that the spindle is or isn't aligned with the quill movement. It does NOT indicate whether the quill movement and the head movement are in the same coplanar relationship in both the X (tilt) and Y (nod) axis.

 

[55fairlane]

Ok guys here is your video, please excuse my voice, I don't feel good, and my poor video skills.

 

[davidpbest]

Ok guys here is your video, please excuse my voice, I don't feel good, and my poor video skills.

I watched your video. Your column is indeed leaning forward about 0.097 degrees. The difference you see going up versus down is probably loose or misfitting gib where the head attaches to the column. I would first take out the gib, deburr it, lightly stone it, and check it for flatness. Then reinstall and snug it up and repeat your test - the discrepancy between up versus down measurements will probably reduce.

That said, best practice for this test is to lock the head with the two levers, zero your indicator, unlock the head and move it up/down, then lock the head again and check the indicator reading. That will suck out any sloppiness in the head-to-column dovetail surfaces. You should also have your XY table locked so that any "sponginess" of those gibs are pressed out with the locking levers - this will ensure your XY table isn't also contributing to the tram error.

You should also check the head movement side-to-side by indicating to the angle-block re-positioned 90° with the indicator pointing in the X (sideways) direction. Then, moving the head up/down will tell you if the column is also leaning one direction or the other. Column tilt can NOT be corrected by just rotating the head.

If you haven't seen my video on the all of the different checks, it might help you make a more thorough and complete set of alignment checks. You can see it here:

To correct the nod, you will need to shim the column where it attaches to the base. Both nod and side-to-side lean need to be taken into consideration when aligning the column.

But first, check the quill movement with the same setup. Put an indicator in or on the quill, indicate to the same angle-block, and observe the indicator reading as you move the quill up/down. Based on your column measurements, if the quill travel is aligned with the head travel, you should see a similar nod conidtion of about 0.005" forward nod over the 3-inches of quill travel.

Looking at the manual for the 728VT, the column is attached to the base by four bolts coming up from under the base. This configuration makes it difficult to shim the column since you have to get the machine base partially off of the base to access the attachment bolts.

Assuming the column is not also leaning side to side, you're going to need something in the 0.002-0.003" of shim under the front edge of the column where it attaches to the base. Making this correction is tedious trial and error, adding/subtracting shims, tightening the column bolts and then doing the head movement checks against a known good square. I've probably done this opearion 50 times on different benchtop mills.

Hope this helps. I have to say that if this is a new mill, I'm very surprised by the amount of nod as delivered.

 

[qualitymachinetools]

No problem with video skills, that video was perfect.

Probably a little movement of the indicator arm moving around etc since its not returning to zero, but overall it gives a very good idea whats going on. A few more thoughts I have:

Before focusing 100% on the column, just to be safe, don't forget to check the Y Axis movement of the table surface. (The best way is to start with the Y Way surface and check Y to Z from there without the table, but its not easy so 2nd best is make sure the table surface is good)

To do this, you would put an indicator with the base on the head or in the spindle, and then the indicator needle would be touching the table surface (Much easier if you have a ground flat to set on the table, but you can get a good idea with just a light touch and dipping in and out of the T Slots), and run it back and forth in the Y. If the table is tilted, you could see the same results. Right now, everything is assuming that the table is dead on zero with the Y Axis. It makes sense that it is, but I would check that to be sure.


If that is where it should be, I am curious if you push on the column, do you get any movement? (Not that movement is bad, I can push on our 20,000 pound VMC Head and move it .003", but it shouldnt be loose is what I mean)


These are all tested and documented at the factory, I will get the spec of what it was then. There was mention of shipping damage when received that was discovered when removing from the crate on this one when we spoke on the phone, but even at that, thinking about it, cast iron doesnt bend.

 

[RJSakowski]

This test will confirm that the spindle is or isn't aligned with the quill movement. It does NOT indicate whether the quill movement and the head movement are in the same coplanar relationship in both the X (tilt) and Y (nod) axis.

This is why you lock the quill and repeat the test using the the column for vertical movement instead. Any adjustments must be done by shimming the base of the column.

The certification specs for my Tormach were relatively good. Perpendicularity of the column to the table surface are speced at .00023"/1" in x and y. My machine tested at .00007"/1" in x and .00010"/1" in y. The perpendicularity of the spindle axis to the table surface are speced at .00020"/1" in x and y and tested at .00005"/1" in x and .00015"/1" in y. However, when facing in the x direction workpiece, the cut pattern indicated an out of tram condition, and there was a clear step in the y direction. A sweep of the table surface showed that the table surface was not parallel to the x and y ways. Further investigation pointed to the table tilting as it was traversed to the left due to the center of gravity located outside the support of the ways. Tightening the gibs reduced the tilt significantly but at that point I couldn't determine if the residual variation was due to the table tilting or to the surface of the table not being parallel to the ways. Because of this issue, I abandoned using a square for checking perpendicularity of the column ways and spindle axis to the x and y ways.

If I was confident that I had eliminated the table tilt, I could have mounted a plate and skim cut a fresh surface to rest my square on; unfortunately I wasn't. This was the reason for using the modified RDM procedure.

 

[Archer60x]

I got my PM728VT last week and here are the results of my Z column and quill going up and down. .0005 DTI that i got today.

I am _not_ Quinn of Blondihacks that is for sure! I kick the tripod 3 times and say a few things wrong.

I am going to check tram and more tomorrow night. I know my tram is good as i used my old DTI and it is about one to two thousandth over the length my noga arm that holds the DTI (about 12inch diameter). I want to use my new DTI tomorrow to make sure it is good to great!

I am still learning so make sure you tell me if i am doing something wrong or if i can test better.

Any suggestions on fixing this 5 thousandths over the quill travel range of 3 inches let me know. I like Quinns method of only using the quill to set the depth of the cut. Do the major moves with the column. Then move the quill (less then one inch) to set the cut depth.

Can i live with approx 1.5-2 thousandths over 1 inch of travel? Now that i know it i will keep it in mind.

Thanks!
Chad

I re-measured everything today on my PM728VT (I am not the original poster of this thread).

I took my known good machinist square and found the squarest size of my 246 block. I have a granite square that comes with papers on order so i can do this measurement again and get more practice. I would use the square, but it has a tapered edge so not easy to run my DTI up and down that edge.

My column was .001 over 5" on this measurement. Past measurement was .0015 over 5 inches.

Quill was much better also. Less then .0005 over the 3 inches of travel that it has. I noticed my quill seemed tighter. I maybe used the handles while the fine feed was engaged. I hope i did not hurt anything.

I will delete my video as even though it is not public i dont want my beginning machining skills to give PM a bad name in any way.

I trammed the head and got to .0005 on X and less than .0005 on Y.

As i am learning more i appreciate everyone input. I locked down each axis before recording the measurement.

Again, i am not the original person that posted. I just happen to have purchased a PM728VT around the same time.

Chad