Micrometric Obsesion

Some time ago I note parallelism of milled parts sometimes was not as good as it could be, and though I didn’t bother, this time I wanted to try to enhance this. So this is about more measurements and tweaks.

Vise Horizontal Parallelism

The first suspicious was the vise, so I begin with it. After some measurements of the vise bed width, lapping was accomplished to enhance the numbers. Here are the before and after.

1. Vise bed width

Lapping did remove most of bottom anodizing, but was worth.

2. After bottom lapping.

Corners get a little over-lapped, maybe this can be reduced by fixing paper to surface (I don’t care anyway).

Vise Vertical Squareness

I took measurements at two points near the middle of the jaw. Before bottom lapping there was a minimal tilt to the left:

3. Column-jaw squareness.

After lapping the tilt was also minimal but change to the right. So I did remove the jaw and softly lap the top. After this almost no tilt was detected (see below), at least in the middle of the jaw.

4. Column-jaw squareness, corrected.

Finally I took measurements at six points. Here’s what I get.

5. Jaw to column measurements.

Vertically was ok, but horizontally there was some concavity. This was not a surprise, as I noticed this before but don’t think this would be an issue.

Test

As a test I mill a small aluminum block and took measurements at the ends. They were 9.341 and 9.343 ; a difference of 0.002 (nice). Previously this difference, for similar parts, was around 0.010, as I remember. But the main enhancement, supposedly, should be in the Y axis, but it wasn’t. The widths vary 0.030 from side to side, in a distance of 9 mm. Too much for me.

6. Test.

So what happened? After a lot of work I found four main reasons:

  • When measuring, I don’t take into account error in squareness of the column to the table.
  • When a part is clamped in the vise, there’s some deflection of the back side.
  • When the movable jaw is pressed, it also lift up a little bit.
  • Some time ago I lap the column’s gib and tough it feeled tight, it happens to be a little wider on the middle.

Some Measurements

Several measurements were taken.

1. Table to column squareness. Measurements every 1cm, over a section of the travel.

7. Table to column measurements.

2. Squareness of the fixed jaw to the column, without load.

8. Squareness without load.

3. Squareness of the fixed jaw to the column, with load at top.

9. Squareness with load.

Here’s some interesting. According to previous measurements (look Figure 5), this difference should be only 6um (and not 24um). Figure 11, part 5, explains this (gray box).

4. Deflection, or how much the jaw goes back with load.

10. Deflection.

With all this I was able to figure out the complete picture. After some struggling with the numbers, I got the diagram below.

11. Summary diagram.

I conscientiously don’t take in account for table unevenings. In previous measurements I get a difference of around 6um from side to side, in the middle of the table (see previous post). This difference act pointing the square some micrometers towards the column.

Deflection compensation and jaw lapping

To compensate deflection, I Mill jaw support surface. As the headstock had a very light angle towards the back (look next section), this provided a compensation angle. Otherwise some padding would have been required.

12. Jaw suport retouch.

Moreover, I found that only the external face of the fixed jaw was ground, and after some examination and measurements I opt for lap it on both sides (this seems to explains concavity). It was an almost impossible job (I don’t recommend to anyone), but after many hours of hard work I get satisfied with the measurements: at least regarding to width, maximum measured variation was of 0.003 (I took measurements at 15 points and check with a good rule).

13. Jaw lapping.

After all this I check again, and the jaw seemed to be compensated (look Figure 11, part 4).

14. Squareness measurement after milling.
15. Squareness measurements, indicator in vertical position.

Then I repeat the test with the aluminum block, milling both sides. I found that the initial difference was reduced only to 0.020, not enough. Something more was happening.

Gib

I did the classical head alignment test and get a damn 0.060mm difference in the Y axis (I did this measurement some years ago, and it was not as bad). It looks as if the headstock was inclined, as this difference can’t be explained by the column deflection alone.

16. Headstock axis totable squareness

After some thinking, a found the problem. Some weeks ago I noted a play in the headstock when lock it in place. After disassembling some wear was detected (look below), seemingly caused by some play in one end of the gib (poor lubrication was not the cause). So I  lap the gib using 320 grit sandpaper, until the fit feeled similar in both sides. After that the play in the headstock was reduced and fit seemed to be improved.

17. Column saddle wear.

But now, trying to explain the numbers, I checked the gib straightness using a rule and a back light, and was clear that the gib was ticker in the middle, on the not lifting side. After straightening using sandpaper, install and recheck, the alignment difference was reduced to 16 um, a lot better. After repeating the milling test, I get a width difference of 12um.

18. Gib adjustment.

Vise Jaw Lift

There were still 12um, and then I noted something: in the milling test I use two parallels, and when the part is held, the one in near the movable jaw gets looser than the other. So I did one more test: press down strongly the part while tighten the vise. After repeating the test one more time, finally I get a difference of 0.001.

Professional guys know well this issue (look here). After you visualize how the part is grip, it’s clear what it’s happening.

19. The final issue.

The lift problem will affect mainly small parts, so I think having a smaller vise should be a must to work with very small parts.

Conclusions

I’m not pretending to have micrometic presicion, but get the most from my machine doing all can be done to improve it. There’s nothing bad with it, it has a very good accuracy for the money, but you can always improve things.

It’s well know that accurate of a machine depends on a build-up of a lot of inaccuracies. But identifying and measuring them is another thing. Though this was a very tedious journey, now I have better idea on where innacuracies are, how they sum-up and how can be reduced.

After a lot of work I was able to enhance the vise a little bit, but now I think that a better deal is to left the sherline one as is and buy a chinese screwless vise for high precision work. Some time ago I bought one, but it’s still waiting for me to make the clamps (look here). It’s worth to note that the sherline one is ok for most of works, and it’s aluminum body allows to be less careful, as opposed to an all-ground steel and heavier vise.

Regarding the gib issue, I believe all the gibs in a new machine can be lapped to improve the fit, but clearly a “good fit” can be misleading if the gib it’s not checked properly.

Milling table tweaks

Having a tooling plate is almost a must for anyone that owns a Sherline milling machine. It helps to protect the table and can be adapted to specifics needs. When I bought mine (A2ZCNC brand), I don’t worry about how well aligned the top surface was; I supposed it was good enough. But someday I took some measurements, and this is the history. Please note this this is not a review about the mill or the tooling table precision, but rather some reference information.

Base Table Measurement

I use my 2um test indicator to get measurements at six points:

So there’s a small tilt on the X axis (around 0.0243º); I don’t know usual this is, but I was not happy with that. Please note this is a three year machine, used lightly, so this wasn’t caused by worn or misuse.

Tooling plate width

I’m really not able to measure how flat the plate is, so I only took a few thick measurements using a digital micrometer.

Looks ok.

Installed Tooling Plate

Here’s what I get with the installed tooling plate, after filing bottom protroudings (see below).

After that I search for a way to enhance this numbers.

First Try

  • Flattening tooling plate sole.  Bottom side of tooling plate holes seemed to have protruding sourrondings. I’ve noticed this before but didn’t take care as if protruding were equals this wouldn’t, at least in theory, have any effect . But one day I noticed some round marks left on the mill table, so I just pick up a file and take out protrudings (I used a small flexible flat file).
  • Padding. To enhance top surface alignment,  I cut some nylon and aluminum foil and play a while.

Here is the best I got after this tweaks.

Milling the plate

Padding was a rather ugly solution, so after some weeks I choose to face mill the plate surface. This was a two-step process, as tooling plate area is larger than max milling area. Please note this was done to fix a milling table issue, not related to the tooling table itself.


After milling lapping was required, so I just pick some 350 and 500 grit sand paper and begin the process. A recently acquired cheap granite surface plate was used as lapping surface.

After around an hour, I got a nice finish, so I tested, and surprise: borders where  lower than center area.

This was in part, I think, due to the fact I did most of the  lapping work in perpendicular direction; bad idea. Anyway this lapping process will always take a little more material from the borders; to compensate for this an abrasive area smaller than the plate should be used (not showed here).

Bad Not too bad

Fixing this required several hours of hard work and and continuous test. Here is the final result.

Not bad.

A final tip: here is how I clean dust in threads.

Aligning pins

Aligning of the milling vise is a tedious process, so to avoid this,  or at least to help, I bored some holes and turn some pins. Two pins at plate sole to fix the table, two to align the vise at center and two to align the vise at the left.

Boring precise holes it’s not and easy task. Hole position must be carefully calculated, and fitting must be tight enough to avoid pins fall apart, and lose enough to allow to remove and put them back by hand. Having this on mind, some micrometers more or less can make the difference. I fail the first time in bottom holes, and the top left holes ended with a little play. As show below, align is ok for most of works (this is a 2um indicator).

Center position Left position

Conclusions

It’s clear that in the manufacturing process of the tooling plate  lapping was done first and threading later, leaving the undesired protroudings. To correct this, filing and/or lapping can be used. I’ve said there were protroudings at the sole, but the top surface also had small protroundings (10 um, not noticeable by eye). I recently bough a Sherline tooling plate (for another machine), and in this is clear that lapping (or some kind of surface grind) was the final step; in fact, the plate comes with some fine dust.

In regard to the milling table tilt, may be this is within expected tolerances for this machine, but must be corrected to help to machine precision parts; 28 um it’s a lot.

After a lot of work I got acceptable tolerances and a nice finish. Its clear that now this tooling plate should be used only in this machine and in the same position always, but this is not a problem.

It would be interesting to hear about similar measurements from others, may be I was unlucky, or may be I’m expecting to much precision from this things. This of course doesn’t change my concept about Sherline products as high quality, super nice machines; in fact  I’m getting parts for the cnc version of this mill.

Runout

Usually I try to work the way I dont need to take too much care of my lathe runout, but its a good idea to have some reference values. Here are some simple measurents I take. I use a swiss 0.002 mm test indicator and some grounded steel rods.

  • Spindle internal tapper: 0.006 mm. Sherline specs states 0.0005″ (0,0127 mm) , so its ok.

  • Spindle play 0.005 mm. Again within sherline specs: 0.0002″ (0,0054 mm).
  • Spindle face: 0.014 mm. This suprise me as spindle face its the reference surface for chucks. There’s no spec for this, buts seems to much to me.

  • 2.5″ 3 jaw chuck: 0.094 mm. Sherline states a max of 0.003″ (0,076 mm) runout, so its a bit out of specs.

  • Another 2.5″ 3 jaw chuck: 0.124 mm. I buy it used and seems it has a lot of use, but don’t think this explain this value.

  • 2.5″4 jaw chuck: 0.080 mm. Almost within specs (buy used but in good condition).

  • 3.1″ 3 jaw chuck: 0.024 mm. Better. This is was also buy used. This was a surprise as visually jaws doesn’t look to met “perfectly” when closed.

  • WW adaptor: 0.006 mm. Seems this doesn’t add noticable runout.

  • WW 3/16 collet: 0.024 mm. This is a lot for this type, but to be fair may be I ruin this (never used) collet as I tight without the steel rod (of course this should never be done). Ups. Maybe should I order a new one.

  • WW 0.25″ collet: 0.014 mm. This time no mistake was made. I read somewhere that no more than 0.0005″ (0,0127 mm) is ok for this collets.
  • Chinese ER16 chuck: 0.040 mm, at 40 mm from spindle nose. That’s a lot; don’t now if its due to poor quality or larger distance from nose. I must say this cheap chuck looks well made, but I did correct back thread as this was so oviously deviated.

  • Threaded ER16 chuck: 0.006 mm. I buy this from a guy on ebay, not chinese, and must re-bore internal cone and front thread to make it usable, so this explain this good reading.

  • ER32 chuck: 0.016 mm. Accounting the spindle face “runout”, cost (around$50), the fact this is mounted in a faceplate and still don’t get the bolts,  this doesn’t seem to be a bad value (will take another reading when buy the bolts).

  • ER32 threaded chuck (“Beal Tool”): 0.006 mm. Sell as pencil chuck for the taig lathe;  I did must face the back (final adjustment was carefully made using sand paper). Its seems a good value, but as I remember there’s a small amount of axial deviation (don’t save those readings).

  • 6 mm milling collet: 0.016, 0.030 and 0.070 mm at 3, 13 and 23 mm from spindle nose.

  • 3/16 mm milling collet: 0.006, 0.004 and 0.010 mm at 3, 13 and 23 mm from spindle nose.

So???

  • I buy ER chucks and collets looking for precision and avoiding marks on work. Now its doesn’t seem a great deal (collets add more runout). Nevertheless ER16 chucks are handy for the mill. And ER32 chucks can work great as vertical fixtures.
  • Jaw chucks seems to have too much runout. Measuring first chuck face runout on border gives me 0.032 mm, so may be I need to re-face spindle (I would lost ER16 threaded chuck precision tough).
  • I would like to buy precision pin gages to be able to measure ER and WW collets (and to take better measurements).

Anyway, how much runout is acceptable depends on your particular needs, and for killing precision a independent jaw chuck or a special-purpose-on-spindle-turned fixture should be the way.

Update: I re-face spinle nose and now face runout is less than 0.004 mm, and first chuck runout drop from 0,076 mm to 0.054 mm and second drop from 0.124 mm to 0.104 mm. Due to some chatter issues I take around 0.1 mm, but sure can be less if well done. I forgot to check spindle align, but it’s better do it before re-face (may be turning a test rod and measuring both sides).