I’ve been struggling in this days.
First, here my nice Z axis delrin nut. This was supposed to be split type, but I get (luckily) a perfect fitting so it wasn’t required. I grind my own threading bit, not bad for free hand.
Turning the Z axis knob feels very soft, better than my manual mill. I’m not interested in measuring backlash by now, so please don’t ask. I also did a delrin nut for the X axis… but I’m still pissed because a stupid error, so I will not show it (seems to work well, though). Just remember…. when threading, always check a table with drill sizes for both, steel and soft metals, like this.
Later, when I test X and Z axis, an annoying idle-motor noise made me pissed again (not near like this, luckily). All the world says that microstepping it’s noisy and you should live with it, but what annoys me was the fact that when I connect only one motor, there was no noise at all. After some fighting with a hammer against my crappy controller, I was able to eliminate the noise (better said, doing it almost inaudible) doing two simple things:
First, I connect the ground from the 5V supply to the ground of the 24V supply (brown cable). Seems I misunderstood STK672-050 datashet, which says you can use separated grounds for logic and power supply.
Second, I add a ferrite bead to the supply wires of every board. Seems to work better with at least a turn. I will bought a bunch on eBay (not snap type, just plain toroidal).
Nevertheless I should note that I’m driving the motors at 2.5A, not 3A as it should be, so maybe I find some noise later, but it’s ok for now. Please remember, I’m talking about idle motor noise; running motors will always do some noise.
I was easily able to get 70 IPM’s, seemingly without losing steps. I hope to do more testing once the whole thing get finished.
Just a last thing… using a laboratory regulated supply I found that when I set a current at 2.3A, the current draw from the supply, for the Z axis going up, was 1.25A. Not a surprise, but an interesting empiric data.
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.
Lapping did remove most of bottom anodizing, but was worth.
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:
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.
Finally I took measurements at six points. Here’s what I get.
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.
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.
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.
Several measurements were taken.
1. Table to column squareness. Measurements every 1cm, over a section of the travel.
2. Squareness of the fixed jaw to the column, without load.
3. Squareness of the fixed jaw to the column, with load at top.
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.
With all this I was able to figure out the complete picture. After some struggling with the numbers, I got the diagram below.
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.
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).
After all this I check again, and the jaw seemed to be compensated (look Figure 11, part 4).
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.
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.
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.
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.
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.
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.
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.
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.
Installed Tooling Plate
Here’s what I get with the installed tooling plate, after filing bottom protroudings (see below).
- 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.
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 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.
A final tip: here is how I clean dust in threads.
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|
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.
Some time ago I buy a used (indeed abused) Sherline manual lathe planning convert it to cnc (a in-course project). The headstock spindle of this lathe had a worn feeling so new bearings were needed. Replacing bearings was easier than expected (no strong hits were required), and here is what work for me.
- Electric stove (something like this)
- Leather gloves
- 1 1/4″ soft metal rod
- A small part to protect spindle from damage when hammer is used.
- Oviously a pair of new 6004ZZ bearings. Mine were Japan made.
- Remove front cover and back nut.
- Set stove to medium level an heat headstock for a few minutes. Don’t now exact temperature, but you should not be able to touch without gloves. Too hot it’s not good of course.
- Wearing the gloves hit the spindle back while holding headstock by hand. Be careful as spindle and front bearing may jump away.
- Use the rod to hit rear bearing from inside.
- Place back front bearing in spindle housing, but in reverse position, an hit spindle back again.
- Wait headstock to cold (20 min or more).
At this point some cleaning and checking is required.
- Clean bearing housings and spindle, using sand paper and oil if required (I use 600 grit aluminum oxide paper). I found some nasty brown coating in the rear bearing, may be some kind of glue.
- Search for nicks in bearing housing borders, and file if required (those may difficult bearing fit).
- Check front bearing fit on spindle; you should be able to move it by hand (a little force may be required).
- Coat spindle, bearings and housings with oil.
This is pretty much the inverse process.
- Heat headstock as before.
- Mount front bearing on the headstock and insert spindle. Hit spindle nose while holding by hand, until complete fit. No too much force should be required. Remove assembly from headstock.
- Mount rear bearing; careful alignment is required. (I should hit a bit as the border had a nick and filing was not perfect).
- Put back spindle, hitting should not be required.
- Ensure bearing fit are ok and wait to cold.
Once the headstock is cold you can mount in the lathe and put back the nut to adjust preload (Sherline instructions here). To me this was a bit hard as the nut was a bit tight, so adjustment was difficult. I adjust preload so there was very low friction added. Measurements were:
- Nose runout: 0.008 mm
- Nose play: 0.003 mm (may be a bit tight but I can’t get any greater)
- Face runout: 0.006 mm
After checking old bearings, I realized that front bearing was severely worn, while rear bearing doesn’t feel bad.
I would like to try 7004 angular contact bearings, but haven’t found a provider (there’s only a ceramic version on ebay, costing around $100).
Now I’m pretty sure the nasty brown coating was bearing retaining compound. I suppose this is required when there’s some play between the assembled parts. Maybe I should have use this for spindle-bearing assembly.
I forgot to say it seems original and new bearings where normal class (or ABEC1). I guess that normal bearings will do the work, but with ABEC3 bearings costing US10 on ebay (as Andy points out), there’s no reason to spare.