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.