I suggest it would also be prudent to make a plate or a sticker adjacent to these four bolts showing what mod has been done and which is the 'master' (concentric) top-hat spacer. If you get the job right, you may not need to work on these screws for a long time, and no-one remembers everything for ever. And the next owner will bless you.

Cheers, Tim

Sorry if this a bit pushy, but here's what I'd do and indeed what I did on my own BF-20….. it's not a quick fix, but it's a good thing to know.

Tram the tilting head to the table with a dti, to ensure she's not rotated left to right.

Tram it front to back, to see if she's tilting forwards or backwards.

Great, lets, say it's perfectly square – you now have two useless pieces of information, relative only to an arbitrary reference because you've assumed your dovetail column is perpendicular in two planes AND you've assumed the rotating circular surface of the vertical saddle is also square in two planes……. (or you used an engineers square, and assumed or accept the inaccuracy of your square/surfaces/measurements) – often with machines of far eastern origin, if they're not square at every step of the way and they don't mind. They just grind a bit off (usually off one single bit) here and there to get the final figures – as soon as your start raising and lowering the head, or rotating the mill head, you're assuming things move square and true, and they probably dont.

Best not to assume anything these days.

so let's go back a step and try to derive accuracy, assuming only reasonable things. Remove the table from the equation, forget it exists for now, so dont worry about that bolted connection . You now have a column, a saddle with a 360 degree surface, and a spindle axis. Its not unreasonable to assume your column dovetail was machined in a single pass and is pretty much square and true, if you've got a micrometer you could check it's parallel, but I don't think you'll gain much.

Imagine now, that this column is laid down in front of you, with the spindle box on the left, as a lathe would be setout. (forget the table exists, remove it from your imagination). Now, you have a lathe without a saddle for a tool post, but who's spindle box can rotate on its own saddle.

To check this rotating face is parallel in two planes, isn't easy, since right and left, you have no column, no reference, and probably isn't necessary unless you're attempting to split the atom in your shed (it's been done…..) but you can check it in one plane ((THIS IS OPTIONAL, NICE TO KNOW KINDA THING)), along the axis of the column. Simply move the saddle to centre, and mount a DTI in the spindle. Rotate the spindle box 360 degrees, taking 4 measurements as you pass the lower face of each dovetail – making sure you don't move the guage.

assuming you're 360 saddle is true….. move on.

now check out rollies dads method of spindle alignment, you can adjust very easily to align it towards or away from you (imagining it's laid down like a lathe) – however if you're spindle is pointing up or down along the axis of the "bed" (again, imagining its a lathe) – this is a more serious issue, it means the alignment of the quill bore, is knackered – the only option is to remachine the circular face on the back to correct the up/down movement – if it's out in the other plane…. it doesn't actually matter that much (imagine it being mounted on a hinge like a door, opening and closing, it doesn't matter where it swings, so long at it's not dropping or lifting along the axis of the column……)

still with me?

no, me neither………

so if you successfully determined that your 360 rotating face, is square, and you've determined that your quill isn't pointing towards or away from column, then you can now go back to square one (almost)

"Tram the tilting head to the table with a dti, to ensure she's not rotated left to right.

Tram it front to back, to see if she's tilting forwards or backwards."

You can't tram left and right to the table, because you don't know if your spindle is aligned to the column unless you've just done the rollies dads method to alight the spindle left/right. So now when assuming you've done that, when you "Tram the tilting head to the table with a dti, to ensure she's not rotated left to right." – the difference is – if you're out, it's the column that's out, and not any errors in the alignment of the rotating spindle box or the trueness of the 360deg spinning face.

The only accuracy you're accepting, is the spindle runnout – which can easily be quantified – you actually already have all the measurements you needed to know from the rollies dads method, so you can acutally offset then values and incorporate this – but seriously, this is getting silly, but its there if you want perfection, 

I'd consider that a mechanical QED (almost)….. now, if you still need that shim, you know you're not offsetting an error in your spindle to column alignment………

that was taxing……. I'd started writing an article for the magazine but I'm a long way from home right now, so don't have it…. this is all off the top of my head, but I think this is right, I spent quite a while working this out – the measurement and checks are actually simple in the end if you think about it……..

Edited By Russ B on 10/11/2017 08:18:56

First fit a bar in the chuck and lock the spindle if possible.

Use a DTI and set the tilt so that you get no motion of the needle regardless of whether you move just the quill or the whole head up and down the column.

This gets quill and column perfectly aligned.

You can now check, and adjust if necessary, the column using normal tramming tests (e.g. dti in the spindle).

You can't easily set the column unless the head is aligned first.

Neil

I use a pretty simple method to align the column ,

Set up a dial indicator on a mag base and clamp it to the side of the head ( not in the spindle ) then set up an angle plate on the table and square it up so the vertical face is at 90 deg to the x axis ( use an engineers square for this ) you can then use the indicator to sweep the vertical face of the angle plate .

Lock the X and Y axis up then take a reading by moving the head up and down the column , compare the reading at the top with the one at the bottom of the angle plate , don't wory about what it shows when you are raising or lowering the head as it will wander all over the place so just use the reading at the top and bottom of the angle plate and lock the gibbs up at each reading to remove any play between the column and head .

Once you are happy that the column is square to the table you tram the spindle in the normal manner .

You can use the same method to check if the column is tilted forward or backward just set up the angle plate parallel to the X axis .

Neil, you missed "if your lucky" inbetween the words quill and column……….and you also just assumed the spindle is running true inside the quill……..

Check the 360 saddle is true on the XZ plane, as described (or by any other means) if this is true to the XZ plane (aka front face of the column) – move on and check the quill is parallel by extending it, and mounting a dti on the column and then run the milling head up and down dialing in the column left to right (nice and easy its a rotating head after all) but also front to back (hand scraping or machining required – not so easy!!) 

once you've got or know the quill is true to the column in the XZ plane and XY plane as above, and you know you're 360 face is also true, only then can you start making assumptions as bold as suggesting the spindle, inside the quill, the mating between the quill and 360 face, and the mating between the face of the 360 and dovetail attaching it the column are ALL in alignment……. and you're still not 100% because you've still not check that the spindle inside the quill is running true, and this is where rollies dads method comes in to play.

For the record, my 360 face was ok on the vertical saddle, but the spindle was out of alignment. I went for the quick option as you suggest here Neil and it came back to bit me, every time I moved the Z axis, I drifted off the X,Y co-ordinate because I'd assumed my quill, and the spindle inside that quill were both true to the column – silly silly mistake. The spindle inside the quill was slighty out, but no big drama, the quill however was pointing towards the column! So if I shimmed the column to get the spindle level to the table, the Y co-ordinate would drift as I wound the head up and down.

Check all this once, and use the machine for life, and take neat little shortcuts as you suggest, with confidence.

Edited By Russ B on 13/11/2017 13:04:58