Bf20L Z-axis dovetail/gib problem

[Hornetb]

Hi guys,

Just wondering if I could get some comments from people who have been down this track before.

I recently purchased a Bf20L and after some delays finally got my stand build and the machine up onto it to begin to start using it. First port of call was to tram in the head and this is where I have struck some difficulties.

I understand how to tram in the head (FWIW I've watched all of Hoss's tramming videos as well and understand what to do - have no problem doing the tramming procedure) but where I have been having problems is the sheer amount of movement when I lock the gib locks. There is just no combination of gib adjustment or any circumstance where I can create a situation where the head can be trammed in at all when you lock the gib locks. On the X and Y, the gib locks just come to a nice solid lock, say 1/5 of turn between loose and on - a nice solid lock up, no need to crank them and obviously not a lot of play and everything works as it should. On the Z however there are problems, in some places the top one comes to a definite solid point (right at the top of travel) but the bottom one is just a spongy mess. You can feel it make contact and start to tighten up but it just keeps pushing the head out of alignment (you can watch it move) and there is no definite positive feeling, you can just feel it pushing the head out of alignment as clearly the gib to dovetail fit is very poor.

My problem mirrors very closely the experiences of Mystichrome here (http://www.cnczone.com/forums/bencht...lp_needed.html), but no fiddling with gib adjustment can get me out of this hell. I've done A LOT of reading on this forum and elsewhere.

I initially thought my gib strip was very undersize or the taper was out of whack (and it may well be) as if I remove the bottom screw completely the gib strip can slide through the head and protrude from the bottom up to nearly an inch, yet even at that extreme travel of the gib it wont cause the head to lock up - the head is free to move up and down even in this past maximum adjustment. So clearly if the gib can slide through the slot so far, if I put the bottom adjustment screw back in it has to push the gib back up about another 1/2" before the threads of the screw can engage and clearly that means that even at maximum adjustment the gib will never do what it is supposed to do and provide a nice sliding friction fit. I hope you follow my explanation.

However with even more testing I started to suspect that perhaps the dovetail was thinner in the middle than at the ends, because it was tight at the top of the travel which is a commonly mentioned observation, so I started measuring. Using dowel pins to measure the dovetail width and with position 1 being closest to the table and position 6 being at the top of the column, these are the numbers I get (each measuring position is roughly 100mm/4" apart).

Head Position / Dovetail Width
1 90.00mm - (3.543")
2 89.96mm - (3.542")
3 89.82mm - (3.536")
4 89.35mm - (3.537")
5 89.92mm - (3.540")
6 90.08mm - (3.547")

So you can see that the Z dovetail is narrower in the middle by up to 0.26mm (0.010") compared to the top. Perhaps not surprisingly the deflection I see when I try to adjust the bottom gib lock is anywhere up to 0.012-0.015" when I place a DTI on the side of the lower part of the head (not an endmill - If you put the DTI on the end of an inserted endmill the actual deflection at that point is anywhere up to or past 0.025"-0.030". You just can't consistently lock that gib and keep any form of tram. I thought the answer was to just give the head a bit of kentucky windage and cock it to the side ~0.012 so that once you snibbed the gib lock in a sort of consistent manner you would hopefully be pretty close, but you just can't do it - at the tool tip you end up anywhere between 0.010 and 0.030 out of tram. I thought maybe I could just allow for that shift and tighten it up pretty snug but depending on how tight you turn it you'll get varying readings, even up past 0.015 on the side of the head or sometimes less depending on where the head is on the Z. I also tried just locking the top gib as it tends not to cause as much deflection (around 0.002" which is what I would probably expect), and then just lightly tightening the lower but I'm still just going round in circles.

I haven't even got to the point of tramming the column to the table, at the moment I'm just trying to get the head aligned to the column. Unfortunately I think its just a futile exercise as it appears 1. the dovetail isn't parallel, and 2. the gib is a such a poor fit. The gib might be such a poor fit due to the bow in the dovetail or it might be a poor fit in its own right just compounding the problem.

So if I adjust the gib at the top of the travel, its sloppy in the middle. If adjust the gib as much as I can in the middle (given that it seems to be undersize) it binds at the top of the travel where the dovetail is thickest.

I've spoken to the store I purchased it from and they say there is no problem, or that I'm adjusting the gib incorrectly but I honestly dont think they even understand what tramming a mill is or why I would want to do it.

Now I understand that these are cheap chinese machines and you take that on board when you purchase them, but where is the line between accepting that you may need to tweak, fine tune and adjust and accept some errors - and then standing back and saying "No - this machine is faulty and not fit for purpose"

What are your thought?

Thanks for reading, I'd be really interested to see what people say. Obviously I understand that people scrape or lap in their gibs and/or dovetails for a proper fit - but am I too far behind the eightball to get out of this mess? I would need to scrape the gib and dovetail but I really need the tools like a proper dovetail traight edge etc which I dont have. I'm wondering if the only answer is to remachine the dovetail and refit a new gib but that is going to be big $$ and should I be accepting that cost? Am I unrealistic in my expectations that I should be able to tram this mill in within a few thou?

Thanks
HB

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[hoss2006]

Who did you get this mill from?
The column is definitely bad and they need to replace it if you're still under warranty.
Give them the measurements you made.
Trying to tram this or use it for much of anything IS futile, unless you have it resurfaced or change to linear rails, search ebay for some used 15-20mm rails.
Since the bow seems to have one low spot does there appear to be any damage to the column, could have been crushed.
Hoss

[Hornetb]

Hi Hoss, thanks for the comments.

I'm glad to hear I'm not being unreasonable and I was coming to the same conclusion that there was just no way I could make a usable piece of equipment out of this as it stands.

I purchased the mill through an engineering place in Melbourne, Australia. I'll go back to them again and re-explain the situation and try again.

The mill should still be under warranty and I have made no modifications to it whatsoever yet, although all the gear is there to do it (DRO's etc, and I do have ballscrews, stepper etc for conversion over the next 6 months or so). But as so far I haven't wanted to drill or modify anything until this is sorted. I don't suspect any damage but I can't rule it out either.

I'm just glad to hear that I wasn't expecting too much because from where I stood I thought this was beyond help but wanted to make sure I wasn't being over expectant of these cheaper mills.

I appreciate the help.

HB