Lathe repair advice requested
I have a problem with the tailstock of my Chinese lathe.
The lathe is a heavy duty GBC 1000-400 which is 1000mm between centres and 400mm swing. It weighs 2 tons. Used for maintenance-repairs on a farm plus steam hobby stuff. I bought it second hand, almost unused.
On checking why I experienced innaccuracy when boring/drilling from the tailstock, I found that the V shaped goove in the base of the tailstock has been badly machined. It should never had got through any sort of quality inspection. The angles of the V are visibly incorrect by 2-3 degrees and the machining itself is very rough. The V needs to be re-machined. The lathe otherwise seems excellent.
The tailstock is accurate in height and level, but it points off to one side.
My plan is to make an insert with a V machined into it. To machine a rebate in the base of the tailstock, cutting out the old V, and fix the new piece into place with cap screws, and grub screws, using the screws to get accurate adjustments. Then when I am happy with the adjustments, to pack the gap between the tailstock and the new insert with some sort of metal repair putty.
The tailstock is very heavy. I need a block and tackle to lift it. It has an offsetting adjustment, so the base of the tailstock is a separate piece from the rest of it, and I presume that it will come apart. The base part with the faulty V is a casting about 300 x 250 x 50mm, so there is enough bulk to remove the faulty V and replace it.
So that is my problem, and my current plan. Any advice or suggestions would be appreciated.
would it be easier to bore the tailstock barrel from the headstock...then sleave it ..if there is enough material there to do it with ..
you could push it along with it attatched the carrage
just an idea
i though every tailstock was adjustable ...for any error that you describe anyways...thats offsetting thoufgh ..and will not cure youre problem
all the best.markj
Last edited by aboard_epsilon; 01-21-2012 at 05:41 PM.
To start take it apart. Now is there a key at a right angle to the ways, move every thing close to the head stock, mount an indicator from the spindle to reach the key now sweep the key and see if its square to the spindle, if not put some shim into one side of the vee until its square, now extend the indicator to the tail stock end of the base touching a machined surface, while holding the shims in slide the base and see if its parallel to the spindle, if not shim to make parallel. After you've done that, using your feeler gauges check the flat way, if its reasonably flat, you're half way there, if the ways are smooth, you now have to make the base match the ways, if you know how to scrape that's what you'll be doing, removing material from the non shimmed side. It doesn't matter if you end up low you'll just shim the tail stock up. The other way is to build up the vee way with, we used Ampco bronze, machine the vee out glue and screw the Ampco to the sides of the vee machine within .003"/.005" oversize and scrape to fit. That's just my $.02 worth.
Since part of the repair is getting the tailstock bore perfectly aligned, this would be a way of helping in that regard. Remove the t/s ram completely and measure the diameter. Then mount some bar in the chuck and turn a section of it to that diameter, long enough to slip the t/s onto and secure it.
You are going to remove material from the bottom of the t/s in order to allow for a new V to be added, so get that done first. Then get the t/s mounted onto the stub you just turned. The trick now is to lay the new Vs on the bedways, slide them under the t/s, then inject a filler between the parts.
I'd be inclined to mix up some epoxy putty and press some between the new Vs and the t/s base at both ends of them. Once that's cured, the new Vs are in the right position. You can now hoist the t/s while loosening the chuck, then you get it upside down so you can finish injecting a filler in the rest of the gap. I'd probably use PC-7, as it's a very tough epoxy material that will stick well, and it doesn't run. Poke it into place until the gaps are filled to your satisfaction, then let it cure for a couple of days. If you want to drill and tap to add some securing screws afterwards, I'd suggest drilling the holes in the Vs first, before getting them added to the base of the t/s. That way you won't be adding stress to the Vs later- the holes are then used to guide the drill bit for the tapped bolt holes in the t/s base.
Note that if you do use the bolts, it will use up some of the contact area between the Vs and the bedways. Don't drill them too close to the ends of the new Vs, as that is where you will want the most contact area. I don't think I'd bother with the bolts myself. For the most part the weight of the t/s will keep the parts from breaking loose anyway.
If your tailstock has provisions for adjusting height and tilt, etc, you might want to clean all this up and add equal-thickness shims all around before you do any of the other work. Then, if required, you can play with the thickness of the shims during the fine adjustment proceedings.
Thanks for those ideas. Afer carefully reading the suggestions from the senior members, I get the feeling that I am roughly on the right track. The idea of holding the TS rod in the lathe chuck while doing the shimming and packing into the gap between the new V piece and the rebated body, makes sense. Then to insert holding capscrews when the packing has set. Sounds do-able. Thanks very much again. I will post some before and after photos if it works.
The use of bronze and scraping sounds like it would be a good solid job, but the marking of the high spots, and multiple times the TS would need lifting off the lathe and manoevering it would be daunting. It really is very heavy.
John from Geelong, Oz.
Aboard Epsilon, your idea about resleeving the barrel would fix the problem for a while, but the rough machining of the V slot has resulted in a lot of high and low spots in the V, and a slight but perceptable rocking movement. I think that the V itself has to be replaced. It really sucks that it passed the Chinese quality control if they have such a thing.
Before removing any metal, check it out closely and more thorough.
Since it has a separate base and provision for setover, then both the ram and base halves have a key/slot thing going on. Find out whether the key/slot stuff between the halves is good or out of perpendicular somewhere before machining anything. Otherwise, you may be just duplicating the alignment that's already there.
In fact, for simplicity's sake I'd just take the halves apart and mill the key off and make a new key which you'll bolt in place. You can just kind of snug it into place so it will still move then reassemble the halves, measure things on the lathe and tap it to slip the key around until it's good. Then once you have it aligned, carefully disassemble and tighten the key bolts down good. Then of course re-check your alignment to make sure it's still good.
This would be much simpler. And if the key/slot arrangement between the two halves is a bit skewed from "perfect" perpendicularity, it won't matter as long as the end result nets you a ram bore perpendicular to the V slot in the base.
Of course a worn V slot like you describe is another problem which sounds like it needs to be fixed. But I'd compensate with the key and slot between the two halves before re-boring and sleeving the ram.
What I'd do:
1) Make sure the bottom seats well and bears evenly on the bed. Typically this is a spotting and scraping process but there's always more than one way to skin a cat I guess. Remember that if you remove material from the V, the flat on the other side won't bear flat against the flat way anymore and that will now (probably) need to be corrected.
2) once that's done, check that the top of the base plate is parallel with the bed. If it's not, then you'll want to make it so.
3) assemble the TS and check alignment. It may be off horizontally and vertically and in height. Check all three. (left/right is built-in adjustable).
4) alter the key & slot if necessary. Shim or mill to adjust height and/or alignment if necessary, or maybe make a tapered plate or something to go between or whatever. I guess my main point is that it would seem to be easier to work on the seam between the two halves to regain correct ram alignment than to do it with the bore or the V.
Last edited by tyrone shewlaces; 01-22-2012 at 03:27 AM.
Start at post #223 in the link and go through post #244, or so. It's pretty straight forward, and easy in concept, the reality is a little different. It's the results you're after.
I might suggest a PICTURE OF THE PROBLEM
While I *think* we all have the right mental picture.......... a real picture makes it certain.
From the description of the repair your going to do you have a mill or have access to one.
I will suggest brazing the V up and then re-machining it to the correct depth and angles to be aligned with the lathe spindle bore. I think that would be easier to do than some ideas presented.
It's only ink and paper