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AES

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I've got the problem with my Chinese Mini Lathe (metal lathe) shown below. And I'm not sure how to fix - indeed, I'm not sure if I should even try to fix it!

In a word, the cross slide lead screw is bent.

DETAIL:

I stumbled the other day while carry a fairly heavy/bulky machine in my workshop. The only thing that finally stopped me from collapsing in a heap on the floor was my lathe - specifically I and the machine I was carrying landed full-tilt on the handle shown at the bottom of the pic below (the lower bright silver handle):

Photo A-C.jpg

PHOTO A

This is the lathe after disassembly to get to the problem. To show the actual position of the subject handle I just loosely re-assembled everything onto the saddle (no gibs), then placed it on the ways, but with the complete apron and the threading/longitudinal traverse lead screw half nut and its engaging lever removed for clarity.

At the time I thought I'd just bent the M6 machine screw that retains the handle, or at worst, bent the handle itself. (At risk of teaching granny, this is the handle used to apply the depth of cut in normal turning operations).

But after stripping I found out that neither the handle retaining screw nor the handle itself were bent at all - it's the smallest turned diameter integral to the lead screw on which the handle is mounted that's bent! (Hereafter, the "handle mount"). See Photo B below - dowel added to show the real deflection - I guess it's about 15 degrees off line:

Photo B-C.jpg

PHOTO B

The whole cross slide lead screw is retained in the saddle casting (steel?) by means of a machined aluminium block (in the pic, just to the right of the vee block). That housing block contains an internal plain bearing which looks like it's pressed into the housing block. The housing block is bolted to the front face of the saddle casting (2 x 6mm machine screws). The depth of cut indicator, which is normally also mounted on the smallest diameter of the cross slide lead screw ("handle mount"), is shown separately (it's just above the now bent out of line lead screw "handle mount").

With some difficulty (and with minimum damage thank goodness), I was able to get the indicator off the lead screw "handle mount" using two custom-cut ply wedges cut on the scroll saw! Gentle tapping with a mallet on the wedges managed to force the indicator over the now bent "handle mount".

But as I see it, getting the housing off the lead screw in its present condition is going to be virtually impossible without damage; I) because of the amount the handle mount is bent out of line; AND II) because the ali lead screw mounting block is about 3 times the "length" of the indicator (see Photo B above).

For obvious reasons (2 dissimilar metals); the relative mass of the (steel?) main saddle casting; and the pressed-in plain bearing in the (ali) housing; I don't see that the use of heat, however carefully applied, will do anything but cause damage.

But please correct me if you think that I'm wrong, but note I only have a couple of small & medium blow lamps, no induction heater coil unfortunately.

So without using heat to me the obvious thing to do is to scrap the complete lead screw/housing assembly and either make new - or buy new.

BUT:

I can't buy new (but see the lathe info at the end for why not, and any inputs on that welcome too).

And as I don't have a working lathe now I can't see me being able to make a new assembly, bearing in mind that the housing and indicator are aluminium, are both marked with accurate calibrations - not to mention that plain bearing in the housing. So I can't see it being all that easy to make WITH the lathe, but I would probably have a go anyway - IF my lathe was working!

ALSO, to add insult to the above injury, not only is the lathe currently disassembled for this problem, but after serving me very well since 2005, the electronics control box will "soon" be now on its way to the USA to get fixed - a complete coincidence, but the control electronics have developed fault/s which I can't fix myself.

So NO lathe, and the only "machine tool" now left to me is my vertical pillar drill (which fortunately is fairly accurate).

So instead of making new, I've come up with the idea of the "straightening tool" (Item A) as shown in the Sketch below:

MTB 3000 CS LS Tool-C.jpg

SKETCH:

MTB 3000 CS-LS Legend 2-C.jpg

LEGEND:
The "plan" is to make the straightening tool shown (Item A) from a suitable MS block or rod from my scrap box. As noted, the above sketch is not to scale, so: the bend angle; the "height" of the straightening tool; and in particular the "steps" between each of the three holes (B, C, & D above) will NOT all be exactly as shown of course.

When making the straightening tool I'll obviously need to take some care, not only with positioning the holes E, F, & G on the block/rod, but also with the depths of holes B, C, & D. But with careful marking out I should be OK with that ("hope springs eternal").

Also, I'm not sure if the 2 additional bolts (Items F & G above) will initially line up with the appropriate holes in the saddle casting or not - but if not initially, then they would be brought into use as soon as the handle mount starts to straighten up enough to allow bolts F & G to be inserted.

But the basic idea (I hope the above sketch makes it clear) is to slowly and equally tighten nuts on bolts E, F, & G, (say half a turn each at a time) thereby slowly "graunching" the straightening tool downwards, so causing the cross slide lead screw handle mount to be forced back into line. This will then allow the cross slide mounting block to be slipped off the cross slide lead screw - just as the lathe designers originally intended.

Well, that's my theory anyway. YOU may well have a better idea! If so "answers on a post card"(or better yet, on this thread!) please.

OTHER IDEAS?:

1. I HAD also thought of just not bothering and leaving the wonky handle alone until I can find a complete new assy to buy. But there the problem is that having forced the cross slide depth of cut indicator off the lead screw, (those custom ply wedges) I seriously doubt I'll be able to get the indicator back where it belongs without serious damage. And I don't fancy trying to use the lathe with little or no indication of the depth of cut I'm actually putting on!

2. I can't yet find any company who can supply spares for this particular lathe (but details are below in case anyone can come up with a possible supplier);

LATHE DETAILS:

Badged "Einhell MTB 3000". (Einhell is a German "badge engineering" firm). Unlike most Chinese mini Lathes, the OEM of this machine is NOT SEIG of Shanghai. The OEM is believed to be either a Chinese company called "Red Dog" or called "Big Dog" - now believed to be out of business.

E-mails to possible Chinese manufacturers have so far produced either "not us" answers, or just NIL replies. Considering Covid I'm not surprised.

The basic lathe spec is "7 inch x 10 inch" as so many of these lathes say they are (actual numbers for mine are approx 6.25 inch dia swing X about 11.75 inches between centres).

As checked with my thread gauges, the cross slide lead screw is 10.0 mm x 1.00 mm pitch (ISO Metric Fine). When the present situation eases I shall contact possible UK and US suppliers for spares.

Meanwhile, if any one has a better method than the above to suggest, or any improvements to the "tool" suggested, I'd very much appreciate a comment. After all this idea IS a bit of a "brute force/mechanical graunch" method - something I've always been taught to avoid unless there's absolutely no other alternative!

I know full well from other posts that there are quite a few members here with far more experience of lathe work than I have, so by all means "have at it".

Thanks in advance
 

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AndyT

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I can't offer an engineering solution, but maybe worth checking - does your house contents insurance include accidental damage?
 

Rorschach

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Since you have a lathe and most of it is working this would be my idea.

Assuming the bent part is a standard size thread of course and I had a bolt in stock.

Take out the lead screw, cut off the bent part, face it, drill and tap for the bolt. Loctite and pin the bolt in the threaded hole, cut off the head and trim to length.
The lead screw shouldn't be taking enough torque to even really need the pin but it would be a pain if the loctite failed so I would install a very small one.

EDIT: Just re-looked at that picture, it isn't a threaded stud is it, my bad, sorry.
 

AES

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@Andy T: Thanks for that, good idea, I hadn't even thought of it! (Of course, IF covered, that'll depend on me finding a new cross slide lead screw and presenting them with a bill - I CAN'T see them paying me for my "labour" if the straightening idea works!).

@Rorschach: Thanks for that also. OK, that was your "starter for ten". Want another go (it's free)? ;-)
 

Rorschach

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Hmmm it's tricky.

You do still have most of a lathe though, you can rotate stock, you can drill it and you can use the compound slide to do some basic turning. Bit of clever thinking maybe.

I am assuming this part is where the handle slides over? That being the case would my threaded stud idea work as a temporary measure to get you back up and running to then machine a new custom stud? What kind of stock do you have on hand? A threaded stud could be made using a die without the need for turning/threading as usual. Use a file to do any material removal.
 

AES

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Yup, it IS tricky Rorschach. Especially as my lathe won't run at all (repair needed for DC motor electronic controller that I can't fix).
 

Rorschach

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Well that *compounds* the problem :lol:

I'll get my coat.
 

CHJ

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1. Cut off the bent portion.
2. Remove leadscrew from mounting.
3. Get someone local to face of the cut end and either:-
4. Drill the leadscrew to form a socket or
5. Turn down a spigot on the end of the leadscrew.
6. Make up new stub handle mount with either a spigot or socked to fit leadscrew.

Basically an inline splice repair.
 

MusicMan

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I have done a leadscrew straightening job before. The principle is that one uses a lathe. mounts the straight part in a chuck (preferably collet) close to the bend but not including it, then runs the lathe slowly so that the bent bit rotates eccentrically. Pushing the eccentric bit inwards then straightens the bend. When it runs true the bend has gone.

The best pushing tool is a piece of wood with a hole in it, not much bigger than the rod. Musical instrument technicians do this all the time, and usually drill holes in mallet handles. Easier to see than to read about, and here's a nice example

https://www.youtube.com/watch?v=RvPTAIAF5EQ

You need to spin it more slowly because you have that lump of metal hanging on it, but you should be able to manage by hand rotating. The point about the hole in the piece of wood (which you use as a lever) is that one side of the hole pushes the rod to straighten while the other side supports it and stops it bending further.

There are lots of other youtube examples of straightening rods, threaded or otherwise, and also there are commercial firms that make a living out of it.

You can even do it without a lathe, just a surface plate, dial indicator and V blocks. See https://www.youtube.com/watch?v=BjJxYBZxNlU

If rotating your lathe by hand won't do it, you could probably use a drill press.

I'd offer to do it for you except that I see you are in Switzerland! (Where you should be using a Schaublin :)).
 

AES

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Thanks to both of you CHJ and Music Man.

A LOT of food for thought there, I'll mull it over for a bit, but I think both ideas have merit and may well be less complex than my idea.

I'd already thought of turning the lathe over by hand and re-mounting the tail stock, thereby using the inherent dead straight line between the two.

But no rush, especially under the present sancumsternces ;-)

Much appreciated. Stay healthy everyone.

Edit for P.S. I'd LOVE a Schaublin MM, but there'^s very few SH ones around (back when I looked in 2005) and new is out of the budget which SWMBO allows for my toys.
 

MusicMan

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Chas's method would work, but is more work and will weaken the joint; not too serious I agree.

But AES could try the straightening method first, which is cheaper and stronger. If it fails, he could proceed to the inline splice without losing anything.

But he is right to study it more and ponder!

cheers

Keith
 

AES

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Right, thanks for those links Music Man, I've watched both and find both very helpful.

But I do have a reservation that in both examples, they're straightening a "long thin" rod, whereas what I need to straighten is "short & fat" rod. The actual dimensions of my job are 8.00 nom dia x 11.5 mm long. Bearing in mind the pressure required using the mallet handle method, the lead screw will need to be pretty well chucked. So not having a set of collets, and as the only part of the lead screw that I can chuck is threaded portion, I'll obviously need some protection for the threads - I'm thinking of some brass shim I've got handy.

But the above are not show stoppers. After some more thought I'll come back and report results, and even if not satisfactory, there's always Chas' "cut it off and start again" method, which should be doable.

So thanks again for the suggestions gents, more later .........
 

MusicMan

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That's a good point about only being able to grip on the threads. Without a collet chuck you will definitely need a shim protection. I used the method on a 12 mm diameter leadscrew bent in a similar way to yours though it does need some force and in one case I simply pushed on it with a piece of wood using the cross slide (which is what you don't have!).

You can buy cylindrical or hexagonal rods with various threads in them. If you can find one to match your leadscrew (if it is 10 mm dia x 1 mm pitch with a regular ISO V thread then look for chandelier fittings!) you could screw it into that and hold the rod in the chuck. Locknut on the end.

But looking at your pics again, I think I would first try clamping on a V block (itself clamped solidly) on the straight bit of rod just before the bend, sliding a close fitting tube over the handle part, and then simply bending it back into line. It won't be perfect but may enable you to get that big aluminium block off. Then the leadscrew itself will be much easier to work on with one of the YouTube methods.

Note also that new lead screws are obtainable at reasonable cost from plenty of suppliers. You don't need a replacement part from the original manufacturers. They are available with a variety of threads and thread forms and ends. Yours looks about the simplest, so searching on lead screw suppliers (after you have identified the thread pitch and type) should be successful. Then you can simply saw it off and discard the old one.

I also once had success with a mallet method. Make a V groove in a length of hardwood (say 50x25 mm x length of screw) and support it on a flat surface. Turn the lead screw til the bend is uppermost then whack it with a lead mallet. If you don't have the latter, use a softwood buffer. It needs quite a whack but can be repeated.

I think you are right not to be in a hurry but to survey the methods and think before attacking it!
 

CHJ

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I think AES's main problem is that he can't get the leadscrew out of the locating block to enable any of the simpler methods of straightening.
 

MusicMan

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CHJ":2m0haf98 said:
I think AES's main problem is that he can't get the leadscrew out of the locating block to enable any of the simpler methods of straightening.
Exactly, which is why I made the last suggestion, of hand straightening the severe bend.
 

AES

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Thanks MM, still thinking ....

... while (simultaneously) photographing all the leads to/from the electronics control box ready to package up and send to the USA (there's a bloke there with a good rep - thru the various mini lathe sites and suppliers over there - who specialises in repairing these controllers). I haven't found a European equivalent, though I suspect it wouldn't be too hard to find someone with more "electronickery" know-how than me. But this bloke has already long-range diagnosed 1, maybe 2, of the tiny adjustable 50 Meg pots which have gone phut, and as he "only" wants USD 50 fixed price for a guaranteed job I'm going to let him have at it.

Ah well, "It all makes work for the working man to do" (Know it? An old Flanders & Swan "special" - one of my English party pieces after I've had enough red wine, though the typical Swiss audiences do have trouble with "technical" terms such as skirting boards and joists).

Must say that though I really do sympathise with the "housebound" right now, I really don't understand those who are "just bored" - there's ALWAYS something to do at home, especially as neither my wife nor I have any symptoms (touch wood), so NO social distancing between us - not even any "unsocial" distancing either, though we're both getting on a bit for all that (so she says) ;-)

"Idle dreaming and Topic wander", sorry.
 

AES

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@CHJ: Yup, spot on Sir. With such a "short/fat" bit to straighten, there's obviously going to be quite some graunching force involved, hence my concern re chucking up on the threads, & using brass shim.

As in my post above (it crossed with yours, sorry), I'm currently making sure I remove the motor controller "correctly" (i.e. so that I can reconnect it in X weeks time!). But meantime I've got an idea that I already have/can make a MS sleeve with internal 10 mm x 1.00 mm ISO fine thread, so that would protect the lead screw thread when chucking it up.
 

Rorschach

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I've got it!

Buy another (bigger preferably) lathe in order to repair this lathe, then you have 2 lathes :twisted:
 
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