The Shop > Tools
Finishing problems
Darren:
Ah, I wonder if others out there have similar probs with these small lathes?
Does anyone else on here use a 7x12 or similar?
I'm wondering if it's just down to the lightness of these machines, or at least the carriage.
Darren
I've threatened the missus that there may be a new addition after xmas, I'm toying with several possibilities.
A larger Smart and Brown, a Cardiff or a Colchester Student. Be nice if I could see something larger in action to make further assessments.
Alas I don't know anyone locally with a lathe, let alone a bigger one. Or a workshop for that matter.
N Wales,
Here's some stuff I did a few years ago, http://homepage.ntlworld.com/darren.dean/index.htm
bogstandard:
Darren,
I am not an expert on this sort of problem on a lathe, but have come across a similar situation on another type of machine.
Has it been like this ever since the machine was new, or is it a fault that has appeared over time?
It looks like the far side of the saddle is 'dragging', that is where the leadscrew is driving the one side, and the other side is lagging behind, then when the lag gets too much, it springs forwards to catch up, and then does the same thing over again. It only takes a few very difficult to see thous to cause a problem. It might not happen when using the handwheel because of the different moment forces involved.
I would try holding the leadscrew side rigid (or just engage the half nuts) and try to move the far side of the saddle backwards and forwards. If there is play there, you should be able to adjust up on the jibs to reduce the fwds/back play. But before doing all that, make sure you are not running dry on the far side, the lube should be on the top edge and the far side plus the far bottom of the slides.
If that isn't the problem, try to only partially engage the half nuts, so that it is only just driving the saddle, and see if the fault is still there. If it disappears, that would point to your problem leadscrew, and you might get around it by adjusting the engagement force on the half nuts. Not an ideal situation, but if it gets rid of it, you are halfway there.
That is all I have, hope it helps with your problem.
John
BTW I am in fairly easy striking distance. Crewe, Cheshire. Plus you have Ralph in N. Wales.
Darren:
Hi John,
Had another good look taking on board your comments. I've had this lathe about two years and it's always done it.
I've always thought there was a lot of movement in the carriage, up/down and rocking horizontally side to side. But then I've little to compare it to.
My S&B is a plain lathe and thus locks its carriage down hard to the bed. Movement is manual only.
On the 7x12 there are adjusters underneath, ie when tightened they pull the carriage down, no gibs as such, just some crude plain bars.
The thing is, the bed seems narrower at the tail than the headstock. If I adjust the "gibs" for mid point along the bed it becomes very slack at the tail and impossible to get near the headstock.
It could be dirt, I doubt it but I'll gunk it all down and give it a good clean and try again.
Will post here when that's done.
Yep, Crew is not too far, Ralph must be in between then.
I'm nr Porthmadog, well almost.....just think model trains that are put to work..... ;D
Regards
Darren
Divided he ad:
Hi Darren, I've been reading your post and was thinking of play in the saddle too... Maybe undulations in the lead screw moving the whole saddle?
Well it's a thought. The tapered bed is a new one on me though!
I've also checked the distances involved.... A good trek!!
http://maps.google.co.uk/maps?f=d&saddr=mold&daddr=A494+to:52.987511,-3.390656+to:Porthmadog,+Gwynedd,+United+Kingdom&hl=en&geocode=%3BFbVWKQMdQo7M_w%3B%3B&mra=dpe&mrcr=0&mrsp=2&sz=10&via=1,2&sll=53.078353,-3.637848&sspn=0.52055,1.450195&ie=UTF8&z=10
(personally I'd grab and drag the blue line over to the 494 Road.... It will travel much faster than the winding roads, no matter what the computer says!)
Crewe works out at just shy of 2 1/2 hrs.
I've only got a 9x20 and John has the now super acurate beastie.... Not too sure of it's size and can't remember the name!!! (the memory is getting worse with age!) it is a nice big fella with a gap bed and I'm sure 20+ inches between centres (I'm sure John will clarify? ;D ) I'm sure visits could be sorted..... You'd have to be ready for a messy cramped workshop where two can barely fit at mine though!! It's only just over 6'6" square! :)
It'll be interesting to see your findings.
Ralph.
bogstandard:
Darren,
The bed must have been machined that way, because when a bed wears, it always does it the opposite to what you have. It always tapers towards the head, and when you adjust, you have trouble getting it to the tailstock end.
Maybe after 20 to 30 years of hard use, it will wear itself parallel.
How long is the bed? Because if you really want to fix it well, it might fit onto my mill, and I know I can get it within a thou parallelism, which must be better than you have now.
This is my new 'beastie' as Ralph calls it.
http://www.chesteruk.net/store/crusader_lathe.htm
John
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