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I’ve noticed the carriage on my U3 can lift slightly. If I grip the front of the carriage I can rock it very slightly from side to side. This allows it to yaw a fraction - it cuts on the back as well as forward stroke.
Assuming that the gibs that hold the carriage down on to the bed were worn (Emco call them ‘keep plates’) I made some new ones (twice as thick - 6mm oilon) - fitted them today but it has made no difference.
I’m thinking it can only be that the base of the carriage is somehow lower than the bed, so the gibs aren’t pressing the carriage down properly on to the prism. I can’t imagine that the underside of the bed can be worn, or the top of the bed (considering the carriage is aluminium).
Has anyone else come across this, and, if so, how did you rectify it?
Thanks!
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I had the same problem with both a U3 and a Compact 5.?? Similar
Emco lathes with the same problem.? I made new keep plated from
brass for the Compact 5, no help, so I measured the width? of the
keep plate pressed on the bottom of the carriage, marked the keep
plate and filed a shallow step that took up the play, leaving less
than .001" play between the keep plate and the bottom of the bed,
measured by lifting the carriage with a dial indicator on it. It
took a lot of careful filing and checking, but it works better. I
think a better repair would be a set of stepped retainer plates
bolted to the bottom of the carriage with adjustable gibs similar
to the one on the cross slide,bearing on the bottom of the bed.?
Someone else did some similar work on a U3 carriage and documented
it on either the unimat forum or the U3 forum.
Chuck Daldry
On 8/21/23 12:31, Peter Brooks wrote:
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I’ve noticed the carriage on my U3 can lift slightly. If I grip
the front of the carriage I can rock it very slightly from side
to side. This allows it to yaw a fraction - it cuts on the back
as well as forward stroke.
Assuming that the gibs that hold the carriage down on to the bed
were worn (Emco call them ‘keep plates’) I made some new ones
(twice as thick - 6mm oilon) - fitted them today but it has made
no difference.
I’m thinking it can only be that the base of the carriage is
somehow lower than the bed, so the gibs aren’t pressing the
carriage down properly on to the prism. I can’t imagine that the
underside of the bed can be worn, or the top of the bed
(considering the carriage is aluminium).
Has anyone else come across this, and, if so, how did you
rectify it?
Thanks!
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Thanks Chuck for the reply.
There is enough room for 6mm of keep plate and the heads of the socket screws, so I suppose it would be possible to have two 3mm plates and grub screws with locking nuts to apply pressure as per the slide gibs…
The other idea I had was to mill a very shallow rebate in the new keep plates (as you did, leaving the portion that makes contact with the bed) and then shim them back up with feeler gauge material until the fit is good. Metric feeler gauges have a useful range going from 0.05 mm (I’ve just ordered four!).
It did strike me (while I was lying in bed of course) that the other thing that could cause this was if the bed was bent, slightly arched - the middle of the carriage would be pressed down on the bed but there could be a gap (and thus play) at the outer edges. So I’m assessing the beds of my two U3s. One (in bits already) is flat but ever so slightly twisted according to my surface plate. I can just get the 0.05 mm feeler gauge under the corner. I don’t think that’s enough to cause an issue.?The other U3 is being dismantled.
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I have always assumed the purpose of the Keep Plates is to stop the carriage falling off, not to hold it down to the bed. In normal use the carriage is held down by the cutting forces. Most larger lathes I've come across work like this. If you have problems with slack in the system when facing off, tighten the clamp on the carriage. That's what its for. In general, any axis that is not moving when making a cut should be locked.
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I could have written this, but I wanted to double check the "big
lathes" in the garage. I believe that one can just grab the apron
and lift - not far, but noticeably.
Another in general: It helps to be aware of forces in general and
what opposes them. In this case the force is down in normal
turning. Does anybody do much "upside down" turning on the back
side of the spindle on Unimats? I know some people swear by
parting off that way on the big lathes.
On 8/23/23 04:14, Keith S. Angus wrote:
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I have always assumed the purpose of the Keep Plates is to stop
the carriage falling off, not to hold it down to the bed. In
normal use the carriage is held down by the cutting forces. Most
larger lathes I've come across work like this. If you have
problems with slack in the system when facing off, tighten the
clamp on the carriage. That's what its for. In general, any axis
that is not moving when making a cut should be locked.
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Thanks both. Maybe I am doing something fundamentally wrong. If I make a cut traversing along towards the headstock, surely the lathe shouldn’t be cutting more material (even a fraction) when reversing back in the other direction (towards the tailstock)? Am I meant to back out of the work at the end of a cut and then reverse (in which case all sorts of issues of backlash and accuracy come into play)? Sorry, I am such a novice!
Talking about the ‘the force is down’ - I have wondered about this recently - should boring actually be carried out on the ‘far side’ of the internal face? This would seem to be the direct equivalent of turning on the outside face.
I’m thinking now of trying the keep plates in more of a gib arrangement, it’s non-destructive anyway.
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I recall the tool taking a fine cut on returning from facing off quite often on various lathes. I think it depends on the metal being cut and the way the tool is ground. I've never worried about it. On the Unimat I'd expect it if I'd taken a deep cut quickly, which could cause some deflection of the toolpost. I do most of my work with hard brass (CZ121) or free cutting alloy (2011) and haven't noticed this happening, although, thinking about it, the things I've worked on recently have not involved much facing off. I also keep the slideways nipped up at all times. I'd like to find a way to lock the carriage without needing to reach for a hex key every time. Less worried about the cross slide since that has a dovetail that can be adjusted to be just right.
I do recall facing off a 250 mm diameter piece of cast iron on a big lathe one day, to make a small surface plate. When I checked the flatness with a straight edge I found it was down 1 mm in the centre. I'd finished up with a shallow cone. Nothing on the lathe suggested this could be corrected by some sort of adjustment. Maybe it had been like that since the day it was built. So I don't expect perfection from any of these machines. If you get high accuracy that's a bonus.
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Need to consider the effects of tool geometry.
1) stiffness...when traversing "forward" the tool deflects backward and results in a shortening of length. The dynamic change of direction from forward to reverse relieves the tool bending and then digs in on the reverse stroke, taking a little bit more off.
2) cutting tool geometry...most hand ground tools are "handed" with back rake not only leaned back but also angled away from the predominant direction of cut.? Hence right and left tools.? Reverse cuts result in negative tool geometry, and can screw up the finish.
3) exceptions...radiused blade tools with straight back rake, but even those will dig in on the withdrawal, due to lateral floppiness.
4) I was always taught to rough in both directions and final roughing done in the preferred direction of the tool.? Then measure with a micrometer and the final few thousanths in the same direction.? For critical parts, even roughing in one direction only.
5) In turning, that little bit of a cut on the reverse is the equivalent of a spring pass.? Best to do that in the same direction.
6) Btw doming or cupping on cross feed is not unusual,? It is usually due to the changing feed rate as you move toward center.? The surface speed of the cut changes as the diameter changes assuming constant RPM.? I have found that taking several very light passes at a higher RPM helps to neutralize the over-feeding as you head towards center.? The cutting tool is progressively taking heavier cuts as you head toward center.? If you take light final cuts at a higher rate of speed then progressively really slow down the feed as you get to? center, you can usually take out a lot of the cupping.? Cutting tools exhibit this effect regularly because they are so floppy in the lateral direction.
I recall the tool taking a fine cut on returning from facing off quite often on various lathes. I think it depends on the metal being cut and the way the tool is ground. I've never worried about it. On the Unimat I'd expect it if I'd taken a deep cut quickly, which could cause some deflection of the toolpost. I do most of my work with hard brass (CZ121) or free cutting alloy (2011) and haven't noticed this happening, although, thinking about it, the things I've worked on recently have not involved much facing off. I also keep the slideways nipped up at all times. I'd like to find a way to lock the carriage without needing to reach for a hex key every time. Less worried about the cross slide since that has a dovetail that can be adjusted to be just right.
I do recall facing off a 250 mm diameter piece of cast iron on a big lathe one day, to make a small surface plate. When I checked the flatness with a straight edge I found it was down 1 mm in the centre. I'd finished up with a shallow cone. Nothing on the lathe suggested this could be corrected by some sort of adjustment. Maybe it had been like that since the day it was built. So I don't expect perfection from any of these machines. If you get high accuracy that's a bonus.
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Does the amount of tool bit deflection change due to the speed of the spindle?
Does the hardness of the metal you are working? with change this?
For my final finish cut, I frequently just push my tool bit holder to take that final .001 + off of the diameter.
Probably not the most precise method of machining but it works.
Tamra
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Sometimes the speed does affect the amount of deflection.? If you run certain metals like stainless and some steel alloys at very high speeds, you can overheat and in effect heat treat them making them glass hard.? That hardness change can destroy the edge of a high speed tool.? Also harder material = stronger material = more deflection.
If you run certain carbide insert tools with a shallow depth of cut and at very high speeds you can rub and overheat and work harden the metal that way as well.
If you are using high speed tools with these small lathes, keeping the edges razor sharp reduces the cutting forces that are transferred from the rotating part to the tool.? This reduces deflection.
If you keep the cutting edges well lubricated that makes them cut more effectively and reduces cutting forces and material welding on the cutting edge.
If you choose to use insert tooling like DCMT inserts it is best to use the aluminum finishing inserts with a sharp edge on all materials.? This reduces cutting forces.
Rigidity = good machine finish.? If you can deflect the cutter with your finger, it is time to try and take out any backlash in all the axes.
Does the amount of tool bit deflection change due to the speed of the spindle?
Does the hardness of the metal you are working? with change this?
For my final finish cut, I frequently just push my tool bit holder to take that final .001 + off of the diameter.
Probably not the most precise method of machining but it works.
Tamra
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Hello: Sorry, have not been around for a while hence this belated answer.
Has anyone else come across this, and, if so, how did you rectify it?
Yes. It is one of those typical, hard to detect Emco Unimat 3 issues which can make your life utetrly miserable. Curiously enough, once I took delivery of my Unimat 3, I scoured the web for months on end for anything and everything Unimat 3 related in english, german, french and spanish to see what had to be done to put it in shape, but never saw this specific problem mentioned. Anywhere. See this rather long and sometimes rambling thread at model-engineer. It boils down to this, posted by a long time member of that forum:
... the mating surfaces of the underside of the saddle held the 'lift strips' a tiny bit away from their ways on the bed, so the saddle could move vertically. Some careful blueing, and needle-file work on the saddle soon had them snug. Problem solved."
Besides working on the mating surfaces, the best solution ends up being ditching the p.o.s. OEM plastic gibs and make yourself a new set in brass. Add a regulation system as the one used in the Sieg C0, which uses a couple of small set screws to adjust the gibs against the rails and that will be it. It is a rather intensive and slow running job but it will be one less Unimat 3 problem to haunt you. Best, JHM
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Hello JHM, thanks for that and for the link to the post on ME. I’m glad I’m not the only one to notice the issue.
I did find some burring at the bolt holes and a bit of dressing with a hand countersink has helped a little I think.
I’m still going to make more of a gib type arrangement (when I get round to it :-) with oilon replacement strips of the same dimensions as the originals, and brass strips above them threaded to take grub screws to apply pressure to the oilon ones below.
I’m keen to use a non-destructive solution, and certainly wouldn’t be filing the saddle. My ham-fisted incompetence could make things considerably worse !
Cheers,
Peter
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Hello:
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On Sat, Sep 16, 2023 at 03:42 PM, Peter Brooks wrote:
... thanks for that and for the link to the post on ME.
You're welcome, hope it was at least it gave you an idea as to where you stand.
... glad I’m not the only one to notice ...
Make no mistake: everyone who comes across the problem notices it as it is a nightmare to try to turn even a small part properly. What was hard/impossible to find was the actual cause. Easily traced to the lack of QC + bad design decisions at Emco. But I digress. Like I mentioned, I searched all over the web for months looking to find references/solutions/modifications/etc. which would address the problem to no avail. While searching for a solution to making a new set of gibs, a member of that forum revealed the origin of the problem.
... burring at the bolt holes and a bit of dressing ...
The problem is actually twofold in origin: on one hand the plastic glass filled gibs (a contentious issue) and on the other the excess material present on the mating surface where the gibs sit. The new set of brass gibs was a start and the iterative process of dressing/adjusting their mating surfaces to get the right height made a world of difference. I was, as you can imagine, scared shitless (cue Stephen Stills/Woostock/1969) but reasoning it out in detail it quickly became clear to me that, even with a new set of gibs, if they did not run at the right height with respect to the guide rails there was no way I was ever going to be able to get it working properly. ie: the carriage would always have vertical movement. As the U3 carriage is made of Zamak, it has very little mass and the only way to keep it in place/down is via a properly adjusted set of gibs.? Best, JHM
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I had the same problem with my Emco Compact 5, an overgrown U3.?
I found that the brass I made did not eliminate the play in the
carriage, so I filed about about .004" from the part of the gib
that contacted the carriage. That raised the gib enough to
eliminate the majority of the play.? I should probably finish the
job by marking and scraping the gib to fit better, but I need to
acquire the skill first.
Charles Daldry
On 9/16/23 13:28, Julius Henry Marx
wrote:
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Hello:
On Sat, Sep 16, 2023 at 03:42 PM, Peter Brooks wrote:
... thanks for that and for
the link to the post on ME.
You're welcome, hope it was at least it gave you an idea as to
where you stand.
... glad I’m not the only
one to notice ...
Make no mistake: everyone who comes across the problem notices it
as it is a nightmare to try to turn even a small part properly.
What was hard/impossible to find was the actual cause. Easily
traced to the lack of QC + bad design decisions at Emco.
But I digress.
Like I mentioned, I searched all over the web for months looking
to find references/solutions/modifications/etc. which would
address the problem to no avail.
While searching for a solution to making a new set of gibs, a
member of that forum revealed the origin of the problem.
... burring at the bolt
holes and a bit of dressing ...
The problem is actually twofold in origin: on one hand the plastic
glass filled gibs (a contentious issue) and on the other the
excess material present on the mating surface where the gibs sit.
The new set of brass gibs was a start and the iterative process of
dressing/adjusting their mating surfaces to get the right height
made a world of difference.
I was, as you can imagine, scared shitless (cue Stephen
Stills/Woostock/1969) but reasoning it out in detail it quickly
became clear to me that, even with a new set of gibs, if they did
not run at the right height with respect to the guide rails there
was no way I was ever going to be able to get it working
properly. ie: the carriage would always have vertical
movement.
As the U3 carriage is made of Zamak, it has very little mass and
the only way to keep it in place/down is via a properly adjusted
set of gibs.?
Best,
JHM
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Hello:
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On Sat, Sep 16, 2023 at 05:48 PM, Charles Daldry wrote:
... same problem with my Emco Compact 5 ...
Ahh ... I see a pattern becomes visible. Not only Unimat 3s.
... brass I made did not eliminate the play in the carriage, so I filed about about .004" from the part of the gib ...
Yes, that's one way to do it albeit with a huge disadvantage: any further adjustments will always require partly dressing the brass gibs. Dressing the gibs' mating surfaces to the level where they should be is a permanent solution. And if and when further adjustment is needed you just turn over the gibs, evenly dress the whole side evenly or make new set.
... need to acquire the skill first.
Same thing I said till I decided to jump in and be done with it. Just have to remember that Prudence, Patience and Constance will be your best friends while at it.? Best, JHM
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Tamra