Trials and tribulations with a 7x12 lathe

[modeldozer]

Hi to all modders,

My first try at a bit of a post on here, hope this is the correct place to post, it will take a while to finish.

This saga started from brand new.  After the cleanup tried to turn some parts from PVC tubing with poor finish and not taking the same cut in and out.  Investigation revealed the gib plates on the saddle to be LOOSE     .  Trying to adjust this it was soon apparent that the bed ways was not parallel top to bottom, being a lot thicker at the tailstock end.  This was corrected manually using various oilstones. The gibs were also adjusted with shims rater than the adjusting bolts.  Since then I was able to turn quite a few parts but always had problems with the finish on mild steel and parting off, even plastics.  Then added a Soba vertical slide for milling and had reasonable success milling aluminium.  Recently I tried to do some simple milling in mild steel with a 8mm end mill with disastrous results, mill broke.  Further investigation showed that under force the cross slide lifts up on the right (gib strip) side. Looking into this showed the gib strip to be to small and actually rotating around the adjusting screws.  After a failed attempt to fly cut a new one on the lathe, frustration,  nearly putting a sledge hammer to the lathe  , as a last resort ended up making a new gib strip old school by hand.  This was actually quite fast about 7 hours spread over 2 days.  Here are some pictures of the process. The final surfaces were lapped with W&D on a piece of thick glass.









Before cutting to size the gib was used with some W&D to clean the dovetails.

Also noted the saddle has picked up some play, so this was removed and then following methods from here and some other sites on the web the saddle was lapped to the bed ways, testing regularly with mechanics blue.





The bed was rechecked wit a micrometer and filed and honed some more for smooth constant fit.



As I do not have a milling machine, new gib pates was hand made from calibrated mild steel flat bar, These are thicker and longer than the originals to add more rigidity. They will also be fastened buy SS studs rater than cap screws. (This is not my idea, seen on the web)


As the bolt holes in the saddle are all over the place a template was drawn to drill them.



Using the modified drillpress as a tapping stand.

For the mods/repairs to the bed and saddle the lathe was striped down and a nasty surprise found in the headgear.





Looks like a gorilla with an angle grinder made the shift fork      .  The detents are also no good, causing the damage to the change gear.  This will be looked into later in the rebuild. Hoping to save funds for taper bearings and metal gear.

Next up is refitting the rack to the bed, this will be shimmed for better engagement.  Unfortunately rain, blocked drains and doctor’s appointments will be delaying this a bit.

[DaveH]

modeldozer,

You are doing a good job The more luv and care you put into it, the more your lathe will give you back

Well done

DaveH

[loply]

This may be too late, but I went through a similar procedure a few weeks ago and here are some notes for you.


Firstly regarding gibs. I fiddled around with mine quite a bit and in the end I replaced all of them - compound, cross slide and saddle - with a new system wherein I tightened the gib down onto a pile of shim stock material. For the cross slide and compound gibs I attached M4 studs into the gib, and the studs subsequently have a nut popped on them and tightened to squish them flat against the shim stock. This works much better than trying to push the gib out and allows repeatable accuracy down to the thinnest shim you've got - in my case 0.01mm.

By this means I was able to achieve a cross slide which has absolutely no wobble in it - not even a hundredth of a mill - yet slides back and forward with complete ease.

Unfortunately the compound has proved more difficult due to the small surface area, but is acceptable.

I also found the gear selector had been wearing on the gears and have since adopted a technique of changing gear ratios then reversing the lever a bit to put the selector in free space, instead of rubbing against a gear. I did also round the edges over quite a bit.

Lastly headstock mods - I replaced bearings with tapered roller bearings and changed the pulley ratios to reduce the low gear top speed from 1150rpm to 750rpm for an increase in torque. Between these two mods the lathe changed and I was able to achieve more or less mirror finish on mild steel with no 'lines' showing on the work piece.



In conclusion I would highly recommend using shims for your various gibs and if you want a good finish would suggest the motor speed + tapered bearings mods.

[andyf]

Modeldozer,

Despite having no personal experience of Sieg or Real Bull mini-lathes (yours being the latter, I think), I am for some reason  a moderator of the Yahoo 7x12 minilathe group.There, some say it's better to keep the original headstock gears, to act as a plastic fuse if a tool crash demonstrates that chuck jaws are hard to machine. And others say that metal gears are noisier.

I'm not advocating that you stick to plastic, but these are points which you might take into consideration.

Andy

[websterz]

This may be too late, but I went through a similar procedure a few weeks ago and here are some notes for you.


Firstly regarding gibs. I fiddled around with mine quite a bit and in the end I replaced all of them - compound, cross slide and saddle - with a new system wherein I tightened the gib down onto a pile of shim stock material. For the cross slide and compound gibs I attached M4 studs into the gib, and the studs subsequently have a nut popped on them and tightened to squish them flat against the shim stock. This works much better than trying to push the gib out and allows repeatable accuracy down to the thinnest shim you've got - in my case 0.01mm.

By this means I was able to achieve a cross slide which has absolutely no wobble in it - not even a hundredth of a mill - yet slides back and forward with complete ease.

Unfortunately the compound has proved more difficult due to the small surface area, but is acceptable.

I also found the gear selector had been wearing on the gears and have since adopted a technique of changing gear ratios then reversing the lever a bit to put the selector in free space, instead of rubbing against a gear. I did also round the edges over quite a bit.

Lastly headstock mods - I replaced bearings with tapered roller bearings and changed the pulley ratios to reduce the low gear top speed from 1150rpm to 750rpm for an increase in torque. Between these two mods the lathe changed and I was able to achieve more or less mirror finish on mild steel with no 'lines' showing on the work piece.



In conclusion I would highly recommend using shims for your various gibs and if you want a good finish would suggest the motor speed + tapered bearings mods.

Can you give us a little more info about the torque improvements you made? I am getting ready to do a top to bottom rebuild of my 7x12 headstock...metal gears, tapered roller bearings, might as well add as much to the job as I can while I'm at it. 

[websterz]

Modeldozer,

Despite having no personal experience of Sieg or Real Bull mini-lathes (yours being the latter, I think), I am for some reason  a moderator of the Yahoo 7x12 minilathe group.There, some say it's better to keep the original headstock gears, to act as a plastic fuse if a tool crash demonstrates that chuck jaws are hard to machine. And others say that metal gears are noisier.

I'm not advocating that you stick to plastic, but these are points which you might take into consideration.

Andy

I will be replacing the spindle gears with metal but leaving the plastic change gear in place as the weak link. It will be much easier to replace than the others in the event of a catastrophic work interruption.

[modeldozer]

Hi to all,

Thnks for the encoragements and info.

The saddle gibs will be shimed again, the adjusting screws were only added to aid adjustment wile lapping.

Andyf,   Thanks for the tip on the dovetail gibs, will be looking into that one.

On the gears I will only put a metal one on the spindle and retain the plasitc one for the layshaft.  Whwt puley ratio did you use and wher did you get the puleys?  My plan was to add a jackshaft reducer as others have, but changing the current ratio sounds a lot easyer.

Abraham

[andyf]

Hi Abraham,

I think the tip on the dovetail gibs came from Loply, not me.

You probably already know about this source of information:
http://www.toolsandmods.com/mini-lathe-mods.html - look under "Lathe mods and tools". Jose Meneses has a Real Bull like yours. Also look at Marty Nissen's 7x14 Lathe Projects, which are mentioned on Jose's home page. It is a shame that Marty never recorded the end of his project.

Andy



 

[loply]

I think the original pulleys were something like 14:20, I replaced them with 12:32's which I bought from bearingboys.co.uk.

The ones I bought were a 5mm pitch thereby necessitating a new drive belt too, as the drive belt that came with the lathe was an unusual pitch. I stuck with the original 9mm belt width.

The 32 tooth pulley was the largest one that could fit on the shaft before it would interfere with the spindle.

Both the pulleys are a chippy metal of some kind, machines somewhat similar to CI but isn't. The larger one needed quite a bit of machining as it had flanges and all sorts on it, but I trimmed it up and cut a slot for the wudruff key using a file.

The smaller pulley I didn't bother with the key and just used the cross screw to transfer thrust, seems to work!

As stated in low ratio the top speed has reduced from over 1100rpm to around 750rpm. For me, the critical speeds of about 350-450rpm were lacking in torque previously, resulting in frequent stalling. The change is very noticeable and it VERY rarely stalls now, but perhaps more importantly it maintains the same speed easier, whereas previously even though it didn't stall it would often momentarily slow down during a cut, resulting in inconsistent finish.

[modeldozer]

Sorry for the mixup.

Loply,  Thanks for the info, mine tends to hunt below 300RPM and have tried to reajusting the control as per the KB manual to no avail, hence thinking of mechanical reduction.

Andy,  thanks for the links, my mods/repairs is mostly based on toolandmods with some of my own ideas throughn in.

The lathe is sold here in Spain under the Einhell brand and is more compatible to the Real bull than the Sieg.

Abraham

[cfellows]

I, too, replaced the spindle gear with metal, but left the plastic gear on the countershaft.  I installed tapered gibs on my carriage, but haven't done anything with the gibs on the cross slide.  So far the factory gibs haven't given me any trouble.

Chuck

[loply]

Sorry to go off on a tangent but this thread leads me to share an idea I nearly tried.

I'm worried that the increased torque of the new lathe will mean I will break the plastic gears again very shortly.

I was surprised how much work it was to replace those gears and really don't want to do it again!

I designed a new gearbox which used belt drives inside the head instead of plastic gears. Using this method I was able to design a system with 3 speeds instead of just two, using quite wide belts so that they won't ever snap, or at least, shouldn't snap before the belt that goes from the motor to the shaft.

I designed the gearbox similar to a motorbike gearbox - ie with three sets of always-connected belts between the two shafts. The pulleys on the lower shaft are on bearings and are free to rotate, unless a wheel with some studs on it is slid along the shaft whereupon the studs mate with matching holes on the pulley, thereby afixing the pulley to the shaft... If you see what I mean 

Because pulleys have a slightly lower 'profile' than gear teeth I was able to make the diameters slightly larger too.

My intention was to end up with 3 ratios - with respective maximum speeds of around 400rpm, 800rpm, 1600rpm.

Using plastic gears it would be senseless to lower the ratio to the point where you could use full motor power at 400rpm as you would almost certainly just break the gear again!

For the moment I haven't bothered but at some point I'd love to do this!

[modeldozer]

loply,
  I was thinking of doeing something similar with the jacshaft reducer and leaving the headgear locked in low.  For the torque involved was thinking all metal gears in the head and a "fuse" pulley in the drive to the head, somewhere where it will be easy to replace.

Mannaged to get some of the other tasks done so had a few hours in the shed this afternoon, din not get a lot done so no pitures today.  More time programed for tommorw.

Abraham.

[loply]

Abraham,

Well just for your interest here is a video I made tonight whilst experimenting with my lathe. I have just finished the headstock upgrades and this is really the first turning I've done.

This video shows me removing 4mm from the diameter of a steel bar in one pass using hand feed, of note is that on the withdrawal of the tool there is NO secondary cutting what so ever, and what may sound like chatter is the tool chattering against the workpiece due to hand feed.

The end finish was pretty good too! Cut was made with a sharp carbide tool so it was a 'liney' finish but pretty damn smooth and extremely consistent. Motor running at full power which was 750rpm in low gear with the modified pulleys.

Also on that note, I have a set of cheap brazed carbide tools. I discovered tonight they are all crap. I reground them to new shapes and angles on my bench grinder and made a BIG difference. Prior to this there was no way I could remove 4mm at a time, in fact it was always hit and miss whether they would cut at all. I found most of them rubbed on the workpiece no matter what as they didn't have enough relief.

The video actually makes the finish look bad so here's a better pic.



The tit on the end of the part is 0.5mm diameter. At one point I turned a 0.5mm piece about 1cm long but then snapped it.

Prior to the upgrades (headstock bearings, motor speed reduced, modified gibs) the machine chattered like crazy and required several 'spring cuts' if you made a 1mm cutt, and it would stall occasionally and generally be dramatic. I usually stuck to removing 0.35-0.4mm from each side of a part max.

I'm totally confident that I could remove from the diameter 5mm or maybe even 6mm now...

Anyway, what I mean is that I would definitely upgrade the bearings and slow the motor speed down! You won't regret it

[modeldozer]

Loply,

Many thanks for the info and encoragement.

Spent most of thr day working on the lathe (post to follow soon), but am having problems with the saddle, no matter how tight i adjust the gibs the saddle can be turned/pivoted left to right by 0.02mm (holdig it front and back and foring in opposite directions). Do not know if this is to much,. if so, it seems I might have to get it remachined or replaced.  GRRRR

Abraham

[loply]

I've got the same problem with my saddle!

Will be interested to know how you get on fixing it.

My gib strip on the side of the 'vee' is not as tight as the opposite one, because I was led to believe it was less critical, but I believe this may be causing the rotation.

I note that if I push the saddle down above the vee that the rotation seems to become impossible.

Are you saying that even with really tight gibs you can still rotate it though?

[modeldozer]

Yes, even so tight it can not move it has 0.01mm rotation, this is why I think the V in the saddle might need a remachine.

[andyf]

You may find some help on sorting out the Vee both here http://www.toolsandmods.com/saddle.html and on the link it gives to Marty Nissen's work.

Andy

[loply]

Provided the angle of the V is precisely 90, I can't see how it should end up rotating?

Because it's a V it clearly can't be oversized or undersized, and I can't imagine the sides of the V not being parallel?

I agree that lapping it is probably the solution but would be interested to know exactly what the problem is.

[modeldozer]

Hi again to all,

Andy and Loply, The saddle have been lapped is as Jose at toolandmods decribes.  Conclutions at this tine is at the end of this post.

A bit more progress.

Made a “shim” for the rack from some 1mm sheet.




Filed a recess for the pinion in the front gib.




Shortened a grub screw to fit under the pinion.





Made a new full chip guard for the apron gears. Also made a thrust washer to prevent the gear eating into the guard.






All fitted.



The saddle was then adjusted for a slightly dragging fit and play checked with a DI.  Up/down with oil on the ways 0.0000 but then found the rotational play mentioned before.



Today only had little time in the shed and the saddle contact with the ways was rechecked with mechanics blue, both sides on the “V” have full contact over the complete with of the saddle, so I am thinking the play is just inherent (Not ridgit enough) of the design.  Will continue on for the moment and see how this affects turning.

All for now

Abraham

[modeldozer]

Hi all,

Spent some time today to invetigate the play, me thinks me found it.  As I had to lap the saddle quite a bit to get good contact the "v" in the saddle ended up wider at the edges than in the middle, an unfortunate side effect of lapping the actual guiding surface while using the same to guide.

For cost comparrison have been in contact with Amadeal and at the cost of a new saddle and having no mill myself I think it might be time to bite the bullet and get a new saddle.

Meantime am racking the grey matter    to come up with an external guiding system for the lapping process.
Any sugestions welcome.

Abraham

[lordedmond]

the saddle should be scraped to a known prism and camel back , not lapped with wet and dry stuck on.

have you checked the squareness of the saddle to the single axis



BTW you are using far to much blue it should only be the faintest smear and the saddle only moved a few mm not slid the entire length

[modeldozer]

lordedmond,

Thank you for the advise.  Unfotuanatly scraping is out of my capabilities due to my disability.  Out of interest though what is a camel back?  Using only very litle blue I found out today trying to detect the problem, learning as I go along (only used it before for setting tooth contact in differentials).

Abraham

[lordedmond]

A camel back is a long straight edge
http://www.schsm.org/SCRAPING.pdf

the blue should only be just discernible on the master part ( non worked bit ) degrease both parts apply blue and wipe 99% off to get an evan smear the lower the worked part on vertically ,then move it a few mm lift off vertically .
your pic show that you have run it end to end and the blue has been shoved off the end , more like a coat of grease ,remember very little and move a little
incidentally you do the ways on the bed with a camel back  then you fit the saddle to the ways, not the other way round , if there are dove tails then you use a prism 


well I need wheels and I can still use a scraper but I was trained when I was an apprentice Large old DC motors ( 100 hp plus ) had brass/babbit bearing but hey that was when I was 15 now 65

good luck with the refurb , IMHO with the cheaper end of machine tool its best to work round the problems as the cast iron is very soft and nearly could be remove with a dumb nail



don't get me wrong I had a 7 x12 it lasted a week ( dangerous electronics , it started up on its own and motored against a hand push ) now my lathe is £10k not touched at all just lube it and USE it to make loco's etc


Stuart

[loply]

Abraham,

I'm not sure if I understand how it ended up wider at the edges than in the middle - though I can see exactly why that would cause rotation.

Since mine is the same and I have not (really) tried to lap the saddle to the ways, I wonder they tend to be like that from the factory. If it was milled on a pretty poor mill the bit could flex inwards as it makes the cut, thus resulting in the V being wide at the edges and narrower in the middle?

Maybe you didn't make it like that, maybe you need to lap it some more in order to fix it?

I am almost tempted to buy a or make a precision V and bond it in there to replace the original. If only I could get a precision V which was only 1 or 2 mm thick, and a matching flat piece to jack up the opposite side, which could do with more contact area anyway.