New lathe arrived today : The ongoing saga

[Neil WyattModerator @neilwyatt]

Brian,

The 'edge' of the dovetail may look curved, but what really matters is whether or not the actual sliding face is flat.

It's quite possible (likely) that the edge has been relieved more at the ends than in the middle.

Neil

[AjohnwParticipant @ajohnw51620]

Posted by Brian John on 16/10/2015 10:04:31:

Mahgnia : yes, it does look very curved on the side away the gib. Might that explain everything ie. why it is much tighter in the middle ? I may take it off again tomorrow and have a closer look. I must admit that I did not see it because I was not looking for it. I had assumed that they could machine the edges in a straight line !

John : I assume that machined edges are machined in a milling machine. What are ''ground'' edges ?

Edited By Brian John on 16/10/2015 10:10:41

Edited By Brian John on 16/10/2015 10:12:50

If they had been ground the machined surfaces would be mostly nice and shiny. The bed on your lathe will have been ground. Looking at the photo I'd assume that the male part has been ground and some tool dabbed on it to make it look like it's been scraped. The female part looks like it's just been milled however the Chinese have used a form of linisher in the past, emery cloth belt that is rotating. These linishers are a bit more "technical" that the usual type.

John

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[Martin KyteParticipant @martinkyte99762]

This may have been mentioned before but unless it hasn't you would do well to remove the lead screw so you can side the cross slide back and forwards by hand if you are doing some serious dovetail slide tuning.

That is the only way you are really going to get any feel for tight spots or looseness. If you want to check what rubs on what a coating of marker pen ink is a good place to start (or micrometer blue if you want to get flash). Coat the face of interest and then refit and slide up and down a few times then remove and see where the ink rubbed off. Checking flatness with straight edges is good to start with but essentially you are trying to fit one dovetail against it's mating half. I would be very surprised if it's not clearance on unrelieved edges, burrs and the like that is the source of the problem.

Good luck Martin

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 16/10/2015 10:04:31:

I assume that machined edges are machined in a milling machine. What are ''ground'' edges ?

.

… Ones produced by a grinding machine !!

MichaelG.

[HopperParticipant @hopper]

Brian, can you post some pics showing the wear patterns on the bearing surfaces of the cross slide and mating dovetails. If the ways are not straight, it might show up in the pics. Another thing to look for is that the corners of the dovetails are not binding. There should be a groove machined along the female dovetail where the two faces meet, or at least the sharp corner on the on the male dovetail so it does not ride on the corner and all load is taken of the flat faces.

Pic of the bearing side of the gib strip might be informative too.

[egaParticipant @ega]

Brian John:

"I am still looking at ''dowelling the gib'' but I have to be honest here in that I do not see how it helps. I must be looking at it the wrong way !"

I doubt if I can explain this any better than GHT in the extract I sent you and in any event the question of the geometric accuracy of the slide should be resolved first. I will mention, however, that the dimples in your gib appear to have been drilled normal to the face rather than in line with the adjusters which I think is undesirable and underlines the need to prevent slip-stick being caused by the poor fit of the adjusters to the dimples. In your setup I would install a dowel between two of the adjusters and a locking screw in the other space all as per the GHT extract.

I think it is very positive that you have now dismantled the slide and can actually address the problem – good luck!

[AjohnwParticipant @ajohnw51620]

The idea of the dowel is to stop the gib strip sliding back and forth as you move the slide. Fitting one is exacting work and could easily finish up worse than it currently is. You could hold the screw in the 3 jaw and file a rad on the end. Ball end like. It will be a better match to the dimples.

The edges of gib strips are usually angled. If so it is possible to put them in the wrong way round. If radiused, chamfered or of a such a width that the angle isn't needed they can go in either way. The edges need to be slightly clear of the castings. If the edges are angled chances are they will foul if put in the wrong way.

Check the edges as per this

There is no problem associated with using the correct type of linishing machine for this sort of thing. The male half of the slide should also have a narrow flat on it's peak.

If you do use a kitchen abrasive cream with oil to lap the slide a little it will probably be easier to remove the nut rather than the lead screw. It looks like it's fixed in place via a screw under the compound slide which will also be easy to remove. Forget mention of serious lapping. All these abrasives will do is lap minute amounts of metal off helping smooth out machining marks. It probably wont get rid of them. They are more like coarse metal polishes really that will leave a polish. If you were in the uk I would suggest CIF which is a cream mixed with some 3 in 1 which in real terms is a light cycle oil. You may need to spend 10 to 20mins or more to get a smooth even drag when the slide is pushed by hand. Then clean up and oil them while they are apart. The nice aspect about removing the lead screw is that the gib can be adjusted correctly and easily while pushing the slide by hand. It's easy to feel the slight drag that is needed this way. It's easy for people to over tighten when the actual lead screw is used to do this.

I wouldn't advice using coarse abrasives. It can be done but once again it's easy to mess things up and will probably result in a gib strip that needs replacing.

John

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[Brian JohnParticipant @brianjohn93961]

Some additional photos. The gib strip is a parallelogram in cross section hence it can only be replaced one way if the dimples are to be on the correct side ie. it cannot be inserted incorrectly. I think those curves on the ends were put there intentionally to help placement of the slide. The female part of the dove tail does look straight although you cannot see it on these photos (night time here).

I think I should have left the gib screws alone ; it was better when the ends were flat. I will buy new ones to replace them.

Edited By Brian John on 16/10/2015 15:09:51

Edited By Brian John on 16/10/2015 15:10:17

Edited By Brian John on 16/10/2015 15:11:42

Edited By Brian John on 16/10/2015 15:16:09

[AjohnwParticipant @ajohnw51620]

The gib strips can be turned end over end with the dimples on the same side Brian and on some lathes that does matter.

The top of the male dovetail wont be in contact with the other part – they run on the angled parts and the flats at the side.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Brian,

I mean no offence to you ['though my intentions towards 'Optimum' may differ]:

The surface finish on the gib-strip is poor, and the scraper marks on the male dovetail are just a joke. … The surface appears to have been rough-machined on a shaper, and then needlessly decorated with a scraper !!

If you want to get this cross-slide working sweetly, I think you have some fettling to do.

MichaelG.

[Brian JohnParticipant @brianjohn93961]

By ''fettling'' , do you mean that something constructive can be done by hand ? I have my doubts that I could do anything with this but I am willing to try.

[Michael GilliganParticipant @michaelgilligan61133]

Brian

Quite possibly, yes …

As others have mentioned, you first need to ensure that [where appropriate] corners are 'broken' so that the surfaces that are supposed to slide together actually do. … this is easily done with a decent quality file, and reasonable care.

You might then consider improving the surface finish of the sliding surfaces … depending on how bad they are, gentle filing may be useful; but hopefully, lapping [as suggested by John W1] will bring a substantial improvement. … The really important dovetail is the one without the gib-strip, because that one aligns the slide with respect to the bed, and you need to maintain that alignment. … The gib adjustment will cover up a few sins on the other side!

The bearing suface of the gib-strip also looks like it could use some attention … ideally it should be a 'mirror' finish, and flat … I restored the one in my Myford, using a fine file, followed an Eze-Lap hone [the red one that I linked for you when we discussed sharpening Carbide tools]

If you can do some close-up photos of various surfaces, it would help 'the team' to advise you.

Note: All of this is 'to be expected' on a modestly priced 'Hobby' lathe, and it is why they have sometimes been described as a set of castings. … It would be easier if we could afford to buy a Schaublin or Levin <etc.>, but most of us cannot.

Take it steady, and look for the areas that you can improve by simpe 'fettling' … in the fullness of time, you will probably go several times round the block as your skills and tooling improve.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

P.S. …. It would be worth you looking at this book,

to appreciate what was [and can be] done with files and scrapers.

MichaelG.

[Brian JohnParticipant @brianjohn93961]

1. Would these tight spots simply wear in over time with regular use ?

2. Once I remove the lead screw (I have not done this yet) from the cross slide then I can push it back and forth freely. Would an automotive lapping compound suffice for this purpose ? I have read that any metal polish is better as it is finer than the lapping compound and gives a better result.

http://www.bunnings.com.au/autosol-75ml-chrome-and-metal-polish_p4460667

3. I have a 600/1000 grit diamond stone. Could I use this on the gib strip ? I also have 1500 and 2000 wet/dry sandpaper which works on brass but I doubt it would do much on steel.

4. Those photos I posted are as close up as I can get with my camera.

 

Edited By Brian John on 17/10/2015 00:53:31

[Brian JohnParticipant @brianjohn93961]

I spent three hours today taking apart the cross slide and then, and ''lapping'' the slides with metal polish (made in Germany so it must be good ! ). I really thought I was getting somewhere. I could feel the tight spot in the middle gradually reducing until things were even right throughout the complete movement of the slide then I would tighten the gib screws slightly and repeat the procedure. I did this for 90 minutes. I put everything back together and the problem is worse. There is huge movement in the cross slide when I grab it with my hands and I cannot get rid of it except by tightening the gib screws up so much that it is hard to turn the wheel.

I am not sure what went wrong here today but I am starting to suspect some other problem not connected with the gib strip or slides. I have my eye on part 213 through which the cross slide lead screw passes. Should this have any back and forward movement in it ? You can see the part in the photo just to the right of the top slide.

Edited By Brian John on 17/10/2015 06:52:29

Edited By Brian John on 17/10/2015 06:53:37

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian John on 17/10/2015 00:41:54:

1. Would these tight spots simply wear in over time with regular use ?

2. Once I remove the lead screw (I have not done this yet) from the cross slide then I can push it back and forth freely. Would an automotive lapping compound suffice for this purpose ? I have read that any metal polish is better as it is finer than the lapping compound and gives a better result.

**LINK**

3. I have a 600/1000 grit diamond stone. Could I use this on the gib strip ? I also have 1500 and 2000 wet/dry sandpaper which works on brass but I doubt it would do much on steel.

4. Those photos I posted are as close up as I can get with my camera.

.

Brian,

I will try to answer your questions by number:

1. Maybe … or maybe it will 'wear out' … it really depends on the naure and location of the high spots. But either way, the gib adjustment would continue to be a problem during the process.
2. Autosol is excellent stuff.
3. Your diamond stone should be good for the gib-strip … I would use it with water, and keep washing the surfaces.
4. Photos are O.K. at that scale … but could we please see some of the angled faces of the dovetails.

Note:

• The 'scratches and digs' in the surfaces are relatively unimportant; it's the high spots that need attention.
• There is no point polishing an uneven surface

I hope that makes sense.

MichaelG.

Edited By Michael Gilligan on 17/10/2015 07:39:32

[mahgniaParticipant @mahgnia]

Brian,

If that part is the nut for the cross slide screw, it should be solidly attached to the cross slide with NO movement between them. The attachment should only be finally tightened after the whole assembly is assembled.

How is the nut attached to the cross slide?

If the movement is between the cross slide screw and the nut, it looks like there is some adjustment in the nut itself to minimise the backlash. There should be one or two small grub screws in the nut that can open or close the slit in the nut to spread or close the slit and take out most of the backlash.

Do not remove ALL the backlash!

Andrew.

[Brian JohnParticipant @brianjohn93961]

There is a grub screw (part 220) which tightens against the side of nut 213. I am going to take the whole thing apart and have another look at it tonight. I think I have been missing something all along.

Why not remove all the backlash ? How much should be left ?

There is another grub screw with a pointed tip 234 : this bears down on the top of the nut 213. Up until now I have not touched that at all. It may be the solution to my problems but at this stage I am not sure how it is supposed to work.

Edited By Brian John on 17/10/2015 08:25:36

[Brian JohnParticipant @brianjohn93961]

There is another grub screw 232 which holds the nut tight. This has always been loose up until now and I thought it was supposed to be like this. It is concealed under the top slide. I have now tightened it up. It is early days yet but this may have been the problem all along.

[Frances IoMParticipant @francesiom58905]

234 appears to widen the slot? between the two sections of the nut thus tightening the thread on the lead screw and removing some of the backlash

[AjohnwParticipant @ajohnw51620]

The reason I suggested you used the technique outlined on the Taig pages Brian is that you would be unlikely to do the lathe any harm. Seems to have been a waste of time suggesting it – several times.

The feed screw nut should be free to move up and down and rotate but be well located in other respects. If not it will probably make your problem worse.

The time to adjust the back lash is when it's a problem and with that style of adjustment it would be an indication that it's time to get a replacement nut as the improvement wont last for long.

The problem with trying to do something with rough gib strips is that they have to be flat and attempts at improving the surface are likely to introduce a curve.

John

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[Brian JohnParticipant @brianjohn93961]

John : the method I used to lap the cross slide is the same as that of the Taig lathes. They use Ajax and oil whereas I use a metal polish but both are mildly abrasive cleaning compounds. I followed the same procedure by progressively tightening the gib screws. Unfortunately I did not do a good job of it (not sure what I did wrong) and I may have to do it again tomorrow. It is interesting that they specifically mention not to use lapping compound which I was initially going to use ; it is too abrasive.

I thought I was doing really well but it now looks like the side closest to me is tight. Before, this was the loose section…..this needs practise !

How do you tell if a gib strip is ''rough'' ie. how smooth is ''smooth'' ? Should it be a mirror finish ?

Edited By Brian John on 17/10/2015 10:39:46

[AjohnwParticipant @ajohnw51620]

I would suggest you leave the gib strip alone.

There is no saying what type of abrasive is in a metal polish. The one in CIF, Ajax, Vim etc is probably softer and mixing oil in to it is important.

If you have tightened the feed nut fixings up so that it can't go up and down or rotate that will probably cause tight spots because the feed screw is unlikely to be exactly aligned with it – that is why they aren't fixed rigidly in place. The nut needs to be able to rotate and move up and down slightly.

If you lap by moving the slide back and forth the ends will receive less wear than the middle as the middle is in contact all of the time so steps have to be taken to even it out.

John

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[mahgniaParticipant @mahgnia]

Brian,

I can't tell for sure from the diagram, but the nut 213 should have little/no movement side to side and fore and aft in the cross slide, but it may be able to rotate to allow it to remain parallel to the cross feed screw over the full movement of the cross slide. The screw 232 when tightened down in the bush 245 may allow this. If the screw when tightened down locks the nut 213 from rotation, you MAY have tight spots when turning the cross slide feed dial. If not, all good.

Screw 220 looks like it may lock the bush 245 from rotating in the cross slide, and screw 234 looks like a backlash adjustment screw, bearing in the slot in the nut 213. Does screw 234 have a tapered end?

You should retain a small amount of backlash in the cross feed screw to minimise wear on the nut/screw assembly (allow lubrication), and to allow easy movement of the cross feed dial for accurate feeding. About 1/16 of a turn would be adequate, at a guess.

Last question, is bush 245 eccentric?

Andrew

Edited By mahgnia on 17/10/2015 11:24:14

[Neil WyattModerator @neilwyatt]

The feed nut should be adjusted and locked in place with the slide fully wound in, if it can wobble there will be unwanted backlash. This ensures that it is 'free' for the whole of its travel.

There is a school of thought that you can use the adjusting grubscrews (there two on mini-lathes) to tilt the nut and remove all backlash. I'm not convinced of this and I have only seen it mentioned a few times and never in a mini-lathe manual.

A 'satin' finish for sliding surfaces is ideal, a shiny surface will hold less oil and be prone to stick-slip.

Neil

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