Cant seem to get a nice finish

Or boring it out oversize, pressing in a bushing and reboring the hole concentric with the worm or gear teeth.

Did the dial guage test, no movement, rock steady

If locking the cross slide makes the problem go away, as you have found, then it is hard to think how something like the feed worm wheel running eccentric could be causing it in this particular instance. It would seem the problem is more likely something to do with the cross slide moving cyclically under the load of cutting forces.

Try a few more tests with the cross slide locked and if it confirms that fixes the problem, you should look more closely at the cross slide before ripping into more complicated things. Rule 1 of troubleshooting: Always eliminate the simple stuff first. (Surprisingly often, this will fix it.)

You said you could not get a happy medium on the gib screw adjustment between too loose or too tight. That sounds like something is not right in the dovetails/gibs/ways area. It might pay to check it all carefully for burrs and sharp edges etc and smooth them down with a dead smooth file or a small slip stone.

The other thing to check, which I have found dodgy on some cheaper lathes is the fit of the gib strip. It should have small indents drilled in it for the screw ends to seat in. The ends of the screws should be domed to fit nicely into those indents. And, very importantly, the gib strip should have clearance at the edge so when you tighten those gib screws it is not pushing the gib strip up against the bottom of the dovetail of the half it is supposed to slide against, thus causing binding. And make sure there is no gap between the horizontal flat surface on the carriage and the mating surface on the cross slide.

This video gives a bit of a run through on things to look for on typical Chinese cross slide set up and the pitfalls that can occur. It shows how an incorrectly fitted gib strip can cause the cross slide to ride up on the edge of gib strip instead of on the flat way surface.

Edited By Hopper on 20/02/2023 08:26:53

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Excellent demonstration

MichaelG.

+1 for that video. Clear & concise - must have a look at some of his others when time permits

Mike

Thanks. Although, I don't recommend his use of channel lock pliers to undo the gib screw locknuts. Poor guy is in America and has not yet heard that you can buy metric spanners for "furn" machinery. But his use of feelers to find where the cross slide is riding is spot on.

I reckon you could file the top off that gib strip if you had to, if you have no milling machine. I notice too that the gib strip doesn't have the drilled divots for the gib screws to engage with. So I guess by nature it will push that gib strip downwards against the base when you tighten them up. Which should work. But is the opposite of the Myford philosophy and GH THomas who took it one step further and added two dowel pins to give absolutely positive gib strip location.

Edited By Hopper on 20/02/2023 10:15:50

How are you locking the cross slide, I thought they had done away with the lock on the 290 as you can't get to it with the DRO in the way?

I moved the DRO to the other side

My gib screws dont directly push onto the gib strip but push what looks like phospher bronze pins onto it.

As I am no longer going away I am going to strip to cross slid off again and will take a photo of the gib stip as I suspect it could be the problem, I will also check, as best as I can, the cross slide for flatness etc

Edited By petro1head on 20/02/2023 14:25:38

Removed the top slide and that seems Nice and flat.

I have ordered a New Cross slide screw and nut from Warco. I also asked about gib strip but it seems they don’t do them now.

Looking at the existing gib stop it looks very worn (see photo below). So it looks like I will have to figure out how to make a new one. Does it have to be steel or could i use somit else?

Edited By petro1head on 20/02/2023 15:56:45

To be honest I think any material would be an improvement, how about a piece of brass? What does the mating piece, the dovetail, look like?

Looks fine to me.

Ok, probable not the right thing to do but thought what the ****. I have machined it nice and flat. Done a temp reassemble and its sooooo much better.

So just waiting for a delivery from Warco of the new lead screw and the big reassemble and test.

Edited By petro1head on 20/02/2023 17:09:16

I agree with the previous correspondents who said it was vibration from the single phase motor you are probably using. I had the same problem and it vanished when I fitted a 3 phase motor. I think the marks will only appear on fine cuts.

Best to avoid brass for gib strips. It's generally a poor choice, it wears fast and gets very grabby when the oil is squeezed out. Bronze is good, steel also, cast iron very good.

That is horrific. Is that on the sliding surface? If so there must be something horribly horribly wrong to do that to a gib strip. You need to check the cause of that before it does it again to your remachined surface. Did you do the checks with a feeler gauge per the video posted above?

Or was that groove in the gib strip machined in there at the factory for the ends of the gib adjusting screws to locate on, on the non-sliding side of the gib strip?

I had the same problem on both my lathes.

On my small lathe is was only visible when moving the top slide. The cause was to much play in the gibs combined with a dry nut/feed screw. After cleaning and oiling the feed screw and nut and adjusting the gibs, the problem was gone.

On my bigger lathe, the problem was only visible when moving the carriage (power feed). It was caused by not enough / no play between the rack and pinion that moves the carriage. The problem was gone after removing the rack. Cleaning the rack, scraping some paint of the bed and removing some high spots (light sanding), under the rack, solved the problem.

Yes i did check and it was fine

The grove was on the mating side. Again i have checked the dove tail and it feels ok

One i have the new lead screw i will do some macining checks. If the problem still exists i will remove the sadle and inspect

Good stuff. One step at a time.

Hard to see how that groove could have been worn in place like that. Maybe it left the factory like it? QC can be a bit hit and miss over there. It will have to improve things with a full smooth face bearing on the dovetail!

Have you tried manual feed rather than powered feed?

Might be worth a try, see if it makes an difference.

I only ever engage the leadscrew for screwcutting, though i'm probably in the minority??

Also you could try~

**LINK**

Works for me every time i'm having problems with finish.

Are we sure that this isn't an issue of material building up on the cutting tool and then breaking off in a repetitive cycle?

Does increasing the feedrate or applying coolant alter the pattern?

Martin.