Lathe Rigidity Issues - Modification Opinions

Quite the response!

I can't address everyone so I guess I will generalise the response, also, I'm originally from the UK. I've owned an old Pooles, Denford Viceroy and Harrison M250. Never had rigidity issues with any of those, but they were good old lathes. Some nice big lathes over here, like 4m long beds or more for cheap, but everything smaller is new Chinese. Going from model lathes to industrial and back to a model lathe is quite a step down. 25HP back to 1HP. Funny that I buy old HSS end mills over here for aluminum, Presto, my home town, take a guess.

I know I'm pushing the lathe a little too much, the original idea was to make one from scratch and have some castings made, wish I had now, but that is a project for another time, but now I don't have access to an industrial machine. Thankfully I will get a decent amount back when it comes to part with this lathe due to the market, plus I have all the original manual parts for it as I'd keep the servos. A little bit of info, 10 inch chuck, 3 inch spindle bore, linear rails, 10 inch swing over slide, 20 inch between centres, gang tooling, 3 or 5HP direct drive, 1500 rpm max. Which brings me to the spindle bore on lathes. If you want a 3 inch bore then you need a 10 inch chuck, that comes with a very long bed, probably over 80 inches. Some CNC's are 36 inches, but the machine is still huge and requires big power. What are peoples thoughts on a hobby / small industrial lathe with a larger spindle bore and shorter bed? I personally hate end pieces, and working at the tail stock if its something long.

So, 1 inch tools are cheap and better quality than the hobby stuff. Round tool holder because it's quicker to make it from round in a lathe rather than mill it, plus round stock is cheaper. Tool post is 3 inch round 120ksi steel. Grooving tool is set length, not adjustable. Insert is 2mm wide, feed 0.04 to 0.12mm/rev, they are intended for light turning. Slowest speed of lathe is 150rpm, but it's belt driven. I will mostly be turning between 1 to 3 inch round, the 5 inch piston was over doing it, but cheaper myself than outsourced, plus it worked out on the mill in the end. I want the lathe mostly for threading, but a 12TPI -1.5 inch thread I still think it's not rigid enough. I agree with the z axis ball screw coupler, I do have to limit the speed because of this. HSS definitely would have been an option, but at 5 inch the surface speed would have been about 200SFM, that's on low carbon steel, in my experience it would not have lasted long, 10 parts total, 2 thou tolerance. HSS is diabolical over here, all comes from China.

I took the plates off the lathe to take a look at the castings, as in the pictures, thankfully it's reasonable unlike the one in Niels post, that is pretty sketchy, can't believe someone would design something like that. I might get a 3HP servo to directly drive the lathe with a timing belt since I will need an encoder for the threading, the motor will then be used on my later CNC project, maybe 5HP depending on the price. Would love to make chips instead of razor wire!

Anyway, only managed a few hours on the lathe, busy day. I took apart the saddle assembly to see if it sat on the bed correctly. Look at the pictures carefully, the vee has contact all the way down, the flat has one tiny contact patch in the corner, looks like this is a big issue. Overall not a lot of contact. I broke out the Sandvik scraper, something I've never done before. I probably took a thou off all surfaces and about 3 thou off the bad corner. Sorry no pictures, my cross-hatching technique didn't look pretty. That's all I managed, but now the slide sticks with oil suction to the bed, I also put force in every direction with a DTI to find nothing moved, I'm happy with that result.

There are two gibs underneath the bed, neither adjustable, but there is a 4 thou gap on both front and back now. I've seen a few recommendations from adjustable gibs to bearings. There isn't much room to play with but I am thinking of something spring loaded, maybe some spring steel strips with a bronze blade? Let me know your thoughts.

I really like the modifications made to the top slides, especially that milled tool post. I have a large piece of steel I'm going to work on in the next day, it will bolt directly to the top slide at its full length. I will then bolt some gang tooling to that, will update in the next 24 hours. It will reduce my swing over the slide to about 6 inches, but I can't imagine ever needing that.

So overall you get what you pay for, and this lathe is better than none at all.

Yes when you see inside the headstock, it does not look overly massive. The word Gibraltar certainly does not come to mind.

It's hard to tell from the pics, but it looks like yours has the common shortcoming of many Chinese lathes: The bed width is narrow in comparison to the spindle height. It looks like the bed width is quite a bit less than the spindle height. So the triangle between the bed shears and the spindle is a tall, narrow based one. That's inherently less stable than many better quality industrial lathes and toolroom lathes that have the bed considerably wider than the distance from bed to spindle, so they have a flattish, broad-based triangle to spread cutting forces over. So between the headstock bulk and the bed wiidth, you are never going to get the type of rigidity you are used to in industry.

But as you've seen, going through and "blueprinting" the machine should tighten it up considerably.

I wouild not go with springs under those saddle lift plates. Cutting forces will overcome those springs. Best to do like Myford and most others and machine either the lift plates or the area where they mount on the bottom of the saddle so strips of shim can be used to get the desired half a thou or so clearance there. Anyway, if yoiu are plunging straight in you should have the carriage locked, which ideally should lock on that rear bed way. Might be an addition to consider if it does not have one.

I know you like that 1" tool and the overhang would be fine on a big rigid industrial machine but is just begging for trouble on these smaller flimsier machines.

If you go through as you are and eliminate all these little things, the sum total should put you ahead of where you were. But you are not going to make a Monarch 10EE out it due to the core design and build factors.

Hi Hopper,

Yes the spindle is definitely higher than the width of the bed, plus due to the diameter of the piece I was turning the tool holder was to the front of the bed. I knew it was dodgy, but sometimes you got to do what you got to do. I really wish I had gone for the part-off blades that are adjustable, I should have known better, but now it is too late.

I totally forgot about the bed lock, and I definitely used it on all of my previous lathes. The big issue now is that it is CNC and I'll be mostly using it for threading, so while I could lock it for grooving, I definitely cannot for threading. I have about 100 threads to cut, 0.75 - 16, that will be its next challenge when I sort out the encoder. I then have some 1.5 - 12 threads to do after than. All in 4140, which is 120KSI or thereabouts.

I will see what the others say about the gibs first before I make my final decision, but I do think you might be right in that the springs will just give, shimming would also be fairly easy to do, I just hope the bed is the same thickness all the way down.

I'm probably going to remove the head in the next day or two to see how that's attached, maybe replace the bolts with some studs. Worst case I'll get some nickel rods and weld it together, I might also put a tiny bit more pre-load on the headstock bearings, although haven't checked those to see if they're too tight or too loose yet, but there definitely is no play in it by hand.

While you have it apart you might as well ditch the Chinese bearings and put in some good quality SKF or similar. Not all bearings are created equal.

If the headstock is removeable from the bed it would be worthwhile to blue up the mating surfaces and scrape for a good matching fit. As you have seen, they are not finished to old time precision standards by the factory so could be moving about a bit on poorly matched surfaces. Then of course you have to check spindle alignment to bed etc.

Welding sounds kinda radical and could bring heat distortion into play etc. A firm seating on the ways should do the job, you would think.

It probably should do those smaller diamter threads better than the big diameter piston job, once you have all the littte things tightened up. Modern insert tooling works pretty well for screwcutting.

Edited By Hopper on 19/03/2021 07:02:14

Simple way to get rid of the 0.004" gap would be to mill that amount off the half of the plate that mates to the carriage but leave full thickness where it runs against the underside of the bed, the step will raise the running surface up 4thou.

Do check that the bed is constant thickness along it's length, may need a bit of scraping on the underside.

Jason,

You said it better than I did

Ed,

First time I have looked at your post but certain things come to mind straight away.

1. Mounting the lathe on a wooden table/cabinet will not help. I would make a substantial RSJ stand for it and bolt it down with adjustments for bed twist.

2. The 4th picture in your OP shows the external gripping chuck jaws at more extension than I would like to see, looks like you need a bigger chuck! Workholding can exacerbate chatter if not secure.

3. Have you considered a rear toolpost? An inverted tool in a rear post will work with you, reducing chatter as the cross slide will lift and tighten onto the dovetails.

Apologies if anyone has mentioned these points previously.

Martin

A good advice to Niels:

Do your homework before touching keyboard. The CQ6128X660A lathe is 280 size with 660mm between points.

This is more or less a Brother of JasonB s.But a smaller 26mm spindle I think.

I am thinking of something spring loaded

No !. Why introduce the possibliity of unwanted movement when load increases ?

Just make the strips (plural - there should also be one under the front shear) fit properly, no need to add fudges that will come back to bite you. When properly sorted, the only way the saddle should be able to move is longitudinally along the bed - no lift, no twisting.

As Jason suggested, the easiest way is to take the 4 thou of the part of strip surface that the bolts to the saddle.

Weld the headstock to the bed - really ?

Nigel B.

AFAIK all 280s have a 26mm through hole on the spindle, only the 290s have the bigger bearings and larger 31mm spindle. The Chinese one-upmanship that has resulted in the big bore 250s doesn't seem to have hit the 280 class yet.

It is  an offence to us underpriviledged citizens in not going to the moon soon countries ,to ofload us so much iron and then put such a small knitting neddle spindle in.

As luck have it I can offer two solutions that do not need any change of headstock.

Please enjoy.

Edited By Niels Abildgaard on 19/03/2021 16:51:32

Edited By Niels Abildgaard on 19/03/2021 16:53:32

Edited By Niels Abildgaard on 19/03/2021 16:54:04

Edited By Niels Abildgaard on 19/03/2021 16:54:41

I should have a two updates today, the first being that I milled down the gib strips for underneath the bed. Thankfully I did not screw the scraping up as I removed 4 thou on one strip and 4.5 thou on the other. That resulted in a lift of 0.8 thou on the front and 0.2 thou on the back, slides nicely on the full length of the bed, this part is at least well ground.

I did some searching last night on standard taper bearings, so I might pick up some SKF ones or even possibly upgrade the spindle slightly as in Niels post to a beefier setup. I have a threaded mandrel which I originally used to hold the piston, then I changed jaws to hold on the outside to see if that improved it, it did not, I also used a live centre for all operations, a good quality one. I do agree my bench is a little underwhelming, but I can sort that after I've tuned the lathe. I was also joking about the welding , plus you need iron hot to weld it good.

I have done some designing for a piece to bolt onto the top slide, made from 6 x 2 x 12.5 steel, a left over, quite a lot of milling to do so hopefully I will get some pictures tonight. I chose lots of slots and tapped holes for lots of arrangements as I intend to use gang tooling. The swing over slide is now 142mm, just enough to do some 140mm diameter tubes, which I have already done without issue.

Little bit of milling and 62 holes later the bottom side is done.

Wow that's shifting some metal. What milling machine do you have.?

I would go with replacing the existing size bearings with SKF.

Bolt on external bearing carriers might allow bigger bearings but are only as rigid as the thinnest wall in the carrier piece. Which is thinner than the headstock casting.

Edited By Hopper on 20/03/2021 03:41:16

I'll have to take a picture of the mill, it's a Fulland Mill, 1985, Taiwan machine, not sure of the model number, about the same size as a Bridgeport. Maximum spindle speed is 3500rpm, so a little limited. I did a 23mm depth of cut, 0.6mm stepover, 80 IPM, 7/16 carbide end mill. Took about 45 minutes to do the milling, and then about 1hr to do the drilling. Running on Mach4 and using fusion 360. Fusion retracts out all the way for each pass whereas Mastercam micro lifts and back traces the previous cut, I think I could have got it down to 30 minute on that software with this machine, but that's the limit, plus I can't afford Mastercam.

I think you're right in regards to the spindle bearings. I have started designing my own machine so don't want to go too crazy with this one, in terms of money. I think the external carrier would add some rigidity, but ultimately the castings will let it down. I have also considered filling all of the castings with concrete, or maybe some kind of sand/resin mix, I think that would also dampen the whole thing, wouldn't be all that expensive either.

Or maybe rather than throwing the baby out woth the bathwater, get some 1" steel plate laser/waterjet cut to shape and weld up your own headstock to fit the existing bed. Looks like you have the milling facilities to bore the bearing holes and cut the ways to sit on the bed.

Ed, have you seen Joe's current lathe build thread?

Before the Myford I had a Mashtroy 220 lathe that gave a horrible finish and would not part anything at any speed. After worrying at it for quite a while I noticed that there was a distinctly gritty feel to the bearings so I too out the spindle and the roller races were quite badly marked even though the lathe was quite new. Then I bugged the supplier to replace the bearings until he sent me some decent ones which transformed it.

I think you said the lathe was CNC? Why do you need a topslide at all?