Leadscrews and other features of Asian milling machines

I am about to buy medium size Asian milling machine.

Its weight is about 1 ton and between few features which are nice I have found one a bit worrying - its leadscrews are turned rather than ground or rolled.

It has a table 1000 x 240 mm, power feed in x and y axis. It can travel 590 mm in axis X and 205 mm in axis Y.

Seller says that I don't need to worry about turned leadscrews because table is quite heavy (approx 60 kg) and ground or rolled leadscrews would quickly lose their advantage, unless they are hardened and further surface treated what would make them very expensive. He is confident that accuracy will be good for long time regardless and he points out that machine has DRO anyway.

DRO is handy but I would like if readuts from dials on handwheels can be trusted as well

Dovetails of this machine are scraped but only on 1 of 2 mating surfaces.

Seller says that 1 scraped surface still holds enough oil to work with second ground one and it is still beter than if not scraped at all.

Mill is trammed well (0.02mm TIR on 200mm diameter), at least in the case of one in display room and machine "eat the meat" without any noticeable fuss or chatter.

Gearbox is quiet.

Has 2 spindles (vertical and horizontal), good selection of speeds beginning from 115 up to 1750 rpm on vertical spindle and from 60 to 1350 rpm on horizontal. You can rise and lower main table total 350mm.

Quill has a strike of 120 mm and is located in head of swiveling type.

2hp motor.

Everything cost equivalent of 5500 queeds including VAT.

What do you think about these worrying "features" of leadscrews and one of 2 mating dovetails surfaces scraped?

What more to look for before buying?

On a mill absolutely no need for dials if you have DRO, trust me I started my career on dials, then optics & then DRO's, you just will not use them, my mill at home doesn't even have a dial on the X axis! Scraping on a hobby mill is usually only on the horizontal surfaces and is cosmetic as best. A turned lead screw is perfectly adequate, a rolled one would be less accurate & a ground lead screw if you could get it would be many thousands of pounds & totally unnecessary, in any case the DRO scale is showing the actual table movement.

Tony

Edited By Tony Pratt 1 on 16/09/2021 15:10:28

For hobby use, or indeed most day-to-day use, I doubt if yo have anything to worry about.

As Tony say,s, no need to go wild for normal use.

Sounds like a nice machine, I like the idea of the two spindles, handy for gear cutting.

As said if you have dro you won’t look at dials. It is quite normal to have one ground and one scraped surface, ground surface is often hard so can’t be scraped. The scraping as said is to create a relieved surface to create oil pockets after first grinding, also used to correct geometrical errors.

+1 for Tony's and larry's comments.

I've always thought that rolled threads were done for strength, wear resistance and rapidity of manufacture rather than specifically for accuracy. I worked in the 70s for a company that made jacking screws for artic trailers, and their thread rollers would run off a 10 foot length of 2-start inch-and-three-eighths Acme in a couple of minutes. I don't know of any particular reason these threads should be more or less accurate than turned.

I never look at the dials on my mill, only the DRO.
For what it’s worth just looked and there is .055” backlash on the Y and just under 0.03” on the X.

Still makes accurate parts as the DRO measures where the table is, not where the screws think it is

Dave

I'd agree, rolled threads are used for mass production as it is fast. They're also used for aircraft bolts for reasons of strength and fatigue resistance. There has been at least one fatal aircraft accident (involving a Stampe) where the replacement component that failed had a cut thread rather than rolled as per the original.

Andrew

I'd draw issue with a couple of the points others have been made:-

1. Scraping should be done on all mating surfaces if at all, not just half of them. Scraping is for precision alignment not just oil control or cosmetics. Flaking will normally only be to half the surfaces and should only be done to the surfaces which aren't visible i.e. not subject to grit falling on them. This is done for oil control and (on the wrong surfaces) for cosmetic reasons.
2. The most important things about the feed screws are how much backlash there is, how much variation from the best to the worst parts of the travel and are there arrangements for adjusting the backlash. this is important when climb milling.

Bear in mind that a top speed of 1750rpm is not optimal for cutters less than 5mm in steel or 25mm in aluminium. Still works, just not optimal.

Many thanks for comments up to date.

They are interesting.

DRO will be a good feature but up to date I am used to handwheels. Lets hope I will quickly get used to DRO as well. I am aware it is superior.

So it seems that flaking for oil control was done rather than scraping.

I have seen installed "hardened, rolled super precision leadscrew" ex McMaster Carr fitted on one S7 Myford lathe.

Owner is very proud of it. and declares that accuracy of screws he makes is now improved.

Regarding speeds of the mill I plan to buy - seller says that after few monhs of initial use I may go up to 60 Hz to get maximum 2100 rpm if desired and machine will take it without trouble. Seems a good idea, but can I trust him? At high speeds cutters are small, loads on bearings low so it looks reasonable. Also the same machines sold under different brand names are likely to be run in US where they have 60 Hz anyway but...

So a Warco WM50 equivalent from somewhere slightly cheaper? Maybe without the universal feature which seems to be a recent enhancement on the Warco ones.

If they can set up to grind one surface they should be able to do 2 so I think it is just that one surface is flaked (the one you can see) 'cos it looks like they care and the salesman doesn't know the difference.