Specification on mill table flatness

[MikeA]

Hello all,

A friend and I have Benchmaster MV-1 vertical mills, these are bench top units originally made in California about 50+ years ago, and got to discussing 'how flat' should the table be.

I had mine surface-ground at a facility that does highly critical work for the aerospace industry, often to ridiculous specs, and am comfortable my table is now as good as it ever was, perhaps better.

Unfortunately there is nothing available I can use as a reference as to what it should be. A question posted to the Benchmaster forum elicited some interesting philosophical responses, but (guess what?) no hard facts.

So I turn to you gentle people - does anyone know what the 'standard' spec would be for mill table flatness, being mindful this is a relatively small table (6 X 18), construction all CI and quite well made here in the US of A.

Even specifications on other mills would be of interest, at least to know what one can expect to find.

Thanks in advance,
Mike

[loose nut]

The Schlesinger limits would probably tell you what you want to know. The following passage was taken from an old post and explains what they are. Where do you find them, look for one of the books, possibly they are on a websites somewhere but I didn't find them. Pretty specialized literature.

"Dr. Georg Schlesinger wrote "Testing Machine Tools", which is the very best book on that subject I've ever seen . . . and I'll include Moore's "Foundations of Mechanical Accuracy" and Connelly's "Reconditioning Machine Tools" among the books surpassed."

[Forrest Addy]

It's not a simple case of table flatness.

Knee mill tables, those whose saddle bearing is shorter than its length, droop over time. That amount of droop varies with length Vs thickness, overhang, metallurgy, time, end loading and other variables. A typical 42" turret mill table may droop 0.005 in 20 years plus what ever wear may be present in the flats of the dovetail way bearing surfaces. A long knee mill table may droop 0.015".

I used to rebuild machine tools and often worked with a master scraper prepping castings for scraping. Knee mill tables I frequently put 0.003" to 0.005" "reverse droop" in the table. After a year or so in service the table would droop to cancel this reverse droop and remain stable for some years after.

If the table is made flat and the underlying dovetail ways are still "drooped" the table surface is no longer parallel o the way surface ie asix mition. Long work clamped to it will machine hourglass shaped. Generally speaking, it's better if the table surface is ground parallel to the way bearing surfaces. Then whatever is clamped to it will be machined parallel. Parts made with a gradual bow are easier to deal with than parts machined with an hour-glass shape. It would e better of course if parts were machined straight but that would be asking for near new condition and few of us bottom feeders have that luxury. My mill was new in 1981 and over time it's drooped 0.003" over the length. I attribute the low droopage to low hours.

I suggest you mike the thickness of your mill's table ends and center. If they are parallel fine. If the table is thinner in the center than on the ends then you will have to compensate when you machine longer work. YMMV.

[Richard King]

I had never heard of a Benchmaster and looked it up. Pretty cool little machine. I Googled it's name and model and found several examples. I am a machine rebuilder and if someone asked me to rebuild your machine I would tell them .0002" per 12" accuracy of the ways as this is standard for manual machines. Or they also are called conventional machines. We scrape or hold CNC and Super Precision machines like Jig Bores and Jig grinders we use the spec of .00005" per 12". If your setting up the gibs you should gap it or set the lost motion gap at a minimum of .0005" to .001". This will assure you will have the proper amount of way oil on the ways. I have seen imported machines recently built in China that are out .008", so if your with-in .001" you'll be fine. Unless your doing aero-space parts on it too. :-) Rich

[MikeA]

Hello Loose, Forrest, and Richard,

First I want to thank you for your well thought out and informative responses. It seems too often responses to questions/requests for information are more idle thoughts or misinformation.

Loose - I found a pdf copy of the Schlesinger book at:http://www.totallyscrewedmachineshop...hlesinger).pdf

It appears to be chock full of valuable information and will take a while to digest.

Forrest - it sounds as though we're on the same page and I've already figured out the planarity between the ways and the table top at the center are the critical measurements, and expect I'm well within the 'safe' or whatever regime.

Richard - thanks for your well thought out response even though you don't have any direct experience with this particular unit; BTW - it really is a pretty neat machine and for those of us operating with severe size constraints the Benchmaster is a good compromise.

Measuring the table using a DTI I appear to be within 0.001" and am more than happy with that. Nope, I'd doing mostly klutz work for the most part and anything <0.003" or so range is more than satisfactory!

All the best,
Mike

[loose nut]

.001" is good. Mills are not surface grinders, so they won't be super flat (standing by to receive incoming flak) and with age/work the table will warp some, how much depends on the table size thickness etc.

[MikeA]

Hello Loose,

Yes, I feel it's about as good as needed (that should divert the flak my way!)

BTW - I've posted this question in two machining forums and no one has provided a specification for any mill. I was able to find one post on a forum that stated the OP had received a new mill from Grizzly (X2 or X3?) and the table has quite a bit of excursion from the published spec, which is 0.002". (Don't recall if that is over the length of the table or one foot.)

Frankly that seems a bit much for a spec, but what do I know? What is the spec of Bridgeports? Does anyone know?

All the best,
Mike

p.s. - I'm only curious and recognize a good machinist will be able to work with whatever the equipment limitations might be and still turn out quality work.

[loose nut]

Mike, the quality of the machine has a lot to do with it and even some high quality brand name machines have problems. Just ask Sir John about his old Bridgeport. If the table is long and the thickness is not substantial then the table can droop at the ends. Most of the high end machines (jig borers etc.), like Deckels, that I have seen (pictures only), seem to have shorter tables, maybe for that reason??? On my cheapy RF45 mill, the table, which isn't that long compared to the base it is mounted on, is ground flat. I ran the indicator over it when it was new to verify it and the pointer barely moved. If I have to run the table to the extreme at one end then I can feel the tightening of the ways, through the leadscrew, as the table droops a bit, it doesn't have to be much but it does happen. Something that has to be lived with unless you have money for really sturdy equipment.

[MikeA]

Hello Loose,

One would think that would be the case - just don't have the input to demonstrate it one way or the other.

FWIW - the Benchmaster is a rugged little bugger and I searched for one for quite some time; I have limited space for my shop and it is almost inaccessible, certainly any thing I want to move into the shop must be able to be broken down into one-man pieces and carried down two flights of stairs with a 180° in the middle.

One other factor in it's favor is the relatively small table - 6" X 18" - with little hanging over the ends!

Does the job for me and I'm very happy with it.

All the best,
Mike