Machine Tolerances

Whether I finally buy a new or used mill and lathe, how do I know what tolerances the machine should be at for things like backlash, runout, etc. For new, I am sure these would change with price/quality. For used, what is the acceptabe maximum for the home machinist (in my case no gunsmithing, simply machine parts for robotics, and who knows what else in the future)

Chuck
I'm not sure there is a definitive answer. Some can live with half a turn on the dials, some would go crazy. A good machinist can make excellant parts with a worn mill, others would just be chopping metal. So it's really what you can live with.

The only machines that will not have at least some backlash are those that use ball screws (which, so I have been told, are not ideal for a manual machine). If you have any backlash at all, you have to deal with it to get any sort of precision (eg, by making sure that you always move in the same direction). Since you always have to deal with the backlash, the issue becomes how much you personally can live with -- unless the screw is worn unevenly, though even then it depends on which screw and on what machine. On a lathe, for example, you wouldn't depend solely on the dial to get to the desired diameter; you would measure to see how the lathe is cutting with the particular tool, speed, and feed that you are using.

If you have a DRO, who cares about backlash -- just read the position from the DRO.

Runout is, IMHO, much more of a concern, depending on where it is. Runout in the spindle of the lathe or mill is going to affect everything you do, so you want to keep it below some acceptable minimum. Depending on the lathe or mill, runout may be reduced by adjusting the bearings/preload.

OTOH, a few thousands of runout on something chucked in a 3-jaw is not unusual, and not something to worry about depending on how you are planning to proceed. I have read that drill chucks often have a couple of thousands of runout as well.

http://mermac.com/ has some guides to buying old mills and lathes.

Backlash gets to be a problem when the nut/screw interface starts to lose rigidity due to the reduced contact area.

Seems esoteric but I owned a lathe where I couldn't control what I was doing. After disassembly, the acme profile was worn to a sharp v profile and the "points" of the male thread had so much spring to it that it was allowing the tool to deflect proportional to the cutting load.

New screw, new nut and it was like having a brand-new machine.

Agreed that backlash on a manual mill doesn't matter, you still have to keep it "going in the same direction" no matter what magnitude.

Also the addition of a DRO (especially on a mill) takes all issues away of using the "lead" and dials on the screws to set your position.

Runout means spindle issues and I would "run" away especially if you are somewhat new to this gig.

Chuck I would suggest you need to think about the work you are going to be
doing on the proposed machine. Only you can set the specifications for that, and
you need to have them set in your mind before starting to shop so you don't spend
extra money buying extra accuracy you don't need, or by wasting money purchasing
a machine you cannot use for the task it is slated for.

Think of the prints being drawn for the parts you will be working on.

There will be dimensions on the prints, and a tolerance associated with
each dimension.

In some cases the tolerance might be, say, +/- a sixteenth of an inche, for
example for the length of a turned diameter. In other cases you may be
specifying +/- 0.0005 or so because a diameter has to fit closely to another
part you already have.

If you put up the tolerance range of the work you are going to be doing
that makes it easier for posters to make sensible suggestions that will help
you in picking out what lathe or milling machine is best.

Jim

Getting What You Paid For.

I was coming at this from a different direction. Let's say you buy a new Lathe or Mill. How do you know what the machine tolerances are supposed to be, so if they do not check out you have a warranty claim.

I don't see these advertised for any new machine tools, with one exception. There is at least one Precision Matthews lathe (I think it was the 13x40HD) that listed these on the MachineToolOnline site. Are these generally available from the dealers on request.

I would expect these tolerances to be much tighter as you move from Grizzly to Sharp to Clausing. I wouldn't expect the machine I can afford to buy for my home shop to be made to the same tolerances as a machine used for industrial work.

So lets say the you can either afford a new Jet, or the equivalent dollars in used American. What tolerances should be the minimum you should expect for your money, and what are the key tolerance measurements to focus on.

Jeff, I will search for some of this in the link you listed. I think I read over this last year, but I need to do it again.

awake said:

...If you have a DRO, who cares about backlash -- just read the position from the DRO.

Click to expand...

It seems to me that you should have the screw holding against the working force otherwise you are only counting on friction to hold the position, not very likely to give a satisfactory result.

Raymond

RDL, if you are feeding the cutter into the work you will have force against the work. You don't feed it in and back off the handwheel, so what you feed in with the DRO will be what is cut off the work.

Chuck, you are looking at machine tolerances in the wrong light. Every new machine is shiped with manuals and a spec inspection sheet. That sheet shows the tolerances of the lathe as it was inspected before shipment. If you believe that sheet that's nice. I would believe American and European manufacturers before Asian manufacturers but there are some dependable Asian companies.

Forget what a lathe is supposed to be when new unless you are buying new.

If you are buying used then you and only you know what you want in accuracy.

If you are a perfectionist then you better buy new the very best precision lathe or mill money can buy otherwise you'll never be satisfied and more than likely that won't satisfiy you.

If you are a realist then you know nothing is perfect and your expectations are reasonable. If so, then start looking for a machine that is in the condition that will turn out work in the tolerances you want.

The ways need to be true and the bed straight. Backlash in the crossfeed and compound handwheel is normal and even a new lathe will have backlash. What you want is to still have some adjustment left in the nuts and the gibs on the crossfeed and compound.

The problem is the wear in the feed and lead screws and bed where the lathe was used most. That is what will give you fits when trying to machine to tolerance. If the lead scres was used in the same place to thread for instance it will not cut accurately there anymore if it is worn.

Then there is the headstock. You don't want the spindle bearings to have slop in them with deflection in the chuck. You should probably pull the top cover and look inside the gear box and run your fingers on the bottom of the gear box to see whats down there, dirt, chips, etc..

Remember that the condition of the lathe when you buy it is the best it will ever be unless you rebuild it. Don't buy a lathe intending to rebuild it, buy one the way you want it. Let someone else buy the one that needs rebuilding. You have to decide if you want to spend your time rebuilding machines or making parts for projects. Many times you'll end up with as much or more money in buying and rebuilding than if you had bought a new lathe.

RDL said:

It seems to me that you should have the screw holding against the working force otherwise you are only counting on friction to hold the position, not very likely to give a satisfactory result.

Raymond

Click to expand...

Yes, I agree completely. You still need to move in the right direction (or lock the table if that is appropriate). What I was trying to say was that with a DRO, it doesn't really matter if you don't get exactly .100 when you dial in .100 -- you're not reading the dial, you're reading the DRO. Sorry that I wasn't clear.

Chuck, I would say that the specs that really matter the most, used or new, are the runout of the spindle and the flatness of the bed/ways -- in that order. If the new machine specifies .005 backlash and you measure .007, I wouldn't bother with a warranty claim, not unless you find something else wrong.

If you haven't read the mermac articles that someone linked to above, I highly recommend them. These were very helpful for me when I got started. Note in particular that if you buy a used lathe, you will very likely have some bed wear ... but it might still be perfectly usable. My old Cinci Traytop has some wear, but I've been able to turn out parts to the limit of accuracy that I can measure.

That last statement is really important -- as a home user, I don't have the special temperature controlled room and ultra-sensitive tools to measure to .00001, nor do I have the slightest need. I am not convinced that I am yet able to get a repeatable measurement within .0001 with my best micrometer -- my "touch" is just not that good yet. But I can get within .0005 if I need to. Measuring within .001 is easy, and hardly ever do I really need anything closer than .001. Actually, much of the time .005 is all I really need (though I still shoot for .001 for pride ).

Note that I'm talking about "absolute" measurements here. As a home/hobby user, the only time I need to be within really close tolerances is when I am fitting parts together -- which I will fine-tune for the perfect fit. If I need to get a super-close sliding fit between two parts, for example, I may not be sure -- or care -- that one part is .5000 instead of .5005, or even .505, but I am sure that I have a sufficiently close fit between the parts -- certainly within .0005, and maybe even better than that.

Carl Darnell said:

RDL, if you are feeding the cutter into the work you will have force against the work. You don't feed it in and back off the handwheel, so what you feed in with the DRO will be what is cut off the work...

Click to expand...

Carl, whenever possible I try not to move into the piece especially on finishing cuts because an over shoot cannot be undone. I was simply making the point that when establishing the tool position it should be held in place by more than the friction of the slides. To minimize inaccurate positions I placed a scale on the Compound (Vectored to the other two axes) which I find more precise than the coarse controls of the Carriage.

Raymond

I worry much more about the runout on a mill than a lathe. A lathe is generally either boring (tool pressure to the operator) or turning (pressure away from). Slop is no problem to deal with. You never backfeed the screw, just keep going in one direction and back up beyond your starting point to go deeper if you overshoot. As long as you don't try to back into a measurement, you generally can get good results even with a lathe that has a half turn of slop. As was stated, you may not be able to take a 3" diam part to .2500" strictly by reference to the dials, but you can rough wildly down to .500 or so and then zero out your dials. That's how I run my L&S. Built in 1918, I can still get it to split thoudsandths when needed, as long as I take my time on those last three or four cuts.

A mill is another story, especially a vertical mill. Facing, slotting, and such it is no problem, but get to cutting inside work, like say a square recess, and backlash is going to deal you fits. Problem is that you MUST reverse your pressure and backfeed the screw during the same cut. In doing so, you are going to get double the error of your slop by the time you reverse at both ends. Same goes for climb milling in either a horizontal or vertical. It'll snatch a table with excess slop and do nasty, awful things before you realize what has just happened.

Maybe US made machines come with runout charts, but not the chinese junk. My Jet 9x20 had no such documentation and neither did the Grizzly mill at the museum. The Grizzly screws had over .100 slop in both X and Y directions fresh out of the crate. Adjustment got that down to under .003, but there were no promises from the maker.

RE ballscrews... my bridge mill has them. They have zero backlash, but there are two problems with them on a manual mill.

First off, they usually are much coarser in pitch than an Acme leadscrew. Mine are 4TPI. I had a friend who runs an engraving shop make me up some 6" diam dials divided up into thousandths so I wasn't shooting in the dark, but reading in .250", instead of .200 or .100, they are still difficult to work with. DRO would hep, BUT......

Main problem is that they will not hold position under load like an Acme. If you get heavy on the cut, they will just transfer the thrust back to the scew and turn the unused motor. On a CNC machine, you get an active braking effect from the servo motors to prevent this, but on a manual, you need to lock each axis seperately as you work to prevent errors. Wish my mill still had handwheels and Acmes to be honest. It's fine for planing work, but will require CNC retrofit to do anything else with total confidence.

As far as slop in the feed screws and wear on same, if you know the machine has wear and the dials will not be true you should always use a dial indicator on the lathe to show what you are really feeding into the cut.

In many of the machine shops I worked in the machines were worn out. Everyone used a dial indicator on the crossfeed of all the lathes to make cuts or you didn't get the size you wanted. There were lathes we couldn't thread on in some shops because the lead screw was to worn and would not cut an accurate thread pitch.

If you are trying to get fine cuts with any lathe I suggest you use a dial indicator and if you are trying to get extremely fine cuts you need a .0005" or .0001" division dial indicator.

Even a brand new lathe can/may have a feed screw and/or nut with inaccuracies in the thread. That will cause you to take more or less material off the work than you intend to.

Unless you have taken a dial indicator and tested the entire distance the crossfeed screw travels for a comparison to the dial indicator and the lathe dial you don't know what spots on the thread may not be accurate.

Anyone that uses a worn lathe and don't use a dial indicator to be sure what they are feeding into the cut is foolish. If you make a trash piece once the boss may not say anthing depending on the price of the material. Make it twice and you'll be outside looking for a new job.

As a matter of fact, one owner said, use a dial indicator, the lathes are worn and the dials are not accurate. What he meant is, if you screw up a piece of metal after I told you this your fired. Over the time I worked for him I found he didn't take fools lightly and you had better not lie to him. We got along real fine.

I have seen and used some real junk to do pretty fair work, so I don't want to hear, it can't be done.

Thanks Again

Thanks Again for all the advise. I fully understand that commercially you would have requirements for the accuracy of a machine tool and buy accordingly. As a novice just starting out, I have a limited budget and will have to live with what that can buy me. Once I gain experience, and decide what I actually want to do long term, I will be able to articulate what I need, and hopefully I will already have it????

Its nice to know that skill and the proper techniques and overcome the short comings of an older machine.

Let's say you buy a new Lathe or Mill. How do you know what the machine tolerances are supposed to be, so if they do not check out you have a warranty claim.

Click to expand...

Some manufacturers supply a test sheet for each mill or lathe that they sell, so you might try contacting vendors you are considering and see if they will supply a sample to check out. The same might hold true for used equipment, though getting hold of samples would probably be harder. Clausing is one brand that used to print test sheets in some of their brochures. Of course if you are looking at used you should be able to evaluate in person before buying and that trumps a test sheet.

Mike

"I have a limited budget and will have to live with what that can buy me...."

You sir have put your finger on the real issue: accuracy costs money. How much
do you want to buy?

To answer the question, tell use what you are doing with the machine. If you
cannot pin that down, tell us what your measuring tools are. Pick one:

Tape measure

Six inch machinsts scale

Plastic dial calipers

Old micrometer you found in the shed

Brand new set of micrometers that have the verniers to read to tenths

Gage blocks and a Moore measuring machine



Somewhere in there is the truth and when you pick your answer that says
if you are working to:

quarter inch

1/64 inch

0.005

0.0005

0.00005

Jim

Of all the threads I ever read, Jim's has to the the best to answer the question.
Well done.

Bought a NEW ballscrew for a cinci lathe a few years back, when cinci was still cinci milacron. Anyway, got it installed and the thing had .006 backlash. Called up CM and was told "yep, that's about right, comp it our with the control". Did some more checking and very few ballscrew makers guarantee less than .005 and it costs big bucks for every .001 after .005.

Moral of the story: we comp'd it out with the old 900 cnc control and everyone was happy.

Really the way to get zero backlash in any mechanism, there have to be two parts preloaded against each other.