|Y C Lui||12/02/2022 14:21:14|
|62 forum posts|
I noticed recently that the machine cannot hold zero. I zeroed the ABS X and Y of the DRO after locating the corner of the vise with edge finder and after using the machine for a while, I located the corner again and found that the drift in the position of the vise corner along the Y axis can be as much as 0.03 mm while that in the X axis is less on average.
This is kind of puzzling for me because the round-vertical-column design of the machine should have more problem in holding the zero along the X axis ( head can swing left and right due to play in the tiny vertical rail ) but in reality the Y axis is having more problem.
At the beginning the drift seems to be kind of random but I finally found the cause today - it is due to thermal expansion of the head. For some reasons, the head can get quite hot during operation and thermal expansion causes the head to extend outward hence the drift along the Y axis. After the machine is allowed to cool down, the original zero position is found to be restored.
This is just a 300 lb bench mill so high precision is not expected. I am just wondering if the same happens to bigger machines and how people deal with it.
Edited By Y C Lui on 12/02/2022 14:50:39
|Clive Foster||12/02/2022 14:43:57|
|3135 forum posts|
Standard industrial way of dealing with thermal drift is to run the machine until its fully warmed up and re-zero before starting critical jobs. Which may take a while.
Perhaps arrange some sort of heating to bring the ehad up to temperature quickly and stabilise it during breaks in work.
|John Haine||12/02/2022 15:58:40|
|4675 forum posts|
Infra-red thermometers are quite cheap these days and useful round the workshop. You could run the machine until it's warm as Clive suggests, and measure the temperature of the head. If you want to avoid running the machine to work it up before use, a small heater could be fitted to keep it warm perhaps.
|1188 forum posts|
the drift in the position of the vise corner along the Y axis can be as much as 0.03 mm ..... This is just a 300 lb bench mill so high precision is not expected.
I suspect that a lot of small milling machine owners would be happy with a little over a thou movement ? What are you looking for or expecting ?
According to the parts book for the FB2 (well mine is a Taiwanese clone badged "FV-320T", but the manual appears to be a copy of the Emco original with the Emco badges redacted) the gearbox lay shafts run in plain bushes & it only has a small quantity of oil (can't recall exactly how much I put in, but probably less than half a pint) so it gets warm in use. Anything that gets warm is going to expand.
I am just wondering if the same happens to bigger machines and how people deal with it.
Oh yes - it definately happens to bigger machines ! How it is dealt with depends on how critical the parts are & how deep the companies pockets, but at one end is running "warm-up" cycles at the start of a shift to warm up ballscrews, spindles etc & build up lubrication films before starting to machine parts. That is how tight tolerance parts are machined where I currently work - 20 minutes rapid traversing the machine through it's working envelope with the spindle running, then run 3 parts with no blank in place, then start producing & the critical dimensions come out the same as at the end of the previous shift. Just put a part on the cold machine & they fail inspection.
Then there is actively monitoring temperatures around the machine & applying compensations to the axes to keep the tool stable relative to the part as temperatures change. I have seen a few variations of this method - Oerlikon CNC horizontal milling machines had a temperature sensor on the bed casting which was used to set the temperature of the lubricating oil circulated around the head casting to limit spindle growth, Or a Kearney & Trekker MilwaukeeMatic horizontal machining centre that had an Invar rod attached to the end of the spindle casting that rested on a dial gauge, with a resolver driven by the gauge needle - any movement of the resolver due to spindle casting growth moved the Z axis to keep the tool position constant WRT the job. Some modern CNC controls have inputs for multiple thermistors & compensations are handled through the PLC on the control.
Or there are temperarture-controlled environments. Only saw two of these while at my last employment (though former colleagues worked on two more temperature controlled sites - both defence related) & only one of them was actually functioning - the former DeVleig works at Red Lane, Coventry, which was turned off by the later owner due to high running costs & a printing press manufacturer in Leeds (Crabtree Vickers IIRC) which maintained the whole machine shop at 19C to better than 1 degree C all year round & had airlocks on all the entrances / exits. The machine I worked on there was a DeVlieg 5K120 jig borer with a 10 foot x 10 foot working area - after we had finished doing a CNC conversion on the machine, the owner had the positioning accuracy checked by an independant laser calibration outfit - it was found that the machine could position anywhere within the 10 foot by 10 foot area within a tenth of a thou - required because the bearing bores for the prining press rollers on the side frames only had a 4 tenths positional tolerance. Newsprint running at 30 mph through the press doesn't tolerate mis-aligned shafts !
|Tony Pratt 1||12/02/2022 19:29:02|
|1964 forum posts|
I have never ever ever worried about ‘thermal drift’ on a manual mill😬
|Robert Atkinson 2||12/02/2022 22:03:51|
1209 forum posts
I once designed the electronics for some 3 axis machines that placed biological samples that were subsequently scanned. Distortion in the pattern was caused by temperature changes including diurnal. These machines were running Renishaw encoders with 1 micron (0.001mm) resolution. We had to accuratelymeaure the temperature of key part of the machine and use software compensation. We used a Renishaw laser interferometer to measure the position of the moving head for calibration. Fortunatly we did not have to do every machine.
|Graham Meek||13/02/2022 10:41:08|
|473 forum posts|
During my apprenticeship years in the air craft industry. Grinding machines were always left running to maintain the spindle bearing temperature.
As regards Concorde there was written somewhere that the shade of white was changed and it made a really significant effect on the expansion during flight. How much that was is no longer in my data bank.
Getting back to the FB 2 having the quill extended and running at top RPM makes the quill expand to a noticeable degree, so much so, that when it comes to retract the quill there is a noticeable resistance. The needle roller bearing is a very efficient pump as regards the oil movement. The quill usually fills up with oil during use and is the first thing to get hot.
Personally I would not worry about this drift. I have never found it a problem, but then I do not use DRO. Are you sure the DRO is not resolving to a higher number. Some of the budget DRO's only resolve to a thousandth of an inch.
|Jouke van der Veen||13/02/2022 10:59:00|
|174 forum posts|
If the carpet in Concorde cabin had to stretch that much passengers must have got nice warm feet during flight 😉. And not only their feet got warm. So the Concorde needed extra cooling or less heating for certain parts/areas? I am a bit familiar with APU and ECS systems in air plaines.
|561 forum posts|
..many a micron mak's a muckle
8694 forum posts
Me neither! Shouldn't be necessary unless working to tight tolerances, which few Model Engineers do.
Tolerances are essential when making interchangeable parts and the likes of push fits between shafts and holes made in different factories. For example, Imperial push fits might be made to Newall Limits, where for a nominal 1" diameter hole, the shaft's HIGH limit is 1" - 0.00025" and the LOW limit is 1" - 0.00075, tolerance 0.0005".
To achieve tight tolerances old school machines need to be warmed up, though a modern automatic might actively measure dimensions with temperature compensation.
As far as I know, no one on the forum works to Newall Limits or similar. Model Engineering plans are rarely toleranced, and if they are we ignore them! I'm often wrong: does anyone on the forum use tolerances?
|1188 forum posts|
The structure of Concorde got so hot special measures had to be take to allow for it.
One of the special measures was that the carpet was made to be able to stretch in one direction - IIRC the growth in length in flight was around 8". My late father was a carpet weaver at Firth Carpets in Brighouse, who made the special "strechy" carpets for Concorde.
|Graham Meek||13/02/2022 12:19:29|
|473 forum posts|
Yes, I do.
While I know of the Newall system of tolerances, I have never used them, but have used their Jig Borers.
Tolerances have been a part of my life since I started work in 1968. The Hole Based system which I have come to know is in constant use by me in my workshop. I think most Home Machinists fight shy of using them because they do not understand how they work.
I am of the opinion that Tolerances are a bit like Screwcutting and Morse Tapers. They are viewed with the Mantra, "to be avoided at all cost". Mastering them is all part of the fun of engineering. We are after all in the 21st Century now, should we not move with the times. I am sure a large percentage use carbide inserts these days, and CNC's, so why not use tolerances?
Not all dimensions on a drawing carry a tolerance, usually it is the fussy bits like shafts in holes, pieces sliding in slots, or hole positions.
I know that if I turn a part to a specific size form the charts, it will fit he hole I have reamed with my desired amount of clearance, or lack of it. Be that Press Fit, Transition Fit, or Running Fit. Many of my projects do not get assembled until all the parts are made. Plus if I have to make a new part at a much later date, (especially know my memory is going), I do not have to have access to the original to measure it. I know exactly what to make the new one because I have the drawing. This is especially so when the part in question might be on another Continent, and yes this has happened with my hobby.
|Y C Lui||13/02/2022 13:31:11|
|62 forum posts|
The DRO on the machine is a cheap one but the resolution is decent ( 0.005 mm ) and so far I have not seen any evidence that it's not working properly. 0.03 mm drift of the origin is not an issue if that is the sole source of errors but that's not the case. The more I use the machine, the more sources of errors I found and they can add up to some unacceptable level if all ignored. That's why I try to compensate for them when possible. Here are some sources of the errors I have found so far :
1) The X and Y rails are not perpendicular to each other. It's off by about 0.02 mm over 90mm.
2) The vertical column is slanted backward by about 0.02 mm over 90 mm.
3) The tram along the Y axis is off by 0.04 mm over 150 mm if the quill is locked. Increased to 0.1 mm max ( but in different direction ) if the quill is lowered by operating the quill handle
4) 2 & 3 added together, the tool tip shifts by 0.01 mm in the Y direction for every 18 mm increase in the length of tool / measurement device.
5) The quill moves forward by 0.1 mm when pressure is applied to the the quill handle.
6) Play in X Y Z rails with that along the Z rail having the most prominent effect.
7) Strange drift in Z depending on the direction of rotation of the spindle.
8) Tool / head flexing : 0.01 mm to 0.018 mm depending on the material of the workpiece.
Thermal drift is a new one just added yesterday. Hope that's the last one ....
Edited By Y C Lui on 13/02/2022 13:42:15
|1188 forum posts|
Seems like you have a pretty dreadful machine there Mr Lui.
To prevent you having to suffer further distress, I will risk all and offer you £100 to take it off your hands.
To be a little more serious - what are you expecting in the way of accuracy & rigidity from a small, bench top milling machine that the Emco brochure describes as being made "as priceworthy as possible" ?
The numbers you are quoting are ballpark for my Taiwanese FB2 clone (x2, as I am am on my second). Both of my clones showed no signs of hand fitting & I am not certain that the Emco machines were hand fitted either - I doubt that the selling price would have allowed that. The Emco brochure makes no mention or claims of accuracy & neither does the redacted Taiwanese copy of the Operator manual / parts book. No test certificate either.
Your points 1 and 2 could be improved or corrected with mild scraping & in all probability 3 & 4 could be improved likewise.
6 should be adjustable with the adjustments provided (gib strips).
5 and 7 are likely what they are and would require more expensive work to rectify ( hard chrome & regrind the quill body for a closer fit - regrind the spindle and maybe replace or re-engineer the spindle bearings).
8 I would take as par for the course with a light weight machine.
The tone of your comments suggest that you were expecting much "better", though I am not sure that any comparable machine would show "better" results ?
|Graham Meek||13/02/2022 20:06:14|
|473 forum posts|
To follow on from Nigel B's comments.
The machine I had before my first FB2 circa 1984, was a Far Eastern Turret mill, not un-like the Myford VMC. This machine was new, yet I could insert 0.25 mm feeler blades between the slideways.
The FB2 was used to re-machine the offending surfaces and the machine was then scraped as it should have been done in the first place. Considering the FB2 is not scraped it is a remarkable piece of engineering. It has done every thing that I have asked of it and more.
I could probably spend ages getting my machine better, but why, it is doing all I ask of it with no problem. The work done on my FB2 ranges from the Back Gates, (see "FB2 Earning its Keep", to Optical and Experimental work for the Medical profession.
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