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Milling Table Flatness - What is acceptable

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Peter Smith 3011/05/2022 10:23:34
5 forum posts
6 photos

Hi, I have finally got around to assembling my mini mill, which originally was a lathe/mill combo. I never used the mill part as it was intended. So saw a milling table that matched exactly the one in the parts catalogue. Basic X and Y axis.

On assembly I first wanted to check that the table ran parallel. As in when you wind the table in the X and Y axis. We can ignore the column as I will tram that later.

Issue is I find that over my full range I see 0.2mm increase over 580mm of travel. In fact for the first 250mm of travel its only increased by 0.01, then from 300 mm it increases exponentially. See first picture.

The Y axis has 0.03mm increase over 200mm (I think this is acceptable?)

If I lean on the table at one end fully extended I get about 0.04mm movement.

So questions;

  1. Is the X axis acceptable for a home hobby work
  2. Is Y axis ok
  3. Is there any adjustment that can be made?

I did stumble across an article in MEW 68 October 2000 page 30 "The Vertical Mill as a surface grinder" by David Machin where he has a table that shows .003" (0.076mm) which to him was very disappointing. But table looks to be half my size.

I might have a go at reproducing what he did to flatten my mill but perhaps that might be out of my league.

Thoughts comment welcome, pictures in next post below.

Peter Smith 3011/05/2022 10:31:37
5 forum posts
6 photos

Picture of the table and measurements

[url=https://postimg.cc/8sz6BXn3][img]https://i.postimg.cc/8sz6BXn3/20220511-165452.jpg[/img][/url]

Picture of the mill in question

[url=https://postimg.cc/G4GW7Q5w][img]https://i.postimg.cc/G4GW7Q5w/20220511-165503.jpg[/img][/url]
Pete Rimmer11/05/2022 13:19:56
1233 forum posts
65 photos

First thing to do is measure the thickness of the table from the top surface to the bottom flat way at the four corners with a micrometer. If they are not all the same you're not going to adjust it out.

Peter Smith 3011/05/2022 13:34:17
5 forum posts
6 photos

Hi ok, I assume from bottom edge of the dovetail as per the picture.

screenshot_20220511_223022-2.jpeg

Pete Rimmer11/05/2022 13:55:19
1233 forum posts
65 photos

Measure from the flat way, the part that is actually in contact.

Thor 🇳🇴11/05/2022 14:11:38
avatar
1628 forum posts
46 photos

Hi Peter,

Schlesinger's limits will tell you what tolerances the industry permits, see here.

Thor

SillyOldDuffer11/05/2022 17:03:20
Moderator
8691 forum posts
1967 photos

Here's Peter's mill - made from the head and column of a combo mill/lathe plus a milling table.

The setup is flimsy compared with a real mill and I suspect everything is flexing under it's own weight. A stiffer support, such a sturdy bench or a proper machine stand, might help considerably.

The figures aren't good, but the mill is usable for small distance cutting, where the error is low, provided it's not flexing. If the problem is flex, cutting will make it worse.

It's unusual to load a mill with work as big as the table, most of what I do is over about 200mm.

I advise against plunging into measurements as the first step because they're difficult to get right. It's extremely easy to lead oneself up the garden path due to faulty measuring technique. Best thing is to try see how well or badly the machine cuts metal. If results are acceptable, job done! If not, the work piece will contain good clues as to what's wrong. Following up the clues measurement is next the step.

Don't worry about Schlesinger Limits. They apply to professional equipment, not hobby compromises!

Dave

old mart11/05/2022 17:23:11
3772 forum posts
233 photos

Hi Peter, as long as you know the limitations of your machinery, and the errors, you will be able to produce very good results. Most work is a lot smaller than the bed, so knowing the sweet spot is the best war to proceed. The drill mill at the museum had an error in the bed slots, so using a pair of keyed vises would have had a Y axis error of 0.007" , 0.18mm over the length of the slot. I milled the entire length of the rear face of that slot in three stages, swinging the round column to the left and right taking zero off one end and the 0.007" off the other, and now the vises can be spaced anywhere as a pair with less than 0.001" error. I only checked the particular slot which suited the 100mm Bison vises best.

When the museum was left the Tom Senior light vertical mill in a legacy, it was not in working order and the top of the bed was not a pretty sight. I sat the bed on four 1-2-4 blocks on the drill mill and and milled off 1.5mm,  0.059", then we spent months rubbing it by hand upside down on the surface table. The surface table had sheets of wet and dry paper stuck on with double sided tape, we ended up using 120 grit paper. Over 90% of the damage has been removed and the table is flat within 0.001" 0.025mm. This could be done with your bed, but is a real challenge to do.

Knowing the error, you could simply shim up one end of a long workpiece if needed.

 

Edited By old mart on 11/05/2022 17:35:06

Edited By old mart on 11/05/2022 17:37:25

HOWARDT11/05/2022 19:21:59
908 forum posts
39 photos

The problem is the ratio of width to length of the table support and gib. When the table is supported at the mid stroke it is in equal balance across it but as you move to full stroke the table will tend the overbalance. Also the alignment of the lead screw and nut can throw the table off causing it to swivel around as it is moved. Getting it all right is very much trial and error, tightening up the gib to strike the right balance of rigidity and free movement.

old mart11/05/2022 19:59:27
3772 forum posts
233 photos

As already mentioned, you need to measure the thickness of the bed at both ends to see if the error is there. If the thickness is the cause of the problem it would be worth visiting some small local machine shops and seeing whether they could put it right and how much it would cost. Make a drawing to show them. I would be happy with a milled surface, we had to hand rub our bed because I had no choice but to mill in 4 stages because of the size relative between the two mills.

Pete Rimmer11/05/2022 20:14:29
1233 forum posts
65 photos

If the error is in the top-to-way thickness there's nothing you can do to adjust it out. It's a very simple check you could even use a digital caliper if there's no micrometer handy. Checkig anything else before this basic check is quite pointless.

not done it yet11/05/2022 20:57:02
6809 forum posts
20 photos

It looks to me as though those run-out figures were initiated (zero reading) at one end of the table and finished at the other?

I might suspect it may be the gib that is not set ideally and the table run-out is increasing as the table balance changes, the highest reading being with the heaviest overhang at the far end.

It might be prudent, if this is the case, to do the same procedure but starting at the other end of the bed as zero - if the same increasing pattern is found in that direction, no amount of scraping of the table will sort that? If that way provides negative readings, then scraping is likely the answer.

Andrew Johnston11/05/2022 21:08:49
avatar
6601 forum posts
701 photos
Posted by Pete Rimmer on 11/05/2022 20:14:29:

....Checkig anything else before this basic check is quite pointless.

Exactly. As SOD says making meaningful measurements is much more difficult than it seems. Often what is thought to be measured isn't anything of the sort.

Andrew

Hopper12/05/2022 00:31:00
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6393 forum posts
334 photos
Posted by Peter Smith 30 on 11/05/2022 13:34:17:

Hi ok, I assume from bottom edge of the dovetail as per the picture.

screenshot_20220511_223022-2.jpeg

 

Measuring this distance with a micrometer should not be too difficult and will at least tell you where your tabletop is at in relation to the ways it runs on. It should be within less than a thou or so (.025mm) if you want to achieve that kind of accuracy in your finished job. Looks like it could be done without dismantling the machine.

If it is out by a similar amount to what you are measuring with your dial indicator, you have most likely found the source of your problem. Send it back as unfit for purpose. Or if you can't do that, take it to your nearest automotive machine shop and have them put it on their cylinder head grinding machine and take a skim over the table top surface (NOT the ways) to bring it into parallel. Or if you want to spend more money you can take it to a machine shop with a precision surface grinder.

 

Edited By Hopper on 12/05/2022 00:33:18

Neil Lickfold12/05/2022 04:08:17
862 forum posts
195 photos

There are many inherent issues happening. It is the reason that planers have a bed that fully supports the entire length of the moving bed section. You are fighting so many things at once. The best option in my view is to have some sort of support frame on either side that helps to support the main base by directly supporting the bed. One way is to have the machine mounted to a piece of thick kitchen counter top and then use a large ball support underneath. This is why I like the Deckel machine that had the solid bed on the ground, and the head was all moving off the back of the machine. I guess its still possible to have the Bed fully supported and then have the entire of the machine floating underneath it with a roller support for the column weight is another way around the issue.

Or you can ignore the bed, and place a plate on top, Mill that area to what you can and call it quits. Make sure that the ways actually do match correctly and do bed down correctly and that the gib is actually in full contact etc on what you have. A really great new to me product for slideways is the grease used on DMG machine tools for their linear slides etc. It is their 000 grease. Quite amazing the difference it had made to just the slides on my mills and lathes.

Neil

Peter Smith 3012/05/2022 06:30:26
5 forum posts
6 photos

Hi, lots of responses so that is greatly appreciated so I will try and answer most of them.

  • @Pete Rimmer, Old Mart and others : I measured the four corners with digital caliper and it seems there is a thickness increase from one end to the other. Looking at my mill as per the picture above, in order from top left 51.74m, top right 51.69, bottom left 51.99 and bottom right 51.94. I equate that to 0.20mm difference between ends. This matches what I am measuring on with DTI. The table thickness increase from top end to the end facing you in the picture.
  • @silly old duffer: Yes its not a real mill more a hobby/mini mill so not a precision machine, but I would like to try and get it close to tolerance before I started to machine. The base is on 1.5 inch cross ply, sheet of steel, then top of the workshop trolley. The base is not sitting on the wheels, its on 2inch pipe with reinforced steel. I copied the design of Robin Renzetti who built a granite surface plate stand. As well as Stefan Gotteswinter has done similar. As I am measuring the machine in static form I would not have thought this would have impact on measurements?
  • I will strip and clean the dovetail faces and gib then re-assembly in case there is gunk in it.
  • Finally it I might be chasing a lost cause here for the cost/value of the machine and just work around the uneveness of it.

Photo of how I measured it.

20220512_085300.jpg

20220512_085435.jpg

20220512_090545.jpg

DiogenesII12/05/2022 06:51:34
561 forum posts
221 photos

Peter, you are measuring across the full width of the table, including the dovetails - the bit you need to measure is from table top to the flat ways 'underneath-of-the-table-top', the flat surfaces that support the table on the shears above the dovetails... ..the dovetail bit doesn't matter..

Edited By DiogenesII on 12/05/2022 06:52:47

Andy_G12/05/2022 08:20:13
avatar
173 forum posts

It would be worth repeating the measurements with the leadscrew removed to rule out the possibility that it is lifting the table as the nut approaches the bearing.

Nicholas Farr12/05/2022 08:30:14
avatar
3360 forum posts
1542 photos

Hi Peter, as DiogenesII says, don't measure to the bottom of the dovetails, you should measure where I've marked with the red line with arrow heads in the photo below and you can see where the table makes contact with the base on the flat surfaces.

table measurement.jpg

The bottom pf the dovetails shouldn't be actually touching anything along their entire length and won't matter if they are not the same measurement at each end.

Regards Nick.

SillyOldDuffer12/05/2022 09:59:51
Moderator
8691 forum posts
1967 photos

Posted by Peter Smith 30 on 12/05/2022 06:30:26:

...

  • @silly old duffer: Yes its not a real mill more a hobby/mini mill so not a precision machine, but I would like to try and get it close to tolerance before I started to machine. The base is on 1.5 inch cross ply, sheet of steel, then top of the workshop trolley. The base is not sitting on the wheels, its on 2inch pipe with reinforced steel. I copied the design of Robin Renzetti who built a granite surface plate stand. As well as Stefan Gotteswinter has done similar. As I am measuring the machine in static form I would not have thought this would have impact on measurements?
  • Finally it I might be chasing a lost cause here for the cost/value of the machine and just work around the uneveness of it.

Hi Peter, no disrespect intended of what you've done! However, the first requirement of an accurate machine tool is rigidity - more the better. For practical purposes, it's always a compromise, because the simplest way of achieving it is to pour several tons of cast-iron: big machines are better than small ones.

Any milling capability is better than no milling capability! And as most of us are constrained by space and money, how close we get to owning the perfect mill is usually sadly compromised. From worst to best, my list is:

  1. Milling vice on a small lathe. (Wobbly, tiny travel)
  2. Milling vice on a big lathe. (Stiffer and more travel)
  3. Combination Mill lathe. (Better than a milling vice, but limited space and travel compared to a purpose built milling machine.)
  4. Small hobby mill.
  5. Large hobby mill (like my WM18)
  6. Medium professional mill of which the Bridgeport is perhaps the star example. By my standards a Bridgeport is wonderful, but professional users sometimes complain they're a bit flimsy. It's all relative!

The typical hobby mill configuration is functional, but relatively light. Picture stolen from Warco - a WM18, which is about as big as this layout can get before the head lift wheel is too high to reach!

wm-18-variable-speed-milling-machine.jpg

This type of mill is pretty spindly compared with a stubby heavy Bridgeport, which also comes with stiffer design features such as the knee.

bridgeport-104044-mill-2.jpg

Nonetheless, all these milling devices are capable of good work, it's just that the operator has to live within their limitations.

My guess is your mill will perform somewhere in the small to large hobby range. Stefan-type improvements will push the performance up, but the machine can't be perfect. Doesn't matter provided the mill cuts metal to your satisfaction!

On the subject of measurement, the more I do the harder I realise it is to get right! Looks simple, but there are a multitude of details liable to lead the junior metrologist deep into the mire. I use a DTI as a comparator to tram my mill over a sweep of about 350mm and to square the vice over its jaw width. I don't take actual measurements. All I can say is the tram is as good as I can get it based on the needle of an inexpensive DTI. Approximately ±0.02mm over 350mm, but I wouldn't bet the farm on it. The real test is how well a fly-cutter performs flattening a plate, which after much fuss and bad language during tramming my mill does satisfactorily. A shortcoming of my mill's simple design is tramming is quite fiddly, so I avoid disturbing it. (A forum suggestion which I haven't tried yet is to tram this type of machine by pushing the spindle down onto a square plate so the head squares itself and then tightening the fiddly bolt.)

In my experience most lathes and mills get to about a thou / 0.02mm without too much fuss and can hold that provided the operator respects the machine's limitations. Getting to about 0.01mm is doable, and, depending on the machine and operator, it might be possible to stretch down to 0.0025 by taking significant care. Unfortunately, achieving ever higher accuracy becomes exponentially harder. The average machine and cutters can't hold high levels of accuracy and the operator can't measure it!

I don't think what's been done is a lost cause unless the goal was super-accuracy, above and beyond what most home workshops are capable of. My advice, set the machine up as best you can and test it by cutting metal. If faults appear, use measurements to pin the causes down, and proceed from there. Avoid the temptation to measure without real-world cutting, because there's a high risk the measurement will be wrong or misleading. Even experts get it wrong!

Dave

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