Quality/Accuracy of Milling Machines

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Geswearf

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In my recent thread in the PM subforum, I asked about the quantitative accuracy of Taiwanese machines over Chinese and it seems this might have been the wrong question (I'm learning). Expanding it a bit, were one looking for a full sized Bridgeport Mill for a machine shop, does the buyer look for a level of accuracy or is the Brand alone all he considers. Do the full sized mill manufacturers toss around the same descriptions I see on the PM website (such as 'ultra precision, etc.) or do individual brands rely on their reputation? I understand the term bridgeport mill has come to mean a particular style of vertical milling machine and there are a number of 'clones', 'knock-offs', other brands that now are synonymous with 'bridgeport mill'.
 
I think the types of bearings used in a new machine will have an impact on accuracy, plus manufacturing tolerances, which are all above my pay grade to discuss. :)
 
Generally us hobbyists buy used machines that have already served some time in industry and are a bit worn.
For hobby use that's usually ok. Yes, the brand matters somewhat but the nature of how the machine was used and
how it was treated matter as well.
Buying a new machine you can usually expect good precision and zero wear but the cost is much higher. If you need to work
to very tight tolerance then I wouldn't purchase a used machine unless you can bring a buddy that knows how to check for wear.
Most used machines will be worn to some degree so you need to know what your requirements are.
 
It is machine/model specific, most machines come with a spec. sheet/QC showing allowable tolerances and actual machine tolerances. That alone does not necessarily tell you the accuracy of the machine. Example is typical spindle tolerance (TIR) is typically +/-0.0002" or better say on a mill or lathe, but it doesn't tell you much about the tolerance/deflection of all the other moving parts that effect accuracy in cutting and at what position. All the mumbo jumbo on wordsmithing, is just that. Machines are built to a price point, the lower that is the more likely the quality suffers.
 
Geswearf said:
Expanding it a bit, were one looking for a full sized Bridgeport Mill for a machine shop, does the buyer look for a level of accuracy or is the Brand alone all he considers.
Click to expand...
The last tool room I was near bought 6 new machines out of a catalog. They had a contract with a supplier. The supplier gave them options. The company gave them a budget. Those two things were balanced, and 6 new imported machines were brought into the plant. All were the wrong voltage. So then they blew the budget buying transformers. I don’t think any specs other than swing and bed length were considered.
 
You will find that as Mark mentioned, price point determines build quality within a range. The designs are similar and often the factories are the same among various brands. If I were buying new, I'd look for a reputable retailer who will stand behind the machine. Not every machine, or even most, are going to be trouble free or of outstanding quality so the reseller is very important. Sharp, Acra, Webb, PM, Kent, Acer, etc are all very similar so find someone who makes you feel warm and fuzzy to deal with and check how well they fix problems. A lightly used or well cared for used machine is where the real savings are. If it came to money being an issue, I'll take a good condition Taiwan machine over new China. any problems with the used machine should have been dealt with and quality control is hit and miss in the Chinese market compared to Taiwan- which is far from perfect.

Dave
 
In general I think the man behind the machine has more influence on accuracy than the machine it self. Even an ultra precise machine will perform very poor when not setup the right way. Even if this machine is setup correctly, results will be crap if it is not used the right way.

My 70 kg mini lathe can be as precise as my 290 kg lathe but on the bigger lathe it is easier to get accurate results. Larger is not always better. For watch making a true watch maker lathe would be more suitable.

I have a Chinese mini mill (130 kg) and, when milling steel, can get an accuracy of 0.02 mm if I measure often (twice per dimension). This accuracy is only the dimension that can be measured using a micrometer. I can't measure squareness, flatness, etc. So maybe I am totally off spec.

Accuracy is not really defined. It can be the diameter of a drilled hole, the distance between 2 holes, or etc.

If you can position a drill at 0.001 mm (according to the DRO) it does not mean you have a 0.001 mm precision drilled hole.

My manual mini mill is far more rigid and accurate than my CNC router even I spent 10 times more time to setup the CNC router.

From following the results of machines bought on different fora, machines that come from a company that has an actual shop (branded machines) are most of the time far better than machines that come from sellers on Amazon, Ebay, AliExpress, etc. Nevertheless, my Artillery 3D printer from BangGood is absolutely OK.
 
I googled “ultra precision bearings”, and there’s a book that’ll tell you about them. It’s $150.
I haven’t ever seen the term used with regard to milling machines except that you tell us it’s on the PM site.
Not very many hobby machinists are going to know the method by which professional machine shops buy their machines. I believe though, that they are like me in knowing that all new machine tools are of acceptable quality. So they probably focus more on capacity and price.
 
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