Accuracy issues with an Excellon Uniline 2000

[sam.price]

Hello

i`m a trainee electronic test fixture manufacturer from Liverpool currently working with an Excellon Uniline 2000 for drilling purposes.

I`ve been having a lot of problems lately with accuracy issues i have to drill through 15mm fibre glass material (or G10). because the machine runs with an ABW110 spindle which is air bearing i have to drill through this material in two runs one 8mm through the top face, then flip the material and drill through the remainder of the material from the back.

The problem im having is the holes i am drilling (usually 1mm/1.4mm/1.75mm wide holes) have a slight step in them and the holes are tight this suggests to me the holes haven't quite met in the middle after flipping the material.

I have spoken to an expert and he seems to think it could be something wrong with the tooling strips i use to hold the material in place, they are very worn and could be causing the problem.

I have included pictures of the machine i`m working with, the bed of the machine which quite clearly has seen some wear and tear, the tooling strips which have also seen better days and one of the holes i have drilled with a very obvious step in the middle. the pictured hole is not a small diameter but is a 19mm hole which is drilled on the machine out of hundreds of 4.35mm holes.

[EddyCurrent]

I think it's your method in general that needs refining, watch some of these videos; http://www.youtube.com/results?search_query=2+sided+cnc

[sam.price]

I don`t think it`s the method im using its a method the company have used for years and its always worked. the job is drawn up in AutoCAD, the same job is then mirrored in AutoCAD and saved as a separate file. Both files are then converted into Excellon drill files. the first file is drilled on the top face of the material and the mirrored file is drilled on the bottom side of the material, the mirroring of the drawing can`t be incorrect in AutoCAD so it must be something on the machine

[JAZZCNC]

Tend to agree with Eddy.

Part Fixturing needs to be perfect when flipping and the slightest error will be highlighted like your seeing. This is made worse in your case because your drilling which in it's self is not an easy thing to do with great accurecy without very carefull attention to detail. Really if you want accurate thru holes then you should be drilling under size from one side or both and then reaming thru from one side.

Personally I would make a Fixture Jig for the part when flipping and fasten the pre drilled side to the fixture jig with the drilled holes using short alignment pins. This way your only working from the fixture plate and because part is located by the holes and hole locations always known by referencing from the fixture plate it's much easier to align and get accurate. The pins in the Jig align the part perfectly and this will give you the best chance.

The Type of drill, length etc along with pre drilling etc and materail changes due to heat etc will all come into play so drilling accurate holes isn't an easything at best of times and with flipping then all new dimension comes into play.!!