At my company we have a bridgeport style 3 axis mill, its a KRV2000 converted to CNC by XYZ Machine Tools and fitted with a Prototrak Control with an R8 spindle. Quill OD is around 3.5” 86mm.

I've been fighting the thing since I started working here due to its piss poor rigidity. Normally we just opt for the smallest cutter that won't make the job take forever and run conservative feeds but even then our tool life is poor.

At the moment we have a program going through to upgrade our mold tools that requires me to cut 4 pockets 120 x 240 x 90 mm (4.72" x 9.44" x 3.54") per tool, so getting the cycle time as short as possible is important so the tool room isn't tied up for weeks. The 90mm 3.54” depth is probably the biggest obstacle because of the length of end mill we therefore have to use.

The ER32 collet chucks we usually use are a complete no go here, with 70mm 2.75” odd gauge length + 110mm 4.3” outside the holder cutter length the overall length is floppier than a stream of piss in the wind. Using a 20mm 0.787” R8 collet and the manual draw bar so it has the clamping force to actually hold on to the cutter I've managed to get it up to 1000mm/min 40IPM, 1mm 40thou step over at the full 26mm 1” cutting length without horrendous chatter, but its only just behaving itself. If i cut with less than the full flute length the chatter is completely unworkable.

See video of test piece before attempting to cut expensive bolster block.


Enough of the background.

I need to up my machine rigidity. BADLY. My machine vendor sells a 30 taper spindle conversion kit which I am strongly considering. The tool holders are typically 50mm 2" gauge length which beats the 70mm 2.75” of the ones I've got now.

What I need advice on is specifically, what is the biggest weakness in my spindle, is it the bearings themselves, the amount of stick out, or the R8 collet. Would I be better off sticking with the R8 and the manual draw bar because it keeps the stick out to the minimum, or converting to a 30 taper. Of course the collet holder is putting all that extra length into the R8 taper, would it be fine going into a 30? I'm just not sure which is the weak link.

If there's any other tricks people know to get a more rigid setup out of a bridgeport I'd love to hear them.

Can't change the machine for a VMC / bigger machine because we don't have the head room in the tool room ceiling wise.

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