[zaebis]

Folks,

Been considering getting one of those knee mills CNC 3 axis converts BP or the like of it. I only have experience with desktop CNC mills (a-la Tormach) and I am not impressed at all with their precision and travels. In view of that I would like to ask those who worked with knee mills converts their take on;

1. accuracy. Can I reliably step by 0.0001" forth and back on X and Y. If not what can I expect?


2. spindle power. With most having around 3hp is it safe to assume that it can give me honest 3hp worth of performance based on machine hp requirements calculations for cutting? (not the case at all with desktop CNCs)


3. rigidity. Most having about 3500 lbs of cast iron in them plus swivel head (which I believe takes away from rigidity) how do they fare in mild steel?


4. Dovetails / Boxways. Which is better for rigidity and accuracy?

5. Once CNC converted is it practical to use them in manual mode given the tight ballscrews?


I understand that some conversions are better than others so let me know which ones you dealt with/recommend/dont recommend.


Thx.

[HawkJET]

What do you want to do? These questions are not very specific. Machine accuracy, resolution, cutter deflection, ambient air temperature, rigidity, material, feedrate, etc., etc., all affect the accuracy of the part.

Bridgeports are usually 2 HP not 3 HP (older ones are 1 1/2). What are you doing that you need "3hp worth of performance" (and what does that even mean)?

A standard Series I Bridgeport weighs about 2000 lbs. A Series I Bridgeport CNC (with BOSS control) weighs about 3000 lbs. The CNC is available with a rigid ram. What machine are you talking about?

The task at hand and budget will define the machine you need - what is the task at hand?

[SCzEngrgGroup]

The answer to your first question is a resounding "No F'ing Way!". *NO* machine will give you that kind of accuracy without being in a thermally-controller environment, AND having linear encoder feedback on all axes, AND being VERY carefully calibrated, AND being programmed and operated by someone with many, many years of experience. Thermal expansion alone will take you FAR out of that range, even on a much smaller machine.

What do you think you'll be doing that would require such precision?

Regards,
Ray L.

[HawkJET]

The answer to your first question is a resounding "No F'ing Way!".

Agreed! I was just trying to sugar coat it somewhat....

[a1diesinker]

The best you can hope for is +-.002
Take a big cut and the head will rotate on the swivel base.
I used one for years to make crankshaft electrodes
The only plus is you get a large amount of X travel a a cheap price

[Malzra]

I'm restoring a BP Interact Series 2 knee mill for my small hobby machine shop and I've put a AJAX CNC KIT in it and I'm quite happy with it. Keep in mind you'll have to talk to Support a "few" times to work out the bugs.

My interact has Boxed ways and a BP Mill repair guy (They Exist!) when he came out to install all new head gears for my mill was telling me the virtues of boxed ways. I see that even new Haas mills use Boxed ways.

The interact series 2 is a monster of a machine weighing in at 5,500 pounds when I got it (It broke the supports of my 7000 pound trailer) but it uses a rigid ram. Even this monster of a machine only has a 2hp motor

and for accuracy a1diesinker pegged it

and manual mode can be achieved if your willing to step outside the box alittle from a conventional machine.

just my two cents!

[zaebis]

Ok folks bear with me.

I am referring to positioning accuracy, not cutting accuracy. I understand that to cut accuratly many other things come into play.

Just the other day I went to my friend's shop where I got my hands on used and abused VF2. So I put a dial indicator in the spindle and put 123 block on the table. Made sure the pin of a dial is parallel to X and used MPG to step 0.0001 left/right while pushing against 123 block. I see no backlash at all on the dial. I could make 5 x 0.0001 steps on X left and then 5 steps right and dial each time would register 0.0001 increment. Also I could interpolate 3" hole with 0.2" endmill to be as concentric as bored hole.

When I use my desktop CNC (a-la Tormach with steppers and P4) I could more or less reliably step by 0.0004". And I consider it to be a hobby CNC at best.

Are you telling me that BP convert will be 0.002" at best in positioning?

The parts I am going to make are mostly one offs but will need to be produced within 0.001". What I am hearing is that it is not possible!?!

[HawkJET]

You need to learn the terms. What you are talking about is backlash.

You may also be talking about repeatability or even resolution.

Accuracy is something else. If you are getting the results you said, that machine is IN NO WAY ABUSED!

Your term "concentricity" is used incorrectly. The term you described is roundness. Concentricity is comparing the relation of the centerline of two different diameters.

Since you are using incorrect terms, you are getting answers to questions you didn't ask. And you are not asking the questions you think you are.

You are also confused about the weights and horsepower of a Bridgeport. It is difficult to answer questions when it appears you don't really know what you are talking about.

Again, the answer you are looking for depends upon what you want to do.

TELL US WHAT YOU ARE TRYING TO DO!

TELL US WHAT MACHINE YOU ARE TALKING ABOUT!