Unable To Hold Diameters

[random_guy]

Hello all,

I have been trying to machine a cylindrical part on my cnc mill and then thread mill it, but have been unable to hold correct diameters, which causes my threads to be either too loose or too tight. Hopefully you can help.

As for the details:

I'm using a 1 HP syil, 3 axis mill in conjunction with hsm express and solidworks. I use Gwizard for my speeds and feeds.

material is 4140 but I have had this problem with aluminum too. The part calls for milling the cylindrical shape then using the single form thread mill to cut the threads. On the other end of the part I perform a boring operation and then use the single form again. While modeling I look up the recommended min and max thread dimensions in machinery handbook.

Material: 4140
Spindle speed: 5000 rpm
Feedrate: 12 in/min for the external cuts and 5 in/min for internal boring
Cutter: .375" 4 tooth flat carbide endmill with 1.5" stickout
Cut Depth: .0375" for the external cuts
Finishing Pass: .005"

My cuts always come out under or over sized by multiple thou, any ideas why?

 

[T Bredehoft]

Can you determine if it's end play in the lead screws or error in the programming, or perhaps a combination of these?

 

[joshua43214]

Could you provide a bit more detail on the machine please?
Charter Oak sells some different varieties of the Syil based around Sieg mills. My understanding was that these machines are meant for engraving and the aluminum is about the hardest material you can put on it. The CO website does not provide any real details about the machine such as stepper vs. servo etc. 0.0375" sounds like a pretty heavy cut for a 1hp machine if it is a stepper system with no closed loop, especially if this is based on a x3.

Either way, if you are having unexplained problems, the best thing is to back all the way up to the basics.
Have you double checked to make sure that the gibs are all properly adjusted, and the ball screws are properly adjusted?

Do you have dimensional errors when making rectangular parts as well? Are round parts actually round, or oval?

 

[derf]

Have you actually measured your cutter with a mic? What it actually measures and what you told the machine may be different.

 

[random_guy]

Can you determine if it's end play in the lead screws or error in the programming, or perhaps a combination of these?

The machine lead screws are pre-loaded to reduce the backlash if that is what you mean. Also I don't believe it is the code. I manually followed the code and it seems to be outputting perfect arcs. The machine was recently switched from serial port to smooth stepper. Could there be a problem transmitting coordinates there?

Joshua,

The machine is based on an x5 so I think it should be able to handle the steel. I am not receiving dimensional errors on rectangular parts as I calibrated the steps and after that was getting linear cuts good to well below a thou.

Derf,

The cutter could be the issue but I measured it and it seemed to be spot on. I just ordered new mills so will get back to you when I rule that out too

 

[derf]

Sometimes even if the tool diameter is spot on, you get some tool push off, from the cutter being slightly dull or maybe the cutter is not quite right for the material. When I have this problem, I usually go back and redefine the tool diameter, essentially lie to the machine about the diameter of the tool until it comes out correct.

 

[stupoty]

Is that a bit of a hih rpm for steel at that size cutter? I'm less good at carbide speeds off the top of my head

How many thousands are you out? You say multiple, is that like 3 tho or 33 ? If it's closer to 3 could the size variance be from the part heating? As i notice you didn't mention coolant.

If it's getting warm during the machining you could try pausing the program between the roughing and finish pass to see if that helps a bit.

Stuart

 

[TommyD]

Was it capable of repeating before you changed it over?

 

[Wobbles]

You probably already know this, but I will re-state the obvious....

You shouldn't cut to the thread diameter by purely reading the tool setting. Since tool deflection comes into play, several passes without changing the cutter position are required. Each successive pass taking a smaller and smaller cut (as the tool deflects less and less) until the correct size is achieved. "Correct size" is usually better tested with the actual mating part, rather than a gauge or thread mic.

The thread sizes in SolidWorks are in tables which you can modify. You might look at temporarily changing those to bring you within .005", and then allowing you to make the last 3-4 passes with very small or no adjustment. You don't have to accept what SW gives you.

Hope this helps.