Machining both sides of a part?

[itsme]

Hi there,

I have a small CNC mill, but haven't done too much work on it since I have converted it. This should be changing shortly.

What I was wondering is how I should be setting my machine up to machine double-sided parts (one-offs or maybe only a couple of each component). The machine has 3 axes and a 4th axis is not an option at the moment. I have read about machining one side and then turning the stock over to machine the other side. This sounds reasonable, BUT how do I accurately align the part once I have turned it over? Would my stock have to be accurately machined so that I can use the edge of it for a reference point? How are smaller, more complex parts normally machined when a 4th axis isn't available?

Sorry for the questions, but it would be useful to know.

Thanks
Warren

[2muchstuff]

You need a jig with locating pins to mount your parts onto. The part will have thru holes in it so that when you flip it over the same holes line up with the same pins.

[mishikwest]

cut softjaws with the profile of the first machined side of your part. part will perfectly nest when flipped, provided you do it right. Cut a "god hole" or other feature in your hard jaw that you can indicate to and use as an origin when programming the new side.

[little bubba]

Its You, that is a really open ended question. It basically boils down to what kind of part are we talking about. There are probably 6 million ways to do it, a simple square part, flip it over up against a stop. Pins that locate already machined features work pretty well. Milled soft jaws that that hold an outside profile work fairly well, and can be very accurate.

I'd ask you what type of part are we talking about, and then you will probably get some better/more specific answers.

[itsme]

Hi,

Thanks for the replies.

The question was fairly (maybe very???) open-ended, because there is no specific part that I am talking about - I just need some general ideas, since I've never really operated a CNC mill before. I think the information so far has already helped a lot. It seems that some kind of jig would be the best way of doing things. I guess I was just wondering if there was an 'easier' way out, but realistically a jig is not that hard to make with a CNC mill. In case you are wondering, most parts I will be making will be for model engines (crankcases, conrods, heads etc.).

Thanks again
Warren

[ctate2000]

Please don't be offended but cnc machining and conventional machining are the same only difference is the device turning the screws. You use the same techniques for work holding. You have to examine the accuracy desired and the geometry to determine the method for holding the part. Holes over pins is good sometimes, do you drill the holes or ream the holes? You may have bore the hole, clamp to the table and indicate the bored hole to the spindle. Next time there will be no holes or it could be an odd shaped casting. Every part will be a little different.

[fredhh47]

I have found that it is often problematic to do both sides of a part in one single operation. For instance, you have to account for the change in part zero (i.e. relative tool length) unless you pre-finish your part thickness, which adds a couple manual operations (and why did we buy CNC's in the first place?) Second consideration is burs. If you're going to flip the part, you have to make sure that there are no burs to interfere with the locating of the part. If you'e making more than one piece at a time (and why did we buy CNC's in the first place?) I often find it easier to do each side as a separate operation. If you ARE making one piece at a time, the setup and programming time will be the single largest factor in your "overhead" which could render the usefulness of the CNC rather moot. Perhaps easier to do the onesies on a manual mill with a rotary table? Just my 2 cents, based on 30+ years of CNC use.
Fred