Rough cuts first?

[RSG]

When I machine complex parts, I handle the roughing operations pretty much the same way I handle the part in finishing. That means I'd also hold the part as you suggested, but the round diameter would not be finished to size--just roughed. When the part was roughed completely, I'd then start over, machining the round diameter first, so it would be used in fixturing, just as it was in roughing. The difference, now, is that the parts should have undergone the vast majority of change that can be expected, so the end result is all diameters and faces remain as they are machined.

H

So as to not tread on another persons thread I thought I'd start a new one regarding roughing out a part prior to finishing.

For those that don't already know I make fishing reels, fairly complicated ones I'd like to think, with tight fitting matting parts and so on. I do this without taking roughing cuts while working on the lathe, my friends at a local machine shop have always used the practice and told me to do so as well. Harold and Glen W here indicated they do too. Perhaps it was from inexperience but that stems from mainly one reason, when I tried it I would get high frequency chatter so bad on certain thin walled sections that it made both the finish and dimensions change. Moving forward many years now I supposed my ability is better but have not bothered to revisit the practice and have found the outcome of my work to be more than satisfactory despite not using the practice. Currently I can reach for any part on the table and assemble a reel without having to try to mate parts together due to not fitting and it doesn't matter how many reels I'm making.

So I have to ask, would revisiting roughing out really add to accuracy of my parts or would it only add to the time.

What does everyone else do?

[Magicniner]

Roughing cuts are to remove a large amount of material quickly before finishing, if they cause issues with flex or resonance it's either not appropriate for the job or alternate tooling/techniques are required.
I make a Titanium part which reqires a 2" length of 1/4" material to be reduced to just over 1/8", I do that job in one pass with a Tangi Flow Roller Box.

[Harold_V]

So I have to ask, would revisiting roughing out really add to accuracy of my parts or would it only add to the time.

I think if fair to make mention that many of your surfaces are cosmetic, and do not create issues if they vary. I'm not discounting the fact that some are critical, and must be within limited parameters.

I hope to not muddy the water here---as you have raised valid points. I just want to make clear the idea that material moves. It moves when it's heated, and it moves when it is changed by machining, in particular, when removal is not uniform. That the movement creates issues with desired parameters, or not, should determine whether you should, or should not, rough. It obviously isn't always a requirement.

One of the best possible reasons to rough parts is when you have tight tolerances to hold. As roughing is generally done with greater feeds, with much larger depths of cut, your tooling often suffers, to say nothing of parts that are heated well beyond ambient temperatures. Holding tight tolerances on parts that have been roughed is much easier than on parts that have not. They are generally back to ambient temperature, and machined with new cutting edges, which have not been subjected to rough cuts.

One thing that should be considered. Your work is with aluminum. While it's subject to the same issues as other materials, it is also relatively free machining. For that reason, your tooling doesn't suffer. Were you to make the same parts from other material types, even mild steel, I think you'd experience issues that would be troublesome. Try making the same pieces from stainless, for example.

To be clear, if what you're doing results in the success you hope to achieve, there is nothing wrong with your approach. However, if you had to work to tight specifications, where there were callouts for flatness, and perpendicularity, and your parts were inspected accordingly, you may discover that surfaces that are functional, and will assemble without issue, may not conform to the specifications. Not suggesting they do not, just that, without the roughing operation, the parts are subject to movement that you can't control.

Roughing parts before finishing is akin to surface grinding by constantly flipping. It's an approach that permits accomplishing a difficult task, that, otherwise, most likely would not be successful. It may not always be required. Use the procedure when you experience difficulty. It works. And, while it may add a little time to machining, it's still faster than doing a job more than once. Look at it as insurance.

H

[RSG]

Thanks for the replies guys!

Harold, I think your answer sums it up for me and I'm happy to continue doing things the way I am as it pertains to my lathe work. I would agree there aren't many tight tolerance components requiring detailed attention accept for the bearing journal which I do follow a very strict process to ensure the right fit.

As for my mill work, I do roughing work first and for good reason. This is just one example from my past project where the black parts had to fit tight. Once the work was complete I made sure to protect the mating surfaces from further disruption so as to maintain the tight fit.

As shown here