Questions about machining tolerance and time

[Tartof]

Hi i am trying to figure out what cutting machines can do in tolerance and time, so that i get a better felling for the cost and to sort out some confusion

1.In an CNC machine are there any difference between the output tolerance between making a surface of 500x500 flat and the drilling of four holes 500x500 from each outer.

2. If i am going to make a surface of 1000x1000 mm^2 flat, who much in time does in vary between the following planarity tolerances, 1 0,1 0,01 in a feature control frame

Thank you in advance

 

[KENAT]

Tartof, believe it or not you're probably not giving enough information for much of an answer.

On the first question, yes I'd expect a significant time different between machining a surface flat over 500*500mm and drilling holes in each corner. Of course you don't say how flat, what surface finish, how thick material/how deep holes, what size holes, what material etc.

On the second question, you may find that for the tighter tolerances you may need to go to a different process. Again you don't give material information, surface finish etc.

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[CheckerHater]

It also may be that desired surface roughness will actually affect your cycle time more than tolerance, because it will require slower feed rate.

Just a thought.

 

[KENAT]

Sorry, when I said surface finish I meant surface roughness.

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[Tartof]

For question 1 my thought was that the head will move the same max distance from origo in both cases. I assumed that the error in accurazy was an function of how long the tool head needs to be moved. Maybe that wasn't such a well thought out question. Anyway what i was looking for was more some general design guidelines for cost in relationship to diffrent tolerances when working mainly with steel.

 

[KENAT]

Yes it travels the same distance, but is it realistic to travel at the same speed when doing significant work compared to just traveling through air? Feed rates come into it which relate back to material, desired surface finish, amount of material removal...

For general machining tips there have been a few posts over time, I suggest a site search may turn up something. I got these results as the first few hits using site search.

thread404-163892
thread404-278305
thread1103-197339

http://www.eng-tips.com/viewthread.cfm?qid=284781

There have been others, some relating to specific materials/processes such as sheet metal but my search kung-fu is week today for some reason!

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[looslib]

The head will not move the same distance in scenario 1.
For drilling, you will be moving the head to 4 points, so the distance calculation is easy.
For milling the surface, you need to account for the size of the milling cutter. In order to do 500x500, you would need a 250 diameter cuter to do it in 2 passes, but the travel distance is only 250 for the stepover, instead of the 500 for the hole distance stepover.
As Kenat pointed out, drilling holes use a rapid feed between points and milling speed has other factors to consider: material, cutter material, rigidity of the workpiece, surface finish, etc.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[KENAT]

Sorry looslib, I was assuming a really big cutter dia, honest;-)

Doh.

More realistically/typically if using say a 25mm cutter then yeah you're looking at 20 or so passes simplistically so 5X the traveled distance for just the corner drilling.

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[looslib]

If the cutter is big enough you can do it in 1 cut. just have to allow for lead in distance to get to the full width of the plate.
Realistically, any cutter over 100-125mm in diameter is hard to load in a CNC machine with tool changer. A 500mm+ diameter cutter would require manual tool change.


"Wildfires are dangerous, hard to control, and economically catastrophic."

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[drawoh]

Tartof,

This is not really a dimension and tolerance question. You would be better off asking this in forum281.

--
JHG