Hello,
We have problem in controlling the dimensions (Od & ID) on lathe machines. We are doing plant maintenance and some of the components have close tolerance of 10 microns (may not close for many people). So some times technicians have to remove 5 or 10 micron in the final cut to get the exact tolerance. But this will ruin the surface quality of the part or may cut extra material (Due to the repeatibility factor/ mechanical accuracy of machine).
Can anybody suggest a better method to control these dimensions? Any possibility to add an attachment that takes care ID and OD? We have lathe that can machine 5mm to 2400mm dia.
Please help.
A 2400mm capacity machine, you're wanting to hold to 10micron (.0004")....and ......"Due to the repeatibility factor/ mechanical accuracy of machine".....Originally Posted by yesornoblr
I'm thinking that pretty well sums it up. I feel sorry for the poor schmuck trying to hold 4 tenths with a 16micro on a 60" OD part using a Mahindra.
1) most lathes can only hold about a 0.02mm tolerance. a heavy cut causes a spring push back on tooling and part where just repeating a cut and not adjusting anything more metal will be cut.
2) i use at least 2 finishing cuts after rough cutting a part.
3) false cutting edge is a buildup of material on cutting edge from metal sticking to cutting edge that can effect tolerances. cutting fluid and carbide insert coatings can help with this as well as certain speeds and feeds.
4) all metal expands and contracts with temperature changes. many a part that measured perfect when warm or hot is undersize when cooled back to room temperature. cutting fluid is another way to better control part temperature. Room temperature also effects part size and can also effect DRO or digital readout calibration. machine components change size too as a machine warms up and or being covered with hot chips
5) some parts have a fuzzy finish and sanding the part with a strip of fine grit abrasive paper, cloth will remove the fuzz. this rough surface fuzz can measure a part 0.01 oversize but quickly wear down or burnish smaller. rubber abrasive sticks held like a pencil eraser can also be used to improver surface finish and remove fuzz
6) grinding a part on a lathe is the only way i know to reliably remove less than 0.02mm amounts
7) a dirty machine can effect precision movements and eccentricity of part in a 3 jaw chuck, many a tolerance problem is also made worse by machines not being kept clean enough
actually if you manage to hold 0.05 mm tolerances all day long on a lathe most would consider that not bad
Sounds like you would have to grind in the final dimension. Also depending on the finish as well.
I for one am getting very bored with these requests for help..then no followup at all. No thanks, no input, no nuthin'.
The guy doesn't mention the size of the part, material, tool material, machine...
I'm for boycotting responses unless there's a prompt reply from the originator...so we know whether it's worth wasting time with the question.
Thats not entirely the way to go. I was cruising thru some threads, found this one and it spurred me to look into building a small grinder for my lathe. So just because the original poster never comes back at all, doesnt mean that someone else may not find the answers usefull.
This thread helped me to understand why my dials are a little off on my lathe now compared to 5 months ago. 30 degrees F now compared to the 80 to 90 F a short couple months ago.
Dear All,
Thank you very much for the reply.
I agree that I have'nt replied but that does not mean that I am not reading or considering your valuable comments. I am new to this and that could be reason for your complaints.
I don't want to take this part on a grinding machine. It does not require a grinding surface quality or tolerance.
We have many components (mainly steels ST32, ST 52, 42Cr etc) where the tolerance has to be controlled within 10 to 30 microns. The quantity of the component is sometimes just a single piece. This force the technician to take extra care while doing final finishing. So to make sure that he wont cut extra material, he keeps some allowance (0.1mm on OD)and machine it. Then he check the dimension and add this in the wear allowance (on CNC machine) and machine it again. But sometimes, he found that the final dimension is short of 20 microns. If he cut this 20 micron, the surface quality is spoiled. This is the problem.
We have different lathe machines (manual NC &CNC) and one of the largest machine have a capacity of 2400mm dia.
As Mr. Fixittt mentioned, I am also thinking about some portable grinding unit that can be fixed on these machines.
Any other ideas please.
Im by no means no expert but try to make the final 2 cuts close to the same depth if possible. Maybe check the diameter and leave .2mm so you can take a .1mm pass then recheck and take the rest. If you are taking a big cut then measuring and then taking a smaller cut the force pushing back on the turret wont be the same and result in too much material being taken off.
I'm with sandrewb on this - you need to take cuts of the same amount each time. When I'm cutting for high precision, I try and rough-turn to get the part to be the final dimension, plus a whole number of cuts. (With my tipped tooling, real cuts .25mm upwards, not tiny skims, so the tip of the tool is properly cutting, not rubbing). Then with each pass, I re-measure and can fine tune the next cut depth by a small fraction so that by the time I reach the final one, I'm confident I will hit the target. I can also be confident that the surface finish I'm achieveing will be matched on the last pass.
I agree the method by sandrewb. But just in case if I am short of 10 or 20micron to the final dimension, how do I get the dimension with good surface?
See post #3 item 6).
Tool post grinder rather than moving to a grinding machine?
Dick Z
DZASTR
Well .0004 or 10 microns would be tough to hold on a large machine, but do-able.
If you really want to do it, don't think a tool post grinder is going to help much.
You need to get the machine checked, laser calibrated, alignments etc.
Then you will need a temp controlled shop. A machine that big is going to move all over the place with temp swing.
Then you need to get a coolant chiller to keep the coolant at ambient temp.
This is not to say you can't do one off parts with this tolerance without the above, but if you want to hold that tight in production, its required i'm afraid.
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