Accuracy problem since fitting DRO

[fizzy]

It's driving me mad!
I always boast how accurate my lathe is. I can turn to within a few tenths easily.
But I always used to do it by creeping up on the diameter and using a micrometer.
Now I have my DRO fitted.
The lathe handwheels match the movement of the DRO perfectly. Very impressive.
But when I take a cut I don't get what I am expecting. I dialled in a 5 thou cut. I got 12 thou off the diameter. It should have been 10. I then tried a 10 thou cut and got 26 . How?
Centering the dials and dro against the work so slack taken up. Using a live centre so it's not deflection.
Maybe I should just disconnect the cross slide. The long axis seems far more usefull

 

[roofman]

It's driving me mad!
I always boast how accurate my lathe is. I can turn to within a few tenths easily.
But I always used to do it by creeping up on the diameter and using a micrometer.
Now I have my DRO fitted.
The lathe handwheels match the movement of the DRO perfectly. Very impressive.
But when I take a cut I don't get what I am expecting. I dialled in a 5 thou cut. I got 12 thou off the diameter. It should have been 10. I then tried a 10 thou cut and got 26 . How?
Centering the dials and dro against the work so slack taken up. Using a live centre so it's not deflection.
Maybe I should just disconnect the cross slide. The long axis seems far more usefull

When quality machinery meets Chinese add ons...never owned one myself but would like one, is it a calibration issue?

 

[Pete.]

Sounds like either the tool is springing or the cross slide is rocking if it's worn. When you're doing the heavier cuts it pushes back more against the tool. As you creep up using smaller cut there's less load against the tool so you cut the amount you dialled in plus the difference in spring between the heavier cut and the lighter one.

Do this as a test:
Aim for a diameter by taking off 50 thou cuts. Say you start at 1" and you are heading for a final diameter of .700", take a 50 thou cut and measure. If you now have .910" you put in 50 thou of cut but you sufffered 5 thou of spring which means your diameter is 10 thou big. Go for another 50 thou cut you should be at .810" (because you added 50 thou cut but have the same spring). Go for a 55 thou cut now and you should be at .700" (because you still have the same amount of spring but now you've removed 10 thou extra from the diameter in your cut).

You should be able to hit the final size dead on once you dial it in with a bit of practice plus the heavy cuts will get you a beautiful finish on good steel whereas a light cut will leave it rough. If 50 thou cuts are problematic try 25 thou then you get twice as many steps to dial in the final cut to size.

 

[Pete.]

When quality machinery meets Chinese add ons...never owned one myself but would like one, is it a calibration issue?

I would have said the same - it might have been that if the dials didn't agree with the DRO. Since the dial and DRO agree it's very unlikely.

My DRO has a correction factor for the scales that is easy to apply by mistake when using one common feature. It got me confused for a while.

 

[Brad93]

Sounds like either the tool is springing or the cross slide is rocking if it's worn. When you're doing the heavier cuts it pushes back more against the tool. As you creep up using smaller cut there's less load against the tool so you cut the amount you dialled in plus the difference in spring between the heavier cut and the lighter one.

Do this as a test:
Aim for a diameter by taking off 50 thou cuts. Say you start at 1" and you are heading for a final diameter of .700", take a 50 thou cut and measure. If you now have .910" you put in 50 thou of cut but you sufffered 5 thou of spring which means your diameter is 10 thou big. Go for another 50 thou cut you should be at .810" (because you added 50 thou cut but have the same spring). Go for a 55 thou cut now and you should be at .700" (because you still have the same amount of spring but now you've removed 10 thou extra from the diameter in your cut).

You should be able to hit the final size dead on once you dial it in with a bit of practice plus the heavy cuts will get you a beautiful finish on good steel whereas a light cut will leave it rough. If 50 thou cuts are problematic try 25 thou then you get twice as many steps to dial in the final cut to size.

pete don't you mean .820?
if youve got 5 thou spring

 

[Pete.]

pete don't you mean .820?
if youve got 5 thou spring

Nope you get the same spring for every cut of the same depth and it's not accumulated, once you've put the first cut in and established the sping in the tool for that depth of cut it's then negated for all subsequent cuts so long as all of your cuts are the same (which is what allows you to dead-nail your finished depth using heavy cuts). You can add/subtract a few thou without affecting it, to make lining up your finish pass easier to figure.

I got angry from over-shooting on light cuts many times before I learned this.

 

[Pete.]

I'm guessing that before Fizzy fitted his DRO he used many more cuts on the dials to approach his finish size but after fitting the DRO it gave him the confidence to throw in some heavier cuts to horse the material off then put in a light pass to finish. That would explain his troubles perfectly.

 

[Brad93]

Nope you get the same spring for every cut of the same depth and it's not accumulated, once you've put the first cut in and established the sping in the tool for that depth of cut it's then negated for all subsequent cuts so long as all of your cuts are the same (which is what allows you to dead-nail your finished depth using heavy cuts). You can add/subtract a few thou without affecting it, to make lining up your finish pass easier to figure.

I got angry from over-shooting on light cuts many times before I learned this.

I don't get that.

You've taken your cut at 50 thou. The part is deflecting your tool 5 thou so you end up 10 thou under.
If you take another cut at 50 thou again the part still deflects your tool 5 thou, so now your 20 thou under?

 

[minimutly]

I don't get that.

You've taken your cut at 50 thou. The part is deflecting your tool 5 thou so you end up 10 thou under.
If you take another cut at 50 thou again the part still deflects your tool 5 thou, so now your 20 thou under?

No you don't. As long as you just add the next cut, the amount added is the actual depth - unless you cut on the return or zero the cutter to the work.

 

[fizzy]

I'm guessing that before Fizzy fitted his DRO he used many more cuts on the dials to approach his finish size but after fitting the DRO it gave him the confidence to throw in some heavier cuts to horse the material off then put in a light pass to finish. That would explain his troubles perfectly.

Yup spot on!
They should sell 1 axis kits for lathes No doubt I will get used to it.
Fiddling with too many different things at once doesn't help either.
I should learn to finish one project properly before starting another.

 

[tigdlo]

can you set the dro to read the diameter of the workpeice? If so take a cut, measure and reset the dro, this should negate tool deflection but your final 2-3 cuts need to be the same.

 

[fizzy]

I would have said the same - it might have been that if the dials didn't agree with the DRO. Since the dial and DRO agree it's very unlikely.

My DRO has a correction factor for the scales that is easy to apply by mistake when using one common feature. It got me confused for a while.

Yes they are spot on. Most impressive.
Once you have taken out the backlash of course. Even though the dials are graduated in thou I can get with a tenth or spot on every time.

 

[Pete.]

I don't get that.

You've taken your cut at 50 thou. The part is deflecting your tool 5 thou so you end up 10 thou under.
If you take another cut at 50 thou again the part still deflects your tool 5 thou, so now your 20 thou under?

No because the first cut left 5 thou on the part due to deflection. Add 50 thou on the dial you're effectively making a 55 thou second cut. Allow for 5 thou deflection again and 50 is taken off.

 

[Other Daniel]

Yup spot on!
They should sell 1 axis kits for lathes No doubt I will get used to it.
Fiddling with too many different things at once doesn't help either.
I should learn to finish one project properly before starting another.

The story of my life

 

[Agroshield]

Do this as a test:
Aim for a diameter by taking off 50 thou cuts. Say you start at 1" and you are heading for a final diameter of .700", take a 50 thou cut and measure.

There should be a small caveat with this advice insofar as your starting diameter and final diameter should not be too far apart.

As the stock diameter reduces, unless you alter the rpm (and no-one ever does), the sfm reduces. Thus, the cutting characteristics will also change.

So do not start with something 2" diameter, do your calibration on the first cut and expect it still to be valid finishing at 1/2" diameter.

With that set up, your sfm will vary by a factor of 4 from start to finish and that is too much.

I have never seen it mentioned, but picking a number out of the air, perhaps start your calibration at 20% bigger than your finished diameter (i.e. 0.600" diameter for 0.500" finished, or when you have 0.100" still to go). An alternative way to do the same thing is to look at how many equal cuts are remaining until you finish and start the process when there are three or four to go.

 

[northwest]

It's driving me mad!
I always boast how accurate my lathe is. I can turn to within a few tenths easily.
But I always used to do it by creeping up on the diameter and using a micrometer.
Now I have my DRO fitted.
The lathe handwheels match the movement of the DRO perfectly. Very impressive.
But when I take a cut I don't get what I am expecting. I dialled in a 5 thou cut. I got 12 thou off the diameter. It should have been 10. I then tried a 10 thou cut and got 26 . How?
Centering the dials and dro against the work so slack taken up. Using a live centre so it's not deflection.
Maybe I should just disconnect the cross slide. The long axis seems far more usefull

This only comes with experience. You will get to know your machine and how much to actually dial on for a given cut, I know from experience how much to dial on on my machine for a given diameter, it is a learned skill in conjunction with your machine. I have a pal who uses a specific machine at work, it is well and truly knackered, he turns out perfect work on it as he is the one who wore it out, they sort of "grew up together". I cannot use the machine at all, the backlash is horrendous.

 

[fizzy]

This only comes with experience. You will get to know your machine and how much to actually dial on for a given cut, I know from experience how much to dial on on my machine for a given diameter, it is a learned skill in conjunction with your machine. I have a pal who uses a specific machine at work, it is well and truly knackered, he turns out perfect work on it as he is the one who wore it out, they sort of "grew up together". I cannot use the machine at all, the backlash is horrendous.

I have always said a good machinest with a knackered lathe is better than a beginner with a brand new one.
My lathe is far from knackered thankfully but I have to learn to change my methods since fitting the dro. I never used to use the dials the way they were intended and yet I could get very accurate work.

 

[Pete.]

There should be a small caveat with this advice insofar as your starting diameter and final diameter should not be too far apart.

As the stock diameter reduces, unless you alter the rpm (and no-one ever does), the sfm reduces. Thus, the cutting characteristics will also change.

So do not start with something 2" diameter, do your calibration on the first cut and expect it still to be valid finishing at 1/2" diameter.

With that set up, your sfm will vary by a factor of 4 from start to finish and that is too much.

I have never seen it mentioned, but picking a number out of the air, perhaps start your calibration at 20% bigger than your finished diameter (i.e. 0.600" diameter for 0.500" finished, or when you have 0.100" still to go). An alternative way to do the same thing is to look at how many equal cuts are remaining until you finish and start the process when there are three or four to go.

You know I included this in my post but re-wrote it several times to make it less confusing. It's a very valid point. I usually go for 5 20thou cuts from 100 thou out with minor adjustments for cuts 2 and 3 and it gets me on the money on the last two cuts.

 

[Pete.]

I have always said a good machinest with a knackered lathe is better than a beginner with a brand new one.
My lathe is far from knackered thankfully but I have to learn to change my methods since fitting the dro. I never used to use the dials the way they were intended and yet I could get very accurate work.

Wait 'till you put a perfect sized bore in a 316 stainless part then turn the OD only to find now the bore has shrunk. That was a lesson hard learned - rough OD and ID before you do your finishing.

 

[fizzy]

Wait 'till you put a perfect sized bore in a 316 stainless part then turn the OD only to find now the bore has shrunk. That was a lesson hard learned - rough OD and ID before you do your finishing.

How does that happen? Material contracting?