Turning to a diameter

[Sshire]

More often than not, when I'm turning a part to fit in a hole, that last cut seems to get me slightly under what I wanted. Even using the compound at 30degrees and taking half thou cuts, I often go too far.
Tips PLEASE!

Best
Stan

 

[Don1966]

I am not by far an expert and have done the same thing more than once. I try to make sure I am reading my caliper correctly and allow my self extra clearance to litely file or sand it the reading I need.

Regards Don

 

[Mosey]

Perhaps the work is hot when you cut and shrinks when it cools?
Or, the last cut is shallower than the previous ones, resulting in different pressure on the tool, yielding a different depth of cut (larger) than expected?

 

[digiex-chris]

When it's really critical, I start sneaking up on the final dimension. For example, in aluminum on my machine I can tolerate a 0.100" DOC, but I consider that a roughing cut on this machine. When I get within 0.050 or so, I take a 0.030 cut, and re-measure. then a .010 cut, then re-measure. then a .005 cut, then re-measure. Then depending on if I want to be even more accurate, I take the rest of the cut based on the measurement, or split it up more.

 

[mklotz]

Plan to make your next-to-last cut the same size as your last cut. This way you'll be able to see how much your last cut is likely to REALLY take off and adjust the last cut accordingly.

Unless you have a really, really rigid setup, taking a last cut of a thousandth or two will be problematic. The work will want to spring away from the tool.

I plan to make my last two cuts 0.005" deep (ie, removing 0.010" from the diameter). When I measure after the first of these cuts and see that the diameter has actually reduced by 0.012", I can then make the final cut 0.004". One more thing. Before making the first of these two cuts make a spring cut (or several) to remove that confusion from the process.

Depending on your equipment your numbers may differ from mine but the technique of last two cuts the same still applies.

 

[b.lindsey]

Stan, how are you measuring the ID of the hole? If using calipers the actual hole size can be .001-.002" larger than the calipers indicate.

Bill

 

[rhankey]

Though it sounds like your problem is as discussed by the others, It's probably worth pointing out hte obvious. Don't forget that the dial on the compound slide will typically take 2 times what the dial says when turning down a diameter. By setting the compund at 30 deg from being parallel to the spindle, a .001" change in the compound dial will take .001" off the diameter of the work piece. As such you may need to set the compound either at an even shallower angle, or make far smaller adjustments with the compound. I think I made that mistake once shortly after getting my lathe a year ago before I thought through the math.

Robin

 

[digiex-chris]

Though it sounds like your problem is as discussed by the others, It's probably worth pointing out hte obvious. Don't forget that the dial on the compound slide will typically take 2 times what the dial says when turning down a diameter. By setting the compund at 30 deg from being parallel to the spindle, a .001" change in the compound dial will take .001" off the diameter of the work piece. As such you may need to set the compound either at an even shallower angle, or make far smaller adjustments with the compound. I think I made that mistake once shortly after getting my lathe a year ago before I thought through the math.

Robin

That;s a good point. I found that my compound, each divison is 0.002", rather than 0.001" like on the rest of the dials. So I needed to set my compound to 2.7something degrees to get tenths instead of 3 or 4.something degrees.

 

[lensman57]

More often than not, when I'm turning a part to fit in a hole, that last cut seems to get me slightly under what I wanted. Even using the compound at 30degrees and taking half thou cuts, I often go too far.
Tips PLEASE!

Best
Stan

Hi,
I still have this problem but things are improving. First I use a micrometer now to measure the diameter of the work specially as it nears the target.
Second as the target diameter approaches I take lighter cuts about 0.05 mm
in the final stages to eliminate the spring in the tool, just before the final cut I run the tool at its present setting along the work piece to make sure that it has cleared all the high spots and then I measure and measure again.
If you have another piece with the correct diameter hole you could try and slip the it on the work ( without removing it from the chuck) to see how it feels.
Use as much coolant and lubrication as appropriate to cool the work, Aluminum expands quite a bit under heavy cuts due to the generated heat.
To give you an example for the head stock pulley to fit on my Taig lathe I need to boil the pulley in water for a couple of minutes and then it just slips over the spindle, 15 seconds out of the water and it will not go on, we are talking about 0.02mm here.
And always remember to measure and measure and measure.
Good luck,


A.G

 

[David Morrow]

On my Myford, and I'm sure on other lathes, if I take a second, third, even fourth pass with the cutting tool in exactly the same place, the lathe continues to remove material - less with each pass. So, when I'm getting really close, I do not adjust my cut depth but make those extra passes to get successively finer cuts. And I do this more slowly with each pass. This may be due in part to the spring back effect noted about but can happen on pretty stout and short pieces. I often do this more than once but it's just so much faster than making the part again.

Make haste slowly.

 

[Mosey]

Stan,
How about trying this...take a number of different cuts such as .001", .002",,, .005", and see which depth of cut gives a nice smooth finish. Also the idea of taking a series of cuts without increasing the depth at all, does finish-off nicely. Then you will have a depth of cut you know to give good finish, and you should use that for you final 2 cuts. (thanks, Marv). The sneaking up routine doesn't work for me, and the professional who guides me also said "that's a no-no".
Mosey

 

[digiex-chris]

is that because in professional work, time is money, so by sneaking up on a dimension you're wasting time? I think it also depends on if you're using carbide or HSS. with a razor sharp HSS bit I can take off minute amounts, but even with a perfectly sharp carbide, I've gotta take 0.003" or nothing. carbide needs more pressure to cut.

 

[n4zou]

It takes a really expensive machine that will allow the accuracy you are striving to achieve. Most of us can't afford a machine of that quality. Most of the shops I worked in when I was working as a Machinist could not afford them as well. To compensate I leave a little extra length on the part I'm turning so I can make a cut and check the size it's going to actually cut. I then make adjustments to compensate for spring and other variables. I always use a dial indicator instead the the machines dial to make these fine adjustments. When doing very precise work on a machine that was never designed for that level of precision I simply leave a little extra material on the work and use fine sandpaper to remove the final few ten thousands of an inch required for the final diameter.

 

[Don1966]

I am glad I responded to this I have learn quiet a bit from all of you. Marc you seem to be a good teacher on this forum. I do like your replies. Since I am a newbie at this you guy have to be right for us to learn things correctly. Good to be here with such great guys thanks.

Don

 

[digiex-chris]

+1, I'm thankful every time. Especially when I find out I'm doing something in a way that's wrong!

 

[Sshire]

Thank you. Thank you everyone.
I'm going to use all of the advice here.
FYI
I'm using my "good" Starrett micrometer.
When I get close, I do 3 or 4 passes with no change (spring passes)
I'm cutting (for this particular engine) brass with 0.5" extending out of the 5c collet
Using a just sharpened insert in a Warner holder.
I'm test fitting the hole onto the piece. The problem is when it almost fits. So it looks like a piece of fine sandpaper on a parallel to bring it down to size.

Love the boiling trick. Can't wait to try that

Thanks again all. I can't believe the different (and all very useful responses). Keep them coming.

Best
Stan

 

[mklotz]

Sandpapering is fine on decorative items and anything where you don't care if the part is truly cylindrical but if you want something really cylindrical, like a piston for a piston valve, lose the sandpaper and learn to turn to size.

 

[lensman57]

Thank you. Thank you everyone.
I'm going to use all of the advice here.
FYI
I'm using my "good" Starrett micrometer.
When I get close, I do 3 or 4 passes with no change (spring passes)
I'm cutting (for this particular engine) brass with 0.5" extending out of the 5c collet
Using a just sharpened insert in a Warner holder.
I'm test fitting the hole onto the piece. The problem is when it almost fits. So it looks like a piece of fine sandpaper on a parallel to bring it down to size.

Love the boiling trick. Can't wait to try that

Thanks again all. I can't believe the different (and all very useful responses). Keep them coming.

Best
Stan

Hi Stan ,
Stan onee thing I forgot to mention in my previous post was that at least in my case, with a small machine like the Taig most of the spring is in the useless 1/4" 20 lead screw in the cross slide, even though I have a dial indicator securley attached to the cross slide to measure the infeed, I still have to measure with the micrometer to make sure that what I am dialling is what I am getting, something to keep an eye on. Also please make sure that your saddle and cross slide are true to the ways and the gibs are well adjusted, it really does make a difference.

Regards,

A.G

 

[doubleboost]

Hi
Been reading this thread with interest
Even the profesional machinists use emery tape to take off tenths of a thou
John

 

[lensman57]

Hi
Been reading this thread with interest
Even the profesional machinists use emery tape to take off tenths of a thou
John

Hi John,
LOL, as a novice and a real amature, I am glad to read your reply. In my case at least if the integrity of the piece is in doubt at least the finish is something to brag about, but joking aside I have on occasions used the sand paper successfully to bring a bar to size and get rid of all the machine marks but only on occasions, odd ones to be true!!!