Improving lathe turning accuracy.

The compound rest holds a secret to improving the resolution of infeed setting on a lathe. This is useful for both turning with a regular cutting tool and when using a tool post grinder.
I think that most lathe uses in Europe and the UK and its old colonies tend to set the compound rest at 0 degs. except when using it for turning tapers. On the other hand I have noticed that many US machinists leave their compound at the angle used for thread cutting, which is just a touch under half the included thread angle. Typically 29 or 29.5 deg for 60 deg thread. Particularly as most hobby machinists only cut threads once in the blue moon it seems rather illogical to leave the compound thread ready.
Is there a better general purpose angle to set the compound rest to most of the time. I think there is and I think that it's 5.7392 deg, that might be a touch fine to set easily so let's say five and three quarters. Basically I set it as close to that as is reasonable by eye. For the non maths phobes that number is arcsin(0.1).

The sketch below shows why I use this angle;
Attachment 16293

We can see that with that angle the inward feed of the toolpost is 1/10 of the travel of the compound slide. We have therefore increased our setting accuracy by a factor of 10 compared to adjusting the tool post position with the cross slide dial. !0 thou on the compound slide is 1 thou infeed. Likewise 1 thou on the compound is a tenth infeed.

Attachment 16294
Here is an real example.

I mentioned in another post that I always like to do an error analysis when working with numbers, so how critical is the calculated angle. Roughly the error in infeed is approximately 4 to 5% for 1/4 deg error in setting. That sounds a lot but let's put it in perspective in comparison to using the cross slide.
Say we set the compound at 5.5 deg instead of the target 5.74 deg then if we move the compound by 10 thou aiming for 0.001" at the work piece we will only get 0.000958" or less than 1/2 a tenth error. Try that with the dial on the cross slide. In any case if you can't set the compound angle any better than that you probably couldn't set the infeed any better than 1/4 thou on the cross slide.

I have been careful to talk about tool post movement or infeed and not actual metal removal. When using a lathe tool, advancing the infeed by 1/10 thou with usually not remove 1/10 off the radius, often it will just rub on the surface. Tool sharpness and material properties have important influences on this and is beyond the scope of this post. However, if you are using a tool post grinder then a 1/10 infeed might actually take 1/10 off, when using a fresh wheel.

A side advantage, though often an important one, is that a small angle on the compound rest gives more clearance when you are working close to the tail stock. Many lathes with the compound at 0 deg. restrict the infeed position of the cross slide.

Attachment 16295 Attachment 16296
Two views of the offset compound rest close to 5.74 deg.

Thanks for watching.

Really interresting post, thanks Tony !
For sure it's true that threading is not my favorite occupation on the lathe :)

One comment would just be that this kind of adjustment would not be measurable for the one who possibly has a DRO on the cross slide. (of course, simple reflection, I'm just thinking about the pertinence of DROs on my lathe by the time being)

thanks again

Quote:

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Originally Posted by Christophe Mineau

Really interresting post, thanks Tony !...........
One comment would just be that this kind of adjustment would not be measurable for the one who possibly has a DRO on the cross slide. (of course, simple reflection, I'm just thinking about the pertinence of DROs on my lathe by the time being)

thanks again

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Christophe,

I was wondering who would be first with that comment. Thanks.
In fact I do have a DRO on my cross slide and it is very useful, it is hiding under the white cover in the photos.
Many DROs have resolutions of 0.01mm (4/10 thou). Of course the best way to use the angled compound when you are chasing accuracy is to have a DRO on the compound rest as well. Cheap digital calipers can easily be fitted but when I need to be very concerned about accuracy I simply use a dial gauge on a magnetic base on the compound rest.
I use the cross slide DRO to get close to size and then use the compound rest to finish off.

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If you use this idea, be careful when using a carriage stop to turn up to a shoulder. Cos(arcsin(0.1)) = 0.995 so the tool will move closer to the shoulder by an amount only a tiny bit smaller than whatever you dial in on the compound.

Nice job Mr Foale! Currently helping a friend build an LSR bike....you're being tracked! <G>

Quote:

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Originally Posted by mklotz

If you use this idea, be careful when using a carriage stop to turn up to a shoulder. Arccos(arcsin(0.1)) = 0.995 so the tool will move closer to the shoulder by an amount only a tiny bit smaller than whatever you dial in on the compound.

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Marv,

You are right and wrong, arccos returns an angle not a relative length. arcsin also returns an angle which we usually express in deg. or radians. If in deg. then it 5.74 but that is an invalid argument for arccos. If in rads then it's 0.1 and arccos = 84.3 deg or 1.47 rads, neither equals 0.995.
However, cos returns a relative length and Cos(arcsin(0.1)) does = 0.995.

Of course you are right about turning to a stop, you must allow for the self evident fact that winding the compound rest in with a shallow angle will invalidate the carriage stop setting.

Quote:

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Originally Posted by mudnducs

Nice job Mr Foale! Currently helping a friend build an LSR bike....you're being tracked! <G>

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what does "being tracked" mean?

Quote:

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Originally Posted by tonyfoale

Marv,

You are right and wrong, arccos returns an angle not a relative length. arcsin also returns an angle which we usually express in deg. or radians. If in deg. then it 5.74 but that is an invalid argument for arccos. If in rads then it's 0.1 and arccos = 84.3 deg or 1.47 rads, neither equals 0.995.
However, cos returns a relative length and Cos(arcsin(0.1)) does = 0.995.

Of course you are right about turning to a stop, you must allow for the self evident fact that winding the compound rest in with a shallow angle will invalidate the carriage stop setting.

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Yes, brain fault on my part. It should have been "cos". I've corrected it in the original post.

Quote:

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Originally Posted by tonyfoale

what does "being tracked" mean?

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Just that we read all your posts and threads Tony.

Tony's 5.74 decimal degrees [or 5° 44' 24"] produces a geometric .0001 movement of the compound; actual movement is theoretical depending on accuracy of dial and thread pitch built into lathe. As he states, an indicator resolve better than the screw & dial alone.

The problem's root? Conversion of manual torque overcoming stiction to rotary motion [screw & dial] into the linear of any slide motion. Add counteracting manufacturing tolerances of equipment itself...

Unless your lathe has [a]very large diameter dials, [b]a vernier, [c]gibs and [d]backlash well set, and [e]screw/ nut combination are very smooth operating, such fine increments are difficult to attain. A 1/10th by lathe cross slide is virtually impossible; while a good old jig bore can do that with no fuss; utilizing the details above. Can you imagine threading Class 3 or 4 fits without a cross slide? Yikes!
DRO's are fine, but size changes in turning and boring are dependent on good/ ideal mechanical conditions to support the digital readings. It matters when only 1 piece of stock is available for a complex part.

Another convenient angle is 30° 00' 00"; generating half-thousandths easily by a sine of 0.5000. Very handy on lathes with direct .001 graduations that affect diameters by .002.

DRO or not, personally I set compounds 0°, 5° 44' 24", 29° 30' 00'', 30° 00' 00'' and/or 90° 00' 00" according to job being run, considering design features with tolerances. Occasionally this requires altering settings to complete a part.
I'd say 29° 30' is my normal setting. Ready to thread, close enough for .0005 size control, and imparts smooth movement for 45° ID/OD chamfer form tools. On larger lathes, mass helps with cut-off tools, 29° 30' is OK. Small machines I set 0° [straight in].

I have used this many times. It is a really good way to "sneak up" on that finish cut.

Quote:

---

Originally Posted by tonyfoale

The compound rest holds a secret to improving the resolution of infeed setting on a lathe. This is useful for both turning with a regular cutting tool and when using a tool post grinder.
I think that most lathe uses in Europe and the UK and its old colonies tend to set the compound rest at 0 degs. except when using it for turning tapers. On the other hand I have noticed that many US machinists leave their compound at the angle used for thread cutting, which is just a touch under half the included thread angle. Typically 29 or 29.5 deg for 60 deg thread. Particularly as most hobby machinists only cut threads once in the blue moon it seems rather illogical to leave the compound thread ready.
Is there a better general purpose angle to set the compound rest to most of the time. I think there is and I think that it's 5.7392 deg, that might be a touch fine to set easily so let's say five and three quarters. Basically I set it as close to that as is reasonable by eye. For the non maths phobes that number is arcsin(0.1).

The sketch below shows why I use this angle;
Attachment 16293

We can see that with that angle the inward feed of the toolpost is 1/10 of the travel of the compound slide. We have therefore increased our setting accuracy by a factor of 10 compared to adjusting the tool post position with the cross slide dial. !0 thou on the compound slide is 1 thou infeed. Likewise 1 thou on the compound is a tenth infeed.

Attachment 16294
Here is an real example.

I mentioned in another post that I always like to do an error analysis when working with numbers, so how critical is the calculated angle. Roughly the error in infeed is approximately 4 to 5% for 1/4 deg error in setting. That sounds a lot but let's put it in perspective in comparison to using the cross slide.
Say we set the compound at 5.5 deg instead of the target 5.74 deg then if we move the compound by 10 thou aiming for 0.001" at the work piece we will only get 0.000958" or less than 1/2 a tenth error. Try that with the dial on the cross slide. In any case if you can't set the compound angle any better than that you probably couldn't set the infeed any better than 1/4 thou on the cross slide.

I have been careful to talk about tool post movement or infeed and not actual metal removal. When using a lathe tool, advancing the infeed by 1/10 thou with usually not remove 1/10 off the radius, often it will just rub on the surface. Tool sharpness and material properties have important influences on this and is beyond the scope of this post. However, if you are using a tool post grinder then a 1/10 infeed might actually take 1/10 off, when using a fresh wheel.

A side advantage, though often an important one, is that a small angle on the compound rest gives more clearance when you are working close to the tail stock. Many lathes with the compound at 0 deg. restrict the infeed position of the cross slide.

Attachment 16295 Attachment 16296
Two views of the offset compound rest close to 5.74 deg.

Thanks for watching.

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Been using this method since finding out about it when i first got me lathe.This tool combined with 1 in 10 set over is invaluable for taking off those pesky tenths and also leave a fine finish on ordinary non free machining mild steel.This one has had much use and sharpening so needs a regrind to regain a bit more hook.Tool is lightly clamped straight into toolpost dead square to work andthe straight cutting edge is set at any angle 45-85 to vertical and swarf comes off like the finest of cobwebs

Quote:

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Originally Posted by Daturat100r

Been using this method since finding out about it when i first got me lathe.This tool combined with 1 in 10 set over is invaluable for taking off those pesky tenths and also leave a fine finish on ordinary non free machining mild steel.This one has had much use and sharpening so needs a regrind to regain a bit more hook.Tool is lightly clamped straight into toolpost dead square to work andthe straight cutting edge is set at any angle 45-85 to vertical and swarf comes off like the finest of cobwebs

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