# Tool Height Tolerance



## epanzella (Jan 24, 2018)

I know cutting tools are usually set on center but I wonder what the tolerance is?  .0001" .001" .010"? All I ever see is "as close as possible", but what's possible can change with finer fixtures and better techniques. I currently use the standing straightedge procedure. Is this good enough? How far should I chase this thing?


----------



## Ulma Doctor (Jan 25, 2018)

it has a lot to do with what you are machining and what you are making and whether you are turning or facing, or parting.
if you were within .001" that would be ideal
you can chase tenths but it's really not necessary for the stuff we are likely to construct


----------



## Tozguy (Jan 25, 2018)

I kinda like to error slightly on the high side as a hedge against deflection and flex in the system between ways and tool.
An adjustable gauge can be set very accurately and makes easy work of setting a QCTP in the 0 to +.0005'' range.


----------



## mikey (Jan 25, 2018)

I use mostly HSS tooling that cuts best when set on centerline so I use a simple height gauge that is on the exact centerline of my spindle. Similar in concept to @Tozguy. When I use inserted carbide, I  use the same gauge but try to remember to adjust the tool holder about 0.005 high. This is to accommodate tangential cutting forces and it works rather well. The harder the material and the deeper my depth of cut, the more important this becomes. You might give it a try.


----------



## Wreck™Wreck (Jan 25, 2018)

Tozguy said:


> I kinda like to error slightly on the high side as a hedge against deflection and flex in the system between ways and tool.
> An adjustable gauge can be set very accurately and makes easy work of setting a QCTP in the 0 to +.0005'' range.



Surely you jest.


----------



## Ray C (Jan 25, 2018)

No matter what tooling is in use (carbide or HSS) I do a face cut and adjust height until the the little dimple in the center cuts off nicely.   Tool holders have a locknut to preserve the height and I snug that enough so it won't change during normal handling.  Since I use mainly carbide inserts, some of those tool holder were probably last adjusted 5 or more years ago.

Ray C


----------



## Fabrickator (Jan 25, 2018)

I use a mag base digital height gauge and follow-up with the "dimple cut-off" method.  I think I bought mine off of Grizzly, but they're sold all over.

https://www.googleadservices.com/pa...hUKEwiYgMXGqvPYAhWNyKQKHfcaCvcQ9aACCDc&adurl=


----------



## Tozguy (Jan 25, 2018)

Wreck™Wreck said:


> Surely you jest.



I knew someone would pick me up on this. Not really jesting but it is a calculation and not an actual measurement.


----------



## chips&more (Jan 25, 2018)

I just eye ball it. Have been all my life, never had a problem. IMHO it’s not a big deal. I do take the time to better align things if I’m facing and don’t what that little tit.


----------



## Rooster (Jan 25, 2018)

I chucked up a piece of 3/4" round brass and with a very sharp HSS tool faced it until i got the tool on dead center. Then i bored the end for a 1/2" X 1/8" rare earth magnet which i epoxied in. I then placed it on the ways and using the same tool scribed a line at the correct height, then re-chucked it and faced it too dead center length. It is very easy too set a tool height by feel or sight


----------



## Jimsehr (Jan 25, 2018)

Search ( one way to find center ) and you will see how to find center by using a depth mic checking down from top of a dia to centerline. Should get within thousand or two with ease.


----------



## RJSakowski (Jan 25, 2018)

There are two angles which are important in determining the performance of a lathe tool; rake and clearance.

The rake angle is the angle between the leading surface of the cutting edge of the tool and a plane  through the axis of rotation of the part and the point of contact of the tool.  If the leading surface slopes away from the cutting edge, it is a positive rake.  A tool can have either back rake, side rake or both.

The clearance angle is the angle between a plane tangent to the part surface at the point of contact of the tool and the trailing surface of the tool.  A tool can have either back clearance, side clearance or both.
	

		
			
		

		
	




The drawing shows a part being turned with a tool coming in on center (blue), above center (green), and below center (red).  The tool is presented horizontally  and when on center has zero rake.  If the tool is moved to an above center position, the rake becomes positive but the clearance angle is such that there is interference with the work, causing rubbing and a tool the won't cut.  If the tool is moved to a below center position the clearance angle has increased but the rake has become negative

Different materials work best within a range of rake and clearance.  Here is a site that lists those angles.  http://www.steves-workshop.co.uk/tips/toolgrinding/toolgrinding.htm  As can be seen, moving a tool up or down from the center line can have a detrimental effect on cutting performance.  By grinding a larger rake and clearance angle, the tool becomes less sensitive to vertical position but the strength and durability of the tool is decreased.  Furthermore, if you are using insert tooling, you have to work with the geometry purchased.

There are two more reasons for running a tool on center.

First,  when off center the angular relationship changes with the part diameter so as a part is being turned down, the rake and clearance are continually changing.  In fact, as you are approaching center there will be a point where you are shearing off the remaining nub or riding under it.

Second, machining off the center line distorts the geometry of the cut.  The tool moves in a horizontal plane but material is removed radially and the radial motion is the horizontal distance times the cosine of the angle formed by the plane through the centerline and the point of contact and the horizontal plane.  And that angle is continually increasing as you get closer to the centerline.

It should be noted that in certain circumstances, it may be desirable to set the tool above or below the centerline as long as proper rake and clearances are observed.  There is a complicated mechanical coupling between the work and the cutting tool that will vary greatly from one machine to another.  This greatly affects chatter and surface finish.  Each setup will have its own sweet spot.

So how accurate do you have to be?  A 3º angle has a cosine of .9986 and a sine of .052.  On a 1" diameter cut, that would amount to a vertical offset of .026" and the cosine error would be negligible for most work.  I use the vertical ruler at times.  I also use a gage like this.

https://www.ebay.com/itm/Lathe-Gage...hash=item5649056a5f:m:m3NZb7KTSDbcKWZzGIrJgqA

There are a number of DIY tool height gages which rest on the cross slide.   A final check is what happens as the cut approaches the center of the part.  If you are low, there will be a nub left or it will break off as you move past center.  If you are over, you will shear the nub off.

Edited to include missing drawing


----------



## projectnut (Jan 25, 2018)

Too many people over complicate setting tool height on a manual lathe, especially when using a QCTP.  First of all I'll assume you have a properly sharpened HSS tool bit or a new Carbide insert meant for the job at hand.  Then just use a straight edge or 6" rule and trap it between the work piece and the point of the tool.  If the tool is low the top of the straight edge will tilt toward the operator.  If the tool is high the top will tilt away from the operator.  Adjust the tool height until the straight edge appears to be perfectly vertical.  This simple method will get you within a few thousandths of center.

Next do a face cut on a piece of scrap material.  if there is a nub left at the center note whether it's above or below the point of the tool.  Adjust in the appropriate direction until no nub remains.  Once on center lock the height adjustment nut (or screw).  If you're using a lantern post style tool holder this procedure must be repeated anytime you change tooling.   When using a QCPT it should be a one time operation until the tool needs to be sharpened or the insert needs to be changed.  The object of using a QCTP (quick change tool post) is to make speedy tool changes.  If you have a quality system once the tool height is set it should be repeatable every time.


----------



## Tozguy (Jan 25, 2018)

Personally I use a tool height gauge all the time because it is easy and a good habit to adopt for consistency. I aim for center unless there is a good reason to stray. For cutting a straight taper, the tool HAS to be dead on center.


----------



## SSage (Jan 25, 2018)

The 6" metal ruler works for me, I center my tools and usually hit it just right. I face first and make sure there is no nipple left. I work at fairly tight tolerances, never felt the need for a specific height tool.


----------



## mikey (Jan 25, 2018)

To each his own and whatever works, works. I use a gauge and there is no guessing. I don't have nubs or have a need to check for them. You can have a QCTP but the tool holders do not sit at the same elevation every time you seat them in the post; you're fooling yourself if you think they do.


----------



## derf (Jan 25, 2018)

I turn lots of rifle barrels, and a dead centered tool will leave you with a mess. I run .015" doc, and .015" pr with a ccmt insert oriented like this ^, and run the tool high as I can get it and still cut. It might even squeal at the beginning of the cut, but if you don't, there will be chatter so bad in the middle section, it will look like rope.
 I know that a lot will depend on the individual machine, but I turned hundreds of barrels without a steady or follower rest using this method.


----------



## Tozguy (Jan 25, 2018)

derf, what speed do you use with that and how long are the barrels?


----------



## mikey (Jan 25, 2018)

derf said:


> I turn lots of rifle barrels, and a dead centered tool will leave you with a mess. I run .015" doc, and .015" pr with a ccmt insert oriented like this ^, and run the tool high as I can get it and still cut. It might even squeal at the beginning of the cut, but if you don't, there will be chatter so bad in the middle section, it will look like rope.
> I know that a lot will depend on the individual machine, but I turned hundreds of barrels without a steady or follower rest using this method.



Interesting to read this, @derf. @Bamban also reported having better finishing results with an increased lead angle on his insert when doing barrels. I use a CCMT/CCGT insert in a 3/8" tool holder and found that raising the tool 0.005" above center cut better for me with deeper depths of cut. I'm pretty sure this counters tangential forces in a similar fashion that raising a boring bar up does. I also angle my cutter like you and Bamban do when doing finishing cuts, especially in aluminum, and get much better finishes. My thinking is that raising the cutter above center reduces tangential forces and angling the cutter reduces radial forces so the cut is more accurate with a better finish. It isn't necessary to angle the cutter for deep cuts but it sure helps finishing cuts, at least for me. Now that makes three of us who don't use inserts the way we're "supposed to".

It is curious that you can cut a long barrel without a taper without follow rest support. I'll have to try it and see how it works.


----------



## derf (Jan 27, 2018)

I usually turn at 500 rpm, actually 538 according to my tach. I can cut them tapered or cylindrical with no big problems. I actually use a taper attachment in conjunction with an off set tail stock for profiled sporter barrels.


----------



## Tozguy (Jan 28, 2018)

thanks derf, much appreciate the info. At the risk of being a pest, may I ask how long a barrel can be done that way and do you use cutting lube?


----------



## derf (Jan 28, 2018)

I've done barrels up to 30", but that requires me to remove my thread dial to get that much travel. I use Ashburn 9000 water soluble coolant.
I didn't mention it before, but this is on a 13" South Bend. It's a good stiff machine and that's probably why I don't have many problems with chatter. I can't get away with this on my heavy 10".


----------



## mikey (Jan 28, 2018)

What I don't get is how you can turn a taper-free barrel over 30" while using carbide inserts without a follow rest. I believe you but it sort of seems to defy physics.


----------



## derf (Jan 29, 2018)

Once you get a few hundred under our belt, you tend to figure things out....


----------



## Wreck™Wreck (Feb 3, 2018)

Close enough for this job.


----------



## umahunter (Feb 9, 2018)

I have  one of these from edge technology it has an adjustable  bubble  you set to your machine then put it in the chuck and level  your tool


----------



## Bob Korves (Feb 9, 2018)

umahunter said:


> I have  one of these from edge technology it has an adjustable  bubble  you set to your machine then put it in the chuck and level  your tool
> 
> 
> 
> ...


What do you do if the work is partially finished in the chuck and you want to change to a different tool holder and/or put a different height tool in it?


----------



## umahunter (Feb 11, 2018)

I have holders for each tool I have so I don't run into that problem but I believe  they make a similar level you don't mount in your chuck you just hold against the stock but since I bought holders for each tool  bit i have I bought this one


----------



## RJSakowski (Feb 11, 2018)

umahunter said:


> I have holders for each tool I have so I don't run into that problem but I believe  they make a similar level you don't mount in your chuck you just hold against the stock but since I bought holders for each tool  bit i have I bought this one



Like this.



https://www.ebay.com/itm/Lathe-Gage...hash=item5649056a5f:m:m3NZb7KTSDbcKWZzGIrJgqA


----------

