Can't hold tolerance with finish cuts

[Glenn Brooks]

John and Russ, thanks. Looks like I have a new procedure.

Glenn

[clive]

For future reference put the dial gauge on the tool post and see how much movement you get by pushing and pulling on it with your hands.

[ctwo]

Like i say tip of tool towards TS so cutting pressure moves it away from the cut,think of a clock face looking down in the tool ,tip between 12 and 1. And lock the compound gib down tight. Then see what the results are.

Trying to visualize this, if it is what I think; I'd have to mount tools on the right side of the post, and that won't work for most things.

[John Evans]

Like i say tip of tool towards TS so cutting pressure moves it away from the cut,think of a clock face looking down in the tool ,tip between 12 and 1. And lock the compound gib down tight. Then see what the results are.

Trying to visualize this, if it is what I think; I'd have to mount tools on the right side of the post, and that won't work for most things.

No tool mounts normally ,just the very point of the cutter is not at 90* to the work. If point is to the left of 90*[to HS] it can tend to dig in with cutting force. If ,on the other hand the point of the tool is just past 90* pointing to the TS cutting forces will push it away from the work there by cutting oversize rather than under. Lots of things can cause this issue tool geometry ,rigidity of the machine/ware and even the material being cut.

[GlennW]

You are referring to positive and negative lead angle.

[Harold_V]

You are referring to positive and negative lead angle.

Not necessarily, although it can be. Depending on the grind of the tool, it can have proper lead angle and still be mounted such that cutting pressure will rotate the tool inwards (towards center). Key to success, here, is for the pivot point of the tool to be such that the cut rotates the tool away from the cut.

I know you know this, Glenn, but some of the new guys may not.

Harold

[GlennW]

I interpreted John's post above as describing lead angle of the cutting edge in relation to direction of the cut, not necessarily the lead angle ground on the tool bit in relation to the shank.

Negative lead angle on a heavy cut would tend to draw the tool into the cut, where positive lead angle would keep pressure on the tool pushing it out from the cut.

But, yes, one could definitely mount a positive lead angle tool to where it was rotated to produce negative lead angle in relation to the direction of the cut. (If that is what you are referring to)

I'm getting worse (if you can imagine that) at understanding and describing mechanical things and motions via text..

[Glenn Brooks]

Key to success, here, is for the pivot point of the tool to be such that the cut rotates the tool away from the cut
Harold

This seems like a really good rule of thumb.

Received a couple of new thrust bearings for the screw, which I will be installing. so will be revisiting this in a few days and report back on what happens...

Glenn

[pete]

Since I haven't read through every post here Glenn this may already have been mentioned. No doubt most know anything that can be done it increase a machines rigidity can only help with getting accurate cuts. One of the more common additions to the Myford Super 7 is mounting a rear tool post to the cross slide to help while cutting off. The cutting action then trys to lift the cross slide and lessens the digging in issue, but likely and just as important the top slide is taken out of the picture completely with that rear mounted cut off tool on lightly built lathes. ROBRENZ over on Youtube is a professional machinist and seems to specialize in very accurate custom work. He's an extremely intelligent and highly skilled machinist.Fairly recently he showed a video of replacing his HLV's top slide with a tool post block to raise the quick change tool post back to the correct center line on his Hardinge HLV. And that's a lathe that's well thought of as being rigid. He did it to get an increase in rigidity at the tool tip and better accuracy on a machine that's also well known as being accurate. More by need to save using up a limited number of quick change tool holders I did the same years ago on my little Emco for a front mounted cut off tool. It surprised me at the time at the change in tool performance compared to using the same tool mounted onto the top slide.

It all depends on your average type of parts as to how often you use the top slide of course. But removing it when it's not needed and mounting the tool directly to the cross slide does make a large difference in how much flex shows up between the lathe bed and the tool point. One less dovetail slide, gib, and eliminating any back lash in the top slides nut and feed screw does make a difference in making repeatable cuts and even that surface finish. ROBRENZ on YT both pinned and blocked his quick change tool post raising block to his HLV's cross slide and that might be pushing it for a home shop. He did that so there's no chance of the block ever moving out of postion of his pre set dro position due to the cutting forces.But there is some real and noticible differences in not using that top slide when it's not needed. With the top slide there's a lot of different clearances that can and do show up at the tool tip. For some lathes and tool positions there's less support under the quick change tool post or what ever else your using at times than what would be ideal. With everything 100% rigid then if the slide moves X amount it should cut dead on to what's dialed in if the movements are accurate. Since nothings ever 100% rigid we do a lot of that adjusting for the depth of cut to get the part size needed. Thought I'd mention this anyway.

But that video wasn't the first time I've run across something about removing the top slide when it's not needed to get better accuracy and that better surface finish. I guess only you can judge if it would be worth a try.

[ctwo]

OK, here is what I was thinking.

I draw a force vector that is perpendicular to the leading cutting edge. That creates a moment around the main tool post bolt. At some lead angle, the force vector would point directly to the tool post bolt. There must be a special name for that angle? It is defined by the angle from the tip/point of the cutting tool to the center of the tool post bolt (relative to spindle axis or it's perpendicular? And then there is side cutting edge angle, which seems like a compliment to lead?). That is where you take the heaviest cuts? It seems that if your lead angle is anything less (or more?), the forces will draw the tool into the cut.

Most of my question marks seem to be a matter of terminology, or reference, I think...?

Usually I am cutting to a shoulder, unless I am going Abom, and who has that kind of luxury?

[Glenn Brooks]

So this thread is a few months old. However, I just stumbled on a fix that might be worth documenting for others with a similar problem. my final solution, as it where.

To recap, my initial problem: Lathe was cutting oversize. No matter what DOC I dialed in, the lathe would cut randomly deeper than my desired dimension. Also cut inconsistently - tooling would cut hi and low and leave tool marks and deep spots in the work.

Solution: had a new cross feed screw and nut made up. Now consistent DOC- wonderful!

Edit: oops, forgot. Also, put a new gib into the cross slide.

The tip off was the variable tool marks left on the surface of the work after each pass - a worn lead screw causes a lathe bit to 'wander' in and out along the work surface as it cuts.

EXCEPT: every time I touched the tip to the work, then turned on the machine, the tooling would jump a consistent thou or two into the work- cutting oversized .002" . Maddening. Many others mention this phenomena on the web, with no apparent solution.

Solution: just installed new thrust bearings on the end of the cross feed screw shaft! Wallah! No more tool jump!!!

Now my new to me SB Fourteen makes finish cuts exactly to DOC I dial in - every time.

So, new cross feed screw, new gib, new thrust bearings = accurate Lathe! Happy days are here again!!!

Glenn

[dbstoo]

Thanks for the follow-up. Makes good sense.