Cutoff Tool

Mikey I must be missing somthing , parting from the rear the work is rotating up , would that not lift the cut off tool and holding assembly's "verses " parting from the front the work is traveling down and the cutoff tool would have downward pressure ?.
I haven't tryed parting from the rear , but that's just what's come to mind . Could the upward forces lift the assembly's into the dovetails resulting in increased stability and less chatter .?
Next time I part I'll get out the indicator , I'm useing a SB 10L and don't get much chatter.
 
Mork said:
Interesting... I would think parting from the back would raise the back of the carriage although I can see how the force could transfer to the front. Maybe taking the play out of the carriage in front will help parting from the back. I'll probably try this again in the future.
Click to expand...

If you think about it, the blade is being lifted when parting from the rear. That force has to go somewhere. It cantilevers that tool holder and pushes down on the carriage, taking up what play there may be. Once that play is taken out the forces are sustained by the entire cross slide and lathe bed and you can't get more solid than that. The best scenario is when the tool holder is bolted solidly to the cross slide, not sitting on top of the compound.

When parting from the front the same thing happens except the forces on the blade push it down, lifting the tool holder up. This lifts the supporting structures and increases clearances. As the tool cuts the tool/blade/cross feed is going up and down so the chances of chatter are increased.

In my feeble brain I think the reason so many guys tell you to force the tool into the cut is because of this lifting thing. Forcing the tool keeps the cutting forces high so there is less oscillation taking place. I don't know that this is true but conceptually it makes sense. It also makes sense to keep overhang to a minimum, tighten up your gibs, make sure you are dead on center height and perpendicular to the work to minimize cutting forces
 
mikey said:
If you think about it, the blade is being lifted when parting from the rear. That force has to go somewhere. It cantilevers that tool holder and pushes down on the carriage, taking up what play there may be. Once that play is taken out the forces are sustained by the entire cross slide and lathe bed and you can't get more solid than that. The best scenario is when the tool holder is bolted solidly to the cross slide, not sitting on top of the compound.

When parting from the front the same thing happens except the forces on the blade push it down, lifting the tool holder up. This lifts the supporting structures and increases clearances. As the tool cuts the tool/blade/cross feed is going up and down so the chances of chatter are increased.

In my feeble brain I think the reason so many guys tell you to force the tool into the cut is because of this lifting thing. Forcing the tool keeps the cutting forces high so there is less oscillation taking place. I don't know that this is true but conceptually it makes sense. It also makes sense to keep overhang to a minimum, tighten up your gibs, make sure you are dead on center height and perpendicular to the work to minimize cutting forces
Click to expand...
Draw a free-body diagram. You'll see that the net force exerted on the carriage by the work when cutting from the rear (doesn't matter what operation) has to be upward and to the rear. When cutting from the front the net force has to be down and to the front.
 
I've done that, John, and I've also tried to measure it, albeit with a dial indicator instead of strain gauges which I don't own. I've proven it sufficiently to myself so I won't argue the point. :)
 
I should expand on my last comment, John. I wasn't trying to cut you off or anything. :(

When parting from the rear the cutting forces push up on the blade's tip, which tends to lift the blade out of the cut. However, it is constrained from doing that by the tool post, which acts as a fulcrum. As these forces try to tip the tool post backwards, which they usually cannot do, they are transferred to the cross slide. Once any play in the cross slide is taken up the force then transfer to the bed. The reverse is true when parting from the front.
 
My lathe has a much beefier rear gib for up ward forces than the front up face of the gib , my old 9x20 didn't have a top and bottom gib at the front at all it just sat on the ways so any harmonics that occurred would probably be bad. I used to explode parting blades on my 9x20 a lot. :-0

Stuart
 
Try a piece of tire innertube or a rubber band on the chuck and a band rapped around the tool block, passive damping sometime works. dum dum putty in the corner of a tool block sometimes works also. These techniques are used on brake drum lathes to eliminate bell ring during turning and truing. Rick
 
I don't see how parting from the back changes the dynamics of the cut compared to cutting from the front except that chips might be able to fall out of the way more easily. However there is no clear advantage to try outsmarting the design of the lathe by cutting upside downtown at the back, if you have a choice, unless there is something inadvertent going on.

Everything you have said mikey in favour of the forces when parting 'upside-down' apply more so to parting from the front.

To my mind the issue is to get the cut as close to the mounting point of the compound on the cross slide as possible. Further, upward pressure will put the fulcrum on the tool post centre bolt. Conventional downward pressure will put the fulcrum point on the outside edge of the tool post body which is closer to the cut.

If cutting upside down somehow puts the whole dovetail system to better use then would someone please walk me through the rationale.

What strikes me is that the weight of the carriage is biased towards the front so that the front way is carrying most if not all of the weight. There might even be some lift on the back way in some cases. What I want to avoid is a set up where cutting forces cause the carriage to teeter on the front way. On a 7+ inch wide bed like mine to get both ways to support the mass of the carriage more evenly, setting the tool post behind the cut might make sense. However I prefer to add a lead weight on the cross slide at the back to prevent teeter on the front way.
 
Last edited:
Mork said:
Here's a tool post I made for my Logan 200 lathe. The AXA post I have has the capacity for a 1/2" cutoff blade and I was hoping a 3/4" blade would be more rigid. The post works but really no better than the 1/2 blade on the AXA post. Does anyone have any suggestions for as to how to stop a tremendous amount of chatter? I've tried high speed and the slowest speeds. The slowest is by far the best but still not great. I also checked the bearing in my head stock... as near as I can tell I have about .001 slop in the bearing which doesn't seem excessive to me.

I suspect I'm expecting a bit to much from a small lathe. The compound is nowhere near as solid and other lathes and the 1.5" head stock probably flexes under load. Still I'm hoping somebody has a good suggestion as to how to improve the chatter I'm getting. Thanks.



View attachment 122765
Click to expand...
Hi, please google dale detrich's cut off tool holder, I made a couple, & they work great, no chatter, clean cuts, you'll be pleased. ken s.
 
Mork said:
Here's a tool post I made for my Logan 200 lathe. The AXA post I have has the capacity for a 1/2" cutoff blade and I was hoping a 3/4" blade would be more rigid. The post works but really no better than the 1/2 blade on the AXA post. Does anyone have any suggestions for as to how to stop a tremendous amount of chatter? I've tried high speed and the slowest speeds. The slowest is by far the best but still not great. I also checked the bearing in my head stock... as near as I can tell I have about .001 slop in the bearing which doesn't seem excessive to me.

I suspect I'm expecting a bit to much from a small lathe. The compound is nowhere near as solid and other lathes and the 1.5" head stock probably flexes under load. Still I'm hoping somebody has a good suggestion as to how to improve the chatter I'm getting. Thanks.



View attachment 122765
Click to expand...
What I've done with some degree of success comes in two parts. 1st, it seems that chatter is always a product of too much tool touching the workpiece. So I grind my tool so that it slopes towards the piece that will be coming off, and when it does seperate, it leaves a nice flat surface on the piece coming off, you can then face off the mound left on the piece in the Chuck if necessary. 2nd, I slow the spindle speed down and keep the tool dug into the metal so I have a constant chip coming off of it, it never gets a chance to chatter like that. I don't know if it's necessary, but since the tool is ground to a point, I worry about getting it hot, so I back it out every several seconds to make sure it's not turning color, that allows me to get a little oil in that slot at the same time. And plenty of lubrication. Good luck.

PS: I've never had the nads to power feed it in, but 1 of these days........
 
You must log in or register to reply here.
Back
Top