Movement in bit when turning

[Maplehead]

Hi All
I am loving my new Jet BDB-919 lathe. Powerfeed is awesome. I notice one thing though that I would like to correct. When I initiate the powerfeed upon turning some rod stock I notice right at the point the bit meets the rod that the bit backs off (gets pushed back), just a bit. Maybe 1/1000. When using the scale on the crossfeed dial, this movement makes it so that my count down to my desired diameter is inaccurate. At most I take off 10/1000’s at a time whether it be brass or steel. The lathe has no issues with this depth of cut. I just want it to be accurate. Is the fix a tightening of the gibs on the crossfeed or on the top slide or tool post.
One last thing to note here. Given the above, you would think that if I set my depth of cuts at .010" and then bit slightly moves back, that the result would be a lesser depth cut, like .009". However, I just did three .010 cuts and I am .005" deeper into the stock than I should be. What is going on here?

 

[stupoty]

Hi All
I am loving my new Jet BDB-919 lathe. Powerfeed is awesome. I notice one thing though that I would like to correct. When I initiate the powerfeed upon turning some rod stock I notice right at the point the bit meets the rod that the bit backs off (gets pushed back), just a bit. Maybe 1/1000. When using the scale on the crossfeed dial, this movement makes it so that my count down to my desired diameter is inaccurate. At most I take off 10/1000’s at a time whether it be brass or steel. The lathe has no issues with this depth of cut. I just want it to be accurate. Is the fix a tightening of the gibs on the crossfeed or on the top slide or tool post.
One last thing to note here. Given the above, you would think that if I set my depth of cuts at .010" and then bit slightly moves back, that the result would be a lesser depth cut, like .009". However, I just did three .010 cuts and I am .005" deeper into the stock than I should be. What is going on here?

Having the tool slightly off centre can alow you to take a different depth of cut than you dialed in , also the tool might be grabbing and pulling itself in a little perhaps.

On the too little cut , any deflection will give you some error. the heavier the cut the more flex you will discover

 

[RJSakowski]

All lathes have a certain amount of flex or spring. Larger, more rigid lathes less so. This is a reason why we make spring passes. .001" is really rather minimasl, especially for a 9x19 lathe. When makeing successive passes, I keep my depth of cut consistent. After the first pass, I mike the diameter and compare that to my expected diameter. That will tell me what the amount of spring is. Making the same depth of cut on successive passes should remove a consistan amount. Keep in mind that if you reduce the depth of cut, you will have less spring and will remove more material. You will need to sneak up on that final dimension as you get close. I like to make a second pass at the same setting before that final adjustment.

Adjusting the gibs, tightening the preload on the thrust bearings, adjusting the backlash on the lead screw nut can all help reduce the flex. Reducing the tool stickout will also help.

 

[benmychree]

You do not say what type of cutting tool that you are using; if negative rake carbide, likely the cutting edges are not very sharp, and deflection can be expected; using positive rake uncoated ground inserts with small point radius, freer cutting can take place with less deflection, using HSS tools with generous back rake such as a ground in chip breaker is even , or can be even more free cutting and work much better with smaller lathes.

 

[Maplehead]

Pictured is what I’m turning with.
Can I purchase already ground M2 HSS .5” tooling? ebay and Amazon looks like it’s all the Asian stuff.

 

[mikey]

Really hard to tell what the nose radius on that insert is from just the pic but if I had to guess, I would say its around 0.015" or so. This matters because the depth of cut an insert needs to take should exceed the nose radius by a few thou if it is to cut accurately and without chatter. If you take a cut that is less than the nose radius then the tool will deflect and that is what I think is going on here. The reason accuracy suffers is because the amount of deflection varies in real time based on numerous factors but the bottom line is that if you need an accurate cut, pay attention to your nose radius.

The other two cutting conditions - speed and feed - also matter. If your work piece is big enough to enable you to set the proper speed for the material you're cutting then getting the depth of cut right will solve a lot of your issues. Then you just need to find the right feed for those conditions and the lathe will cut accurately.

There is a lot to cutting with inserts. None of it is hard but you have to attend to it or accuracy suffers. You have a little lathe that is not all that rigid, fast or powerful so it is in your best interests to learn about your tooling. Like Benmychree said, HSS would be an attractive option for you.

 

[Mitch Alsup]

One last thing to note here. Given the above, you would think that if I set my depth of cuts at .010" and then bit slightly moves back, that the result would be a lesser depth cut, like .009". However, I just did three .010 cuts and I am .005" deeper into the stock than I should be. What is going on here?

When seeking up on a turned diameter, I measure the current diameter with a micrometer (after having turned at least 2 passes off the raw stock).
I then dial in 1/3rd of the difference on the cross feed dial and take a cut.
I then measure the then current diameter and determine how much of a cut was taken, compared to the indication on the dial.
Then I dial in 1/2 of what is left (which SHOULD be 1/3rd of the original) on the cross feed dial and take a cut.
I then measure the diameter and verify that the cross feed dial is linear,
This last time, I take the measurement and dial it in on the cross feed dial*, and take the final cut.

See, what you are doing is developing a relationship between the dial readings (always suspect) with precise measurements and developing a plan to sneak up on a target without over doing it.

(*) if the turned diameter is to be polished I leave 0.000,5" for sanding.

 

[WarrenP]

Pictured is what I’m turning with.
Can I purchase already ground M2 HSS .5” tooling? ebay and Amazon looks like it’s all the Asian stuff.

Grizzly.com has a nice set of HSS bits already ground at different angles, etc. Worked well for me.

 

[Maplehead]

I purchased a set if pre-ground 8mm tools and they do seem to cut better than the carbide insert bits. Also, I don’t see the bit moving back upon entering the turning cut. Seems like a win.

 

[Janderso]

There is a lot to cutting with inserts.

Right you are Mikey. Thanks for the tip about the depth of cut needs to exceed the nose radius. I had forgotten that rule.
I am using HSS more on finishing and more delicate work but you can't beet quality carbon inserts for removing a lot of material.
They like aggressive feed rates too for good chips and finish. IMHO