Tiny lathes will humble you.

[addertooth]

Long ago, when the Earth was still bubbling, I was a machinist. In my local area the bottom fell out of the machining and fabrication market, so I switched back to electronics and design. I had always wanted to build a small shop (for hobby work), with no plans of having large equipment. The fact my small shop area only had 115 Volt AC available set some constraints. But all that said, I had heard horror stories of how badly some of the 7 by (10/12/14/16) lathes chattered and assumed it was horror stories told to scare the children. After all, a proper tool grind, tool height, keeping the extension from the tool post short, the right cut depth, and feed and speed always dealt with chatter on the lathes I used to operate.

I recently purchased a 7 X 16 lathe, and have been in the process of levelling and trueing the lathe. I did some "play cuts" with aluminum and brass without a problem in the world. I decided to make a real part out of cold rolled steel, and the humbling began immediately. It seemed to have very narrow ranges of feed and speeds which seemed to keep the carbide "chatter free", and often that was at the upper end of what the carbide would tolerate. I realize the standard rejoinder is "get a bigger lathe"; that mantra is quite familiar. However, due to space, a bigger lathe really isn't going to happen (I have to leave the space for a mill and a few other tools).

Tomorrow I am going to try some changes in my grind to see if that will make things happy, but I must say, I had expected cold rolled steel to be a bit easier. It will probably end up being something simple. I remember dealing with super hard bearing brass eons ago, it actually required the carbide to be slightly below the center line to cut cleanly without chattering (that and a reverse rake). At any rate, there is a sweet spot to be found, and I am sure it is out there somewhere.

 

[benmychree]

Carbide tools may be the cause of your problems; if you have speeds low enough for HSS, it may be worth a try.

 

[addertooth]

Benmychree,
Yep, that was on my to-do list for tomorrow. The lathe is the "high torque" version of the lathe, with decent low-end grunt.

 

[benmychree]

I like a tool that has a semi circular chip curler/breaker ground in the whole length of the cutting edge; in high school we were taught to do that on the corner of the 12" grinding wheel. I only use those tools on my 9" Monarch Junior lathe.

 

[dbstoo]

Small lathes do best if you keep a few things in mind. You've already mentioned the need for minimizing stickout. Another common problem is adjusting the gibs to eliminate lift of the compound and the cross slide.

If you are using carbide, try negative rake. It will let the insert cut with less HP.

Dan

 

[addertooth]

Yes, the gibs are tight, verging on too tight, but they won't get properly adjusted until I put the new brass gibs in, and do my full tear down on it (soon). It came from the factory on the tight side for virtually every adjustment.

To cover some other potential suggestions, the steel stock is resting on a steady rest with ball bearings to reduce flex.

The part I am making is a keyed collar which goes between the bearings and the gear on the chuck spindle. I am getting ready to switch out to tapered bearings, and the stock keyed collar will be too long. I ordered a spare keyed collar, but as it is made of a plastic, I was concerned it was too compressible to keep a consistent pre-load on tapered bearings.. thus why a new one is being fabricated from steel.

 

[benmychree]

Small lathes do best if you keep a few things in mind. You've already mentioned the need for minimizing stickout. Another common problem is adjusting the gibs to eliminate lift of the compound and the cross slide.

If you are using carbide, try negative rake. It will let the insert cut with less HP.

Dan

Negative rake?? that is not the way it works, positive rake is well known to reduce the power to drive a cut; nearly the only reason to use negative rake is for interrupted cuts.

 

[erikmannie]

Working with mild steel on my 1HP, 10” swing benchtop lathe, I reaped some rewards by reducing RPMs by more than you would believe as well as being very fastidious about having the cutting tool centered (obvious, I know). An experienced machinist came to my garage last Sunday & those were two of his tips.

Lately, I have been migrating to carbide. I usually use the slowest power feed dial. With carbide, I can increase RPMs (as opposed to HSS) and the work goes more quickly.

 

[dbstoo]

Negative rake?? that is not the way it works, positive rake is well known to reduce the power to drive a cut; nearly the only reason to use negative rake is for interrupted cuts.

I'll have to take your word for it. I always get it backwards. I blame dyslexia. In this case I actually looked it up online before posting in order to get it right. Fortunately I can recognize the one that works properly and use it. In many cases the chip breaker serves as the back rake.

 

[benmychree]

Working with mild steel on my 1HP, 10” swing benchtop lathe, I reaped some rewards by reducing RPMs by more than you would believe as well as being very fastidious about having the cutting tool centered (obvious, I know). An experienced machinist came to my garage last Sunday & those were two of his tips.

Lately, I have been migrating to carbide. I usually use the slowest power feed dial. With carbide, I can increase RPMs (as opposed to HSS) and the work goes more quickly.

Yes, if the particular lathe is up to it, carbide can speed metal removal, but I see all to many folk using negative rake tools on low power/rigidity machines because they esteem how many cutting edges that they may offer rather than how well they preform under the conditions that the machine may provide; forget about how many edges are offered, concentrate on how freely they cut.