Trouble parting off mild steel Southbend 9A

[Threadkiller]

Good morning everyone. I am clearly doing something wrong. I am a super beginner when it comes to running a lathe. The other day I was trying to part off some spacers out of 1.5" mild steel round bar with some 1/8" HSS parting bit and I couldn't do it for the life of me. I would either stall the lathe which obviously was me trying to take too deep of a cut, but before I would take a cut that deep the bit wouldn't cut anything. One time I snapped the bit because I had too much "stickout" for lack of a better term out of the tool holder. I don't know if I didn't grind the HSS correctly (probably) but I ground it and cooled it off with water between grinds and I had what I thought was a sharp point on it to cut. No matter what I tried the lathe would either stall, bind and slip the belt or I would break the bit. Any help would be greatly appreciated.

 

[tincture500]

Could be a combination of issues.
Is the cutting tip in the middle or just slightly below the center of the work piece? Too low the bar will try to roll over the tool
The cutting angle of the tool is ground with adequate relief
Try to get the cutter mounted as close the the cross feed saddle as possible
Slow the rotation down and look to see if you have something going on at the point of contact visually.
Start slow and increase speed



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[4ssss]

I'll give you some solutions since I don't know the real problem. The motor may be stalling. The belt may be slipping. The cut off tool needs to be on center. #1 a new motor is needed. #2 clean the grease and oil from the belt and the pulleys, and tighten the belt. #3 Put the cut off tool on center. Too high and it rubs, too low and it will pick up the piece being cut off and grab it breaking the tool and knocking the part out of the chuck. Also grind the cut off tool with an angle in front facing whichever side you need to face clean so the part is cut off clean while it's still being held by the turned diameter. Use cutting oil. Run it slow. Using the back gears on your lathe will also help in belt slippage.

 

[Bob Korves]

Beyond what has been stated above, the parting blade must be at a true 90 degree angle to the spindle axis, the carriage must be locked down, the compound should have the gib locked on a light lathe like yours, and you should check for looseness in the compound, cross slide, saddle, spindle, and work mounting. When grinding the parting blade, consider the angle that the blade is mounted on the tool post. If it is not level with the cross slide, you must compensate for the mounting angle when you grind the parting blade. The tool should be fed at a rate that gives a continuous chip, not rubbing and not digging in. Once it is cutting, keep it cutting. Some pics of your setup might shed some light on your problem, if it is possible for you to post some. If it is any consolation, most of us have had parting off problems at times. After great success for a long time, I am also currently having problems with it myself. First get the basics right, then get the technique right, and things should improve. Parting off is sometimes how we are kept humble.

 

[jocat54]

I had very little luck parting with my SB9A-just too light and loose I think. I have had pretty good luck parting with my 12x36--bigger and newer(tighter) but some times it just seems to be no stars in alignment. This is using the same HSS parting blades I used on the SB9A and with QCTP.
It is probably me causing the problem--tool grind and positioning of the parting blade.

 

[tq60]

D all of the above....

The lathe is not the most rigidly built but good enough when limitations known.

First the shape of parting tool is critical and we may not be able to explain it well but Google "controlled cut with saw blade" which is related to wood cutting and it may give you the general idea.

Here is where we are going.

The small lathe has some backlash here and there that all sum together.

When parting you are pushing the tool into the work so forces are against the backlash so all is well.

If the tool is too aggressive where it can take a larger bite it could move forward into that bite as there is nothing holding it back.

A control cut blade in a radial arm saw limits tge bite allowing easy cuts without the saw trying to climb over the material.

Changing the angles at the front of the tool to cause it to limit the bite may help, we never parted much on the small lathes before as we had difficulties like yiu, we would gut a groove then use saw.

On bigger units we part just fine.

Others will come along to advise possible tricks in angles or setup.

Try cutting just below center as that point a downward force would reduce depth of cut.

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[fradish]

Though not strictly parting advise, if you have a leather belt, you might want to consider
upgrading to an automotive serpentine belt. I used to use belt dressing on the leather belt on
my SB9C to try to keep it from slipping, but the serpentine belt is much better. However I have
to say for a beginner, having a belt that slips a bit isn't the worst thing imaginable. The serpentine
will still slip occasionally, but I find it makes using the lathe much less frustrating.

I know most people will scarf and glue the serpentine belt, but since I had alligator clipping I just
used that. I was used to the click-click from the leather belt and it doesn't bother me. It has
held up fine for about a year now. I don't run with a huge amount of tension on the belt. Just
enough so that it doesn't slip during normal use.

 

[Paul in OKC]

Small lathes can be tricky. Hell, parting off is tricky anyway! I used to hate it with a passion, especially with HSS blades. SHoot, I was teaching a class about a year ago at a Vo-Tech, and was doing a part off. Got to show the students that even with 40 years of experience I can still snap off a blade with the best of them . It is doable, good suggestions given above, Tool on at a hair below center, and definitely at 90 degrees to the part. Light but constant feed pressure, lubrication.

 

[woodchucker]

I had very little luck parting with my SB9A-just too light and loose I think. I have had pretty good luck parting with my 12x36--bigger and newer(tighter) but some times it just seems to be no stars in alignment. This is using the same HSS parting blades I used on the SB9A and with QCTP.
It is probably me causing the problem--tool grind and positioning of the parting blade.

Tighten your gibs, align your blade square to the part, center the cutter, hone the edge. I have no problem parting with mine. I do tighten the gib screws on my compound, and I do use plenty of cutting fluid. My 9a will cut like a pro using HSS. I don't stick out much, and just keep moving the blade stickout further and further as I get deeper. A 9a is fully capable.

 

[woodchucker]

Try cutting just below center as that point a downward force would reduce depth of cut.

This is a religious matter. some like it high, some centered, some low.
I found that when I went low, I crashed the lathe. The part would ride up over the blade (caused by the lifting action). I find centered, or early in the cutting a few tenths high are safe. When you get close to the center, you need to be more centered. I never go low after a few scary crashes.