PM1236 Surface finish issues

Hello,
I recently took delivery of a 1236 lathe [...] I end up with a very rough surface finish.

Hmm, spindle preload (or lack thereof)? "Something" appears to be vibrating. Are you locking the compound?
 
As others have said, Hardware steel is definitely gummy and will give you a poor finish - as will most 1018 bought from a steel yard - although you can work with the latter if you have a really sharp and honed cutting edge.

You show one photo with wavey marks in the finish. I have had similar results when thin diameter work protrudes out to far from the chuck. I think the rule of thumb is support all work extending over 3 diameters from the chuck. So 3/4" OD round stock - hold the end with a Live center in the tailstock for any length over 2 1/4" . Makes the work much more rigid and produces better finish.

The other thing to try is sharpen your HSS tool bits periodically, including touching up the top(chip breaker area). And manually run the cutting edge over an oil stone. Also round off the pointy end with a small radius - (1/32 to 1/16" r) to insure the cutting tip doesn't individually score the work, leaving randomly deeper cuts. I took a close look at a couple of brazed carbide bits recently and learned they all have a tiny flat edge at the pointy end, causing a smooth finish with a flat overlapping cut, versus a series of tiny deep v cuts - that look and feel rough to the touch. So look geometry also.

LAST thing that might help. Put a dial indicator on your tool holder and watch the dial as you take a cut. The tool itself should be rock steady. If not, if the tool moves even .001" in and out during the cut, you have something wrong with the cross feed screw and the bit will produce deeper then shallower cuts - resulting in a rough finish. Maybe tighten up your cross feed thrust bearings. You have a new lathe, so wear likely not an issue, but it's very interesting to watch the dial indicator a couple of times to see what is happening with the tip of the tool.

Good luck. I am sure this will sort itself out with a little experimentation.

Glenn
 
How do you lock the cross slide on a lathe with DRO? The scale covers the set screw. Im not sure if there is another way to lock it in place?

Screen Shot 2017-04-01 at 7.15.16 AM.png
 
Cal Ron, I have no experience at all with this machine, so may be all wet. In your photo, are you sure the cross slide lock you point to, is actually intended to lock the cross slide.? Usually this set screw is a gib tensioning screw, designed to position the gib to reduce side to side movement. You could of course lock down the cross slide by over tightening, but normally just adjust the gib for finger free movement of the compound. On my machines the cross slide screw and thrust bearings take the load and do not move of their accord.

The only time I lock down my cross slide is when taking a facing cut, across the work. Then I tighten a nut that bears through the apron against the ways.

Maybe PM machines are designed differently??

Glenn
 
I could be wrong, but that picture was taken straight from the owners manual that came with the lathe. I will contact PM to see what they recommend.
 
I have a PM1236 also, yes that is the cross slide lock. I never use it anymore cause my DRO scale covers it now. I never have an issue with finishes like mentioned & having it unlocked. I do keep the compound slide locked though when not using it.
 
I do keep the compound slide locked though when not using it.

The compound is more often than not the weakest link in terms of rigidity when it comes to hobby lathes. I replaced mine with a slab of steel and the results are nothing short of impressive.

In the picture below, you can see a piece of mystery (and very gummy) steel being turned to a nice shiny finish. This was my first cut after replacing the compound with a block of steel. Closer to the spindle, you can see my previous attempt with the compound installed.

View media item 96125
 
From what I can see, none of them look too bad other than that one piece of steel with the chatter in it? Lots of good advice as always here. As far as the cross slide lock, yes its covered by the DRO, but that wont matter with surface finish. One thing to check, make sure the tool post is tight to the compound. Just had a few recently where the mounting nut was a little too high, and it wasnt clamping the tool post down tight. Anyone else who has a new 1236 too, we found it on a few of them in the last shipment. Think we got them all but might not have.
 
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