Striped finish problem

[camaro_dan67]

I have a 9 inch south bend with the flat belt drive and a gearbox I added to it. Its driven by a 1.5 hp motor that I replaced the original 3/4 hp with. My problem is a striped finish no matter what I do. I tried different tooling with a different radius on the tools. I tried different depths of cut but nothing seems to help. I also tried different feed rates, oil, coolant. I just cant get a nice finish. What else do I try?

 

[FamilyTradition]

Could you post a pic of your setup and the "striped finish" you are referring to? Are you getting chatter?

Also type of material you are trying to cut, carbide or HSS cutters. There's so many variables that go into it.

One of the first things to do is make sure you are supporting the workpiece properly. Make sure it is properly clamped in the collet or chuck.

If it is a long, skinny workpiece you might need to center drill the end first and run a center, and/or use a steady rest. Rigidity is your friend and lack of it is the source of all sorts of problems.

Double checking all the nuts and bolts on your tool post/tool holders doesn't hurt, either. Sometimes the best of us miss even the smallest things. (Of course I don't claim to be among the best, maybe that means I don't miss the small things that much?)

 

[camaro_dan67]

I will add a pic tomorrow. I usually use c6 brazed carbide for o.d. turning. It kind of resembles a thread. Its like a continuous line of chatter in a helix on the surface.
I have a feeling what I'm seeing is motor vibration running through the whole lathe. I think a pic will help everyone here understand it better.

 

[texasgeartrain]

Based on description, the point on cutting tool is too sharp of a point. Widen the contact point of cutting tool, or adjust tool angle so that the tool tip contacts work surface on a wider portion of the tool.

 

[Screwmachine]

I'm guessing you have a single phase motor on it? If you go (or go back) to a 3phase it will likely solve your problem.

An older thread with some great info from Forrest Addy here.

 

[metalmagpie]

Are the bolts holding on your gearbox loose?

 

[SLK001]

Its driven by a 1.5 hp motor that I replaced the original 3/4 hp with

First off, a 1.5 HP motor is too much for a 9". If it works, go back to the 3/4 HP motor. Your SB has simple sleeve bearings and your new motor is simply too much for them. Now, on to your stripe problem. Do you have any runout on your spindle? What material are you cutting, what tool, etc, etc,etc? A picture or two of your setup will help greatly in resolutions to your problem. Did you convert a Model B or C to a Model A?

 

[john.k]

Striped ,variable diameter finishes are often caused by using soft draggy steel,of the kind you might find in a scrapyard labelled as ideal for hobbyists...........If you want to test the lathe,best to use brass rod,as there is never any BUE with brass.

 

[Illinoyance]

It can also be caused by an irregular feedrate possibly due to worn rack and pinion.

 

[camaro_dan67]

I got around to taking some pics. I noticed it was worse when taking a finish cut.

 

[camaro_dan67]

After reading the thread suggested by screwmachine I'm leaning towards the vibration of the motor causing it. A few years ago I put a steel plate between the rear pulley/motor carrier and the lathe because carrier was sliding twards the lathe. Everything is mounted to a board most people would use for a countertop. The mounting holes are oversized so it would slide. It might be time for a new lathe bench.

 

[chipss]

Let's see the cutting tool. That could be the issue as there appears to be a lot of harmonic chatter in your cut. I don't buy the single phase motor routine as it's belt driven which slips and absorbs a lot of the 'power pulse' cogging of a single phase motor. Especially on such a small machine. Do the headstock bearing clearance test and you may find them loose.

Put a dial indicator on the machine with it acting straight down on the chuck near the bearing.
With a piece of rod in the chuck sticking out about 6 inches pull up with 75 pounds force.
You should only see about .0015 deflection therefore the clearance. Same test for the rear bearing. Then reshim as needed.

 

[camaro_dan67]

Here is the tool.

 

[Scottl]

I will add a pic tomorrow. I usually use c6 brazed carbide for o.d. turning. It kind of resembles a thread. Its like a continuous line of chatter in a helix on the surface.
I have a feeling what I'm seeing is motor vibration running through the whole lathe. I think a pic will help everyone here understand it better.

That's what I was wondering. The fact that it is not the original motor makes me wonder if there's a balance issue such as a bent armature.

Another possibility is that doubling the horsepower exposed an issue with the lathe not seen with the old motor.

 

[chipss]

Tool and the holder look good. Will assume you've played with the height of the tool to the workload. Sometimes those brazed tools that don't have any positive rake can cause problems as they require more tool pressure to cut, and these flimsy little 9 inch south bends don't like it. I have seen it on mine. If you grind a high speed bit with some rake in it I'll bet the problems will disappear as it requires many times less tool pressure.

This style of insert tooling for aluminum works very well on most materials especially on these light machines. Notice how much rake these have and how sharp the cutting edge is that minimises tool pressure. https://www.ebay.com/itm/152867686200?epid=16012269089&hash=item23979fc738:g:UhkAAOSwj1haXSwr

Putter around with checking the headstock bearing clearance. You might be surprised. *then maybe not if they're nice and tight.

edit:
Snug up the gib screws on the compound and adjust the crossfeed gib. As the tool chatters it is compounded by any vibration of loose slides as the whole saddle can then vibrate.

 

[jim rozen]

The tool has a large radius on the nose and being the standard brazed carbide unit it has a turned edge on it - basically you're turning with a negative rake round nosed tool.

1) get a diamomd wheel and sharpen it properly, or
2) learn how to sharpen a HSS tool and use that.

There is nothing wrong with: your spindle bearings, your motor, or your drive belt. You just need a cutting tool that's positive rake and minimal nose radius.

 

[FamilyTradition]

After seeing the photos of work and tool - I'll be third to say - play around with your tool grinding (at least BEFORE fiddling with the other stuff)

Yes you could f around with the bearings, use a different material, change out the motor, scrape the bed, draw a pentagram on the floor and summon a demon to make the tool you have work with the machine you have, BUT...

... looking at the tool FIRST may save you the time/money/frustration of screwing around with all those other things. Only when you have reasonably exhausted that aspect of it, you can then start looking at other things. If that is factory edge on the tool or you were not the one to grind it, I wouldn't trust it to work better than a butter knife.

 

[camaro_dan67]

I tried more rake on a different tool, here's what I got.

 

[10KPete]

That looks like a bent lead screw moving the carriage around as it travels. Every body laugh now but I'm here to tell you that I've seen that quite a few times in the last 60 years. If you measure the pitch of your stripes it will likely match your lead screw.

 

[john.k]

I think its caused by the single phase motor........these motors dont run at constant speed unless fully loaded,lightly loaded they accelerate /decelerate .