Problem's with a parting tool.

[trainman142]

Hello

I'm having some problem's using a parting tool on my 7x10 mini lathe, and would like some advice on how to fix it.
Whenever the tool enters a workpiece smaller than a 1/2 inch it cut's it just fine,but any thing bigger than that,it start's screaming like s@#t,and sometimes even stop's the chuck from spinning and make's the belt slip,but the tool is always at the center hight of the workpiece.
Any advice will help.

p.s. Alex

 

[stevehuckss396]

Hello

I'm having some problem's using a parting tool on my 7x10 mini lathe, and would like some advice on how to fix it.
Whenever the tool enters a workpiece smaller than a 1/2 inch it cut's it just fine,but any thing bigger than that,it start's screaming like s@#t,and sometimes even stop's the chuck from spinning and make's the belt slip,but the tool is always at the center hight of the workpiece.
Any advice will help.

p.s. Alex

Try slowing down the RPM's. I parted a 3 inch diameter the other day and I put the lathe down to the lowest speed possible until I was better than half way thru. After half, I was able to increase the speed. The closer to center you get, the faster you can part off. If you had more spindle power you could increase the speed you feed the tool in but with the little lathe we are stuck reducing speed and increasing torque.

 

[Anko]

look if your tool is sharp, and look the shape too, it has to be larger in the cutting edge and thinier in the rest, this made the tool not to jam when is pluggin into the workpiece

saludos

 

[gbritnell]

Alex,
Cutting off is one of the hardest things to learn relative to turning metal. Each metal has it's own characteristics and therefore requires a different set of speeds and feeds.
First off the tool edge must be sharp and on center. If the edge is not sharp all the way out to the edges it starts to wedge in the piece. The screaming you hear can be the sides of the tool scraping on the uncut metal. Along with this is setting your tool square to the axis of the cut. If it's the least bit out of square you will be trying to push the tool sideways and that will also produce a squeal. Next is the speed, as with any metal the removal is relative to the surface speed of the metal being cut. When you start at the outside the speed should be somewhat lower. Then as you approach the center the speed should increase. The problem with this is that not many of us have the ability to change speeds on the fly so the one given is that it's always better to cut slowly than fast. Now the feed rate. This is where experience and finesse come into play. As long as the speed is not too high and you start your cut and find that it's chattering somewhat the cure is to feed more, not less. By more I don't mean crank the heck out of the crossfeed wheel I just mean that you have to cut under the chatter to smooth it out.
One last thing, understand how each metal cuts. For example 12L14 steel cuts wonderful when you're turning it but start cutting it off and it can bind up instantly.
For any type of steel I always put a periodic drop of cutting fluid on it as I cut off. This will help lubricate the small particles and chips that could otherwise fuse between the tool and the stock.
Once you have mastered cutting off you're on your way to being a machinist.
gbritnell

 

[Ned Ludd]

It might not be Alex's fault that he has trouble. There is a slight manufacturing fault with some of the Mini-Lathes. The top slide gibs can fall short of an acceptable level of competent manufacture. The problem is that the "male" part of the dovetail is very sharply pointed, whilst the female part is rounded. It works, sort of, with small loads but can move with heavier loads as in parting, causing nasty dig-ins.


Anyone going to the Ally-Pally show next month, will be able to see a mini-lathe used for parting on the SMEE stand. Also anyone wanting to try the new Arc Euro Super C3 (generously donated to SMEE by Arc Euro Trade, thanks guys) can come along and have a go, but you might have to wait your turn (pun fully intended) with the youngsters. Last time, SMEE's "hands on a lathe" proved very popular with youngsters and a few adults too.
Ned

 

[chbeyer]

Hi Alex,

don't worry cutting off with a small lathe is a tedious task sometimes and there can be many reasons for that. The carriage should be locked when you cut in any case. If your lathe runs in reverse mode you can also mount the cutting blade upside down in the toolpost like that the chips easier fall out off the cut.

Just my 2cent

cheers
christoph

 

[bronson]

I have done alot of parting off with a mini-lathe i always used low gear and find that the thinnest partoff tool works the best with quite a bit of feed more then you think. There was someone on this board that uses hack saw blades and grinds them like cut off tools and made a holder for them this works really well on a mini-lathe. Maybe someone can point out the thread i saw this in, pretty sure it was in the tool section.

 

[deverett]

Bronson

'twas I who showed my parting toolholder using a piece of hacksaw blade.

http://www.homemodelenginemachinist.com/index.php?topic=7600.msg80704#msg80704

If using bimetalic blades use the edge with the ground off teeth, but I prefer to use all-hard blades.

Dave
The Emerald Isle

 

[Ken I]

As gbritnel states - less speed more feed.

At the first sign of chatter I up the feed - doing it manually of course using my left hand for feed (I'm left handed - I suppose that helps) while applying lubricant with a brush in my right hand.

Width is also a killer, my mini lathe point blank refuses to cooperate with 3mm or 1/8" blades - but 2.5 seems fairly happy.

I have a part off holder for (wait for it) old Stanley knife blades (0.7mm wide) with the sharp edge ground right off - no side clearance (best get it dead square) - suitable for parting off thin wall sections only and I generally only use it on brass or aluminium but have parted off some bushes made from cap screw steel (tough stuff).

The front edge must also be square (no front rake) otherwise it wanders, you also can't "tickle" the sides of the cut for the same reason the blade is not stiff enough and it flexes away - position it and part off - one shot is it.

When parting off bushes I typically get it to touch the face, pull back - dial in the width of the bush plus the width of the blade and go.
If its go to be dead accurate - leave a bit on for subsequent finishing of the parted end - obviously.

If you try this please grind off all of the orignal sharp edge otherwise you are going to cut yourself (guess how I know this ?).

Ken

 

[jpeter]

Put a prop under the blade. take a piece of scrap just the right length to prop up the tool just back of cutting edge. This will increase the rigididity of the machine a lot. Try it.