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Parting off question

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Blue Heeler16/09/2020 07:03:01
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Is there a way to make the stub be on the material that still in the chuck and not have the stub on the part that you've just parted off?

Regardless of any changes I make in my parting off tool, the stub is always on the parted off piece.

JasonB16/09/2020 07:08:54
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I see from previous posts you are using an insert blade, rather than use GTN ( Neutral) you can get GTR and GTL blades which are angles right and left and will tend to leave the pip on one side or the other

Blue Heeler16/09/2020 07:12:12
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Posted by JasonB on 16/09/2020 07:08:54:

I see from previous posts you are using an insert blade, rather than use GTN ( Neutral) you can get GTR and GTL blades which are angles right and left and will tend to leave the pip on one side or the other

Thanks Jason, I'll have a look and see if I can get some of those inserts.

I have a lot of small repetitive pieces that I have to part off occasionally. If these inserts are hard to come by, I might go back to using a HSS blade. Appreciate the reply.

Clive Foster16/09/2020 08:53:31
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Its a question of rigidity.

The part in the chuck is nice and rigid but the part being cut off is unsupported and effectively flapping around in the wind. So when the stub gets very small its easier for it to break off at the stiffly held end than to be cut.

Narrow blades and high speeds tend to be better but our smaller manual lathes may not be able to operate in the best range. As always with carbide inserts GTR and GTL blades work best at their design speeds and feeds. Something worth checking before you buy as there are some very demanding ones about.

The traditional way to avoid a stub on a parted off part with a HSS tool is to grind the end at an angle to as to put the narrowest part of the stub up against the bit being cut off. Unfortunately this creates a side force which tends to bend the blade so you need a relatively thick blade or tool to make straight parting cuts of any depth. Thick blades are hard work for small lathes, maybe too hard.

Clive

mechman4816/09/2020 12:18:51
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...'The traditional way to avoid a stub on a parted off part with a HSS tool is to grind the end at an angle to as to put the narrowest part of the stub up against the bit being cut off. Unfortunately this creates a side force which tends to bend the blade so you need a relatively thick blade or tool to make straight parting cuts of any depth. Thick blades are hard work for small lathes, maybe too hard.'

This is why you will always get a convex face on the chuck side of the work, conversely concave on the parted off piece. When parting off I try to leave approx' 0.5 mm for final facing to size of the parted piece.

George.

JasonB16/09/2020 12:32:39
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If your small parts are less than 12mm dia then the Nikcole Minithin holder and inserts are good and rigid even with a handed insert though I mostly just use a neutral one. Being only 1mm wide you don't load up the machine or waste material

Phil P16/09/2020 13:08:25
639 forum posts
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Jason

Can you reveal where to buy those holders and inserts please.

Thanks

Phil

Henry Brown16/09/2020 13:21:47
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When I operated a capstan lathe with a bar feed collet chuck back in the day we used to intend to have a stub on the parted off piece as the tool was designed to face the next part and put a chamfer on, then the bar was pulled out for the next part. When the batch first end was done the facing tool was set so the parts could be just popped into the collet and faced off, and chamfered, if required in one pass. I vaguely remember asking why it was done this way and was told it was down to tool rigidity and repeatability. If I remember correctly the parting blade was about 4mm wide.

JasonB16/09/2020 15:18:26
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I got mine from MSCdirect (UK), if you keep an eye on their monthly flyers they often have the holders and some of the common inserts discounted

Vic16/09/2020 15:55:48
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Posted by mechman48 on 16/09/2020 12:18:51:

...'The traditional way to avoid a stub on a parted off part with a HSS tool is to grind the end at an angle to as to put the narrowest part of the stub up against the bit being cut off. Unfortunately this creates a side force which tends to bend the blade so you need a relatively thick blade or tool to make straight parting cuts of any depth. Thick blades are hard work for small lathes, maybe too hard.'

This is why you will always get a convex face on the chuck side of the work, conversely concave on the parted off piece. When parting off I try to leave approx' 0.5 mm for final facing to size of the parted piece.

George.

I use one of these blades on my 8” x 14” lathe and haven’t noticed any bending issues.

**LINK**

Blue Heeler16/09/2020 23:58:30
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204 forum posts

Appreciate the replies one and all.

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