Drilling and tapping carriage for rear tool post

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GEARS

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Gentlemen, I think the bigger problem has not been mentioned. That is “forgetting it’s there” especially on small lathes. I have seen nearly finished work like crank shafts with multiple journals ruined in a split second.
 

awake

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<regarding setting the parting tool on center-line>
I have seen it in writing from reputable sources (once on the Sandvik website even) although I do set mine as bang-on as I can. Too much away from the centreline against the rotation direction is obviously bad for dig-in and too much in the direction of rotation can lead to the work 'climbing over' the tool. I believe the recommendations I've seen that advocate for 'slightly high' (for a front toolpost) such that the deflection in the toolpost and the tool itself result in the cutting being done exactly on centre. Probably easy to refine in batches of hundreds on a CNC machine but not easy to do at home.
I have a parting tool that not only has a "T" shape to allow additional clearance, but also has a "U" shape along the top surface, which helps to curl the chip inward so that it is less likely to catch. However, the "U" shape means that it is a bit hard to determine where the actual cutting edge is, or should be set - at the bottom of the U? Or at the top? I find I have to set it with the top of the U a bit above center, so that the bottom of the U is on center.

Not sure if that made sense trying to describe it in words ... if needed, I'll go take a picture!
 

Shopgeezer

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I have seen that U shape in parting tools but how on Earth would you sharpen it?
 

ShopShoe

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FWIW,

What I will say is not very "scientific," and I think science is a good thing, in general.

It took me awhile to figure out parting, but I can say I've got there at this point.

My parting blades are like you describe above in post #24. I have a mini-lathe, so they are small and do small work. I set the blade with the trick of putting a 6-inch scale (ruler) between the work and the point of the cutter and making sure the tool height is adjusted so the scale sits vertically, or very slightly away from vertical toward the operator. What this does, in my opinion, is even out all the variables to create the best situation for me, in my shop, on my lathe, with my cutting tool. An old trick, but works for me and is simple to do.

In addition, I pay special attention to keeping the tool sharp. In parting a dull blade will dig or climb with the greatest of ease.

--ShopShoe
 

BaronJ

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Hi Guys,

Shopgeezer: The blade has a flat face so that is the only bit to sharpen.

Shopshoe: That is a very old trick and one worth remembering. Being lazy, I commonly use it to center a drill over a round surface !
 

awake

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Right, for a U-shaped blade, you only sharpen the front face; don't touch the top. Shopshoe, I used the ruler trick all the time to set / check blade height (and once in a while to center a drill on a round surface!), but at least on my lathe, if I set my U-shaped blade that way, it does not perform well - I have to set it higher than I would normally set a blade so that the bottom of the U is closer to the center line. It took me quite a while (lots of failed experiments, shall we say) to determine that that was what worked best on my lathe. As always, though mileage varies!
 

Shopgeezer

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Struggling a bit with the U shape. If the front face is flat, where is the U? Along the top edge? Seems that it would have two sharp points at the apex of each arm of the U. That would suggest digging in to the work. Sorry to be thick on this but I have never seen or heard of that shape.
 

awake

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Struggling a bit with the U shape. If the front face is flat, where is the U? Along the top edge?
I'm not surprised, since I was confusing myself trying to explain it! But you've got the idea. Here are a couple of pictures that should help. First, here is a close-up of the business end of the parting tool:

IMG_8062.JPG

And here is a closeup of the chip that it produces:

IMG_8063.JPG

Notice that the chip is curled up not just like a spiral (well, normally it would look like a "flat spiral," but this got pulled out of shape when I was extracting it from other chips in the chip bin), but also from side to side, if that makes sense. To say it another way, the chip is curled up like a spiral around the axis and is curled into a slight u-shape across the axis. The idea is that the latter curl makes the chip slightly narrower than the width of the parting cut, helping to keep the chip from jamming.

Seems that it would have two sharp points at the apex of each arm of the U. That would suggest digging in to the work.
Yes, it seems like it would. I'm guessing the reason it doesn't dig in has to do with three things: 1) The advantage of the curled chip as described above; 2) the fact that the U is fairly shallow; and 3) the "horns" are sharp and small, so rather than digging in, they easily slice through the steel. Please note that I don't know any of these for sure - just my guesses!
 
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BaronJ

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Hi Guys,

Shopgeezer: If you have a close look at carbide insert parting blades, the carbide insert has a spoon shaped front edge. Apart from chip breaker action it causes the chip to fold away from the sides of the cut.

Andy: I've experimented with making a groove like you show, using a Dremal tool with a cutting disc. Admittedly without too much success. I found that as long as the blade was dead square and truly vertical, getting it on center height cured most problems. It was only after I went to a rear toolpost that the minor issues went away. Having said that plenty of cutting fluid particularly with aluminum is a must.

I went through a spell of blade jamming when parting aluminum, and later found out that if the aluminum smeared on the sides of the parting blade I would get jams. I now use a squirt bottle and diesel fuel for lubrication. It works better than WD40 in this application. Particularly if the aluminum is a bit gummy.
 

fcheslop

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Another fan of the U shaped parting blades,Used them for many years in a rear tool post the cantilever version that over hangs the cross slide to give more room between the tooling
 

goldstar31

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Most. of us 'oldies' who are both very old, live alone and are ill in this virus which actually started in February at the Chinese New Year. My health people will not visit, are loathe to telephone and a raft of ' ot is not my thing' On the Home Front( I lived through a 6 year war) and then into uniform.
Now things are breaking down and cannot be replaced.
Sheer and utter frustration reigns and Heaven help those who 'start interfering'
So whilst I am in a good mood, might I add a few hopefully pertinent things?
Initially, there is little difference between a Myford and a Sieg. I have both- and for the SiegC4 I've been sensible enough to buy a mill drill attachment for the price that I paid for the second hand lathe itself.
Tooling, if you know what you are doing, will happily swop.
My new second hand Myford is just running. The split phase motor is now OK and I bust one of the Tufnol cluster gears because the spindle belt was streched, oul soaked and frankly- bloody iseless. Some oone had added a no slip concoction to make matters worse and I changed it all for link belting and had to invnt a special tool to aid my arthritic fingers.
So it's all ready to take the rear parting tool which has lived and performed adequately on 'all sorts of lathes'. It is one of those tools which reached the end of sensible modifications and that is that.
If someone wants to add a new dimension of tungsten carbide tooling to do the same job but at greater expensem that is up to them.

All that I have to do with my aged contraption is to 'give it a lick' on the grinder. fit it and apply an even older dollop of lard oil- and heigh ho, what's the problem.
This U or vee thing os nothing new, I bought a cheap box of assorted hss tools unseen at a show and found then all with chipbreakers- ready ground. These will go on my homemade ratchet 4 position tool holder to face the rear one. If I can be minded, I can make a bit for the rear tool holder which will accept a conventional tool- and sort of work as crude capstan lathe. Whether it is stuck on either lathe is immaterial.
What is important in ALL machining is 'that little bit' at the end of each tool.
I tried a couple of very expensive dedicated inserts in the commissioning( fiddly about) stage- and the words in my lonely little workshop were-- What a load of bloody crap?'
 

SmithDoor

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The only time I use rear tool post was on long runs on turret lathes and screw machines
On engine lathe I use a quike change tool post for short runs
The rear tool post just would get in the way

Dave

I am making a rear mounted tool post. I will have to bore holes in the carriage for 1/4-20 cap screws. I am wondering if cobalt drills or even HSS drills will work. Also , will HSS endmills work for a counterbore. Only 4 holes are needed. I will start with #6 drill and try to tap the hole.
mike
 
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