Parting off blade

I think it was in 'Instructables'.

Geoff

It is not only parting blades that can be made this way, I have in the past used this technique to make various special cutting tools. I picked up some bargain blades a number of years ago and have been nibbling away at them for ages to make lathe cutters. It is a very cheap way of getting carbide cutting tools.

Doug

Latest MEW has an article on using cut up saw blades

I used something like this only recently,when I needed to cut narrow groves to take cir clips. Having nothing else to hand,I used a short length of broken hacksaw blade fitted to a holder held in the tool post. The holder was simply a piece of scrap 5/8" square with a slot cut in it and two small screws to clamp the blade. I t worked a charm,and replacements are cheap.

HSS slitting saws, but the principles are the same.

Neil

Perhaps being a novice, I find "parting-off" to be the most imprecise, "fingers-crossed and hope" operation on the lathe. I've given up with it. I now just cut a V-groove where I want to part off - to a depth that provides a guide slot for a hacksaw blade to run in, and then just gently run the hacksaw back and forth while the lathe is turning. It works - and I know it will work every time - and the finish is good enough to only need a light facing-off if necessary. Because the V-groove operation works so well - I'm thinking of grinding a V-point onto the front edge of a parting tool to see if that makes the classical method work better for me.

I find that a V groove in the front of a parting tool can help. Don't be frightened get stuck in.

Roy

As Roy's post suggests, most parting problems are caused by lack of confidence. Few of us would worry about taking a normal cut the same depth as the width of our parting tool. I think most problems come from two things:

Chips jamming in the cut, lubrication or a shaped tool tip help here.

Lack of positive feed, so the tool alternately rubs then grabs. When it grabs all the factors that upside down toolposts, rear toolposts and 101 other 'solutions' come into play.

Confident feed is the route to success for most of us. This is on my mini lathe:

I've discovered high speed parting off under power feed with carbide works brilliantly - but on aluminium a constant supply of lubricant is essential top stop it welding to the tip. 500rpm for 50mm 6082 is easily achievable but <300 rpm reduces the tendency to weld.

Neil

This is PB parted, 4" diameter, surface finish OK Standard blade insert.

Have you tried a rear tool post holder set up ?

George.

I’ve tried the old style HSS blades and carbide inserts but find the T shape HSS blades with chipbreaker to be the best.

**LINK**

On a small lathe without coolant carbide inserts can work out expensive.

I'm a rear tool post man on both my lathes.

Parting off and grooving is no longer feared!

JIm

On the manual lathe parting off is done with 3mm carbide inserts. Dry on cast iron and brass, flood coolant for everything else. To avoid chatter and other nasties minimum powered feedrate is 4 thou per rev, I normally use 6 thou per rev. There's no problem going deep:

For grooving on the manual lathe I use HSS, the carbide inserts chatter if you try and creep up on an important dimension.

On the repetition lathe I use HSS blades. I don't drop the speed for parting off, so normally done at 1000rpm. Feed is by hand, but the swarf is around 8-10 thou thick.

I haven't bothered with a rear toolpost, never felt the need. Ok, so the parting tool on the repetition lathe is at the back, but it's the right way up and the normal running direction of the lathe spindle is backwards. So it doesn't count.

Andrew

Edited By Andrew Johnston on 12/06/2018 19:34:08

I have used a secondhand HSS parting off blade in the rear toolpost for fifteen years. Dig ins are almost unknown.

Used a shop made rear toolpost on the Myford for several years before that.

The last time that I used an inserted tip parting tool in the front toolpost, (last week!) it dug in twice and broke the tip each time. This was an expensive prelude to using the hacksaw and then facing. (The normal, four way indexing, back toolpost had been removed to display at an exhibition!)

There are various theories advanced as to why the rear toolpost method is superior, but valid or not, in practice it works better for me!

A steady infeed is important, don't be heavy handed, but don't be frightened.

Howard

I'm sure I saw the carbide saw blade cut up for parting tool idea in an ancient back issue of MEW some years back. Have had an old blade sitting under my lathe since then waiting to be chopped up. But my standard HSS 1/2" x 3/32" mounted upside down in the rear tool post will cut up to 2" diameter steel without dramas so have not bothered. I usually dribble a little cutting oil/water mix on it as I go. Run the lathe one speed slower than I would for turning, so about 100 rpm for 2" steel bar. Steady infeed is indeed important too, as Howard says, although I like to briefly withdraw the tool from the groove every now and then, let it cool off and the chips clear and the cutting oil get in there..

Edited By Hopper on 14/06/2018 02:53:41

Edited By Hopper on 14/06/2018 02:58:21

Hopper,

that is one beautiful rear toolpost. Love it.

It came with my old M Type Drummond when my old man bought it many years ago. Presumably made by a previous owner as I've never seen another like it. It's a beauty. Solid as.

Edited By Hopper on 14/06/2018 06:51:02

One evil, but easily overlooked, gotcha when parting off with tools held in QC or 4 way tool posts where the cut axis is significantly offset to the tool post fixing is the significant twisting force on the whole assembly. I've lost a couple of parting blades and an insert holder that way when the QC post shifted slightly under the temporary extra loading of a chip hanging up. Both times in difficult materials.

Fairly sure that the root cause in my case was using synthetic cutting oil. Rocol Ultracut at book dilution to be specific. The stuff eventually gets everywhere, is seriously slippery and a right pain to clean off fully.

Finally twigged to that problem after wondering for several years why my Pratt Burnerd 180 mm precision 3 jaw chuck, bought new and not abused, didn't seem too be holding as well as it used to. Full strip & rebuild didn't fix the problem so I showed the gripping faces to a fine rotary wire brush. After some fairly aggressive attention the faces looked good and subjectively had a proper "plain metal" feel. Instant fix. Grip back to what it always was.

A more common reason for tool posts being prone to twist is top slide distortion. Its important that the post sits down on the outer parts of its base rather than close to the centre. Clearly the further out from the centre the main contact points are the more leverage it has against twisting. Most tool posts have the centre relieved to help ensure that. However the relieved centre provides no support to the topside around the stud so after many years the top can distort slightly significantly reducing full pressure contact area. This is quite likely with Tee slotted top slides when used with a short Tee nut. Especially so if the nut is shorter than the relieved centre portion of the tool post base. Much better to use a long Tee nut extending the whole width of the slide. The top slide on my big Pratt & Whitney had suffered badly in this fashion. Had to re-work the whole Tee slot, both top and bottom, for a secure hold and plenty of contact area. Previous owners just used a bigger spanner which eventually proved counter productive.

A thin card or alloy "gasket" under the tool post often helps as being able to deform slightly to take up small errors.

Clive.

Edited By Clive Foster on 14/06/2018 09:26:08

As a bit of bits and braces double security parting off tool. I bought one of the new style parting tools with the carbide insert, and since I needed a holder any way I mounted it upside down, getting the best of both worlds.Can't be too careful parting off.