Myford lathe tools, positive & negative top rake

[NigeParticipant @nige81730]

I have been using the set of Myford lathe tools that came with the lathe. The tools have a curve machined into the bottom face which fits into a matching curve in a tool 'holder' that sits underneath. The cutting tip height is adjusted by sliding the around the curve tool in the holder which changes the angle of THE WHOLE TOOL tipping the front up or down to get the height right.

Now, in another thread where I was having problems cutting it was suggested that my tool geometry might not be correct. I had checked that the tool had rake, not a lot but some, in all the right directions but i was getting really 'manky' ragged cuts even with oil to help the cut. I thought it might be down to the steel a black mild steel bolt (BS 4933 DIN 603/555). I changed to a piece of EN1A but although the results improved a little it was still very ragged. Then I took a close look at the tool in its cutting position and was suprised to find that allthough I had ground it with a little positive top rake the act of tipping it forward to get the cutting height right had was effectively presenting the tool with negative rake.

The answer was to grind the tool with a lot more positive rake such that with the tool tipped forward to get the height right it was still presenting with negative rake. This improved the finish on both bits of steel quite a lot. I now understand the negative comments I have seen occasionaly about these 'rocking boat' Myford tools.

Sorry the photo is upside down could a mod fix this please but I think you can see the positive rake effect.

Edited By Neil Wyatt on 30/07/2017 22:06:43

[NigeParticipant @nige81730]

[Clive FosterParticipant @clivefoster55965]

Nige

Suggest you ditch the curved bottom boat tools in favour of conventional flat bottomed ones. The interaction between tilt, grind depth and cutting angle makes it very difficult for a novice to establish consistent performance when changing tools, ten times worse after a regrind.

Two slot blocks are easily made from stock bar and plate. They hold the tool well with very repeatable position and, if you make several, interchanging pre-loaded blocks makes an adequate poor boys QC system. If you do go the interchangeable block route its probably a good idea the set up simple measuring gear so you can set the tools up off the machine. Far easier to see what you are doing and, given a set of sorted to size shims, direct measurement makes shim selection and height setting almost trivially easy.

Clive.

[NigeParticipant @nige81730]

Thanks Clive. I think a QCT is a priority on the shopping list

[Neil WyattModerator @neilwyatt]

For now, grind some more back-rake onto the top of the tool as well and it will cut much more freely..

[RobboParticipant @robbo]

When new, Myford quick-setting tools were ground with sufficient top rake to allow for swivelling of the tool to meet centre height.

What frequently then happened was that users subsequently sharpened the tool and re-ground the top rake to the usual standard for a tool used with packing, thus making them unsuitable for the swivel base.

The top of the tool should only have been given a "quick touch" with most grinding done on the front and sides.

[thaiguzziParticipant @thaiguzzi]

Posted by Neil Wyatt on 30/07/2017 22:08:55:

For now, grind some more back-rake onto the top of the tool as well and it will cut much more freely..

+1.

Wot he said.

[Neil WyattModerator @neilwyatt]

Looking at the scale on the side of that tool – did it come with the lathe?

I suspect it might be a carbon steel tool of similar vintage to the lathe itself, it doesn't look like HSS.

If you want quick, cheap success, order a set of 8mm or 10mm pre-ground HSS tools. These will need packing, but will work well and will give you a better idea of the angles and finish you need on the tool tip to get decent results.

[Martin ConnellyParticipant @martinconnelly55370]

The support shown also increases the overhang of the tool compared to packing. The smaller the overhang the better the finish. Excess overhang can also contribute to chatter.

Martin C

[NigeParticipant @nige81730]

Robbo and Neil: Thanks guys I did grind some more top rake into it. I don't know if it came with the lathe originally, I suspect they might have been bought afterwards though it is from a Myford set of tools, photo below. I wonder if they are carbon steel as I am having to regrind that tool quite often. I suspect it may have been overheated in a previous life as I am very careful with grinding only taking light cuts and cooling in water after every pass. I do have another collection of tools which are definitely HSS (second photo) but I think you are right Neil, a ready ground set would get me started, thanks for that tip

and the other 'set'.

Any comments on this second set, which would be good for what sort of cut would be welcomed please

[Anonymous]

Posted by Nige on 31/07/2017 10:47:11:

Any comments on this second set, which would be good for what sort of cut would be welcomed please

That's a pretty bizzare set of shapes. There's internal and external threading, but most of the rest seem off the wall shapes. Probably specials.

You'd be better off sticking with the Myford set, or buying some new HSS blanks and starting from scratch.

Andrew

[Neil WyattModerator @neilwyatt]

Cripes. Antiques…

I would put them aside, buy a new, affordable HSS set to learn with.

The Myford ones may be brazed on carbide tips, I would need to see a close up. If carbide they would be difficult to grind. Cheap ones like this are troublesome on small lathes, these may be better quality, but need a different approach to HSS to work, if they are carbide, save for later.

WD marked (the arrow), so ex-services.

Top to bottom:

• Fine parting
• LH finishing and reverse facing
• Internal threading (probably 55 degrees)
• Internal boring
• Light parting
• Fine LH finishing (looks like it's upside down)
• Threading
• Knife tool (probably intended for facing/RH corners)
• Getting into very small corners tool.

My guess is they were a collection used for making screwed fittings, probably in brass/gunmetal due to the apparent lack of top rake.

[thaiguzziParticipant @thaiguzzi]

Ca'nt see from the first photo exactly, but that blue set "may" be butt welded HSS, in which case, persevere with that set.

Basic HSS tool shapes and angles are online in the South Bend "how to run a lathe" book, which are similar to the Boxford owners book, and again the LH Sparey book.

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