Question about G H Thomas's rear toolpost

Hi
Can someone tell me why the GHT rear toolpost has the parting blade
pointing down at an angle
instead of 90 deg to the lathe center line ?
I dont see other ones with the same setup - was it a bad idea ?
Tim
Reply to
TMN
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Gives you a trivial way to adjust it to center height - extend the blade to reduce height, retract it to increase. No need to fart around with shims etc.
The new Taig cutoff tool, which fits at the rear, works that way too.
Regards, Tony
Reply to
Tony Jeffree
-more importantly it also provides back rake without having to grind the top edge of the tool bit. All explained very clearly in GHT's excellent book.
Mike
Reply to
mike.crossfield
Good question - good answer!
There was a nice display of GHT's tools and gizmos at Leamington last October. (Shame about the horrible display case they used.)
Reply to
Myford Matt
The blade is angled down by seven degrees, roughly equivalent to the top rake for steel if the blade were inserted horizontally. This way, one simply sharpens the front edge of the blade, the rake is then automatically set when the tool is inserted into the angled toolholder. Clever or what! --
Chris Edwards (in deepest Dorset) ..."There must be an easier way...!"
Reply to
christopher
I have not seen the Thomas tool holder being discussed but I assume that the actual cutting blade is of a constant section. I use an Eclipse parting tool holder which holds its blade horizontally. It does mean that it has to be correctly set on the centerline and it had no top rake but it does have one BIG adavntage. The blade can be progressively extended in stages to part through large diameter material. I have parted off 60mm diameter mild steel several times even though the blade is onlt about 1.5mm wide.
Ian Phillips
Reply to
Ian Phillips
More significantly, It puts top rake on the parting tool without having to grind it. This is significant, since if you grind top rake into a parting blade, when the blade wears, you would have to grind back to completely untouched metal....
As the blades taper from top to bottom, when you grind away the top, the ground part is less wide than the top of the blade. If you used the ground section as the new cutting edge the tool would bind.
IIRC this was GHT's reasoning in the book.
HTH
Mark Rand RTFM
Reply to
Mark Rand

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