GHT Rear Toolpost 8deg milling cutter

[Salty SpudsParticipant @saltyspuds]

[Salty SpudsParticipant @saltyspuds]

Evening folks, I'm in the middle of a Hemingway GHT Rear Toolpost kit for the S7. I'm very much on a (re)learning curve WRG to lathe work, so please be gentle & use lots of lubrication.

The turret requires a slot milling in it to carry the parting blade, straight forward enough…however, the slot needs undercutting by 8deg on the top & bottom edges to create a dovetail for the parting blade to seat into. GHT blithely mentions in passing that he simply makes up a cutter out of silver steel to perform the undercut…hmmm. The supplied Hemingway drawings show the cutter in its completed state with no info how to make it. I'm assuming that this is something a more experienced bloke would take in his stride, but it's beyond my current skillset. I've searched the forum & the internet for either guidance or an off the shelf readymade 1/4" 8deg dovetail cutter, all to no avail.

Any help would be appreciated.

Oh, and a happy new year.

[David George 1Participant @davidgeorge1]

Hi Salty I made a rear toolpost a while ago and made a cutter from a slot drill. It was a straight flute cutter about 10mm diamiter I ground one side away and then by hand ground the angle using a protractor to check angle. I then tried it out to check angle adjust and checked again.

Happy New Year too all🤞🏻🍻

David

[Tony Pratt 1Participant @tonypratt1]

I made the rear toolpost years ago & did make a special cutter but I'm sure it would have been bigger than 1/4" diameter but maybe not? Google '1/4" 8 deg dovetail cutter' there are loads of hits on router bits, one may be suitable.

Tony

[Martin ConnellyParticipant @martinconnelly55370]

Any dovetail cutter of 8° or more will do. If you have one of more than 8° then just tilt the workpiece by the angle difference.

Martin C

[JasonBModerator @jasonb]

Single point tool in a small flycutter will also do the job

[BizibilderParticipant @bizibilder]

You could also cheat and grip the work at an angle of 8°. Then cut a groove at the top and/bottom of the tool slot using a regular small diameter slot drill or a slitting saw. The groove you cut will penetrate slightly into the back (vertical when in use) wall of the cutter slot.

[John PurdyParticipant @johnpurdy78347]

Here are a couple of pictures of the one I made when I made mine many moons ago. Its made from 3/8" drill rod (silver steel). The 8 deg. angle was first cut in the lathe then the teeth were cut using a 1/4" end mill indexing around by eye. The back of the teeth were then backed off with a file to leave a land at the cutting edge of about 1/32". Hardened and tempered back to light straw.

John

[Clive Brown 1Participant @clivebrown1]

Here's mine. Again 3/8" dia. silver steel, as per GHT. 2 flutes, An hour or so to make.

[old martParticipant @oldmart]

I have made three, 2 off 26mm and 1 off 32 mm for standard industrial parting blades. I used a standard dovetail cutter and angled the work to leave the required angle for the mini dovetail. As Martin Connelly has already suggested.

 

Edited By old mart on 31/12/2020 20:48:04

[Chris Evans 6Participant @chrisevans6]

When I made my holder some years ago I seem to recall 7 degrees as a requirement. I used a 14 degree inclusive angle dovetail cutter from my woodworking router set. High spindle speed and very light cuts worked. Difference of 1 degree to your 8 degree requirement may not be noticed over the depth of the slot if you use 14 degree inclusive ?

[old martParticipant @oldmart]

I have just looked at industry standard parting blades in 26 and 32 mm depths, and the angle of the edge is 150 degrees. That is like the roof of a building with a shallow pitch of 150 degrees. That means the angle of the holder should be 15 degrees. The op's holder may not be a standard profile, but needs clarifying.

Edited By old mart on 31/12/2020 22:27:50

[bernard towersParticipant @bernardtowers37738]

Have to say that John and Clive’s are just the same as mine and it worked very well and as they say not difficult to make.

[Clive Brown 1Participant @clivebrown1]

Posted by old mart on 31/12/2020 22:25:55:

I have just looked at industry standard parting blades in 26 and 32 mm depths, and the angle of the edge is 150 degrees. That is like the roof of a building with a shallow pitch of 150 degrees. That means the angle of the holder should be 15 degrees. The op's holder may not be a standard profile, but needs clarifying

The GHT toolpost is designed to hold "Eclipse" type HSS blades. These have a single chamfer at 8 deg. across the full width of the upper and lower faces.The cutting edge is formed at only the one end.

The insert blades are double ended, so, unlike the Eclipse, have the double chamfer to go either way in the holder.

Edited By Clive Brown 1 on 31/12/2020 22:53:18

[Phil PParticipant @philp]

I made a holder for an inserted tip parting blade that fits my QC rear toolpost on the Myford.

Here it is along with my drawing if it is of any interest.
I didn't bother with an angled cutter, I used a slitting saw set over at the angle I needed.
As all the cutting forces are upwards, I clamped the blade using a split at the bottom.

Phil

Edited By Phil P on 31/12/2020 23:22:27

Edited By Phil P on 31/12/2020 23:23:52

[Salty SpudsParticipant @saltyspuds]

Wow chaps! thanks for taking time to reply, it's much appreciated. plenty for me to chew on there. I had already toyed with some of the suggestions e.g. tilting the workpiece, using a router bit, a fly cutter etc. But I wasn't sure how feasible these were. Having only recently acquired the lathe, I'm having to buy tooling as the need arises, so I don't have a "pool" of kit to fall back on, I have to use what I have, or buy it. I do have some router bits which I could probably bodge up the cutter from. I'll let you know how I get on. Thanks again for the help & suggestions.

Dave

[Salty SpudsParticipant @saltyspuds]

Thanks to all the encouragement & pointers from the collective, I decided to "have a go", after all, the point of the exercise is to improve my skills & knowledge & hopefully produce something useful in the process. I worked with what I had & hopefully produced a workable result, time will tell.

[Martin ConnellyParticipant @martinconnelly55370]

Looking good. Before heat treating are you going to relieve the centre like John above has done?

Martin C

[old martParticipant @oldmart]

Phil P's drawing of his quick change blade holder shows the standard angle, 75 (15) degrees.

Getting the op's 8 degrees is just as easy assuming he has a mill.

Edited By old mart on 03/01/2021 18:27:41

[Salty SpudsParticipant @saltyspuds]

Posted by old mart on 03/01/2021 18:26:56:

….. is just as easy assuming he has a mill.

Edited By old mart on 03/01/2021 18:27:41

Whoa! steady on, I've only just got the lathe. I still dream about owning a mill.

[old martParticipant @oldmart]

Ok, with just the lathe you must have been doing some milling to get this far.

Think about tilting the work to the 8 degrees and then using a small cutter, say about 3mm diameter. It will leave an indentation, but that will not affect the way the clamping works at all.

Look closely at Phil P's second picture.

 

 

Edited By old mart on 03/01/2021 20:48:37

Edited By old mart on 03/01/2021 20:50:13

[John PurdyParticipant @johnpurdy78347]

Salty

When I made mine it was made on the lathe only before I had the luxury of a mill. The flutes were cut using a 1/4" milling cutter in the chuck and holding the embryo cutter in the tool post. The centre was relieved using an form tool ground on the end of a tool bit plunged into the end. The relief allows the cutter to cut more freely without rubbing in the centre.

John

Edited By John Purdy on 03/01/2021 22:05:27

[Phil PParticipant @philp]

Just for completeness here is the information on my other parting tool holder that takes a conventional HSS blade which is retained with a wedge.

These have given trouble free service for a few years now.

Phil

[not done it yetParticipant @notdoneityet]

Just for completeness (from my experience), – my advice would be to avoid those ‘cutters at an angle’ designs like the plague. I have one (bought as part of a set when I bought a QCTP but I only used it a couple of times. I know overhang with parting tools needs to be kept to a minimum. Changing the extension of the blade needs the tool height resetting every time the cutter is moved. No thanks, that type is not for me. I’ll stick to a parallel holder!

[Peter SansomParticipant @petersansom44767]

I made the rear tool post nearly 30 years ago. From memory once I had machined the dovetail the cutter was then filed the cutter before hardening. One good thing at the time was I had access to a heat treatment shop who hardened it in a salt bath.

The cutter worked with light cuts. Saw it in the last 12 months, took a while to remember what the cutter was.

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