GHT Rear Toolpost 8deg milling cutter

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.

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

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

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

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

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.

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

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

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

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 ?

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

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.

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

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

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

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.

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

Martin C

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

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

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