Rear parting toolpost for Myford S7LB

**BaronJ** · 3rd July 2015, 07:12 AM

Hi Guys,

I've not been totally idle for the past few weeks ! Apart from working on my van/rv conversion, I had the need to replace my old parting off toolpost. Since I could only part off 12 mm diameter with it and the need arose to be able to part off 1 inch diameter material. I happened to have a nice 1/16" inch by 3/4" inch parting off blade so I thought that I would press it into service.

I obtained a 120 mm length of 50 mm diameter bar and machined three flats on it. Using these flats I milled one end to create the key that fits into the slots on the Myford cross slide. I cheated a little and used the original clamp bar from the old toolpost, just cutting a short length of M8 all thread to suit the hight of the new toolpost.

Then carefully measuring the centre hight I machined a 3 mm deep by 22 mm wide groove across the left hand face, looking from the front of the lathe. Then I drilled two holes right through the bar with M4 clearance holes, setting the hole centres 4 mm from the top edge of the groove and 30 mm apart. Using these two holes as pilots I drilled them out 10 mm diameter with a ten degree taper 20 mm deep so that the top of the tapers broke into the machined groove by two mm. The idea being that the tapers would grab the top of the parting blade and secure it in place. In hindsight one millimetre would have been plenty. I used a 10 degree dovetail cutter to create a taper on the bottom edge of the groove and to accurately set the tool hight so that it was a couple of thou below centre hight.

I turned two tapered studs with M4 threaded shafts from silver steel, so that they would sit a few mm above the surface of the bar and the taper would grip the top edge of the parting off blade locking it into the groove.

The photographs show the various bits and pieces. I didn't take any pictures of the actual machining, mainly because of its experimental nature.
But it works and works very well. Certainly better than I expected. It makes parting off a breeze, The last picture is one of the makers details of the parting off blade.

**krisfarm** · 3rd July 2015, 10:19 AM

Hi BaronJ
A nice simple design and nicely machined.A rear mounted inverted tool post are the most ridged for parting on any sized lathe and work really well on small machines. I helped a mate of mine fit one on a small Seig lathe and now he can part off. The only suggestion I would make would be to machine a small flat on both of your blade lock down bolts so as to increase the area of the stress point on the blade as they are prone to snapping at this point, with just the small edge of the lock down bolt doing all of the holding I found that the blade soon bit into the bolt and worked loose after a bit of use.
Bob

**BaronJ** · 4th July 2015, 02:29 AM

Hi Bob,

That is a good idea ! I can already see marks where the parting blade has dug into the taper. I reckon a quick rub with a smooth file should be enough. I'll have to put a small flat onto the edge of the bolt as well, then I will know which way is up

Thankyou for your kind comments.

Originally I was just going to make a pair of "Top hat" bolts but was concerned that they wouldn't force the blade down into the V groove at the bottom, with the danger that it might spring free.

**krisfarm** · 4th July 2015, 07:58 AM

Hi BaronJ
The file made flat should do it,just make sure it is wide enough to allow it to all ways compress the blade as you tighten. You made the right decision about needing to maintain a tight fit, any movement would soon result in a broken blade as parting off requires very high tool pressure.I always use a good supply of oil as well and a sharp tool and parting turns into a joy.
Regards Bob

**nadroj** · 4th July 2015, 10:23 PM

Congratulations, that's a neat job.
One thing puzzles me - is it possible to adjust the tool height?
Or is it not so important to worry about?

Jordan

**KBs PensNmore** · 4th July 2015, 11:53 PM

Once it's set to the centre height, why would you want to change it. If for some reason you'd want the cutter a bit lower, grind the front edge of the cutter down a bit.
Kryn

**nadroj** · 5th July 2015, 01:11 AM

So, the blade is used as is, without grinding before use?
Most parting blades I've seen don't have a level "top" edge though. Rather they have a bevel all along one edge, or the whole top is at an angle.
Also, wouldn't it cut better if it had some back rake?
No doubt it's a workable system, but I'm curious as to how it all works.

Jordan

**BaronJ** · 5th July 2015, 05:45 AM

Originally Posted by nadroj

Congratulations, that's a neat job.
One thing puzzles me - is it possible to adjust the tool height?
Or is it not so important to worry about?

Jordan

Hi Jordan,
No the tool hight cannot be changed without removing metal from the base of the toolpost. in fact the parting off blade was originally about 10 thou too high. i had to give the base a rub with a file to drop it down and onto centre.

Re. The top edge of the blade, yes it is angled all down its length, and you are right there is no back rake. in hindsight I do wonder if cutting the groove for the blade would have been better done at a slight angle to create some. in any event It does work extremely well. i've parted off inch diameter silver steel bar with it without a problem. But I will try grinding a small relief behind the tip to see if it makes any difference.

**nadroj** · 5th July 2015, 07:39 AM

Thanks BaronJ,
Good to have confirmation that it works well as is.

Jordan

**DSEL74** · 5th July 2015, 11:02 AM

Do you have to worry about the chuck spinning off if it is running in reverse for the rear parting tool……….Oh just go it the lathe runs in normal direction with the parting tool upside down.

**BaronJ** · 6th July 2015, 07:54 AM

Hi Guys,

Dean:
You got it

Jordan:

I put a small flat, a couple of millimetres long angled about 3 or 4 degrees right behind the cutting edge and honed the face with a stone. It does make a difference ! The blade does cut a little better. Now I don't know if that is because the slight angle that was on the cutting edge has now gone or because of the added rake. Either way it is definitely an improvement.

**nadroj** · 6th July 2015, 08:51 AM

Thanks BaronJ,
That fits more with what I've read in text books, and how I do it - having some back rake for an easier slicing action.
Interesting that it's still quite effective even with no back rake.
Of course, once you start modifying the shape, you lose the constant height feature. And, to recover a blunt edge, you have to remove more than you would if it wasn't ground with back rake.
I wonder if there's a way to design a parting tool to provide adjustment, without losing too much rigidity - and without resorting to an angled blade?

Jordan

**KBs PensNmore** · 6th July 2015, 09:26 PM

Originally Posted by nadroj

I wonder if there's a way to design a parting tool to provide adjustment, without losing too much rigidity - and without resorting to an angled blade?
Jordan

Just a thought, would it be possible to make a QC mount for the rear post, and that way adjustment would be available, regardless of angle of the cutting tip.
Kryn

**nadroj** · 6th July 2015, 11:10 PM

Maybe a quickly adjustable, like a Norman patent toolpost, dedicated for parting off use?

Jordan

**KBs PensNmore** · 7th July 2015, 12:53 AM

Don't know what a Norman patent one is, but it sounds good.
Kryn

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