Thread: Grade 8.8 M16 Studding - Machining

Whilst I cant comment on the type of tooling (though Ive got some supermax at school, its M2 and its pretty average in my experience, but I didnt give it a huge shot before moving to Sutton M42), I can comment on the metal youre working with. Bolts are usually manufactured by rolling and upsetting, both processes requiring a metal that is quite malleable. Usually, metals that are more focussed on malleability tend to not machine as well.

I would suggest trying some wks45. It is 11% cobalt. I use seco branded stuff bit I think there are plenty of different manufacturers.

As for carbide, I don't think you really have the power to get the.most from a negitive rake tool, so I would just go with an sclcr type tool with ccmt inserts or something like that. Buy decent brand inserts though, not the crap h&f sell.

Cheers,
Ew

Yeah, carbide would be the better option, although grinding some nose radius to your HSS tool will help a little.
Try putting a .7to 1mm radius to help strengthen the cutting edge.

Hi,

Try the Crobolt from Eccentric Engineering, its a lot tougher than standard HSS and fits in the holder. It's expensive though.

Ben.

Grade 8 has to be a tough carbon alloy steel. I found 8.8 and 12.6 bolts to usually machine very well and with a fine finish using CCMT inserts. I do most of these jobs dry, but would spin fast enough for light blue chips. I have not tried HSS (at least not in the past 20 years, and before I do not remember). But I could imagine even HSS-Co loosing its edge pretty fast when cutting high tensile bolts.

That said, high tensile fasteners (8.8 and above) are heat treated, the heads are forged, and the threads are rolled - not cut - to improve strength. That could make for some variation in hardness just below the thread surface, and affect finish. Also be aware some cheaper Chinese fasteners may be stamped 8.8 alright, but could be made of recycled soup cans.

Guys, thanks for the comments.

Crobolt is something I have seen reports on, definitely worth a try, together with some brand-name (cobalt) HSS.
The positive rake carbide are now on the list also.

However, I am not so sure about the benefit of some tip radius.
I tried a couple of different tip shape/approach angle combinations and could not get any improvements, always alternating between tearing and rubbing.
Once I got to a couple of degrees negative approach (could face with it) and a minimum DOC 0.1 with a very slow speed, it all came good (at least acceptable).
It seems the tool must dig-in and take a cut, else it skids off.
I am sure the presence of nice steady curls of continuous swarf is an indication of a good tool/job combination.

The problem is, by the time an acceptable setup is arrived at, the size has been reached and so no further opportunities to experiment are available.
John.

> However, I am not so sure about the benefit of some tip radius.
> I tried a couple of different tip shape/approach angle combinations and could not get any
> improvements, always alternating between tearing and rubbing.

This could be more a matter of lathe rigidity than material being cut. The larger the tip radius, the greater the cutting forces, the more deflection away from the cut.

> Once I got to a couple of degrees negative approach (could face with it) and a minimum DOC 0.1 with a very slow speed,
> it all came good (at least acceptable).
> It seems the tool must dig-in and take a cut, else it skids off.

The sharpness of your cutting edge (not to be confused with the tip radius) limits how shallow your depth of cut can be before it begins rubbing instead of cutting. That is why HSS cutting edges are honed after grinding. But that is all futile if the tool gets blunt before the pass is finished.

> I am sure the presence of nice steady curls of continuous swarf is an indication of a good tool/job combination.

Yes it is. That is easy with a large DOC, but needs a very sharp tool with a small DOC.

> The problem is, by the time an acceptable setup is arrived at, the size has been reached and so no further opportunities to experiment are available.

That is an art - not so easy to get to a precise diameter if only a large DOC produces a fine finish. If you have 100 pieces to make, it maybe worthwile stuffing up a few samples until you get it right repeatably and first time.

For a one off or just a few, I think this job would be much easier with something like a CCMT carbide insert. If you need to finish to a near ground surface finish and to exact dimension, use a CCGT insert for the last pass, it can handle very shallow DOC's without rubbing. That way you can approach the final dimension slowly and in very small increments without loosing finish quality.

That is all assuming you have a lightweight lathe with limited rigidity.

Another vote for carbide. CCMT, CCGT are good , I also like the WNMG inserts, the ones you can flip over and get 6 edges per insert.

As Ewan suggested, go for recognized brands, like SECO, etc..

Ray

As a final comment, I purchased a 12x12 mm threading carbide insert kit from H&F and successfully cut the M10 thread required to fit the topslide.

The new toolpost is now installed and first cuts will be attempted tomorrow.
If all seems Ok, the original supplied stud will be machined down as a virtual copy of the temporary stud.

The temporary will be put aside as a back-up spare.
I went the temporary route because I was not convinced the BXA sized toolpost was the correct size for the machine, much nicer to sell tools in original conditions.
But, the BXA 'looks' Ok on the topslide and anything smaller would have inadequate reach, so, looks like it is a keeper.

I will take the advice from you all to select some carbide tooling for the new toolpost.
Not sure if 16x16 though, as the tool holders are only just 16 mm with substantial clamp screw hole burrs preventing the direct fitting of a 16 mm tool.

The M250 is not a large lathe and perhaps 12x12 tooling is the correct size.

John

Interestingly, this topic came up just before I was going to buy some M14 Grade 8.8 bolts to make some hinge pins for a rolling block rifle I'm rebuilding.

I was wondering if I'd be able to machine them ok with HSS, which is all I have, and after reading this thread I got a little worried that I wouldn't be able to turn them. Well, for whatever reason I needn't have worried as they cut beautifully with just standard HSS. I was taking quite heavy cuts (for my Hercus), finish was excellent, I was able to work up to a final cut of 0.001" and I hit my dimensions perfectly. I also made some small custom screws from the same stock and it threaded lovely as well.

I can't identify what brand HSS I had as they were just random pieces that came with the lathe. Some were Supermax, others were probably good quality but with no markings on them. I have some generic HSS from ebay that I'm yet to grind and try but I doubt it will be that great but hopefully I'll be surprised.

Anyway, that was my experience cutting grade 8.8 bolts with HSS. I'd not hesitate to do it again.

I'm curious about carbide but most comments I read say the hercus can't push them hard/fast enough, but I still might try some of the high positive rake ones for aluminium.

Originally Posted by Hornetb

I was able to work up to a final cut of 0.001" and I hit my dimensions perfectly.

Hi, the question of tip radius was raised by Clubman7.
What were the tool/machine parameters for your roughing and finishing cuts ?

John.

I'm by no means an experienced machinist and a lot of what I do is trial and error so take that for what its worth, but I did try a few different tools that I had ground and some cut better than others. I tried ones with a bit more radius thinking it would be easier on the machine and give a better finish but found they didn't cut so great.

The one I did have the most success with had very little radius at all (ie. next to none) and was pretty much your standard HSS tool grind and probably pretty much matches what's written for steel here http://www.steves-workshop.co.uk/tip...olgrinding.htm but I don't tend to measure them exactly when grinding but more eye ball it. I'll look more closely at it tonight, but with the very tight point on it I was able to take the last light cuts quite accurately (taking off ~0.001"). I ran my hercus 9a at the lowest speed (without back gear - which I would guess is around 60 rpm, workpiece was 14mm diameter) and used thread cutting oil as lubricant and a pretty slow feed rate for a better finish for the final cuts. I still used a file and sandpaper (with my ways covered) to get the fit I wanted but that was probably more to do with the fact I wanted a very high finish quality and also that I didn't know the exact dimension that I needed so had to hand fit the pin. The first pin took me probably an hour and a half to make due to the fitting, but the second pin probably took me 20mins because I knew the dimensions

I probably tend not to measure or note things so much as I go but more work off what the machine is telling me and what sort of cut/finish I'm getting. To make the screws I had to knock 14mm bolts down to about 5mm and given they were only about 10mm long I just plunge cut with the parting blade to knock off a bunch of the material and then came back in with the tool mentioned above to clean it up and square it to the shoulder.

So, maybe not a lot of help and likely I just hit on a good combination, but I can just say I was surprised at how easily it machined.

Hi

If you wish to have a fine radius on the tip of your HSS lathe tool, something like .5 mm or less . A good method is to use a diamond plate - use a rolling action on the tip as you drag it across the plate . You then hone the edge with a arkansaw stone . I find that the grinding wheel is just too coarse and more often than not if I press a fraction too hard , the radius is too much Mike