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gr 8.8 ht bolt steel which carbide tip

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brian jones 1105/10/2021 15:19:39
347 forum posts
62 photos

Can anyone suggest how to select a proper carbide tip (10mm shank) for turning gr8,8 steel on a Myford

I have struggled with ones that came with those tool sets on EB but of course I am deeply suspicious about the quality of the tips - they will cheat you if they can , its a national sport)

I found a 60deg diamond tip seemed to loose its edge quickly and I ended up with a poor cut finish on 20mm dia taking 50mu cut at 1500 rpm

These carbide tips are still a bit of a mystery to me but I couldnt touch this steel with HSS

Any advice please

Brian

JasonB05/10/2021 15:50:20
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Tickling it with 0.05mm cuts is not the way to use inserts, for a TCMT try to cut at least 0.25mm, if you want to take finer cuts to sneak up on a finished size then use the ones intended for non ferrous which are TCGT

The shank of your tool won't affect the tip much, it's the size of tip that your 10mm shank tool takes that will affect overall size then the material dictates most of the rest

Andrew Johnston05/10/2021 17:20:42
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Posted by brian jones 11 on 05/10/2021 15:19:39:

.....couldnt touch this steel with HSS......

Something wrong there. Here's the head of a 12.9 M12 bolt I just machined with a 5/16" square HSS knife tool that just happened to be lying next to the lathe:

m12_bolt.jpg

Running at 540rpm with a feed of 4 thou/rev and DOC of 10 thou. The finish looks worse than it is real life. A few measurements gave an average surface finish of 1um Ra, which is about as good as I get when turning.

The tool sets from EB can go in the round filing cabinet. There are two rules for buying cutting tools:

1. Don't buy cheap cutting tools - applies to everything from files and hacksaw blades to coated carbide endmills

2. See rule 1

Perversely the rules apply even more to smaller lathes as they don't have the power and rigidity to overcome poorly ground or less than sharp tools.

In general I'd agree with JB regarding carbide inserts. They can create good finishes with small DOC, depending upon the material, but for carbon steels they prefer more like 10+ thou DOC and high surface speeds.

Andrew

SillyOldDuffer05/10/2021 17:23:09
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7675 forum posts
1693 photos

Never tried to machine an 8.8 bolt before, so in the interests of science:

dsc06529.jpg

I skimmed the top lip with a square insert, an interrupted cut bouncing off the hex head. Tough test on tough stuff, but carbide chewed into it reluctantly at 1900rpm without breaking. The motor is 1100W, but it wasn't working hard. Didn't remove much metal because I bought the bolt for a proper job and didn't want to spoil it.

8.8 bolts are now on my 'difficult but possible' list. I'd rather work with machinable steel!

Dave

Edited By SillyOldDuffer on 05/10/2021 17:23:32

Andrew Johnston05/10/2021 17:31:29
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Posted by SillyOldDuffer on 05/10/2021 17:23:09:

...8.8 bolts are now on my 'difficult but possible' list....

Better not try 12.9 then!

What was the provenance of the bolt? My experience of buying import tooling is that the SHCS fitted are made of cheesium, but may be some of them are tougher than expected? I don't worry about shortening bolts with a hacksaw when required.

Andrew

brian jones 1105/10/2021 17:31:32
347 forum posts
62 photos

I had to trim down an m24 bolt to a bit of 19mm shank so I could mount it into my collet

It wasnt a happy experience and I am not versed in carbide use but the surface finish was more like it had been smeared but i got my OD after a while. Its pushing Maureen to DOC 0.5mm. I kept think the tip was blunt - what it felt like - then I suspected the PRC tip of course

gr8.8 is tough stuff I didnt dare try HSS

 

but the question is, where to get a reliable tip for the job - there is soooooooooo much bs out there it leaves you totally confused

Edited By brian jones 11 on 05/10/2021 17:34:58

Ady105/10/2021 18:19:21
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4810 forum posts
717 photos

I've never got it to cut nicely, it always kind of deforms as you remove material and you get a lot of heat

Edited By Ady1 on 05/10/2021 18:20:01

Old School05/10/2021 19:06:34
392 forum posts
39 photos

I use 8.8 and 12.9 regularly sharp hss works well and if you want a really good finish do as Jason B says. Use good quality bolts as a source of material and the same goes for the inserts if you use them.

old mart05/10/2021 19:30:10
3392 forum posts
210 photos

8.8 is only one step up from a gutter bolt in strength and hardness, so should be easy with anything you try.

JasonB05/10/2021 19:30:19
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Well at great expense I've sacrificed an 8.8 bolt (screw actually) of unknown parentage and taken a few video clips.All carbide at approx 1130rpm and 0.4mm (0.015thou) DOC All on a far eastern benchtop hobby machine. In order of appearence:

CCGT 060202 @ 0.04mm/rev (0.0015thou)

CCMT 060202 @ 0.04mm/rev

CCGT 060208 @ 0.04mm/rev

CCGT 060208 @ 0.08mm/rev (0.003thou)

CCGT 060208 @ 0.02mm/rev best finish but lost that bit so will post finish tomorrow

Lastly a random HSS bit not even on ctr height at 600rpm and 0.02mm/rev

Sorry about variable focus the moving chuck and swarf were confusing the camera but you get the idea.

have a look at APT, they sell inserts in two if you click on hobby use the CCGT inserts came from them, CCMT is Kennametal.

bernard towers05/10/2021 19:41:49
336 forum posts
89 photos

8.8 and better is absolutely OK to machine with HSS, your turning speed looks a tad high.

DC31k05/10/2021 19:47:59
586 forum posts
1 photos

I wonder if it is worth noting that 'Grade 8.8 steel' as such does not exist.

Fasteners are graded as 8.8, etc. but you cannot buy steel stock in this manner.

Maybe someone who knows the grades could give us an approximate comparison of bolt grades and steel grades.

Thanks.

Dave Halford05/10/2021 19:48:31
1816 forum posts
19 photos

The simple answer is stop buying woodworking quality 200mm blanks

Nicholas Wheeler 105/10/2021 20:34:14
769 forum posts
52 photos

I think the problem with turning down bolts is the the thread, not the material. Once you get past that, there's no problem. So a deep cut that takes off most, if not all, of the thread is the way to go.

Nicholas Farr05/10/2021 20:57:37
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3040 forum posts
1382 photos
Posted by DC31k on 05/10/2021 19:47:59:

I wonder if it is worth noting that 'Grade 8.8 steel' as such does not exist.

Fasteners are graded as 8.8, etc. but you cannot buy steel stock in this manner.

Maybe someone who knows the grades could give us an approximate comparison of bolt grades and steel grades.

Thanks.

Hi DC21k, maybe the scan below may help.

scan_20211005.jpg

Regards Nick.

Nicholas Farr05/10/2021 21:01:36
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3040 forum posts
1382 photos

Hi, this may also be of interest.

scan_20211005 (2).jpg

Regards Nick.

Edited By Nicholas Farr on 05/10/2021 21:30:11

DC31k05/10/2021 21:26:01
586 forum posts
1 photos

Thanks for the scan.

Looking around, it appears that a Grade 8.8 bolt is roughly equivalent to EN19 or 4140 steel. The steel has a similar ballpark UTS but its yield strength is lower. Were EN19 a bolt, it would be graded approximately as 8.6 rather than 8.8.

SillyOldDuffer05/10/2021 21:57:22
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7675 forum posts
1693 photos
Posted by DC31k on 05/10/2021 19:47:59:

I wonder if it is worth noting that 'Grade 8.8 steel' as such does not exist.

...

It is worth noting. The Grade defines the strength of the fastener, which could be made of any suitable high-tensile steel. The cutting properties of any given bolt depend on the alloy it was made from, so how well it machines is in the lap of the gods.

Whatever high-tensile steel meets the fastener spec will do. One source suggests most modern 8.8 bolts are made from Boron Steel - basically mild-steel with a dash of Boron in it. The resulting alloy is cheap and strong but it doesn't machine well, and it welds badly.

If a Grade 8.8 bolt cuts OK with HSS, it's not made of Boron Steel. When HSS fails, try carbide!

Bolts being made of different steels probably explains why people are reporting different results. My bolt gave carbide such a hard time I don't think HSS would cut it, but I'm prepared to believe HSS would be OK on other 8.8 bolts. I've had such bad experiences machining unknown alloys I avoid scrap, instead buying in metal I know is good to machine.

Dave

Circlip06/10/2021 09:45:30
1382 forum posts

Used to "Make" M16 LH thread allen screws from standard M16 on machine breakdowns. Screws/bolts had enough plain shank under the head to achieve this. NEVER used a carbide to do this, plain old HSS and ALWAYS got an excellent surface finish.

Regards Ian.

brian jones 1106/10/2021 09:52:44
347 forum posts
62 photos

Well JB if I had obtained anything like the results you showed on your vid, I wouldnt have posted this thread

I have formed the simple idea that using carbide

a) the tips are not sharp as per HSS, they are deliberately rounded slightly becasue a sharp edge in carbide would quickly chip and fail, so rounding gives better strenght

b) because of a) above using carbide is like having to punch through a tough thin skin on the work piece to get through to a softer core which can be cut properly. Its counter intuitive to HSS viz with hacksaw you dont put more pressure and saw faster when you feel the blade is blunt

The pic below shows my poor results, note the ridges and how the surface appears smeared like its been rubbed by the tool

I can only assume that the tool tip with pressure reaches a high surface temp that softens the skin and lets mtl be rubbed off below - what an appalling thought

and this was with a new insert

20211006_090749.jpg

I dont know what insert code I was using cos I only bought a set of tools

https://www.ebay.co.uk/itm/133809788614?hash=item1f27af56c6:g:JmoAAOSwDD5g5DuQ

the code wasnt specified

As you can see its the worst kind of rubbish work and if thats all i could get I would give up

Maureen wasnt happy with even a mere 5 thou DOC

I did check the bolt and could use a file on it so not fully hardened

A puzzle?

Brian

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