Machining HT bolts

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compact8

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I tried the other day to make the part ( a special T nut ) on the left side of the photo from M10 12.9 HT bolts the and quite some difficulty was encountered :

1) work hardening ! The parting tool was carbide and I need to be very determined in feeding the tool or the surface will become so hard and so quickly that the tool wouldn't cut no matter how large the feeding force was. I used to believe 304 stainless steel is the worst in this regard but this material is clearly much more stubborn.
2) Not really an issue but when I retract the drill bit from the hole ( 5 mm ), some significant resistance was felt. It appears that either the hole is not straight or the diameter is smaller than that of the drill ?
3) This is major. I tapped the hole with an HSS M6 tap and after going in for about 10 mm, the tap couldn't be turned any further. Not sure what the cause is. Should I use drills of larger diameters ?

Repeated on 8.8 grade HT bolts. No joy.

Then I tried the same operations on softer mild steel stock ( 1018 I believe ) using the same tools and no problems were encountered.

Is this machining difficulties expected for HT bolts ?
 

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At some level of hardness, it is necessary to anneal (or partially anneal) before machining. It is usually easy to soften the metal enough by heating to a dull red and then cool slowly. I have used two ways to slow the cooling: bury the part in ashes, or lay it on ceramic insulation and cover with another layer of ceramic insulation. For unknown steels, this process may and may not result in truly annealed steel but has worked fine for my purposes.
 
compact8 said:
I tried the other day to make the part ( a special T nut ) on the left side of the photo from M10 12.9 HT bolts the and quite some difficulty was encountered :

2) Not really an issue but when I retract the drill bit from the hole ( 5 mm ), some significant resistance was felt. It appears that either the hole is not straight or the diameter is smaller than that of the drill ?
3) This is major. I tapped the hole with an HSS M6 tap and after going in for about 10 mm, the tap couldn't be turned any further. Not sure what the cause is. Should I use drills of larger diameters ?
Click to expand...
Recommended tap drill for M6-1.0 in Steel/SS/Iron is 5.40mm/#4; 5mm works for Aluminum, Brass & Plastics, but even 12L14 would be a tough go.
 
When I was given a bunch of head bolts from a Caterpillar 3406 engine (3/4 NC) I discovered that they are stronger than Grade 8, and machine like butter. They are not hard, and for many applications, that is a good thing. They are very strong, and very tough. Parts made with them will bend, rather than break. Too hard a material is brittle, but it does resist wear better.

These bolts are not supposed to be reused for head bolts on the 3406, so they are discarded by any decent repair shop. I get them for free, and many places will sell them for scrap price, by the pound. The easy machining, combined with the low price, offset any time it takes to remove extra material to make smaller parts. The surface finish is very much like leaded screw stock.

I tried to use some head bolts from a 7.5L (460) Ford engine. They were hard, abrasive, and didn't give a decent finish. Don't bother trying!
 
What Mark said.

Is there a specific reason you want hardened T-nuts?

If something goes south I would much rather break and/or bend the holding components than the machine.
When bad things happen, something WILL give somewhere.

I used 1018, brass, and aluminum to make my hold down components. I have taken to big of a bite and broke/bent hold downs.
 
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