Tips/Techniques Cutting High Strength Steel Forging

[wmetfab]

I have a job to do for a rancher in the area. They broke the drawbar on their tractor, and can't find a replacement at all. They could get a new tractor but that would be an expensive fix. So they bought a drawbar at a junk yard that they figured they could modify. It needs to be cut to length, milled down to fit the receptacle, and a hole for the locking pin drilled out. A few days of messing with that and it suddenly became my project.

I looked at it and said, that will be hard on endmills, and I've no idea how hard it will be to drill. So I will need to buy endmills and drills at your expense plus my time - which could be several more days. We settled on 2 grand or my cost plus markup whichever was less. That's a kings ransom for such a simple job but still cheaper than a new tractor.

First up was the cutoff. Broke a 20 tpi Starrett Intense blade doing that. Replacement on their tab already on order. I switched to a cutoff saw with a grinding wheel for steel in it. Wore out a 14" blade doing that. But succeeded after about 4 hours of patience. Could not just cut - had to cut and chill to soften the steel enough to cut. Would have used a cutting torch but too afraid to ruin the metal properties. Grinding with coolant seemed much safer. That part of the job is done but took a whole afternoon.

Next up is cutting the bar size down. Took a few passes with a Carbide endmill. Not happy.

My Question: I am using 1/2 inch 4 Flute Carbide Endmills to plane it down about 3/8 inch. I'm shooting for 1/8 per pass. I tried HSS which wouldn't touch it. My endmills are not happy cutting this high strength forging. They were happiest at 1100 rpm and a feed rate that just loads the cutter by feel. Faster chatters, slower stops cutting (I assume it's hardening the metal). It's also unproductive. Even as is, this will take all weekend and I won't have the balls to charge what the job is really worth.

What speed would you run a half inch carbide endmill at in a high strength steel forging like this?

Note - It's probably also case hardened. So future passes might be easier. Won't know till I get there.

I am not equipped to soften the metal and then reharden it. Nor do I want to try.

Hi Sus
Can u post a picture of what it is that you are working on

 

[historicalarms]

My plan is to drill it with a 1/4 carbide drill first, then 3/8. Then switching to a sharpened 1/2" concrete drill. Then switching to a carbide insert boring bar.

Night be late to the party but this is my 2 cents worth on drilling hard steel.
30 yrs ago. long before I owned any machining equipment a buddy of mine & I started converting milsurp rifles into sporter/hunting rifles. the availability & cost of the P14-P17 Enfields were our easiest procurements... we used common drill press's, side grinders , Dremmel tools & lots & lots of emory cloth...we didnt have the option of just finding a better cutting tool.
One of the hardest parts of a change-over was drilling & tapping those super hard action bridges to accept scope bases. The method we used was learned from the " Gunsmithing Kinks " series of books published by Brownells ( one of the most entertaining series of books you will ever read...and learn a lot in the process).
We start with an action clamped in a dripp press vise with the spot you want to drill under the quil, cut the head off an ordinary hardware nail as close to the size of hole you want to drill....(in our case a #6-40, in your case 1/2 "). Chuck into the Jacobs and touch it down to the spot to be drilled, now turn drillpress on ^let er buck. friction will turn your rod red hot but keep er going for quit a while. Y our work piece will not turn red much but leaving the rod turning for a while draws the temper down in a small area from the original 60-80 Brinel down to somewhere under 35 ...that hardness can be drilled by a good quality twist dril.
The size of workpiece we used was much smaller than yours but Im betting it will still work, but you may have to do it in layers...I cant remember ever breaking a drill bit or tap in one of those hard actions after learning this process...GOOD LUCK

 

[trevj]

Well, some of that learnen been done! A little more to go. I am not sure what to tell you about the drilling. I have noticed when drilling stainless, that skipping the step drilling seems to work better, and sometimes the same for unknown steels. It seems that the wider the land/cutting edge engagement, the better. Many times just a clearance hole for the drill tip works way better then step drilling. Many times just to get past the edge/corner of the clearance hole will be a bear, the loading in one concented spot often destroys that part of the cutting edge, leading to failure of the drill bit, and work hardening of the metal being worked.
This leaves the corners of the cutting edge of the drill bit to try to finish the hole to size. The failures there are many also, to say nothing of material breaking off the drill bit and lodging in the metal being drilled.
Many times I have ground the cutting edge nearly to no rake to make the edge stronger, and removed some of the heel to reduce rubbing and heat build up. Some bits are ground somewhat this way from the maker.
I am sure you know a lot/all of this, but hard to say.
Anyway good luck, will be thinking of this while at the meet.

One thing that I found across the board, was that drill breakthrough was when most of the really bad things happened. Having a sacrificial block of a similar material, under the workpiece, almost always helped! It keeps a load on the rest of the drill cutting surfaces, rather than reaching that point where the drill can deform the remaining surface and pull it up the flute rather than cutting it, which causes jams.

I too have found step drilling problematic, and only use it if I really do not have the means to apply any real power to the part. I would much rather use a pilot the size, more or less, of the drill's web at the tip, and run straight to size, or as close as can.

@Susquatch , I would not have recommended slow revs and high pressure with solid carbide, as that can crush the cutting edge off, if too low and too slow. Glad it worked out anyways! Power feed was almost always my go-to for heavy or potentially miserable drilling conditions, have used drill presses so equipped, used the power feed on the milling machine's knee (Z axis power feed) or on a lathe either feeding the work against a drill in the headstock, or with a drill mounted on the tool post and fed under power in to the work. It makes the cut much evener, taking away the opportunities to get too much and overload the drill, as well as keeping a constant feed pressure (titanium hardens faster than a hookers heart, if you let the tool rub for just about any time!).

 

[Susquatch]

Nightmare over. It's done.

After using the concrete drill, I installed a boring head and went at it with that. Tough to get going but persistence paid off. Just takes a lot of pressure. More than feels right.



As with the other stuff, it also liked low speed. Depth of cut was about 15 thou (30 thou on diameter). Lots of pressure, and only about 20 rpm. Not happy anyplace else.

I know it seems odd, but if she doesn't like fast, try slow. If low pressure doesn't cut, try lots! This certainly isn't a normal material. I shouldn't be surprised that it liked abnormal processes.

All done, pin installed.

Can u post a picture of what it is that you are working on

Here is the whole assembly including the finished draw bar. The top left bracket bolts to the bottom rear belly of the tractor. The drawbar is inserted into that bracket and then the round pin is inserted to lock the drawbar in place.



There is also a cradle bracket not shown above that is part of the three point hitch assembly. The drawbar can be pinned externally there to stop it from pivoting. Here is an assembled photo. You can't see the belly bracket in this photo. It's at the far end of the drawbar.

 

[Susquatch]

Several missing pieces of tooling bear looking into.

1. Hard steel cutoff abrasive wheels. (diamond?)

2. Hard steel bandsaw blades

3. Carbide drill bits in step sizes through to 3/4 or so.

4. Larger Indexible insert drills in step sizes through to 1.5" or so.

5. Short boring bars with inserts for boring heads. Maybe flats for screws too.

 

[Susquatch]

I would not have recommended slow revs and high pressure with solid carbide, as that can crush the cutting edge off, if too low and too slow. Glad it worked out anyways!

Neither would I. But reading the references you suggested for Cole's high pressure drilling, I found myself wondering how and why it worked. I still don't know how or why, but now I do know that it does work. With more pressure and even slower speeds, perhaps even better than what I experienced myself.

I applied quite a bit of pressure - bordering on scared shitless that I might break my quill rack.

Same goes for speed. I was crazy low. A great outcome of a VFD and a VFD Rated Motor.

I must say that I was shocked at how well it performed. Thanks to you, I am now on a learning mission to discover why and how this worked.

 

[mbond]

Obviously I am too late to help, but for anyone else cutting holes in hard metal, here are a couple of thoughts

Assuming the final hole you need is large, one way of 'drilling' a pilot hold is using an oxy acetylene torch. It can then be cleaned up with a boring head etc. Wet rags can protect the temper in the metal away from the hole

Slow rotation with high axial pressure is a _results_ amplification technique when the material is hard compared with the tool. Think about hoeing mud versus hoeing hardpan - you have to go slower and press harder to achieve results. If you have enough power, you don't have to go slower, but you probably don't have enough power (think a D9 versus a hand hoe) and you probably want to allow heat dissipation by staying slow. And yes, the chance of breaking the tool goes way up because you are putting more power through the tool. This effect is amplified when the material is more 'tough' than 'hard' as in the case of strain aka work hardened metal. I know about this effect in theory, can call up the blackboard diagrams, and describe it in much more detail in engineering terms, but I have never done it in practice.

 

[Chicken lights]

This will seem like it's out of left field but this forum should be used to that by now

A very little bit of research turned up forged snap binders will generally bend, under abusive drivers. Cast, on the other hand, will simply break

@Susquatch holds the opinion that...offshore....forged can be done cheaper, with crappier steels, to produce a cheaper product to sell. (or selling at a loss to produce but that's politics) I know Crosby binders (were? are?) were forged for greater strength, and much much more expensive as a result. Made in the USA.

I don't think my chain hooks are cast, so I think his theory holds true on offshore chain hooks forging being so much cheaper.

But I have broken cheap snap binders, so I still think they are cast. My snap binders on the truck have a couple bent handles, they are Columbus-McKinnon, and again 4-5 times the money versus offshore snap binders

Friendly argument, just to poke the bear

 

[Susquatch]

Assuming the final hole you need is large, one way of 'drilling' a pilot hold is using an oxy acetylene torch. It can then be cleaned up with a boring head etc. Wet rags can protect the temper in the metal away from the hole

Never too late. The deed is done but I hope the analysis and learning is just beginning.

I considered using an OA torch to both cut the bar and to start the 1-1/8 hole. But because the bar is about 2" thick, I figured the internal heat transfer path would bypass wet rags and wreak havoc. That's just a guess of course, but it seemed like a reasonable concern at the time so I kept the OA in the bottles.

Slow rotation with high axial pressure is a _results_ amplification technique when the material is hard compared with the tool. Think about hoeing mud versus hoeing hardpan - you have to go slower and press harder to achieve results.

This is VERY insightful. I like it. It lends itself well to the modelling I often do in my head and might explain a lot of things. I'll prolly dream about it tonight and perhaps have more and better comments tomorrow. For now, it seems to be an excellent perspective. Thank you!

If you have enough power, you don't have to go slower, but you probably don't have enough power (think a D9 versus a hand hoe) and you probably want to allow heat dissipation by staying slow. And yes, the chance of breaking the tool goes way up because you are putting more power through the tool. This effect is amplified when the material is more 'tough' than 'hard' as in the case of strain aka work hardened metal. I know about this effect in theory, can call up the blackboard diagrams, and describe it in much more detail in engineering terms, but I have never done it in practice.

My big knee mill with VFD & 1000:1 motor provides more than enough torque for either extreme. If need be, I could even change gears to get more. It's not a D10, but it acts like a D8..... LOL!

I didn't try the boring bar at higher speeds, but I did try the face mill and 1/2" carbide endmill at higher speeds. Neither showed any sign of being maxed out for torque - they just didn't cut and/or were not happy. Lower speeds accompanied by more pressure worked better and was happier too. It felt like a D8 that was just happy to be working.

I don't really know when the end mill got damaged. It could have been during the higher speed runs when there were a lot more red hot sparks. In hind sight, I should have checked it more often.

Even damaged, it did a great job of drilling as per @thestelster's suggestion.

I can't really comment on the material property curves till I think about it a bit more. At first blush, I think that forging is both hard and tough. Not as hard as the tooling but perhaps a lot tougher. I like your mud analogy. I wish someone would do some high speed microphotography of this particular cutting process. The insights that kind of footage provided for regular steel was amazing. I would dearly love to see that on a forging.

Great food for dreams, and its that time of day.

Thanks again!

 

[Susquatch]

This will seem like it's out of left field but this forum should be used to that by now

None of them know that you were here yesterday and saw the drawbar from hell first hand. If they did, they would know exactly why the bar and your binders are the same subject after all.

A very little bit of research turned up forged snap binders will generally bend, under abusive drivers. Cast, on the other hand, will simply break

@Susquatch holds the opinion that...offshore....forged can be done cheaper, with crappier steels, to produce a cheaper product to sell. (or selling at a loss to produce but that's politics) I know Crosby binders (were? are?) were forged for greater strength, and much much more expensive as a result. Made in the USA.

This discussion about binders came about because @Chicken lights and I have both seen failed Chinese binders, but never failed a domestic one. Bent yes, snapped no. This leads to the thinking that the Chinese ones are either cast or poor quality forgings. The price difference is huge so either possibility exists

I don't think my chain hooks are cast, so I think his theory holds true on offshore chain hooks forging being so much cheaper.

Chain hooks are not too different from binders. The good ones are high quality forgings.

But I have broken cheap snap binders, so I still think they are cast. My snap binders on the truck have a couple bent handles, they are Columbus-McKinnon, and again 4-5 times the money versus offshore snap binders

I've never broken a chain hook. I have broken chain though.

I am Inclined to think it's the same issue. Cost and quality standards in China are not the same as domestic. For that matter so is the liability.

One thing I do know is that high quality forgings are amazing! Cheap ones..... not so much.

Friendly argument, just to poke the bear

Anytime Dave. I don't think this stuff is political but you never know.

 

[mbond]

Probably the idea of being maxed out for torque would look different if the same cutting pressure were applied at 10x speed.

Materials generally can be cut within an envelop, but this is a large topic and I'm not sure how much I should get into it. Then again, the Leafs are already losing, so I might have plenty of time

 

[Susquatch]

Probably the idea of being maxed out for torque would look different if the same cutting pressure were applied at 10x speed.

Good point. I didn't have anywhere near the same pressure applied at high speeds. It was also a different scenario - side milling vs drilling and boring.

What are leafs?

 

[Chicken lights]

None of them know that you were here yesterday and saw the drawbar from hell first hand. If they did, they would know exactly why the bar and your binders are the same subject after all.



This discussion about binders came about because @Chicken lights and I have both seen failed Chinese binders, but never failed a domestic one. Bent yes, snapped no. This leads to the thinking that the Chinese ones are either cast or poor quality forgings. The price difference is huge so either possibility exists

Chain hooks are not too different from binders. The good ones are high quality forgings.
I am Inclined to think it's the same issue. Cost and quality standards in China are not the same as domestic.
Anytime Dave. I don't think this stuff is political but you never know.

True, I guess I forgot a minute we delved into test reports and certs, if an offshore company gives a cert, it's very different from a domestic company giving a cert

Again, no politics, the discussion arose around steel quality compared to country of origin, and how other countries can fudge numbers

I try to source USA/Canada products for my work, as I feel the quality is better, as evidenced by the drawbar

I've hauled steel I-beams made offshore and domestic and learned somewhat about the certs and liability that way, it's no dig at offshore it's just a neutral discussion on quality

 

[Susquatch]

Again, no politics, the discussion arose around steel quality compared to country of origin, and how other countries can fudge numbers

I tried to find those gauge block certs we talked about. Still empty handed. I'll keep looking.

Can you imagine trying to sue a Chinese company when a chain hook failed?

 

[mbond]

as an aside, now that Canada has no domestic production of structural steel, and our building code stipulates standards in metric while our only reasonable source is from the US , where they produce to Imperial, there is an entire industry in attesting that material already tested in one unit system is also valid in the other. They do nothing but multiply and stamp the results, but it's work if you can get it I suppose

 

[mbond]

Good point. I didn't have anywhere near the same pressure applied at high speeds. It was also a different scenario - side milling vs drilling and boring.

What are leafs?

The Toronto Maple Leafs hockey team. Today is the first playoff game for them. They always loose, the question is how long it takes

 

[Chicken lights]

I tried to find those gauge block certs we talked about. Still empty handed. I'll keep looking.

Can you imagine trying to sue a Chinese company when a chain hook failed?

good luck

again, no politics, but when the country of manufacture has no recourse for providing a poor product / that doesn't meet domestic quality control/certs/liability, it's hard for domestic companies to compete

I think that's about as far as we can take this, forum wise, but it was originally a steel forming process versus cost discussion, with the focus being on forged versus cast to produce items

edit: because the drawbar was probably forged quality versus cheap steel produced today

 

[mbond]

Adam Smith described the concept of the invisible hand. Given a choice, and enough time to figure it out, ordinary people will decide what the appropriate level really is

 

[mbond]

good luck

again, no politics, but when the country of manufacture has no recourse for providing a poor product / that doesn't meet domestic quality control/certs/liability, it's hard for domestic companies to compete

I think that's about as far as we can take this, forum wise, but it was originally a steel forming process versus cost discussion, with the focus being on forged versus cast to produce items

edit: because the drawbar was probably forged quality versus cheap steel produced today

forging is a technique of strain hardening generally

 

[mbond]

At the risk of too saying just too much again

All metal is composed of small crystals. Much larger crystals exist with other materials. Most stone is silicate crystals. Diamonds are carbon crystals. And there are many more. But metal is remarkable because of the small size of the directly ordered atomic groupings - crystals. Probably this results from the nature (number?) of the atomic valence electrons but we are getting into too much quick sand.

If I should continue, someone say yes