trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

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SteveM
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trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by SteveM » Fri May 22, 2020 4:23 pm

Ordinarily, John Saunders is all about CNC, but he has done some videos at Paul Debolt's shop.

In this one Paul uses a 3-3/4" trepanning tool to get most of the work done getting to a 4" bore:
https://www.youtube.com/watch?v=2HEruBtaC1c

If you look at the tailstock, he's driving the tailstock with the carriage.

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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by RSG » Sun May 24, 2020 7:54 am

so cool!
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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by pete » Sun May 24, 2020 12:40 pm

Interesting video and I hadn't known the HP requirements were as high as they are for that operation. I'd watched a few of this guys videos so knew about large deep hole trepanning https://www.youtube.com/watch?v=-ZPvfoVTdqE&t=195s

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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by RSG » Mon May 25, 2020 1:50 pm

That's crazy Pete!

My question is what would be the reason they need to trepan a hole that large and deep in the first place? Also, could they not buy something cored from the mill to a close enough size already, then just bore to fished size?
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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by choprboy » Mon May 25, 2020 5:49 pm

Wilks does lots of special alloys as well as standard steel. Trepanning is done for a whole array of different reasons, including:
- special alloys that are not produced as cored product from the mill
- sizes that are not readily available from the mill (particularly for one-offs, no mill is going to manufacture 20" diameter, 4" wall, 4140HT when the annual sales will be 100', but they may be selling 10,000' a year of 4140HT 20" diameter bar in machinery that is readily adaptable to other sizes)
- reuse of core material for additional products (particularly for expensive alloys)
- coring of specially produced forgings (see the video of coring a square block forging)
- extracting samples for destructive testing where every unit must be tested (see Wilks' videos on coring titanium forgings for aircraft landing gear)

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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by pete » Tue May 26, 2020 1:23 am

Yeah it is isn't it RSG, I was amazed the first time I watched some of those videos. Choprboy pretty much nailed it. In one of the videos it was mentioned the cored part was headed to the bottom of the North Sea so really high pressure off shore oil wells is another use. I expect there's not all that many company's with the experience that can do trepanning that large and over those lengths like in the video link I posted. Even less that can do the hard and expensive super alloys like that Inconel. So there's probably pretty good money in it. I'd also bet when it doesn't quite go as planned it's a real costly day. Correctly done there's probably a big time saving over conventional drilling/ single point boring if that could even be done at some of those lengths. Plus you still have that core with expensive materials and not a pile of chips. For anything size and surface finish critical they probably hone as the next step.

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liveaboard
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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by liveaboard » Tue May 26, 2020 1:54 am

I bought some steel recently; mostly solid rod, and one cored rod cutoff.
Later when I went over the costs, I realized the cored piece that was just a little more expensive [by weight] was just about the same price as the solid rod by size.
Obviously if you need it hollow, there's a significant saving. I was just impulse buying.
Boring out rod to make tube sure is boring, but the amount of different stock I'd need to have if I used cored material is beyond the resources of my home shop.
So is trepanning I think.

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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by BadDog » Tue May 26, 2020 5:27 pm

If you grind your own tools, and the trepan isn't too deep, then it isn't all that different from boring on a lathe. However, the mounting of the tool is extra overhead on the mill (build/buy). I've only done it a couple of times, but even though it had a learning curve, it was ultimately successful.
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liveaboard
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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by liveaboard » Tue May 26, 2020 5:36 pm

Sometimes I used a holesaw and it went very well, even for a large hole [3"]
Another time a tooth broke off and was stuck in the grove, took me 20 minutes to get it out and the saw was junk.

Maybe I'll try making a trepanning tool next time.

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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by BadDog » Tue May 26, 2020 6:13 pm

Think of it this way. A trepanning tool is just a single tooth hole saw... :lol: :wink:

I was building a large fabrication this last weekend that needed a variety of 1", 1.25", and 1.5" round tubes welded together to complete the structure. Right near the end my nice hole saw spit out a chunk containing about 15% of the teeth. Luckily I didn't have to dig it out, but I finished the final several "fish mouth" cutts of tha size with the missing piece. I just went easy on the feed rate, and worked like a champ and got the job completed...
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liveaboard
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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by liveaboard » Thu May 28, 2020 2:16 am

One time I needed a 90mm hole at a 45 degree angle in a 400mm pvc round tank.
After some sleepless nights, I made this;
home made holesaw1.jpg
home made holesaw2.jpg
It has 2 teeth but I think 1 did all the work.
The mandrel is a cutting tool I made earlier, 25mm rod with a morse taper end that fits into the drillpress. It's just pressed into this, so when I was done I pressed it out again.

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Bill Shields
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Re: trepanning, or how to bore a 4" hole on a lathe without boring a 4" hole

Post by Bill Shields » Fri May 29, 2020 3:10 pm

liveaboard wrote:
Tue May 26, 2020 5:36 pm
Sometimes I used a holesaw and it went very well, even for a large hole [3"]
Another time a tooth broke off and was stuck in the grove, took me 20 minutes to get it out and the saw was junk.

Maybe I'll try making a trepanning tool next time.
been there...done that...taught me a lesson to never purchase cheap hole saws unless I am cutting non-metallic.

There is nothing so expensive as a cheap tool.... :?
Too many things going on to bother listing them.

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