Machining welded seam off inside surface of pipe

[SpoonsbySven]

Hi Folks,
I'm planning to make a wood-fired forge based on the Whitlox forge: http://whitloxhomestead.com/.

The air supply for this forge is based on two 600mm steel pipes that run along the bottom of the V-shaped channel, with one pipe fitting fairly snugly inside the other. For the purposes of my question, you don't need to know why thre are two pipes, but if you're interested, check out the website.

I've managed to scavenge a piece of 35mm handrail with an ID of 28mm, and will probably (have to) buy a 600mm piece of 28mm OD pipe. The problem is that there's a welded seam in the inside of the larger pipe which prevents the inner one from fitting, so I'm trying to think of a way to "machine" the seam off. I don't have access to a metal lathe.

The best idea I can come up with is to buy a 28mm holesaw like this:

, mount it onto a 300mm piece of threaded rod and drill from both ends of the 600mm pipe. I imagine I will have to clean up the first inch or two of the pipe manually so that there's a clean surface for it to reference off, but provided I go carefully, I think it might work. What to you guys think? I'm open to better ideas!

Does anyone know what size thread the holesaw is likely to take? Are they fairly standard? The one I pictured above doesn't provide any information. Screwfix do a 29mm holesaw, which takes a 3/8in 20UNF thread. I don't particularly want to have to buy a 300mm length of threaded rod just for this_ I have loads, but of course it's all metric.

Cheers folks.
Sven

 

[Myfordman]

This is likely to be a horrible job aggravated by the welded seam being harder than the pipe walls. I don't really have a suggestion as to a sure way to proceed.

Good luck

 

[adidat]

Sounds like a bad idea and a good way to break a wrist... could be done on a lathe with a fly cutter type settup on mandrel

Adidat

 

[jimmy_s]

I think you would need to machine it out on a metal lathe.

I have a Harrison M300 lathe in the garage and could probably drill it out or machine it out with a boring bar. Only problem is I am in Dunfermline and the Forth bridge is broken.

If there's no one closer I am happy to give you a hand with it.

Jimmy

 

[dickm]

How good a fit do the pipes need to be one with the other? If you don' t need a close fit, would filing off the seam be good enough? Not easy but with a long half-rond file, a lot safer than the holesaw idea, IMHO.

 

[t8hants]

One option is to groove the surface of the inner tube with an angle grinder and a 3mm slitting disc this will allow the weld seam to slide along the valley you have created.
Take it slow and steady, but your hole saw idea will result in tears before bed time.
If you are just creating the blast pipe tuyere, you could consider using square tube as the inner or outer.
One of the most frequent question I used to get as a steel stockist was which tube fits in what size, and most don't, but you can put square into round and vice versa

 

[Retire2004]

Hi, Google "cold drawn seamless tube" (sometimes called hydraulic tube) some suppliers give a chart showing O/D and I/D sizes and offer a cutting service.
Regards Tudor

 

[woodpig]

I built a V shaped Charcoal forge some years back using home made refractory. It only had one pipe running along the bottom. Not sure why you need two pipes?

This was mine. Sadly I binned it during a house move. It worked well though and quite capable of forge welding temps.

 

[SpoonsbySven]

Gents,
Thank you all for your comments. In response:

Myfordman: Oh no! If there was anyone who I was hoping for a solution from, it was you! Thank you for responding anyway.
Adidat: I think you're right that the chance of the holesaw catching is high. I'd imagined putting it into my battery drill, on a reasonably soft torque setting to minimise the chances of a broken wrist.
Jimmy S: Thank you, that is very kind. I may well take you up on that. I'm hoping to start this over Christmas, and am just assembling materials. There is no rush for the pipes, and it could easily wait until the FRB is fixed (in the New Year...?) and a time that suits you.
Dickm: I don't really know how good the fit needs to be, but I'm hoping for less than 0.5mm clearance all round, and the closer the better, provided the inner pipe can still move inside the outer. It may come to a half-round file, but that's a last resort. The main problem with that idea is getting right down to the middle of a 600mm long pipe.
T8hants: Unfortunately, the inner pipe needs to rotate inside the outer (explanation below), so slitting the outer one won't work. But thanks for the suggestion- it's a good option for telescoping tubes.
Retire2004: Thanks for that suggestion. I had tried to find something like that, but was obviously not using the right search terms. I'll investigate, and see what the costs are like. I'm trying to spend as little as possible on this, and have managed, so far, to scavenge almost all of the materials.
Woodpig: Nice forge! I remember seeing it in a previous post. The reason for having two tubes is so the length of the forge can be varied by inserting a partition at the desired length. The "two pipe tuyere" allows for the number of air holes to be varied by rotating the inner pipe. It's difficult to explain, but if you're interested, have a look at this video on YouTube, at about 20 seconds in: https://www.youtube.com/watch?v=4kWiJPBirVM.

I guess I may be overcomplicating things by trying to make it variable length, but I'd like the capacity to forge longer things, although I expect most of my forging to be under 300mm. As I will be using bellows (and therefore leg power!) as an air supply, I'd like to make the most efficient use of the air supply.

I'd still like to try the holesaw idea (although I'm much less optimistic about chances of success now, and I will grin and bear any "I told you so's" . If it doesn't work, at the worst I will have wasted a few quid on a holesaw. Are there any views on how standard the threads on these things are?

Thanks again, and I'll let you know how I get on...
Sven

 

[woodpig]

Ah, I see now Sven. If it were me I'd just drill more holes than I need and just drop bolts into those not needed before firing it up. I found that using charcoal only relatively small holes were required, certainly smaller than those in the video. Good luck with the build!

 

[Inspector]

I would try a dowel or curtain rod with a slot cut in it. Slip some 60 grit sand paper, or better, cloth backed, in the slot and wrap it around the dowel. Put it in the pipe and spin it with a drill. A honking big screw with the head cut off or a steel rod epoxied in the end of the dowel should work to hold with the drill chuck. The sandpaper being wrapped so the end trails while spun. As the end wears, rip off the worn bit and continue. Basically a flap sanding wheel for cheapskates. :wink:

 

[Mike.S]

In a similar vein to what Inspector suggested, why not try a flap wheel, perhaps mounted in a flexible drive shaft (like on a dremel) which can be pushed or pulled through the pipe. Or maybe a cylinder honing/honer tool? Either buy ready made or if you have say a 25mm drum sander just wrap something round it and then some sandpaper to bring it to the desired diameter.

 

[chaoticbob]

Hi Sven. Sorry to give another 'if I was going there, I wouldn't start from here' response, but like others I think the holesaw plan is doomed to failure. You would need quite a rigid setup to make this work. Even if the cutter is on, say, half inch threaded rod, it'll be very floppy by metalworking standards at 300mm extension. The cutter teeth will hit the weld seam, and try to move away, then the teeth on t'other side will bite, the whole thing will chatter horribly. And anyway you won't be able to hold the arrangement parallel to the bore with a hand drill. I suspect that you are hoping the cutter will be supported on it's circumference by the inner wall of the tube, but it would have to be a very good fit for that to happen - once chatter sets in all hell breaks loose. Well that's my prediction based on experience of trying to do something similar on welded scaff tube using a metal lathe, but I'd be delighted to be proved wrong!

If you can find drawn tube for the outer it would be much easier for your friendly local engineer to turn the inner to a slide fit than t'other way round.
Thanks for the link to forges - I'm hoping to build something on the same lines myself.
Robin.

 

[TFrench]

Couldn't you just grind a small flat on the inner pipe? If it doesn't need to rotate this will also keep it locked roughly in the right position.

 

[SpoonsbySven]

Thanks very much for the more recent batch of responses.
OK, you've convinced me, the holesaw is a bad idea #-o Chaoticbob, I think your prediction sounds pretty accurate, you summed up all the doubts I had!

I did some research into Retire2004's suggestion of using "cold drawn seamless" tube, and could probably get what I needed for £20-£40 quid. Best supplier I could find for short bits was an ebay seller. If it were essential, i think I might go for that, but the more I think about it, the more Woodpig's suggestion of having a single tube, and just dropping bolts into the unused holes makes sense.

In addition to being tight-fisted, part of the satisfaction of this project will be using entirely scavenged/repurposed materials :wink:

Thanks again.
Best,
Sven
PS If I ever find myself with a piece of pipe with an ID that matches one of my holesaws, I'm still going to have a go. Sometimes you just have to try things yourself ! If I avoid breaking my wrists, I'll let you know how I got on!

 

[Hitch]

We had a machine for doing such a job at a previous place of employment.

Basically a cutter was pushed into the tube or box section, by means of a pneumatic ram, in a similar manner to broaching, the seam was just scraped away.

However, if you have 35mm tube/pipe.... with 28 i/d...
Will a bit of 26.9 CHS fit inside it without bothering the seam? I suppose you want quite a good fit.

 

[SpoonsbySven]

Cheers Hitch,
Yes, I found reference to that method on the Practical Machinist site (for square tube, but same principle). I thought of fashioning something with a carbide tip from a circular saw blade... but I'm missing the pneumatic ram element, and would have to "persuade" it with a big hammer.

I don't really know what tolerance I would be looking for, but I imagine something like 0.5mm all round, so max OD of the inner pipe would be 27mm. The inner pipe needs to rotate easily, and not get jammed up by its of ash/charcoal etc, but still make a reasonable seal so not too much air is lost.

Thanks for the mention of CHS- I hadn't come across that either, but good to know that Circular Hollow Section is another option.

I've pretty much decided that I'm going to give up on the "tube within a tube" idea for this forge. As I said in my last post, I will do some experiments at some point if/when I find some bits the right sizes.

Thanks again.
Sven