Perfect hole in stainless

[Alberto-sp]

Hello.

I had a 304 stainless steel pipe. OD 16mm and ID 12mm. I need to increase the internal diameter to around 12,35. The length is 75mm the objective is tu fit inside a heater cartridge with aluminium casing.

I have a small cheap mini lathe (no the cheapest one, but not the best). I'm using the biggest boring bar that fits in the hole and fixed to the QCTP to allow making each cut in 1 pass, so the rigidity is not the best but it is always the same.

I tried using adjustable hand reamer, but is not very good working in stainless (it is not worth to buy a good one, I need it just for this project), so I think I can only do it with the boring bar. I'm using these inserts



In the past, these time of cuts weren't the best, so this time I'm planning to approach to it's final diameter veeeery slow. Making more a more passes at the same depth of cut to smooth the surface and try to finish with a scouring pad.

I mean, when I make a pass, I remove material, but if y repeat the pass without increasing the depth of cut I keep removing material, bit every time less and less. The objective is to make the best fit possible and maximize the surface contact between the inside of the pipe and the heater cartridge.

What do you think? Any idea or advice?

Thank you

Best regards


Edited: I add again the picture of the inserts

 

[GeneT45]

First, don't try to go too light on passes. Trying to take too little off, particularly with a flexing bar, will just burnish the surface. You may do better with a relatively heavier cut (for your lathe).

Second, your bar is clearly flexing. If it cuts on a "spring pass" (a pass without further depth of cut) but doesn't cut on a second spring pass you might be OK. If it just keeps cutting a bit more each time you need to do something different.

If I were having such a problem I'd be looking to use a sharper tool. You could try a ccGt insert which would hopefully have a sharper edge. There are inserts for aluminum which are very sharp, but many have such pronounced acuity that they might not work for boring. You could also try sharpening the insert if you have the means or switch to a HSS cutter. With a sharper tool you might be able to take a light cut without flexing your boring bar.

GsT

 

[Ischgl99]

You’ll need to check the data sheet on the back of the insert packages to see what the minimum depth of cut is to see if any of those are suitable. A 0.175mm depth of cut for one pass is small for many inserts and you’ll likely need a fine finishing insert to do that without it rubbing. I would probably try this with some of my aluminum inserts since those have a nice sharp edge that works pretty well at shallow cuts. I use Korloy H01 grade CCGT inserts for that. Using a carbide boring bar will help with chatter and flex over a steel one. A better option than inserts is a solid carbide boring bar. The sharp edge and high stiffness makes them suitable for things like this. Micro100 sells them here, don’t know what brands are available in your country. https://www.micro100.com/products/tool-details-bb-3603000

Since you are not taking much off the bore, it might be easier to use a cylinder hone and just grind it out with the course stones. I don’t know if they are available for bores as small as 12mm in your country, but something like this: https://www.autozone.com/test-scan-...4in-adjustable-brake-cylinder-hone/531780_0_0

If you can’t find the adjustable hone small enough, then these should be available in the size you need: https://www.grainger.com/product/WEILER-Flexible-Cylinder-Hone-For-467G45

The smooth surface of a ground bore might work better for your heat transfer, but you might not be able to grind all the way to the bottom, so you’ll need to see if that would cause a problem in your application.

 

[Eddyde]

Slow and shallow is not the way to go with stainless as it can work-harden if the tool just rubs on the surface Which is what a carbide insert will do if it's not cutting deep enough. A 0.175 mm (.0068") depth is one pass with an insert at a steady, even feed. Once it hardens your only hope is to get the tool under the hardened metal. I agree with the above advice on using HSS or a brazed carbide boring bar, those would be my first choice.

 

[WobblyHand]

I'd use a CCGT insert with the smallest nose diameter possible. A small nose radius will allow shallower cuts, and it will reduce deflection. For boring I use the sharpest inserts I have, either carbide or HSS. I also bought a carbide boring bar (have 3 of them now) which allow deeper holes with less deflection than steel boring bars. My smallest is 6mm and uses CCGT inserts. Got it from AliExpress. My other carbide bars use TPCB inserts with a 0.002" nose radius. They allow me to take very fine cuts.

 

[rwm]

I think something is being missed here. That CCxx insert only has a 7° relief angle. I don't think there's any way geometrically it can fit into a 12 mm hole without dragging the heel of the insert. I have definitely run into this problem. You could either try a boring bar that does not use an insert or try grinding the heel of the insert away to get a larger clearance angle. Examine your insert under magnification and see if there's evidence of heel dragging.

See this post here from someone smart.

Post in thread 'Boring Head- Minimum ID'
https://www.hobby-machinist.com/threads/boring-head-minimum-id.112706/post-1136950

 

[WobblyHand]

I think something is being missed here. That CCxx insert only has a 7° relief angle. I don't think there's any way geometrically it can fit into a 12 mm hole without dragging the heel of the insert. I have definitely run into this problem. You could either try a boring bar that does not use an insert or try grinding the heel of the insert away to get a larger clearance angle. Examine your insert under magnification and see if there's evidence of heel dragging.

See this post here from someone smart.

Post in thread 'Boring Head- Minimum ID'
https://www.hobby-machinist.com/threads/boring-head-minimum-id.112706/post-1136950

Good point, that is a small hole.

 

[GeneT45]

I think something is being missed here. That CCxx insert only has a 7° relief angle. I don't think there's any way geometrically it can fit into a 12 mm hole without dragging the heel of the insert. I have definitely run into this problem. You could either try a boring bar that does not use an insert or try grinding the heel of the insert away to get a larger clearance angle. Examine your insert under magnification and see if there's evidence of heel dragging.

See this post here from someone smart.

Post in thread 'Boring Head- Minimum ID'
https://www.hobby-machinist.com/threads/boring-head-minimum-id.112706/post-1136950

I missed that entirely. I'd definitely go to HSS. Or grind more relief, if you're able.

GsT

 

[Alberto-sp]

Thank you all for your detailed answers and effort. Following your advice I have bought aluminium inserts with the smallest nose diameter I have found. Also 2 flex home brush. One if 12,7 and other of 14mm.

About the 7° relief angle, I started wit 11,5mm ID an worked well, or at least I think that, but I'll be more careful and I'll check it in the next try ( when the new inserts arrive)

 

[WobblyHand]

Thank you all for your detailed answers and effort. Following your advice I have bought aluminium inserts with the smallest nose diameter I have found. Also 2 flex home brush. One if 12,7 and other of 14mm.

About the 7° relief angle, I started wit 11,5mm ID an worked well, or at least I think that, but I'll be more careful and I'll check it in the next try ( when the new inserts arrive)

When you get a chance, try reading some of this: https://www.hobby-machinist.com/threads/a-boring-primer.75721/
I learned a lot from Mikey's thread.