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dfox1787
11-05-2018, 11:27 AM
Hi guys

I have a hobby mat mini lathe im pretty new to using it.

Im trying to bore a nice smooth hole which is fairly deep but i cannot get a smooth finish.

The outside finish is really nice. if i reduce the length of the boring bar i.e bringing it closer to the tool post the finish improves. Ill upload some images later you can see what i mean.

My question is really if i am using the correct type of boring bar or if there is a way easier way of getting the nice smooth interior finish.

mekanik
11-05-2018, 01:07 PM
What material are you trying to cut, if it's aluminum you might be best with a HSS tool as opposed to the tipped tools.
Regards
Mike

dfox1787
11-05-2018, 01:50 PM
What material are you trying to cut, if it's aluminum you might be best with a HSS tool as opposed to the tipped tools.
Regards
Mike

its steel bud. Im making a mould injection machine so i need it to be a smooth as posisble. I just did have a thought. I havent check the height of my cutting tool to make sure its set on center. Im wondering if that is causing an issue.

mekanik
11-05-2018, 03:03 PM
Yes tool needs to be @ centre height,would still use HSS tool.I tend to run a fairly slow speed to prevent vibration in the boring bar,when you get to the bottom of your bore reverse the feed and you will find it will probably take a fine cut on it's way out. some sort of cutting oil will help.

dfox1787
11-05-2018, 03:11 PM
Yes tool needs to be @ centre height,would still use HSS tool.I tend to run a fairly slow speed to prevent vibration in the boring bar,when you get to the bottom of your bore reverse the feed and you will find it will probably take a fine cut on it's way out. some sort of cutting oil will help.

Thank you ill give that a go later. I am running my lathe @ 2000 rpm. I have either 250, 500, 1000, or 2000 rpm options.

mekanik
11-05-2018, 03:31 PM
Try 250 see how it goes, it's a bit of trial and error depending on material/machine stiffness ect. just have a play

m_c
11-05-2018, 10:11 PM
By the sounds of it, you want a less springy boring bar.
Use as large a boring bar as possible, and if you really do have to use a relatively long overhang from the tool post, a carbide bar will help as it's not as springy.

As mekanik says, HSS should help, as it's a sharper edge, and not relying on a blunt edge with brute force to cut the metal. Although one thing you could try if you have an insert bar, is use an aluminium insert, as they typically have sharper edges than inserts for steel. You won't be able to take as big a cut, and the insert won't be as durable, but should produce a smoother finish.
And a key thing with inserts, is keep them cutting. Don't let them rub.

dfox1787
13-05-2018, 05:47 PM
thank you for responding.

Still struggling with it not sure if i reach the limitation of my lathe if its my tools. Ill see if i can find some more time to have another go.

24211

24212

24213

cropwell
13-05-2018, 06:15 PM
Have you got a fixed steady to support the end of the tube ?

dfox1787
13-05-2018, 07:10 PM
Have you got a fixed steady to support the end of the tube ?

i havent no. i could look at making one if that is what i need

m_c
13-05-2018, 07:27 PM
You've got a major chatter problem.

I would invest in a bigger boring bar, and cropwell's suggestion of a fixed steady would also help.

dfox1787
13-05-2018, 07:31 PM
You've got a major chatter problem.

I would invest in a bigger boring bar, and cropwell's suggestion of a fixed steady would also help.

any suggestions for a boring bar. Not sure where i would get a fixed steady for my lathe.

m_c
13-05-2018, 08:28 PM
any suggestions for a boring bar. Not sure where i would get a fixed steady for my lathe.

I would go for at least a 16mm bar, or even 25mm, provided it'll fit in your lathe.
And I prefer ones that take CCxT inserts.
I wouldn't be too bothered about brand, as there's not much to choose from with cheaper bars.

It'll also likely be worth making sure all the slides on the lathe are adjusted properly as well, as with that kind of overhang, any play will be magnified.

Depending on how much you use the lathe, it could be worth fitting a quick change tool post, as they'll also give a far more secure mount for any tools, than the bog standard tool holders. I've personally always used ones from Arc Euro Trade, so I know I can buy extra tool holders.


Regardless of all that, you'll need to be aware that even with the best setup, you do risk ending up with a tapered bore. A more consistent method for accurate boring on lathes, is to mount the workpiece on the cross slide, then spin a boring bar between centres.

spluppit
14-05-2018, 03:49 AM
If you have a 4 jaw chuck use it! You have a lot of extension from the chuck this will do you no favours at all. A 4 jaw will give you a LOT more rigidity. It's hard for anyone to categorically say what is the problem without being in front of the machine once all the basics have been covered. It may well be you are just reaching the limit of rigidity of the machine with the work length you have. The machine will have a limit like all machines.

I would do as has been suggested and use a hss boring bar you do not need to buy anything, they are easy to make. You can use a bit of mild steel with a drilled and reamed hole in the end set at 45 deg or 90 deg for a through hole and a grub screw, insert a bit of Hss and grind with a decent amount of top rake as you having problems, i would suggest about 20 deg to start. High rake will help in this situation as it reduces the shear load. This kind of home made bar works very well in difficult situation and works in a contrary fashion to the reasons explained about rigidity. I do not agree a solid carbide boring bar will help on a manual machine. As long as the bar is strong enough to do the job and resist the cutting force and being mild steel this means it has properties to absorb shock because mild steel En3b or even En1a these material are ductile, ductile materials absorb shock and dampen and all you guys know what that means to a machine tool. Harder materials tend to and will resonate more.

This sounds crazy i know but logic dictates this when you think about it. There is not a comparison using these kind of carbide tools on small machines compared to what carbide is generally designed for which is pressure cutting. This works fantastically on the right machines but not all, and manual not being one of them generally speaking when problems are arising such as you are experiencing now. It's the wrong tool for the job on a baby machine..... as you are probably working out or will soon find out.

You also need to learn how to grind tools if you own a small lathe and you don't know how. It's a vital skill you can not be without. Plenty of advice is available on that subject.... but its often overly complicated on the internet. A properly ground tool with no honing or stoning will give a very good finish if done correctly.

Good luck.

mekanik
14-05-2018, 10:47 AM
As sluppit has recommended make you own boring bar, i would get some square section bar(square section will be clamped to topslide) turn a section slightly longer than the intended bore down to a diameter that will give you a reasonable clearance to clear cuttings then do as instructed regarding the HSS tip. remove the original toolpost and use packing ect to get your tool to centre height and and secure the bar on your topslide with a strongback. 4 jaw is essential, you will have to go softly softly to remove the chatter with light cuts,
once you have removed them you might be in with half a chance, my lathe is slightly bigger than yours but with that amount of overhang the weakest link will be the topslide, if that doesn't work, you will need to remove the topslide and mount the boring bar direct onto the cross slide.
Regards
Mike