Mini Lathe Rear Tool Post

Could have saved yourself the trouble if you followed my link posted 3 days ago

When parting off the two alu pulleys I have just made I had the tool dig in a couple of times, nothing too bad and I just carried on with the feed and they parted off ok. However it did highlight the lack of rigidity in a small mini lathe as I was quite surprised at how much the tool post flexed/kicked when it happened.

So I am going to go with making a heavier duty cross slide to hopefully improve the rigidity, I think that and a solid rear tool post must offer more rigidity for parting off. Having said that I suspect that most of the lack of rigidity is above the cross slide through the top slide and tool post. To be fair I havnt tried a front mounted parting tool inverted which a few recommend. I like the idea of a heavier cross slide though which can have T slots something I dont currently have. I can see it useful for job or tool mounting and give the lathe a bit more capability.

I was considering making the cross slide from EN3B but with dovetails/T slots to cut it would be easier machining in cast iron. A question I have, is cast iron available as flat bar..? I have been searching but can only seem to find round or square and I need a size that I can get 7 inch x 3 inch x 1 inch out of.

Ron, M-machine list 100 x 180 which they sell by the inch, I'm sure they would cut you off say 30mm to allow for clean up. Also bear in mind the rectangular stuff has a radius corner so allow for that..

Thanks Jason, thats ideal I will give them a call Monday, their material list is impressive to say the least.

Ron.

Large lathes also suffer in this department. The forces involved are considerable and metals bend. My WM280 is much more heavily built than a mini-lathe and I've still managed to flex the tool-post. Parting off with a Gibraltar style rear-post, I've seen the whole saddle move slightly during a dig in. Much heavier industrial machines can also run into trouble.

Manually pushing on various parts of a lathe with a DTI attached to detect movement is revealing. Truth is it's impossible to completely stiffen a machine - good design and heavy construction help, but they can't stop it entirely. The Eiffel Tower, which is an unusually stiff structure, bends up to about 5" during a severe storm.

By all means have a go, I'm sure you can improve the lathe but don't expect it to solve all your problems.

Dave

Hi Dave, points taken, I know I cant cure the lathe of its lack of rigidity but as you mention I hope to improve it. I am quite sure that a heavier duty cross slide with a decent rear tool post will be more rigid than a parting tool front mounted in the tool post. To me the top slide and tool post just look to be a weak area. They work fine in normal turning providing you dont push them too hard but just not man enough to be heavily loaded.

I will give it a go, it will be interesting to see how it turns out.

Ron

I bought one of these for my little lathe.

I am quite happy with it.

https://www.youtube.com/watch?v=grtiFa56Bpw

Impressed by the video of the F o R tool in a C2.

So pleased with the rear toolpost on the Bl12-24, want to fit one to the C3. (bit frightened of making a complete new Cross Slide, as Neill has done )

So the F o R may be possibility, unless I could come up with a post that bolts on the rear of the Cross Slide wsithout restricting movement.

Time for thinking cap.

Howard

This might do the trick. Postage may exceed the initial cost, but they ship promptly **LINK**

* Danny M *

Really impressed with M-machine, just phoned them and ordered a 180 x 90 x 30mm piece of cast iron for the cross slide. Its priced by the inch but they only charged me £3 for the extra 5mm in thickness. The lady that took my order was very pleasant and polite, she said she would phone back with the price and she did ten minutes later.

A pleasure to deal with, I will certainly be going back again.

Thanks for the link Jason.

This is probably a bit premature but I should have my 30mm slice of 180mm x 90mm cast iron tomorrow. I am assuming it is going to have two cut faces unless its the end of the bar. Would I be right in thinking that the uncut sides and ends could be difficult to machine, just thinking of the best approach as it will be the first time with cast iron.

Maybe, maybe not. My experience of cast-iron is that it's machinability varies between very good (but dirty) and impossible. There are three potential problems:

• Ad-hoc cast-iron as used to make weights and drain covers is often full of impurities. All that matters to the foundry is that it pours into the mould and makes a casting that looks OK. Inclusions, blow-holes and other faults are likely. Pot-luck as to whether this variety of cast-iron machines or not.
• Cast-iron is also made to a specification as a proper engineering material. There are perhaps 30 or 40 different alloys available for different purposes, and not all machine well. However engineering cast-irons are much less likely to cause workshop problems than cheap castings.
• Cast-iron is prone to develop a hard-skin caused when the outside of a casting (including bar-stock) is cooled quickly. Sometimes cast-iron is chilled deliberately, more often it happens accidentally.

I expect your bought cast-iron will be 'good stuff' inside, but it might have a very hard outer skin. The hard outer skin can damage HSS but carbide will cut it. Once through the skin, cast-iron is usually easy to machine. (Unless it's an old sash-weight!)

My main source of cast-iron is a block from an old hot-air heating system. The metal inside is good but the block has an extremely hard skin perhaps 3 or 4 mm deep - it takes the teeth off a hacksaw. I cut the block with an angle grinder and scarify the skin with it. Once the skin is off, it cuts delightfully.

Finally, I advise taking extreme measures to contain the mess! Cast-iron is a mixture of carbon, iron, and carbides. Cutting cast-iron releases a lot of finely powdered graphite, it's black as hades, goes everywhere, and sticks. The carbide particles make a first-class grinding paste - you don't want them on your lathes sliding parts. Cover everything up and try and contain the shavings close to the chuck.

Dave

Thanks for that Dave that is helpful, I know the grade is GR17 and I will be doing all the machining on the mill. I was hoping to flycut the two faces with a HSS tool.

Ron

Edited By Ron Laden on 27/01/2019 11:26:44

If it's Meehanite ( and it probably will be), it's a pleasure to machine. There is a hint of it being very slightly harder for about a millimetre at the surface but nothing a cutter can't cope with.

GR-17 looks ideal - I'm jealous!

Fly-cutting the faces with HSS will be a trial if they have a hard-skin, my guess is they won't and you'll do OK. Apart from the skin and junk in the very cheap stuff, cast-iron is soft: HSS cuts it easily.

Mostly I use HSS cutters on my milling machine. However, I bought a carbide milling cutter for roughing out hard stuff. If the cast-iron was hard, I'd skim it with carbide first to remove the skin and then switch to HSS. The hard skin varies tremendously in depth - 4mm is by far the deepest I've encountered, it's usually much less, or absent. Most of my experience of cast-iron has been with scrap types likely to have a skin, your purchase is likely to be less troublesome!

Please let us know how it goes; my money is on quick success in the workshop followed by domestic misery due to the sudden appearance of black smears throughout the house!

Dave

You will hardly notice any difference between the outer surface and the rest of the bar.

Thanks guys, I,m quite looking forward to using it.

Ron

If its any help, when I machine cast iron, a powerful magnet is placed where the swarf is most likely to fall, and covered by a sheet of newspaper or plastic sheet (can be melted by hot swarf!). The idea, as you might guess, is to hold the swarf against the paper with the magnet. From time to time, or when finished, the paper is removed and used to funnel the cast iron swarf to where ever you want put it (in the garden around roses, in the bin etc, as you fancy)

Howard

Thinking about the machining I need to achieve a top and bottom face which are not only flat but true and parallel to each other, I am intending to flycut them and the blank is not going to be flat to start with. If I clamp to the table to machine the first side I can see it will spring back when its released. Should I shim it to the table to start with or is there a better approach.