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Machining Cast Iron?

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Andrew Tinsley31/10/2020 17:40:09
1216 forum posts

The preferred material for back plates has always been cast iron. Quite a few people believe that mild steel is a NO NO! Stating that it is much more prone to seize up on a threaded nose of a lathe.

I believed all this, until Ketan had a few quiet words, basically saying that mild steel backplates were fine. I have since used Arc mild steel backplates with no problems.

Andrew.

Nigel Graham 202/11/2020 18:30:52
810 forum posts
16 photos

Bo'sun:

I don't machine cast-iron very often but when I do, I use moderate to low speeds. That helps control the mess!

Cast-iron is often used for machine parts because it has a better vibration-damping quality than mild-steel.

Yes you could use mild-steel for a back-plate, as far as I can see, and I dare say some do. After all it is not very large and is heavily clamped so not very likely to ring.

If you are prepared to machine a casting but would prefer steel to iron then you may as well buy a sawn slice of free-cutting mild steel. As you say, buying it ready-machined would put the price up but more significantly a back-plate is normally finish-turned to match the lathe and chuck anyway.

Dennis Pataki02/11/2020 21:44:48
10 forum posts

Bo'sun, I'm thinking concentricity. Not familiar with your particular project, but if part is machined mounted on the lathe spindle for which it is intended, concentricity is pretty much assured.

Regarding material, free cutting mild steel might be a good choice too.

Howard Lewis02/11/2020 22:06:57
3815 forum posts
3 photos

It was reckoned that in the shops where cylinder blocks were machined that the haze contained a complete block casting.

I had greased plates, a foot square, situated in the shop where we machined V8 blocks. Having washed off the dust from the plate, weighed in the cleanliness cage, and calculated the floor area of the shop, there was, indeed, a block floating around above us, in very finely divided form.

Although I didn't work in the machining area, any time spent there during the summer, meant that my brow and head were a nice shade of yellow buy the time that I got home.

Howard

old mart02/11/2020 22:13:02
2251 forum posts
165 photos

I bought a 4" steel backplate on ebay with the 1 3/4" x 8W thread for Smart & Brown, it got fitted in the centre of a faceplate, and works fine, although I always keep the spindle end lightly lubricated. All the others except the opener for male stepped collets have been made in CI. I have bought backplates with the 1 1/2" Boxford thread to rework to S & B size as long as the boss diameter is nice and thick. It saves a lot of machining mess.

Bo'sun03/11/2020 08:31:46
239 forum posts
Posted by Nigel Graham 2 on 02/11/2020 18:30:52:

Bo'sun:

I don't machine cast-iron very often but when I do, I use moderate to low speeds. That helps control the mess!

Cast-iron is often used for machine parts because it has a better vibration-damping quality than mild-steel.

Yes you could use mild-steel for a back-plate, as far as I can see, and I dare say some do. After all it is not very large and is heavily clamped so not very likely to ring.

If you are prepared to machine a casting but would prefer steel to iron then you may as well buy a sawn slice of free-cutting mild steel. As you say, buying it ready-machined would put the price up but more significantly a back-plate is normally finish-turned to match the lathe and chuck anyway.

Good morning Nigel,

Having a back plate that matched the lathe and collet chuck would be great, but for whatever reason, Warco supply the back plate to suit the lathe, but it needs machining to suit the Collet Chuck. No doubt they would say it gives you the choice to use any Collet Chuck.

Bo'sun03/11/2020 08:45:32
239 forum posts
Posted by Dennis Pataki on 02/11/2020 21:44:48:

Bo'sun, I'm thinking concentricity. Not familiar with your particular project, but if part is machined mounted on the lathe spindle for which it is intended, concentricity is pretty much assured.

Regarding material, free cutting mild steel might be a good choice too.

Hi Dennis,

Very pleased with the result. 0.006mm TIR on the tapered bore of the ER32 Collet Chuck. I think much of it was probably luck.

Nigel McBurney 103/11/2020 09:35:02
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763 forum posts
3 photos

Cast iron has always been used for backplates,probably since chucks as we know them were invented,most backplates have a smaller diameter boss to contain the thread plus a lot thinner larger diameter to take the chuck,so if a steel sawn blank is used a lot of material ends up in swarf whereas with a cast backplate there is little material waste,cast iron was also cheaper to produce ,and there were lots of foundries in this country.Steel was more expensive as there were far fewer sources of manufacture.And in the days of carbon steel tooling and HSS tooling it was easier and quicker to produce components in cast iron,just think about it threr were no HSS hacksaw blades to cut off thousands of backplates ,Think how long it would take to cut off the blank for a 12 inch chuck.The ability of cast iron to absorb vibration may have been effective on very large backplates ,but on our small machines I would think that the material may not matter though having worked a lot with cast iron I would think that the steel spindle with a cast b/plate is less liable to "stick".Nowadays with fewer foundries ,availability of mild steel with short leadtimes .high speed manufacture with CNC and new cutter material ,for smaller batches of say 4 or 5inch backplates for the hobby market there is probably little advantage of cast iron over steel. I like machining cast iron,did a lot during my apprenticeship,though if machine tols are not kept clean severe wear can occurr particularly in the days before hardened bedways were available, I have seen the bedways on a worn plano mill look like a piece of wood or chair leg that has been scatched for years by a cat,

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