Machining a lathe backing plate - hard cast iron blank

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dansawyer

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The project is to machine a backing plate for a South Bend 9. I purchased a semi finished plate. The plate blank fits cleanly on the spindle and I started to finish the edge. All that happened was a lot of noise and some sparks. There may have been a thin cloud of very small black dust.
A quick review indicated the plate is likely cast iron. (I was not expecting that when I ordered the blank). Is there anything practical I can to with this plate? It appears to be very hard. HSS bits barely touch it. Carbide makes high pitched sound but it definitely does not machine.
I am comfortable machining a steel plate. I do not believe I have the skill to cut internal threads. Is there a source for semi finished steel back plates?
(BTW: It took the better part of 2 days but I was able to get a really stuck back plate off the lathe. It took repeated doses of penetrating fluid followed by heat on the plate and medium blows. That almost made up for the cast iron fiasco).
 
most likely you're turning it too fast. I'm guessing it's a 5 or 6" back plate, so you'd need to be down in the 100rpm range or so for HSS, maybe double that for carbide.

Cover your ways with a paper towel or similar too, cast iron dust is pretty abrasive and you don't want it getting between 2 sliding surfaces.
 
I would persist in trying to machine the backplate you have and consider it a learning opportunity.
If you are using brazed carbide use a grade meant for cast iron (C2 maybe) and sharpen it for the task.
It is also important to have as rigid a set up as possible. Photos of your set up would help us help you.
 
Follow up: Yes, I was running it too fast. I have been mainly machining small T6061 parts and I was using that setting. The backplate is 8 inches in diameter.
I was using carbide out of the box. I am not sure of the grade. It also may have needed sharpening.
I will try to send photos tonight.
 
thanks for the follow up. You wouldn't be the first - I wiped out the tip of a HSS tool in one pass first time I tried machining cast iron :)

Good rule of thumb is (4 x surface feet per minute)/ diameter of work piece (lathe) or tool (mill). For HSS I typically use 100SFM for alu and 50SFM for steel/ CI ('cos the math is easier). So for a 1" steel round, you're looking at 4 x 50/ 1 = 200rpm. For an 8" CI disk, it'll be more like 4 x 50/ 8 = 40rpm (so higher than my own guestimate!). You can always go up in speed from there, but those numbers are good places to start.

Carbide can be 2-4x the speed, but I usually start at 2x and go up from there. Your work and the chips will soon tell you if you're going to fast, by screaming at you :)
 
Steel would be a bad choice for a backplate, as it could gall the threads on the spindle when trying to remove it, that is one reason that they are made of cast iron.
 
@dansawyer,

I believe most back-plates are cast iron.

Correct your spindle speed as above, and sharpen up a new HSS blank.
You can do this!

-brino
 
benmychree said:
Steel would be a bad choice for a backplate, as it could gall the threads on the spindle when trying to remove it, that is one reason that they are made of cast iron.
Click to expand...

mebbe, but my 4 jaw has a home made steel backplate and it's never caused me issues. Clean and oil the spindle/ chuck threads before you use it and you're good to go.
 
It is the nature of cast iron to have a "crust" that is very difficult to cut through. Once the crust is broken through, it machines as "normal" cast iron. Iron castings are usually oversized in some dimensions to compensate for this. I'm sorry I can't provide the details you need, loss of memory is one of the side effects of age. That, and I never was a machinist, just getting into it when I retired.

One big issue is to cover the shears(ways) when working cast iron. It is very abrasive and will act like lapping compound on the machine surfaces. This is doubly so for cutting through the crust, which also will contain some silica from the molding sand. I do remember the machine shop at the "Pipe Shop" during the late '70s. Experienced machinists cussing a blue streak when they were taking the first cut on an iron casting.

"We" were casting ductile iron, which has a different chemistry from "gray" iron. When the ends of the pipe were to be for "flanged" pipe, the "spigot" end had to be threaded and both ends drilled and trued for bolting together. Actually, "they" were casting the pipe, I just kept the machines running. Again, I'm sorry I can't provide the details you need. Just some insight into why and how steels differ from iron castings. Probably someone more experienced will provide useful details.

Bill Hudson​
 
my back plate on the four jaw is mild steel, home made and so far has never galled or stuck.
Cutting the internal thread was a bit of a learning process but I got there.
 
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