Speeds & Feeds, what's the relationship?

D

dml66

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I've tried to find an answer to the subject question, nothing has popped up.

Here's an example:

A Micro-100 cutter suggests, for carbon steel, including 12LXX as is the case here, an SFM of 200, and for a 0.021-0.280 diameter workpiece, a roughing feed rate of 0.002". Based on the SFM, I'd need to set the spindle speed to about 2800 rpm.

Let's say I can meet the feed rate but I cannot meet the speed rate, lets go halfway and assume my lathe is limited to 1400 rpm.

If I wanted to use this cutter even though the suggested spindle speed is limited to half the suggested speed, would it be safe to say I'd be closest to optimal as I can get by reducing the feed rate by half?

Thank you.
 
It's all about chip load per tooth, or in the case of a lathe, chip thickness, this can be discerned with a slide rule calculator in the case of milling, or just a seat of the pants approach in the case of lathe work, whatever works best with what is being done on what machine. Not enough info to give any definitive advice, but on small diameters, go as fast as you can RPM wise and adjust the feed so as to get the result you desire.
 
matthewsx said:
Bear in mind most speed/feed guidelines are intended for commercial shops and machines. Our hobby machines often won't do the rates that are recommended.

John
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That's why I ask about scaling; can a speed and feed pair of parameters be scaled down, and if so, is the scaling factor linear, or more complex?
There's no incentive for the commercial shops to work on a conversion factor, if there is one.
 
I always use the recommended, or close RPM, and wing it on the DOC, and feed, listening, and feeling how the machine is working. All of the math is beyond my my mental abilities, I am absolutely math challenged, I just try to remember what works. You will do fine, listen to John above(benmychree).
 
Keep in mind that speeds and feeds were primarily developed as an answer to the question “How fast can we remove metal without destroying the tool?”. So lot of the studies were based on quantity of metal removed before replacing or resharpening the tool, with the goal of doing so in the least time possible for production speed.

But factors like finish, machine rigidity, work hardening, dissipating cutting heat, available HP, the shape of the chips, and wear and tear on the machine and operator are all effected by speeds and feeds, so boiling down speeds and feeds to a simple formula is just a starting point. Picking the right answer depends a lot on what you to achieve, and is part of the art of machining, with a healthy dose of trial and error.
 
Karl_T said:
Not positive, but I think it just scales down feed linearly to keep chip load constant.
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This would be my assumption.

I generally do a rough rpm calculation just to avoid burning up a tool. More of an issue with HSS than carbide. After a while it becomes intuitive.

As a manual machinist running manual machines, I have NEVER calculated chip load or feed rate. Start low and increase until something undesirable happens. Undesirable includes chatter, poor surface finish, machine bogging down, cutter breaks.
 
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