Unknown Steel Speeds, Feeds, DOC?

[MaverickNH]

I have a Hemingway Machinists’ Hammer Kit that comes with undefined steel, bronze and aluminum, but I suspect the materials are chosen to be readily machinable. That said, the LMS calculator spits out 1910-2500 RPM (max for the 7x16 is 2500 rpm) to turn 0.5” down to 1/4” with HSS for Steel, Free Machining and 1950-2290 rpm for Steel, Low Carbon, so not much difference. But Steel, Alloy calculates to 610-1220 rpm.

I’ll use Tap Magic as cutting fluid and take shallow cuts. I’ve read that DOC should be no more than 2/3 of the cutting radius, so for a 1/32” radius that’s 0.02”. That’s about a dozen passes to turn 0.5” down to 0.25”.

What are the signs I’m cutting too slow/fast vs too deep?

BRET

 

[RJSakowski]

I’ll use Tap Magic as cutting fluid and take shallow cuts. I’ve read that DOC should be no more than 2/3 of the cutting radius, so for a 1/32” radius that’s 0.02”. That’s about a dozen passes to turn 0.5” down to 0.25”.

What are the signs I’m cutting too slow/fast vs too deep?

BRET

Seven passes @ .02" will take a .5" diameter down to .25". You would be removing .04" from the diameter with each pass.

I personally don't usually use feeds and speeds but go by feel. The limitation on a small lathe is usually the available hp and rigidity of the machine. What I don't want to do is to stall the lathe, possibly breaking something in the process. For rough stock removal, I usually feed manually as the lathe will tell me if I'm feeding too fast. I finish up with power feed as I'm closing in on my final dimension and looking for fine finish. This also saves wear and tear on my gears, lead screw, and half nuts.

 

[francist]

I don’t really watch the numbers anymore either except for depths of cut, and my machine will tell me if I’m on the mark there or not. But at the beginning when you first start out you do look for some guidelines to tell you if you’re in the zone or not. One thing you can do, regardless of whether you’re experienced or not, is look at the colour of the chips coming off the cut. Ideally I’m looking for them (assuming steel) when they’re just turning into the straw to dark straw range. If they’re hitting the pan dark blue or purple-black I’m going too fast.

 

[MrWhoopee]

I always calculate surface speed, if only roughly. It prevents burning up the tool. I have never in my career calculated feed or DOC. Those things are dictated by surface finish requirement and the capability of the machine. They are arrived at by starting low and turning it up until chatter sets in, the machine starts to stall or something breaks.

Basic calculation for mild steel (80-100 sfm) is (cutting speed x 4)/diameter
Using 80 sfm (being conservative) gives a starting point of 640 rpm and twice that at the finished diameter.

 

[MaverickNH]

I always calculate surface speed, if only roughly. It prevents burning up the tool. I have never in my career calculated feed or DOC. Those things are dictated by surface finish requirement and the capability of the machine. They are arrived at by starting low and turning it up until chatter sets in, the machine starts to stall or something breaks.

Basic calculation for mild steel (80-100 sfm) is (cutting speed x 4)/diameter
Using 80 sfm (being conservative) gives a starting point of 640 rpm and twice that at the finished diameter.

I’ve read that speed (rpm) to high or feed rate too low can cause chatter and surface finish degradation. On a 7x16 mini-lathe with 0.004”/rev, using the auto-feed to do short 5/8” turning cuts doesn’t seem plausible.

Today, I was cutting that 0.5” steel down to 0.25” for a threaded end of a hammer handle and the final finish was poor. I guess I could have reversed the lead screw and done my final shoulder plunge with the spring pass by auto-feed. But as a threaded end, the finish won’t be a concern.

I’ll have a better chance using auto-feed to taper the handle. Lower rpm will make it easier to use the auto-feed, as I have to yank the half-nut lever up in time to not overshoot the cut.

 

[woodchucker]

I never calculate speeds and feeds. I go by feel, the machine will talk to you, listen and you'll know. The chips will talk to you as well.

If you were doing large production runs, it would be important to know speeds and feeds, so you can get consistent results, and maximize the tool life and minimize production time per unit. In a hobby shop, paying attention to speeds and feeds alone is not learning. I'll bet most old timers didn't use speeds and feeds for one off parts.

 

[benmychree]

The fact is that the faster you cut, the better finish you will get, limited by the speed the tool will stand, at work we made a run of coil mandrels about 2 ft in diameter, using ceramic insert tools at quite a fast speed, the resulting finish was shiny it was so smooth, the cut was about 4 ft long, I think and there was no tool wear apparent. The mandrels were used to coil thin sheet metal from a rolling mill.

 

[RaisedByWolves]

Seven passes @ .02" will take a .5" diameter down to .25". You would be removing .04" from the diameter with each pass.

I personally don't usually use feeds and speeds but go by feel. The limitation on a small lathe is usually the available hp and rigidity of the machine. What I don't want to do is to stall the lathe, possibly breaking something in the process. For rough stock removal, I usually feed manually as the lathe will tell me if I'm feeding too fast. I finish up with power feed as I'm closing in on my final dimension and looking for fine finish. This also saves wear and tear on my gears, lead screw, and half nuts.

This!

All day long, throw the book out and learn your machine/material.

 

[MaverickNH]

My HSS tool has a 1/32” radius. It seems contact angle would matter in DOC. An 0 degree AR tool drives the tip while into the workpiece at about 9 o-clock. Should I rotate the tool CCW by 30-45 degrees?