PM 1340 Feed Rates

[darkzero]

I know this is an old post but I've wondered this myself.

Can someone explain to an idiot like me how the feed rate is calculated?

I have always just divided the TPI into 1 to find the decimal equivalent but the chart above shows completely different results.

For example I was machining last a part last night using D-6 on the Norton gearbox. The chart on my PM1340GT indicates that D-6 is the gear position for 48 TPI (Threads Per Inch). If you calculate that out isn't is 1/48= .020833 or roughly .021"? Yet in the chart above for D-6 it shows .0068".

Can someone please explain?

Thank you.

I haven't read the entire thread so forgive me if I'm totally missing the question but TPI is not the same as feed rate. TPI would be teeth per the distance of an inch so you can caluculate the travel but the feed rates listed on the chart is when using the power feed rod instead of the leadscrew, feedrate is Inch Per Revolution. So if you set your gearbox to say .005", the carriage should travel .005" for one complete revoution of the spindle. You can somewhat verify this with an indicator or DRO with the power feed engaged & turn the spindle by hand.

 

[mksj]

TPI is carriage moved by the lead screw, feed is fed by the feed rod, the ratio of lead screw to feed rod carriage movement is something like 3.1:1. So D-6 is 48 TPI or 0.0208 IPR, divide by ~3.1 to get the feed rod IPR of the carriage which would be ~0.0067 IPR.
Mark

 

[zmotorsports]

Awesome.

Thank you Will and Mark, I really appreciate that. Been using this lathe for several years now and know what settings work well for various materials and RPM's but never really calculated the feed rates out until I started playing with calculations the other night and simply couldn't come up with what others were stating for feed rates.

Thank you very much.

Mike

 

[xr650rRider]

I just finished Clough42 Electronic Lead Screw on my PM-1340GT and the feed rates just didn't match the front of the machine.

This is the actual feed rate chart, one on machine is incorrect. I put Norton gearbox on D-6 and use 1:1 pulley ratio and 1:1 ratio to encoder. I used a Servo that is rated for 1.2 NM @ 3000 RPM, so using 6:1 ratio I have plenty of torque. I used a JMC 400 watt Servo and changed internal stepper pulses to 500 using servo software.

Code changes
#define LEADSCREW_TPI 8
#define STEPPER_MICROSTEPS 6
#define STEPPER_RESOLUTION 500
#define STEPPER_MICROSTEPS_FEED STEPPER_MICROSTEPS * 4.2405

Using the example above D-6 being a 48 TPI, the feed rate would be 1/48 = .0208"/Rev, when you change to feed ratio, the feed would be .0208/4.2405 = .00491"/Rev which matches the updated worksheet on Precision Matthews website.

 

[parshal]

If I'm reading that chart correctly it's quite a bit different. Is the IPR graph for carriage or cross feed?

The chart shared earlier (attached) has D-6 at .0068" for carriage and .0024" for cross.

 

[xr650rRider]

It's for carriage, verified using DRO and turning by hand. 4.2405 is the constant ratio between threading and feeding. Not sure if earlier PM-1340GT had different gear ratios or not. I thought mine must be a unicorn, then found the spreadsheet on Precision Matthews website and it's been there for a while. Using the gearbox, what I thought I had been feeding, turned out to be completely incorrect.

 

[mksj]

One is the carriage movement off of the lead screw, the other is the carriage movement with the feed rod, cross slide would be 1/2 the feed rod. That is a pretty gross discrepancy, had I known that when I had my 1340GT I would have been more aggressive on my feeds.