Soft jaws

[MLDJR]

I just saw this thread and from my math, 492RPM gives 515 SFPM at 2" radius. Is that correct?

I think it more like 250 SFM

 

[garysue46]

true you have to preload the jaws. I was always told to chuck on a diameter closte to the size you're going to turn. Hold it at the back of the chuck jaws, and bore in as far as needed.

 

[Wreck™Wreck]

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.



I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.



Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544

 

[brino]

@MLDJR,

My old LeBlond is running at 492

The circumference of the work is C = 2 * pi * radius = pi * dia.
Since we need the circumference in feet (not inches) to match with our surface FEET per minute:
C[ft] = pi * dia.[in] / 12
since pi ~= 3 (close enough for these calculations)
C[ft] ~= 3 * dia.[in] /12
C[ft] ~= 3/12 * dia[in]
C[ft] ~= 1/4 dia.[in] or dia.[in] / 4

The SFPM is the circumference(in feet) multiplied by the RPM:
SFPM = C[ft] * RPM
SFPM = dia.[in] /4 *RPM

Then using the above approximations;
492 RPM is about 31 SFPM at 1/4" dia. (1/8" radius)
492 RPM is about 492 SFPM at 4" dia. (2" radius)
(so I agree with @Randall Marx within a bit of slop due to my simplifications)

For cutting mild steel with a HSS tool bit I aim about 80 SFPM.
80 SFPM is about 1280 RPM at 1/4"dia. (1/8" radius)
80 SFPM is about 80 RPM at 4" dia. (2" radius)

So your original 492 RPM was actually slower than required at the inner diameter, but way too fast at the 4" outer diameter. Too slow is usually okay. Too fast can be a big problem.

(Don't forget to convert the diameter in inches to feet in the calculation!)

-brino

 

[Chuck K]

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.



I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.



Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544