# Soft jaws



## MLDJR (Feb 11, 2017)

Morning guys,

Hobby guy here so I need advice.

I purchased a set of Shars soft jaws for a 6" Union 4 jaw chuck I had laying around the shop. So I roughed out the pocket I need and I seem t be having a problem finishing it. I tried a carbide insert tooling, HSS and both seem to dull out quickly and leave a terrible finish. 

What am I not getting here??

Thanks in advance.


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## MLDJR (Feb 11, 2017)

30 views any input?


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## pineyfolks (Feb 11, 2017)

What set did you get? I don't see anything listed on their website.  Are you preloading them while you're cutting them?


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## 4GSR (Feb 11, 2017)

Yep, you have to preload the jaws or  they just sit there and bounce around while boring them out.  A flat washer of an appropriate size chucked on in back of the chuck jaws works great for preloading jaws.


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## Bill Gruby (Feb 11, 2017)

Preload is a must. There is no way to stop the jaws from moving without it.


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## MLDJR (Feb 11, 2017)

Oh yeah I definitely had them preloaded. I used the:
*6" x 1.259"HT American Standard Tongue & Groove Soft Steel Chuck Jaw Set*
SKU202-6255
Jaws.

Just can't seem to get a nice finish. When I start our in the center, the finish is nice. But when I get to the outside of about a 2" radius the cutter seems to be not cutting! Its like the cutter goes dull. Almost like the material gets harder towards the outside!


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## brino (Feb 11, 2017)

All,

I did the SKU look-up and here's the direct link:
http://www.shars.com/6-x-1-259-ht-american-standard-tongue-groove-soft-steel-chuck-jaw-set-1

What spindle speed are you running?
Have you tried slowing it down for the bigger diameter?
The surface feet per minute is very different over that 0-2" radius.

-brino


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## MLDJR (Feb 11, 2017)

Thank you brino I wasn't sure how to post a link.

My old LeBlond is running at 492


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## MLDJR (Feb 12, 2017)

Just a followup to say thanks for the help guys. I went out to the shop last night and was successful!

I ended up dropping the speed and I ground a new HSS cutter and it went very well.


Thanks for the help.


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## Randall Marx (Feb 14, 2017)

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


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## MLDJR (Feb 14, 2017)

Randall Marx said:


> 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


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## garysue46 (Feb 16, 2017)

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.


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## Wreck™Wreck (Feb 16, 2017)

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


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## brino (Feb 16, 2017)

@MLDJR,



MLDJR said:


> 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


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## Chuck K (Feb 17, 2017)

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


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