Let's talk Kerfs (curving your foam)

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Re: Let's talk Kerfs (curving your foam)

Postby GPW » Wed May 17, 2017 4:03 pm

Page one explains kerfs and spacing ….
Rusty, did you try running your saw blade backwards … :thinking:
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Re: Let's talk Kerfs (curving your foam)

Postby rustytoolss » Wed May 17, 2017 8:26 pm

Never thought about running the blade backwards :thinking:
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Re: Let's talk Kerfs (curving your foam)

Postby rowerwet » Fri May 19, 2017 6:51 am

I cut almost all of my foam with a Saber saw.
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Re: Let's talk Kerfs (curving your foam)

Postby KCStudly » Fri May 19, 2017 12:14 pm

You need a blade with not too many teeth and plenty of clearance for the "chips". The foam cuts so easily that the saw needs to be able to take a chip and eject it. If the tooth count is too high and the chip clearance too low, you won't clear the chip and will start creating friction, which will melt the foam and escalate the problem.

General purpose blade should work. I think, IIRC, I have a 40T Irwin in my table saw. Cuts both foam and wood well.
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Re: Let's talk Kerfs (curving your foam)

Postby lthomas987 » Sat May 20, 2017 4:23 pm

rustytoolss wrote:Had I known more about doing kerfs. I would have bought some 1" xps for one section of my roof (2pcs stacked). But not knowing I bought all my foam in 2" thick. I will need to make a 18" inner radius 90 degree curve. I'm going to try this with a piece of scrap 2" foam first. To see how this go. If I don't feel like it's going to work out well. I may have to make my radius in long small sections. Can anyone tell me the spacing for kerfs / and depth of cuts for 2' foam / and number of cuts needed?
Another question I have is. I bought a fine tooth blade for my 10" table saw. Thinking it would give me better cuts in the foam. But I have problems when I cut the foam. The saw will grab the foam , almost like a kick back thing. So I have been using my circ saw instead . I don't get any kick backs with the circ saw.The circ saw has an old general use blade.
Could the fine tooth blade on the table saw be creating my problems ? I actually cleaned off all the foam residew off the side surfaces of the table saw blade thinking that would help prevent the grabbing problem. But it made no difference.


I got a 12" radius in 2" foam. Using a course blade in my table saw. Cuts 1.5" deep.


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Re: Let's talk Kerfs (curving your foam)

Postby rustytoolss » Mon May 22, 2017 7:12 pm

lthomas987 wrote:
rustytoolss wrote:Had I known more about doing kerfs. I would have bought some 1" xps for one section of my roof (2pcs stacked). But not knowing I bought all my foam in 2" thick. I will need to make a 18" inner radius 90 degree curve. I'm going to try this with a piece of scrap 2" foam first. To see how this go. If I don't feel like it's going to work out well. I may have to make my radius in long small sections. Can anyone tell me the spacing for kerfs / and depth of cuts for 2' foam / and number of cuts needed?
Another question I have is. I bought a fine tooth blade for my 10" table saw. Thinking it would give me better cuts in the foam. But I have problems when I cut the foam. The saw will grab the foam , almost like a kick back thing. So I have been using my circ saw instead . I don't get any kick backs with the circ saw.The circ saw has an old general use blade.
Could the fine tooth blade on the table saw be creating my problems ? I actually cleaned off all the foam residew off the side surfaces of the table saw blade thinking that would help prevent the grabbing problem. But it made no difference.


I got a 12" radius in 2" foam. Using a course blade in my table saw. Cuts 1.5" deep.
So you had no problem bending a radius on 2" foam ?


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Re: Let's talk Kerfs (curving your foam)

Postby rustytoolss » Mon May 22, 2017 8:01 pm

lthomas987 wrote:I have a spreadsheet that does all the math for you if you want to give it a try I could share it. It's not well documented at this point but I would be glad to clean it up some next week.

https://docs.google.com/spreadsheets/d/ ... Hs/pubhtml

I found your spreadsheet for doing Kerfs. But I cant seem to make it work. I'm doing a 18" inside radius (90 degrees) in 2" foam. And like you said before cutting 1.5" deep.
How can I make the spreadsheet work ?
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Re: Let's talk Kerfs (curving your foam)

Postby Don L. » Mon May 22, 2017 9:39 pm

You may know this but just in case, run a few strips of packing tape on the outside of the bending foam board, perpendicular to the kerfs. Pull the tape off once everything is glued and set.
The tape really helps stop breakage.
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Re: Let's talk Kerfs (curving your foam)

Postby lthomas987 » Mon May 22, 2017 10:46 pm

rustytoolss wrote:
lthomas987 wrote:I have a spreadsheet that does all the math for you if you want to give it a try I could share it. It's not well documented at this point but I would be glad to clean it up some next week.

https://docs.google.com/spreadsheets/d/ ... Hs/pubhtml

I found your spreadsheet for doing Kerfs. But I cant seem to make it work. I'm doing a 18" inside radius (90 degrees) in 2" foam. And like you said before cutting 1.5" deep.
How can I make the spreadsheet work ?


There is something wacky with the permissions. I'll sort it out at the office tomorrow.


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Re: Let's talk Kerfs (curving your foam)

Postby lthomas987 » Tue May 23, 2017 10:07 am

https://docs.google.com/spreadsheets/d/ ... sp=sharing

Try this link I think the other one was the direct link for me.
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Re: Let's talk Kerfs (curving your foam)

Postby rustytoolss » Tue May 23, 2017 9:00 pm

lthomas987 wrote:https://docs.google.com/spreadsheets/d/1Kk7c9hEz1XWNd5RT-SXk2GimpYZOMOlkoKAXUTULIHs/edit?usp=sharing

Try this link I think the other one was the direct link for me.


Thank you So Much :D
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Re: Let's talk Kerfs (curving your foam)

Postby OP827 » Wed May 24, 2017 9:12 pm

Note that 1/8"=0.125", just my 2c
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Re: Let's talk Kerfs (curving your foam)

Postby rustytoolss » Thu Jun 01, 2017 8:15 pm

lthomas987 wrote:https://docs.google.com/spreadsheets/d/1Kk7c9hEz1XWNd5RT-SXk2GimpYZOMOlkoKAXUTULIHs/edit?usp=sharing

Try this link I think the other one was the direct link for me.

I just made a test piece of 2" foam using your spreadsheet information. Works like a charm :thumbsup: . Very happy :D . I will start on the actual roof piece on Monday. Thanks
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Re: Let's talk Kerfs (curving your foam)

Postby lthomas987 » Fri Jun 09, 2017 9:08 am

rustytoolss wrote:
lthomas987 wrote:https://docs.google.com/spreadsheets/d/1Kk7c9hEz1XWNd5RT-SXk2GimpYZOMOlkoKAXUTULIHs/edit?usp=sharing

Try this link I think the other one was the direct link for me.

I just made a test piece of 2" foam using your spreadsheet information. Works like a charm :thumbsup: . Very happy :D . I will start on the actual roof piece on Monday. Thanks


I am glad to hear it!


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