Compression bits

[andre]

This might sound like a dumb question, but when using a compression bit is it neccessary to cut through in one pass? If you do it in two passes do you still get the benefit of the down cut for the top of the piece?

[harryball]

It depends on if the first cut is deep enough to engage the down spiral. You might want to look at a mortising bit. It is similar to a compression but the upcut part is only a much smaller portion at the very bottom of the bit.

Robert

[Brady Watson]

Andre,
Robert is dead on the money. For a deeper explanation into compression spirals etc, go read the most recent Brady's Tricks -> Poor Man's Compression Spiral (http://www.shopbottools.com/Brady%27s_tricks.htm)

-B

[andre]

Brady
I have read your article and the two bit method is what I was leaning towards. I have found that abrand new straight bit gets pretty clean results if you get the feed speed right. I am building a euro style kitchen with slab doors and I need the cleanest possible cut on the doors. Robert what bit are you refering to, Are you talking about a straight mortising bit?
Do you have a source?

[zeykr]

I've had good luck with Vortex Viper mortise bits: http://www.vortextool.com/standard/productDetail.cfm?groupID=272

I use for sign making - seems to leave a nice bottom but shears the mask well when doing area clears.

[harryball]

I purchased from Onsrud and Centurion Tools
Onsrud
1/4" cutters part# 60-111 and 60-121
3/8" cutters part# 60-113 and 60-123

Centurion Tools ( www.centuriontools.com (http://www.centuriontools.com) )
Their online information is not all they offer, call Fred or Ken. Great company and people with good prices.

Robert

[paco]

Andre,

Onsrud series 60-100 and around. Mortising compression is 60-100M I believe. If you get compression tools with a 1/4" CED, I recommend you avoid single flute as they are too "flexy" compared to larger CED which many prefer in single fluted design. 2 flutes 1/4" CED are very well balanced and #!*? aggressive tool!

[andre]

Thanks for all the info.
I guess I will just have to buy a couple and try them to see what works best.

[jeffreymcgrew]

Andre: with the compression spirals pay close attention to your chipload. We found that they are much more likely to break due to heat than a straight flute bit if your chipload is off.

[daski]

We cut 3/4 melamine using two passes of 3/8 each.
I have a Porter Cable 3 1/4 HP on a PRT standard. We set the router to 10, ooo RPM and cut at 2"/ min and the shavings are cool. If we cut slower they are hot. I've used standard compression and they work great. I have just started using mortise compression from Centurion. They work very well. We have found that with 1/4 bits we get about 20 thou of bit deflection.

[andre]

I did some practice runs today and got great results with a down cut on the first pass and an up cut on the last. Perfectly clean. I am using a .25 bit with a 1/2 shank. I thought deflection might be an issue with a 1/4 shank.
I was running at 4 inch per sec at 19000, I think that may be a little high on rpms, the bit was hot, no burning though.
Another question, is the goal not to have a hot bit? how much does it matter. I'm used to running full rpms and feeding at an equal pace. I have had burning on mdf, but not plywood yet.

[harryball]

The goal is to keep the bit to a cool working temp, get "proper" sized chips and maximize your cutting speed while balancing cut quality. You do this with proper chip load. If you are getting powered dust things are moving way too fast and the bit will heat up. The heat weakens the bit making it dull faster and more prone to breaking.

I'm running 6 ips at 17k RPM through 1/2" ACX plywood with a two flute compression. That's about a .010 to .011 chipload. Which is a little lower than prescribed but it sounds better and the chips being ejected look better.

My only issue is when I'm cutting bat outlines. The tool moves much slower ramping around all the curves so I manually change the bit speed to 14k RPM. I *should* go to 12k RPM for the bats but I don't because if I forget to bring the RPMS back up to 17k (I have to do all this manually) it will still cut at 6 ips on the rest of the project just with a higher chipload.

It's an art. I have a mortise compression I use, has the same chiploads listed... but I run it at 15000 and cut at 6 ips... again I look at the chips and listen.

When I'm setting the cut speed here is what I do...

1) what does the math say? use the chipload for the bit, plug in the numbers and get a range.

2) Using my vast less than one years experience ;-) I try to guess at something reasonable, do I have a lot of curves, am I going to try for one pass, which bit am I using etc... and choose something in the range from #1. I know if I have a lot of curves that running at 8 ips is not an option... I figure 3 ips will be top speed and go from there.

On the other hand, with no curves my max RPM is 18k and I may choose to set that and maximize cutting speed. I may have hold down issues and want to move slow, I may be trying for a smooth polished edge... all of this plays in.

In the end I try to choose something reasonable and maybe conservative for the first cuts.

3) I take the results from 1 and 2 above and cut the file watching it cut. I consider the chip size, heat generated on the bit, sounds, burn marks and adjust from there.

I've never had to slow down a cut file on my first attempt. I'm generally conservative and work up toward speed. The production files I'm currently cutting at 6 ips I started at 3 ips and improved from there. I did reach 8 ips at one point but I noticed my cut time was no longer decreasing thus I knew ramping as limiting me and I was only reaching 8 ips for a few feet of actual cutting, so I settled on 6 ips.

I didn't mean to go on, I know chiploading can seem like smoke and mirrors, but it does get easier with time.

Robert

[andre]

I have been going by sound, and running at 19,000 to 21,000. It seems like an art indeed and takes finess. I feel the bit at the end and try for warm but not to hot to touch. I need to start taking notes!