Compression Bits for Newbies?

[scottp55]

Just had someone ask me something I know ZILCH about :(
I don't know what wood or material he's cutting, or even if he Needs 2" CEL?
Thought I'd copy/paste his edited E-mail here as I always like Real World recommendations from people I trust. He's in OZ and I know many people don't ship, and IF they do it's outrageous.
Not really anything I see myself using in the immediate future.
He has a 2013 Desktop with 1 hp spindle and Z-Upgrade.
Thanks all,
scott

Hi Scott,

Do you use compression bits at all? They tend to be larger and quite expensive. The size alone would not seem to suit the sort of work you do, but I just wondered. I'm planning to get some longer depth of cut straight bits - over 2". The compression cutters I've seen don't go that far - the largest cost over $200. Yikes.

Cheers

[MogulTx]

I use compression bits from time to time... but he needs to know WHY he wants a compression bit. What is he trying to accomplish? If he doesn't know, he needs to back up and do some research. Most of the bits I use are 15-35... I have used a couple $85 bits... large "hogging" bits for foam. They were overkill.

Tell him not to make it any more complex than he needs to.

Monty

[Coogara]

Scott always beats me to the punch on posting. I start with him because we chat so much offline :)

Anyway, I was the one who asked Scott the question initially. I'm about to start working with some nice, hard very dry red gum 2" thick. So I'm likely to end up purchasing some up/down cut 1/2" end mills of suitable flute length, but was wondering about the benefit of using a compression bit. The price of these is confronting and the flute length isn't as long as the Onsrud end mills, so it's more of an intellectual question, no thaving used them before. Mix into the equation that I'm woring on a Desktop with spindle.

Cheers

Graeme

[bob_dodd]

First let me say I never used them , I have used up and down cut bits from Lowes about $20 ( Bosch .25 X 1 ) which both seem to work fine for general use
I seem to remember that Brady did a write up on this subject , either on the forum or on shopbottool about compression bit hope this helps

[ssflyer]

Compression bits incorporate both an upcut and downcut portion. They are mainly used for sheet goods to give a good finish on both top and bottom of the cut.

The first cut needs to be deeper that the upcut lower portion, so that the downcut part is cutting the top of the material. The the final pass (Unless you are cutting full depth to begin with) has to go through, into the spoilboard so the upcut portion is cutting the bottom edge properly.

Sorry for the rambling reply -think I'm coming down with the flu... :(

[scottp55]

Graeme, One of the sparsest "Wood Databases" entries I've seen:)
http://www.wood-database.com/lumber-identification/hardwoods/river-red-gum/
But, the color and mention of "Fiddleback" has my interest:)
http://en.wikipedia.org/wiki/Eucalyptus_camaldulensis

Sounds a little like Bloodwood(but not as dense), Think I might save the bucks for brand new sharp bits:) And "Go with what you Know" for starters.

[Coogara]

Hi Scott,

I have several slabs of red gum in the shed including some fiddleback. The fiddleback is one of my favourites for making pens and other turned products. It produces a lovely finish.

As for the compression bits, yep, I understand their purpose. Really wondering about the extra benefit for dealing with an especially hard wood - haven't worked any red gum on the CNC before. But I recall our Beckwith friend saying he had worked some - slower speeds, IIRC. For now I will no doubt stick with up/down mills.

Cheers

Graeme

[Tim Lucas]

Compression bits give a nice cut on both sides of plywood as said, but I don't see any reason to use them on solid wood - a down-cut bit will keep you from splintering the top side and your table should keep from splintering the bottom side. Just my thought.
Tim

[Coogara]

Good advice, Lucas. Makes a lot of sense.

Graeme

[Tim Lucas]

Thanks, I cut solid maple and use down cuts - some times I wont cut all the way through, and then run it in my drum sander to take off the rest. works real good on small parts with tabs.

[Coogara]

Hi Lucas,

I was just reading another thread where someone was asking about cutting small parts. One of the suggestions was not cutting all the way through then taking the back off with a band saw, but I like the idea of using the drum sander too.

Cheers

Graeme

[barrowj]

I use the onion skin method many times with hardwood, I have a 24" dual drum sander that I use to take off that last little bit. This leaves a nice finish and you don't have to worry about tabs or cutting into your spoilboard.

Joe

[bob_dodd]

Link to Brady's article on compression bits and useage
http://www.shopbotblog.com/2007/03/a-tale-of-three-bits%E2%80%A6and-the-poor-man%E2%80%99s-compression-spiral-trick/

[ssflyer]

Another option that I've used, if you don't have a drum sander - cut most of the way with your downcut bit, then switch to an upcut for a full depth pass. You get the benefit of a compression bit w/o the expense.