What is the right - best bit ?????

[garyb]

Due to geometry in tooling and the material, the direction of cut makes a big difference on the finish cut.
Old rule of thumb " if the off-cut scrap has a better cut finish than the part, then your cutting the wrong direction"

Here's an example, a mortise compression tool should be ran in a conventional direction for the best part finish, but using say a low helix finisher for the same cut will give the better part finish in a climb direction
Gary

[woodshop]

Really appreciate your explanation. I get it.

Will have to try this.

(Great help, Gary)

[dlcw]

Another thing a climb cut does: because of the bit rotation and feed, the bit is pushed AWAY from the cut line. If you cut all but a tiny "skin" of material on the climb, then reverse the direction of cut to conventional, the bit will be pulled towards the cut line. Because the bit is cutting very little material, the flex in the machine is negligible. This gives you a cut perfectly on the line and VERY clean.

The Shopbots flex. Nothing you can really do about that except minimize it. Either way, there is still flex. This two direction cutting process reduces the effects of the flex to a level acceptable in woodworking. Probably not acceptable in the metal world though.

It takes longer to cut a sheet of plywood but the the end result is much better. I learned this the first time I did a bunch of cabinets. I used only conventional cut with a 3/8" compression bit. Every part needed to have a little edge sanded off. This took a lot of time and manual labor. Back 10 years ago, I was told of this two-pass cutting method. The next set of cabinets I did, the results were ten times better. No sanding. Parts were ready to go the the edgebander without any sanding. Eliminated all the sander labor to create a clean edge on the plywood parts.

[Martin Reid]

May I ask, please, what is a "Climb Cut?"
Can that be explained?

Thanks in advance.

Dear Dave

Conventional Vs Climb.jpg

Those that have used hand routers with a fence will know that machining with the cutter (conventionally) the router and the fence pull together. When on the other hand you machine against the cutter (climb) the cutter tends to pull away from the fence.

So why would you do this?

If you are machining the edges of a quarter sawn timber that has a tendency to spelch or split then although it is difficult to do by hand and down right dangerous on a hand fed wood working machine, climbing against the cutter can prevent spelching.

Also if you are machining around all the edges of a piece then choosing where you start can help to eliminate spelching.

Sincerely and in good faith
Martin Reid

[woodshop]

I really get this and the great explanations offered.

So... You guys are sending two file paths to cut. The first is a "Climb" cut at 1/32" deep.
The second is a "Convential" cut and finishes all the way through the ply.

Are you merging those two cut paths?
If so, how do you ensure the climb cut is first, conventional is second in the Merged file?

[steve_g]

Dave…
Because climb cuts have less propensity to pull out splinters and since conventional cuts are smoother, I cut most of the depth as a climb cut and finish with a conventional cut …

Here’s a typical cut for me in ½” Baltic birch:

First path climb at .225” deep and .010 allowance
Second path climb at .450” deep and .010” allowance
Third path, full depth and on the line… Some of us call this an onion skin cutting strategy.
This toolpath is generated as a single file in Vectric products… under 2D profile toolpaths, edit the passes to three and specify the last to be .050”. Under separate last pass, specify the offset from the line for all passes but the last and check “reverse direction”. I’m assuming you chose climb as the original direction of cut.

If this quick example is confusing, let me know and I’ll be more specific with screen shots!
SG

[knight_toolworks]

another advantage to doing the climb cut is if your bit is getting dull and or the bit jams it will pull away from the part not into it.

[woodshop]

Not confusing here. Good explanation.
I did not know I could edit each pass independently from each other.
Hmm... Onion Skin...Cool!

Yep, will try this ASAP.

Totally understandable, by the way. Great explanations.

Over the past two years, we've been cutting through 5/8" maple 1/4" at a time (5 passes) in conventional mode.

[coryatjohn]

>> Over the past two years, we've been cutting through 5/8" maple 1/4" at a time (5 passes) in conventional mode.

Do you mean 5/4 maple? With 5/8", you've just dug a nice groove in your spoilboard...

[steve_g]

Dave

“We cut maple plywood everyday. Using a 1/4" straight cutter”

I was re-reading posts and just noticed this… Gary Beckwith did recommend a mortice compression bit in one of his earlier posts and I thought more discussion might be in order!

The bit he recommended is a spiral cutting bit. A spiral cutting bit has a cutting edge in contact with the wood at all times, not just when a flute passes by as happens with a straight cutting bit. The spiral bit is a smoother cutting bit because of this feature!

The bit he recommended is a compression bit… that means the bottom of the bit is up cut while the top is down cut. This feature minimizes splintering of the top and bottom veneers by drawing the cutting forces toward the center of the material. The two-flute design he recommended is referred to as 2+2 when you’re shopping… it means 2 up 2 down. I’ve had 1+1 bits in 1/8” diameter and 3+3 in ½” diameter bits.

The term mortice compression means the up-cut portion is very short… maybe even less than ¼”. This allows you to cut shallow rabbets or grooves and still get the benefits of a compression bit! In any case, your first pass must be deeper than the up-cut portion or the top veneer will be pulled up giving a nasty cut!

Spiral cut mortice compression bits aren’t a home depot item… you’ll need to keep a few on hand to assure your supply. The retail woodworking hobby stores may or may not have one in stock… but not at reasonable price! Gary Beckwith reps a quality product in the Onsrud line. Many folks here like https://www.centuriontools.com. If you live in a metropolitan area, often you can get one or two day delivery from Amazon and most of us have a favorite eBay source.

I hope this little tutorial wasn’t insulting… it certainly wasn’t intended that way! Feel free to ask for clarification on anything I’ve muddied up!

SG