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Insert Tab A Into Slot B

Posted: Sat Dec 19, 2015 4:03 pm
by zeeway
More and more of my projects these days involve making parts that must fit into each other. In making chairs and stools very often there is a pocket or a slot that mates with a tenon or a shaped end of a leg.

Too often, in my impatience, I remove the female piece from my machine, only to find I must spend a lot of time with chisels or files or sandpaper to make the parts fit.
In my designs, I may leave a theoretical clearance of .010 inch per side. But in real life, even though my machine is calibrated as well as I can adjust it, I find that pockets and slots always cut to the small side, while parts machined on the outside always cut to the large side. My guess is that my .250 cutter is a bit undersize, or flexes, or something, making the size less than perfect.

So, my tip to myself and others is to keep that female part attached to your machine, until you have checked the fit with the mating part. It is certainly much easier to recut a fit dimension on the machine than to do it by hand.

How do you address this issue?

Angie

Re: Insert Tab A Into Slot B

Posted: Sat Dec 19, 2015 4:15 pm
by Turtle49
I have to ask. Do you use climb or conventional for your finish cuts?
I have found that when I select conventional for my finish cuts I get a more accurate size. The climb cut tends to flex the bit away from the vector. While the conventional will load and flex the bit parallel to the vector.

Perhaps that is affecting your results.

Re: Insert Tab A Into Slot B

Posted: Sat Dec 19, 2015 5:16 pm
by cac67
Under sized bits are very common. I used to measure the bit I was going to use and put the actual size in before calculating the tool path.

Re: Insert Tab A Into Slot B

Posted: Sat Dec 19, 2015 5:54 pm
by 4DThinker
When it matters, I'll take my digital caliper to the bit I intend to use. Onsrud bits will usually be exact or very close, but bits from anyone else can vary wildly.

4D

Re: Insert Tab A Into Slot B

Posted: Sat Dec 19, 2015 8:36 pm
by zeeway
I will admit that I have not paid any attention to whether the cut was climb or conventional. I'll give that a shot as well as measuring the bit. Thanks for the help. But even with that, I will still proceed with caution.

Angie

Re: Insert Tab A Into Slot B

Posted: Sun Dec 20, 2015 1:29 am
by mtylerfl
zeeway wrote:,,,

So, my tip to myself and others is to keep that female part attached to your machine, until you have checked the fit with the mating part. It is certainly much easier to recut a fit dimension on the machine than to do it by hand.

How do you address this issue?

Angie
Hi Angie,

Our good friend Tim Merrill has always advised this - i.e., cutting the male part first, then cut the female part, keeping the female part attached to your spoilboard to check the fit of the two. If the male part fits too tightly, then you can easily modify your original file, then recut the female pocket/dado/slot by a tiny offset amount until it does. This is just one of the many, many golden nugget tips I've/we've learned from him.

Another great tip is one I learned from Gary Campbell for determining if, or by how much, an End Mill bit is undersize. He recommends performing a plunge cut (drill) or slot cut with say, your 0.25" EM in some scrap and measuring the HOLE (or SLOT width) itself. The size of the real-world cut is often a better determinant than just measuring the bit alone.

Re: Insert Tab A Into Slot B

Posted: Sun Dec 20, 2015 11:04 pm
by zeeway
Thanks for those comments, Michael. Maybe I should print those out and keep them posted in my shop, so I won't forget. And thanks to Tim and Gary (once removed).

Angie