Calibrating Depths when Changing Tool Bits

When you replace an old dull bit with a sharp new fresh bit, how do you compensate for the difference in length of the tool? May 13, 2013

Is there a fixture that lets you install a sharp bit into the tool holder at the same length as the previous dull bit?

Forum Responses
(CNC Forum)
From contributor H:
There aren't too many options. Back in the day people would get their tools with a thread in the shank of the tools. A set screw with a locking nut was inserted and adjusted to match the same height of the existing tool. You will still have some variance in there. It will get your very close. Another option is using a tool pre-setter and measuring the tool in the tool holder. A lot of people use this to measure the tool beforehand and make it easier for the operator to just enter the new numbers for the tool height.

From contributor M:
One other possibility is replaceable tip tooling, but it limits you in many situations. You also can in some cases bottom the tool out into your tool holder and get very close each time. I think though the best option is a touch tool device. Many routers come with this preinstalled and integrated with the machine controller so that length compensation is very easy and very precise. I do not know offhand where you can get one aftermarket, but I do remember discussion on this a few years ago.

Unfortunately the machine I use now does not have touch tool and I use a device like described above. It is not a lot of trouble, especially if you have a few extra tool holders about. Since there are no good automatic options for diameter compensation I am aware of and I have to compensate the tool for diameter anyway, manual length compensation is not the end of the world for me. This device is pretty nice and the company is very reputable. I have it set up on a granite surface plate in a rollout drawer near the machine. It works very well.

From the original questioner:
My son is a machinist and we are designing a fixture that has an adjustable stop above the locked in tool holder. In this way the bit can be set to the same dimension each time without re-measuring, etc.

From contributor D:
You don't need to design a fixture. A built-in function on a standard height gauge is a lock. Tighten it down and use it to set your next tool to same depth.

From contributor H:
The only problem I have run with a fixed height gauge is that when you put a tool in and tighten the collet nut. The tool can end up a few thousands of an inch less than the desired height.

From the original questioner:
I just want to set the bit to the same height each time. If the drop from the fixed stop is the same each time, that would be fine.