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Important information about successful internal turning.
Anyone who has tried boring a hole has quickly discovered it can be one of the most difficult aspects of machining. Vibration developing from improper setups can lead to failure very easily.
To preface, length to diameter ratio determines much in your success or failure when attempting internal turning. Here are a few rules of thumb when selecting a boring bar for your application. Assuming favorable conditions.
Steel boring bars have an effective length to diameter ratio of 3x1. A 25mm boring bar can generally bore 75mm deep without issues.
Carbide boring bars have an effective length to diameter ratio of 6x1, a 25mm boring bar can generally bore 150mm deep without issues.
Carbide reinforced and dampened boring bars have an effective length to diameter ratio of 10x1, a 25mm boring bar can generally bore 250mm deep without issues. (The main picture on this post explains the internals, see the second image if for comparison between a smaller Carbide bar and a larger steel bar. Both bars though different in size, can bore roughly the same depth successfuly)
Greater internal turning depths are possible with each style of bar with fine tuning. This can take time to find a successful recipe though.
•For successful internal turning it is normally best to shorten the bar overhang as much as possible.
•Choose a smaller corner radius insert than the chosen depth of cut
•Choose a sharp positive insert with an open chip breaker.
•Use coolant to evacuate chips or compressed air
•Modulate the cutting speed to break any harmonics that develop.
•Use a cylindrical clamp to hold as much of the bar as possible, set screws make less contact and do little to absorb vibrations.
•Support the workpiece fully
•Reduce cutting speed and increase feed rates if vibrations begin.
These are just a few key aspects of internal turning, more in depth information will be available here soon. We plan to create categories of machining tips like this so they can be accessed easily.
Anyone who has machined much plastic knows it is impossible to break a chip, especially in UHMW. The long stringy chips can be outright dangerous!
Instead of simply turning plastic, mill a slot just above the finished part size. This slot will act as a relief point for the Chips to fall away freely due to the interrupted cut. Mill several slots for even smaller Chips.
If you don't have a Live Tooled Lathe, simply prep your stock on a Mill beforehand. The time saved not clearing stringers and added process reliability will be worth it!
Often times, a part may be held with one end inside of workholding which can prevent reference orientation from being established.
Unless you have a way of handing a part off for second operation work. (E.g.Lathe Sub Spindle) Reference flats are a great way to orient future operations. For one off parts like this, setting up several tools would have taken much longer than simply milling a few reference flats and drilling a hole. This part required features be oriented 90° from each other. As you can see, the C-Axis orientation of this part could easily be established in reference to the existing geometry.
A demonstration of finishing steel under conditions that are not ideal.
This recipe can make "sneaking up on a tolerance" possible with steel, shallow depths of cut normally lead to poor surface finishes.
Here is the insert used, different manufacturers may have similar inserts available. https://www.sandvik.coromant.com/en-g...
Starting parameters for this insert should be 1100-1500 SFM .005" FPR Depth of cut Greater than the .0156" tool nose radius. The parameters for this less than optimal demo skim pass were.
1100 SFM .001" FPR .004" DOC I hope this helps anyone struggling with surface finish when turning steel.