HOME MACHINE SHOP Lathe Center Height Gage
A robust lathe center height gage built using "off the shelf"
"stainless steel" parts and minimal machining.
ALIGN IT ONE TIME
USE IT FOREVER!
Height gage is trued to the machine center line using the machine centers.
The height gage is locked in position with brass thumb / Allen screws serving
as a "center standard" for thousands of uses without any further adjustment.
CONSTRUCTION
Parts:
Part sources:
Thor Labs - Posts Thor Labs - Right Angle Clamp ENCO - Magnetic Base
Fabrication:
The ENCO magnetic base has an 8mm 1.25 pitch threaded hole that is close in
diameter and pitch to a 5/16-18 thread so the hole in the ENCO magnetic base is
re-tapped to 5/16-18. A 5/16-18 set screw is used to interface the Thor Labs TR3
vertical post to the ENCO magnetic base. The The 1/4-20 threaded hole in the
Thor Labs TR3 post will be enlarged by drilling to 17/64 to accept 5/16-18 threads.
Drilling the 17/64" hole in the Thor Labs TR3 (3") post:
Tapping the 5/16-18 threads in the Thor Labs TR3 (3") post:
The 5/16-18 x 3/4" set screw is installed in the Thor Labs TR3 (3") post using
threadlock:
Tap the 5/16-18 threads in the hole in the magnetic base using a bottom type
tap or a plug type tap. A taper type tap will not allow the thread cutting to
penetrate into the hole in the magnetic base. Grinding the taper off of the
end of a tapered tap will essentially convert the tap to a plug type tap.
Remove the stainless steel set screws from the Thor Labs TR3 (3") post and
the Thor Labs TR6 (6") post and save them for a future project:
Install the Thor Labs TR3 (3") post in the 5/16-18 threaded hole in the ENCO
magnetic base along with a 5/16" washer . A 7/64" Allen wrench may be
inserted in the hole in the Thor labs TR3 post and used to tighten the TR3 post
The Thor Labs TR6 (6") post will be positioned horizontally and will have a
milled slot or notch that is half the diameter of the TR6 post in its depth.
The inner face of the slot or the notch will be the surface that is the reference
contact surface for checking cutter height. The nominal diameter of the TR6 (6")
post is 0.4990" and the nominal depth of the milled slot or the notch is 0.2495".
Ideally, the diameter of the TR6 post should be measured with a micrometer and
the slot or the notch should be milled to a depth that is half the value of the
measured diameter. The slot or notch should be at least 1/2" wide and may be as
wide as 1". The edge of the slot or the notch should be at least 7/16" or greater
from the end of the post so that the post's center hole does not interfere with the
slot or the notch. The slot shown was milled in passes with a depth of cut of 0.025"
at 400 RPM with a slow feed rate of 1 IPM (0.067mm/rev) using ENCO cutting oil.
Within 0.010" of the final dimension, the depth of cut of milling passes were
reduced to arrive exactly at the final dimension.
ENCO cutting oil is a very inexpensive compared to RapidTap or TapMatic.
Alignment / Calibration:
The radial position of the Thor Labs TR6 post in the RA90 clamp is set so the flat
inner surface of the milled slot is alligned parallel with the base of the height gage.
A stack of parallels can be used to assure parallelism or a 1-2-3 block with a gage
pin or drill bit shank. The brass thumb / Allen screw is tightened with a 3/16" Allen
wrench to prevent any rotation of the TR6 post in the RA90 clamp.
Once the radial position of the Thor Labs TR6 post has been alligned to the
Thor Labs RA90 clamp, the TR6 post is mounted on centers on the lathe which
assures that the TR6 post is alligned to the machine center line.
The Thor Labs TR3 post that was assembled to the magnetic base is then inserted
into the remaining hole in the RA90 clamp. The magnetic base is positioned on
the cross slide and the magnetic clamp is enabled. The brass thumb / Allen screw
is tightened with a 3/16" Allen wrench. The height gage is removed from the lathe
centers and is ready for operation!
Height gage parked on cross slide awaiting use.
Operation:
The height gage with the magnetic base enabled will provide a rigid "center"
reference to gauge and adjust cutter height. The cutter height is adjusted so the tip
of the cutter lightly contacts the milled flat surface of the TR6 post.
