It has always been a challenge to grind a lathe tool to an exact form, for example a acme thread. Much of the difficulty comes in attempting to set the angles correctly using the factory supplied fence with built-in protractor. Not only are these generally flimsy in construction, but the graduations are very coarse, and tend to be inaccurate.
Repeated attempts to grind tools to a "line-out" fit with thread gages are always troublesome for beginning machinists. To avoid this problem we first constructed a fence known to be at exactly right angles. This is not adjustable. This fence was then used to align the cross-slot in the table as closely as possible to be perpendicular to the grinder spindle. The fence was then used to support and guide a diamond dresser across the "down" side of the wheel. By this procedure, even if the table slot is not exactly perpendicular to the spindle, the face of the wheel [on the "down" side] and the table slip will be parallel.
The fence was made from some CRS pieces we had available. The project provided our students with practice in precision measurement, stock squaring in a mill, use of wigglers and center finders, drilling and tapping, and use of corner rounding end mills.
A top view of the fence in the guide slot. The key was machined to be a
very close sliding fit in the guide slot. Note that the fence was deliberately
kept short so a tool bit could extend over either side of the tool block to be
ground on the "down" side of the wheel.
A bottom view of the fence. Note that the key is slotted into the body
to prevent movement. This was assembled using "red" Loctite to
prevent any movement. After the Loctite had set , the key was clamped in a
carefully aligned mill vise with the face on top of the jaws and extending
slightly over the end of the vise. The face was then edge milled exactly
perpendicular to the sides of the key. the sides of the key and the face of the
fence are perpendicular as far as we can measure using angle irons and a tenths
Because the fence is not adjustable, another way to set the grinding angles
was required. A block was fabricated from a 1" X 2" X 3.5 "
[25 X 50 X 90 m/m] piece of CRS that was available. The block was machined
on all surfaces. The front was beveled to about 25 degrees which is
slightly greater than the maximum table angle we use. A 3/16 lip was left
at the top for alignment with a thin machinists protractor. (5) 10 X 32
holes were tapped at the front of the block to accept the clamping screws.
Two clamps are used. One parallel to the front of the block for end
grinding of the tool, and a longer one parallel to the sides of the block for
edge grinding of the tool. The clamps were made of 1/2 inch [12.5 m/m] MS
keystock that was on hand. The longer clamp functions similarly to a
parallel jaw clamp. The front screws, in any one of three positions,
retains the clamp and a 1/4 X 20 setscrew provides the clamping force. The
block has been left soft and some burrs have been raised by the set screw in the
long clamp, however these are well clear of the tool alignment surface and do
not affect accuracy.
A top view of a tool being aligned for end grinding using the cross clamp
Another view showing the front bevel
A top view showing a tool being aligned for side grinding using the long
clamp. Because the block was squared on all sides the protractor can be
used on any of the top edges to align the tool.
With these jigs even our beginning students can easily grind threading tools to "line-out" gage fits even with acme tools. One of the first student projects is to grind a standard set of lathe tools including a national v thread tool. We use 1/4 X 1/4 M2 as standard, however the jigs will accept up to 1/2 inch with no problems, although these take considerably longer to grind. One student has taken several pieces of 1/2 keystock and milled a 1/4 X 1/4 slot to adapt his QC 1/2 X 1/2 tool holders to easily accept 1/4 X 1/4 HSS bits.
Top side rake is ground by rotating the tool on the block. We do not normally grind top back rake but use back-rake holders [see picture on other page]. Where back rake is required, for example in boring bar tools, we generally use a die grinder and grind in "hook."
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last updated by GmcD on 27-May-14 14:59
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