Machining a rear mounted cut-off tool [parting -off tool, if you are in the U.K.] holder for the Emco Compact 10 lathes.

Based on our experience, this should be one of the first tools the new lathe owner should make.

Second term student Troy Stephens uses a dial height gage and surface plate to layout the tool holder blank.  The partially completed cut at the other end of the blank occurred because we forgot the holder was not symmetric, and had to be mounted with a specific side to the chuck.  Fortunately, all that was lost was a little time.
Third term machining student Chris VanHouten using a magnetic sine bar and dial test indicator to set the tool holder blank to be machined with exactly 4 degrees back rake.  This appears backwards because the tool cut-off tool is "upside down" in a rear mounted holder.  Several people have pointed out that Chris is not wearing safety glasses. No machining is occurring.  Chris is "tapping" the vise with a small lead block (not in the picture) which is attached to the mill table with one bolt so that the dial test indicator doesn't move when he cranks the table in-and-out.  The tip of the indicator is against the magnetic sine bar. The 2-1/2 inch magnetic sine bars are somewhat expensive [c.180$US] but are worth the money if you do this type of set-up.
The first cut being made for the "T" of the "T" type cutoff-tool (on the correct end).  Note the use of two magnetic base dial indicators to insure maximum accuracy.  This eliminates [or at least allows compensation for] shifting when the column and table locks are tightened.  Vise appears crooked to table because it is set for 4 degree angle.  The extra hold-down was removed in this shot for a better view.  The next picture shows the over the top clamp fabricated from a piece of the 1 inch square CRS we had left over from the parallel jaw clamps.  The hold-down bolts are 3/8 carriage bolts with the heads modified with a belt sander to fit the table t-slots.  Not a good idea with higher power machines but very satisfactory for this use.  Price is right too.
Troy Stevens measures the depth of the cut for the blade under the "T" of the cut-off tool.  Troy just purchased the Mitutoyo depth-micrometer set and wanted to try it out.  The dept of the cut as measured by the dial indicator was within 0.0002 of the digital depth micrometer reading.  
Keith Morris machines the slot for the blade of the cut-off tool to the correct depth and width.  Keith is a practicing machinist, and is honing his skills in precision lay-out and tool/fixture design/production at FPC.  Keith is a real asset to the class and a resource for the other students.

This holder as machined will take any commercial 11/16 tall [17.46 m/m] "T" style cut-off tools such as P-3-N, P-3,  P-4, and P-5-S, although we will use the P-3 with a 1/8 inch [ 3.18 m/m] "T".    The blade shown is a T-15 tool from Somma [p/n BL844V].  This is  much more wear resistant tool than  we require, but part of some donated tooling.  Also included was Somma carbide blade which as successfully cut hardened steel.   The "hinge" hole shown in the other end will be drilled and a slot cut to allow the holder to "pinch" the tool when bolted down to the cross-slide.

Sorry about the reduced quality pictures that follow -- 
Following are taken using my personal "budget" camera.  
Other pictures taken with school's top of the line Sony.  

Bottom of holder showing modified carriage bolts. Extra slot for tool was a mistake that we caught before the we got too far along.  Unsightly but does not affect the operation.  Lessons learned: measure three times, ,verify orientation twice, cut once. 
Another view of the modified carriage bolts. [3/8 inch for EMCO slots]  Belt sander modification is adequate. (Note that you have to take "a little off the top.")  Most likely not the thing to use on your 20 inch swing CNC lathe, but for typical home shop lathe these are more than adequate and the price is right.  

Showing the tool slot and the relief? slot to allow clamping the tool.  This slot done the old fashioned way with a hacksaw.  It is centered on the "spring" hole, and thus appears to high on one end and too low on the other because of the 4 degree rake built into the (upside down) tool. Note clearance slot in bottom of tool slot for T style cut off blades.  Be sure to machine deep enough for the widest blade you will ever use.
Front view [head stock side] of the completed cut-off tool holder showing 4 degree positive rake [Remember tool is upside down because it is on the back side of the part.]  Clamp slot is actually straight across holder.

Another view of the slot for the cut-off tool.  Note the slightly deeper slot at the bottom of the slot to clear the "T" of the T-style cut-off tools. This makes sure the side of the blade is flat against the tool holder and perpendicular to the work.

Another view of the tool slot from part side of the holder showing the undercut for the T. 
A top view of the holder. One carriage bolt has been shortened.  If you think loosening due to vibration might be a problem you can use nyloc type nuts.  We did not find this to be a problem, but it could be in higher volume use.  Although not shown, bolt holes are spaced so that close [SAE] washers will just fit.

Showing the holder with a cut-off blade installed.  View from the headstock side, part to the right..  Blade is extended about the design distance.  One drawback with this design is that you can't adjust the blade protrusion.  The tip will go above/below center height as you deviate from the design extension.
This view is from the back side of the holder.  Swirl marks around "spring" hole show where chips from drilling wore off the dykem. 

FWIW -- the red background in these pictures is my Waterloo box [aka "small goody locker"]

The only modification I would suggest is to include a cap screw [1/4 inch adequate] to hold the tool in position after the height of the tool is adjusted by sliding it in and out of the holder, using a new sharp dead center in the tail stock as the gage point.  Note that only a few thousandths of height (vertical) adjustment is possible with a 4 degree slot (before blade is over/under extended) so you need to be as close as possible when you lay-out and machine the holder.  

 

click here to see more rear cut off tool holders. ==> SHOW ME MORE <==
(graphics intensive > 2 minutes @ 28.8kB)

Feel free to email me with any questions, comments, suggestions, etc.

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last updated by GmcD on 27-May-14 14:59
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