Hi fellow home shop machinists, Can somebody please refer me to a link on how to accurately grind 60 degree lathe bits. Consistancy is what I'm looking for. I need to grind several single cut threading bits from blanks.
I currently hand grind. Now that I have a hand held microscope, I now know that my points are not true 60 degrees when matched to a fish tail guage.
I'm not threading for aerospace, but would like to acheive a perfect
Extremely handy for use in grinding and setting screw cutting tools. Has a table of double depths of threads for determining size of tap drills. Made of spring-tempered steel with satin chrome finish. Gaging surfaces ground. Number of threads per inch scale. American Unified National Standard, 60°
Many (most?) machinists' use the centre gauge and hand-grind the tool to this gauge. Tedious. But practice makes perfect, or at least better.
Next step up would be a bench grinder with adjustable table and fences. Some are advertised in Home Shop Machinist. Or roll your own as they say. An acquaintance of mine built such a device using a pool pump motor and a 5" dia. cup wheel.
The cat's meow is a tool and cutter grinder, commercial or home-made. With this you can make perfectly good AND bad tools under perfect control:-))
I have a Quorn T&C grinder that I finished from someone's start. It is overkill for lathe & shaper tools, but most versatile for all sorts of cutter sharpening.
An alternative would be to look for a single lip cutter grinder such as made by DECKEL. These are designed to grind from solid single lip engraving cutters, and to sharpen them. It is quite easy to adapt one of these to grind lathe tool bits, not only for threading, but also parting blades with proper relief, grooving tools for Circlips and Orings, etc. and accurate angles on turning tools for difficult-to- machine materials.
These grinders are sometimes advertised on Ebay, Kijiji, or Craig's List. Prices vary up to $2000 or so, but in this economic climate I'd offer $500 for one in good shape and complete.
Part of the problem is that the angle you're grinding is a compound miter. You don't grind a 60 degree angle to get a 60 degree cutter when a clearance angle is involved. Perhaps someone has algebraic formulas to compute this, but I am lazy and just model the tool shape and let the CAD software solve numerically:
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In this example, for a minus 10 degree clearance angle, to get a 60 degree cutter, you actually grind a 62.95 degree angle.
Indeed, to machine grind a threading tool you have to know the compound angles to use on the various angle scales of the grinder:-))
To hand-grind proceed as follows:
1) on a blank toolbit grind the front relief angle; make this 12 degrees or so from the vertical if the tool holder holds the tool horizontally. Use an adjustable protractor for this.
2) blue this ground surface (the relief angle) and scribe a vertical line in the middle. Mark the 60 degree angle on top.
3) now grind the LHS surface (flank) of the threading tip so that the ground surface front edge forms a line parallel to the line scribed on the front relief. Carry on grinding until the LHS approaches the scribed line on the front. During this operation use a protractor to ensure that this second surface forms a 30 degree angle with the LHS vertical surface (long edge) of the tool bit.
4) now grind the RHS flank the same way, using the centre gauge to check the 60 degree thread angle, while ensuring that the front edge meets the edge of the LHS flank. Keep the centre gauge parallel to the top of the tool bit during this check.
5) use a hand stone to put a small flat at the tip of the tool. For exact work the width of this flat should be 1/8 of the thread pitch.
6) for cutting brass, cast iron, leaded cold rolled, C1144 aka stressproof, no rake angle is required. For machine and tool steel an
8 degree or so rake angle helps a lot. This angle should be ground so that it runs back at right angles from the LHS flank without affecting the elevation or horizontality of this LHS top edge. (It's easier to show than explain).
Doing it this way ensures a side relief angle without having to measure it. This tool is suitable for cutting a single start right hand thread.
Richard, I would like to thank you for the tool grinding pdf. I mistakenly gave thanks to Wolfgang for this file. The geomotry is very helfful. Thank you.
Not sure what you are asking but the usual procedure for cutting threads is to grind a tool bit to a 60 deg. point with normal side and end relief. If you want side rake then grind it as once the sides are ground changing the top surface doesn't change the side angles.
The tool is then set with the center line of the tool at right angles to the surface being threaded. The compound is set to 30 deg., or very slightly less.
Using the cross feed the tool is advanced until the tip of the bit just touches the work. Either a threading stop is then set or, more usually, the cross feed dial is set to zero.
Feeding the tool into the work is then done by use of the compound while the cross feed is used only to retract the tool between cuts. The zero setting on the cross feed dial, or the threading stop, makes it easy to return to your base setting.
As tool feed is done at 30 deg. to the work surface it means that the threading tool cuts only on the forward edge resulting in a smoother cut.
Setting the compound to slightly less then 30 deg. means that as the tool is advanced by the compound the path is not exactly parallel to the rear edge of the tool resulting in a very light, scraping, cut being taken by the rear edge of the tool bit. Some argue that it results in a smoother thread. Cheers,
Yes you do. The tool is set horizontal, i.e., parallel with a line drawn across the ways. No back rake is normally used. Top rake is usually not used.
Having said that if very large threads are cut one might grind a roughing tool with top relief and rough the thread nearly to depth and then set a finish tool to cut the last few 1/1000's but it is not usually done.
Sorry, that's not true. As you move the edge away from center, it most certainly does change the angle.
Worst thing is it also eliminates the tool from cutting on the back side, which is a vital function performed by threading tools. That's the reason it is fed at less than half the thread angle, and not at half angle as you suggested.
Threading tools should not be ground with rake unless calculations are introduced to compensate for the change in angle. It is minor, but it does exist. Remember, too, that rake eliminates the ability of the tool to take a full cut, which is very desirable when cleaning up a thread as it nears size.
Compound miters with two degrees of freedom are tricky. Hard to argue with CAD models that give correct results. The -10 degrees must be measured normal to the resulting cutting edge axis. Maybe your assumptions are different.
Compound angles are calculated thus; Primary angle = arctan(tan A x cos B) Clearance angle = arctan(sin A x sin B)
The formula will give you the "RESULT" of a secondary angle being introduced. So...
If you set your vice to 30 (60 degrees included) primary angle "A" and then adjust for 10 degrees clearance "B" the result would be (arctan(tan(30) x cos(10)) = 29.621 degrees primary angle or about .756 (.378 x 2) degrees short of the 60 you need.
If your "laying out" a tool to grind offhand, then just lay out the 60 and set the table for clearance and grind to the layout line.
I looked at your PDF forever trying to figure out why you show 20 degrees total clearance against a 60 angle needing 3 more degrees and I come up with about half that....
Try to section "parallel" to the tool top and see if the result changes. This is just confusing the heck out of me....
No use to the OP, but the way I do it is to set the bit to the correct half angle from horizontal in the vice (30deg in this instance). then tilt the vice to give the clearance angle. Then grind on the surface grinder. Since the vice is being rotated about the same axis as the 30deg angle, the two settings are completely independent. As I said, no help with a bench grinder, but trivial on a surface grinder or T&C grinder.
Addendum:- Make a side table with guide rail for the bench grinder and the use a cup wheel. Then the correct angles can be generated with ease.
I guess that depends on what coordinate system "set the bit" and "tilt the vise" means. That is, what are the axes of tilt and rotation in relation to the planes of the table and the grinding surface, and whether these angles are independent vs compounding. On a Baldor or Chinese tool grinder:
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The table tilt axis is parallel to the plane of the grinding surface, so the (half) angle on the sliding miter is not the (half) angle ground on the bit, unless the table tilt is at 0 degrees. The point of my CAD computation was to solve for the miter angle (62.95 degrees) that yields 60 degrees on the bit when compounded by a -10 degree tilt on the table.
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