Drill grinding problem

My question is a bit off topic here but I have found that the skill of those in this topic far exceeds the others!!!
I have a Darex M5 drill grinder, actually 3 of them. Currently I'm using 2 of them to do 3 operations on the drill. Point angle, Split point and Negative rake grind. The M5 will do 2 different grinds so I do a third in another grinder so I don't have to re-chuck the drill.
I'm currently grinding new 1/4" Michigan taper length drills with 118° point angle into 135° Split Point with negative rake to drill acrylic. I test the drills to insure they drill onsize by drilling acrylic with WD-40 lubricant (as being used by my customer).
If I grind the 135° and split point I get .250 hole size but if I add Neg. rake hole size opens to .255 to .260. I tried the grind on a new jobber length Precision Twist Drill and complete grind produced .250 hole. I had a few packages of 12 drills, one package produced .250 to .252 holes and within my limit.However other packages do not test within limits of +.003
Any suggestions? Any thought as to why? I also did some 118° by simply split pointing existing 118° drill with same results .250 hole diameter but when adding the neg rake hole size increased. The 135° + SP grind was done on same grinder, I reversed the operation and did 135° and Neg. rake on the same grinder Slit the point on the other with same results.

Newbie question: what's negative rake on a drill?

Newbie speculation: If it's negative rake right at the cutting edge, could it be plowing the hole open rather than shearing it?

When cutting vinyl siding with a power miter saw, the practice is to install the blade backwards to avoid shattering the material on the cut line. This essentially results in extreme negative rake. Is it the same idea when drilling through acrylic?

I don't know much about drilling acrylic but on other softer plastics that my customers drill if the drills are not sharp enough the material will tend to push out the plastic and then close in smaller than the drill when it is withdrawn but every case is different .
I know what a Darex grinder looks like but use a Christen four facet grinder for some drills and I have a Winslow H.C. made for grinding large quantities of drills that grind a number of types of points .
Winslow calls their version of a 4 facet point a Prycon point that is what I use most of the time .
Products | Winslow Engineering Inc.
The cams for these point are not standard with the Winslow but are an optional extra.
Are you using the negative rake to prevent the drill from grabbing when it beaks through the far side of the acrylic ?
One old Dormer booklet suggests the increased point angle will help prevent breakout and grabbing on thin material if the full diameter of the drill is cutting before the point breaks through,
My way of thinking would be that the negative rake is impeding the cutting action on the drill and causing it to wander oversize but that is just a guess.
An other thing I have read is that when drills are made their flute shape will vary with the point angle so that the Ideal flute shape for a 135 degree point is not the same as one for a 118 degree point so while you can grind a different angle point on a drill that is no guaranty that it will drill the same as drill with flutes designed for that point angle.
I can't remember where I read this but I think if you study the drills you may see the difference in flute shape and also perhaps the helix angle or spiral of the drill.
It is hard to offer concrete suggestions with out seeing the drills or seeing them working on the intended operation.
I have found that there can be so many variables to contend with in the drills , drilling machines , setups and operators if the drills are hand fed that you often have to go against the convention of what is supposed to work in theory and make a few trials and go with what works for you.
Regards,
Jim

Confused a tad as a 118 and a 135 drill ideally would have differing flute shapes.
Are you using a camera or other inspection device to look straight onto the tip after your grind and the lip geometry?
One can get into belly, cave or run/twist at the outside.
Very,very low dollar USB cameras can do this tip check.
Bob

I'm not using a camera, I do have optical comparator and machinist microscope but didn't use them but did use magnifying glass and didn't see anything that I would think caused problems.
I have in the past when needing to drill plexiglass put a flat surface on the edge of the drill. Also when drilling bronze. The flat will counter the very positive rake of the drill's helix. Yes I assume my customer wants it to prevent cracking when the drill breaks thru. My customer sells the drills. I do large sizes with point angle 90° and the flat on the edge. I don't know why in the past I was also grinding the 1/4" the same 90° but now they requested 118° split pointed and 0° rake angle. Also the same grind with 135° point angle. and yes a few 90°. I have no problem grinding 118° drills to 135° SP drills. Prior to grinding 0° they drill on size. The problem is when grinding the 0°rake. I had 4 envelopes of 12 drills each, one envelope ground with no problem the drills in the other 3 are giving me problems. I have been trying setting changes, grind 1 maybe 2 OK then oversize again. I'm using Diamond Wheels, they do a great job. The 0° rake is small, about 1/64" wide. I'm not sure why they want the drills ground as they request, I do notice that the surface of the drilled hole when using WD-40 are surprisingly very smooth. Of the 48 drills I have 24 complete.

Is the zero rake grind shifting the center point of the drill? Try touching up the cutting edges AFTER the zero rake grind. Alternatively try grinding the zero rake on the outside 3/4 of the cutting edge. That way it will not disturb the center point from the point splitting.

I think Illinoyance has a good point. The most common cause of oversize holes is the two flutes not being equivalent: either the point shifted one side or the actual angles being slightly off and one flute grabbing more than the other.

Perhaps the best would be to take a picture under the microscope, rotate the drill exactly 180° and take a second picture. Then overlap the two pictures (it works best if you tint one red and the other green, leaving them semi-transparent).

Paolo

I'm using diamond wheels with sharp corners. When split the point there is a step. The back side of the flute is split along the point on both sides, when grind 0°rake the cutting edge is flattened about 1/64" wide and does not continue past the split point. Before grinding the 0° the drill looks exactly like split point drill I have that were new ground by PTD, GTD or any other good drill manufacturing company. When testing prior to 0° the hole is on size. When I grind the 0° it will then produce oversize holes. I did try grinding the 0°tapered so that it gets smaller and disappears near the point.Got 3 on size holes after 10 drills ground. I move settings and get a few good holes but after 1 or 2 drills the hole gets larger. I look at the drills thru magnifier and those the drill over size look exactly like those that drill on size. So far out of 48 drills I have 29 that drill on size. As I make changes I get a few to grind on size then it changes. If I for example test the hole +.006, grind again +.006 and again +.006. Make no changes try another drill and get +.001, Try another drill +.010!! I'm using a Toolmaster Vert. Mill in excellent condition with Albrecht chuck also in excellent almost new condition to test the drills. Tomorrow I see if I can put a drill in optical comparator.

incorporate drill bushing close to workpiece in process

JHOLLAND1 said:

incorporate drill bushing close to workpiece in process

Click to expand...

I'm not using the drills, my customers sells them to hobbyist that are probably using drill press probably made in China. My customer wants drills that tested to drill on size so when someone complains the drill made a big hole it was not the drill! I understand that part of the problem is the long taper length drill that can flex easy.

A drill wants to orbit. Suppose the two flutes are not cutting evenly. The one taking the larger cut is in effect making a climb cut which pulls it into the work piece.
Use a standard jobbers length drill about 3/32" to drill into clear plexiglas and watch the drill go round in a circle, making a spiral walled hole. I can read the ridges with my Diatest indicator and even feel them as the split ball moves over them. The natural tendency is to orbit and if you give it half a chance, it will. I suspect that the drills giving you trouble have uneven flutes.

Bill

Yes Bill I do understand that off center drill will cut over size, I have intentionally ground them to make an oversize hole, actually I was surprised how much over size I could get a 2" drill to cut oversize!
However the sides of the holes are very smooth, no cork screwing. I ground and shipped quite a few of the 1/4" drills 90° with 0° rake that drilled .250" I also ground the 118° drill 135° split point and tested, hole size was .250" when I added 0° rake test hole size .255" or more! Also the sides are smooth no cork screwing. (drilling clear acrylic)

I'd think the oversize is the reaction of the long twist drill to.the pressure of the negative rake causing it to not accurately track the point.

I had 36 drills 15/64" same length by New York Twist Drill to grind 1/2 of them 118° SP 0° rake and the other 1/2 135° SP 0° rake and they came out good! Have to do the same with 1/4" X 48 by Michigan Twist Drill. 118° SP 0° rake are completed! Problem is with 135°. A while ago I had a problem with 5/8" Diameter 90° with 0° rake. I made a block that the drill chuck fit into (Drill chuck is the chuck provided by Darex to hold the drills) Put a dial indicator on the end of the cutting edge. Checked which lip was more than the other. Put the chuck back in the grinder pushed a little harder and removed material only from the + side. Did the same with those that would not grind on size. Worked for a few so out of 24 I have 8 remaining that will not drill on size.
Because I marked the chuck with magic marker where the drill margin was so as to be able to know which side was + I noticed drills that are now giving me a problem are not in the same location! To locate the drill in the chuck I insert the chuck and drill in the setting fixture, knife edge fingers touch the cutting edge and a pointer on the chuck align with the intended grind angle scale marking then tightened.. The drill should be in the same location every time! The drill protrudes out of the chuck about 3/8", in that distance the edge of the flute changes about 1/4". So the helix has to be different!

Might not be the heilx but where your finger touches the stock.
Flute shape.
Ideally lenghtwise your finger should be set just barley inside the finish grind length and ground to contact outside regardless of flute style.
You can blue a drill and then roll it across a piece of paper to measure helix as a simple check. (in the old days we used carbon paper).
If you change the OD of a drill in say a centerless or step type grind you change the helix angle.
Bob

Darex grinder has the knife edge fingers adjustable for size of the drill. A scale and pointer is used to set the drill size for the knife edge. Also setting for the chuck is aligned as per the intended grind angle. The odd part is the location in the chuck changes drill to drill. The location I see change is back 3/8" from the front cutting location. When I was doing larger drills and had problems I found the helix cut was different on either side, not the helix angle, the spiral was the same. There is something different with the drills. I had 4 envelopes of 12 drills each. One envelope ground with NO problem.