Profile Grinder Overview

One machine that is integral to a molding company’s operations is a profile grinder. Profile grinders allow us to make new molding knives as well as to sharpen dull or damaged molding knives.  This is a precision grinding machine that utilizes a grinding wheel, a stylus, a template, and a moving carriage to grind shapes into molding knives. The carriage holds the cutterhead (with knives) and the template. The stylus is aligned to the grinding wheel, and the stylus rides on the template and allows you to move the carriage back and forth and in an out to replicate the shape of the template onto the molding knives.

Topics Overview
1) Template
2) Cutting out the knife stock
3) Installing the knives in the head
4) Setting up the grinder
5) Coolant
6) Grinding
7) Cleaning up the head

All molding knives made with profile grinders start off as templates.  Templates are the shape of the knives and are made out of plexiglass or thin steel.  They are shaped to the exact profile of the molding knives to be made.  For us, we make our templates out of acrylic (plexiglass).  We’ll first draw the knives in Vectric software and then use a cnc with a 2mm end mill to cut the templates out.  After the template is cut out, I’ll generally refine the edge slightly with a hard sanding block to make sure there’s no chatter that telegraphs into the knives.

After the template is made, we’ll proceed onto the knife stock. We carry varying width knife stock in various grades of tool steel. We’ll use softer, less expensive tool steel for shorter runs. For longer runs, we’ll use more expensive harder tool steel. When choosing the width of knife to make for our blanks, one critical dimension is the depth of cut (DOC). From the furthest out point on the knife to the furthest point in, that dimension is the DOC. The bigger the depth of cut, the wider the knife stock to use. 

Figuring out how wide knife stock
Our cutterheads have a 1″ deep pocket on them. That means 1″ of the knife stock is inside of the cutter head. When using the knife grinder, we need about 1/4″ sticking out beyond the edge of the head for us to clear the rest. So for simple math, we can add our DOC to our 1 1/4″ minimum width to get the knife stock thickness. So for example, say we have a DOC of 1/2″. We would add 1 1/4″ and 1/2″ together and see that we need 1 3/4″ knife stock.

Figuring out the length
Next, we look at the width. For simple math, we add 1/4″ to the width of the profile. Say we’re making knives for a 3 1/4″ baseboard. We’ll add 1/4″ to the width and come up with 3 1/2″ knives. We’ll take our knife stock and mark 3.5″ on it and cut it off with a metal chop saw. I’ve found short quick plunge cuts with a pause between plunges the quickest way with the least amount of overheating.  

Truing up the knives on the tool grinder
Once the pair of knives is cut out, we’ll true up the edges on a tool grinder. We index our knives to the edge of the cutterhead, so we want to make sure that side is square and true, or our index can be off.  We’ll use the tool grinder to make that edge square.  We’ll generally lightly true up the opposite edge, but this side is much less critical than the index side.  Lastly, we want to make sure they’re both the same length.  If one is longer, it can cause the moulder head to be out of balance which can cause all sorts of problems from bad cuts to bad bearings. So we’ll use the tool grinder to adjust the length so they’re both the same.

The next step after cutting the knives is to mount them into a cutterhead. For most of what we do at Loxaco, we use 12 degree heads, so you’ll want to use a 12 degree head that’s around the same length as the knives. We have different length heads from 2″ up to 9″. When installing the knives in the head, there’s a couple of factors that we need to consider. First is which side to index. We always index to the inside edge, however, a top head and a bottom head may use the same head but they spin in opposite directions and index on opposite edges. So to properly index, you’ll need to know which head the knives are for and then index to the inside for that head.  

The knife stock that we use is called corrugated knife stock. It’s called that since it has a series of corrugations down the back. The moulder head has matching corrugations. These corrugations help lock the knives in place. One concern is making sure that each knife is projected out exactly the same amount as the other one. A simple way to check this is by counting the ribs projected above the head. If they’re the same, then you’re good to go. For a fresh set of knives, we should be fully seated in the pocket so setup is fairly straight forward. However, after a few re-grinds, the knives get shorter and thus rib counting becomes standard practice.  

With one knife in place, slide the gib in and lightly snug one gib screw. I’ll generally use a molding knife floating around the bench to hold flat on the index side of the head. I’ll then loosen the gib screw previously tightened enough that the knife and gib can slide back and forth. I’ll slide them down snug against the knife I’m using to index them. Once they’re both flush, I’ll start to tighten the gib screws. One thing to make sure of is that the gib or knives don’t walk as you tighten the screws. Sometimes they try and shift slightly. Once you’ve confirmed a good index, go ahead and snug the rest of the knives. Once both knives are set in the pocket and all screws are snugged, we’ll come back with a torque wrench and snug each screw up to 22 foot pounds.  

With the head ready, it’s time to set up the grinder. We’ll need to mount the head onto the shaft which then gets mounted onto the carrier assembly. We slide the head onto the shaft and then use a series of spacers to allow clamping pressure under the nut. The nut doesn’t have to be super tight, there’s no spinning involved, it just needs to be snug enough that the head doesn’t spin freely as we rotate the shaft. Next, we’ll add on the bearing and then screw on the handle. Set the shaft assembly on carrier frame. The long handle goes to the right, the “wheel” style handle goes on the left. As you drop the shaft in place, make sure you slide it to the far left within the carrier to index it off of the axial stop. Once the shaft is set, put the clamps over the top and snug them down. Don’t go too tight, you still want to be able to rotate the shaft freely.  

With the shaft in place, we’ll need to mount the template. This needs to be a semi-precise setup with our machine. Basically, it needs to be the closest set of screw holes to properly aligned, and then there’s a little bit of adjust on the stylus. To do this, we’ll rock the carriage forward and set the knife on the tool rest. We’ll slide the head over until the index side of the head/knife is just touching the side of the wheel. We’ll then hold that in place while setting our template in rough positions looking at which holes line up best. The template has an index side also, and this is the side with the V-notch cut out. So we’ll align the index side of the template to the stylus, and then find the nearest screw holes that line up. We’ll note which screw holes they are, pull the carriage frame back, and then install the two bolts. I’ll generally put in one bolt, lightly snug it up, then do the other one the same. Next I’ll make sure the back edge is parallel and tight (make sure there’s no debris inside). The acrylic template holes are cut in an oval, which give us a little room to slide the template back and forth. I’ll generally slide the template in the most opportune direction and then snug down the bolts. These need to be snug enough that the template won’t shift during lots of back and forths.  

We now have a rough set up. Our next step is to align the stylus to the index side of our template. However, before we do that, let’s double check that the stylus and wheel are correct for our application. For most of what we do, we use a 5mm wheel and a 5mm stylus. For deep roughing cuts, I’ll generally keep it on the rounded edge (even if there are square corners). For something small with light cuts and square inside corners, I may start with the square edge of the stylus and a square edge on the wheel. For super intricate regrinds, we have a 1/16″ wheel and stylus. So in most cases, we’ll use the 5mm rounded stylus. Just like before, we’ll rock the head forward, place a knife onto the tool rest, and slide it over until the index side is snug against the side of the wheel. Now, we’ll unlock the stylus and crank it over until it’s contacting the index side of our template. I’ll then spin the head back and forth while tweaking the stylus over until the knives are just barely kissing the side of the wheel. Snug up the stylus lock.  

Last, we’ll need to move the stylus in or out to align the stylus/template to the grinding wheel/knives. For this, we’ll first look at the wheel. We want the wheel about 1/16″ off of the tool rest. We may have to unlock the wheel lock and rotate the wheel in (as the grinding wheel wears, it gets farther from the tool rest, so we will regularly have to adjust it in). Bring the wheel into position and then tighten the lock. Now we can bring the stylus in or out. We make our templates and our knives slightly oversized, with the excess being opposite the index side. We’ll use this excess in this part of the setup. Move the knife to the side opposite the index edge towards the end of the knife. This should align with a flat part of the template. Bring the stylus in or out until the stylus is contacting the template and the knife is contacting the wheel. If you bring the stylus too far out, the wheel won’t be contacting the knife. If you have the stylus too far in, the wheel will be contacting the knife but the stylus won’t be touching the template. So you’re looking for all 4 items to be in contact at the same time.

When we grind on tool steel, it generates heat.  Too much heat can cause changes to the steel reducing its hardness, which means it will get dull faster.  To help keep the temperature down while we grind, we use a liquid coolant sprayed at the top and bottom of the knife during grinding operations.  This is a mixture of water and grinding coolant.  Occasionally the lines get debris in them and the flow will be reduced, so when this happens, we’ll disconnect the lines and blow backwards through them with compressed air.  Once they’ve both got good flow, I’ll generally try and align the top spray to cancel out the bottom spray when there’s no knife present.  Otherwise it makes a mess and sprays all over.  

Your setup is complete and coolant is flowing. Time to start grinding. I find I prefer to work left to right, so I’ll pick a knife and start making depth cuts on the left side. Until we get to the final grind, we don’t grind sideways. We’ll make a series of overlapping depth cuts all the way across the profile removing 95% of the excess, leaving a few peaks in between depth cuts. After the depth cuts are complete, I’ll go back through and knock off the peaks that remain between the depth cuts. Remember how we discussed the wheel getting smaller as you grind? Time to find out how much. Go back to the original start point and notice that the wheel will no longer be touching the knife. Move the stylus back until the grinding wheel just barely contacts the knife. I’ll then start at the right side and work left, making a series of overlapping mini depth cuts. At this point, this knife is roughed out. Rotate the head and repeat this process for the next knife.

Finish grinding
We use the same wheel for our finish grind. We just make a series of very light passes across the surface that remove any deep gouges left from our rough grind. Also, we’ll need to even out our knives. With wheel wear, our knives may be slightly different lengths. So before we start our finish grinds, I’ll go back to the first knife and reposition the wheel/stylus to barely touching the knife. I’ll then rotate to the second knife and start my finish grinds on the second knife. If I find there is more than .030″ being removed, I’ll do mini depth cuts again all the way across. I’ll then start working the wheel side to side to even out and peaks or valleys. Grinding wheels are weak with side pressure, so we want to be very gentle so we don’t make the wheel explode. I’ll work back and forth across each knife, bring the stylus in a tiny bit, then repeat the process. I’m looking for an even shine across the edge with no distortions, valleys, or nicks. 

Once both knives are good, it’s time to square up the inside corners if there are any. For this, we rotate our stylus 180 degrees to the square edge. I like to use the index side of the template to make sure the stylus is properly aligned before tightening it down. We’ll then need to put a square edge on the wheel. On our Wadkin grinder, we can unlock the tool rest and rotate it 180 degrees down, which brings the diamond point into position. We may need to extend the diamond point out to touch the edge of the wheel. Once it does, we’ll rotate it back and forth slowly which removes material from the wheel creating a square edge. We’ll go back and forth, extend the diamond out a touch, then go back and forth again until the edge of the wheel is square. Swing the tool rest back into position and then lock it down. Since we just removed material from the wheel, we’ll need to reposition our wheel/knife and stylus/template in/out adjust until we’re just barely sparking off on the knife. Now grind the inside corners gently on both knives. I’ll generally do a few super light passes with the square edge across everything to make sure it blends nicely.  

One last step that I like before calling a set of knives complete is to hone them. We use ceramic rods in various shapes and work them back and forth across the grind and then flat across the back.  This does a mini polish and helps to break off the wire burr left behind by the grinding.  On the ground edge, it can even out minor imperfections.  

Once we’re done grinding, we’ll need to remove the cutterhead from the shaft. After it’s removed, I like to use compressed air to blow off all of the coolant and to dry it off. Be careful, the knives are sharp.  At this point your cutterhead is ready to be installed on the moulder.

We’ve put some information together about profile grinder troubleshooting.  Click here to go to the troubleshooting page.