Onsrud Bits, your thoughts

[wcsg]

I know quite a few people here use Onsrud bits and I'd like to get your opinion to make sure I'm using them right.

I usually use Belin bits but I thought I'd try Onsrud since I know have a closer local supplier.

I'm using a 52-285 series double flute upspiral bit at 18,000 RPM's and 3 IPS. I'm cutting some 3/4" MDF for my kids school projects, just lil pumpkins. Man are these bits loud and ear piercing. This is my first use of DF bits. I broke 2 already. I'm back to using my belin single flute bits and the dust collector is louder than the material being cut, and that's what I'm use to. The cutting harmonics even seem more stable with my Belin single flute spiral bit. Is the Onsrud bit just loud because of the geometry or is that a double flute thing?

I'm going to try the super O bit next by Onsrud but I'm a little disappointed if these bits break that easy, or it could be me and applying the wrong feed speeds etc

[richards]

Erik,
A chipboad of 0.015 (which is what I use for MDF) calls for the RPM to be 6,000 when your feed speed is 3-ips. A 1-flute cutter would need the RPM to be 12,000.

At 18,000 RPM with a 2-flute cutter, you should be feeding at 9.0-ips. With a 1-flute cutter, you could feed as slow as 4.5-ips

[wcsg]

WOW! Am I not doing this right from their chart.

On their chart it shows a 52-200 series to be at a .005-.007 chipload and recommended speeds to be at 18,000 RPM's (Which sounds a little high to me for a DF bit) so per thier chart

18,000 x 2(Z or 2 Flute) x .006 = 216 IPM or 3.6 IPS

What bit are you using, 6000 seems a little to slow but then I'm not use to cutting to much wood. The slowest I've done to date is PVC at 8K RPM's @ 1.3 IPS with a sigle flute spiral

[richards]

Erik,
The chart I have been using lists higher chiploads than the current one listed on the Onsrud site.

On the chart that I use, MDF has a minimum chip load of 0.008 and an average chip load of 0.015. Particle board has a minimum of 0.010 and an average of 0.025. Solid wood (dense), has a minimum chip load of 0.006 and an average of 0.015. Plastics and aluminum are both in the 0.005 range.

The problem that I've found when using a very small chip load is heat. The chips are too small to carry off the heat and the cutter burns up in a hurry. Usually, except on CMT cutters (which always seem to screech on my machine), screeching means that I'm being too conservative and need to slow down the spindle or speed up the feed rate.

You are right; however, 6,000 is slow for my 3hp spindle. There is little torque at that speed. I've got my VFD/spindle set up for constant torque between 12,000 and 18,000 RPM, so I try to keep the spindle speed at least 12,000 RPM. To do that, I have to keep the feed speed higher or use a 1-flute cutter.

I've used several Onsrud cutters and like them very much. However, AA Carbide in Salt Lake City, where I buy most of my cutters, sells an excellent 3/8-inch up or down spiral cutter for less than $20.00 that gives me excellent cuts and long life - and they are quiet. Typically, I run them at about 12,500 RPM and 5-8 inch per second, depending on the brand of MDF that I'm cutting.

One thing to keep in mind. If you have a lot of arcs or frequent axis changes, you will want to keep the spindle speed as low as practical and the feed speed moderate because the Shopbot is going to be spending a lot of its time in the 'ramp' zone - meaning that it will automatically be running at very modest speeds. On the other hand, if you're cutting mostly panels with long straight cuts, higher spindle/feed speeds are more practical.

[wcsg]

Mike I appreciate your help on this btw.

I'm doing 8" pumpkins and ghosts for my kids school halloween projects. If you can imagine a simple profile then inside profile for the eyes and mouths. I don't think going 5-8"IPS is suitable.

I might be a wuss but I've never cut past 3-3.5"IPS on my shopbot before. Like you said for panels it would appear to be more suited.

Also I noticed I forgot to mention I was using a 1/4" bit with a 1/4" CED, I know the 3/8" bit would get a bigger chip per the chart (Onsrud) .008-.010.....I think I'm confusing myself.

For small minor detailed projects (objects, arcs, signs)I'm guessing stick with single flute. Furniture and panels use double fluted spiral bits then. I can't see using a DF bit @ 12K RPM's and traveling at 6IPS to cut a few pumpkins.

Let me see If I have this right then.

I look down at a onsrud chart and for a 52-285 1/4" CED series it says to use a chipload of .006 - .008 for MDF

using 12RPM's on my 3hp spindle it's

12k x 2 x .008 = 3.2 IPS

So now your saying I should use .015 for a chipload? So I can get that with the same bit or do I need a bigger one? CAn I just simple plug it in the formula and get

12k x 2 x .015 = 6 IPS

I need to check out that site where you get yours from salt lake city.

[richards]

Erik,
Your formula with a chipload of 0.15 is correct.

(Are you aware that there is an excellent Chip Load Calculator under the Tools Tab in the SB3 program? If you click on Chip Load Help, you'll see the numbers that are similar to those that I use.)

Actually, I use the same chip load formula regardless of the diameter of the cutter, but I normally use 3/8-inch cutters as my standard size. The main advantage of the 3/8-inch cutter is that it has MUCH less flex than the 1/4-inch cutter when I'm making deep cuts in 3/4-inch material.

I don't think that AA Carbide has a web site, but I'll list their address and phone number:

AA CARBIDE INC 1275 South 2100 West Salt Lake City UT 84119
801-486-4881
800-870-4881.

The part number for the upcut spiral is C0010109. The part number for the downcut spiral is C0010110.

[johnm]

Folks -

Got a problem - not directly SB related, but darn near, and hoping someone here will have some good ideas...

We made some 5' long notched sticks on the 'bot, and after the CNC work, they needed roundovers on the sides and ends. I set up the router table and off we went.... doing the sides was fine....

The problem, is that the ends are cut in a semi circle and need to be rounded over top and bottom. I am trying to do this on the router table but am getting very uneven cuts and burning. What would you all suggest I do to create roundovers on a radius? I guess I could do this on the 'bot, but "Z" issues seem SO troublesome, especially with a profile cut, that frankly, I'm snakebit... Too many times, I've gotten the "oh, yeah, that happens sometimes" in relation to a SB Z problem, that I am very sceptical of doing sensitive Z work with the machine....

I have attached two pics, one showing the burning and unevenness of doing it by hand on the router table, and a second shot showing the radius I am trying to round over.

Any ideas? I'd be happy to do this on the router table or 'bot, but don't have a lot of blanks to spare for testers or "oh, yeah, that happens sometimes" kind of mistakes. These issues now have a nickname in our shop..... A "shop botch"....

Thanks in advance!

John & Rose
5100
5101

[jeff_guinn]

John,
The wood appears to be oak which of course is quite hard. Chances are you are moving much slower on the radius round-overs which is creating a lot of heat. Make multiple,more shallow cuts & if your table router has speed control,slow it down.It appears that heat & kickback are the culperts.
Jeff

[johnm]

Folks -

I've mis-posted the above.... sorry...

John

[jeff_guinn]

John,
I forgot to mention that you can install a pin or bolt in your router table top within several inches of the bit & use this as a fulcrum to control the end radius cuts.
Jeff

[srwtlc]

John,

Another problem that can happen with your situation is if you're using a roundover bit (with a guide bearing) that has a radius that is the same as or more than the thickness of your part. After you roundover one side (which works fine), and flip the part over to do the other side, the bearing no longer has the proper edge to ride on and will dive deeper, burn, and make a stepped edge. Using a smaller radius roundover will help to keep some of the original edge for the bearing to ride on.

If that's not the problem, be sure you have a sharp cutter, and slow down the router some. I typcally make a cut like that by first back cutting the edges to the break of the corner and then conventional cutting the ends with a final pass all the way around (on the router table). Get some good grippy push blocks so you have a firm hold on the part and are able to push it along without hesitation or losing contact with the part as you go around the corners. It takes some practice to back cut, but it will save a lot of lost edges. The push blocks are the secret to feeding the part with no vibration and with firm authority. ;-)