Doing some hand drilling and tapping into my machine.. advise fishing

[photomankc]

I'm going to need to drill a couple holes for my scale mounting bracket into my mills saddle. I'm not going to be able to do it on a machine so I will need to drill with a hand drill. I'm going to make a guide block to help drill it straight. What really makes me pucker up is tapping the holes. I have some hardware store plug taps but I'm plannig to get something better for this job and I need bottoming taps anyway. Last thing I need is a crap tap busted in a blind hole in my machine.

Should I get something like a spiral flute tap? It's a 1/4"-28 thread I'm putting into it. Any cool gizmos that can help me drill the hole nice and square in a tight space? I figured a simple wood block would be of great help but would make it hard to see if I was on the punch mark. Just fishing for tips before I start poking holes in my expensive machine.

[ctate2000]

I would make a drill fixture thae clamps onto the table and holds a drill bushing. You can make everything nice and square that way. The saddle will not be hard to drill and tap. I would suggest using a four flute modified bottoming tap and lots of oil. I would also suggest making the drill fixture in way that would allow you to change the bushing so that you can use it as a tap guide.

No prob, you will be fine.

[metalworkz]

Hello,
The main thing is to get the tap started perpendicular to the surface to begin with and then it will basically follow the path it is started on. That way the tapping work is more visible and you and clear chips easily without trying to hold a guide in position. I have what amounts to a cylindrical steel sleeve about 1.25" dia with a .75" dia. hole in the center that fits the knurled tap tightening sleeve and it works very well, but I find that once the tapping is started to a depth of say .125" or so the guide can usually be removed and that allows you to finish up and get a better look at the work. You will still need to use caution not to apply any pressure other than vertical to the surface but it is not that hard if you proceed carefully. I assume the machine is made of cast iron and it may be somewhat of a granular consistency so the chips may not be the same as steel etc. but the threads should be fine. Use a good tapping fluid for work with the type of metal and it will make the job a bit easier. I have also had fairly good results using crisco for turning, drilling and tapping steels and similar metals, but I prefer something thinner for tapping so the chips can be removed more easily. Here is a picture of my tap guide in use.



Regards,

[icecreamjones]

Another thought to consider would be a roll form tap if you could get one. I have found that they are way easier to get started square because they don't grab. (Of course if your not familiar roll forms, you would use a larger drill size.)

[universalfab]

I have a milwuakee magnetic based drill press that would probably work perfect for that. If there is room that is probably the best most accurate way of doing it. I use that thing on all kinds of installation jobs and it works great.

[metalworkz]

Hello,
Yes a roll tap will be a better alternative to a cut tap and is especially suited to bottom tapping as it does not cut chips that will fall to the bottom of the thole. I did not mention the roll tap alternative as they are not usually available at the common hardware store, but you should be able to get one from a tool supply vendor. Be sure to get the correct drill size for the tap as it takes a larger diameter hole and the tap basically rolls the thread form into the drilled hole. You can achieve very good thread quality with roll taps and not having to deal with the normal chip buildup from a cutting tap is really a plus. In the precision sheet metal shop where I worked we used the roll thread type extensively and had better results than with the cut taps. The fact that they do not produce chips helps with production work as you don't have the usual chip build up to worry about and watch for when using them. They seem to be a bit stronger as there are no flutes ground on the sides of the tool to weaken it, but improper forces exerted sideways will still possibly break the tap.
Regards,

[icecreamjones]

Roll forms are pretty awesome, we also use them 95% of the time because threads we make are between #2 and #10 size (very small!) and they just don't break unless you bottom out. They even kick butt on stainless steel with proper lube.
A formula I use for the drill size is:
nominal tap size minus half the lead
so a 1/2-28 would be .250-(.0357/2) or about .232 which would be letter "a" drill or 15/64"

[photomankc]

I'd heard that the roll taps were not for hand tapping. If that is not the case I'd be willing to order them. The space is pretty tight and I'd be doing it horizontal. I'll post up a picture here to show the location we're talking about.

I found this:

Amazon.com: WoodRiver Drill Guide With 6 Bushings: Home Improvement

Amazon.com: WoodRiver Drill Guide With 6 Bushings: Home Improvement

I'd imagine I could find a way to clamp that down in position if I can find proper bushings for the tap drill and tap. I could make something similar too but I'm not sure I want to if a $14 bit will work for it.

I may be able to make something though to hold bushings that I can find in the mail order places.

[metalworkz]

Hello,
I have never heard that roll taps were not intended for hand tapping, and I did it on a daily basis when with the prior employer. At that time I was the lead person in charge of the area doing all the tapping for a San Francisco bay area precision sheet metal shop with many Silicon valley customers. We used them in multiple head tapping machines, tap heads (Procunier) individual tapping heads, hand tapping pneumatic guns, and anything that needed to be done by hand with excellent results. I believe Ballax was the mfr. for the taps we used and I have a Ballax roll tap drill size chart I will try to find, scan and upload if it will be of any help. Are the holes the ones along the front of the mill table to support/mount the Caliper DRO? For 2 simple mounting holes of this sort I would think even a steady hand drill without drill fixture would probably get you bye, and drilling straight in horizontally should be easier to hold vertical to the surface than doing similar holes in a vertical orientation. If you don't use a fixture with bushings be sure to get a good center punch on location and start with a small drill diameter and step up in increments to the tap size drill. Remember the smaller drills will bend fairly easily so only push hard enough to keep the bit cutting and back out of the hole several times to relieve the chips during the drilling process. If the drill does drift off location you can usually angle the next step drill so as to move the hole center in the right direction to correct for any drift, and by just correcting a small amount each step you can measure the location and see if it is right. You should have enough tolerance to still end up with good mounting holes even if you don't go for the extra cost of bushings and a fixture. Naturally if cost is not a problem them the bushings and fixture may be the most accurate way to be sure it all works out well. I will see if I can find the drill size chart for roll taps and post it here.
Regards,

[icecreamjones]

yeah, you can definitely hand tap with roll forms. Makes no difference. Balax are also the ones we use.

[metalworkz]

Hello,
Here is the chart I was talking about for roll thread drill sizes. I hope this somehow helps you with your project.



Regards,

[photomankc]

Awesome! Thanks for the chart. That will really help. I hear nothing but good about roll taps so I may give that a whirl.

I was actually originally hoping to use the existing holes where the pointer was mounted but the clearances would only allow for a single bolt for the electronics slider to mount to the saddle. I didn't like that idea so I need to drill two holes about 1.313" from the center. I actually just used a ruler and the bracket attached to the scale itself with some transfer punches to center punch the correct locations.

My build thread is here if you want to see a bit more detail of what I am doing.
http://www.cnczone.com/forums/showthread.php?t=102938